Question

Describe how starting torque of electric induction motor may be improved by using each of the following.

Wound rotor motor

Double cage motor.

Wound rotor motor.

This motor has a conventional stator but the rotor has a 3-phase winding (similar to its stator winding) which is connected to 3 slip rings mounted on the shaft. An external 3-phase resistor bank is connected to brushes on the rotor slip rings. The motor is started with maximum resistance to give high starting torque.

The resistance is gradually reduced until at full speed all the resistance is OUT. Thus the motor runs with full speed small slip and high efficiency.

Double cage motor.

The motor has 2 cage winding outer cage and high resistance winding to develop high starting torque and an inner cage of low resistance for high efficiency and low slip at full load speed. Both cage winding develop their own torque and the shaft torque is the sum of these torque values.