Index
3
AIRCRAFT ELECTRICAL SYSTEMS ................................................... 3-3
3.1
AIRCRAFT BATTERIES .................................................................... 3-3
3.1.1 Battery workshops .......................................................... 3-4
3.1.2 servicing lead acid batteries ............................................ 3-7
3.1.3 Servicing ni-cad batteries ................................................ 3-11
3.2
AIRCRAFT BATTERY SYSTEMS ....................................................... 3-17
3.3
DC POWER SYSTEMS .................................................................... 3-27
3.3.1 DC generation................................................................. 3-27
3.3.2 Voltage regulation ........................................................... 3-42
3.3.3 System indication ............................................................ 3-49
3.3.4 System management ...................................................... 3-50
3.4
SYSTEM PROTECTION ................................................................... 3-53
3.4.1 Current Protection ........................................................... 3-53
3.4.2 Undervoltage Protection.................................................. 3-53
3.4.3 Load sharing ................................................................... 3-56
3.4.4 Alternators ...................................................................... 3-61
3.4.5 Starter generators ........................................................... 3-61
3.5
AC POWER SYSTEMS ..................................................................... 3-65
3.5.1 AC generation ................................................................. 3-65
3.5.2 practical generator construction ...................................... 3-68
3.5.3 star & delta systems........................................................ 3-71
3.5.4 Voltage regulation ........................................................... 3-73
3.5.5 Frequency control ........................................................... 3-75
3.5.6 System layouts ............................................................... 3-85
3.5.7 System control and management .................................... 3-88
3.5.8 Paralleling ac generators ................................................ 3-88
3.5.9 Load sharing ................................................................... 3-91
3.5.10 Ac system protection..................................................... 3-97
3.5.11 Ac system indications ................................................... 3-101
3.6
POWER CONVERSION .................................................................... 3-103
3.6.1 inverters .......................................................................... 3-103
3.6.2 Transformer rectifier units ............................................... 3-110
3.7
CIRCUIT PROTECTION DEVICES ...................................................... 3-113
3.7.1 Fuses .............................................................................. 3-113
3.7.2 Circuit breakers ............................................................... 3-115
3.7.3 Current limiters ............................................................... 3-116
3.7.4 LIMITING RESISTORS ................................................... 3-117
3.8
OTHER POWER ............................................................................. 3-118
3.8.1 Auxiliary power unit ......................................................... 3-118
3.8.2 External AC & DC power................................................. 3-121
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-1
THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-2
3
AIRCRAFT ELECTRICAL SYSTEMS
3.1 AIRCRAFT BATTERIES
The most common forms of secondary cells used in aircraft are Lead Acid and
Nickel Cadmium.
Lead acid cells have a nominal voltage of 2 Volts, therefore a typical 24 volt aircraft
battery would consist of 12 cells connected in series. The active material in the
positive plates is Lead Peroxide and in the negative plates, Spongy Lead. The
electrolyte is dilute sulphuric acid with a specific gravity of between 1270 and 1280
when fully charged.
In a typical Lead acid battery the cells are formed within a single block of composite
material. Interconnection of the cells is achieved by connections embedded inside
the block, the only exposed conductors being the two terminals for connection to the
aircraft electrical system.
Nickel Cadmium cells have a nominal voltage of 12 volts, therefore a typical 24 volt
aircraft battery would comprise 20 cells connected in series. The active material in
the positive plates is Nickel and in the negative plates Cadmium. The electrolyte is a
solution of potassium hydroxide and distilled water, with a specific gravity of
between 1240 and 1300.
In a typical Ni-Cad battery, individual cells are mounted in a metal case that
incorporates 2 venting outlets, carrying handles, a quick release connector and a
lid. Each cell is separated from its neighbour by its moulded plastic case and
electrically connected by nickel plated steel links between the terminals.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-3
3.1.1 BATTERY WORKSHOPS
Batteries are maintained and tested in specialised battery workshops. Although no
two workshops are identical the general maintenance and testing procedures are
standard and apply to all workshops, as do the safety requirements.
There are two types of battery in common use i.e. lead acid and Ni-Cad (alkaline)
and therefore two types of procedure and workshop. It is important to prevent
contamination of lead acid cells by NiCad cells and vice versa, therefore batteries
and their maintenance equipment must be kept apart. This is achieved by having
separate charging rooms, not transferring equipment from one charging room to
another, and by clearly identifying the equipment and areas with notices.
Lead acid contamination will destroy an Alkali battery, contamination may be
indicated by the electrolyte having a blue tint.
All battery workshops are CAA approved, and whilst no two are identical there are
general requirements for both lead-acid and alkaline workshops.
3.1.1.1
Battery Charging.
The battery charging board should contain an ammeter, a variable resistor and
supply terminals. A digital voltmeter and hydrometer are also required for taking
measurements.
Charging board supplies are higher than the battery voltage, the voltages used
depend on how many cells the board is designed to have attached to it
simultaneously. Lead Acid batteries require a final charging voltage of 2.8 volts per
cell, Ni-Cad batteries, 1.75 volts per cell.
Batteries must be connected to the charging board with the variable resistor set to
maximum and the supply switched off.
The number of batteries that can be connected to the charging board in series
depends on the charging board supply voltage. When batteries are connected in
series, they should have the same capacity, be at the same state of charge and
have the same charge rate. If batteries have different charge rates, the lowest rate
should be used.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-4
Parallel connection of batteries to one pair of output terminals is NOT
permitted. It is not possible to determine the current through each battery, unless
individual ammeters are used for each one.
Cables used to connect batteries to the charging board should be of sufficient
current rating and fitted with the appropriate terminals for connection to both the
battery and the charging board.
If charging is interrupted for any reason, the charger should be switched off and the
battery disconnected.
If a battery within a group reaches a charged state before the remaining batteries,
the following procedure should be used:

Increase the resistance value to a maximum and switch of the charger
supply


Disconnect the battery and reconnect the remaining batteries
Ensure the resistor is set to maximum, switch on the charger supply and readjust the current to the required value.
3.1.1.2
Capacity Testing.
The capacity of a battery depends not only on its age, but on the amount of current
drawn from it. The greater the current drain, the lower the battery capacity.
Therefore, the nominal rating of aircraft batteries includes an hourly rate.
The capacity (Ah rating), is the amount of energy the battery should be able to
provide from new, or reconditioned, until the end of it's useful voltage on load. The
AH rating is divide by the hourly rate to give the continuous current on which this
rating is based. The hourly rate is the length of time the battery should be able to
provide this current.
i.e. A 40Ah @ the 1hr rate battery, should be able to provide 40 amps continuously
for 1 hour. Whereas a 40Ah @ the 10hr rate battery will be able to supply 4 amps
continuously for 10 hours.
If either battery is discharged at a higher current it will not provide the full capacity of
40Ah.
As the capacity of a battery varies with age, capacity tests need to be carried out at
regular intervals, normally every three months, however a capacity check should
also be carried out:

After initial charging

Whenever the battery capacity is in doubt
To measure capacity, the battery is fully charged and connected to a suitable
discharge control panel. The panel should incorporate a variable load resistor, an
ammeter and an ampere-hour meter. A digital voltmeter is also necessary for
measuring cell or terminal voltages.
The battery is discharged at the hourly rate until fully discharged. Aircraft batteries
are considered discharged when the cell voltage drops to 1 volt for Ni-Cad cells, or
18 volts for Lead Acid cells.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-5
When the battery is fully discharged, the capacity is calculated using the following
formula:
Actual capacity = Measured A-h rating x 100%
Rated A-h rating
Ex.
A 40 A-h at 1hr rate battery would be discharged at 40 amps until fully
discharged, with the time taken to discharge being recorded.
If the time taken to discharge was 48 minutes, the capacity would be
calculated as follows:
40 x 48 = 80%
40 x 60
For a battery to be returned to service it must have a minimum capacity of
80%. When new, some batteries will achieve more than 100%.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-6
3.1.2 SERVICING LEAD ACID BATTERIES
There are many reasons for a battery arriving at a battery workshop. The reason
should be clearly stated on the battery paperwork. Some reasons could be:
A new battery requiring servicing prior to initial issue
Low voltage
Failure to maintain charge
Aircraft voltage regulator fault
Overheating
Leaking
Aircraft heavy landing
Routine service
On arrival at the workshop, the battery must first be visually inspected for condition.
3.1.2.1
Charging
The charging of lead acid batteries may be related to the condition in which they
arrive at the workshop:

dry uncharged

filled, uncharged and requiring an initial charge

in service. Requiring workshop service or recharge
3.1.2.1.1
Dry uncharged batteries
The general procedure adopted for a dry, uncharged battery arriving at the battery
workshop is a follows:
1. Fill the battery with electrolyte.
2. Allow the battery to stand.
3. Charge.
4. On completion of the charge.
5. Allow the battery to cool.
3.1.2.1.2
Filled uncharged batteries
Batteries of the solid block variety normally arrive at the battery workshop in this
form when new. In this condition only the positive plates are formed, the battery
remaining inert until prepared for use. The general procedure adopted for these
batteries is as follows:
1. Check electrolyte level.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-7
2. Charge.
3. Allow to cool.
3.1.2.1.3
In service batteries
1. Check the electrolyte level.
2. Charge the battery.
3. Allow the battery to cool.
4. Check the electrolyte level.
3.1.2.2
Fully charged indications
On reaching a fully charged state, and whilst still connected to the charging board, a
lead acid battery will display three distinct features, the:

battery will gas freely

specific gravity of the electrolyte will remain constant

terminal voltage will remain constant at 30 to 32.4 volts for a good condition
24 volt battery
As a battery ages, the final terminal voltage will fall. If the terminal voltage of a 24
volt battery remains constant at less than 28.5 volts when the charge current is still
flowing, the battery must be considered unserviceable and should not be put back
into service.
With a solid block type battery it is difficult to measure the specific gravity of the
electrolyte, therefore the fully charged state is indicated by gassing and confirmed
by carrying out an open circuit voltage check.
3.1.2.3
Open circuit voltage check
Exact procedures for open circuit voltage checks vary. One method, using a 24
volt, 18 ampere-hour battery, is as follows;

Connect the battery to a 20 amp load for approximately 15 seconds.

Measure and record the battery terminal voltage.

Remove the load and immediately measure the terminal voltage.


If the battery is in a good condition the increase in voltage should be a
minimum of 1 volt.
The charge state of the battery is indicated by the off-load voltage;
25.1 to 25.8 volts - the battery is fully charged
24.5 to 25.1 volts - the battery is ½ to ¾ charged
24.2 to 24.5 volts - the battery is ½ charged
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-8
The procedure for a 12 volt battery is the same. The condition of the battery being
indicated by voltages equal to half those shown above.
3.1.2.4
Capacity testing
After charging, a capacity test should be carried out. The battery is discharged at
its hourly rate until the SG reaches 1150, or the terminal voltage drops to an
average of 1.8 volts per cell. The capacity is then calculated, and if below 80%, the
battery cannot be returned to service.
3.1.2.5
Insulation Test
An insulation resistance test should be carried out:

at periods specified in the maintenance schedule

whenever electrolyte leakage or a damaged cell case is suspected
To carry out an insulation test, the battery must be fully charged, the cell tops and
case must be dry and the battery must be secured by its normal fixtures to a metal
plate.
A 250 volt insulation tester is then connected between one terminal and the metal
plate, and a resistance measurement made. The minimum permitted value is 1M,
if the resistance value is less than this, the battery should be inspected for damage
and moisture, the cell tops and case should be checked to ensure they are dry and
the test should be repeated. If the resistance value is still low, a leak check may be
carried out.
3.1.2.6
Leak Test
A leak test may be carried out as a matter of routine, or when a leak, or damaged
cell case is suspected.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-9
The leak tester simply consists of a small hand operated pump, fitted with a
pressure gauge. Prior to use, the pump should be checked to ensure it has no
leaks, and the battery case should be visually checked for cracks.
To carry out a pressure test, the vent plugs are removed one at a time, and the
tester is pushed firmly against the cell openings. A pressure of 2 p.s.i. (14 KN/m) is
applied to each cell in turn by slowly operating the pump. The pressure being
indicated on the gauge. The pressure of each cell is then monitored for a period of
15 seconds to ensure it doesn't drop. Goggles must be worn whilst performing a
leak check and all vent plugs must be replaced after the test.
3.1.2.7
Preparation for Service
Preparation for service consists of:

Ensuring all work has been carried out.

A final inspection, ensuring the vent caps are fitted.

Applying a light smear of acid free petroleum jelly to the connectors.

Completing all documentation and certifying that the battery is ready for
service. Every battery has a service record, this provides a comprehensive
history of the battery. The exact content and layout varies between service
organisations.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-10
3.1.3 SERVICING NI-CAD BATTERIES
The possible routes taken by a Ni-Cad battery when it enters the battery workshop
are better shown in a flow diagram. See below.
Inspect
Signs of arcing or burning
YES
NO
Discharge to 1V per cell on
known load
Allow battery to rest / cool
Totally discharge battery.
Strip the battery down and
inspect cells, case and links.
Replace as necessary
Charge at the 1 hour rate
During charging, cell
voltages should be within 1
volt of each other.
YES
NO
Carry out cell balancing
Carry out capacity check
Is capacity at least 80%
Carry out
cell
balancing
Carry out
capacity
recycling
YES
Allow battery to rest / cool
Charge at the 1 hour rate
NO
NO
During charging, cell voltages
should be within 1 volt of
each other.
Are all cells
serviceable
NO
Discharge
battery and
replace cells as
necessary
YES
Complete all paperwork
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-11
YES
3.1.3.1
Inspection
When a battery is received at the workshop, its service record should be located. A
new service record will have to be raised if the battery is new, or has not been
serviced at the workshop before.
Once a service record is obtained, the battery should be inspected;
WHEN WORKING ON THE BATTERY WITH THE COVER REMOVED, GREAT
CARE MUST BE EXERCISED TO ENSURE THAT TERMINAL CONNECTING
LINKS ARE NOT SHORTED. NO JEWELLERY SHOULD BE WORN.
3.1.3.2
Electrical leakage paths
Potassium carbonate crystals and other contamination on the cell cases can lead to
electrical leakage paths. Contamination of any form must therefore be removed. It
should be possible to remove most deposits using a clean cloth soaked in demineralised water and a stiff non metallic brush. In some cases it may be
necessary to scrape deposits loose using a non-metallic scraper.
The only way of detecting leakage paths is to measure the discharge current, this
can be achieved by connecting a meter between the positive terminal of the battery
and an exposed part of the metal case. With the meter set on an appropriate range,
(approximately 1 amp) the current measured should be within the limits specified by
the manufacturer, typically 0.02 amps.
If the limit is exceeded, the battery should be thoroughly cleaned and the current remeasured. If the limit is still high, the battery must be stripped, cleaned and
reassembled. Before the battery can be stripped it must be totally discharged.
If the leakage current is high, it is possible that a cell is leaking. A leaking cell can
be found by measuring the voltage between each cell connecting link and the
battery case. The lowest voltages will be found either side of the faulty cell.
A suspected leaking cell must be replaced.
3.1.3.3
Charging
After inspection, the battery should be discharged at the 1 hour rate until the cell
voltages drop to 1 volt.
Once a battery is discharged, or after a battery has been rebuilt, it must be charged.
Constant current charging is used in the workshop, with the current being applied at
one rate for a fixed period of time, or at a number of different rates, each for its own
time period. The total charge given to the battery will be equal to 140% of the
battery capacity.
Before charging the battery, the vent caps must be loosened, but left in place in the
cell tops. During the final stages of the charge period, gassing will occur and the
electrolyte level will fall. Excessive gassing should be avoided, however, a certain
amount is necessary to ensure the battery reaches the fully charged state.
The charge state of a Ni-Cad battery cannot be determined by the terminal voltage,
nor by the specific gravity of the electrolyte. The only way to ascertain the charge
state is to carry out a measured discharge.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-12
DURING CHARGING BATTERIES OR INDIVIDUAL CELLS, CARE MUST BE
TAKEN TO ENSURE THERMAL RUNAWAY DOES NOT OCCUR.
At periods during the charge cycle, cell voltages should be measured using a digital
voltmeter. The voltages of individual cells should all be within approximately 01
volts of each other. If individual cell voltages differ by more than the manufacturers
recommended amount, cell balancing must be carried out.
Differences in cell voltages can develop over a period of time, especially on
constant voltage charging systems and is referred to as unbalanced cells.
Variations in cell voltage reduce battery capacity and can result in the reverse
charging of a cell when the battery is fitted to an aircraft.
3.1.3.4
Cell balancing
This is a process of controlled discharge of cells to monitor that they all will
discharge at a relatively constant rate. Once discharged the cells are then charged
and the cell terminal voltage is monitored.
Any weak or short circuit cells found during the discharge process must be replaced
before the battery is recharged.
3.1.3.5
Voltage recovery check
A voltage recovery check can be used to find high resistance short circuits or
damaged connection within the cells. The test can be carried out at the end of a
cell balancing test, prior to recharging the battery, or at any other time, by
discharging the battery, applying shorting links and leaving the battery to stand for
16 to 17 hours.
After leaving the battery standing for the required period, individual cell voltages are
measured to ensure they are below the minimum required voltage specified for the
battery. A typical value would be 02 volts.
The shorting links are then removed and the battery allowed to stand for a further
24 hours. At the end of this period, the cell voltages should have recovered to a
value specified in the maintenance manual. A typical value being 1.08 volts.
On completion of charging and any additional tests, the battery is allowed to stand
in order to cool prior to being given a capacity test.
3.1.3.6
Capacity test
Capacity testing is completed in the same manner as for a lead acid battery. The
discharge normally being carried out at the 1 hour rate.
In order to be certified for use, the battery must give at least 80% of the nameplate
capacity, or the minimum authorised design capacity, whichever is greater.
Some American batteries have new capacities substantially higher than the
nameplate capacity. Once the nameplate capacity of these batteries is no longer
obtainable, the battery must be rejected. A list of these batteries can be obtained
from the CAA.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-13
The TRUE capacity must always be recorded and therefore a full discharge is
necessary. During the final stages of the discharge, the cell voltages drop very
quickly and great care must be taken to ensure no cell goes into reverse polarity. As
each cell reaches 1 volt it should be shorted using a 1 resistor.
3.1.3.7
Capacity Recycling
If a battery fails the capacity test, a capacity recycle can be carried out. Discharge
rates vary, therefore reference must be made to the appropriate manual.
Generally the battery is discharged at the 1 hour rate. As each cell voltage drops to
1 volt, it is shorted using a 1 resistor. Once all the cells are shorted, the battery is
allowed to stand for a minimum of 16 hours, preferably 24 hours, before removing
the links and recharging the battery at the recycling rate specified by the
manufacturer.
Approximately 5 minutes into the charge, the cell voltages are measured and
distilled water is added to any cell with a voltage greater than 1.5 volts.
After a further 5 minutes, any cell above 1.55 volts or below 1.2 volts must be
rejected and replaced.
After 20 hours, the cell voltages are measured and recorded and electrolyte levels
are adjusted as necessary. After 24 hours, the cell voltages are again measured
and compared with the reading taken at 20 hours. If the 24 hour voltage is more
than 0.04 volts less than the 20 hours reading, the cell must be rejected.
3.1.3.8
Replacing cells
Cells will require replacement for the following reasons:
a.
a suspected leaking case.
b.
an internal short circuit
c.
failing a cell balancing test.
d.
failing an insulation resistance test.
e.
failing a capacity recycle.
Should more than six cells in a battery require replacement, it is generally
recommended that all the cells are replaced. Serviceable cells removed from a
battery can be grouped with cells of the same potential difference from other
batteries, and used to replace unserviceable cells in other batteries.
To remove a cell from a battery, the battery must be fully discharged, the cell
connecting links must be removed and the vent cap should be loosened. An
extractor tool is then fitted to the battery terminals and the cell withdrawn vertically
from the battery. After removal the vent cap must be retightened.
Replacement cells must be identical to those already fitted in the battery and must
therefore:
a.
have the same part number.
b.
be from the same manufacturer.
c.
have the same capacity.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-14
New cells should be fitted using only a steady downward force. Mallets or other
tools must NOT be used to force a cell into the case.
Once new cells are fitted, the connecting links should be re-connected and torque
loaded to the required value. To confirm that cells are correctly connected, a cell to
cell voltage check must be carried out.
Having had cells replaced, the battery must be recharged and given a further
capacity test before it can be released for service.
Any rejected cells must be correctly disposed of, or clearly identified „FOR
GROUND USE ONLY‟.
3.1.3.9
Preparation for Service
Preparation for service consists of:

Ensuring all work has been carried out.

