Question

Explain speed control method of 3 phase induction motor by the following methods: (1) Pole changing (2) Rotor resistance control (3) Stator voltage

Answer 1

1) Pole Changing Method: 

Synchronous speed is given by,

From the above equation of synchronous speed, it can be seen that synchronous speed (and hence, running speed) can be changed by changing the number of stator poles. This method is generally used for squirrel cage induction motors as squirrel cage rotor adopts itself for any number of stator poles. Change in stator poles is achieved by two or more independent stator windings wound for different number of poles in same slots. 

ii)Rotor Resistance Control Method: This method is only applicable to slip ring motors, as addition of external resistance in the rotor of squirrel cage motors is not possible. The speed of the motor can be decreased by adding external resistance to the rotor. Under normal running condition T ∝ s/R₂, where R₂ is rotor resistance per phase. Therefore, for same load torque, slip can be increased (i.e. motor speed can be decreased) by increasing the rotor resistance and vice versa.

iii)Stator Voltage Control Method: As in three phase induction motor, the torque produced, T ∝ sE₂² where E² is rotor induced emf and E₂ ∝ V. Thus, T ∝ sV² , which means, if supplied voltage is decreased, the developed torque decreases. Hence, for providing the same load torque, the slip increases with decrease in voltage and consequently the speed decreases and vice versa. Ultimately speed depends on the supply voltage.