In a synchronous generator, a prime mover rotates the rotor magnetic field while the stator windings remain stationary. The changing flux linkage of those windings induces a balanced AC voltage.

Frequency, speed and poles

The generated electrical frequency is fixed by rotor speed and pole count:

$$f=\frac{Pn}{120},$$

where $n$ is mechanical speed in revolutions per minute and $P$ is the number of poles.

A generator connected to a fixed-frequency power system must therefore rotate at the corresponding synchronous speed.

Power flow

Mechanical shaft power enters the machine. Electromagnetic torque opposes the driving torque while electrical real power is delivered from the stator terminals.

Increasing real electrical output requires greater mechanical torque from the prime mover.

Excitation

Rotor excitation controls the strength of the rotor magnetic field and therefore influences the internal generated EMF.

When connected to an AC network, changing excitation primarily changes terminal reactive-power exchange and voltage behavior, while mechanical input primarily controls real power.

Three-phase generation

Three stator windings displaced spatially by $120^\circ$ produce three induced voltages displaced by $120^\circ$ in electrical phase.

Synchronous generators are therefore a natural source of balanced three-phase electrical power.