Why slip ring induction motor is not started with its slip ring short circuited?

The rotor of the slip ring induction motor is connected to the external resistances. When the motor is started the total external resistance get added to the rotor winding resistance and the value of the resistances get gradually decreased with the acceleration of the motor and becomes zero when the motor attains its full rated speed.The purpose of adding external resistance to the rotor is to increase the starting torque and to limit the starting current of the motor.

The wound rotor has less resistance as compared to the rotor resistance of the squirrel cage induction motor. The resistance of the wound rotor is increased by adding the external resistance when the motor is started to increase the starting torque and to decrease the starting current.

If the slip rings of slip ring/wound rotor induction motor are shorted at start, a huge current will flow in the stator as the rotor resistance is very low and the breaker will trip with the over current fault. If the breaker does not trip, the electrical power network will experience a heavy stress and the motor is likely to get damaged in this situation.

The flash over in the slip ring is a typical problem that occurs with slip ring motors. The slip ring must be cleaned periodically by blowing the carbon particles in the slip ring chamber. At the time of starting, the maximum rotor voltage is produced at the slip rings and any carbon or dust particles, moisture at the slip rings may cause insulation breakdown.

If slip ring motor is to be operated in the polluted environment, the slip ring must be cleaned periodically to maintain the creepage distance.

In a nutshell, a slip ring induction motor can not be started with its slip ring shorted.  

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