In a synchronous motor, the stator produces a rotating magnetic field while the rotor carries its own magnetic field. In steady operation, the rotor field remains locked to the stator field and rotates at synchronous speed.

Torque angle

The rotor and stator fields are not generally perfectly aligned while the motor supplies torque. Their angular separation is often called the torque angle or load angle.

Increasing mechanical load increases the angle required to produce enough electromagnetic torque, while the steady rotational speed remains fixed by supply frequency and pole count.

No steady slip

Unlike an induction motor, a synchronous motor does not need a nonzero slip in normal steady operation:

$$n=n_s.$$

A change in load therefore changes the field angle and transient motion rather than establishing a new steady speed slightly below synchronous speed.

Loss of synchronism

There is a maximum sustainable electromagnetic torque for a given operating condition. If the load exceeds this limit, the rotor can no longer remain locked to the rotating stator field and the machine loses synchronism.

Excitation and power factor

For an electrically excited synchronous motor, changing rotor excitation can change reactive-power exchange and stator power factor while the real mechanical load remains essentially the same.

This ability to control reactive behavior is a distinctive feature of synchronous machines.