Starting Methods of Induction Motor (3-PHASE)

Starting Methods of Induction Motor (3-PHASE) Points : starting methods of induction motor (3-PHASE) The three-phase induction motor is started with the help of any one of the following types of starters:
1. Direct-on line starter. (Used up to 5H.P. motor)
2. Star/Delta starter. (Used from 5 to 20 H.P motor)
3. Auto-transformer Starter (Used from 7.50 to 50 H.P. motor) a) Direct Line Starter i. L₁, L₂, L₃ stands for 3 - supply terminals, M₁, M₂, M₃, represents motor leads connected to the starter.
2. It does not decrease the starting current, but provides an easy on and off system.
3. D.L. Starter saves the motor from overload, single phasing and sudden starting.
4. This starter is used for motors up to 2 H.P. where the machine starts on a tight load.

b) Star-Delta Starter Fig. (b). is shown to illustrate the operation of the starter in Fig (a) more elaborately.
(i) In start position the stator windings are connected in start. In star connection. Phase voltage = 1/3 Line voltage. Thus low voltage is supplied to the motor and so the current in the beginning.
(ii) When the motor has accelerated, the starter is quickly moved to the Run position, thereby changing the connections of stator windings to delta position.
In delta connection. Phase voltage = Line voltage.
Thus full voltage is now applied to the motor.
(iii) This starter is used for motors between 6 to 20 H.P.

c) Auto Transformer Starter (i) The line current at starting is reduced due to the transformer action.
(ii) The torque can be varied depending upon the number of tappings.
(iii) Motor is permanently connected in delta and three terminals from the stator are brought to the starter.
(iv) In starting position, the stator is connected to the line terminals through the auto-transformer, which reduces the voltage to a pre-determined value.
(v) When the motor has accelerated, the starter is changed over to the running position, thereby connecting the stator to the thus line voltage.
(vi) It is used for starting squirrel cage motors of 20 H.P. or above.

(d) Auto-Star Delta Starter i. Motor terminals are connected to A₁, B₁, C₁, and A₂, B₂, C₂ respectively.
ii. On pressing the start button no-volt coil is energized starting the motor on star.
iii. Due to thermal effect, time switch closes after a few seconds. It energizes the coil on the right side causing the motor to run in delta.