Faculty of Engineering and Technology, University of Buea
EEF480: Control of Electrical Machines
Lecture No. 1: Traditional Starting Methods
Outline:
Introduction
Direct On-line Starter
Stator Resistance Starter
Autotransformer Starter
Star-Delta Starter
Rotor Resistance Starter
Motor Protection
3-point starter
4-point starter
2-point starter
I.
Introduction
When a motor is switched on, there is a high inrush current from the mains which may, especially if
the power line section is inadequate, cause a drop in voltage likely to affect receptor operation. This
drop may be severe enough to be noticeable in lighting equipment.
An induction motor is similar to a poly-phase transformer whose secondary is short circuited. Thus, at
normal supply voltage, like in transformers, the initial current taken by the primary is very large for a
short while. Unlike in DC motors, large current at starting is due to the absence of back emf. If an
induction motor is directly switched on from the supply, it takes 4 to 8 times its full load current and
develops a torque which is only 1.5 to 2.5 times the full load torque which put the mechanical
elements under considerable strain. This large starting current produces a large voltage drop in the line,
which may affect the operation of other devices connected to the same line. Hence, it is not advisable to
start induction motors of higher ratings (generally above 25kW) directly from the mains supply. Some
sector rules prohibit the use of motors with direct on-line starting systems beyond a given power.
This exclusive starting current has to be prevented because:
• It causes sudden depression of voltage of supply (large voltage drop occurs) system causing
disturbances to other loads connected in the system.
• It would cause heavy sparking at the brushes which may destroy the commutator and brush gear.
• Due to heavy inrush of current at start there is possibility of damage of the motor winding.
There are several starting systems which differ according to the motor and load specifications. The
choice is governed by electrical, mechanical and economic factors. The kind of load driven is also
important in the choice of starting system.
At starting the motor takes large amount of current which is nearly 25 x full load current. This large
amount of current cannot be allowed to flow in a motor even for a short period of time. This exclusive
starting current has to be prevented because
Various starting methods of electrical motors based on Manual Starting, Across the Line Starting
Reduced Voltage Starting, used to reduce currents and torque are described below.
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Faculty of Engineering and Technology, University of Buea
II.
AC Starters
1. Manual Starter
A manual motor starter is package consisting of a horsepower rated switch with one set of contacts
for each phase and corresponding thermal overload devices to provide motor overload protection.
• The main advantage of a manual motor starter is lower cost than a magnetic motor starter
with equivalent motor protection but less motor control capability.
• Manual motor starters are often used for smaller motors-typically fractional horsepower
motors but the National Electrical Code allows their use up to 10 Horsepower.
• Since the switch contacts remain closed if power is removed from the circuit without
operating the switch, the motor restarts when power is reapplied which can be a safety
concern.
• They do not allow the use of remote control or auxiliary control equipment like a magnetic
starter does.
2. Across the Line Starter
Also, called magnetic starter, it is a package consisting of a contactor capable of opening and
closing a set of contacts that energize and de-energize the circuit to the motor along with additional
motor overload protection equipment. Magnetic starters are used with larger motors (required above
10 horsepower) or where greater motor control is desired.
The main element of the magnetic motor starter is the contactor, a set of contacts operated by
an electromagnetic coil.
a. Direct On-line (DOL) Starter
This is the simplest mode, where the stator is directly connected to the mains supply (See
Fig.1). The motor starts with its own characteristics.
In spite of its advantages (simple equipment, high starting torque, fast start, low cost), direct
on-line starting is only suitable when:
✓ the power of the motor is low compared to that of the mains, which limits interference
from inrush current,
✓ the machine to drive does not need to speed up gradually or has a damping device to
limit the shock of starting,
✓ the starting torque can be high without affecting machine operation or the load that is
driven.
Its drawback is the high overcurrent.
L1 and L2 are indicating lights to show the presence of supply and the running of motor, respectively.
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Faculty of Engineering and Technology, University of Buea
Figure 1: DOL-Power Circuit
Figure 2: DOL-Control Circuit
b. Direct On-line Starter with Forward and Reverse Directions
Figure 3: DOL-FR-Power Circuit
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Figure 4: DOL-FR-Power Circuit
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H1, H2, H3 and H4 are indicating lights to show the presence of supply, the fault, the running of motor in
forward and in reverse directions, respectively.
