Question

A 4-pole, 3-phase, 60 Hz induction motor is operating at a speed of 1500 rpm. The frequency of the rotor current of the motor in Hz is __________

(a) 5

(b) 4

(c) 2

(d) 7

This question was addressed to me during an interview.

My question is taken from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives

Answer 1

Correct choice is (b) 4

To explain I would say: Given a number of poles = 4. Supply frequency is 60 Hz. Rotor speed is 1500 rpm. Ns = 120×f÷P = 120×60÷4 = 1800 rpm. S=Ns-Nr÷Ns = 1800-1500÷1800 = .166. F2=sf=.166×60=4 Hz.