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A 3-phase induction motor runs at almost 1000 rpm at no load and 950 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) 30 revolution per minute(b) 40 revolution per minute(c) 60 revolution per minute(d) 50 revolution per minuteThe question was posed to me in final exam.My question is based upon DC Motors topic in division Characteristics of DC & AC Motors of Electric Drives

Answer»

The correct answer is (d) 50 revolution PER minute

The explanation: Supply frequency=50 HZ. No-load speed of motor = 1000 rpm. The full load speed of motor=950 rpm. Since the no-load speed of the motor is almost 1000 rpm, hence SYNCHRONOUS speed near to 1000 rpm. Speed of ROTOR field=1000 rpm. Speed of rotor field with respect to rotor = 1000-950 = 50 rpm.



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