A final inspection, ensuring that vent caps are fitted and connecting links are
correctly secured.

Ensuring that the cell tops are clean and dry.

Fitting the battery cover and any associated seals or mats.

Completing all documentation and certifying that the battery is ready for service.
The battery record for Ni Cad batteries differs slightly from the lead acid battery
in that it generally has a column for each cell. Exact formats and contents differ
between maintenance organisations.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-15
THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-16
3.2 AIRCRAFT BATTERY SYSTEMS
The majority of light aircraft employ a single battery for both engine starting and as
a source of emergency power. Some large commercial aircraft are also fitted with
only a single battery, this being used for both APU starting and as a source of
emergency power, however many large aircraft have separate batteries for each
task.
Some aircraft have multiple batteries connected in parallel to give extra capacity,
and a few aircraft employ a switching arrangement that connects the batteries in
parallel for normal operation and series for engine starting.
3.2.1.1
Fixtures and Fittings
The fixtures and fittings employed on battery systems depend very much on the
type of battery used and the type of aircraft to which it is fitted. The following points
are of a general nature.
3.2.1.1.1
Mountings
The batteries are normally mounted on special trays located in special
compartments, these provide protection for the aircraft structure, ventilation and
heat dissipation.
3.2.1.1.2
Ventilation
Ventilation systems are used to remove battery gasses that escape from the cells
via the vent caps and to assist battery cooling. The ventilation system is connected
directly to the battery case, or to the battery stowage compartment.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-17
Various types of ventilation system exist. In the simplest form, one side of the
battery is connected to a ram air inlet and the other to an overboard drain or vent.
The ram air passing across the battery or through the battery compartment removes
both gasses and heat. In other systems the same flow is achieved by using a
venturi on the outside of the aircraft, or by using pressurised air from the cabin. A
non-return valve must be fitted to any system using cabin pressure so as to prevent
a flow of gasses into the cabin when the aircraft is on the ground and
unpressurised.
3.2.1.1.3
Acid traps
In lead acid systems an acid trap may be fitted after the battery to catch any acid
that has escaped from the battery vents.
3.2.1.1.4
Battery connection
To avoid the use of long cables with their associated voltage drop, the battery is
normally located near to the main battery bus or distribution point.
Connection of the battery to the aircraft system may be by simple eye end lugs that
are secured by nuts and bolts, or more commonly by a special connector. Various
types exist, but all consist of a plastic housing containing 2 shrouded pins, the
sockets being located on the battery. The connector is either “push-in” or employs
a threaded lead screw. Both require some form of locking.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-18
3.2.1.2
Light aircraft battery systems
Light aircraft battery systems generally comprise:

A lead acid battery

Battery contactor

Battery Master Switch

Hot Battery Bus
A bus is simply a distribution point. It is where power from a source is distributed to
the aircraft loads requiring it. Buses can be hard to identify, especially on light
aircraft where they may simply take the form of a terminal block. On larger aircraft
with multiple circuit breakers (CB‟s), the bus comprises a copper strip that
interconnects the live terminals of the CB‟s to the power source.
The battery system is connected as shown in the diagram below. The main bus
provides the interconnection point between the battery system and the generation
system, and is the point to which most of the aircraft equipment is fitted.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-19
3.2.1.3
Large Aircraft Battery Systems
The main battery system fitted to a large aircraft has the same basic components as
the light aircraft system, however, there are likely to be more busbars. Two large
aircraft battery systems are shown below. Although the two layouts look different,
the primary difference is the form of battery charging employed. This will be
investigated in the next chapter.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-20
3.2.1.4
Aircraft Battery Charging
On the majority of light aircraft, the battery is charged directly from the output of the
generator, rather like a car battery. Providing the generator output voltage is
greater than the battery voltage, current will flow from the generator, via the main
bus and battery bus, through the battery and then back to the generator via the
earth return.
The size of the charge current depends on the potential difference between the
generator terminals and the battery terminals, which in turn depends upon the
voltage regulator setting and the state of charge of the battery.
Some large aircraft employ a similar system, the constant voltage d.c. Source being
provided by transformer rectifier units (TRU‟s) as opposed to d.c. generators.
Overcharging is prevented by current limiters fitted in the cables between the buses
and the batteries.
Although battery chargers may be found on some light aircraft, they are more
common on large aircraft. There are numerous types of charger but only two will be
examined in more detail. It should be noted that whilst exact charging methods
vary, all chargers commence by using some form of pulsed or constant current
mode and then switch to a constant voltage mode. When in constant voltage mode
the system operates in the same manner as a light aircraft system or a TRU type
large aircraft system. The charge current being determined by the difference in
potential between the charger terminals and the battery terminals.
Aerodynamics, Structures and Systems (CE)
Page 3-21
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
3.2.1.4.1
Boeing 767 charger
The battery charger used on the 767 uses 115v 3 phase 400 Hz a.c. and can
completely charge a battery in 75 minutes. The charger has two modes of
operation; charge mode and transformer rectifier mode.
The charge mode has 2 regions:
a constant current region where the current is maintained constant at 38 
2A and the voltage varies 20 - 36V.
a constant voltage region where the voltage is maintained constant at
27.75V and the current varies 0 - 38  2A.
The charger senses the terminal voltage of the battery, if below 23V it switches to
constant current mode. The battery voltage will then rise to the temperature
compensated inflection point. Under normal ambient conditions this will be when
the battery voltage has reached 31V, under low temperature conditions it may be as
high as 36V.
The period from switch-on to the inflection point is called the base charge and is
memorised by the charger.
Having reached the inflection point the battery is given a proportional overcharge,
this being based on a percentage of the base charge time. The charger then
switches to constant voltage mode.
Battery charging commences independent of battery voltage:
When ever power is initially applied.
if the charger power is interrupted for longer than 0.5 seconds in any mode.
if the sensed voltage drops below 23 volts.
if the charger has operated for longer than 0.5 seconds in transformer
rectifier mode and is then switched to charger mode.
The charger will shut down:
if the input voltage is greater than 134 volts or less than 94 volts. The
charger will return to normal operation as soon as the undervoltage clears
and 10 to 25 seconds after an overvoltage condition.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-22
when the sense and control cable between the charger and battery is not
connected. Charger returns to normal operation when the lead is reconnected.
when battery power is not connected. Unless the voltage sensed at the
battery terminal is greater than 4 volts charging will not commence.
if the battery temperature reaches 63ºC (145ºF). Charging will commence
when the battery cools to 57ºC (135ºF).
in the event of an internal over-current or inverter imbalance, this causes a
cyclic shutdown of fixed duration.
3.2.1.4.2
Boeing 747 charger
On the Boeing 747, charging of a discharged battery starts at 60 amps. As the
battery terminal voltage increases the charge current falls. At 26 amps 33 volts,
charging stops, the current reducing to zero. The time taken to reach this first
switch off point will not be longer than 50 mins.
As the battery discharges, the terminal voltage gradually decreases. At 28 volts the
charger switches on again, the current rising to 35 amps, the voltage to 30 volts.
The battery voltage increases and the current decreases, at 33 volts 26 amps the
charger switches off again.
This cycle is repeated seven times. The charger then switches to a constant 28 volt
output.
The timing of each pulse is determined by the time taken for the charge current to
fall to 26 amps, generally only a few seconds.
The time between pulses is governed by how long it takes for the battery voltage to
fall from 33 volts to 28 volts, this will be somewhere between 30 seconds and 30
minutes.
With the charger at 28 volts and the battery also at 28 volts, the charge current will
be less than 0.5 amps.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-23
3.2.1.5
Effects of low Battery Voltage
If the terminal voltage of a fitted Ni-Cad battery drops to a very low value, the
battery will be charged as previously described, however, when the voltage drops
too low, it is possible for a single cell to reach zero volts. If a cell is allowed to reach
zero volts and the load current is maintained, the voltage across this cell will
reverse. When the charger commences charging, the cell will be reverse charged
and the battery will not reach full capacity.
3.2.1.6
Thermal runaway on fitted batteries
Although thermal runaway affects both lead acid and Ni-Cad batteries, Ni-Cad‟s are
more susceptible. This has already been discussed in the section on Ni-Cad
battery construction. Once fitted to an aircraft, there are several system fault
conditions that could lead to thermal runaway, among these are:

an incorrectly adjusted voltage regulator

frequent or lengthy engine starts

loose cell connecting links

low electrolyte levels

the use of poorly regulated ground support equipment

high charging currents

unbalanced cells
3.2.1.7
AWN 82 and AWN 88
Air Worthiness Notices 82 and 88 are titled electrical generating systems, however,
they have a great deal to do with batteries.
Investigations at the time of their introduction revealed inadequacies in the
generator failure warning systems on aircraft. It was possible for a generator failure
to go unnoticed for a considerable length of time, resulting in serious depletion of
the battery.
AWN No 82 states requirements for the modification of certain aircraft. The
modification requires a visual display that gives a warning when the battery starts to
support the main electrical load, or when the generator output is lost at the busbars.
In either event, the battery must be able to support essential services for a minimum
period of 30 minutes. This period includes 10 minutes for the pilot to notice the fault
and carry out the necessary load shedding drills, and 5 minutes for landing. When
ensuring the battery can supply the aircraft loads for the required period, several
points must be taken into consideration:

flight conditions must be assumed to be cruise at night

some form of attitude reference must remain operable

essential communications must remain operable

essential cockpit lighting is required


the pitot heater must remain operable (if aircraft cleared to fly in icing
conditions)
any services essential to safe flight must remain operable
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-24

the fact that not all services can be shed
In addition, to allow for partially discharged or older batteries, only 75% of the
nameplate capacity may be used for the battery capacity calculations. Also, the
voltage time characteristics for the battery may not be extended beyond 21.5 volts
for a 24 volt battery or 10.75 volts for a 12 volt battery.
Where calculation reveals that the battery is not capable of providing the necessary
power, consideration must be given to fitting either a larger battery or additional
batteries.
To carry out the calculations necessary to ensure that the battery complies with
AWN No 82 or 88, a copy of the aircraft load analysis is required. This details the
current drawn by all the electrical systems on the aircraft. If a load analysis is not
available, the current drawn by aircraft equipment must be calculated or measured.
3.2.1.7.1
An example of the AWN 82 compliance calculations
Assuming a 25 A-h battery, the necessary calculations are as follows:
Battery - 25 A-h = 1500 amp / minutes
A.
Battery capacity available
= 75% of 1500 amp / mins
= 1125 amp / mins
B.
Pre load shed power consumed
(calculated from a/c load analysis)
= 400 amp / mins
C.
Cruise load
(calculated from a/c load analysis)
= 15 amps
D.
Landing power consumed
(calculated from a/c load analysis)
= 125 amp / mins
Cruise duration
= A - (B + D)
C
= 1125 - (400 + 125)
15
= 40 minutes
Total duration
= 10 + 40 + 5 mins
= 55 minutes
Average load
= 1125 amp / mins
55 mins
= 20 amps
Allowable increase
(10% of average load)
= 2 amps
Check the effect of a 2 amp load increase on battery duration.
cruise duration
= 1125 - (420 + 135)
17
= 33 minutes
Total duration after Mod
= 10 + 33 + 5 mins
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
= 48 minutes
Page 3-25
When carrying out an electrical modification, or series of modifications, the increase
in load current must be calculated. If the increase in load current is less than 10%
of the average load, and the battery capacity is at least 35 minutes, the modification
can be carried out without immediate amendment of the AWN 82 Battery Capacity
Analysis. It can be left until the next C of A renewal. If the increase in load current
is greater than 10% of the average load, the Battery Capacity Analysis must be
carried out in full.
3.2.1.8
Battery System Maintenance
3.2.1.8.1
Inspection of Battery Compartments
Batteries and battery stowage areas should be inspected at periods specified in the
aircraft maintenance schedule. The following points are general.
Security.
Damage.
Battery receptacle.
Overheat.
Cables.
Electrolyte levels.
Vent caps.
Ventilation system.
Acid traps.
Battery charger.
Voltage regulator.
3.2.1.9
Electrolyte spillage
When electrolyte is spilt in an aircraft the immediate aim must be to prevent spread.
Using a moistened, clean rag, soak up the electrolyte. Rinse the area taking care
not to spread the electrolyte or contaminate below floor areas. Any carpets that
become contaminated should be removed as soon as possible, ensuring that the
area below is decontaminated.
After rinsing test for contamination by using universal PH tester. If none is available
use Litmus paper. Then ensure area is thoroughly dried.
If the electrolyte is suspected of having penetrated blind or complex structures use
neutralising agents to ensure contamination is removed.
For Lead acid electrolyte spillage use either sodium bicarbonate powder or wash
down with solution of sodium bicarbonate. Remember, sodium bicarbonate is itself
mildly corrosive to light alloys and therefore requires thorough rinsing.
For Ni-Cad electrolyte spillage, rinse down the area with a 5% solution of acetic
acid.
Having used neutralising agents, rinse, test and dry affected areas.
An Acceptable Deferred Defect (ADD) should be raised detailing the area and the
level of cleansing carried out. The ADD should call for further inspections at 24
hours and 14 days. If corrosion is found, remedial action must be taken.
Consideration must always be given to further inspections, especially if any doubt
exists.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-26
3.3 DC POWER SYSTEMS
Although d.c. power is used on large aircraft, d.c. generation in generally limited to
light aircraft systems. Direct current on large aircraft is normally obtained from
TRU‟s, devices that convert alternating current into direct current.
3.3.1 DC GENERATION
D.C. generation is covered more fully in Modules 3 (Electrical Fundamentals) and 4
(Electronic Fundamentals). The following notes are therefore for revision purposes
only.
3.3.1.1
Induction of an emf
If a conductor is moved at right angles to a magnetic field, an emf is induced in the
conductor. If an external circuit is connected across the conductor, a current will
flow. The direction of the current flow depends on two factors:

direction of the magnetic field

direction of relative movement between the conductor and the field
The direction of conventional current flow can be determined using Fleming‟s Right
Hand Rule as shown below.
Fig 35
The size of the generated emf (E) depends on three factors:

strength of the magnetic field - B

effective length of the conductor in the field - l

linear velocity of the conductor - v
E = Blv
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-27
The strength of the magnetic field and velocity of the conductor can be combined
into one value referred to as „the rate of change of flux‟. Increasing the linear
velocity, or the field strength, increases the rate of change of flux and therefore the
induced emf.
The formula then becomes
3.3.1.2
d
E = l dt
Basic generator
In its simplest form, a generator consists of a single loop of wire rotated between
the poles of a permanent magnet. The rotating part of the machine is called the
rotor, the winding having the emf induced in it, the armature. The external circuit
is connected to the rotating armature via two brushes that run on two slip rings at
one end of the rotor shaft. Current flow around the circuit is then possible.
As the loop rotates, an emf is induced in the conductors on either side. Using
Fleming‟s right hand rule, it can be seen that the currents flow in opposite directions
on each side of the loop, but are in the same direction around the loop.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-28
Maximum emf is only induced in a conductor when it is moving at right angles to a
magnetic field. Therefore, the loop will also only have a maximum emf induced in it
when it is moving at right angles to the field. When the loop is moving parallel to the
field ( i.e. the wire is moving in line with the lines of flux), no emf is induced in it. At
any position between these two extremes, there will be a proportion of maximum
emf (E) induced in the loop.
The instantaneous value of the emf (e) induced in the loop is given by:
einstantaneous = Emax sin 
where Emax = Blv and  is the angle of the conductor with respect to the field.
As the loop passes the point of zero induced emf, the direction of movement of the
conductor in the field reverses. The conductor that was moving upwards through
the field, is now moving downwards through the field. Reversal of the direction of
movement causes a reversal in the direction of the induced emf, and a reversal of
the resultant current flow.
3.3.1.3
Frequency
As the loop rotates, the emf rises to a maximum in one direction, then falls to zero
and then rises to a maximum in the opposite direction, before once again falling to
zero. One complete revolution is one cycle.
The number of cycles completed per second is called the „frequency‟. The faster
the loop is rotated, the more cycles per second and the higher the frequency. In
this simple generator, the frequency only depends on the number of loop
revolutions per second.
3.3.1.4
Direct Current Generation
The output from the simple single loop generator described, changes polarity every
time the loop rotates 180 degrees and is therefore of little use as a direct current
generator.
To make the current flow in the same direction through the load, the connections to
the external circuit must be swapped over every time the loop moves past the point
of zero induced emf. This is achieved by using a split ring instead of two slip rings.
The split ring is like a single slip ring cut into two halves.
Each end of the loop is connected to one half of the split ring. Two brushes are still
used to connect the rotating loop to the stationary external circuit.
The split ring has 2 functions:

Firstly, to transfer current from the rotating loop to the stationary external circuit.

