Synchronous generator working principle

A synchronous generator (alternator) works on Faraday's law of electromagnetic induction: an emf is induced in a conductor when there is relative motion between the conductor and a magnetic field.

Working:

1. The rotor carries a field winding supplied with DC through slip rings, which creates alternate N and S poles.
2. A prime mover (turbine or engine) rotates the rotor at constant synchronous speed $N_s$.
3. The rotating field flux cuts the stationary three-phase armature conductors in the stator slots, inducing an alternating emf in each phase.
4. The three phase windings are placed $120^\circ$ apart in space, so the emfs are $120^\circ$ apart in time, giving a three-phase output.
5. The frequency depends on the speed and the number of poles:

$$f=\frac{PN_s}{120}$$

For 50 Hz and 4 poles, the speed is 1500 rpm. The rms emf per phase is $E=4.44f\Phi T_{ph}K_w$. Because the rotor speed is locked to the frequency, the machine is called synchronous.