A synchronous machine contains two magnetic-field systems that rotate together in steady operation.

Stator

The stator carries multiphase AC windings. A balanced three-phase supply produces a rotating magnetic field whose mechanical speed is the synchronous speed

$$n_s=\frac{120f}{P},$$

where $f$ is electrical frequency and $P$ the number of poles.

Rotor

The rotor produces its own magnetic field. This field may come from DC excitation supplied to rotor windings or from permanent magnets.

Common rotor geometries include salient-pole rotors with visibly projecting poles and cylindrical rotors with a smoother magnetic surface.

Synchronous operation

In steady synchronous operation, the rotor magnetic field remains at a fixed angular displacement relative to the rotating stator field. The rotor therefore turns at exactly the same mechanical speed as the stator field.

The rotor does not need relative slip to induce its main field, unlike an induction machine.

Excitation

Changing rotor excitation changes the strength of the rotor magnetic field. This changes the internal generated EMF and the electromagnetic interaction between rotor and stator.

The detailed terminal behavior produced by that change depends on whether the machine is operating as a motor or as a generator and is developed in those operating units.