Secondly, the periodic switching of the external circuit to maintain current flow in
the same direction through the load. The switching takes place when the loop is
moving parallel to the lines of flux and therefore has no emf induced in it.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-29
Using a single loop generator and split ring, the output will be as shown above.
It can be seen that the current in the load is always flowing in the same direction.
However, because the current falls to zero twice per cycle, it is of little practical use.
The problem can be overcome by adding a second loop, positioned at 90 degrees
to the first. To connect the 4 ends of the loops requires the split ring to be cut into
4 segments, one end being connected to each segment. With 4 segments (or
more), the split ring is called as a commutator. Using two loops and a four
segment commutator prevents the output falling to zero every half cycle, but the
output amplitude still varies considerably.
A more steady output can be obtained by adding more loops, each with its own two
commutator segments.
3.3.1.5
Ring Wound Generator
Although no longer used, the simple construction of a ring wound generator makes
it ideal for explaining the operation of a multi coil machine, as described above.
The rotor consists of a laminated iron cylinder onto which 8 equally spaced coils are
wound. The junction between each pair of coils is connected to a segment of the
commutator (i.e. two different coils each have one end connected to a single
commutator segment) . The number of commutator segments equals the number of
coils. This being true for the armature windings of all d.c. machines.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-30
The brushes are drawn inside for clarity and positioned to enable switching of the
coil when zero emf is induced in it, that is when it is moving parallel to the lines of
flux.
The metal of the rotor has a very low reluctance. The flux of the main field flux
therefore flows through it, rather than through the airgap in the centre. The parts of
the coils on the inside of the rotor therefore, do not cut any flux and have no emf
induced in them.
The low reluctance of the rotor also creates a radial field in the airgap, as opposed
to the linear field in previous diagrams.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-31
The radial field means the conductors of the armature are moving at right angles to
the flux for a longer period of time and therefore producing maximum emf for longer.
This results in a flat top to the output waveforms as shown above.
The 8 coils are split into two parallel paths of four, each group of four coils being
connected in series. Because one set of four coils is moving up through the main
field and the other set is moving down through the field, the emf's induced in each
set of four coils is in opposite directions, but the emf‟s are in the same direction with
respect to the brushes.
The emf induced in four coils is as shown below. The emf in the other four coils
(not shown) is in the opposite direction but in the same direction with respect to the
brushes. It can be seen that the emf no longer falls to zero and has negligible ripple
on it.
As stated, the ring wound generator is no longer used. Although simple in
construction, there are difficulties in winding the coils through the rotor, and half of
each coil has no emf induced in it and is effectively wasted.
3.3.1.6
Practical generators
A practical generator would usually have at least four main field poles. Two poles
being reserved for small machines.
The armature coils are wound, insulated and tested before being fitted into slots cut
in the rotor. The coils are constructed in such a manner that each will pass along a
slot in the armature and return along another slot that is approximately one pole
pitch away. This type of armature is called a drum wound armature.
Pole pitch is a term used to describe the angle between one main pole and the next
main pole of the opposite polarity. The pole pitch of a two pole machine will be
180, the pole pitch of a four pole machine, 90
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-32
The top right diagram identifies four individual coils, a/a1, b/b1, c/c1 and d/d1.
Each coil passes along a slot and returns to the same end of the armature along
another slot approximately 90 around the armature. The emf induced in each side
of the coil is again in opposite directions, but assisting around the coil.
When all of the windings are fitted, each slot will contain two halves of two different
coils (an out side of a coil and a return side of another coil). These individual coils
need to be interconnected and then attached to the appropriate sections of the
commutator. There are two ways of doing this, resulting in two different types of
drum wound generator, the lap wound and the wave wound.
3.3.1.6.1
Lap Wound generators
In a lap wound generator, the end of each coil (the return side) is bent back and
connected to the start of the next coil. i.e. (using the previous diagrams) a1 would
be bent back and connected to b, b1 would be bent back and connected to c etc.
The two ends of any coil are therefore connected to adjacent commutator
segments.
The diagrams below show the construction of a four pole, lap wound generator.
Each numbered box represents a commutator segment and the lines between each
box, an armature winding. It can be seen that each armature winding is connected
between sequentially numbered commutator segments.
This form of construction is used on large, heavy current machines and not normally
found on aircraft. The number of parallel paths always equals the number of
brushes and the number of field poles. So the example shown has four brushes,
four parallel paths between those brushes and four field poles (not shown).
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-33
3.3.1.6.2
Wave Wound generator
In a wave wound generator, the end of each coil is bent forward to the start of
another coil approximately one pole pitch around the armature. i.e. (using the
previous diagrams) a1 would be bent forward to connect to the winding in the slot
below d1. The ends of one coil are therefore connected to commutator segments
two pole pitches away.
The diagram below shows the construction of a four pole, wave wound generator.
The machine has two parallel paths and two brushes, irrespective of the number of
pole pieces. This is the form of construction used in smaller machines, and
therefore commonly found in aircraft d.c. generators.
It should be noted that wave wound generators require an odd number of coils and
commutator segments in order to interconnect all of the coils.
3.3.1.7
Internal Resistance
A d.c. machine has resistance due to the:

armature windings

brushes

commutator
 brush to commutator surface contact
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-34
This is called internal resistance and can be measured across the terminals of the
generator.
For the purpose of calculation, the internal resistance is represented as a single
value in series with the generated emf. As in the case of batteries.
Due to internal resistance, the generators terminal voltage varies with load current.
As load current is increased the voltage dropped across the internal resistance
increases and the terminal voltage decreases.
When supplying a load current, the terminal voltage (V) equals the generated emf
minus the voltage dropped across the internal resistance.
V = emf + Ir
3.3.1.8
Armature Reaction
When armature current is flowing, a field is produced around the armature windings.
The overall field of the machine is then produced by the interaction between the
main field and the armature field.
The armature field is at 90 degrees to the main field of the machine and therefore
distorts it as shown. This distortion is called armature reaction.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-35
Armature reaction distorts the lines of flux away from points A in the diagram,
towards points B. The field normally operates near to saturation and therefore the
field strength at points B cannot increase, however, the field at points A does
decrease in strength. The overall effect is a weakening of the field and a reduction
in the generator output voltage.
Distortion of the field also means the magnetic neutral axis (MNA) or electric
neutral axis (ENA) is moved around in the direction of rotation, away from the
machines geometric neutral axis (GNA). The GNA is the axis that physically
divides the machine into two halves.
Movement of the ENA away from the GNA means commutation no longer occurs
when zero emf is induced in the windings. A simple way of overcoming the problem
is to move the brushes around to align with the new ENA. Unfortunately, the
position of the ENA depends on the amount of distortion, which in turn depends on
the size of the armature current. The greater the armature or load current, the
greater the distortion of the field and the further around the brushes need to be
moved. Therefore, this form of correction is only suitable for fixed load generators.
The best way of reducing or even eliminating armature reaction is to fit
compensating windings. Compensating windings are small windings, wound in
series with the armature and fitted into slots cut in the pole faces of the main fields.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-36
When armature current flows, current flows in the compensating windings producing
a field that is equal in strength, but of opposite polarity to the armature field, thereby
cancelling it.
With careful design, correction can be achieved for all values of armature current,
keeping the magnetic neutral axis on the geometric neutral axis and restoring the
overall strength of the machines field.
3.3.1.9
Reactive Sparking
The diagrams below represent the movement of the commutator under a brush.
In the lower left diagram, coil 1 is shown just prior to being shorted by the brush, the
current flowing at maximum value from left to right. Coil 2 is shown just having
been shorted by the brush, current flowing at maximum value in the opposite
direction from right to left.
When coil 1 is shorted by the brush (centre diagram), the current must drop to zero,
ready for it to go to maximum value in the opposite direction when it comes of the
brush (right diagram).
Unfortunately the coil has inductance and when shorted produces a back emf called
reactance voltage. The reactance voltage tries to maintain current flow in the coil,
preventing it dropping to zero whilst shorted by the brush. This results in an excess
of current in the coil when it leaves the brush, which produces a spark from the
trailing edge of the commutator to the brush. The sparking is called reactive
sparking.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-37
It should be noted that sparking at the commutator may also be caused by other
factors such as:

worn or sticking brushes

incorrect brush spring tension

commutator flats

proud mica between commutator segments
A simple way of overcoming the problem is to increase the resistance of the
brushes. Increasing the brush resistance reduces the time constant of the inductive
circuit, allowing the current to collapse to zero during commutation. Unfortunately,
increasing the resistance of the brushes produces a power loss and increases the
overall resistance of the machine, thereby producing greater variations in terminal
voltage with changes in load current.
A better way of overcoming the problem is to use emf commutation. The purpose
of emf commutation is to neutralise the reactance voltage that leads to the reactive
sparking.
A simple way of achieving emf commutation is to advance the brushes beyond the
magnetic neutral axis. This means the coils are under the influence of the next
main pole before being shorted and will therefore have an emf induced in them.
The induced emf will be of opposite polarity to the reactance voltage and will
oppose it, thereby reducing the current in the coil and allowing time for it to drop to
zero whilst shorted by the brush.
Unfortunately advancing the brushes is only good for one value of armature or load
current, if the armature current increases, the brushes must be advanced further.
Therefore if the load is variable the brush positions must be continually changed.
Advancing the brushes also increases the demagnetising effects of armature
reaction.
A better way of applying emf commutation is to fit commutating or interpoles
between the main poles of the machine. Interpoles have the same polarity as the
next main pole and are connected in series with the armature.
Interpoles induce an emf into the shorted coils that exactly cancels the reactance
voltage, thus allowing the current to fall to zero instantly. Being in series with the
armature means the reactance voltage is always eliminated irrespective of the value
of armature current.
By careful design the interpoles can also be used to eliminate the effects of
armature reaction in the interpole region.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-38
3.3.1.10
Generator Construction
The size and weight of generators vary considerably, but all are constructed in a
similar manner to that shown below.
The field assembly consists of a cylindrical frame, or yoke, onto which the pole
pieces are bolted. Wound around each pole piece is a field coil. The yoke has a
low reluctance and provides a path for the main field of the machine. To reduce
circulating currents the pole pieces and sometimes the yoke of the machine are
laminated.
The armature core also provides a path for the main field and is also of low
reluctance and laminated. The armature windings are located in slots cut in the
core, being wedged in with insulation to prevent them being thrown out by
centrifugal forces.
The armature windings are connected to risers attached to the commutator
segments. The commutator consisting of copper segments separated by mica
insulation.
The brush gear assembly consists of a holder and rocker. The holder allows the
brushes to slide up a down without allowing them to move laterally, the rocker
allows the brushes to be rotated around the commutator so they can be positioned
on the magnetic neutral axis.
3.3.1.11
Generator Classifications
Generators are usually classified by the method of excitation used. There are three
classifications; permanent magnet, separately excited and self excited.
A Permanent Magnet Generator has limited output power and an output voltage
that is directly proportional to speed. It therefore has limited use in main power
applications, but will be found in secondary power systems and as a source of field
supply in some a.c. generators.
Aerodynamics, Structures and Systems (CE)
Page 3-39
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
A Separately Excited Generator has its field supplied from an external source. the
output voltage being controlled by varying the field current. The only external power
source on an aircraft is the aircraft battery. The battery provides emergency power
and should not be used to excite a generator, therefore, although a satisfactory form
of construction, it is not generally found on an aircraft.
Self Excited Generators supply their own field current from the output of the
generator armature, again, the output voltage is controlled by varying the field
current. Initial excitation is provided either from residual magnetism or the aircraft
battery, the battery being re-charged from the generator output. This group may be
subdivided into three sub-groups; series, shunt and compound.
For a self excited generators to self excite; the generator must have residual
magnetism or a supply from the battery, and the field once formed, must assist this
initial magnetism. If the field produced opposes the initial magnetism, the fields will
cancel out and the generator will fail to excite, the output voltage falling to zero.
The only way to reverse the output voltage of a self excited generator is to reverse
the polarity of the initial magnetism. If the supply to the field winding, or the drive
direction is reversed, the excitation will oppose the residual magnetism, the field will
be lost and the output voltage will fall to zero.
3.3.1.12
Series Generator
Series generators have a field winding consisting of a few turns of heavy gauge wire
connected in series with the armature.
On 'No-load', there is no armature current and therefore no field current. The only
voltage generated is due to residual magnetism within the fields.
As the load current increases, the field current increases and the terminal voltage
rises. The increase in voltage more than compensates for the losses due to
armature reactance and internal resistance. As the load current increases, the
voltage continues to rise until saturation of the field occurs.
A series generator therefore has a rising characteristic and is generally only used as
a line booster.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-40
3.3.1.13
Shunt Generator
Shunt generators have a field consisting of many turns of fine wire connected in
parallel with the armature.
On 'No-load', the terminal voltage is a maximum. As the load current increases, the
terminal voltage decreases due to the resistance of the armature and armature
reactance. If the load is increased beyond the operating limits of the generator, the
output voltage will „turn-under‟ or drop to zero. This is because the load starts to
shunt or short the field winding. Turn-under provides a good safety feature should
the generator be shorted.
The shunt generator has a falling characteristic and is used for the production of
d.c. power on aircraft.
For a shunt generator to self excite additional conditions to those already mentioned
must exist:

The field resistance must be below a critical value.

The load resistance must not be too low or it will short the field.
3.3.1.14
Compound Generator
Compound generators have both series and shunt field windings and fall into one of
two categories:

Differential compound generators, in which the two fields are wound so as to
oppose each other.

Cumulative compound generators, in which the fields are wound so as to
assist each other.
The differential compound generator is generally used where a high initial voltage is
required, but only a low running voltage. Devices such as arc welders or arc
lighting may use this form of generator.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-41
Cumulative compound machines can be made to produce over, level or under
compounding. Under compounding is more common in aircraft generators, the
output voltage falling slightly as the load current is increased.
3.3.1.15
Ventilation
The maximum output from a d.c. generator is determined primarily by its ability to
dissipate heat. Methods of cooling vary. Large, low powered generators are
normally cooled naturally by convection and radiation. Smaller, high power
generators will need some form of cooling system that blows or sucks air through
the generator. This may be achieved using ram air from a propeller slipstream, or
directly from movement of the aircraft through the air, or by a fan attached to the
rotor shaft of the generator.
3.3.2 VOLTAGE REGULATION
The output voltage from a d.c. generator must be kept constant irrespective of
varying engine speed and changes in the load current.
Control of the output voltage, or voltage regulation, can be achieved by varying the
engine speed or the generators field strength. The practical problems of varying the
engine speed make the latter the preferred method.
A voltage regulator controls the strength of the main field by controlling the field
current. Exactly how this is achieved varies, some use a continually variable
resistor in series with the field, others switch a resistor in and out of the field circuit
and some simply switch the field circuit on and off.
The three most common voltage regulators are the vibrating contact, carbon pile
and transistorised. Although the first two have, in most cases, been superseded by
the latter.
3.3.2.1
Vibrating Contact Voltage Regulator
The vibrating contact voltage regulator works by continually removing and replacing
a short circuit across a resistor that is in series with the field winding.
The regulator consists of a field resistor, in series with the field, a pair of normally
closed contacts that short circuit the field resistor and an operating solenoid that is
connected across the generator output.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-42
When the generator is first started, its output voltage is low and the magnetic pull of
the solenoid is small. The contacts are kept closed by the pull of the spring, short
circuiting the field resistor, allowing maximum field current to flow and the generator
output rise.
When the generator output voltage reaches the regulated value, the pull of the
solenoid is sufficient to overcome the spring tension, opening the contacts and
inserting the field resistor in series with the field. The field current and field strength
both decrease.
The reduction in field strength causes the generator output voltage to fall, resulting
in a reduction in current flowing in the solenoid. Once again, the pull of the spring is
sufficient to overcome the pull of the solenoid and the contacts close, short
circuiting the field resistor.
Under normal conditions the contacts will open and close approximately 50 to 200
times per second maintaining the voltage at its regulated value.
The regulated voltage can be varied by increasing or decreasing the tension of the
spring. An increase in tension produces a higher generator output voltage, a
decrease in spring tension produces a lower generator output voltage.
A problem with the basic vibrating contact type voltage regulator is that when the
contacts open, the air gap between the solenoid and iron armature attached to the
contacts is lost. The loss of the airgap causes a large increase in the density of the
flux around the coil. For the spring to pull the contact closed again, the flux density
needs to decrease considerably, which can only be achieved by a large drop in the
generator output voltage. If left uncorrected, this problem produces a large ripple
on the generator output voltage. The simplest way of overcoming the problem is to
fit a brass nipple to the top of the solenoid, to maintain an airgap, and reduce flux
density. A better way is to use an accelerator winding.
When the contacts are opened, the accelerator winding is open circuited, making its
field collapsing completely. The collapse of the accelerator field greatly reduces the
pull of the solenoid, allowing the spring to quickly re-close the contacts.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-43
3.3.2.2
Three Unit Vibrating Contact Voltage Regulator
The unit shown below contains a vibrating contact voltage regulator (to the right), a
current limiter (in the centre) and a reverse current cut out (RCCO). These
additional items provide protection for the d.c. system, and should not be studied in
detail at this point, but returned to after studying the d.c. power system.
The generator output voltage is sensed at terminal G, current flowing through the
current limiter and then through the voltage coils of the voltage regulator and
RCCO. Until the RCCO closes, the generator is disconnected from the aircraft
battery and loads.
As the generator output voltage increases, the current flowing in the current and
voltage coils of the RCCO increases. At a pre-determined value, sufficient field is
produced to close the contacts of the RCCO connecting the generator to the aircraft
battery and loads. When the contacts close, load current flowing in the current coil
of the RCCO assists the voltage coil in holding the contacts closed.
The current flowing in the shunt (voltage) coil of the voltage regulator is determined
by the generator output voltage. At the regulated voltage, the pull of this coil will be
sufficient to open the contacts, inserting a resistor in series with the field and open
circuiting the accelerator winding. Operation of the voltage regulator is as
previously described.
If the load current reaches a pre-determined maximum value, the contacts of the
current limiter will open. This inserts the resistor into the field circuit, reducing the
generator output voltage and subsequently the output current (Ohms Law V/R=I).
All the time an excessive load is present, the current limiter contacts will vibrate in a
similar manner to the voltage regulator contacts.
When the engine is shut down, or if the generator output voltage falls for any
reason, current will flow from the battery to the generator. This reversed flow of
current through the RCCO current coil, causes its field to oppose the field produced
by the voltage coil. The fields cancel and the contacts of the RCCO open,
disconnecting the generator from the battery.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-44
3.3.2.3
Carbon Pile Voltage Regulator
The carbon pile regulator has a number of carbon discs that are arranged in a pile,
and placed in series with the field winding.
The resistance of the carbon pile depends on the compression applied to it.
Increasing the compression decreases the pile resistance and decreasing the
compression increases the pile resistance.
Increasing the pile resistance
decreases the field current and vice versa.
Under static conditions, the carbon pile is compressed by a plate control spring,
attached to which is a soft iron armature. Under the armature is a solenoid, the coil
being connected across the generator output.
As the generator output voltage increases, the current flowing in the solenoid coil
increases, as does the pull on the soft iron armature.
At the regulated voltage, the force of the spring and the pull of the solenoid are
balanced.
Any further increase in the output voltage causes the solenoid to overcome the
spring force, decreasing the compression on the pile, increasing its resistance and
decreasing the generator output voltage.
As the output voltage falls, the pull of the solenoid reduces and the compression on
the pile increases, causing its resistance to decrease. The decrease in resistance
causes an increase in field current and an increase in the generated voltage and the
process repeats. Eventually the regulator settles down at the value of compression
required to provide the correct output voltage.
Adjustment of the regulated voltage can be achieved by adjusting a trim resistor
located in series with the voltage coil of the solenoid. Increasing the resistance
causes an increase in the generator output voltage and vice versa.
Changes in the temperature of the solenoid coil will affect its resistance and
subsequently the generators output voltage. A ballast resistor located in series with
the trim resistor provides compensation.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-45
Changes in temperature also effect the physical size of the pile housing, varying the
amount of compression on the pile. Supporting the pile on a bi-metallic washer
provides compensation.
3.3.2.4
Adjustment
Three adjustments are provided on the voltage regulator, however, the engineer on
the line may use only one of them, the trim resistor value.
The trim resistor consists of a small variable potentiometer and provides the
engineer with a means of adjusting the regulated output voltage. Trim resistors
allow approximately 1.5 volts of adjustment.
The voltage coil circuit resistance can also be adjusted by varying the ballast
resistance. The ballast resistor is fixed and set by the manufacturer. When
selecting the ballast resistor, the manufacturer will set the trim resistor to its central
position.
The magnet core airgap is set by the manufacturer or workshop and provides for
optimum regulation at the nominal controlled voltage.
The initial compression on the carbon pile is also set by the manufacturer or
workshop and determines the degree of regulation and stability factor of the
regulator. This adjustment is regarded as the characteristic setting of the regulator
and ensures that over the working range of the pile, the spring force and magnetic
force balance irrespective of the armature position.
The carbon pile voltage regulator eliminates the electrical noise of the vibrating
contact type voltage regulator, and has no contacts to wear or burn. However, the
carbon pile regulator provided fairly poor regulation due to mechanical stiction and
poor temperature compensation, even with a ballast resistor.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-46
3.3.2.5
Transistorised voltage regulator
The problems associated with carbon pile and vibrating contact type regulators are
overcome by using transistorised regulators. These regulators are small, reliable,
cheap, have very rapid response times and no moving parts. They also radiate no
electrical noise.
In the above example, current will flow to the base of TR2 and through the potential
divider network R1, R2 and RV1 when the alternator switch is placed in the on
position. If the alternator output voltage is less than the required value, the potential
on the zener diode will be insufficient to make it conduct and TR1 will be off.
The base of TR2 will be positive with respect to the emitter, causing the transistor to
turn on, turning on TR3 and allowing field current to flow in the alternator field
winding.
The output from the alternator is rectified within the machine to produce d.c. power
which is used to supply the aircraft electrical system. As the output voltage
increases, the voltage applied to potential divider R1, R2 and RV1 also increases.
At the regulated voltage the zener diode conducts, switching on TR1. TR1 turning
on causes TR2 and TR3 to switch off, open circuiting the alternator field and
reducing the alternator output voltage.
As soon as the output voltage starts to fall, the zener diode will again stop
conducting. TR3 will switch off, TR1 and TR2 will both switch on and the field
current will be restored, increasing the alternator output. Rapid switching of the
transistors will maintain the alternator output voltage at the regulated value.
3.3.2.6
Alternate Transistorised Voltage Regulator
In order to explain the operation of the above voltage regulator it is necessary to
establish conditions that will produce an output from the generator. To produce an
output, field current must enter the generator at terminal F1. The earth return being
through terminal F2. Therefore transistor T2 must be turned on.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-47
For T2 to be on, its base must be negative with respect to the emitter (PNP), so T1
must be off. T1 will be off if the base is not negative with respect to the emitter.
This condition will exist if the base and emitter are at the same, which will be the
case if no current is flowing through resistor R4. Therefore the zener diode D1
cannot be conducting.
Under these conditions field current is flowing and the generator is producing a
terminal voltage that is below the regulated voltage. Applied to the cathode (left) of
the zener D1 is the generator output voltage and on the anode (right) is a proportion
of the generated voltage as determined by the potential divider R1, R2 and R3.
As the generators output voltage increases, the voltage on the cathode (left)
increases at a faster rate than the voltage on the anode, developing a potential
difference across the zener. At the regulated voltage, the potential across the zener
will be sufficient to cause breakdown and conduction.
When the zener conducts a voltage drop is produced across R4 making the base of
transistor T1 negative with respect to the emitter, turning it on. Due to the voltage
drop across diode D2 the base of transistor T2 goes positive with respect to the
emitter and T2 turns off. Open circuiting the field.
The zener now has a decreasing voltage on its cathode (left) but the voltage on the
anode is held high by capacitor C1, thus zener D1 quickly ceases to conduct,
returning the circuit to its original condition ready for the sequence to repeat.
Adjustment of the output voltage can be achieved by varying the position of the
wiper on variable resistor R2. This increases or decreases the value of the
generated voltage required on the cathode (left) of the zener in order to cause
conduction, thereby varying the regulated voltage.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-48
3.3.3 SYSTEM INDICATION
3.3.3.1
Current Monitoring
To enable monitoring of each generators load current, individual ammeters are
generally provided on the control panel.
Shunts are used in the main load carrying cables so that only small gauge cables
need to be run to the ammeter.
The shunts can be located in the positive feeder cables, which requires the use of
two cables for the ammeter, or in the earth returns to the generators, which only
requires the use of a single cable.
3.3.3.2
Voltage Monitoring
A single voltmeter is normally provided for monitoring the generator output voltages.
To enable the meter to be connected to the required generator a rotary switch is
provided.
On some systems the switch may have additional positions to enable the voltages
at various points on the d.c. system to be monitored.
3.3.3.3
Warning Lights
To comply with the requirements of AWN‟s 82 and 88 and for safety reasons, each
generator must have a generator failure warning light. On single generator
systems, the warning light will be red. On a multi generator systems each generator
warning light will be amber, and a red caption will be used in the event of total
generator failure.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-49
Shown is a simple warning system that is wired through a pair of auxiliary contacts
on the generator contactor. Some systems may use auxiliary contacts on other
relays, but the system is still warning that the generator is no longer supplying the
electrical system.
On modern alternator systems, the generator failure warning is normally operated
by a „low voltage sensing unit‟ that senses the voltage of the main bus. The
warning should illuminate on a falling voltage between 25 and 255 for a 24 volt
system and between 125 and 13 volts for a 12 volt system. The warning should
also automatically extinguish on a rising voltage 05 volts above these values. The
indication can be steady or flashing, with a flash rate between 50 and 100 cycles
per minute.
3.3.4 SYSTEM MANAGEMENT
Once the necessary switches are made in the cockpit, operation of the d.c.
generation system is automatic, however the pilot must still have overall control.
This control is provided in the form of a field switch to open circuit the generator
field and a generator switch to take the generator 'off-line'. Both switches may be
incorporated into a single assembly.
When the generator is supplying the aircraft electrical system it is said to be
'on-line', if disconnected from the electrical system for any reason, the generator is
'off-line'.
Other switches may be available to enable connection, disconnection and
interconnection of individual bus bars. These will vary from system to system.
Items concerned with the monitoring and control of the d.c. electrical system
include:

