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.
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.