(a)
Figure 5: (a) T vs N and (b) I vs N
𝑇𝑑
= 0.6 𝑡𝑜 1.5
𝑇𝑛
𝑎𝑛𝑑
(b)
𝐼𝑑
= 4 𝑡𝑜 8
𝐼𝑛
3. Reduced Voltage Starters
Reduced Voltage Starting connects the motor windings/terminals at lower-than-normal line voltage
during the initial starting period to reduce the inrush current when the motor starts.
➢ Reduced voltage starting may be required when:
- The current in-rush form the motor starting adversely affects the voltage drop on the
electrical system.
- needed to reduce the mechanical “starting shock” on drive-lines and equipment when
the motor starts.
➢ Reducing the voltage reduces the current inrush to the motor and also reduces the starting
torque available when the motor starts.
➢ All NEMA induction motors can will accept reduced voltage starting however it may not
provide enough starting torque in some situations to drive certain specific loads.
If the driven load or the power distribution system cannot accept a full voltage start, some type of
reduced voltage starting scheme or solid state (electronic) starters must be used.
Reduced voltage starters can only be used where low starting torque is acceptable or a means
exists to remove the load from the motor or application before it is stopped.
Here, the voltage applied to the motor is reduced by ballast chokes or resistors, as is the
starting current. The starting torque is reduced by the square of the current reduction.
a. Stator Resistance Starter
In this case, cost-efficient resistors are used instead of the chokes. This method is less helpful
in reducing the starting current for the same torque requirement, because the motor torque
reduces as a value of the square of the voltage and the voltage applied to the motor increases
only due to the motor’s reduced current consumption during increasing speed.
It is better to reduce the ballast resistor step by step during start. But this requires
considerably more switchgear.
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Faculty of Engineering and Technology, University of Buea
Another possibility is the use of encapsulated wet (electrolytic) resistors. For these resistors,
the ohmic resistance reduces in line with the temperature increase caused by the starting
current’s heating capability.
With this system (Fig. 6), the motor starts at reduced voltage because resistors are inserted in
series with the windings. When the speed stabilizes, the resistors are eliminated and the
motor is connected directly to the mains. This process is usually controlled by a timer. This
starting method does not alter the connection of the motor windings so the ends of each
winding do not need outputs on a terminal board. The resistance value is calculated
according to the maximum current peak on starting or the minimum starting torque required
for the resistance torque of the machine to drive. The starting current and torque values are
generally:
𝑇𝑑
𝐼𝑑
= 0.6 𝑡𝑜 0.85
𝑎𝑛𝑑
= 4.5
𝑇𝑛
𝐼𝑛
During the acceleration stage with the resistors, the voltage applied to the motor terminals is
not constant but equals the mains voltage minus the voltage drop in the starting resistance.
The voltage drop is proportional to the current absorbed by the motor. As the current
weakens with the acceleration of the motor, the same happens to the voltage drop in the
resistance. The voltage applied to the motor terminals is therefore at its lowest on starting and
then gradually increases.
As the torque is proportional to the square of the voltage at the motor terminals, it increases
faster than in star-delta starting where the voltage remains constant throughout the star
connection.
This starting system is therefore suited to machines with a resistive torque that increases with
the speed, such as fans and centrifugal pumps.
It has the drawback of a rather high current peak on starting. This could be lowered by
increasing the resistance value but that would cause the voltage to drop further at the motor
terminals and thus a steep drop in the starting torque.
On the other hand, resistance is eliminated at the end of starting without any break in power
supply to the motor, so there are no transient phenomena.
The characteristics of the motor is illustrated in Fig. 7.
An alternative to the stator resistance starter power circuit diagram is shown in Fig. 8.
