The rotor windings automatically switch from AC to DC at synchronous speed.

Slip becomes negative once synchronous speed is reached, which increases torque. 

The rotor must reach synchronous speed so the stator current can reverse direction.

The rotor always runs faster than synchronous speed to generate additional torque.

If the rotor reaches synchronous speed, there will be no relative motion between the rotor and the rotating magnetic field.

With the rotor operating at synchronous speed, no EMF is induced in the rotor conductors, and hence no torque is produced.