Question

4.6. A 460-V, 75 kW, 4-pole, delta-connected, 60-Hz, three- phase induction motor has a full-load slip of 5 percent, an efficiency of 92 percent and a power-factor of 0.87 lagging at full-load. At start-up, the motor develops 1.9 times the full load developed torque but draws 7.5 times the rated current at the rated voltage. This motor is to be started with an auto- transformer reduced-voltage starter. Assume the stator resistance and the stator core loss to be negligible. Calculate (a) the output voltage of the starter circuit required to reduce the starting torque until it is equal to the full-load developed torque of the motor, (13.0 marks) (b) the referred rotor resistance and the referred equivalent reactance of the motor, (5.0 marks) (c) the motor starting current and the current drawn from the supply with the voltage calculated in (a) (6.0 marks)

Answer 1

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