Question

Which of the following methods of starting is required for slip ring induction motors of high rating?
1. Star delta starters
2. Direct on line starters
3. Auto-transformer starters
4. Rotor resistance starters

Answer 1

Correct Answer - Option 4 : Rotor resistance starters

• Rotor resistance starter is used in a high rating slip ring induction motor
• It uses an external resistance/phase in the rotor circuit so that the rotor will develop a high value of torque.
• High torque is produced at low speeds when the external resistance is at its higher value.
• At the start, supply power is connected to the stator through a three-pole contactor and, at the same time, an external rotor resistance is added
• The high resistance limits starting current and allow the motor to start safely against a high load.
• Hence if resistance is inserted in the rotor circuit of a slip ring induction motor, then compared to direct line starting the starting current reduces but starting torque increases.
• Rotor resistance starter is used for all ratings of slip ring induction motors.

• We are unable to put external resistance to the rotor of the cage induction motor because of short-circuiting of the rotor bar.
• Hence, different kinds of starters such as Star delta starters, Direct online starters, Auto-transformer starters are used according to power rating in the cage induction motor.