SOLUTION

The starting torque of the synchronous motor is zero.

Consider the rotor is at rest and the stator field is rotating at the synchronous speed at 100% slip. Suppose the N pole of the rotating field in the stator is approaching at S pole of the rotor then the magnetic force will tend to turn the rotor in the direction opposite to the rotating field When the same N pole of the rotating field has just passed the rotor S pole, the magnet force will tend to pull the rotor in the same direction as the rotating field. These opposite interactions balance out, producing a zero net torque on the rotor.