# Electrical Machine 1 - Lecture ```ELECTRICAL MACHINE 1
Chapter 1
Electrical Machinery Generalization
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
• The two (2) most frequently used types for rotating electrical machines are:
(a) Generators – mechanical energy is converted into electrical energy.
(b) Motor – electrical energy is converted into mechanical energy.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
( GENERATOR )
A
+

Tm


S

S


Te
 
N
N
B
_



ELECTRICAL
APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
S
( MOTOR )

A
+ S

DC
SUPPLY
v

Te
 
N


B
N _



TL
APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
• Two other types of rotating electrical machines not used so often are:
rotary converters and frequency converters.
Basic Operations:
• Electric Generator – it is driven (rotated) by a mechanical machine usually
called a prime mover ( can be a steam turbine, a gasoline engine, an
electric motor, or even a hand-operated crank). A generator action can
take place when, and only when, there is a relative motion between
conducting wires (usually copper) and magnetic lines of force.
• Electric Motor – it is supplied with electrical energy and develops torque,
that is a tendency to produce rotation.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
• All rotating electric generators consists essentially of two important parts:
(1) An even set of electromagnets or permanent magnets.
(2) The laminated steel core containing current-carrying copper wires which
is called an armature winding.
• In dc-generator, the armature winding is mechanically rotated through a
stationary magnetic fields created by electromagnets or permanent
magnets.
• In ac-generator, the electromagnets or the permanent magnets and their
accompanying magnetic fields are rotated with respect to the stationary
armature winding.
• In dc-motor, current is sent into the armature winding, the latter is being
placed inside a set of radially supported magnet poles.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
Field Structure
for a dc machine
Field Structure for a
ac machine
FIG 1: Field Structure for a dc generator (rear) and ac machine (foreground)
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
• In the illustration of previous slide:
(a) dc-generator – shows many main and commutating poles bolted to the
outside yoke; also visible is the brush rigging on the far side.
(b) ac-machine – the two rings mounted on the shaft; the two ends of the
entire field winding are connected to these rings so that stationary
brushes riding on the latter can “feed” direct current into the rotating
structure.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
Wound Stationary
armature
FIG 2: Wound Stationary armature for a low speed ac generator. A field
similar to that depicted of the ac –machine in figure 1 would be rotated
inside this frame.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
FIG 3: DC Motor – The armature commutator and the carbon brushes are
clearly visible on the near side.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
• In an ac motor, current is sent into the armature winding, which is usually
placed in a stationary laminated iron core; the rotating element may or may
not be set of magnet poles.
FIG 4: A rotary converter
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
• A synchronous type, in which a small dc generator is placed on the shaft
extension to supply dc excitation to the rotating field.
• The rotary converter, electrical energy of one form is changed into
electrical energy of another, the usual arrangement is to change ac energy
into dc energy although the reverse is sometimes done.
FIG 5: A small frequency converter set
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
• The frequency converter, has very limited application; its function is to
change ac electrical energy at one frequency into ac electrical energy at
another frequency.
How to do this?
Two rotating machines are directly coupled together; one of them operates
as an ac motor when connected to an ac source having a given frequency,
while the other, driven machine functions as an ac generator to deliver
electrical energy at some other frequency. The stationary part of the ac
generator is also connected to an ac source, the same supply to which the
motor is connected.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Rotating Electrical Machines
DC MACHINES
YOKE
BRUSH
AXIS
.
.
S
FIELD
WINDING
+
+
_
+
+
BRUSH
+ + +
.
.
FIELD
N
POLES
MAIN FIELD AXIS
+
+
+
ARMATURE
ARMATURE
CONDUCTORS
APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
1. BODY OR MAGNETIC FRAME OR YOKE
2. POLE CORE AND POLE SHOES
3. FIELD or EXCITING COILS
4. ARMATURE CORE
5. ARMATURE WINDING
6. COMMUTATOR
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
7. BRUSHES
8. END HOUSINGS
9. BEARINGS
10. SHAFT
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
Bearing
Pulley
Brush
Body / Yoke
End Housing
Shaft
Armature
Commutator
Field Core
Brush
holder
Field Winding
MAIN CONSTRUCTIONAL FEATURES
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
ARMATURE
YOKE
+
SHAFT
FIELD POLE
&amp; COIL
BRUSH
-
COMMUTATOR
MAIN CONSTRUCTIONAL FEATURES
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
OTHER MECHANICAL PARTS :
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
1. MAGNETIC FRAME or YOKE
The outer cylindrical frame
to which main poles and
inter poles are fixed and
by means of the machine
is fixed to the foundation is
called YOKE.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
MAGNETIC FRAME OR YOKE serves two purposes:
a) It provides mechanical protection to the inner parts of the machines.
b) It provides a low reluctance path for the magnetic flux.
The yoke is made of cast iron for smaller machines and cast steel or fabricated
rolled steel for larger machines.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
2. POLE CORE AND POLE SHOES
The pole core and pole shoes are fixed to
the yoke by bolts. They serves the following
purpose :
a) They support the field or exciting coils.
b) They distribute the magnetic flux on the
armature periphery more uniformly.
c) The pole shoes have larger X- section, so,
the reluctance of the magnetic path is
reduced. The pole core and pole shoes are
made of laminated steel assembled by
riveting together under hydraulic pressure.
POLE CORE
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
2. POLE CORE AND POLE SHOE
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
3. FIELD or EXCITING COILS
Field coils or exciting coils are used to
magnetise the pole core. Enameled copper
wire is used for the construction of these
coils. When direct current is passed through
these coils/ winding, it sets up the magnetic
field which magnetise the pole core to the
reqd. flux.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
3. FIELD OR EXCITING COILS
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
4. ARMATURE CORE
 Armature is a rotating part of the DC machine, reversal of flux takes
place, so hysteresis losses are produced. To minimize this loss, silicon
steel is used for the construction.
 The rotating armature cuts the main magnetic field , therefore an e.m.f is
induced in the armature core. This e.m.f circulates eddy currents in the
core which results in eddy current loss in it.
The armature core is laminated to reduce the eddy current loss.
 Armature core serves the following purposes:
a) It houses the conductors in the slots.
b) It provides an easy path for magnetic flux
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
4. ARMATURE CORE
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
5. ARMATURE WINDING
The no. of conductors in form of coils
placed in the slots of the armature and
suitably inter connected are called
winding .
This is the armature winding where
conversion of power takes place i.e. in
case of generator, mechanical power is
converted into electrical power and in
case of a motor, electrical power is
converted into mechanical power.
ARMATURE WINDING
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
5. ARMATURE WINDINGS
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
6. COMMUTATOR
It is the most important part of a DC machine and
serves the following purpose :
i) It connects the rotating armature conductors to the
stationary external circuit through the brushes.
ii) It converts alternating current induced in the
armature conductors into unidirectional current in
the external load circuit in generating action and it
converts alternating torque into unidirectional
torque produced in the armature in motoring
action.
The commutator is of cylindrical shape and is made of
wedge shaped hard drawn copper segments. The
segments are insulated from each other by a thin sheet
of mica. The segments are held together by means of
two V-shaped rings that fit into the V-grooves cut into
the segments. Each armature coil is connected to the
commutator segment through riser.
COMMUTATOR
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
6. COMMUTATOR
MICA INSULATION
COPPER SEGMENT
RISER
END RING
SHAFT
METAL SLEEVE
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
6. COMMUTATOR
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
7. BRUSHES
They form the connecting link between
armature winding and the external circuit.
The brushes are held in particular position
around the commutator by brush holders.
BRUSH
+
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
6. BRUSHES
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
8. END HOUSINGS
They are attached to the ends of main
frame and support bearing . The front
housing supports the bearing and the
brush assembly whereas rear housing
supports the bearing only.
END HOUSING
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
9. BEARINGS
The function of the bearing is to reduce
friction between the rotating and
stationary parts of the machines.
These are fitted in the end housings.
Generally, high carbon steel is used for
the construction of the bearings.
BEARINGS
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Main Constructional Features
10. SHAFT
The function of shaft is to transfer
mechanical power to the machine or from
the machine. Shaft is made of mild steel
with maximum breaking strength. All the
rotating parts like armature core,
commutator, cooling fan etc. are keyed to
the shaft.
SHAFT
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Understanding Armature Windings and Field Poles
 The armature windings of all types of motors and generators, whether dc
or ac are always wound on laminated steel cores of good magnetic
permeability.
 Alternating voltages are always generated in the windings of ac and dc
generators:
(a) In the ac generator, the generated alternating electromotive force (emf)
