Question

A center ammeter connected to the rotor end circuit of a 6-pole,50 Hz, induction motor makes 45 complete oscillations in a minute. Then the rotor speed and the speed of stator field w.r.t. rotor is?

(a) 985, 15

(b) 970, 1000

(c) 985, 985

(d) 985, 970

This question was posed to me at a job interview.

I want to ask this question from Starting of Polyphase Induction Motors-II topic in section Starting of Induction Motors of Electrical Machines

Answer 1

Right option is (a) 985, 15

Explanation: Rotor frequency  = s*f1 = 45/60 = 0.75 Hz.

                                s = 0.75/50 = 0.015

        rotor speed = 1000(1-0.015) = 985 rpm.

   Speed of stator field w.r.t. rotor = Ns-Nr = 1000-985=15 rpm.