Synchronous electric motors have dual excitation. Electrical power is supplied to both the field winding and the winding of the armature. When this is done, torque is developed at only one speed, which is the synchronous speed. At any other speed, the average torque is zero.
The synchronous speed is the speed at which the rotor and the field of flow of the rotor are stopped, that is, the two move in synchronism. To better understand this, we must consider that the conventional two-pole synchronous motor has a DC voltage applied to the rotor winding and a three-phase AC voltage. The three-phase voltage produces in the stator a field that rotates with a speed of 60 Hz.
The DC (or zero frequency) currents in the rotor establish a two-pole flux field (like a magnet) that accompanies the stator when the rotor is rotating, but when there is a pair of rotating armature poles and a pair of Rotor poles stationary, the motor does not present torque, that is, does not have the force to rotate. This is the reason why a synchronous electric motor alone has no starting torque, so it needs an auxiliary motor.