is transmitted directly to the load.
(b) In the dc generator, the generated alternating emf is first rectified by the
commutator and its brushes, that is changed to direct current before it is
(c) The ac motor receives its energy directly from an ac source and, without
any change whatever in form, uses it as alternating current in its winding
to develop torque.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Understanding Armature Windings and Field Poles
Why Steel cores have good magnetic permeability?
From magnetization curve,
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Understanding Armature Windings and Field Poles
Cont… armature windings
(d) In dc motor, however direct current is delivered to the brushes but flows as
alternating current in the armature winding after passing through the brushes
and commutator.
• The electromagnets (field poles). There are always an even number of them
in a given machine, and each one consists of a laminated steel core, of
rectangular cross section surrounded by one or more copper coils.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Understanding Armature Windings and Field Poles
Cont… field poles
• The spread out portion of the pole core, or shoe permits the magnetic flux to
enter the armature core over a wider area than would be possible with a core
having straight sides.
FIG 6: Pole shoe
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Understanding Armature Windings and Field Poles
 The type of field structure are:
(a) Stationary field type – the electromagnets are bolted to a yoke as shown
below so that they project radially inward toward the rotating armature.
Interpoles
FIG 7: Stationary field type of machine
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Understanding Armature Windings and Field Poles
(b) Rotating Field Type - is driven by a slow speed prime mover, the
electromagnets are bolted to a hub fastened to the shaft as shown below,
so that they project radially outward toward the stationary armature core;
this construction is called salient-pole field.
When the alternator is driven by a high
speed turbine, the field winding is placed
In a slotted core; this construction is called
non-salient pole field.
FIG 8: Rotary field type of machine
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Alternating Current Generators
 Generally called an alternator. The speed of rotation of the field must be
kept absolutely constant, since frequency of the generated voltage in
cycles per second is directly proportional to the speed.
 The voltage developed by an ac generator varies greatly with changes
 To maintain a constant voltage at the load, it is customary to equip an ac
generator with a regulator, a device that tends to maintain the terminal
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Alternating Current Generators
 There are two reasons why ac machines can be built in large sizes and
(1) No commutator is required, a commutator being a very definite limiting
factor in the construction and operation of a dc generator.
(2) The armature winding can be placed in the stationary part of the
machine, the stator, where it is possible to provide good insulation
strength for the high voltage winding.
• One of the most important reasons for the use of ac generators in
comparatively large power systems is that alternating current can be
transformed efficiently from one voltage to another by the use of
transformer.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Types and Operating Characteristics of Direct Current Motors
There are three (3) general types of dc motors on the basis of the kind of
excitation used.
(1) Shunt Motor – if comparatively high resistance field winding of many
turns of fine wires is employed for this function, it is connected in parallel
with the armature.
(2) Series Motor - when an extremely low-resistance field winding of very
few turns of heavy wire is used, it is connected in series with the armature.
(3) Compound Motor – a machine that is excited by a combination of a shunt
field connected in shunt with the armature and a series field in series with
the armature.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Types and Operating Characteristics of Direct Current Motors
SELF EXCITED DC MOTORS ( DC SHUNT MOTORS )
F
Ia
If
IL
A
M
FF
+
E
Ra
V
SUPPLY
AA
_
DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS )
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Types and Operating Characteristics of Direct Current Motors
SELF EXCITED DC MOTORS ( DC SERIES MOTORS )
Y
YY
Ia
+
A
M
AA_
ISE
+
IL
E
Ra
V
SUPPLY
_
DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS )
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Types and Operating Characteristics of Direct Current Motors
SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS )
ISh
Y
YY
Z
A
Rsh
ZZ
Ia
M
ISE
IL
SUPPLY
E
Ra
+
V
AA
_
DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS )
LECTURE 9 OF 40
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Types and Operating Characteristics of Direct Current Motors
The following general operating characteristics distinguish the three types of
motor from one another.
(1) On the basis of the same horsepower and speed rating, a series motor
develops the highest starting torque and the shunt motor the least, while
the compound motor falls between the first two.
(2) The overload capacities follow the same general order as for starting
torque.
(3) The speed variation for changes in load are the least for the shunt motor,
the greatest for the series motor, and somewhat larger than the shunt
motor for the compound.
(4) Both the shunt and compound motor operate at very definite stable
speed when all mechanical load is removed, while the series motor, and
somewhat larger than the shunt motor for the compound machine.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Types and Operating Characteristics of Direct Current Motors
There are three methods that may be employed to alter the speed of a dc
motor.
(1) By changing the flux through resistance control.
(2) By changing the voltage across the armature through resistance control.
(3) By changing the voltage across the armature when it is supplied with power
from a separate voltage-controlled generator.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Starting Direct Current Motors
 At the starting period, the armature current is usually much higher than
normal. During this starting period, arcing at the commutator is likely to be
very severe and can caused burning.
What do we need to prevent burning at the commutator during start-up?
An external resistance (rheostat) must be added to limit the current in the
armature circuit, where the current must pass between brush and
commutator and where the serious effects of poor commutation are likely to
result. The value in ohms is chosen to limit the armature current for about
1.25 to twice the rated amperes at the instant the machine is started.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Starting Direct Current Motors
 As the motor accelerates, the armature generates an opposing emf called
a counter electromotive force (counter emf) which causes the current to
fall; this counter emf has the same effect in lowering the armature current
as would an impressed voltage across the armature terminals.
 The counter emf will not exceed for about 80% to 95% of the impressed
voltage. Counter emf ≠ impressed voltage
 Use automatic starters with relay contact to cut out the limiting resistance
as the motor speed up
 Small machines up to &frac34; HP, starters are not employed because the
starting current is generally low.
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
Commutating Poles for Direct Current Machines
 When current passes between brushes and commutator in dc machine,
generator or motor, the ends of those coils joined the commutator
segments that are bridge by the brushes are short circuited for an instant.
= extremely low resistance = generated voltage is zero.
 If previous condition above in which the generated voltage ≠ 0, will cause
excessive arcing and commutator burning which lead to motor failure.
 To meet the requirement Isc =0, there is a correct brush position. In order
to do this, the use of special, narrow poles located between large main
poles suited for variable loads called interpoles. ( see figure 7 of previous
slide)
ELECTRICAL MACHINE 1
Electrical Machinery Generalization
References
 Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New
York: McGraw-Hill.
 Norton, R. (2011). Machine Design: an integrated approach. (4th ed.).
Boston: Pearson.
 Norton, R. (2012). Design of Machinery: an introduction to the synthesis
and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill.
 Sharma, C.S, &amp; Purohit, K. (2012). Design of machine elements. New
Delhi: PHI Learning Private Limited.
 Gope, P.C. (2012). Machine design: fundamentals and applications.
New Delhi: PHI Learning Private Limited.
ELECTRICAL MACHINE 1
Chapter 2
Direct Current Motor Principles
ELECTRICAL MACHINE 1
Direct Current Motor Principles
Principle of Motor Action
 When an electric motor is in operation, it develops torque, which in turn
can produce mechanical rotation.
The principle of motor requires:
(1) The presence of magnetic lines of force.
(2) Current through conductors lying in the magnetic field.
(3) Force, and therefore torque is produced.
ELECTRICAL MACHINE 1
Direct Current Motor Principles
Principle of Motor Action
Illustrating how motor action is produced by the interaction of the magnetic
fields created by the main poles and the current- carrying conductors
ELECTRICAL MACHINE 1
Direct Current Motor Principles
Principle of Motor Action
Illustrating how motor action is
produced by the interaction of the
magnetic fields created by the main
poles
and
the
current-carrying
conductors
ELECTRICAL MACHINE 1
Direct Current Motor Principles
Principle of Motor Action
 To change the direction of rotation of a dc motor can be done by either:
(1) The direction of current flow through the conductors.
(2) Reversing the polarity of the field.
 A motor will not reverse its direction of rotation if both the field polarity and
the direction of the current flow through the armature changed.
Interchanging the two line wires connected to a dc motor effects both
these changes at once, and does not cause the motor to reverse.
ELECTRICAL MACHINE 1
Direct Current Motor Principles
Force and Torque Developed by Direct-Current Motors
 The force action exerted by a current-carrying conductor placed in a
magnetic field depends on the following:
(1) The strength of the main field
(2) The value of the current through the conductor.
Resultant Non Uniform Magnetic Field = main field +flux set up by current
carrying conductor.
