The rotor of the slip ring induction motor is connected to the external resistances. When the motor is started, the total external resistance gets added to the rotor winding resistance, and the value of the resistances gradually decreases 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 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 decrease the starting current.
If the slip rings of the slip ring/wound rotor induction motor are shorted at the 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 heavy stress, and the motor will likely get damaged.
The flashover in the slip ring is a typical problem with slip ring motors. The slip ring must be cleaned periodically by blowing the carbon particles in the slip ring chamber. When starting, the maximum rotor voltage is produced at the slip rings, and any carbon or dust particles or moisture at the slip rings may cause insulation breakdown. If the slip ring motor is to be operated in a 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 if its slip-ring is shorted.