generator contactor and switch

ammeters

voltmeter

warning lights

differential contactor
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-50

pilot or undervoltage relay

field switch
3.3.4.1
Generator switch and contactor
The generator contactor isolates the generator from the aircraft loads and battery.
There are two reasons for using a generator contactor:

Firstly. Direct current generators used on aircraft are either compound wound
machines with mainly shunt characteristics, or shunt wound machines, neither of
which will build up to voltage if started on load. Therefore the generator must be
allowed to build up to voltage before connecting it to the aircraft loads.

Secondly. When stationary, the generator has a very low resistance. If it were
connected directly to the main d.c. bus, a large current would flow from the
battery to the generator when the battery master switch was operated. Apart
from trying to motor the generator, the excessive current would also damage the
cables.
Although the generator switch provides ultimate control of the generator contactor,
the switch is normally left in the 'on' or 'auto' position. Operation of the contactor
being controlled automatically by the d.c. system. This will be examined later.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-51
THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-52
3.4 SYSTEM PROTECTION
3.4.1 CURRENT PROTECTION
The d.c. system is protected from excessive current demands by fuses and circuit
breakers positioned at various points throughout the system. In addition some
aircraft have a current limiting relay that works on a similar principle to the voltage
regulator. The current limiter may be located in the same unit as the voltage
regulator, as seen previously, or housed in a separate unit.
3.4.2 UNDERVOLTAGE PROTECTION
Undervoltage occurs as a consequence of shutting down an engine and is therefore
a normal occurrence. Early protection involved the use of Reverse Current
Cut-Outs (RCCO) or Reverse Current Circuit Breakers, these being located in the
voltage regulator as previously discussed, or in a separate unit.
RCCO‟s had a series and a shunt coil and were designed to connect the generator
to the bus at a pre-determined output voltage. It was therefore possible for the unit
to connect the generator to the bus before its output voltage had reached the
required value. If this happened, the flow of current from the battery to the
generator through the RCCO series coil would trip the generator off-line. However,
when the generator was disconnected from the bus, although the series coil was
open circuited, the shunt coil was able to re-energise the RCCO and re-connect the
generator to the bus. This cycling would continue until the generator voltage
reached the bus voltage. The effects of this cycling caused contact wear and
burning and on occasions welded the contacts together.
If the contacts welded together, a large current would flow from the battery to the
generator when the generator was shut down. This current not only tried to motor
the generator it was also liable to damage the cables. Another more serious
problem was that the reverse current could reverse the polarity of the residual
magnetism of the generator, it was then possible for the generator to come up to full
voltage with the wrong polarity. The current flowing in the shunt coil of the RCCO
would cause it to energise and connect the generator to the bus, rapidly exhausting
the battery unless a fuse in the system blew.
These problems resulted in the RCCO being replaced by a device that would only
interconnect the generator and bus if the generator output voltage was greater than
the bus voltage. Such a device is called a differential cut-out and was essential on
multi-generator systems. In some instances the RCCO was retained as a form of
back-up protection, with only the series coil being used.
3.4.2.1
Differential cut-out Relay
The normal method to protect a circuit is by use of reverse current relays. These
cut-out devices serve as an automatic switch which trips whenever generator
voltage falls below battery voltage, e.g. at start up and shut down times or when the
generator has failed.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-53
The relay has two coils wound onto the same core and a spring controlled armature
and contact assembly. The shunt winding has many turns of fine wire, connected
across the generator output. The series winding has a few turns of thick wire and is
in series with the supply line. The series winding is connected to the contact
assembly which is held open by a spring.
As the generator starts, the output level increases to a predetermined voltage and
the shunt winding produces enough electromagnetic induction to attract the
armature and close the contacts. The cut-out will trip should the voltage level from
the generator fall sufficiently for the contacts to be broken.
3.4.2.2
Undervoltage relay
The differential coil of the differential cut-out is made of very fine wire in order to
make it sensitive. When the generator is 'off-line' and the battery is connected to
the main bus, the battery voltage across the differential coil and may cause it to
burn out. To stop this occurring, an undervoltage or pilot relay is used.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-54
The undervoltage relay is connected between the generator switch and the
differential coil and senses the generator output voltage. When the generator
output reaches approximately 22 volts, the relay energises and connects the
differential coil between the generator and the main bus. The generator will then be
brought 'on-line' as previously explained.
3.4.2.3
Overvoltage Protection
An overvoltage could occur if there was a fault in the voltage regulator or the
excitation circuit. To prevent the overvoltage being applied to the aircraft electrical
system protection is provided.
Several methods are used but generally it is the voltage drop across the generator
field windings that is sensed. When a pre-determined level is reached, the field is
either open circuited or has a resistor placed in series with it.
In the system shown above, an overvoltage situation energises the overvoltage
relay connecting a supply to the latch relay. Operation of the latch relay does two
things:

the top contacts open, inserting a resistor in series with the field winding,
thereby reducing the field current and subsequently the generator output
voltage.