 A force of one dyne will be exerted upon a conductor 1 cm long carrying a
current of 10 amp when placed under a pole the area of which is 1 sq cm
and producing one line of force( flux density = one line per sq. centimeter)
ELECTRICAL MACHINE 1
Direct Current Motor Principles
Force and Torque Developed by Direct-Current Motors
′
𝐹 =
𝐵′ 𝑥 𝐼 𝑥 𝑙′
10
dynes
where: B’ = flux density, lines per sq. centimeter
I = current in conductor, amp
l’ = length of conductor, cm
If the units F, B, and l are specified in more practical terms, that is pounds, lines
per square inch, and inches* respectively becomes
𝐹=
𝐵
6.45
𝑥 𝐼 (𝑙𝑥2.54)
10 𝑥 980 𝑥453.6
=
𝐵𝑥𝐼𝑥𝑙
11,300,000
lb
ELECTRICAL MACHINE 1
Direct Current Motor Principles
Significance of Back- EMF
• In accordance with the laws of electromagnetic induction, e.m.f. is
induced in them whose direction, as found by Fleming’s Right- hand Rule,
is in opposition to the applied voltage.
Consider a Shunt Wound DC Motor
ELECTRICAL MACHINE 1
Direct Current Motor Principles
Significance of Back- EMF
 Back EMF depends among other factors upon the armature speed.
 If the speed is high, back emf (Eb) is high. Hence, the armature current
(Ia) is small.
 If the speed is less, Eb (back emf) is less. Hence, more current (Ia) flows
which develops motor torque.
𝑇 = 𝑘 𝑥 &Oslash; 𝑥 𝐼𝐴 lb-ft
 where: T = torque develop, lb-ft
&Oslash; = magnetic flux, maxwells
Ia = armature current
ELECTRICAL MACHINE 1
Direct Current Motor Principles
General Voltage Equation for Direct Current Motors and Generators
Factors to consider:
(1) number of poles
(2) number of armature conductors
(3) number of parallel paths
(4) armature speed
(5) flux cuts for each conductor
------------------------------------------= Generated Voltage and Back EMF
𝐸=
&Oslash; 𝑥 𝑃 𝑥 𝑟𝑝𝑚 𝑥 𝑍
𝑎 𝑥 60
𝑥 10−8 , volts
ELECTRICAL MACHINE 1
Direct Current Motor Principles
General Voltage Equation for Direct Current Motors and Generators
where: Eg or Eb = total generated voltage or back emf
&Oslash; = flux per pole, maxwells
P = number of poles, an even number
rpm = speed of armature, revolutions per minute
Z = total number of armature conductors effectively used to add to
resulting voltage
a = number of armature paths connected in parallel (determined by
type of armature windings)
Note: 1 weber = 10−8 maxwell
ELECTRICAL MACHINE 1
Direct Current Motor Principles
General Voltage Equation for Direct Current Motors and Generators
Cont….
For a simplex wave-wound
No. of parallel paths = 2
No. of conductors (in series) in one path = Z/2
For a simplex lap-wound
No. of parallel paths = P
No. of conductors (in series) in one path = Z/P
ELECTRICAL MACHINE 1
Direct Current Motor Principles
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Problem 1: A conductor is 8 in long and carries a current of 140 amp when
placed perpendicularly to a magnetic field the intensity of which is 58,000 lines
per square inch. Calculate the force exerted by the conductor.
Problem 2: The armature of a dc motor has 648 conductors 65 percent of which
are directly under the poles where the flux density is 48,000 lines per square
inch. If the core diameter is 7 in and its length 4 in and the current in each
conductor is 20 amp. Calculate (a) the total force tending to rotate the armature
(b) the torque exerted by the armature in pound feet.
Problem 3: A dc motor has an armature containing 192 conductors, 70 percent
of which lie directly under the pole faces at any given instant. If the flux density
under the poles is 52,000 lines per square inch and the armature diameter and
length are 12 in. and 4.5 in respectively, calculate the current in each armature
conductor for a torque of 120 lb-ft.
ELECTRICAL MACHINE 1
Direct Current Motor Principles
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Problem 4:
A four pole machine generates 250 volts when operated at 1500 rpm. If the
flux per pole 1.85 x 10^6 maxwells, the number of armature slots is 45, and
the armature winding is simplex wound.
Calculate:
(a) the total number of armature conductors
(b) the number of conductors in each slot.
ELECTRICAL MACHINE 1
Direct Current Motor Principles
Commutation of Direct Current Motors
Sketch showing the directions of the currents in the conductors of four pole
motor for clockwise rotation. Note the dividing axes midway between
ELECTRICAL MACHINE 1
Direct Current Motor Principles
Commutation of Direct Current Motors
 Commutator and brushes act as an inverter, that is to change direct
current to alternating current, because the current in the armature
conductors must be alternating if rotation in the same direction is to
continue.
Note:
1. In the dc generator the commutator and brushes function to change
internally generated alternating current to a load applied direct current.
2. In the dc motor, the commutator and brushes perform an inverse function
by changing the externally applied direct current to alternating current
flowing in the armature conductor.
ELECTRICAL MACHINE 1
Direct Current Motor Principles
References
 Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New
York: McGraw-Hill.
 Norton, R. (2011). Machine Design: an integrated approach. (4th ed.).
Boston: Pearson.
 Norton, R. (2012). Design of Machinery: an introduction to the synthesis
and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill.
 Sharma, C.S, &amp; Purohit, K. (2012). Design of machine elements. New
Delhi: PHI Learning Private Limited.
 Gope, P.C. (2012). Machine design: fundamentals and applications.
New Delhi: PHI Learning Private Limited.
ELECTRICAL MACHINE 1
Chapter 3
Direct Current Generator Principles
ELECTRICAL MACHINE 1
Direct Current Generator Principles
Principle of Generator Action
 If it is assumed that the magnetic lines of force leave a cylindrically
shaped pole core, faces and and pass across the air space (called the air
gap) and hence into the rotating armature, it is clear that the moving
conductors cut the lines of force as they are rotated mechanically. =
generated voltage.
Principle of Generator requires:
(1) The presence of magnetic lines of force.
(2) Motion of conductor cutting the flux.
(3) Voltage is generated.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
Principle of Generator Action
 The magnitude of the generated voltage is directly proportional to the rate
at which a conductor cuts magnetic lines of force.
 It implies that higher voltages may be generated by moving conductors
more rapidly across lines of flux, by increasing the number of flux lines
across which the conductors move, or by increasing both the speed of the
conductors and the flux across which they move.
 1 volt is generated for every 100,000,000 (10^8) lines cut per second.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
Principle of Generator Action
ELECTRICAL MACHINE 1
Direct Current Generator Principles
Principle of Generator Action
𝐸𝑎𝑣𝑒. =
&Oslash;
𝑡 𝑥 108
, volts
where: Eave. = average generated voltage in a conductor.
&Oslash;
= total flux cut
t
= time, seconds, during which the cutting takes place
Note : 1 weber = 10−8 maxwell
 The number of parallel paths, determines the current rating of the
generator, whereas the number of series conductors per path is a
measure of the terminal voltage of the machine.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
Principle of Generator Action
Example:
120 cells,1.5 volts each with 5 amps /path
No. of parallel
paths
E, volts
I, amps
P, watts
2
90
10
900
4
45
20
900
6
30
30
900
8
22.5
40
900
10
18
50
900
12
15
60
900
Therefore:
The power rating is independent of the manner in which the conductors are
connected.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Problem No.1: A four-pole d-c generator has an armature winding
containing a total of 648 conductors connected in two parallel paths. If the
flux per pole is 0.321 𝑥 106 maxwells and the speed of rotation of the
armature is 1,800 rpm, calculate the average generated voltage.
Problem No.2: In the first problem, calculate the rated current in each
conductor per path if the power delivered by the armature is 5kw.
Problem No.3: A magnetic coil produces 100,000 maxwells with 2,000
turns and with a current of 2 amperes. The current is cut-off and flux
collapses in 0.01 sec. What is the average voltage that will appear across
the coil?
ELECTRICAL MACHINE 1
Direct Current Generator Principles
General Voltage Equation for Direct Current Generator
Factors to consider:
(1) number of poles
(2) number of armature conductors
(3) number of parallel paths
(4) armature speed
(5) flux cuts for each conductor
------------------------------------------= Generated Voltage
𝐸𝑔 =
&Oslash; 𝑥 𝑃 𝑥 𝑟𝑝𝑚 𝑥 𝑍
𝑎 𝑥 60
𝑥 10−8 , volts
ELECTRICAL MACHINE 1
Direct Current Generator Principles
General Voltage Equation for Direct Current Generator
where: Eg = total generated voltage
&Oslash; = flux per pole, maxwells
P = number of poles, an even number
rpm = speed of armature, revolutions per minute
Z = total number of armature conductors effectively used to add to resulting voltage
a = number of armature paths connected in parallel (determined by type of
armature windings)
 For a simplex wave-wound generator
No. of parallel paths = 2
No. of conductors (in series) in one path = Z/2
 For a simplex lap-wound generator
No. of parallel paths = P
No. of conductors (in series) in one path = Z/P
ELECTRICAL MACHINE 1
Direct Current Generator Principles
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Problem No.1: An 85 kw six-pole generator has an armature containing 66
slots, in each of which of 12 conductors. The armature winding is connected
so that there are six parallel paths. If each pole produces 2.18 𝑥 106
maxwells and the armature speed is 870 rpm, determine the generated
voltage.
Problem No.2: How many armature conductors are there in a generator
given the following information: &Oslash; = 2.73 𝑥 106 maxwells; P= 4; rpm = 1,200;
a = 2; Eg = 240 V?
Problem No.3: A four-pole generator, having wave-wound armature winding
has 51 slots, each slot containing 20 conductors. What will be the voltage
generated in the machine when driven at 1500 rpm assuming the flux per
pole to be 7.0 mWb ?
ELECTRICAL MACHINE 1
Direct Current Generator Principles
Direction of a Generated Voltage
 The direction of the generated voltage in a conductor as it is rotated to cut
the lines of force produced by the electromagnets in a generator, will
depend upon two factors:
(1) direction of the flux, which is determined by the magnet polarity.
(2) the direction of motion of the conductor or coil.
Lenz’s law states, the direction of the generated voltage in the coil is such
that it tends to produce a current flow opposing a change of flux through the
coil.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
Direction of a Generated Voltage
MAGNETIC FIELD
A
B
Q

