Question

A 2-pole, 3-phase, 50 Hz induction motor is operating at a speed of 400 rpm. The frequency of the rotor current of the motor in Hz is __________

(a) 43.33

(b) 42.54

(c) 43.11

(d) 41.47

This question was posed to me in final exam.

This is a very interesting question from Effect of Starting on Power Supply, Motor and Load in portion Starting of Electric Drives

Answer 1

Correct answer is (a) 43.33

For explanation: Given a number of poles = 2. Supply frequency is 50 Hz. Rotor speed is 400 rpm. Ns = 120×f÷P=120×50÷2 = 3000 rpm. S=Ns-Nr÷Ns = 3000-400÷3000 = .8666. F2=sf=.8666×50=43.33 Hz.