the lower contacts close, connecting a latching supply to the relay so that as the
generator voltage falls and the overvoltage relay de-energises, the field resistor
is not removed.
To reset the generator, the reset switch must be operated, this removes the latching
supply and allows the relay to de-energise. If the fault condition is still present, the
overvoltage relay will trip again. A reset should only be attempted once, if the unit
trips again, further investigation must be carried out before attempting additional
resets.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-55
The overvoltage protection unit shown below consists of two relays, one having a
pair of normally open contacts, the other a pair of normally closed contacts.
The generator field supply passes through the normally closed contacts to the
voltage regulator. As the field voltage rises due to an overvoltage fault, the potential
across the two relay coils increases, increasing the current flow through them.
At a predetermined level, both relays will energise. The left pair of contacts open,
removing the generators field supply and decreasing the generator output voltage.
The right pair of contacts close, providing a hold on supply for the two relay coils.
To reset the system the generator switch must be selected to off and then to on
again. If the fault is still present the unit will trip the field supply again.
3.4.3 LOAD SHARING
In a multi generator system the generators must share the total aircraft load equally.
If one generator output voltage goes slightly high, that generator will take more load.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-56
The increased load causes a drop in the terminal voltage which is corrected by the
voltage regulator. As the voltage is increased, the generator takes more of the load,
which again produces a reduction in output voltage. This process will continue until
the generator is supplying the total aircraft load, and the other generator is
supplying nothing.
3.4.3.1
Split Busbars
A simple way of preventing load imbalances is to use split busbars. Each generator
is connected to its own bus and the aircraft loads are split equally between the
busbars. In the event of a generator failure the lost bus is connected to a
serviceable bus via a coupling contactor. To prevent the coupling contactor being
closed accidentally, the energising supply is routed through the auxiliary contacts of
the generator contactor.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-57
3.4.3.2
Load Sharing or Equalisation
Another method of ensuring loads are shared equally is called load sharing or
equalisation. This entails comparing the generator load currents and varying their
output voltages to correct for any imbalance. If a generator is taking too much load,
reducing its output voltage will force it to take less load. If a generator is not taking
enough load, increasing its output voltage will force it to take more load.
Using carbon pile voltage regulators, load sharing is achieved by putting an another
coil, called an equalising coil, around the same core as the voltage sensing coil.
The field produced by the equalising coil is designed to assist, or oppose the field
produced by the voltage sensing coil, depending on the direction of current flow
through it.
In a two generator system, the equalising circuits are connected as shown below.
The point where the equalising circuits join is called the star point, each circuit
being connected in parallel.
When the loads on the generators are equal or „balanced‟, the voltage drop across
the compensating windings of each generator is the same, and the potential at the
top of each winding is the same. When the potentials at the top of the
compensating windings are the same, no current flows in the equalising loop.
Whenever load current is flowing from the generators, the potentials at the top of
the compensating windings will be negative with respect to earth. If the generators
are not supplying load current, the potentials will be zero.
When the loads on the generators are not the same, the loads are said to be
„unbalanced‟. When the loads are unbalanced, the voltage drops across the
compensating windings are different, creating a potential difference across the
equalising coils that results in a current flow..
Assuming No 1 generator starts to take more than its share of the load, the voltage
at the top of its compensating winding will go more negative with respect to earth.
Because the total aircraft load is fixed, the other generator will have to take less
load. The reduction in load on No 2 generator will cause the potential at the top of
its compensating windings to go less negative with respect to earth (it will still be
negative with respect to earth).
The potential difference between the tops of the compensating windings causes a
current to flow around the load-sharing loop and through the equalising coils as
shown below.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-58
The field produced by No 1 equalising coil
assists the field created by the voltage
sensing coil, reducing the compression on
the carbon pile, increasing its resistance.
The increase in resistance, decreases the
generators field current and therefore
reduces its output voltage. The reduction
in output voltage causes the generator to
take less load.
In No 2 voltage regulator, the equalising
field assists the field created by the voltage
sensing coil.
This increases the
compression on the pile, decreases its
resistance, increases the generator output
voltage and increases its share of the load.
When the loads are balanced the
potentials at the compensating windings
will again be equal and no current will flow
in the equalising loop.
The equalising loop is connected to the star point via contacts of the pilot or
undervoltage relay. This prevents operation of the load sharing loop until the
generator output voltage has built up to at least 21 volts.
3.4.3.3
Transistorised Load Sharing
The load sensing circuit of a transistorised voltage regulator system works in the
same manner as the carbon pile type system. In the diagram, the voltage drop
proportional to load current is sensed across a potential divider network that is
placed across the interpole windings, but operation is as previously explained.
Again, the voltage at A will be negative with respect to earth. The greater the load
current, the more negative with respect to earth this voltage becomes.
In a transistorised voltage regulator, the voltage at A is used to vary the voltage on
one side of a zener diode in the voltage regulator. Changes in voltage A, change
the voltage required on the other side of the zener to cause break down. The
voltage applied to the other side of the zener is the generator output voltage,
therefore regulation takes place at a different generator output voltage.
Using the second transistorised voltage regulator explained in the notes. The
voltage at the compensating windings of No 1 generator is applied to the potential
divider network of No 1 regulator as shown below.
An extra resistor R4 has to be added to the potential divider network to prevent the
shorting of resistor R6.
If No 2 generator takes too much load, No 1 generator (shown) will take too little
load and the voltage at A will go less negative with respect to earth, increasing the
voltage at point B.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-59
The voltage applied to the cathode (left) of the zener must now be increased in
order to make it breakdown and conduct. This voltage is the generator output
voltage. Therefore, the generator output voltage will increase, as will its share of
the aircraft load.
If No 2 generator takes too little load, No 1 generator will take too much load and
the voltage at point A will go more negative with respect to earth, decreasing the
voltage at point B.
It now takes less voltage on the cathode to make it break down and conduct. With
a lower output voltage, the generator will take less load.
When the loads on the generators equalise, the potentials at A and B revert back to
their normal values, still negative with respect to earth, and regulation takes place at
the correct generator voltage.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-60
3.4.4 ALTERNATORS
3.4.5 STARTER GENERATORS
Many gas turbine aircraft are equipped with starter-generator systems. These
starting systems use a combination starter-generator which operates as a starter
motor to drive the engine during starting, and after the engine has reached a selfsustaining speed, operates as a generator to supply the electrical system power.
The starter-generator unit shown below, is a shunt generator with an additional
heavy series winding. This series winding is electrically connected to produce a
strong field and a resulting high torque for starting.
Starter-generator units are desirable from an economical standpoint, since one unit
performs the functions of both starter and generator. Additionally, the total weight of
starting system components is reduced, and fewer spare parts are required.
The starter-generator shown below has four windings; (1) A series field, (2) a shunt
field, (3) a compensating field, and (4) an interpole winding. During starting, the
series, compensating, and interpole windings are used. The unit is similar to a
direct-cranking starter since all of the windings used during starting are in series
with the source. While acting as a starter, the unit makes no practical use of its
shunt field. A source of 24 volts and 1,500 amperes is usually required for starting.
When operating as a generator, the shunt, compensating and interpole windings are
used. The series field is used only for starting purpose. The shunt field is
connected in the conventional voltage control circuit for the generator.
Compensating and interpole windings provide almost sparkless commutation from
no load to full load.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-61
The following diagram illustrates the external circuit of a starter-generator with
undercurrent controller. This unit controls the starter-generator when it is used as a
starter. Its purpose is to assure positive action of the starter and to keep it
operating until the engine is rotating fast enough to sustain combustion. The control
block of the undercurrent controller contains two relays; one is the motor relay,
which controls the input to the starter. The other, the undercurrent relay, controls
the operation of the motor relay.
To start an engine equipped with an undercurrent relay, it is first necessary to close
the engine master switch. This completes the circuit from the aircraft's bus to the
start switch, to the fuel valves, and to the throttle relay. Energising the throttle relay
starts the fuel pumps, and completing the fuel valve circuit gives the necessary fuel
pressure for starting the engine.
As the battery and start switch is turned on, three relays close. They are the motor
relay, the ignition relay and the battery cu-tout relay. The motor relay closes the
circuit from the power source to the starter motor; the ignition relay closes the circuit
to the ignition units; and the battery cut-out relay disconnects the battery. On this
particular aircraft, opening the battery circuit is necessary
because the heavy current drain of the starter motor would damage the battery.
This is not the general case, the majority of aircraft are designed to be started using
the battery, this enables the aircraft to be independent of ground resources. The
battery will however be disconnected from the bus when the ground power is
connected and care must be taken to ensure the ground power unit is capable of
supplying the current required by the starter motor.
Closing the motor relay allows a very high current to flow to the motor. Since this
current flows through the coil of the undercurrent relay, it closes. Closing the
undercurrent relay completes a circuit from the positive bus to the motor relay coil,
ignition relay coil, and the battery cut-out relay coil. The start switch is allowed to
return to its normal "off" position and all units continue to operate.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-62
As the motor builds up speed, the current draw of the motor begins to decrease,
and as it decreases to less than 200 amps, the undercurrent relay opens. This
action breaks the circuit from the positive bus to the coils of the motor, ignition and
battery cut-out relays. The de-energising of these relay coils halts the start
operation.
After the procedures described are completed, the engine should be operating
efficiently and ignition should be self-sustaining. If, however, the engine fails to
reach sufficient speed, the stop switch may be used to break the circuit from the
positive bus to the main contacts of the undercurrent relay, thereby halting the start
operation.
On a typical aircraft installation, one starter-generator is mounted on each engine
gearbox. During starting, the starter-generator unit functions as a d.c. starter motor
until the engine has reached a predetermined self-sustaining speed. Aircraft
equipped with two 24 volt batteries can supply the electrical load required for
starting by operating the batteries in a series configuration.
The following description of the starting procedure used on a four-engine turbojet
aircraft equipped with starter-generator units is typical of most starter-generator
starting systems.
Starting power, which can be applied to only one starter-generator at a time, is
connected to a terminal of the selected starter-generator through a corresponding
starter relay. Engine starting is controlled from an engine start panel. A typical start
panel (see diagram below) contains an air start switch and a start switch.
The engine selector switch shown has five positions ('1, 2, 3, 4, and off'), and is
turned to the position corresponding to the engine to be started. The power selector
switch is used to select the electrical, circuit applicable to the power source (ground
power unit or battery) being used. The air-start switch, when placed in the "normal"
position, arms the ground starting circuit. When placed in the "air-start" position,
the igniters can be energised independently of the throttle ignition switch. The start
switch, when in the "start" position, completes the circuit to the starter-generator of
the engine selected to be started, and causes the engine to rotate. The engine start
panel shown above also includes a battery switch.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-63
When an engine is selected with the engine selector switch, and the start switch is
held in the "start" position, the starter relay corresponding to the selected engine is
energised and connects that engine's starter-generator to the starter bus. When the
start switch is placed in the "start" position, a start lock-in relay is also energised.
Once energised, the start lock-in relay provides its own holding circuit and remains
energised providing closed circuits for various start functions.
An overvoltage lockout relay is provided for each start-generator. During ground
starting, the overvoltage lockout relay for the elected start-generator is energised
through the starting control circuits. When an overvoltage lockout relay is
energised, overvoltage protection for the selected started- generator is suspended.
A bypass of the voltage regulator for the selected starter-generator is also provided
to remove undesirable control and resistance from the starting shunt field.
On some aircraft, a battery lockout switch is installed in the external power
receptacle compartment. When the door is closed, activating the switch, the ground
starting control circuits function for battery starting only. When the door is open,
only external power ground starts can be accomplished.
A battery series relay is also a necessary unit in this starting system. When
energised, the batteries are connected in series to the starter bus, providing an
initial starting voltage of 48 volts. The large voltage drop which occurs in delivering
the current needed for starting, reduces the voltage to approximately 20 volts at the
instant of starting. The voltage gradually increases as starter current decreases
with engine acceleration and the voltage on the starter bus eventually approaches
its original maximum of 48 volts.
Some multi-engine aircraft equipped with starter-generators includes a parallel start
relay in their starting system. After the first two engines of a four-engine aircraft are
started, current flow for starting each of the last two engines passes through a
parallel start relay, which shifts the battery output from series to parallel. When
starting the first two engines, the starting power requirement necessitates
connecting the batteries in series. After two or more engine generators are
providing power, the combined power of the batteries in series is not required.
Thus, the battery circuit is shifted from series to parallel when the parallel start relay
is energised.
To start an engine with the aircraft batteries, the start switch is placed in the "start"
position. This completes a circuit through a circuit breaker, the throttle ignition
switch and the engine selector switch to energise the start lock-in relay. Power then
has a path from the start switch through the "bat start" position of the power selector
to energise the battery series relay, which connects the aircraft batteries in series to
the starter bus.
Energising the No 1 engine's starter relay directs power from the starter bus to the
No. 1 starter-generator, which then cranks the engine.
At the time the batteries are connected to the starter bus, power is also routed to
the appropriate bus for the throttle ignition switch. The ignition system is connected
to the starter bus through an overvoltage relay, which does not become energised
until the engine begins accelerating and the starter bus voltage reaches about 30
volts.
As the engine, turned by the starter, approaches approximately 10% r.p.m. the
throttle is advanced to the "idle" position. This action actuates the throttle ignition
switch, energising the igniter relay. When the igniter relay is closed, power is
provided to excite the igniters and fire the engine.
When the engine reaches about 25 to 30% r.p.m. the start switch is released to the
"off" position. This disconnects the start and ignition circuits from the engine start
cycles, and the engine accelerates under its own power.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-64
3.5 AC POWER SYSTEMS
3.5.1 AC GENERATION
The generation of an alternating current has already been examined in the section
on d.c. generation. The rules concerning the size of the generated emf and the
direction of current flow are as previously described.
Instead of using a commutator to ensure the current flows in one direction through
the load, the load is connected via slip rings and the current flow is alternating, as
shown below.
3.5.1.1
Output Voltage
The instantaneous value of emf induced in the loop is given by:
e(instant) = E(max) sin 
where E(max) = lv and  is the angle of the conductor with respect to the
field.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-65
3.5.1.2
Output Frequency
Referring back to our simple loop it can be seen that, if the loop was rotating at 120
revs per second, then the output frequency would be 120 Hz. It therefore follows
that the frequency of the output of an ac generator is directly proportional to its
speed of rotation.
Another factor which determines the output frequency of the ac generator is its
physical construction. A generator with 4 field poles will produce two complete
cycles of output for each revolution of the shaft.
Similarly, a generator with six field poles will produce three complete cycles for each
revolution and so on. A cycle is complete whenever a conductor has passed under
the influence of two dissimilar magnetic poles.
From the foregoing it will be seen that the output frequency of an ac generator is
given by:
F = Revs per second × No of pairs of poles
The speed of rotation is normally given in revolutions per minute (rpm), therefore
the output frequency of is calculated from the following formula:
Frequency =
where;
NP
60
N is the speed of rotor rotation in RPM
P is the number of pairs of poles
From the foregoing, it will be seen that one cycle is completed in:
360 mechanical degrees for a two-pole machine,
180 mechanical degrees for a four-pole machine,
120 mechanical degrees for a six-pole machine,
90
mechanical degrees for an eight-pole machine, and so on.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-66
It is therefore necessary to use electrical degrees when referring to angular motion
in the cycle. One cycle = 360 (electrical) degrees. It is not usual to use the word
„electrical‟ in this respect, but the concept should be clearly understood.
3.5.1.3
Effects of a Resistive Load
When a resistive load is placed on an a.c. generator armature reaction occurs. If the
generator is of the rotating field type, then the field is distorted against the direction
of rotation as shown below. If the load is increased, armature reaction is increased
and the field is distorted further.
A resistive load also tends to slow the generator down, this results in both the
output frequency and voltage decreasing. The output can be restored by providing
more drive torque to overcome the extra load.
3.5.1.4
Effects of an Inductive Load
If an inductive load is placed on a generator the current in the stator lags the voltage
by 90, causing the stator field to move around 90. The stator field now opposes
the main field, producing a weaker field and a reduction in output voltage.
The voltage can be restored by increasing the field current, however this will
generate additional heat in the machine.
3.5.1.5
Effects of a Capacitive Load
If a capacitive load is placed on a generator, the stator field is advanced by 90 and
now assists the main field, this increases the overall field strength, increasing the
generator output voltage.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-67
This can be corrected, without adverse affects, by decreasing the field current.
Most aircraft systems have inductive loads and a lagging power factor.
3.5.2 PRACTICAL GENERATOR CONSTRUCTION
Two forms of construction are used for alternating current generators, the rotating
field type and the rotating armature type. Although the rotating field type generator
is the one most commonly used for main power production, both types will be met
later in the course.
3.5.2.1
Rotating Armature Type
A rotating armature generator is constructed in a similar manner to a d.c. generator.
The field is located on the stator and the emf is induced in windings located on the
rotor. The output is then taken from the generator using slip rings as previously
described.
3.5.2.2
Rotating field type
It is possible however, to obtain the same output by rotating the field inside
stationary windings that are located around the frame of the machine, the output is
then taken from the stationary armature, or stator.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-68
This type of generator is called a „rotating field generator‟ and has several
advantages over the rotating armature type:

Because the output windings are now stationary they are no longer subject to
high centrifugal forces and can therefore be larger.

By having the output windings on the outside of the machine there is more room
for good insulation and higher voltages can be used.

With the output windings on the outside of the machine they are more easily
cooled and can therefore carry larger currents.

Using a rotating field only requires the use of two slip rings and two brushes,
also the current required is relatively small.
These advantages mean a larger output can be obtained from a smaller machine.
3.5.2.3
Single phase generator
A single phase a.c. generator consists of a single output winding wound on a pair of
poles and a rotor fitted with either a permanent or an electromagnet. The
electromagnet is energised from a d.c. supply via brushes and slip rings.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-69
When the rotor is driven, emf's are induced in the stator windings. When the
windings are connected to a load, current flows. The output frequency is dependent
on the speed of rotor rotation and the number of poles on the rotor. If the generator
shown was rotated at the same speed, but had two pairs of poles the frequency
would double.
3.5.2.4
Two phase generator
A two phase generator consists of two output windings wound on separate pairs of
poles and a single, common, rotor.
The two output windings are located at 90 to each other, so that when maximum
emf is induced in one winding, zero emf is induced in the other winding.
The output from the generator will be two voltages of equal amplitude and
frequency, but, phase displaced from each other by 90.
3.5.2.5
Three Phase Generator
A three phase a.c. generator has three sets of output windings, each being
physically displaced from the other two by 120. The rotor is the same as that used
in a single phase or two phase generator.
The Three phase a.c. generator is really three single phase generators on one
stator, all using a common field. Due to the construction of the machine, the emf's
generated in each of the windings is phase displaced by 120 degrees, as shown.
The normal order of rotation is:
Red
Yellow
Blue
1
2
3
A
B
C
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
If two phases are reversed
then motors and control
circuits will try to operate in
reverse.
Page 3-70
If required, the three single phases can be used independently, however this is not
common practice. The windings are normally connected together in one of two
ways, called star or delta. Whether star or delta depends on the way the windings
are connected at the generator output terminals.
3.5.3 STAR & DELTA SYSTEMS
The three armature windings of a three phase generator can be connected in two
ways. Firstly, the end of one winding can be connected to the start of the next, so
that the three windings form a triangle. This form of connection if called a Delta
system. The alternative, is to connect the same end of each armature winding to a
common point and take the other end of each winding to an output terminal. This
form of connection is called a Star system. The star system is a four wire system,
as a wire is also taken from the common point to an output terminal.
3.5.3.1
Delta Connection
A Delta system is a three wire system, one wire coming from each of the armature
winding interconnection points. In a delta connected system:
VLINE = VPHASE
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-71
ILINE = 3 x IPHASE
Or
ILINE = 173 x IPHASE
A delta connected system has no neutral line and is generally used on small
generators supplying virtually fixed, balanced loads.
3.5.3.1.1
Balanced loads
If the currents in each phase are equal in size and phase displaced from one
another by 120 degrees, the loads are said to be balanced. Under balanced
conditions, the loads on each phase are identical
3.5.3.1.2
Symmetrical loads
If the phase voltages are the same magnitude, and phase displaced from one
another by 120 degrees, the system is said to be symmetrical. Aircraft systems are
naturally symmetrical.
3.5.3.2
Star Connection
Although a star connected system is considered to be a four wire system, if the
loads are balanced, the neutral line need not be connected. The neutral line only
carries out of balance currents.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-72
The neutral, although connected to earth, should not be confused with the earth in a
three pin plug which is there for protection. Under the majority of conditions, an
aircraft star connected system will have current flow in the neutral line.
The voltage from the neutral line or star point to the other end of each phase
winding is called the phase voltage, the voltage from one phase to another is called
the line voltage.
In a star connected system:
VLINE = 3 x VPHASE
or
VLINE = 173 x VPHASE
and ILINE = IPHASE
The frequency is always expressed as the frequency of a single phase.
In aircraft a.c. systems the phase voltage is 115V and the line voltage is 200V. On
some systems the frequency may be variable, or wild, on a controlled frequency
system the frequency is 400 Hz.
With a star connected generator two possible systems are available:

three single phase systems each operating at phase voltage

a single three phase system operating at line voltage
If the instantaneous values of two phases are added together to produce a line
voltage and the process is repeated for the other phases three line voltages will be
produced. Each line voltage will be displaced 120 degrees from the other two. One
point to note is that there is a 90 degree phase angle between a phase voltage and
its opposite line voltage, this relationship is used in several control and monitoring
systems.
3.5.3.3
Power in AC Systems
In star and delta connected loads the power dissipated in each phase is given by
the formula:
PPhase = VPhase x IPhase Cos  Watts
If the system is balanced and symmetrical then the total power is three times the
above value.
3.5.4 VOLTAGE REGULATION
Also located within the GCU is the voltage regulator. The excitation field of an a.c.
generator may be provided in one of two ways, firstly by application of a constant,
variable level current, as in a d.c. generator and secondly by application of a pulse
width modulated (PWM) signal.
When generators are operated in parallel additional control, circuitry must be added
to the voltage regulators, this will be examined later.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-73
3.5.4.1
Pulse Width Modulated Field Supply
Using this method, the field would normally be supplied with a PWM signal with a
mark to space ratio of 1:1, the average current being half the maximum amplitude.
The PWM signal would generally be obtained by chopping the rectified output from
the permanent magnet generator.
The generator output voltage is monitored by connecting it to a rectifier in the GCU.
The d.c. level produced being compared with a reference voltage produces a d.c.
level that can be monitored.
If the output voltage falls, the GCU increases the field strength by increasing the
mark : space ratio of the signal applied to the field. This increases the average
current applied to the field, increasing the field strength, thereby increasing the
generator output voltage. If the generator output increases, the mark: space ratio is
reduced, decreasing the average current applied to the field and decreasing the
generator output voltage.
3.5.4.2
Variable Field Current Method
This method is similar to that used for controlling the output voltage of a d.c.
generator. The output voltage is applied to a three phase, full wave bridge rectifier
and the resultant d.c. level is sensed. The d.c. voltage will have a specific level
when the generators output voltage is correct, if less, the generator output voltage is
low and vice versa.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-74
The d.c. level is then used to vary the field current, this may be achieved using a
carbon pile voltage regulator as shown above, or transistorised voltage regulator.
3.5.5 FREQUENCY CONTROL
3.5.5.1
Constant Speed Drive Unit
The output frequency of an a.c. generator is a function of the speed at which it is
driven. The purpose of a constant speed drive (CSD) is to produce a constant
speed output from the varying speed of the engine. The CSD must keep the
generator speed within its limits over the full normal speed range of the engine.
The CSD is mounted on and driven by the engine accessory gearbox. The
generator may be mounted on the forward face of the gearbox but it is still driven by
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-75
the CSD, a shaft passing through the gearbox. The generator shown is air cooled,
however, the majority of high output generators are oil cooled.
The CSD is normally attached to the gearbox by a quick attach/detach ring, this
reduces the time taken to replace a CSD.
3.5.5.2
Integrated Drive Generators
On some aircraft the CSD and generator are combined in one unit called an
Integrated Drive Generator (IDG), this has the advantage in that it is more
compact and weighs less.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-76
A CSD or IDG is not normally fitted to an auxiliary power unit (APU) because its
speed is fairly constant.
There are various types of CSD fitted to aircraft, some use a purely hydraulic
method of transmitting torque, others use a hydro-mechanical method, these notes
concentrate on the hydro-mechanical type.
3.5.5.3
Speed Control
There is large range of input speeds that the CSD must contend with, from ground
idle up to 'take-off' rpm, therefore at times the CSD must add speed to the engine
speed and at others it must subtract speed in order to maintain a constant output
speed. Control of speed is referred to as:

Overdrive - when the CSD is adding speed to the input speed of the engine.

Straight drive - when the input speed is correct.