A A
P
B
0o
MAGNETIC FIELD
A
B

A
+
Q
P
B
_
e
30o
t
MAGNETIC FIELD
A
B
+
Q

A
B
P
_
e
60o
t
MAGNETIC FIELD
A
B

+
Q A
B
P
_
e
90o
t
MAGNETIC FIELD
A
+
Q
BA
A
B

P
_
e
120o
t
MAGNETIC FIELD
B
A
+

BA
Q
A
P
_
e
150o
t
MAGNETIC FIELD
B
A
+
Q

B A
P
A
e
180o
t
MAGNETIC FIELD
B

B
+
A
Q
P
A
_
e
210o
t
MAGNETIC FIELD
B
A
+
Q

B
A
P
_
e
240o
t
MAGNETIC FIELD
B
A

+
Q B
A
P
_
e
270o
t
MAGNETIC FIELD
B
+
Q
AA
B
A

P
_
e
300o
t
MAGNETIC FIELD
B
+
Q
AA
B
A

P
_
e
300o
t
MAGNETIC FIELD
A
B
+

A A
Q
B
P
_
e
330o
t
MAGNETIC FIELD
A
B
Q

A A
P
B
e
360o
t
ELECTRICAL MACHINE 1
Direct Current Generator Principles
Direction of a Generated Voltage
Thus, it is seen that for a clockwise rotation the side of the coil under north
pole will always have a voltage direction away from the observer, while the
side of the coil under south pole will always have a voltage direction toward
the observer.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
The Elementary Alternating-current Generator
Elementary two-pole ac generator
ELECTRICAL MACHINE 1
Direct Current Generator Principles
The Elementary Alternating-current Generator
 In general terms, the frequency of the alternating current in cycles per
second is P/2 x revolutions per second.
𝑓=
𝑃
2
𝑋
𝑟𝑝𝑚
60
=
𝑃𝑥 𝑟𝑝𝑚
120
where: f = frequency
P= number of poles
rpm= speed in revolutions per minute
, cps
ELECTRICAL MACHINE 1
Direct Current Generator Principles
The Elementary Alternating-current Generator
 Illustrating the relations between the number of poles and the generated ac
frequency in cycles per revolution.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Problem 1: An ac generator has six poles and operates at 1,200 rpm. (a)
What frequency does it generate? (b) At what speed must the generator
operate to develop 25 cycles? 50 cycles?
Problem 2: How many poles are there in a generator that operates at a
speed of 240 rpm and develops a frequency of 60 cycles?
ELECTRICAL MACHINE 1
Direct Current Generator Principles
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Problem No. 3: Thirty six dry cells are connected in four parallel groups of
nine cells in series per group. If the voltage and current rating of each cell is
1.45 volts and 4 amp respectively, (a) calculate the voltage, current and
power rating of the entire combination (b) Recalculate the problem for nine
parallel groups of four cells in series per group.
Problem No.4: A six dc pole generator has an armature winding with 504
conductors connected in six parallel paths. Calculate the generated voltage
in this machine if each pole produces 1.65 𝑥 106 maxwells and the armature
speed is 1,800 rpm.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Problem No. 5: Calculate the voltage generated by a four pole dc machine
given the following particulars: number of slots in the armature=55; number
of conductors per slot=4; flux per pole= 2.62𝑥 106 maxwells; speed = 1,200
rpm, number of parallel paths in armature = 2.
Problem No. 6: A four pole machine generates 250 volts when operated at
1,500 rpm. If the flux per pole is 1.85 𝑥 106 maxwells, the number of armature
slots is 45, and the armature winding has two parallel paths, calculate (a) the
total number of armature conductors (b) the number of conductor in each
slot.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Problem No. 7 : The speed of the generator in Problem No. 4 is decreased to
1,350 rpm. (a) What will be the generated voltage if the flux per pole is
maintained at the same value, i.e. 1.85 𝑥 106 maxwells? (b) To what value of
flux per pole should the excitation be adjusted if the generated voltage is to
remain the same, i.e. 250 volts?
ELECTRICAL MACHINE 1
Direct Current Generator Principles
The Commutation Process of DC Generator
 The elementary two pole dc generator with two segment commutator.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
The Commutation Process of DC Generator
• As an emf’s in the conductors mn and pq reverse direction, the semi rings, to
which they are connected, automatically change places under stationary
brushes. It follows therefore, that the polarity of the brushes does not change.
• The magnitude of the current change, but there will be no reversal of current
• The process performed by rectifying the alternating current – changing the
internal alternating current to an external direct current – is called
commutation.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
The Commutation Process of DC Generator
 Illustrating how the rectified current pulsates in two, four and six pole
generators.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
The Commutation Process of DC Generator
 Illustrating the combined additive effects of two or three coils. It shows how
the voltage is increased and how the waveform tends to become smoother.
ELECTRICAL MACHINE 1
Direct Current Generator Principles
References
 Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New
York: McGraw-Hill.
 Norton, R. (2011). Machine Design: an integrated approach. (4th ed.).
Boston: Pearson.
 Norton, R. (2012). Design of Machinery: an introduction to the synthesis
and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill.
 Sharma, C.S, &amp; Purohit, K. (2012). Design of machine elements. New
Delhi: PHI Learning Private Limited.
 Gope, P.C. (2012). Machine design: fundamentals and applications.
New Delhi: PHI Learning Private Limited.
ELECTRICAL MACHINE 1
Chapter 4
Direct Current Motor Characteristics
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Operating Differences between Motors and Generators
GENERATORS
MOTORS
1. When in operation, it is driven by mechanical
machine such as engine, water turbine or even
electric motors producing a current in an electric
circuit.
1. When in operation, it is “fed” by an electric
current from an electrical source of supply
develop a torque produces mechanical rotation.
2. The load on the generator constitutes those
electrical devices that convert electrical energy
into other forms of energy.
2. The load on the motor constitutes a force that
tends to oppose rotation and is called a counter
torque.
3. The voltage of a generator tends to change
3. The speed of rotation tends to change as the
4. The voltage of the generator can always be
adjusted by doing either or both of two things: a)
changing the speed b) changing the strength of
the magnetic field.
4. The speed of rotation can be changed by
varying either or both of two things: a) the
strength of the magnetic field b) the voltage
impressed across the armature terminals.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Operating Differences between Motors and Generators
GENERATORS
MOTORS
5. The speed of rotation is usually quite constant, 5. The impressed emf across the motor terminals
since the speed of the mechanical prime mover
is substantially constant, except in the case of
is generally fixed by the governor controls.
special motors in which the power supply
constitutes a separate source.
6. Frequently operated in parallel with others to
supply power to a common load.
6. Usually operate as single independent units to
drive their individual loads although they may be
connected in parallel or in series for the purpose
of particular jobs at varying speeds.
7. Are started without electrical loads; the
procedure is to bring them up to speed, adjust
the voltage and then close the main switch that
permits the machine to deliver current.
7. May not have a mechanical load when they
are started. It is quite customary for a motor to
start a load that is often equal to or greater than
the rated name plate value.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Classification of Direct Current Motors
 There are three general types of motors: series, shunt and compound.
 For the purposes of classification, it is convenient therefore to indicate
how a motor behaves between no load and full load by using such a term
as constant speed and variable speed.
 In general, if a change from no mechanical load to full load causes the
speed to drop approximately 8 percent or less, the motor is said to be of
the constant speed classification. (shunt)
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Classification of Direct Current Motors
 Motors in which the speed changes by greater values than indicated as
falling into variable speed classification. (series and compound)
 Whenever the speed of a motor can be controlled by an operator who
makes a manual adjustment/automatic control equipment, it is said to be
 The speed changes inherently as a result of a modification of the loading
conditions and referred to as a variable speed type.
 Constant speed-adjustable speed – shunt motor with field rheostat control.
 Variable speed–adjustable speed – series motor with a field rheostat.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Counter Electromotive Force (Counter EMF) Voltage Generated by a Motor
 With the armature rotating as a result of motor action, the armature
conductors continually cut through the resultant stationary magnetic field, and
because of such flux cutting voltages are generated that experience force
action.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Counter Electromotive Force (Counter EMF) Voltage Generated by a Motor
 The generated voltages are indicated by crosses and dots below the
circles and are in directions opposite to the flow of current. This is called
counter emf or back emf.
 This counter emf can never be equal to, and must always be less than,
the voltage impressed across the armature terminals.
 This can mean only that the armature current is controlled and limited by
the counter emf. By Ohms Law,
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Counter Electromotive Force (Counter EMF) Voltage Generated by a Motor
 Formula:
𝐼𝐴 =
𝑉𝐴 −𝐸𝐶
𝑅𝐴
, amp
where:
𝐼𝐴 = armature current
𝑉𝐴 = impressed voltage across the armature winding
𝐸𝐶 = counter emf generated in armature
𝑅𝐴 = resistance of armature
 Counter emf depends upon two factors: (1) flux per pole &Oslash; (2) the speed
of rotation S in revolutions per minute.
if, Ec = k&Oslash;S
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Counter Electromotive Force (Counter EMF) Voltage Generated by a Motor
where k = is a proportionality constant that depends upon the number of
armature conductors, the type of armature winding and the number of poles
becomes,
𝐼𝐴 =
𝑉𝐴 −𝑘&Oslash;𝑆
𝑅𝐴
 Ec is directly proportional to &Oslash; and S.
amp
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM No.1 : A 115 volt shunt motor has an armature whose resistance
is 0.22 ohm. Assuming a voltage drop across the brush contacts of 2 volts,
what armature current will flow (a) when the counter emf is 108 volts? (b) if
the motor is increased so that the counter emf drops to 106 volts?
PROBLEM No.2 : A compound motor operates at a speed of 1,520 rpm
when the voltage impressed across the armature terminals is 230. If the flux
per pole is 620,000 maxwells and the armature resistance is 0.43 ohm,
calculate: (a) the counter emf and the (b) armature current (Assume a value
of k = 2.2 x 10^-7 and a brush drop of 2 volts)
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM No. 3: If the load on the motor in problem No. 2 is increased so
that the armature current rises to 64 amp. What will be the speed of the
motor, assuming that the flux increases by 6 percent?
Note: The counter emf developed in the armature of a motor is usually
between 80 and 95 percent of the voltage impressed across the armature
terminals:
• Ec is high percentage of the Va, will operate most efficiently.
• Ec is small percentage compared with Va, will have low efficiency.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
• A motor will develop the greatest power when the counter emf Ec is a
maximum.
