Question

A 3-phase induction motor runs at almost 140 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) 80 revolution per minute

(c) 90 revolution per minute

(d) 70 revolution per minute

The question was asked during an interview for a job.

My question is taken from Methods of Starting Electric Motors topic in section Starting of Electric Drives

Answer 1

The correct answer is (c) 90 revolution per minute

Easiest explanation: Supply frequency=50 Hz. No-load speed of motor= 140 rpm. The full load speed of motor=50 rpm. Since the no-load speed of the motor is almost 140 rpm, hence synchronous speed near to 140 rpm. Speed of rotor field=140 rpm. Speed of rotor field with respect to rotor = 140-50 = 90 rpm.