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Faculty of Engineering and Technology, University of Buea
Figure 6: Stator Resistance Starting- Power and Control circuits
Figure 7: Characteristics of the inductor motor
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Figure 8: Alternative power circuit diagram
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Faculty of Engineering and Technology, University of Buea
b. Stator Choke Starter
During the off state, the motor resistance is low. A
large percentage of the mains voltage is reduced at
the ballast chokes. Therefore, the motor’s starting
torque is considerably reduced. As the torque
increases, the voltage applied to the motor increases
due to a reduction in current consumption and the
vectoral voltage division between motor and the
ballast reactance. This leads to an increased motor
torque. After a successful run-up, the chokes are
short-circuited. The starting current is reduced
depending on the required starting torque.
Figure 9: Stator choke starting power circuit diagram
c. Autotransformer Starter
An autotransformer starter enables the start of squirrel-cage motors using a reduced starting
current, since the voltage is reduced during start. Contrary to a star-delta connection, only
three wires to the motor and 3 motor connections are required. This connection is particularly
widely used in English-speaking countries.
The motor is powered at reduced voltage via an autotransformer which is bypassed when the
starting process is completed (Fig. 10). The starting process is in three steps:
• in the first place, the autotransformer is star-connected, then the motor is connected to
the mains via part of the autotransformer windings. The process is run at a reduced
voltage which depends on the transformation ratio. The autotransformer is usually
tapped to select this ratio to find the most suitable voltage reduction value. In order to
adapt the motor start characteristics to the torque requirement, autotransformers are
usually equipped with three selectable tappings (e.g. 80%, 65%, 50%),
• the star connection is opened before going onto full voltage (motor almost reaches its
rated torque). The fraction of coil (transformer’s partial windings) connected to the
mains then acts as an inductance (chokes) in series with the motor windings. This
operation takes place when the speed balances out at the end of the first step,
• full voltage connection is made after the second step which usually only lasts a fraction
of a second. The piece of autotransformer winding in series with the motor is shortcircuited and the autotransformer is switched off.
The values obtained are:
𝑇𝑑
𝐼𝑑
= 1.7 𝑡𝑜 4
𝑎𝑛𝑑
= 0.5 𝑡𝑜 0.85
𝑇𝑛
𝐼𝑛
The starting process runs with no break in the current in the motor, so transient phenomena
due to breaks do not occur. However, if a number of precautions are not taken, similar
transient phenomena can appear on full voltage connection because the value of the
inductance in series with the motor is high compared to the motor after the star arrangement
is open. This leads to a steep drop in voltage which causes a high transient current peak on
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Faculty of Engineering and Technology, University of Buea
full voltage connection. To overcome this drawback, the magnetic circuit in the
autotransformer has an air gap which helps to lower the inductance value. This value is
calculated to prevent any voltage variation at the motor terminals when the star arrangement
opens in the second step. The air gap causes an increase in the magnetizing current in the
autotransformer. This current increases the inrush current in the mains supply when the
autotransformer is energized. This starting system is usually used in LV for motors powered
at over 150kW. It does however make equipment rather expensive because of the high cost of
the autotransformer.
Note: There is an interlock between KM1 and KM2.
Figure 10: Autotransformer starter-power
circuit diagram
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Figure 11: Characteristics of induction motor - T vs N and
I vs N
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Faculty of Engineering and Technology, University of Buea
4. Star-Delta Starter
This starting system (Fig. 12) can only be used with a motor where both ends of its three stator
windings are fitted to a terminal board. Furthermore, the winding must be done so that the delta
connection matches the mains voltage: e.g. a 380V, 3-phase supply will need a motor with 380V
delta and 660V star coiling.
The principle is to start the motor by connecting the star windings at mains voltage, which divides
the motor’s rated star voltage by √3 (in the example above, the mains voltage at 380V = 660V / √3).
The starting current peak (SC) is divided by 3: SC = 1.5 to 2.6 RC (RC rated Current).
A 380V / 660V motor star-connected at its rated voltage of 660V absorbs a current √3 times less
than a delta connection at 380V. With the star connection at 380V, the current is divided by √3
again, so by a total of 3.
As the starting torque (ST) is proportional to the square of the supply voltage, it is also divided by 3:
ST = 0.2 to 0.5 RT (RT Rated Torque).