PROBLEM No. 4: The armature of the 230-volt motor has a resistance of
0.312 ohm and takes 48 amp when operating at a certain load. (a) Calculate
the counter emf and the power developed by the armature. (b) If the
armature resistance had been 0.417 ohm, the other conditions remaining the
same, what would have been the values of Ec and the power developed in
the armature? (Assume a brush of 2 volts in both cases)
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Note: In starting a dc motor, if Ec is zero or very small, as the motor is
coming up to speed, a resistance must be inserted to take place of Ec, as
the speed increases, resistance may be cut out gradually because Ec rises;
finally when the motor has attained normal speed, all resistance can be cut
out of the armature circuit.
PROBLEM No. 5: The armature of a 230 volt shunt motor has a resistance
of 0.18 ohm. If the armature current is not exceed 76 amp, calculate: (a) the
resistance that must be inserted in series with the armature at the instant of
starting; (b) the value to which this resistance can be reduced when the
armature accelerates until Ec is 168 volts; (c) the armature current at the
instant of starting if no resistance is inserted in the armature circuit. (Assume
a 2-volt drop at the brushes)
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Starting Resistors Connection
 Connection of Starting Resistor
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Starting Resistors Connection
 In the case of very small motors, usually up to about &frac34; hp, no starting
resistor is necessary. There are two reasons for this:
(1) The resistance and the inductance of the armature winding are generally
sufficiently high to limit the initial rush of current to values that are not
particularly serious.
(2) The inertia of a small armature is generally so low that it comes up to speed
very quickly, thereby minimizing the serious effect that might otherwise result
from a high sustained current.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Starters for Shunt and Compound Motors
 Their primary function is to limit the current in the armature circuit during the
starting or accelerating period.
 There are two standard types of motor starter for shunt and compound motor:
(a) three point type
(b) four point type
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Starters for Shunt and Compound Motors
 Three point starters are not
completely
satisfactory
when used with motors
whose speeds must be
controlled
by
inserting
resistance in the shunt field.
Why?
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Starters for Shunt and Compound Motors
Schematic Wiring Diagram
(Three-point starter connected
to shunt motor)
• If sufficient resistance is cut
in by the field rheostat, so
that the holding coil current
is no longer able to create
sufficient electromagnetic
pull to overcome the spring
tension the starter arm will
fall back to the OFF position.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Starters for Shunt and Compound Motors
(Four point starter connected
to compound motor)
 If this starter is compared with
the three point type, it will be
observed that one important
holding coil has been removed
from the shunt field circuit and
in series with a current
protecting resistor r, has been
placed in a separate circuit in
parallel with armature and
field.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Starters for Shunt and Compound Motors
Schematic Wiring Diagram
(Four point starter connected to compound motor)
• This starter are divided into three
parts:
(1) The main circuit is through the
starting resistor r, the series field
and the armature.
(2) The second circuit is through the
shunt field and its field rheostat.
(3) The third circuit is through the
holding coil and the current
protecting resistor r.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Controllers for Shunt and Compound Motor
Schematic Wiring Diagram
(Controllers for Shunt and Compound
Motor)
 To eliminate providing a separate
mounting for field rheostat,
controllers are available that
incorporate
both
starting
resistors and field rheostats in
single, panel mounted units and
make it impossible to start shunt
or compounding motors with any
resistance cut into the field
rheostat.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Automatic Starter for Shunt and Compound Motor
 The counter emf type of starter , a number of relays are connected across
the armature where the counter emf increases as the motor accelerates,
and the former are adjusted to pick up the predetermined values of
voltage.
 The time limit starter, a group of relays are timed to operate at preset
intervals of time, by means of devices that function mechanically,
pneumatically or electrically.
 The current limit starter, the relays are designed so that they are sensitive
to current changes in the armature circuit.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Automatic Starter for Shunt and Compound Motor
 Wiring Diagram of a Counter EMF Starter connected to shunt motor
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Automatic Starter for Shunt and Compound Motor
 Wiring diagram of a time limit acceleration starter connected to a
compound motor.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Automatic Starter for Shunt and Compound Motor
 Wiring diagram of a current limit acceleration starter connected to a
compound starter.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
 If the mechanical load on the motor changes, either the torque or the
speed, or both, must change. The control of the speed of the motor is
generally exercised through the medium of flux adjustment or control.
 When a load is applied to a motor, the natural tendency is to slow down
because of the opposition to motion called counter torque. The reduction
𝑉 −𝐸
of speed results in the increase of the armature current, since I = 𝐴 𝐶 .
𝑅𝐴
This means that any increase in mechanical driving power must be met
by a corresponding increase in electrical power input to the armature.
 Loading a motor always results in two changes: (1) reduction in speed ;
(2) an increase in armature current. Vice-versa will occur when the motor
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
will operate at a speed only slightly higher than the normal speed,
between 2% and 8% higher than the normal speed.
will result in a rise in speed of about 10% to 25% .
does attempt to race, or operate at a very high speed.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Torque Characteristics of Direct-Current Motors
 The torque developed by a motor, the tendency of the motor to produce
rotation depends on two factors:
(1) The flux created by the main poles.
(2) The current flowing in the armature winding.
 The torque is independent of the speed of rotation.
𝑇 = 𝑘 𝑥 &Oslash; 𝑥 𝐼𝐴 lb-ft
where:
T = torque (usually in lb-ft)
k = proportionality constant
&Oslash; = flux per pole in maxwells
Ia = total armature current
=
0.1173
108
x
𝑃𝑥 𝑍
𝑎
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Torque Characteristics of Direct-Current Motors
hp
2𝜋 𝑥 𝑟𝑝𝑚 𝑥 𝑇
=
33,000
where:
rpm = speed of the motor
T = torque in lb-ft
Note: just multiply the formula above by 746 watts when converted to watts unit.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Torque Characteristics of Direct-Current Motors
• The developed torque of a motor varies with the armature current Ia of
the motor:
(1) The torque of a shunt motor depends only upon the armature current;
assuming that the shunt field current is not changed by field rheostat
adjustment, the torque is independent of the flux. Therefore, a graph
indicating the relation between torque and load should be straight line
(T= k1Ia).
(2) The torque developed by a series motor depends upon the armature
current and the flux that this current produces in passing through the
series field. T= k2Ia^2 when the motor series field is not saturated.
(3) The torque of compound motor combines the torque load characteristic
of shunt and series motors.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Torque Characteristics of Direct-Current Motors
 Characteristic Torque vs. armature current curves for three types of dc motor.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO. 1 – (a) Calculate the torque in pound feet developed by a dc
motor, given the following particulars: poles = 4; total number of armature
conductors = 828; flux per pole = 1.93 x 10^5 maxwells; total armature
current = 40 amp; winding = wave. (b) What will be the horsepower of the
motor when operating at a speed of 1,750 rpm?
PROBLEM NO. 2 – A four pole shunt motor develops 20 lb-ft of torque when
the flux per pole is 700,000 maxwells. If the armature winding has 264
conductors and is wound wave, calculate the total armature current.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO. 3- To change the shunt motor of example no. 2 into a
compound motor, the main poles are wound with a series field. What torque will
this motor develop when the armature carries 65 amp if the series field
increases the flux by 22 percent ?
PROBLEM NO. 4 – A series motor develops 62 lb-ft of torque when the current
is 48 amp. Assuming the flux varies directly with the current, calculate the
torque if the load increases so that the motor takes 56 amp.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO. 5 – The speed of the 50 HP (37.3 kW) series motor working on
500V supply is 750 rpm at full-load and 90% efficiency . If the load torque is
made 350 N-m and a 5 ohm resistance is connected in series with the machine,
calculate the speed at which the machine will run. Assume the magnetic circuit
to be unsaturated and the armature and field resistance to be 0.5 ohm.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Torque Characteristics of Direct-Current Motors
 The torque developed by a motor at the instant of starting is called
starting torque.
 Among the three types of motors, in terms of percentage:
(1) Series motor have approximately 500% starting torque
(2) Compound motor have approximately 250% starting torque
(3) Shunt motor have approximately 125% starting torque
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
• From previous lecture,
s=
Va −Ia Ra
, 𝑟𝑝𝑚
k&Oslash;
PROBLEM NO.1 – The armature of a 230 volt shunt motor has a resistance
of 0.30 ohm and takes 50 amp when driving its rated load at 1500 rpm. At
what speed will the motor operate if the load is completely removed, when it
is running idle, a condition under which the armature current drops to 5
amp? Assume that the flux remains constant and that the brush drops at full
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO. 2 – A 220 volt long shunt compound motor has an armature
resistance of 0.27 ohm and a series field resistance of 0.05 ohm. The full
load speed is 1,400 rpm when the armature current is 75 amp. At what speed
will the motor operate at no load if the armature current drops to 5 amp and
the flux is reduced to 90 percent of its full load value? Assume brush drops to
be the same as in problem no. 1.
PROBLEM NO. 3 – A 25 hp 240 volt series motor takes 93 amp when
driving its rated load at 800 rpm. The armature resistance is 0.12 ohm, and
the series-field resistance is 0.08 ohm. At what speed will the motor operate
if the load is partially removed so that the motor takes 31 amp.? Assume that
the flux is reduced by 50 percent for a current drop of 66 2/3 percent and that
the brush drop is 2 volts at both loads.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Speed Characteristics of Direct-Current Motors
Characteristic speed vs. horsepower output curves for three types of dc
motor:
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
Speed Regulation of Direct-Current Motors
