A 50-hp, 550-V shunt motor has an armatureresistance, including brushes, of 0.36Ω . Whenoperating at rated load and speed, the armature takes75 A. What resistance should be inserted in thearmature circuit to obtain a 20 percent speedreduction when the motor is developing 70 percent ofrated torque? Assume that there is no flux change.

A 50-hp, 550-V shunt motor has an armatureresistance, including brushes, of 0.36Ω . Whenoperating at rated load and speed, the armature takes75 A. What resistance should be inserted in thearmature circuit to obtain a 20 percent speedreduction when the motor is developing 70 percent ofrated torque? Assume that there is no flux change.

Delmar's Standard Textbook Of Electricity

7th Edition

ISBN:9781337900348

Author:Stephen L. Herman

Publisher:Stephen L. Herman

Chapter32: Three-phase Motors

Section: Chapter Questions

Problem 6RQ: Name three factors that determine the torque produced by an induction motor.

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Concept explainers

Synchronous Motor

The synchronous motor is an AC electric type motor where at a steady-state the shaft rotation is synchronized with the supply current variable-frequency. To an integral number of AC cycles, the rotation period is exactly equal. The stator of the motor in the synchronous motor contains multi-phase AC electromagnets due to which a magnetic field is created that rotates in time with the oscillation of the line current. The rotor contains a permanent magnet and the rotor rotates in synchronization with the stator field at the same rate and provides the second synchronized magnetic field which is rotating in nature as a result. When the synchronous motor is supplied with independently excited multi-phase AC electromagnets on both the rotor and the stator, it is termed as a doubly-fed machine. The most widely used AC-type motors are the synchronous AC motor and the induction AC motors.

Question

A 50-hp, 550-V shunt motor has an armature
resistance, including brushes, of 0.36Ω . When
operating at rated load and speed, the armature takes
75 A. What resistance should be inserted in the
armature circuit to obtain a 20 percent speed
reduction when the motor is developing 70 percent of
rated torque? Assume that there is no flux change.

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