The motor speed stabilizes when the motor and resistive torques balance out, usually at 75-85% of
the rated speed. The windings are then delta connected and the motor recovers its own
characteristics. The change from star connection to delta connection is controlled by a timer. The
delta contactor closes 30 to 50 milliseconds after the star contactor opens, which prevents shortcircuiting between phases as the two contactors cannot close simultaneously (interlock).
The current through the windings is broken when the star contactor opens and is restored when the
delta contactor closes. There is a brief but strong transient current peak during the shift to delta, due
to the counter electromotive force of the motor.
Star-delta starting is suitable for machines with a low resistive torque or which start with no load
(e.g. wood-cutting machines). Variants may be required to limit the transient phenomena above a
certain power level. One of these is a 1-2 second delay in the shift from star to delta.
Such a delay weakens the counter-electromotive force and hence the transient current peak. This can
only be used if the machine has enough inertia to prevent too much speed reduction during the time
delay.
Another system is 3-step starting: star-delta + resistance-delta. There is still a break, but the resistor
in series with the delta-connected windings for about three seconds lowers the transient current. This
stops the current from breaking and so prevents the occurrence of transient phenomena.
Use of these variants implies additional equipment, which may result in a significant rise in the cost
of the installation.
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Faculty of Engineering and Technology, University of Buea
Figure 12: Y-Δ starter power circuit
Figure 13: Y-Δ starter control circuit
Figure 14: Star-Delta power circuits
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Faculty of Engineering and Technology, University of Buea
5. Rotor Resistance (or Slip Ring motor) Starter
A slip ring motor cannot be started direct on-line with its rotor windings short-circuited, otherwise it
would cause unacceptable current peaks. Resistors must therefore be inserted in the rotor circuit
(Fig. 15) and then gradually short-circuited, while the stator is powered at full mains voltage. The
resistance inserted in each phase is calculated to ascertain the torque-speed curve with strict
accuracy. The result is that it has to be fully inserted on starting and that full speed is reached when
it is completely short-circuited.
The current absorbed is more or less proportional to the torque supplied at the most only a little
greater than the theoretical value.
Figure 15: Slip ring motor starter-power and control circuits
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Figure 16: Rotor resistance starter-power circuit with 3 sets of resistance bank
6. Part winding motor starting
This system (Fig. 17), not widely used in Europe, is quite common in the North American market
(voltage of 230/460, a ratio of 1:2). This type of motor has a stator winding divided into two parallel
windings with six or twelve output terminals. It is equivalent to two “half motors” of equal power.
On starting, a single “half motor” is
connected directly at full mains
voltage strength, which divides the
starting current and the torque
approximately by two. The torque is
however greater than it would be
with a squirrel cage motor of equal
power with star-delta starting. At the
end of the starting process, the
second winding is connected to the
mains. At this point, the current peak
is low and brief, because the motor
has not been cut off from the mains
supply and only has a little slip.
Figure 17: Part winding starting
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7. Summary table of 3-phase motor starting systems
This starting system requires the motor to be specifically sized.
Figure 18: Comparison of starting methods
8. Motor Protection
The summary in the table in figure 19 shows the possible causes of each type of fault, the probable
effects and inevitable outcome if no protection is provided. In any event, motors always require two
protections:
• protection against short circuits,
• protection against overload (overheating).
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Faculty of Engineering and Technology, University of Buea
Figure 19: Summary of possible faults in a motor with their causes and effects
III.
DC Starters
Basic operational voltage equation of a DC motor is given as: Vt = Ea + IaRa and hence, Ia = (Vt - Ea)/Ra.
Now, when the motor is at rest, obviously, the back emf Ea = 0. Hence, armature current at the moment of
starting can be given as Ia = Ea/Ra. In practical DC machines, armature resistance is basically very low,
generally about 0.5 Ω. Therefore, a large current flows through the armature during starting. This current is
large enough to damage the armature circuit.
Due to this excessive starting current:
➢ the fuses may blow out and the armature winding and/or commutator brush arrangement may get
damaged.
➢ very high starting torque will be produced (as torque is directly proportional to the armature
current), and this high starting torque may cause huge centrifugal force which may throw off the
armature winding.
➢ other loads connected to the same source may experience a dip in the terminal voltage.
