Question

A 3-phase induction motor runs at almost 1500 rpm at no load and 900 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) 300 revolution per minute

(b) 400 revolution per minute

(c) 600 revolution per minute

(d) 500 revolution per minute

The question was posed to me in semester exam.

The above asked question is from DC Motors topic in portion Characteristics of DC & AC Motors of Electric Drives

Answer 1

The correct option is (c) 600 revolution per minute

To elaborate: Supply frequency=50 Hz. No-load speed of motor = 1500 rpm. The full load speed of motor=900 rpm. Since the no-load speed of the motor is almost 1500 rpm, hence synchronous speed near to 1500 rpm. Speed of rotor field=1500 rpm. Speed of rotor field with respect to rotor = 1500-900 = 600 rpm.