starting methods of squirrel cage induction motor in detail with diagram

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10. Starting Method for Induction Motors
A 3-phase induction motor is theoretically self starting.
The stator of an induction motor consists of 3-phase windings,
which when connected to a 3-phase supply creates a rotating
magnetic field. This will link and cut the rotor conductors
which in turn will induce a cu
ent in the rotor conductors and
create a rotor magnetic field. The magnetic field created by the
otor will interact with the rotating magnetic field in the stato
and produce rotation.
Therefore, 3-phase induction motors employ a starting
method not to provide a starting torque at the rotor, but
ecause of the following reasons;
1) Reduce heavy starting cu
ents and prevent motor from
overheating.
2) Provide overload and no-voltage protection.
There are many methods in use to start 3-phase induction
motors. Some of the common methods are;
Direct On-Line Starter (DOL)
 Star-Delta Starte
 Auto Transformer Starte
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 Rotor Impedance Starte
 Power Electronics Starte
Direct On-Line Starter (DOL)
The Direct On-Line (DOL) starter is the simplest and the
most inexpensive of all starting methods and is usually used
for squi
el cage induction motors. It directly connects the
contacts of the motor to the full supply voltage. The starting
cu
ent is very large, normally 6 to 8 times the rated cu
ent.
The starting torque is likely to be 0.75 to 2 times the full load
torque. In order to avoid excessive voltage drops in the supply
line due to high starting cu
ents, the DOL starter is used only
for motors with a rating of less than 5KW
There are safety mechanisms inside the DOL starte
which provides protection to the motor as well as the operato
of the motor.The power and control circuits of induction moto
with DOL starter are shown in figure(1).
* K1M Main contacto
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Power Circuit Control Circuit
Fig.(1): power and control circuits of I.M. with DOL starte
The DOL starter consists of a coil operated contacto
K1M controlled by start and stop push buttons. On pressing the start
push button S1, the contactor coil K1M is energized from line L1.
The three mains contacts (1-2), (3-4), and (5-6) in fig. (1) are closed.
The motor is thus connected to the supply. When the stop push
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utton S2 is pressed, the supply through the contactor K1M is
disconnected. Since the K1M is de-energized, the main contacts (1-
2), (3-4), and (5-6) are opened. The supply to motor is disconnected
and the motor stops.
Star-Delta Starte
The star delta starting is a very common type of starte
and extensively used, compared to the other types of the starters.
This method used reduced supply voltage in starting. Figure(2)
shows the connection of a 3phase induction motor with a star –
delta starter.
The method achieved low starting cu
ent by first
connecting the stator winding in star configuration, and then
after the motor reaches a certain speed, throw switch changes
the winding a
angements from star to delta configuration.
By connecting the stator windings, first in star and then in
delta, the line cu
ent drawn by the motor at starting is reduced
to one-third as compared to starting cu
ent with the windings
connected in delta. At the time of starting when the stato
windings are start connected, each stator phase gets voltage
୐ , where ୐ is the line voltage. Since the torque developed
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y an induction motor is proportional to the square of the
applied voltage, star- delta starting reduced the starting torque to
one – third that obtainable by direct delta starting.
 K2M Main Contacto
 K3M Delta Contacto
 K1M Star Contacto
 F1 Thermal Overload Relay
Fig.(2) Induction Motor with Star Delta Starte
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Auto Transformer Starte
The operation principle of auto transformer method is
similar to the star delta starter method. The starting
cu
ent is limited by (using a three phase auto
transformer) reduce the initial stator applied voltage.
The auto transformer starter is more expensive, more
complicated in operation and bulkier in construction when
compared with the star – delta starter method. But an auto
transformer starter is suitable for both star and delta
connected motors, and the starting cu
ent and torque can
e adjusted to a desired value by taking the co
ect
tapping from the auto transformer. When the star delta
method is considered, voltage can be adjusted only by
factor of .
Figure (3) shows the connection of a 3phase induction
motor with auto transformer starter.
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Fig.(3) shows I.M with auto transformer starter.
Rotor Impedance Starte
This method allows external resistance to be connected to
the rotor through slip rings and
ushes. Initially, the roto
esistance is set to maximum and is then gradually decreased
as the motor speed increases, until it becomes zero.
