Question

Explain the effect of reduced voltage on standard squirrel cage mtor with respect to each of the following.

Burn out.

Starting current.

Starting torque.

Speed.

During normal running of an AC squirrel cage induction motor, if the applied voltage is reduced then to sustain the load [same power], the motor will draw more current from the supply and the stator winding temperature will increase due to higher current flowing.

The starting current of induction motor depends upon the value of supply voltage and standstill impedance of staor winding. Since the starting torque of an induction motor is proportional to the square of supply voltage [T α (V)2]therefore there is corresponding and greater drop in starting torque and in starting current.

T α (V)2

If V decrease then T decrease

For star connection V = VL / √3

Torque becomes 1/3^rd that of delta

The run up time depends on the starting torque developed and the load on the motor possibility of stalling of motor.

With reduction of voltage, the motor draws more current from the supply to sustain the load almost at the same speed. However with considerable reduction of voltage, the speed of motor drops, when the developed torque is unable to sustain load torque. If the speed drops to such a level that it crosses the maximum torque point, then the motor falls back to very low speed and stops.