Question

A 3-phase induction motor runs at almost 50 rpm at no load and 25 rpm at full load when supplied with power from a 50 Hz, 3-phase supply. What is the corresponding speed of the rotor field with respect to the rotor?

(a) 25 revolution per minute

(b) 20 revolution per minute

(c) 10 revolution per minute

(d) 30 revolution per minute

This question was addressed to me at a job interview.

The origin of the question is Starting topic in portion Starting of Electric Drives

Answer 1

Correct option is (a) 25 revolution per minute

To elaborate: Supply frequency=50 Hz. No-load speed of motor= 50 rpm. The full load speed of motor=25 rpm. Since the no-load speed of the motor is almost 50 rpm, hence synchronous speed near to 50 rpm. Speed of rotor field=50 rpm. Speed of rotor field with respect to rotor = 50-25 = 25 rpm.