A large DC motor will pick up speed rather slowly due to its large rotor inertia. Hence, building up the
back emf slowly causing the level of high starting current maintained for quite some time. This may cause
severe damage. To avoid this, a suitable DC motor starter must be used. Very small dc motors, however,
may be started directly by connecting them to the supply with the help of a contactor or a switch. It does not
result in any harm because they gather speed quickly due to small rotor inertia. In this case, the large
starting current will die down quickly because of the fast rise in the back emf.
To avoid the above dangers while starting a DC motor, it is necessary to limit the starting current. So, a
DC motor is started by using a starter. A resistance is introduced in series with the armature for very start
duration of starting period only, which limits the starting current to a very safe value. This starting
resistance is gradually cut out as the motor gains speed and develops the back emf which then regulates the
speed of the motor.
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There are various types of dc motor starters, such as 3-point starter, 4-point starter, no-load release
coil starter, thyristor controller starter, etc.
The basic concept behind every DC motor starter is adding external resistance to the armature winding
during starting. From the followings, 3-point starters and 4-point starters are used for starting shunt wound
motors and compound wound motors. For series motors, 2-point starters are used for starting.
1. 3-Point Starter
The internal wiring of a 3-point starter is as shown
in the figure. When the connected dc motor is to be
started, the lever is turned gradually to the right. When
the lever touches point 1, the field winding gets directly
connected across the supply, and the armature winding
gets connected with resistances R1 to R5 in series.
During starting, full resistance is added in series with
the armature winding. Then, as the lever is moved
further, the resistance is gradually is cut out from the
armature circuit. Now, as the lever reaches to position
6, all the resistance is cut out from the armature circuit
and armature gets directly connected across the supply.
The electromagnet 'E' (no voltage coil) holds the lever
at this position. This electromagnet releases the lever
when there is no (or low) supply voltage.
It can be seen that, when the arm is moved from the
position 1 to the last position, the starter resistance gets added in series with the field winding. But, as
the value of starter resistance is very small as compared to the shunt resistance, the decrease in shunt
field current may be negligible. However, to overcome this drawback a brass or copper arc may be
employed within a 3-point starter which makes a connection between the moving arm and the field
winding, as shown in the figure of 4-point starter below. When the motor is overloaded beyond a
predefined value, 'overcurrent release electromagnet' D gets activated, which short-circuits
electromagnet E and, hence, releases the lever and the motor is turned off.
2. 4-Point Starter
The main difference between a 3-point
starter and a 4-point starter is that the no
voltage coil electromagnet (E) is not
connected in series with the field coil. The
field winding gets directly connected to the
supply, as the lever moves touching the
brass arc (the arc below the resistance
studs). The no voltage coil (or Hold-on
coil) is connected with a current limiting
resistance Rh. This arrangement ensures
that any change of current in the shunt field
does not affect the current through hold-on
coil at all. This means, electromagnetic pull
of the hold-on coil will always be sufficient
so that the spring does not unnecessarily
restore the lever to the off position.
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A 4-point starter is used where field current is to be adjusted by means of a field rheostat for the
purpose of operating the motor above rated speed by reducing the field current.
3. 2-Point Starter
Construction of DC series motor starters is very
basic as shown in the figure. 2-point starter is used in
series motors because in case of series motor, the
armature winding and field winding are connected in
series. Therefore, series motor achieves dangerous high
speed. So, series motor should not be started without
any load. Two-point starter construction is very much
similar to the combination of rheostat with a tap
changing. The starting resistance and no-load release
coil are connected in series with the armature of a series
motor. When a series motor is given a supply, the
handle is moved from OFF position stud no.1 ie) full
resistance is given at starting. Therefore, inrush of high
starting current to the series motor is reduced. Then
starting resistance is gradually cut down and the motor gathers speed, which will then develop back emf.
The start arm is simply moved towards right to start the motor. Thus, maximum resistance is connected
in series with the armature during starting and then gradually decreased as the start arm moves towards
right. This starter is sometimes also called as a 2-point starter. The no load release coil holds the start
arm to the run position and leaves it when the voltage is lost.
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