The change is speed of a shunt or compound motor between full load and
no load is referred to as regulation. When this regulation is referred to the
full load or rated speed of the motor expressed in percent , it is called a
percent speed regulation.
𝑆𝑁𝐿 − 𝑆𝐹𝐿
% 𝑠𝑝𝑒𝑒𝑑 𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 =
X 100%
𝑆𝐹𝐿
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Problem 1: The no-load speed of the compound motor is 2,200 rpm. At what
speed will it operate at full-load if the percent speed regulation is 18%?
Problem 2:
A 550 V long shunt compound motor has an armature resistance of 0.815
ohms and a series field resistance of 0.15 ohm. The full-load speed is 1,900
rpm when the armature current is 22 amp. (a) At what speed will the motor
operate at no-load if the armature current drops to 3 amperes with the
corresponding drop in flux to 88 percent of the full-load value? (Assume a
brush drop of 5 volts at full-load and 2 volts at no-load. (b) Calculate the
percent speed regulation of the motor.
ELECTRICAL MACHINE 1
Direct Current Motor Characteristics
References
 Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New
York: McGraw-Hill.
 Norton, R. (2011). Machine Design: an integrated approach. (4th ed.).
Boston: Pearson.
 Norton, R. (2012). Design of Machinery: an introduction to the synthesis
and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill.
 Sharma, C.S, &amp; Purohit, K. (2012). Design of machine elements. New
Delhi: PHI Learning Private Limited.
 Gope, P.C. (2012). Machine design: fundamentals and applications.
New Delhi: PHI Learning Private Limited.
ELECTRICAL MACHINE 1
Chapter 5
Direct Current Generators Characteristics
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Types of Direct Current Generators
• There are three general types of self-excited dc generators:
(1) Shunt Generator – if the excitation is produced by a field winding that is
connected to full, or nearly full line voltage.
(2) Series Generator – if the excitation originates in the field winding connected
in series with the armature, so that the flux depends upon the current
(3) Compound Generator – if the excitation is produced by two field windings,
one connected to the full, or nearly full, line and the other excited by the line
or armature current, a comparatively large current.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Types of Direct Current Generators
Series field
Shunt field
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Types of Direct Current Generators
 DC Generators are divided into:
(1) Self-Excited – the shunt field of a shunt or compound generator may be
excited by current supplied to its own armature.
(2) Separately-Excited – the shunt field of a shunt or compound generator may
be excited by or connected to an outside, i.e. separate source of supply.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
• When a shunt or compound
generator operates without
load – that is, when it is driven
by a prime mover, is properly
excited, and has none of the
voltage will appear at the
terminals that are normally
connected to the electrical
devices, provided two factors
are being considered: (1) the
speed of rotation and (2) the
flux.
Separately excited shunt generator connections to
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
 The graph shown below is the direct proportionality relationship between
the no-load generated voltage Eg and the speed RPM with constant
excitation.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
 To determine the relationship of the no-load generated voltage Eg and field
current, it is important to note that this relationship is not a direct one for all
changes in excitation, because magnetic saturation sets in after the field is
increased beyond a certain value.
E1
Eh
E E2
El
Saturation Curve or
Magnetization Curve
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
 Thus if E = 100 volts at 1,500 rpm, Eh will be 110 volts at 1,650 rpm
[(1,650/1500) x 100= 110] and El will be 90 volts at 1,350 rpm [(1,350/1500) x
100= 90]
 If saturation curve is determined by experiment by one speed, it is quite
possible to calculate values for another such curve at any other speed by the
simple method of proportionality.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Building Up the Voltage of a Self-excited Shunt Generator
Self excited shunt generator
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Building Up the Voltage of a Self-excited Shunt Generator
• Four Requirements for Build Up:
(1) The first requirement for the build up process is a small voltage resulting
from residual magnetism. A generator will not build up if the residual flux
is insufficient; in 110 and 220 volts generator, this flux should be such a
magnitude that about 4 to 10 volts is developed. Flashing the field is the
solution for the new or one that has lost its residual flux because of a long
period of idleness must be separately excited to create necessary
magnetism.
(2) A second requirement for build up is a field circuit resistance that is less
than the so-called “critical value” for the speed used in operating the
generator.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Building Up the Voltage of a Self-excited Shunt Generator
(3) A third requirement is that the speed be high enough for the shunt field
resistance used. It should be stated that a generator is usually operated at
some definite speed originally fixed by the manufacturer.
(4) Finally, there must be a proper relation between the direction of rotation and
the field connection armatures. If a generator will not build up when operated
clockwise, for example, it will build up when the direction of rotation is
counterclockwise, assuming other conditions to be satisfied.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Brush Contact Drop
 It is the voltage drop over the brush contact resistance when current
passes from commutator segments to brushes and finally to the external
load. Its value depends on the amount of current and the value of contact
resistance. This drop is usually small and includes brushes of both
polarities. However, in practice, the brush contact drop is assumed to have
following constant values for all loads.
0.5 V for metal-graphite brushes.
2.0 V for carbon brushes.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
Problem 1: A short-shunt compound generator delivers a load current of 30 A at
220 V, and has armature, series-field and shunt-field resistances of 0.05 Ω , 0.30
Ω and 200 Ω respectively. Calculate the
induced e.m.f. and the armature current. Allow 1.0 V per brush for contact drop..
Problem 2: A long-shunt compound generator delivers a load current of 50 A at
500 V and has armature, series field and shunt field resistances of 0.05 Ω , 0.03 Ω
and 250 Ω respectively. Calculate the generated voltage and the armature
current. Allow 1 V per brush for contact drop.
Problem 3:In a long-shunt compound generator, the terminal voltage is 230 V
when generator delivers 150 A. Determine (i) induced e.m.f. (ii) total power
generated and (iii) distribution of this power. Given that shunt field, series field,
diverter and armature resistance are 92 Ω , 0.015Ω , 0.03 Ω and 0.032 Ω
respectively.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Behavior of a Shunt Generator Under Load
One of the most important characteristics of any generator is its behavior
with regard to the terminal voltage when the load current is increased. For
shunt type generator, the voltage always falls as more current is delivered
to the load. There are three reasons for this:
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Behavior of a Shunt Generator Under Load
(1) As more current is delivered by the armature, the voltage drop in the
armature Ia Ra increases, thus making a lower emf available at the load
terminals.
(2) When the armature terminal voltage falls, the field winding suffers a
corresponding reduction in current, which in turn, reduces the flux; the
latter further reduces the generated emf.
(3) When the armature winding carries increasing values of load current, the
armature core becomes an electromagnet, apart from the effect of the
main poles; this electromagnet action of the armature core reacts with the
main field flux further to reduce the flux, the result being that the
generated emf suffers an additional drop.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Behavior of a Shunt Generator Under Load
 It should be clearly understood
that the generated voltage, which
depends upon the flux(other
factors remaining unchanged), is
always greater than the terminal
voltage or load voltage by exactly
the amount of the voltage drop in
the armature circuit. This is why it
is important to keep the armature
circuit resistance as low as
possible.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Behavior of a Shunt Generator Under Load
 A convenient standard of reference used to measure the performance of a
generator is referring the change in voltage between full load and no load
(VNL to VFL) to the full load voltage VFL. This is called percent voltage
regulation.
Percent regulation =
𝑉𝑁𝐿 − 𝑉𝐹𝐿
𝑉𝐹𝐿
𝑋 100%
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO. 1: A d.c. shunt generator has an induced voltage on opencircuit of 127 volts. When the machine is on load, the terminal voltage is 120
volts. Find the load current if the field circuit resistance is 15 ohms and the
armature-resistance is 0.02 ohm. Ignore armature reaction.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO. 2: An 8-pole d.c. shunt generator with 778 wave-connected
armature conductors and running at 500 r.p.m. supplies a load of 12.5 Ω
resistance at terminal voltage of 250 V. The armature resistance is 0.24Ω and
the field resistance is 250 Ω . Find the armature current, the induced e.m.f. and
the flux per pole.
PROBLEM NO. 3: The voltage of a 100 kw 250 volt shunt generator rises to
260 volts when the load is removed. What full-load current does the machine
deliver, and what is its per cent regulation?
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO. 4: A 25 kW 230 volt shunt generator has a regulation of 8.7
percent. (a) What will be the terminal voltage of the generator at no load? (b) If
the change in voltage is assumed to be uniform between no-load and full-load
kilowatts, calculate the kilowatt output of the generator when the terminal
voltages are 240 and 235 volts.
PROBLEM NO. 5: A 4-pole, lap-wound, d.c. shunt generator has a useful flux
per pole of 0.07Wb. The armature winding consists of 220 turns each of 0.004
Ω resistance. Calculate the terminal voltage when running at 900 r.p.m. if the
armature current is 50 A.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Controlling the Terminal Voltage of Shunt Generators
 It is possible to prevent the terminal voltage V from changing as the load
changes by merely adjusting the field rheostat as the voltage changes in
accordance with the load. This can be done manually or by automatic
regulators.
 Automatic regulators are designed and constructed in several ways, but all
operate on the fundamental principle that a terminal voltage change is
accompanied by an inverse change in flux. Some of these are:
(1) Tirrill Regulators – a pair of short circuiting contacts are connected
across a portion of the field rheostat and are actuated by the
electromagnetic action of a relay.
Note: In practice, the field rheostat was usually adjusted so that terminal
voltage was about 25 to 35 percent below rated value when the machine
was delivering full load current with relay contacts held open.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
A Tirrill Regulator
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Controlling the Terminal Voltage of Shunt Generators
