Question

An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 720 rpm. The frequency of the rotor current of the motor in Hz is __________

(a) 2

(b) 4

(c) 3

(d) 1

I had been asked this question during an online interview.

This key question is from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives

Answer 1

The correct answer is (a) 2

For explanation: Given a number of poles = 8. Supply frequency is 50 Hz. Rotor speed is 720 rpm. Ns = 120×f÷P=120×50÷8 = 750 rpm. S=Ns-Nr÷Ns = 750 – 720÷750 = .04. F2=sf=.04×50=2 Hz.