Question

A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. The number of poles of this motor is?

(a) 4

(b) 6

(c) 5

(d) 2

The question was asked during an interview for a job.

This interesting question is from Rotor Frequency in section Induction Machines of Electrical Machines

Answer 1

Correct choice is (a) 4

Easy explanation: P = 120f/N = 120*50/1440 = 4(1/6). The number of the poles should be whole number and must be even. So number of poles must be 4.