Underdrive engine.
3.5.5.4
when the CSD is subtracting speed from the input speed of the
Construction
The CSD consists essentially of two positive displacement, axial slipper piston type
hydraulic units and a mechanical axial geared differential which performs the speed
summing function.
3.5.5.5
Hydraulic System
The hydraulic system consists of:

charge pump

scavenge pump

charge relief valve
The charge pump Supplies oil to the:

cylinder blocks

governor

control piston

lubricating system
The hydraulic units are both the same physical size, one having a variable angle
swashplate the other a fixed angle swashplate, the fixed angle swashplate
producing a fixed displacement. The hydraulic units rotate independently and are
positioned on opposite sides of a common stationary port plate.
The variable displacement hydraulic unit runs at a fixed ratio with respect to the
transmission input speed. The swashplate angle of this unit is continuously variable
in both directions from full negative swashplate angle, through zero angle, to full
positive swashplate angle, therefore the displacement of the variable unit is
continuously variable from zero to full rated displacement in both directions.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-77
The fixed displacement unit is driven by oil delivered from the variable displacement
unit, the fixed unit will therefore run at any speed from zero to full rated speed in
either direction.
The working pressure between the two hydraulic units is proportional to the torque
being transmitted to the generator. At lower input speeds the variable displacement
unit acts as a hydraulic pump, this supplies flow to the fixed displacement unit which
provides rotation that is added the input speed through the geared differential.
At input speeds above straight through drive the variable angle swashplate is set to
allow negative displacement of the variable unit, the working pressure in this case is
manipulated to allow the fixed displacement hydraulic unit to be motored by the
differential, this results in speed being subtracted from the input speed, the variable
unit is then acting as a motor.
3.5.5.6
The Differential
The differential is of the folded type with planet gears in the centre and output ring
gears on the outside, to complete the assembly the planet gears rotate about their
own axes and also revolve about the centreline of the planet gear carrier.
The variable displacement unit is also driven by the transmission input. The fixed
displacement hydraulic unit is hydraulically coupled to the variable unit and also
connected to the differential through the input ring gear. The output ring gear of the
differential is connected to the transmission output.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-78
Constant speed of the output ring gear is maintained by either adding to or
subtracting from the speed of the planet gears. This is achieved by controlling the
direction of rotation and speed of the input ring gear. The governor and pumps are
driven of the constant speed gearing.
3.5.5.7
Operating in Overdrive
If the input speed is lower than required, the transmission hydraulically adds speed,
the system is then operating in overdrive.
A torque load is imposed on the output ring gear by the output gear, input torque is
supplied by the input gear turning the carrier shaft. If there were no torque on the
input ring gear it would run freely allowing the output to stop.
Because the carrier shaft to ring gear ratio is 2:1 the speed of the input ring gear
would be double that of the carrier shaft.
If the input ring gear is Constrained to zero speed the output ring gear will run at
double the speed of the planet gears.
If the input ring gear is forced to rotate in a direction opposite to that of the carrier
shaft the output ring gear will run at a speed that is more than double the carrier
shaft speed.
If the input ring gear is allowed to rotate in the same direction as the carrier shaft,
then the output ring gear will run at a speed less than twice that of the carrier shaft.
Thus the differential is a 'speed summer' or adding device, that is controlled
through the input ring gear to add or subtract from the speed of the engine
accessory gearbox.
In overdrive the variable displacement unit acts as a hydraulic pump. The governor
ports control oil to the control piston which in turn positions the swashplate, the oil is
then compressed as the pistons are forced into the rotating cylinder block producing
high pressure (working pressure) oil that is ported to the fixed hydraulic unit. The
fixed unit functions as a motor, being driven by the high pressure oil.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-79
High pressure oil from the variable unit forces the fixed unit pistons to slide down
the face of the fixed swashplate, this causes the cylinder block to move in a
direction opposite to the carrier shaft, thus adding speed to the engine accessory
gearbox speed.
If the input speed supplied to the transmission is enough to produce the required
output speed the transmission drives the alternator directly through the differential,
this is straight through drive. At straight through input speed, torque is transmitted
through the geared differential unit. The variable unit swashplate will be slightly
offset from zero angle, this allows some pumping action to be accomplished and
leakage losses to be made up.
If the input speed exceeds that required the transmission hydraulically subtracts the
necessary speed, when the transmission is subtracting speed it is operating in
underdrive. When operating in underdrive the variable unit functions as a motor.
The governor ports oil away from the control cylinder, this causes the swashplate to
be positioned so the volume for accommodating oil in the piston bores on the high
pressure side is increased, consequently oil flows from the fixed unit to the variable
unit, the fixed unit acting as a pump.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-80
The units pistons are forced into the cylinder block as they slide up the inclined face
of the swashplate, this causes high pressure oil to be pumped to the variable unit
allowing the cylinder block to rotate in the opposite direction to overdrive operation.
The opposite block rotation allows the input ring gear to turn in the same direction
as the carrier shaft, thus speed is subtracted from the speed of the engine gearbox.
3.5.5.8
CSD Governor
If the speed changes outside the required limits oil is ported from the controller,
this in turn adjusts the variable displacement unit swash plate.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-81
The typical governor consist of a
spring biased, flyweight operated,
hydraulic control valve. A rotating
sleeve in the governor is driven by
the CSD output gear and is
therefore responsive to the output
speed of the CSD. The flyweights
are pivoted on the sleeve and move
a valve stem against the bias of a
spring, this stem is located within
the sleeve.
Above the flyweights is an
electromagnet, this provides a
means of electrically trimming the
CSD for torque adjustment. The
electrical power system is therefore
able to adjust the CSD's torque for
parallel operation of a.c. generators.
With an increase in load the CSD
output would tend to slow down, this
causes the governor to command
an increase in torque to increase the
output speed back to its correct
value. If the load is decreased then
the reverse would occur, hence the
governor maintains the required
torque output as well as a constant
speed.
3.5.5.9
Oil Temperature Indications
The CSD has its own self contained oil system which provides a means of:

lubrication

transferring heat

transmitting torque
Oil is cooled in an external cooler the temperature being sensed at two separate
points, at the exit point to the cooler and the entry point on return from the cooler.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-82
Oil temperatures are displayed on the flight deck, various forms are used but oil out
(from CSD) and oil rise temperatures are always displayed, with oil in temperature
being displayed on some systems.
To obtain the oil rise temperature it is normal to have to press and hold a push
switch located on or adjacent to the temperature gauge. The rise in oil temperature
reflects the amount of work being done within the CSD.
Oil pressure within the CSD is monitored by a charge pressure switch, should the
pressure drop below a predetermined value this switch will illuminate a low
pressure warning light on the flight deck. In event of illumination the CSD is
disconnected from the engine drive.
Some CSD's have a thermal switch in the oil reservoir, this monitors the oil
temperature and operates either the low pr warning or a separate caption if the
temperature exceeds a pre-determined value.
If the fault can be identified as an overheat then the generator load may be reduced,
if it is not possible to identify an overheat, or the fault persists then the CSD is
disconnected.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-83
3.5.5.10
CSD Oil Level
The oil level of the CSD must be inspected at the times specified in the
maintenance manual or if a CSD fault is reported. The CSD must never be
overfilled as this will lead to speed control problems.
3.5.5.11
CSD Disconnection & Re-connection
Selection of a guarded switch on the flight deck energises the disconnect solenoid
of the CSD.
The solenoid enables a spring loaded pawl to move into contact with the threads on
the rotating input shaft. The input shaft is then driven away from the input spline
shaft, removing the drive to the CSD.
Reset is achieved by pulling the reset handle mounted on the CSD. Some IDG's
have a disconnect indicator on the IDG.
It is important that the CSD is only disconnected with the engine running and
only reconnected with the engine stationary.
3.5.5.12
Over Speed Sensing
The CSD governor also acts as an overspeed sensor and will cause the
transmission to assume minimum speed selection if the maximum permitted speed
is exceeded. The CSD will be locked in this condition until automatically reset when
the engine is shut down.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-84
3.5.5.13
CSD Maintenance
Maintenance is carried out in conjunction with an approved engine engineer, some
points to be checked are:

excessive oil consumption

blocked filters

magnetic probes

unstable or erratic frequency when a load is connected
The CSD's performance is monitored on a ground run, where voltage, frequency, oil
pressure and oil temperature should be noted.
3.5.6 SYSTEM LAYOUTS
The a.c. system is automatic in operation and is self monitoring, however the pilot
must have overall control, this is achieved by the provision of various switches that
enable generators, CSD's and bus bars to be disconnected and / or reconnected as
desired.
There are three basic a.c systems:

Constant frequency with non paralleled busbars

Constant frequency with paralleled busbars

Variable frequency or frequency wild with split busbars
A Busbar is a point between the Generator circuit breaker and the load circuit
breakers where the power is distributed from. In simple terms it is a distribution
point and may consist of copper bars on the back of circuit breakers or a terminal
stud.
Most modern a.c. systems are constant frequency with either paralleled or
non-paralleled busbars. The basic system will include:

ammeters

voltmeters

frequency meters

failure lights and magnetic indicators

switches - GCR / Field / BTB
3.5.6.1
Basic Non Parallel System
Shown below is the layout of a non parallel a.c. system consisting of two main
generators, an auxiliary power unit generator and an external power facility.
Under normal operating conditions each generator supplies its own busbar via its
Generator circuit Breaker (GCB). The generators are three phase, star connected
with neutral, only one line is drawn for clarity.
The standby bus is a single phase bus that is normally supplied from one of the
main busbars. In the event of the loss of both main generators, the standby bus
would be powered from the inverter.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-85
Normal flight configuration is with both GCB's closed, both Bus Tie Breakers
(BTB's), the Auxiliary Power Breaker (APB) and the External Power Contactor
(EPC) open. Under these conditions each generator is supplying its own busbar.
If a generator fails, its GCB will open, disconnecting the unserviceable generator
from its busbar. In order to restore power to the lost bus both BTB's will close, both
busbars are then supplied from a single generator.
If the APU was then started, the BTB of the good system would open before
allowing the APB to connect the generator to the unserviceable bus.
During ground maintenance, either the APU or ground power can be used to power
the complete electrical system via both BTB's and either the APB or EPC.
3.5.6.2
Priority Systems
When a second power source is selected, an automatic priority system chooses
which one is to be used to supply the system. Two priority systems exist; a system
whereby the power already on the aircraft has priority, (with this system the existing
supply must be switched off before the new source will be allowed onto the aircraft),
and a system whereby the entering power has priority, this system will automatically
disconnect the power already on the aircraft and connect the entering power
source.
Constant frequency, non parallel systems may be found on helicopters, the
generators being driven of the main rotor gearbox or the engine, both of which are
running at a constant speed. These systems would be non-parallel because of the
lack of load sharing facilities.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-86
3.5.6.3
Load Shedding
Load shedding is basically removing loads from the generators by switching off
equipment and systems Only non essential systems such as galley electrical loads
and hot food containers will be shed.
There are two main reasons for load shedding:

Firstly to prevent overloading a power source.

Secondly to ensure that essential services are maintained.
Load shedding may be automatic or manual, an automatic load shed taking place
when:

There is a reduction in the power sources available. i.e. when a main generator
fails.

The APU generator is supplying the aircraft electrical system and it is required to
start an engine.

A generator overload occurs.
Some systems will automatically reset the shed loads when the condition causing
the load shedding has passed, other systems require manual resetting of the load.
Non essential loads generally consist of hot food containers and galley electrical
loads. An Electrical Load Control Unit (ELCU) is used on some aircraft to control
the shedding of galley loads.
The galley relay is relaxed for an automatic reset load shed, whilst the ELCU supply
contactor is locked out for latched fault. The ELCU supply contactor is reset by a
discrete signal from the reset switch, or by removing and reapplying power to the
ELCU.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-87
3.5.7 SYSTEM CONTROL AND MANAGEMENT
3.5.8 PARALLELING AC GENERATORS
3.5.8.1
Conditions for Paralleling
In order for a.c. generators to be paralleled, (connected together on the same bus)
certain conditions must exist:

output voltages must be the same

output frequencies must be the same

the generator outputs must be in phase

the phase rotation of each machine must be the same, A / B / C - red / yellow /
blue
The diagram below represents two single phase generators about to be connected
together on the same bus.
If any of the above conditions are not met, there will be an a.c. voltage across the
contacts. The frequency of the voltage is known as the beat frequency and will
depend on how far out of synchronisation the two generators are.
If two lamps are connected across the contacts they will flash on and off at the
same rate as the beat frequency. If all of the above conditions are met there will be
no voltage across the contacts and the lamps will be off.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-88
The time to close the contacts is when the lamps are off. If the contacts are closed
when the lamps are on, a large circulating current will result, this will pull the
generators into synchronisation but may damage the generators or drives.
The lamps dark method of paralleling is the basis for all systems. Originally the
lamps were provided on the generator control panel to enable the engineer
manually parallel the generators, now the system is automatic.
3.5.8.2
Manual Paralleling of A.C. Generators
Shown below is an aircraft system used to manually parallel three a.c. generators.
Two synchronising lights are used, one connected between the generator phase A
output and the phase A synchronising bus, the other between the phase B output
and the phase B synchronising bus.
The generator connected to the lights is determined by the position of a rotary
switch, this switch also determining which generator is connected to the frequency
meter and voltmeter.
Prior to starting an engine all the GCB's will be open and all the field relays and
BTB's will be closed.
On starting No1 engine the rotary switch is selected to position 1, this connects
phase C of No1 generator to the synchronising lights and phase B to the voltage
and frequency meters. The synchronising lights, flashing at 400 Hz, will appear ON.
No1 generator frequency control knob is then adjusted until the required frequency
is obtained on the frequency meter.
Having set No1 generator, the rotary switch is selected to position 2, this connects
phase C of No2 generator to the synchronising lights and phase B to the frequency
and voltage meters. The output frequency of No2 generator is then adjusted to the
same value as No1 generator using the frequency control knob.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-89
Once the frequency of No2 generator is set, No1 generator GCB is closed, this
connects No1 generator to the aircraft loads and the top of the synchronising lights.
Since the bottom of the synchronising lights are connected to the No2 generator
output, they will now be subjected to the beat frequency determined by the degree
of synchronisation. Providing the generator output frequencies were set correctly,
the lights should now be flashing.
The output frequency of No2 generator is then adjusted until the lights are flashing
at the slowest possible rate, then whilst the lights are off, No2 generator GCB is
closed, paralleling the two generators.
The third generator is then paralleled in a similar manner to the No2 generator.
3.5.8.3
Automatic Paralleling A.C. Generators
The above diagram shows a simplified method of automatically paralleling a.c.
generators, the device does away with the need for synchronising lights and
relieves the pilot from having to close the GCB's at the right instant. The device will
close the GCB when:

The frequency difference between generators is less than 4 Hz.

The phase voltage difference is less than 10 V.

The out of phase angle is less than 90 degrees.
Transformer T1 is connected between the C phase of the two generators and is
therefore receiving the modulated waveform or beat frequency that exists between
them.
The output from the transformer secondary is half wave rectified by diode D1 and
applied to C1 and R1. C1 will charge with the modulated wave and discharge
through R1 as the waveform dies away.
The RC time constant of this circuit allows C1 to fully discharge if the beat
frequency is less than 4 Hz, but not is it is above 4 Hz. Above 4 Hz there is
sufficient base - emitter voltage to cause Q1 to conduct, its collector is almost at
earth potential and the zener is not conducting.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-90
If the beat frequency drops to 4 Hz or less, capacitor C1 is able to discharge
completely, this reduces the base - emitter voltage of Q1, turning it 'off'. When Q1
is switched 'off' its collector voltage rises causing the zener Z1 to breakdown and
conduct, switching 'on' Q2.
When Q2 is switched 'on', relay RL1 is energised and power passes through the set
on contacts to energise the GCB, paralleling the two generators.
3.5.9 LOAD SHARING
As in a d.c. system it is necessary to ensure that the total aircraft load is shared
equally between the generators. The simplest way of achieving this is to employ a
non-parallel a.c. system.
On a non-parallel system each generator supplies its own bus. The aircraft loads
are then connected to the buses in such a manner as to ensure even load
distribution. In the event of a generator failure, power is maintained by closing a
BTB and connecting the 'dead' bus to another serviceable bus.
Failure of a generator may result in load shedding and the loss of non-essential
systems.
When a.c. generators are paralleled, they must also share the aircraft loads equally,
therefore a load sharing system must be used.
There are two types of load that may be applied to an a.c. generator:

Real load, due to resistive components within the electrical circuits.

Reactive load, due to capacitance and inductance in the electrical circuits.
Both types of load must be shared equally between the generators, therefore two
load sharing systems are required, one for real load, one for reactive load. The load
sharing systems are generally referred to as load sharing loops.
3.5.9.1
Real Load Sharing Control
Real load sharing is determined by the relative speeds of the generators, the speed
being determined by the governor setting on the CSD.
If one generator is running slightly faster than the others, it will start to take more of
the real load and the remaining generators will take less. If the situation is allowed
to continue uncorrected, the generator will eventually take all of the load.
Real load sharing control must therefore control the generator speed, in order to do
this it must be on the CSD and must control the generator drive torque. Increasing
the drive torque of any generator taking too little load and decreasing the torque of
any generator taking too much load.
3.5.9.2
Reactive Load Sharing Control
Reactive load sharing is determined by the relative sizes of the generator output
voltages, these being determined by the setting of the voltage regulators.
If one regulator is set slightly higher than the mean system voltage, that regulator
will try to raise its generators output voltage and subsequently the system voltage.
This results in the generator taking more of the reactive load, leaving the other
generators with less reactive load. If the situation continues unchecked, the
generator will eventually take all of the reactive load.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-91
Reactive load sharing control must therefore control the current delivered to the
generators field, to achieve this it must vary the voltage regulator setting. Increasing
the field strength of any generator taking too little reactive load and decreasing the
field strength of any generator taking too much load.
3.5.9.3
Principle of Operation of the Load Sharing Loops
The operating principles of both load sharing loops are the same, the only
difference being in the use of the error signals that are produced. It is therefore only
necessary to examine the operation of a single loop and then to look at the
application of the error signals.
Two protection systems also use load sharing loops that work on the same
principles. It is therefore possible to find a total of four such loops on an aircraft a.c.
distribution and control system. The two protection systems will be examined later.
Each load sharing loop employs one current transformer (see annex A for) per
paralleled generator. The transformer is placed on one of the main, three phase,
feeder cables from each generator, the phase used is immaterial, but it must be the
same phase on each generator. The transformers are then connected together to
form a loop as shown below.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-92
Connected across each transformer are a pair of BTB auxiliary contacts, a pair of
GCB auxiliary contacts and a resistor, known as a Burden resistor. The auxiliary
contacts used are in the BTB and GCB associated with the particular generator and
the resistors all have the same value.
Although the loop shown has four generators, loop layout is the same irrespective of
the number of generators used.
Under normal operating conditions the BTB and GCB auxiliary contacts would be
open, the generators paralleled and load current would be flowing along each phase
of each feeder cable.
If the total aircraft load was shared equally between the paralleled generators, the
current flowing in the same phase of each generator would be the same, as would
the emf induced in each of the current transformer secondaries.
The emf induced in each current transformer (the polarity being assumed) will try to
push a current in three directions:

through its own Burden resistor

around the loop, through the other current transformers

around the loop, through the other Burden resistors.
In the example shown, the current that each transformer tries to push around the
loop, through the other Burden resistors, will be one third of the current that it tries
to push through its own Burden resistor. To each transformer, the other three
resistors appear to be in series, these three series resistors then being in parallel
with its own resistor.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-93
In a three generator system the current that each transformer tries to push around
the loop will be half what it is trying to push through its own resistor.
If the currents that each transformer is trying to push around the circuit are
vectorially added, it can be seen that a current flow around the loop exists, but there
is no current flow through the Burden resistors. This condition can only exist if the
generators are sharing the total aircraft load equally.
It should be noted, in this type of system, that the loop current is always the average
of the current transformer currents. If each transformer produces 0.5 amps, the loop
current will be 0.5 amps.
If the system becomes unbalanced, with one generator taking more load, the other
generators will take less. The emf's induced in the current transformers will change
accordingly, as will the currents they try to push around the circuit.
Under these conditions the currents that each transformer tries to push through the
Burden resistors no longer cancel, producing a resultant current flow, as shown
below.
The size of the current flow in the Burden resistors depends on the size of the load
imbalance and the direction is determined by whether the generator is taking more
or less load. Therefore, the current flowing in the Burden resistors can be used as
an error signal to indicate when the generator loads are unbalanced.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-94
The above explanation of loop operation only takes into account simple changes in
generator load current, it must be remembered that changes in phase angle, supply
voltages, types of load etc. will all affect the loads taken by each generator and may
produce an imbalance.
If there is a real load imbalance, real current flows in the Burden resistors. If the
load imbalance is reactive, reactive current flows in the resistors. It is necessary
therefore to determine whether the imbalance is real or reactive in order to apply the
appropriate correction.
It should be noted that, if the voltage drop across the Burden resistor is detected,
this voltage will be in phase with the current through it, be it real or reactive. If the
Current transformer is on phase 'C'. Real current in the Burden resistor will produce
a voltage that is in phase with phase 'C' voltage. If the current in the Burden
resistor is reactive, the voltage across it will be in phase or anti phase with line
voltage B-C, and at 90 to phase 'C' voltage.
3.5.9.4
Real Load Sharing
The real load sharing loop is identical to the loop used in studying the principle of
loop operation.
The error signals produced at the Burden resistors are used to modify the generator
drive torque. This is achieved by applying the signals to the electromagnet, located
under the flyweights, in the CSD governor.
By applying signals to the electromagnet it is possible to simulate an increase or
decrease of centrifugal force acting on the flyweights.
If a generator is taking too much real load, the signal from the Burden resistor is
used to simulate an increase in the centrifugal force on the flyweights. This causes
the governor to reduce the drive torque in order to slow the generator down, as the
generator slows, the load taken by it reduces.
If the generator is taking too little real load, the signal from the Burden resistor is
used to simulate a reduction in centrifugal force. This causes the governor to
increase the drive torque in order to increase the speed of the generator, as the
speed increases, the generators share of the real load increases.
3.5.9.5
Reactive Load Sharing
The principle of operation of the reactive load sharing loop is the same as that of the
real load sharing loop. The error signals produced are applied to the voltage
regulator in the GCU to modify the generator excitation, increasing the excitation of
any generator taking too little reactive load and decreasing the excitation of any
generator taking too much reactive load.
Systems used vary, two examples are shown below.
In the first system, Burden resistors are used in the same way as in the real load
sharing loop. If there is an imbalance in reactive loads a current will flow in the
burden resistors, the direction indicating whether the generator is taking too little or
too much load, the size of the current indicating the magnitude of the imbalance.
The flow of current through the Burden resistor creates voltage drop across it, this
voltage is in phase with the reactive component and is applied to the primary of a
transformer in the voltage regulator. Also applied to the primary of the transformer
is line voltage BC.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-95
The transformer secondary windings are connected in such a manner as to produce
two different outputs:

