Types of starters of 3 phase induction motor

1. Directorate of Technical
Education,
Tamilnadu.
Thanthai Periyar Government
Institute of Technology,
Vellore–632002
Department of ECE

2. Types of starters of 3 phase
induction motor
Unit 4
Presented by
M.P.Sasirekha

3. Why starter needed ?
• Most large induction motors are started directly
on line, but when very large motors are started
that way, they cause a disturbance of voltage on
the supply lines due to large starting current
surges.
• To limit the starting current surge, large induction
motors are started at reduced voltage and then
have full supply voltage reconnected when they
run up to near rotated speed.

4. Definition of Motor Starter
• It is the device connected in series with the
motor to decrease its starting current and then
increase it as the motor starts rotating gradually.
• It consists of a connector that acts as a switch to
control the flow of current to the motor and an
overload unit which measures the flow of current
through the motor and controls the halting of the
motor in case of large current being drawn.

5. Principle of Motor Starter
• The current drawn by the motor can be
controlled by reducing the back emf (possible
by reducing the supply voltage) or by
increasing the rotor resistance during the
motor starting.

6. Types of Motor Starters
• Direct Online starter
• Star Delta starter
• Auto Transformer starter
• Stator Resistance Starter
• Rotor Resistance Starter

7. Direct Online starter

8. Direct Online starter

9. Direct Online starter
• It is simple and cheap starter for a 3-phase
induction motor.
• The contacts close against spring action.
• This method is normally limited to smaller cage
induction motors, because starting current can be
as high as eight times the full load current of the
motor.
• Use of a double –cage rotor requires lower
staring current( approximately four times) and
use of quick acting A.V.R enables motors of 75 Kw
and above to be started direct on line.

10. Direct Online starter
• An isolator is required to isolate the starter
from the supply for maintenance.
• Protection must be provided for the motor.
Some of the safety protections are over-
current protection, under-voltage protection,
short circuit protection, etc. Control circuit
voltage is sometimes stepped down through
an autotransformer.

11. Star Delta starter

12. Star Delta starter
• A star delta starter is the most commonly
used method for the starting of a 3 phase
induction motor. In star delta starting an
induction motor is connected in through a star
connection throughout the starting period.
• Then once the motor reaches the required
speed, the motor is connected in through a
delta connection.

13. Star Delta starter
• A star delta starter will start a motor with a
star connected stator winding. When motor
reaches about 80% of its full load speed, it will
begin to run in a delta connected stator
winding.
• In star connection, the current drawn is 0.58%
of the normal current, and also the phase
voltage is reduced to 0.58%. Thus the torque
is reduced.

14. Auto Transformer Motor Starting
• Operated by a two position switch i.e. manually /
automatically using a timer to change over from
start to run position.
• In starting position supply is connected to stator
windings through an auto-transformer which
reduces applied voltage to 50, 60, and 70% of
normal value depending on tapping used.
• Reduced voltage reduces current in motor
windings with 50% tapping used motor current is
halved and supply current will be half of the
motor current.

15. Auto Transformer Motor Starting
• Thus starting current taken from supply will only
be 25% of the taken by DOL starter.
• For an induction motor, torque T is developed by
V2, thus on 50% tapping, torque at starting is
only (0.5V)2 of the obtained by DOL starting.
Hence 25% torque is produced.
• Starters used in lager industries, it is larger in size
and expensive.
• Switching from start to run positions causing
transient current, which can be greater in value
than those obtained by DOL starting.

16. Auto Transformer Motor Starting

17. Rotor resistance starter
• This type of control is used in the slipring
induction motors. An external variable
resistance is connected in the rotor circuit.
• At the time of starting this variable this
resistance is set to the highest possible value
,to the motor current at starting.
• The three moving contacts are interconnected
to form a start point for the resistors.

18. Rotor resistance starter
• To ensure that the motor cannot be started
until all rotor resistance is in circuit, an
interlock is fitted which prevents the
contactors from being closed until this
condition is fulfilled.

19. Rotor resistance starter

20. Stator resistance starter
• A starter resistance is connected in each line
series with each phase winding of the stator.
Initially all the starter resistances are kept in the
“Start” position so that they offer their maximum
resistance.
The switch is turned on to connect the three
phase ac supply to the stator winding.
As the motor accelerates, the starter resistance is
reduced by the moving the variable contact of
the resistance towards the “Run” position.

21. Stator resistance starter

22. reference
• https://www.brighthubengineering.com/hvac/74
957-starting-methods-for-induction-motors/
• https://www.elprocus.com/motor-starter/
• https://www.polytechnichub.com/types-of-
starters/
• https://www.electrical4u.com/direct-online-
starter-or-dol-starter/
• https://www.electrical4u.com/star-delta-starter/
• https://www.marinerdesk.com/motor-starter-
types-working-circuit/

23. THANK YOU