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A 3-phase induction motor runs at almost 1100 rpm at no load and 640 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) 430 revolution per minute(b) 440 revolution per minute(c) 460 revolution per minute(d) 450 revolution per minuteThe question was posed to me in an interview for internship.I'm obligated to ask this question of Effect of Starting on Power Supply, Motor and Load in division Starting of Electric Drives

Answer»

Correct choice is (c) 460 REVOLUTION per minute

Explanation: Supply frequency=50 Hz. No-load SPEED of motor = 1100 rpm. The full load speed of motor=640 rpm. Since the no-load speed of the motor is almost 1100 rpm, hence SYNCHRONOUS speed near to 1100 rpm. Speed of ROTOR FIELD=1100 rpm. Speed of rotor field with respect to rotor = 1100-640 = 460 rpm.



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