V (BC) + Burden Voltage

V (BC) - Burden voltage
Both of these voltages are rectified and applied either side of R1, creating a biasing
voltage that is used to change the regulator setting, increasing the excitation if the
generator is providing too little load, decreasing the excitation if it is taking too much
reactive load.
If there is no load imbalance, the Burden voltage will be zero and the two rectifier
outputs will be the same. Under these conditions no biasing signal is developed
across R1 and the regulator setting is not altered.
Any real load imbalances produce voltages in the Burden resistors that are 90
degrees out of phase with line voltage BC. When these voltages are combined by
the transformers, the magnitudes of the two resultant signals are the same,
therefore no bias signal is produced across R1.
The second system uses a mutual reactor, this device produces a 90 degree phase
shift between the primary current and the secondary voltage.
The primary winding of the mutual reactor is connected in the sensing loop in place
of the Burden resistor. Any load imbalances cause a current to flow in the primary
winding, producing an emf in the secondary that lags the current by 90 degrees.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-96
If the current in the primary is due to a reactive load imbalance, the secondary
voltage produced will subtract from, or add to the phase C voltage that is passing
through the mutual reactor secondary winding, increasing or decreasing the output
of the rectifier. The output from the rectifier is then applied to a magnetic amplifier
to increase or decrease the generator excitation.
3.5.10 AC SYSTEM PROTECTION
The protection of a.c. power systems takes many forms:

Under-voltage

Over-voltage

Under current

Over current

Differential
3.5.10.1
Over & Under-Voltage Protection
The first form of voltage regulation is the voltage regulator, however, to protect
against regulator faults, the generator output is also monitored for excessively high
and low voltages.
Generally an over-voltage or under-voltage fault will cause the GCR to trip, open
circuiting the generator field supply and reducing the generator output to zero, this
in turn will cause the GCB to trip.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-97
3.5.10.2
Over Excitation & Under Excitation
On a parallel a.c. system further protection is required for an over and under-voltage
faults, this protection is called over and under excitation protection. With paralleled
a.c. generators, an over or under-voltage condition on one generator will cause the
complete a.c. system voltage to increase or decrease. The problem is determining
the generator that is causing the fault.
The solution lies in the fact that, if the output voltage of one generator is increased,
that generator will take a larger share of the reactive load, if its voltage is
decreased, it will take less reactive load. Protection is achieved by using another
sensing loop (a third loop) to monitor the reactive load supplied by each generator.
If a generators output voltage starts to increase, the generator will start to take more
reactive load, the other generators will start to take less load. Under normal
operating conditions this will be resolved by the reactive load sharing loop.
If however there is a fault in the voltage regulator, or the load sharing loop, the load
imbalance will continue. The imbalance being sensed by the third loop. When the
error signal reaches a pre-determined value the GCU lowers the over-voltage trip
level of the generator taking too much load. When the system voltage reaches the
lowered over-voltage trip level, the GCU opens the generators BTB:

removing the generator from parallel operation, leaving it powering its own bus
only,

closing the auxiliary contacts of the BTB, shorting out the loop current
transformers for that generator and completing the loop for the remaining
generators,

restoring the over-voltage trip level to its original, higher value.
If the fault was in the load sharing loop, the system will now continue to operate,
with the one generator supplying its own load bus and the remaining generators
sharing the rest of the aircraft loads.
If the fault was in the voltage regulator, not the loop, the generators output voltage
will continue to rise. When the normal over-voltage level is reached, the GCU trips
the GCB, disconnecting the generator from its bus. To restore power to the bus, the
GCU closes the BTB, connecting it to the remaining paralleled generators.
3.5.10.3
Frequency Protection
The method of frequency protection depends on whether the system is parallel or
non-parallel. In a non parallel system the protection may be on the CSD or
contained within the GCU.
With CSD protection, an over-speed or under-speed condition will cause the CSD to
be put into full under-speed mode, tripping the low speed switch, as the generator
output falls the GCB is tripped, removing the generator from its bus.
With GCU protection, the GCU trips the GCB directly, removing the generator from
its bus.
In a parallel system protection is a little more complicated. When the frequency of
one generator is increased, that frequency is felt throughout the a.c. system, the
problem is trying to ascertain which generator is at fault.
If the speed of a generator increases, the generator starts to take a larger share of
the real load, if the speed decreases it starts to take less real load, this will normally
be resolved by the real load sharing loop.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-98
If however there is a fault on the load sharing system, the generators speed will
continue to increase or decrease as will its share of the real load.
Protection is achieved by another real load sensing loop, the fourth sensing loop.
When the error signal produced by this loop reaches a pre-determined level, the
BTB is tripped by the GCU, removing the generator from parallel operation. If the
fault is on the CSD and not the load sharing loop, the generators speed will
continue to increase or decrease.
With an over-speed condition, the CSD over-speed sensor will put the CSD into full
under-speed mode. In either case, the low speed switch on the CSD will eventually
operate, tripping the GCB. The BTB is then closed to provide power to the
disconnected bus.
In some systems the GCU will trip the GCR instead of the GCB. Loss of the
generator field causes the generator output to fall and the GCB to be tripped.
3.5.10.4
Differential Protection
Differential protection is used throughout the aircraft for line to line faults and line to
earth faults. All modern aircraft have a logical approach to differential protection
and will attempt to isolate the fault with minimum loss of services.
The principle of operation is quite simple, if the current leaving a source is the same
value as that which arrives at the distribution point, then no fault exists.
The system uses two current transformers, one to measure the source current, the
other to measure the current at the distribution point.
When no fault is present, the emf induced in each current transformer is the same
and the resultant current flowing in the resistor is zero. With no current flowing in
the resistor, no voltage is dropped across it and no trip signal is produced.
Under fault conditions, the emf's induced in the CT's are different, producing a
resultant current flow in the resistor. Current flowing in the resistor causes a voltage
drop across it, producing a trip signal. The trip signal is passed to the GCU, which
will trip the breakers necessary to isolate the fault.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-99
Differential protection is connected in a variety of ways, however the basic principle
of operation is always the same. To ensure errors are not made, reference must
always be made to the maintenance and wiring diagram manuals.
3.5.10.5
Merz-Price Protection
Merz-Price protection is another form of differential protection found on some
aircraft a.c. distribution systems. It provides protection for faults between phases,
or between one phase and ground.
Connections for a single phase are as shown above, connection of the CT's on the
other phases being identical. The two CT's are connected in series opposition, in
series with two relay coils.
Under no fault conditions the emf's induced in each CT are equal and opposite and
no current flows in the relay coils.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-100
Under fault conditions, the emf's induced in each of the CT's is different producing a
resultant current flow that energises the relays. Once the relays are energised the
feeder cable is isolated, isolating the fault.
Once the cable is open circuited, no current flows and no emf's are induced in the
CT's. To prevent the relay contacts closing again latched relays are used.
3.5.10.6
Open Phase Protection
Open phase protection also uses current transformers, the current in each phase
being measured. If the current in one phase is almost zero when the current in the
other two phases is high, the system assumes a fault has occurred and will trip the
appropriate breakers.
3.5.10.7
Shorted Rotating Diodes
A shorted rotating diode in the generator would produce a ripple on the main
generator output. The GCU monitors the generator output and trips the GCR if a
ripple is detected.
The same form of protection is used on the PMG output.
3.5.11 AC SYSTEM INDICATIONS
Power is normally indicated on Watt-Var meters that are provided for each
generator. Real power is generally displayed, with reactive power being displayed
on selection of a switch.
The indicator uses the relationship between a phase voltage and its opposite line
voltage to produce the two displays on a single meter. There being a phase angle
of ninety degrees between the two.
When used to indicate real power, the voltage coil of the meter is connected to
phase B of the generator and the current coil is connected to a CT also on phase B.
Under these conditions, real current flowing in phase B will produce a field in the
meter current coil, that is at 90 to the field produced by the voltage coil. As the
fields react, the pointer moves to indicate real power.
Any reactive current flowing in phase B will produce a field in the current coil that is
anti-phase to the field produced in the voltage coil. No interaction of fields takes
place and the pointer remains stationary.
To indicate reactive power, the voltage coil of the meter is connected across phases
A and C of the generator. This shifts the field in the voltage coil by 90. Any
reactive current flowing in phase B now produces a field in the current coil that is at
90 to the voltage coil field, causing the pointer to move to indicate reactive power.
Any real current in phase B produces a field in the current coil that is anti-phase
with respect to the voltage coil field. No field interaction takes place and the pointer
remains stationary.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-101
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-102
3.6 POWER CONVERSION
With the exception of transforming a.c. at one voltage to a.c. at another voltage,
there are two basic forms of power conversion used on aircraft electrical systems,
a.c. to d.c. and d.c. to a.c.
Conversion of d.c. to a.c. is achieved using devices called inverters, the conversion
of a.c. to d.c. transformer rectifier units.
3.6.1 INVERTERS
There are two main reasons for converting d.c. to a.c;

To produce a source of frequency controlled a.c. for use on either d.c. or
frequency wild a.c. powered aircraft.

To produce an emergency a.c. power supply. This may be for use on a
frequency wild or a constant frequency a.c. powered aircraft.
There are two basic types of inverter, rotary and static.
3.6.1.1
Rotary Inverters
A rotary inverter consists basically of a d.c. motor driving an a.c. generator. As
there are various types of both motor and generator, there are several different
rotary inverters, of which the following are the primary types:

