Question

A 38-pole, 3-phase, 80 Hz induction motor is operating at a speed of 12 rpm. The frequency of the rotor current of the motor in Hz is __________

(a) 75.2

(b) 76.1

(c) 79.2

(d) 79.6

The question was posed to me in a national level competition.

This intriguing question originated from Speed Control of Series Motor in section Speed Control of Direct Current Motors & Induction Motors of Electric Drives

Answer 1

Right choice is (b) 76.1

The explanation is: Given a number of poles = 8. Supply frequency is 50 Hz. Rotor speed is 720 rpm. Ns=120×f÷P=120×80÷38 = 252.63 rpm. S=Ns-Nr÷Ns = 252.63-12÷252.63 = .952. F2 = sf = .952×80 = 76.1 Hz.