Question

A 3-phase induction motor runs at almost 70 rpm at no load and 50 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) 20 revolution per minute

(b) 30 revolution per minute

(c) 40 revolution per minute

(d) 50 revolution per minute

I had been asked this question at a job interview.

Origin of the question is Speed Control of Series Motor in division Speed Control of Direct Current Motors & Induction Motors of Electric Drives

Answer 1

The correct answer is (a) 20 revolution per minute

The explanation is: Supply frequency=50 Hz. No-load speed of motor = 70 rpm. The full load speed of motor=50 rpm. Since the no-load speed of the motor is almost 70 rpm, hence synchronous speed near to 70 rpm. Speed of rotor field=70 rpm. Speed of rotor field with respect to rotor=70-50= 20 rpm.