Permanent magnet

Rotating armature

Inductor
3.6.1.1.1
Permanent Magnet Inverter
Shown below is a single phase, permanent magnet inverter.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-103
Located at one end of the machine is a d.c. motor, this consists of a field winding
mounted on the case and an armature winding mounted on the rotor. D.C. power
for the armature is supplied via a commutator and brushes.
At the other end of the machine is a permanent magnet a.c. generator. This
consists of a six pole permanent magnet that is located on the rotor and an output
winding that is wound on a pair of poles on the case of the machine. Putting the
output winding on the stationary part of the machine allows the a.c. power to be
taken directly from the generator without the use of slip rings and brushes.
When d.c. power is applied to the motor field and armature windings the rotor starts
to turn, rotating the permanent magnet inside the stator winding. As the lines of flux
from the permanent magnet cut the stator windings an emf is induced in them.
The output frequency depends on the number of poles on the magnet and the
speed at which it is rotated. The output voltage depends on the strength of the
permanent magnet, this being rather limited. This type of inverter can be made
multi-phase by fitting additional stator windings.
3.6.1.1.2
Rotating Armature Inverter
The d.c. motor is a four pole compound wound machine, the four field poles consist
of four shunt windings, made of many turns of fine wire, with four heavy gauge
series windings wound on top.
The speed of the motor is controlled by a centrifugal switch mounted on the rotor of
the machine. When running speed is reached, the switch short circuits a resistor
that is in series with the shunt windings.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-104
The generator is a three phase, four pole, star connected machine, the output being
taken via brushes and slip rings. A single phase generator would only require two
brushes and slip rings.
The output voltage is controlled by varying the generator field current, this is
achieved using a carbon pile voltage regulator. As in d.c. generation, the pile is
placed in series with the field winding and the compression is varied by the current
passing through the voltage sensing coil. The d.c. for the voltage sensing coil is
obtained by rectifying the output from a single phase of the generator output.
3.6.1.1.3
Starting rotary inverters
Most inverter motors are compound wound machines with mainly shunt
characteristics and cannot therefore be started on load.
Larger inverters require some form of starting circuit to prevent overheating of the
armature at low rpm. Starting circuits put a resistor in series with the armature for
starting, then remove it once the motor is running. A common form of starting circuit
is the 'T' start circuit.
At start-up the motors rotor is stationary and therefore producing no back emf.
The potential across the relay coil is therefore almost zero and no current flows
through it.
With no current in the relay coil the relay remains un-energised putting the resistor
in series with the armature.
As the motor speed increases, the back emf and the potential across the relay coil
increase. Eventually the potential across the coil will be sufficient to cause a current
flow through it that will energise the relay. When energised, the relay contacts short
the field resistor, removing it from the armature circuit.
3.6.1.1.4
Voltage control
The output voltage of an inverter is controlled by varying the generator field current.
The control methods used are the same as those used on the main power
generation systems, carbon pile or transistorised regulators.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-105
In the example shown below a carbon pile voltage regulator is used, the pile being
in series with the generator field. Pile compression is controlled by a voltage
sensing coil that is connected across the output of a full wave bridge rectifier, the
rectifier producing a d.c. level proportional to the sensed a.c. output voltage.
The output voltage can be varied by adjusting the variable resistor RV1.
3.6.1.1.5
Frequency control
The output frequency of a rotary inverter depends on the speed at which the
generator is driven. The generator is driven by a d.c. motor, therefore the output
frequency is controlled by varying the speed of the motor.
The majority of inverter motors are shunt wound machines, the speed being
controlled by varying the motors field current. A variety of methods are used to
sense the frequency and control the current:
Contacts controlled by the operation of mechanical bobweights are used to short
circuit a resistor in series with the field
The changing impedance of a resonant circuit across the inverter output is used to
vary field current
The output of a PMG on the inverter shaft is used to sense inverter speed, current
control being achieved by a transistorised circuit.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-106
3.6.1.1.6
Failure warning
Aircraft fitted with rotary inverters generally have an inverter failure warning. Many
of these are operated by a torque switches working on the drag cup principle.
A rotating field is produced by connecting the three phase inverter output to three
star connected windings.
The rotating field induces emf's in the drag cup, producing eddy currents. The fields
produced by the eddy currents react with the main field and rotate the cup. A cam
attached to the cup also rotates, the movement being restrained by a spring.
If the phase rotation, output voltage and output frequency are correct the cam will
hold open the pair of contacts that operate the inverter failure warning.
If a phase is lost, the phase rotation is incorrect, or the frequency or output voltage
decrease, the eddy currents and resultant drag cup field strength will decrease.
The control spring torque will then be greater than the torque produced by
interaction of the fields and the cup will not rotate as far, allowing the switch to close
and operate the inverter failure warning.
3.6.1.1.7
Inductor Type Rotary Inverter
The inductor type rotary inverter shown below uses a rotor of soft iron laminations.
Grooves are cut laterally along the surface of the rotor to provide poles that
correspond to the number of stator poles.
Field coils are wound on one set of stationary poles, the a.c. armature coils on the
other set of stationary poles. Not shown on the diagram is a third set of coils, the
motor armature coils, these are wound on the rotor and supplied via a commutator
and brush assembly.
When d.c. is applied to the motor field and armature the rotor starts to rotate. As
the poles of the rotor align with the stationary poles a low reluctance path is
established for the motor flux. The flux passes from the field pole through the rotor
poles to the a.c. armature poles and back to the field poles, in this situation there is
a lot of flux linking with the a.c. output coils.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-107
When the rotor is aligned between stationary poles there is a high reluctance path
for the flux and the number of flux linkages is greatly reduced.
The increasing and decreasing flux density in the stator induces an emf in the a.c.
coils, which when connected to a circuit produces an alternating current flow.
The frequency of this type of inverter is controlled by the speed and number of
poles, the voltage by the strength of the d.c. stator field.
3.6.1.2
Static Inverters
Static inverters use solid state components and are smaller, lighter and more
reliable because of the lack of moving parts.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-108
When power is applied to the above circuit, the current rises according to the
circuits time constant. Whilst the value of d.c. current is changing, an emf is
induced in the secondary winding creating a current flow.
If the polarity of the primary current is reversed, the emf and current flow in the
secondary winding will reverse.
By continually switching the d.c. an a.c. output is produced. This principle is used
by a static inverter to convert d.c. to a.c.
Alternately switching SCR1 and SCR2 'on' and 'off' produces an alternating current
in the secondary winding.
Control of the SCR's is maintained by a square wave oscillator, this will alternately
turn on the SCR's at 1.25mS intervals, keeping the frequency constant at 400 Hz.
When SCR1 is turned 'on it conducts, the current rising at a rate dependent on the
circuit time constant. Whilst the current is rising an emf is induced in the secondary
winding, producing a current flow. Once conducting, capacitors C1 and C2 start to
charge, right plates positive.
1.25mS after SCR1 was turned on, the oscillator turns SCR2 on. When SCR2 is
turned 'on', its anode (top) drops to almost earth potential, the voltage drop across it
being negligible. In order for the capacitors to maintain their charge the left plates
go negative, switching 'off' SCR1.
The current through SCR2 now rises at a rate dependent on the circuit time
constant, this induces an emf of opposite sense in the secondary winding,
producing a current flow in the opposite direction.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-109
1.25mS later SCR1 will be turned 'on' and the process repeats.
Voltage control is maintained by switching the SCR's 'off' before peak voltage is
reached. This is achieved by switching 'on' SCR3, the control signal coming from
the voltage regulator.
When SCR3 is switched 'on', its anode drops to almost earth potential, making the
inside plates of both capacitors drop to earth potential.
Assuming SCR1 was conducting and the capacitors were charging right plates
positive, the potentials being 0v / 5V / 10V. When SCR3 is switched 'on' the
potentials would change to -5V / 0V / 5V, switching 'off' SCR1.
When SCR2 is switched 'on' (1.25mS after SCR1 was switched 'on'), the potentials
across the capacitors would be -10V / -5V / 0V, switching 'off' SCR3, making it
ready to be triggered again for switching off SCR2.
3.6.2 TRANSFORMER RECTIFIER UNITS
Transformer rectifier units (TRU's) are a combination of static transformers and
rectifiers used to convert a.c. power into d.c. power.
The majority of modern aircraft are fitted with TRU's as the main source of d.c.
power.
Each main a.c. bus supplies one TRU. The d.c. outputs are then used to supply
individual busbars or may be connected in parallel.
TRU's are designed to work on a regulated three phase input of 200 volts at 400
Hertz and provide a continuous d.c. output of between 100 and 500 amps at 28
volts.
The unit shown, consists of a transformer and two three phase, full wave, bridge
rectifier circuits, that are mounted in separate sections of the case.
The secondary windings are wound in star and delta, each being connected to
individual bridge rectifier assemblies made up of six silicon diodes.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-110
An ammeter shunt is connected in the output side to enable the current to be
measured. To provide overheat protection, thermal switches are provided at the
transformer and rectifier assemblies. These are supplied from an external source
and normally operate at approximately 150 to 200 degrees centigrade.
Cooling of TRU's is normally by natural convection although some units have an
integral fan for forced air cooling.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-111
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-112
3.7 CIRCUIT PROTECTION DEVICES
A short circuit, overload or other fault condition occurring in the circuit formed by
cables and components of an electrical system, could result in extensive damage
and failure of equipment. For example, excessive current flow caused by a short
circuit at some section of a cable left unchecked, will generate heat in the cable
which will continue to increase until something gives way. A portion of the cable
may melt, thereby opening the circuit so that the only damage done would be to the
cable involved. However, it is likely that much greater damage would result; the
heat could char and burn the cable insulation and that of other cables forming a
loom, and so would cause more short circuits and the possibility of an electrical fire.
It is essential therefore to provide devices in the network of power distribution to
systems, and having the common purpose of protecting their circuits, cables and
components. Protection devices normally used are fuses, circuit breakers and
current limiters. Other devices are provided to serve as protection against such
fault conditions as reverse current, over-voltage, under-voltage, over-frequency,
under-frequency, phase unbalance, etc. These devices may generally be
considered as part of main generating systems, and those associated with d.c.
power generation, in particular, are normally integrated with the generator control
units.
3.7.1 FUSES
A fuse is a thermal device designed primarily to protect the cables of a circuit
against the flow of short-circuit and overload currents. In its basic form, a fuse
consists of a low melting point fusible element or link, enclosed in a glass or
ceramic casing which not only protects the element, but also localizes any flash
which may occur when “fusing”. The element is joined to end caps on the casing,
the caps in turn, providing the connection of the element with the circuit it is
designed to protect. Under short-circuit or overload current conditions, heating
occurs, but before this can affect the circuit cables or other elements, the fusible
element, which has a much lower current-carrying capacity, melts and interrupts the
circuit. The materials most commonly used for the elements are tin, lead, alloy of tin
and bismuth, silver or copper in either the pure or alloyed state.
The construction and current ratings of fuses vary, to permit a suitable choice for
specific electrical installations and proper protection of individual circuits. Fuses are,
in general, selected on the basis of the lowest rating consistent with reliable system
operation. For emergency circuits, i.e., circuits the failure of which may result in the
inability of an aircraft to maintain controlled flight and effect a safe landing, fuses are
of the highest rating possible consistent with cable protection. For these circuits it is
also necessary that the cable and fuse combination supplying the power be
carefully engineered taking into account short-term transients in order to ensure
maximum utilization of the vital equipment without circuit interruption.
Being thermal devices, fuses are also influenced by ambient temperature variations.
These can affect to some extent the minimum “blowing” current, as well as
“blowing” time at higher currents, and so must also be taken in account. Typical
examples of fuses currently in use in light and heavy-duty circuits, are shown in Fig.
7.l(a)-(b) respectively. The light-duty fuse is screwed into its holder (in some types a
bayonet cap fitting is used) which is secured to the fuse panel by a fixing nut. The
circuit cable is connected to terminals located in the holder, the terminals making
contact with corresponding connections on the element cartridge. A small hole is
drilled through the centre of the cap to permit the insertion of a fuse test probe.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-113
Fuses are located accessible for replacement, and as close to a power distribution
point as possible so as to achieve the minimum of unprotected cable.
The heavy-duty or high rupturing capacity fuse shown above lower is designed for
installation at main power distribution points. It consists of a tubular ceramic cartridge within which a number of identical fuse elements in parallel are connected to
end contacts. Fire-clay cement and metallic end caps effectively seal the ends of
the cartridge, which is completely filled with a packing medium to damp down the
explosive effect of the arc set up on rupture of the fusible elements. The material
used for packing of the fuse illustrated is granular quartz; other materials suitable for
this purpose are magnesite (magnesium oxide), kieselguhr, and calcium carbonate
(chalk). When an overload current condition arises and each element is close to
fusing point, the element to go first immediately transfers its load to the remaining
elements and they, now being well overloaded, fail in quick succession.
In some transport aircraft, the fuse holders are of the self-indicating type
incorporating a lamp and a resistor, connected in such a way that the lamp lights
when the fusible element ruptures.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-114
3.7.2 CIRCUIT BREAKERS
Circuit breakers, unlike fuses or current limiters, isolate faulted circuits and
equipment by means of a mechanical trip device actuated by the heating of a bimetallic element through which the current passes to a switch unit. We may
therefore consider them as being a combined fuse and switch device. They are
used for the protection of cables and components and, since they can be reset after
clearance of a fault, they avoid some of the replacement problems associated with
fuses and current limiters. Furthermore, close tolerance trip time characteristics are
possible, because the linkage between the bi-metal element and trip mechanism
may be adjusted by the manufacturer to suit the current ratings of the element. The
mechanism is of the “trip-free” type, i.e. it will not allow the contacts of the switch
unit to be held closed while a fault current exists in the circuit.
The factors governing the selection of circuit breaker ratings and locations, are
similar to those already described for fuses.
The design and construction of circuit breakers varies, but in general they consist of
three main assemblies; a bi-metal thermal element, a contact type switch unit and a
mechanical latching mechanism. A push-pull button is also provided for manual
resetting after thermal tripping has occurred, and for manual tripping when it is
required to switch off the supply to the circuit of a system. The construction and
operation is illustrated schematically above. At (a) the circuit breaker is shown in its
normal operating position; current passes through the switch unit contacts and the
thermal element, which thus carries the full current supplied to the load being
protected. At normal current values heat is produced in the thermal element, but is
radiated away fairly quickly, and after an initial rise the temperature remains
constant. If the current should exceed the normal operating value due to a short
circuit, the temperature of the element begins to build up, and since metals
comprising the thermal element have different coefficients of expansion, the
element becomes distorted as indicated in (b). The distortion eventually becomes
sufficient to release the latch mechanism and allows the control spring to open the
switch unit contacts, thus isolating the load from the supply. At the same time, the
push-pull button extends and in many types of circuit breaker a white band on the
button is exposed to provide a visual indication of the tripped condition. The
temperature rise and degree of distortion produced in the thermal element are
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-115
proportional to the value of the current and the time for which it is applied. The
ambient temperature under which the circuit breaker operates also has an influence
on circuit breaker operation and this, together with operation current values and
tripping times, is derived from characteristic curves supplied by the manufacturer.
After a circuit breaker has tripped, the distorted element begins to cool down and
reverts itself and the latch mechanism back to normal, and once the fault which
caused tripping has been cleared, the circuit can again be completed by pushing in
the circuit breaker button. This “resetting” action closes the main contacts and reengages the push-button with the latch mechanism. If it is required to isolate the
power supply to a circuit due to a suspected fault, or during testing, a circuit breaker
may be used as a switch simply by pulling out the button. In some designs a
separate button is provided for this purpose.
Some circuit breakers incorporate a separate manual trip push button. A cover may
sometimes be fitted to prevent inadvertent operation of the button.
In three-phase a.c. circuits, triple-pole circuit breakers are used, and their
mechanisms are so arranged that in the event of a fault current in any one or all
three of the phases, all three poles will trip simultaneously. Similar tripping will take
place should an unbalanced phase condition develop as a result of a phase
becoming “open-circuited”.
The three trip mechanisms actuate a common push-pull button.
3.7.3 CURRENT LIMITERS
Current Limiters are designed to limit the current to a pre-determined amperage.
They are thermal devices but have a high melting point which means they can carry
a considerable overload current before rupturing. For this reason they are used
mainly in heavy –duty power distribution circuits. A typical current limiter (shown
below) incorporates a fusible element which is a single strip of tinned copper, drilled
and shaped at each end to form lug type connections with the central portion
“waisted” to the required width to form the fusing area.
A typical current limiter (manufactured under the name of “Airfuse”) is shown below.
It incorporates a fusible element which is, in effect, a single strip of tinned copper,
drilled and shaped at each end to form lug type connections, with the central portion
“waisted” to the required width to form the fusing area. The central portion is
enclosed by a rectangular ceramic housing, one side of which is furnished with an
inspection window which, depending on the type, may be of glass or mica.
The central portion is enclosed by a rectangular ceramic housing, one side of which
is furnished with an inspection window which may be glass or ceramic.
Aerodynamics, Structures and Systems (CE)
Page 3-116
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
3.7.4
LIMITING RESISTORS
Limiting Resistors provide another form of protection particularly in d.c. circuits in
which the initial current surge is very high, e.g. starter motor and inverter circuits,
circuits containing highly-capacitive loads. When such circuits are switched on they
impose current surges of such a magnitude as to lower the voltage of the complete
system for a time period, the length of which is a function of the time response of
the generating and voltage regulating system. In order therefore to keep the current
surges within limits, the starting sections of the appropriate circuits incorporate a
resistance element which is automatically connected in series and then shorted out
when the current has fallen to a safe value.
The diagram above shows the application of a limiting resistor to a turbine engine
starter motor circuit incorporating a time switch; the initial current flow may be as
high as 1500 A. The resistor is shunted across the contacts of a shorting relay
which is controlled by the time switch. When the starter push switch is operated,
current from the busbar flows through the coil of the main starting relay, thus
energizing it. Closing of the relay contacts completes a circuit to the time switch
motor, and also to the starter motor via the limiting resistor which thus reduces the
peak current and initial starting torque of the motor. After a pre-determined time
interval, which allows for a build-up of engine motoring speed, the torque load on
the starter motor decreases and the time switch operates a set of contacts which
complete a circuit to the shorting relay. From the diagram, with the relay energized
the current from the busbar passes direct to the starter motor, and the limiting
resistor is shorted out. When ignition takes place and the engine reaches what is
termed “self-sustaining speed”, the power supply to the starter motor circuit is then
switched off.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-117
3.8 OTHER POWER
Maintenance generally requires the use of some form of electrical power. In some
circumstances power may be obtained from the main engine driven generators, or
by running the APU, if fitted. In the majority of cases, running the main engines or
APU is impractical or not possible, this results in the need for some other form of
power.
3.8.1 AUXILIARY POWER UNIT
Auxiliary power units (APU's) are used on many types of aircraft to provide electrical
power and a source of pressurised air thus making the aircraft independent of
ground support equipment. On most aircraft the APU may be used in flight as a
back-up.
An APU is a self contained unit which generally consists of a small, constant speed,
gas turbine engine coupled to an accessory drive gearbox. The gearbox provides a
means of driving a generator of similar type & power rating to main engine
generators. The APU generator is not normally paralleled to the main generators, if
it is then it becomes the master generator.
The gearbox also drives the APU accessories such as:

fuel pump

oil pump
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-118

tacho generator

centrifugal switch
The centrifugal switch is used to control:

starting

ignition

the governed speed indication circuit

and to provide over-speed protection.
The APU is located in an unpressurised compartment of the fuselage, usually in tail
section, the compartment is separated from remainder of the fuselage by a firewall.
Access to the compartment is usually obtained through hinged cowlings positioned
either side of the APU or through a single panel below the APU.
Air for the APU compressor is drawn in through single or twin intakes. The intake
sections usually have electrically actuated doors that are controlled by the APU
master switch to ensure the correct operation during starting and shut down
sequences. Some systems use an electromagnetic control valve which when
operated allows pneumatic ram air to open and close the doors.
Door positions are usually detected by micro switches or proximity switches, these
switches being used to operate indicator lights on the flight deck.
The APU compressor discharges air into the plenum chamber which is connected to
the air conditioning system and main engine starting system, this way the engine
bleed air that is supplied is automatically regulated to prevent overloading of the
APU. On some APU's any air that is not required is bypassed into the engine
exhaust duct. Bleed air may also be used to provide intake anti icing for the APU.
3.8.1.1
Fuel System
Fuel for the APU is supplied from one of the main fuel tanks via a solenoid operated
valve, the fuel is regulated by a Fuel Control Unit (FCU) which controls the
acceleration, deceleration and speed of the APU.
3.8.1.2
Starting
The engine is started using an electric starter motor and a High Energy Ignition
system, both being controlled by the master control switch via a centrifugal switch.
Power for starting is taken from the main battery or the APU battery.
3.8.1.3
Cooling
Cooling and ventilation of the APU compartment is normally by an accessory
gearbox driven fan. Air may also be ducted from the fan to be used for cooling of
the a.c. generator and engine oil.
3.8.1.4
Oil System
A self contained oil system lubricates all the gears and bearings within the APU. Oil
pressure, quantity and sometimes temperature are monitored.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-119
3.8.1.5
Fire Protection
Fire detection in the APU compartment is normally achieved by a continuous wire
detector working on a change of resistance or capacitance with heat. When
triggered, the system will operate warning lights and horns and shut down the APU.
A single shot fire extinguisher is usually used to extinguish the fire, however, aircraft
with a centre mounted main engine may have a crossfeed system allowing the main
engine extinguisher to be discharged into the APU compartment.
3.8.1.6
Operation
All the switches, warning lights and instruments for starting, stopping and normal
operation are located on control panels on the flight deck and in compartments
accessible from outside the aircraft.
The APU can normally only be started from inside the aircraft but for safety reasons
can be shut down from either control panel.
Operation of APU is monitored by an Exhaust Gas Temperature (EGT) indicating
system and either an hour meter or an elapsed running time indicator.
Other items monitored depend on the installation but will include one or more of the
following:

starting current

engine rpm

generator output voltage and frequency

generator bearing temp
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-120

engine oil temperature
A connection for an APU test facility or fault indication module may also be
included.
The operating characteristics of the APU are such that injury and damage to aircraft
and equipment are possible, it is therefore necessary to observe certain safety
precautions and carry out any checks prescribed in the approved manuals for a
particular installation.
3.8.2 EXTERNAL AC & DC POWER
Whether the power requirements are d.c., a.c. or both depends entirely on the
particular aircraft and its main power system. Generally large, commercial aircraft
only require a.c. power, albeit in large quantities, and light aircraft only require d.c.
power, there are however a group of aircraft that require both a.c. and d.c. power.
If d.c. ground power is used on larger aircraft, it is generally only used in place of
the aircraft battery. On smaller, d.c. powered aircraft, the ground supply is used to
power the complete aircraft electrical system.
The electrical power is provided by Ground Power Units (GPU‟s), or obtained from
Ground Power Supplies and is connected to the aircraft electrical system via an
external power connector, or receptacle, located on the lower fuselage.
There are two basic forms of External Power connector, one for d.c. the other for
a.c. Both consist of a series of sockets that are shrouded by a rubber plug, the pins
being located on the aircraft.
Ground power connectors on aircraft are located above ground level, therefore,
when fitted the cable must be supported.
3.8.2.1
DC Power Connector
The d.c. power connector is similar in construction to the a.c. connector except that
it only has three pins and sockets. The three sockets in the ground power unit plug
provide 28V positive, 28V negative and a single interlock pin.
The interlock pin allows an auxiliary 28 volt supply from the ground power unit to be
connected to the aircraft. This supply is used to operate the ground power
contactor, connecting the main d.c. supply to the aircraft.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-121
As with the a.c. connector, should the plug work loose, the shorter interlock pin and
socket will open circuit first, removing the supply to the ground power contactor.
Operation of the contactor will cause the ground power to be disconnected from the
aircraft.
3.8.2.2
AC Ground Power Connector
The a.c. connector has six sockets in the plug. The four large sockets connect the
three phase a.c. and its neutral. The two smaller and shorter pins provide a ground
power interlock.
Generally the two short pins are short circuited within the ground power unit supply
plug. This enables d.c. power to be routed out of the aircraft and back into the
aircraft via the shorting link. The d.c. is then used to operate a.c. power relays, that
connect the a.c. power to the aircraft.
If the socket becomes disconnected whilst power is applied to the aircraft, the two
shorter pins loose contact first. This breaks the supply to the interlock relays,
causing the a.c. external power contactor to de-energise, disconnecting ground
power from the aircraft. Without the interlock, the supply would be broken at the
connector, resulting in arcing between the pins and sockets.
The interlock also ensures that the ground power connector is correctly fitted before
allowing power onto the aircraft.
3.8.2.3
Paralleling of Ground Power
Ground power cannot be connected in parallel with any other power sources.
Similarly, if an aircraft is capable of being supplied from two ground power units
simultaneously, the two power units cannot be paralleled. To connect d.c. or a.c.
power sources together requires the use of some form of load sharing system,
these are not provided on ground power units.
To ensure that paralleling of GPU's does not take place, interlocks are provided.
This is achieved by routing the ground power contactor energising supplies through
the auxiliary contacts of the other power contactors, i.e. BTB's, GCB's and APB.
3.8.2.4
Ground Handling Busbar
Certain types of aircraft have a busbar that is only powered while the aircraft is on
the ground. This ground handling bus supplies services that are only required for
ground servicing of the aircraft, e.g. for cargo door motors, wheel well lighting and
cabin vacuum cleaner sockets.
Power is supplied to the ground handling bus directly from the GPU under the
control of the Bus Power Control Unit (BPCU). There are no switches to control the
power and no indications.
Before the power is allowed onto the bus, the BPCU will ensure that the supply
voltage, frequency and phase rotation are correct, that there are no differential
faults and that the aircraft is in 'on ground' mode.
3.8.2.5
Connection of Ground Power
External power is under the control of the BPCU.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-122
The selector switch for ground power is located on the main electrical control panel
in the flight deck, together with power available light and possibly a power on light.
At the ground power receptacle there is a 'not in use' light and possibly power
connected light.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-123
When ground power is connected to the aircraft, the BPCU checks the supply for
correct frequency, voltage and phase rotation. If satisfied the power available light
on the flight deck and, if fitted, the power connected light at the service panel are
illuminated.
Operation of the ground power switch will then connect ground power to the aircraft.
Once ground power is supplying the aircraft, the 'not in-use' light on the ground
service panel will extinguish and if fitted, the 'in-use' light on the flight deck control
panel will illuminate.
As with APU power supplies, two priority systems may be found. On some aircraft
the BPCU will only connect the ground power if no other power is on the aircraft, on
others, the BPCU will automatically de-select the existing power and apply the
ground power.
While the external power is supplying the main busbars, the BTB's are closed and
the GCB's are tripped. If a main engine is then started, whether or not the main
generator is connected to the aircraft power system depends on the priority system
employed. On some aircraft the BPCU will prevent the main generator being
brought On-line until the ground power is switched off. On other aircraft the BPCU
will automatically deselect the ground power contactor, allowing the GCU to bring
the main generator On-line.
Interlocks are provided throughout the aircraft power supply system to prevent
inadvertent interconnection of power supplies.
3.8.2.6
Indications
In addition to 'power available', 'not in use' and "power on" indications, the
frequency and voltage can be monitored using the main voltmeter and frequency
meters. This may entail selection of ground power on a meter switch. On some
systems it is also possible to monitor the current supplied by the GPU. the indication
being provided by a dedicated ammeter located on the ground power control panel.
Aerodynamics, Structures and Systems (CE)
Part 3 - Electrical systems 1 ATA 24
By COBC – Issue 1 - 11 February, 2014
Page 3-124