(2) Diactor (direct-current) unit – the same functions are accomplished w/o
vibrating contacts but by special rheostatic elements that are acted upon
directly by a unique design of torque elements; the latter, energized by the
varying potential is made of tilt stacks of plates at different angles so that the
proper values of resistance are connected across a manually set rheostat in
series with the shunt field.
(3) Saturable Reactor – adapted to voltage regulators for generators. This
extremely magnetic amplifier is made to act upon a special control field
wound directly over the main field of the generator. In this way the total flux
may be adjusted to the proper value without the necessity for making
changes directly in the main field.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Saturable Reactor (Magnetic Amplifier)
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Controlling the Terminal Voltage of Shunt Generators
 The tendency on the part of a shunt generator to lose voltage with
increasing values of load may be minimized by operating the machine at a
lower speed. When this is done, the field flux must be increased which
means that the iron portions of the magnetic circuit are more highly
saturated.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Controlling the Terminal Voltage of Shunt Generators
 The result is that the rise in voltage between full load and no load is not
so great at the reduced speed.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Controlling the Terminal Voltage of Shunt Generators
 An objection to operation at lower speed, however, is that the machine is
likely to overheat because:
(1) The copper loss in the field is increased.
(2) Ventilation, that is cooling, is not as good because the fanning action of
the armature is reduced.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
 The shunt field excitation is usually more or less steady and is affected only
slightly as the terminal voltage fluctuates. The effect of the series field is
quite variable, however, since its ampere turns depend upon the load
current; when the load current is zero, it produces no component of flux and
when the load current is high, it creates appreciable component of flux.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
 Cumulative Compounded - the shunt and series field coils around each
of the main poles should be connected that they create flux in the same
direction if the tendency of the generator to lose voltage is to be
counteracted.
 Differential Compounded – the action of the series field must oppose, that
is ‘buck’ the shunt field.
 Testing of a compound generator is the same as the procedure for testing
the shunt machine.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
(1) Flat-compound – if the terminal voltage at no load and full load are equal.
Transmission distance between the generator and the load is short.
(2) Over compound – if the full load voltage is higher than the no-load value.
Few series field ampere turns vs shunt field ampere turns.
(3) Under compound- if the full load voltage is less than the zero load
voltage. Many series field ampere turns vs shunt field ampere turns.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
 By connecting a very low resistance shunt directly across the series field,
the no load voltage may be brought up to almost any desired value to
meet individual demands.
 The low resistance shunt is called diverter because it diverts, or bypasses,
part of the load current through a section that creates no flux. Thus, the
series field is less effective in creating flux to boost the generated emf to
an extent determined by the diverted current.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
 Then, the relationship between the diverter resistance and series field
resistance to their respective currents are shown below:
𝐼𝑆𝐸
𝐼𝐷
=
𝑅𝐷
𝑅𝑆𝐸
𝐼𝐿 = 𝐼𝑆𝐸 + 𝐼𝐷
 It follows that:
𝐼𝑆𝐸 = 𝐼𝐿 ∗
𝑅𝐷
𝑅𝐷 +𝑅𝑆𝐸
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Where 𝐼𝑆𝐸
𝐼𝐷
𝑅𝑆𝐸
𝑅𝐷
= series-field current
= diverter current
= series field resistance
= diverter resistance
• In practice, the diverter material is manganin, german silver, or any other
high-resistivity material with a low temperature-resistance coefficient.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO.1: The series field of a compound generator has a resistance
of 0.018 ohm. If the full-load current is 120 amp and it is necessary to divert
36 amp so that the full-load voltage will be brought down to the desired
value, calculate: (a) the value of the diverter resistance, (b) the length of a
square manganin wire (resistivity =265) whose cross sectional area is 15,616
circular mils.
PROBLEM NO.2: Each of the series-field coils of a four-pole 50 kw 250 volt
compound generator has 6 &frac12; turns of wire. The resistance of the entire
series field is 0.012 ohm, and the diverter resistance is resistance is 0.036
ohm. Calculate the number of ampere-turns of each series-field coil at full
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO.3: A long shunt compound generator has a shunt field winding
of 1000 turns per pole and series field winding of 4 turns per pole and
resistance 0.05 Ω . In order to obtain the rated voltage both at no-load and fullload for operation as shunt generator, it is necessary to increase field current by
0.2 A. The full-load armature current of the compound generator is 80 A.
Calculate the divertor resistance connected in parallel with series field to obtain
flat compound operation.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
 When a generator has a single field that is connected in series with the
armature, the load current is simultaneously the excitation current; called
series generator, its voltage will depend upon the current delivered to the
 On open circuit, when the load is zero, the series field ampere turns is
likewise zero and the generated voltage is that due to cutting of the
residual flux , the residual value Er.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
 The terminal voltage Vt will rise as the load amperes are increased and
continue to do so as long as the resultant generated voltage rises more
rapidly than those factors already noted tend to reduce it.
 At loads that are considerably above normal values, the iron portions of
the magnetic circuits become highly saturated, under which condition the
subtractive effects exceed the slowly rising generated emf; the terminal
voltage begins to drop.
 Some applications of dc generator: used in the dc system for voltage
boosting purposes or to minimize the leakage currents in grounded dc
systems so that electrolytic action in underground structures may be
reduced; in Europe it is sometimes employed in the Thury high-voltage dc
systems for transmission of electrical energy.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Armature Reaction in DC Generators
 When the armature of a dc generator carries a load current, it becomes
an electromagnet, apart from the magnetic effect induced in it by the
main poles.
(a) Main pole flux distribution
(b) Distribution of emfs and currents in the armature windings
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Armature Reaction in DC Generators
 If load is delivered by the armature, the current directions in the armature
winding will be exactly the same as those indicated for the emfs if the
brushes are positioned on the commutator so that they are on the exact
neutral plane with respect to the armature conductors being short
circuited.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Armature Reaction in DC Generators
 In (a) the ends of the armature coil connect to commutator segment
midway between the coil sides, the brush neutral lines up with the center
of the poles.
 In (b) the ends of the armature coil connect to commutator segments in
line with one of the coil, the brush neutral lines up with the magnetic
neutral of the main poles.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Armature Reaction in DC Generators
(a) Chamfered pole shoes
(b) pole lamination with one pole slip
 Summary: Armature reaction is:
(1) Produced by the load current in the armature conductors that results in
the magnetic field whose direction is displaced 90 electrical degrees with
respect to the main field.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Armature Reaction in DC Generators
(2) Depends upon and directly proportional to the load current.
(3) Shifts the neutral plane in the same direction as that of the rotating
armature.
(4) Reduces the total flux.
(5) Can be counteracted by the used of specially shaped pole lamination.
 Interpoles of DC Generators
These are narrow poles placed exactly halfway between the main poles and
directly in line with the load magnetic neutral, usually called the mechanical
neutral.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Interpoles of DC Generators
The exciting winding for these poles are always permanently connected in
series with the armature because interpoles must produce fluxes that are
directly proportional to the armature current.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Compensating Windings of DC Generators
 In some large machines, and in those in which the fluctuations are violent,
the cross magnetizing action – armature mmf distorts the main field, can
become severe enough to cause flashover between plus and minus of
brushes.
 It maintains a uniform flux distribution under the faces of the main poles.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
A 2,500 kW, 600 volt 16 pole generator has a lap wound armature with
2,360 conductors. If the pole faces cover 65% of the entire circumference,
calculate the number of pole face conductors in each pole of a
compensating winding.
.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Need for Operation of Generators in Parallel
 Power plants will generally be found to have several small generators
rather than single units capable of taking care of the maximum peak loads
(both dc and ac).
 The several units can then be operated singly or in various parallel
combinations, on the basis of the actual load demand. (Reason: keep
maximum efficiency, continuity of service, maintenance and repair
problems, and additions to plant capacity as the service demands
change.)
 Uninterrupted service recognized as economic necessity.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Operation of Shunt Generators in Parallel
 The shunt generators have “drooping” voltage vs. load characteristics, i.e.
the voltage drops as the load increases.
 For two shunt generators have identical external characteristics: if the
voltage changes in both by exactly the same amount for the same percent
of change in load, then the two machines will divide the total load in
proportion to their relative capacities.
 For two shunt generators not similar external characteristics, then if both
have been adjusted for the same voltage at rated load currents, one
generator will deliver a larger percent of its rated capacity than the other.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Operation of Shunt Generators in Parallel
 Characteristics voltage vs. percent rated current of two shunt generators
operated in parallel with external characteristics not the same
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Operation of Shunt Generators in Parallel Sample Problems
As the load increases, it is necessary to:
(1) Connect a larger generator than A in parallel with the latter, after which
line.
(2) Connect another generator in parallel with A and have two machines
operate jointly to supply the total load. The procedure are the following:
(a) Generator B is brought up to speed by its prime mover.
