why slip rings are used in motors?

by Barely Bates technician

What happens when external resistance is added?

In the case of a squirrel cage induction motor, the rotor resistance is very low so that the current in the rotor is high, which makes its starting torque poor. But adding external resistance, as in the case of a slip ring induction motor, makes the rotor resistance high when starting, thus the rotor current is low and the starting torque is maximum. Also the slip necessary to generate maximum torque is directly proportional to the rotor resistance. In Slip Ring Motors, the rotor resistance is increased by adding external resistance, so the slip is increased. Since the rotor resistance is high, the slip is more, thus it's possible to achieve “pull-out" torque even at low speeds. As the motor reaches its base speed (full rated speed), after the removal of external resistance and under normal running conditions, it behaves in the same way as a squirrel cage induction motor.

Advantages of slip ring induction motors

  • The main advantage of a slip ring induction motor is that its speed can be controlled easily.
  • "Pull-out torque" can be achieved even from zero RPM.
  • It has a high starting torque when compared to squirrel cage induction motor. Approximately 200 - 250% of its full-load torque.
  • A squirrel cage induction motor takes 600% to 700% of the full load current, but a slip ring induction motor takes a very low starting current approximately 250% to 350% of the full load current.

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About Barely Bates Junior   technician

0 connections, 0 recommendations, 10 honor points.
Joined APSense since, October 15th, 2018, From Ahmedabad, India.

Created on Oct 15th 2018 07:34. Viewed 303 times.


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