Question

A 16-pole, 3-phase, 60 Hz induction motor is operating at a speed of 150 rpm. The frequency of the rotor current of the motor in Hz is __________

(a) 20

(b) 40

(c) 30

(d) 10

I have been asked this question during a job interview.

This key question is from Speed Control of Shunt or Separately Excited Motors topic in section Speed Control of Direct Current Motors & Induction Motors of Electric Drives

Answer 1

Right option is (b) 40

The explanation is: Given a number of poles = 16. Supply frequency is 60 Hz. Rotor speed is 150 rpm. Ns=120×f÷P=120×60÷16 = 450 rpm. S=Ns-Nr÷Ns=450-150÷450=.666. F2=sf=.666×60=40 Hz.