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The rotor impedance starting mechanism is usually very
ulky and expensive when compared with other methods. It
also has very high maintenance costs. Also, a considerable
amount of heat is generated through the resistors when cu
ent
uns through them. The starting frequency is also limited in
this method. However, the rotor impedance method allows the
motor to be started while on load. Figure (4) shows the
connection of a 3phase induction motor with rotor resistance
starter.
Fig. (4) Shows the I.M.
with rotor resistance
starter.
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Example (9):
It is desired to install a 3-phase cage induction motor restricting
the maximum line cu
ent drawn from a 400 V 3-phase supply
to 120 A. if the starting cu
ent is 6 times full load cu
ent, what
is the maximum permissible full load kVA of the motor when
i. It is directly connected to the mains
ii. It is connected through an auto-transformer with a
tapping of 60%
iii. It is designed for used with star-delta starter.
Solution:
i. Direct-on-line starting
Maximum line cu
ent, ୐
Starting cu
ent ୱ୲ ϐ୪
Since the maximum line cu
ent drawn from the supply is 120A
୤୲ ୤
Maximum permissible rating of the moto
୐ ୤୲
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ii. Auto-transformer starting
ୱ୲
ଶ
ୱୡ
ଶ
୤୲
ଶ
୤୲
୤୲ ଶ
Maximum permissible rating of the moto
୐ ୤୲
iii. Star-delta starting
ୱ୲ ୤୲
௙௧ ௙௧
Maximum permissible kVA rating of the moto
୐ ୤୲
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11. SPEED CONTROL OF INDUCTION MOTORS
The speed of an induction motor is given as
N = 120f/p (1-S).
So obviously the speed of an induction motor can be controlled
y varying any of three factors namely supply frequency f,
number of pole P or slip S.
The main methods employed for speed control of induction
motors are as follows:
1. Pole changing
2. Stator voltage control
3. Supply frequency control
4. Rotor resistance control
5. Slip energy recovery.
The basic principles of these methods are described below
Pole changing
The number of stator poles can be change by
Multiple stator windings
Method of consequent poles
Pole amplitude modulation (PWM)
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The methods of speed control by pole changing are
suitable for cage motors only because the cage roto
automatically develops number of poles equal to the
poles of stator winding.
1. Multiple stator windings
In this method the stator is provided with two separate
windings which are wound for two different pole
numbers. One winding is energized at a time. Suppose
that a motor has two windings for 6 and 4 poles. For 50
Hz supply the synchronous speed will be 1000 and 1500
pm respectively. If the full load slip is 5% in each case,
the operating speeds will be 950 rpm and 1425 rpm
espectively. This method is less efficient and more
costly, and therefore, used only when absolutely
necessary.
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2.Method of consequent poles
In this method a single stator winding is divided into few
coil groups. The terminals of all these groups are
ought out.
The number of poles can be changed with only simple changes
in coil connections. In practice, the stator winding is divided
only in two coil groups. The number of poles can be changed
in the ratio of 2:1.
Fig.(1) shows one phase of a stator winding consisting of 4
coils divided into two groups a – b and c – d. Group a –
consists of odd numbered coils(1,3) and connected in series.
Group c – d has even numbered coils (2, 4) connected in series.
The terminals a,b,c,d are taken out as shown.
Fig. (1) Stator phase connections for 4 poles
fig. (1-b)
fig. (1-c)
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The coils can be made to ca
y cu
ent in given directions
y connecting coil groups either in series or parallel shown in
fig. (1-b) and fig.(1-c) respectively.
With this connection, there will be a total of 4 poles giving
a synchronous speed of 1500 rpm for 50 Hz system. If the
cu
ent through the coils of group a – b is reversed (fig.2), then
all coils will produce north (N) poles.
In order to complete the magnetic path, the flux of the pole
groups must pass through the spaces between the groups, thus
inducing magnetic poles of opposite polarity (S poles) in the
inter – pole spaces.
Fig. (2) Stator phase connections for 8 poles
fig. (2-b) series connection
fig. (2-c) parallel connection
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. Stator Voltage Control
The torque developed by an induction motor is proportional
to the square of the applied voltage. The variation of speed
torque curves with respect to the applied voltage is shown in
fig.(3). These curves show that the slip at maximum torque...