What happens when external resistance is added?
In case of squirrel cage induction motor, the rotor resistance is very low, so that the current in the rotor is so high; which makes its starting torque poor. But adding external resistance in case of a slip ring induction motor, makes the rotor resistance high during starting, thus the rotor current is low, thus 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, thus the slip is increased. Since the rotor resistance is high, the slip is more, thus 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, under normal running conditions, it behaves in the same way as squirrel cage induction motor. 
Thus these motors are best suited for very high inertia loads, which requires a pull-out torque at almost zero speed and accelerate to full speed with minimum current drawn in a very short time period.
Advantages of slip ring induction motors
•The main advantage of slip ring induction motor is that its speed can be controlled easily.
•"Pull-out torque" can be achieved even from zero r.p.m
•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.
What happens if the motor is started as a normal induction motor?
If the slip motor is started with all the slip rings or the rotor terminals shorted, like a normal induction motor, then it suffers extremely high locked rotor current, ranging up to 1400%, accompanied with very low locked rotor torque as low as 60%. So, it is not advised to start a slip ring induction motor with its rotor terminals shorted.
In my next article, lets discuss on various starting methods & speed control of a slip ring induction motors.
•"Pull-out torque" can be achieved even from zero r.p.m
•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.
What happens if the motor is started as a normal induction motor?
If the slip motor is started with all the slip rings or the rotor terminals shorted, like a normal induction motor, then it suffers extremely high locked rotor current, ranging up to 1400%, accompanied with very low locked rotor torque as low as 60%. So, it is not advised to start a slip ring induction motor with its rotor terminals shorted.
In my next article, lets discuss on various starting methods & speed control of a slip ring induction motors.
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