AC THREE PHASE MOTOR - Practice Mode

1) Why slip ring induction motor is fitted with wound rotor?

To reduce the slip To control the speed To reduce the losses To get high starting and running torque

Correct Answer: To get high starting and running torque

2) What is the function of timer in automatic star delta starter?

Trip at over load Switch ON at pre set time Change from star to delta Switch OFF at pre set time

Correct Answer: Change from star to delta

3) Which instrument is used to measure insulation resistance of a 3 phase induction motor?

Megger Multimeter Shunt type ohmmeter Series type ohmmeter

Correct Answer: Megger

Explanation:

The Megger is the instrument uses for measuring the resistance of the insulation. It works on the principle of comparison, i.e., the resistance of the insulation is compared with the known value of resistance. If the resistance of the insulation is high, the pointer of the moving coil deflects towards the infinity, and if it is low, then the pointer indicates zero resistance. The accuracy of the Megger is high as compared to other instruments.

Megger is used to test the new wiring installation.

4) What is the starting current of an A.C 3 phase induction motor?

1 to 2 times of full load current 2 to 3 times of full load current 4 to 5 times of full load current 5 to 6 times of full load current

Correct Answer: 5 to 6 times of full load current

5) Which method is used to control the speed of 3 phase induction motor from stator side?

By cascade operation By rotor rheostat control By injecting emf in rotor circuit By changing the applied frequency

Correct Answer: By changing the applied frequency

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