Question

A 32-pole, 3-phase, 70 Hz induction motor is operating at a speed of 112 rpm. The frequency of the rotor current of the motor in Hz is __________

(a) 40.2

(b) 46.1

(c) 40.1

(d) 40.6

This question was addressed to me by my college professor while I was bunking the class.

The doubt is from Induction Motors Speed Control in portion Speed Control of Direct Current Motors & Induction Motors of Electric Drives

Answer 1

Correct option is (c) 40.1

Explanation: Given a number of poles = 32. Supply frequency is 70 Hz. Rotor speed is 112 rpm. Ns = 120×f÷P=120×70÷32 = 262.5 rpm. S=Ns-Nr÷Ns = 262.5-112÷262.5=.573. F2=sf=.573×70=40.1 Hz.