(b) Field switch B will closed, whereupon the voltage will build up. Caution:
It is important that the polarity of the incoming generator be exactly the
same as that of the line polarity. “plus to plus and minus to minus”.
(c) After proper adjustments are made, quickly close switch SB. This places
generator B in parallel with generator A.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Operation of Shunt Generators in Parallel Sample Problems
(d) To shift the load from A to B, it is necessary merely to manipulate both field
rheostats simultaneously, cutting in resistance in the field of A and at the same
time cutting out resistance in the field of B.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO.1: In the figure of the previous slide, generator A has a rating
of 150 kW and generator B has a rating of 200kW, both at 230 Volts.
Calculate the kilowatt output of each machine and the total kilowatt load
when the terminal voltage is 240 Volts.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO.2: Two shunt generators each with an armature resistance of
0.01 Ω and field resistance of 20 Ω run in parallel and supply a total load of
4000 A. The e.m.f.s are respectively 210 V and 220 V. Calculate the bus-bar
voltage and output of each machine..
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM NO.3:
Two shunt generators, each with no-load rating of 140 volts, are operated in
parallel. Both machines have external characteristics which are straight
lines over their operating ranges. Machine 1 is rated at 350 kW, and its full
load voltage is 135 volts. Machine 2 is rated 400 kW at 127 volts. Calculate
the operating voltage when two machines supply a total current of 6000 A.
How is the load divided between the two.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Operation of Compound Generators in Parallel
 When two compound generators are to be operated in parallel, it is
necessary to use essentially the same wiring as that employed for shunt
machines, except that an equalizer connection must be added.
 An equalizer is a very low-resistance copper wire that joins together
identical ends of the series fields, the other ends of which are connected
together after the main switches have been closed.
 If the equalizer is removed, instability would result if machines are over
compounded because any tendency on the part of one machine to
assume larger share of the total load than it should is immediately
accompanied by an increased in its generated emf, the other machine in
drooping load, generates less voltage because the series field flux is
increased in the first and decreased in the second.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
Operation of Compound Generators in Parallel
 The equalizer connects the series fields in parallel, and this results in a
division of the total current in a ratio inversely proportional to the
resistance values of the two fields.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM : Two compound generators A and B, fitted with an equalizing
bar, supply a total load current of 500 A. The data regarding the machines
are :
A
B
Armature resistance (ohm)
0.01
0.02
Series field winding (ohm)
0.004
0.006
Generated e.m.fs. (volt)
240
244
Calculate (a) current in each armature. Shunt currents may be neglected.
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
244
ELECTRICAL MACHINE 1
Direct Current Generators Characteristics
References
 Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New
York: McGraw-Hill.
 Norton, R. (2011). Machine Design: an integrated approach. (4th ed.).
Boston: Pearson.
 Norton, R. (2012). Design of Machinery: an introduction to the synthesis
and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill.
 Sharma, C.S, &amp; Purohit, K. (2012). Design of machine elements. New
Delhi: PHI Learning Private Limited.
 Gope, P.C. (2012). Machine design: fundamentals and applications.
New Delhi: PHI Learning Private Limited.
ELECTRICAL MACHINE 1
Chapter 6
Efficiency, Rating, and Application of Dynamos
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
Power Losses in Dynamo
 The power received by a dynamo is called its input. In generator, it is a
mechanical power, while in a motor it is electrical power.
 The power delivered by a dynamo is called its output. In a generator, it is
electrical power, while in a motor is it mechanical power.
 The difference between the power input to a machine (watts) and its
power output (in watts) is called the power loss.
 There are two general classifications of power losses in electric
machines:
(1) Those that are caused by the rotation of the armature (rotational losses)
(2) Those that result from a current flow in the various parts of machine
(electrical losses)
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
Power Losses in Dynamo
A. Rotational losses are divided into five parts:
(1) Bearing friction
(2) Brush friction
(3) Wind friction, usually called windage.
(4) Hysteresis
(5) Eddy Currents
 Hysteresis loss takes place in the revolving armature core because the
magnetic polarity in the iron changes in step with the changing positions
of the magnetic material under the various poles. In modern dynamo, it
depends upon the flux density in the armature core iron, the speed of
rotation, and the quality of the magnetic iron.
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
Power Losses in Dynamo
 Eddy Current loss result because the generated voltage in the iron near
the outside surface are greater than those closer to the center shaft
because of the higher speed; the difference in potential then causes of
current to flow in the iron. To minimize eddy current, laminating or slicing
the armature core and then coating each lamination with a high
resistance varnish.
B. Copper losses occur when there is a current flow through the various
copper circuits.
(1) Armature winding loss (𝐼𝑎2 𝑅𝑎)
(2) Brush contacts between copper commutator and the carbon brushes.
(3) Shunt field
(4) Series field
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
Power Losses in Dynamo
(5) Interpole field
(6) Compensating winding field
(1) The distortion of the flux because of the armature reaction.
(2) Lack of uniform division of the current in the armature winding through
the various parts and through individual conductors of large crosssectional area.
(3) Short circuit currents in the coils undergoing commutation.
 Reasonable value arbitrarily : 1% of the output of the machine with the
rating 150kW (200HP) or more. For smaller rating machines, the stray
without much loss of accuracy.
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
Power Losses in Dynamo
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
Efficiency of DC Motors
 The efficiency of dc motor is the ratio of the mechanical power output,
converted to watts, to the electrical power input.
ℎ𝑝 𝑜𝑢𝑡𝑝𝑢𝑡 𝑥 746
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝑥 100%
𝑤𝑎𝑡𝑡𝑠 𝑖𝑛𝑝𝑢𝑡
Since watts input = (hp output x 746) + watts losses,
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
ℎ𝑝 𝑜𝑢𝑡𝑝𝑢𝑡 𝑥 746
𝑥100%
ℎ𝑝 𝑜𝑢𝑡𝑝𝑢𝑡 𝑥 746 + 𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠
or written in another way,
𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 1 −
𝑥 100%
ℎ𝑝 𝑜𝑢𝑡𝑝𝑢𝑡 𝑥 746 + 𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
Power Stages Motor
Over all or commercial efficiency = C/A
Electrical Efficiency
= B/A
Mechanical Efficiency
= C/B
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM 1: A 15 –hp motor has a total loss of 1,310 watts when operating
at full load. Calculate the percent efficiency.
PROBLEM 2: A 25 - hp motor has an efficiency of 84.9 percent when
delivering three-quarter of its rated output. Calculate the total losses at this
PROBLEM 3: A d.c. shunt machine while running as generator develops a
voltage of 250 V at 1000 r.p.m. on no-load. It has armature resistance of 0.5
Ω and field resistance of 250 Ω . When the machine runs as motor, input to
it at no-load is 4 A at 250 V. Calculate the speed and efficiency of the
machine when it runs as a motor taking 40 A at 250 V. Armature reaction
weakens the field by 4 %. (ch29-1024)
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM 4: A 20-hp (14.92 kW); 230-V, 1150-r.p.m. 4-pole, d.c. shunt
motor has a total of 620 conductors arranged in two parallel paths and
yielding an armature circuit resistance of 0.2 Ω .
When it delivers rated power at rated speed, it draws a line current of 74.8
A and a field current of 3 A. Calculate (i) the flux per pole (ii) the torque
developed (iii) the rotational losses (iv) total losses expressed as a
percentage of power. (ch29-1025)
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
Efficiency of Direct Current Generators
 The efficiency of a dc generator is the ratio of the electrical power output
𝐸𝑇 𝑥 𝐼𝐿 to the mechanical power input converted to watts. As a percentage,
this statement may be written as:
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝑤𝑎𝑡𝑡𝑠 𝑜𝑢𝑡𝑝𝑢𝑡
𝑤𝑎𝑡𝑡𝑠 𝑖𝑛𝑝𝑢𝑡
𝑥 100%
Since watts input = watts output + watts losses,
𝑤𝑎𝑡𝑡𝑠 𝑜𝑢𝑡𝑝𝑢𝑡
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝑥 100%
𝑤𝑎𝑡𝑡𝑠 𝑜𝑢𝑡𝑝𝑢𝑡 + 𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠
𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 1 −
𝑥 100%
𝑤𝑎𝑡𝑡𝑠 𝑜𝑢𝑡𝑝𝑢𝑡 + 𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
Power Stages Generator
A
B
C
Over all or commercial efficiency
= C/A
Electrical Efficiency
= C/B
Mechanical Efficiency
= B/A
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
Efficiency of Direct Current Generators
 The two methods of determining the efficiency of a generator are:
(1) by directly measuring the total power output and the total power input.
(2) by making certain necessary tests from which the various power losses
are determined, then applying the formula of efficiency.
 Procedure in determining the conventional efficiency of the generator:
(1) Measure the resistance of the armature.
(2) Measure the resistance of the interpole field. (including compensating
winding if there is)
(3) Measure the resistance of the series field.
(4) Measure the shunt field resistance.
(5) Measure the rotational (stray-power loss) = 𝐸𝐴 𝑥 𝐼𝐴
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion)
PROBLEM 1: The following information is given in connection with a 10kW 250
Volt long shunt flat-compounded generator: shunt field resistance = 125 ohms;
armature and interpole resistance = 0.4 ohm; series field resistance= 0.05 ohm;
stray power loss = 540 watts; brush drop at full load (assumed) = 2 volts. Calculate
the efficiency of the generator at full-load.
PROBLEM 2: A 312.5 kW, 550 volts DC shunt generator has an armature
resistance of 0.05 ohm and field resistance of 55 ohm. The stray power loss for this
machine at rated voltage and speed are 5000W. Neglecting the brush loss,
calculate the following: (a) the generated voltage at rated output (b) The generated
power at rated output (c) The electrical efficiency at rated output (d) The
conventional efficiency at rated output (e) The load current at which the maximum
efficiency will result (f) The load power at maximum efficiency.
ELECTRICAL MACHINE 1
Efficiency, Rating, and Application of Dynamos
References
 Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New
York: McGraw-Hill.
 Norton, R. (2011). Machine Design: an integrated approach. (4th ed.).
Boston: Pearson.
 Norton, R. (2012). Design of Machinery: an introduction to the synthesis
and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill.
 Sharma, C.S, &amp; Purohit, K. (2012). Design of machine elements. New
Delhi: PHI Learning Private Limited.
 Gope, P.C. (2012). Machine design: fundamentals and applications.
New Delhi: PHI Learning Private Limited.
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