Generators generate electricity. Some generate direct currect(d.c.) and some, called alternators, generate alternating current(a.c.). Both types of generator take rotational energy and turn it into electricity.

Generators work on the principle that when a coil of wire is moved in relation to a magnetic field, an electromotive force (e.m.f.) is induced in the coil. As the name suggests, an e.m.f. is a force that tries to make electrons move. If the coil is part of a complete circuit then the electrons will be able to move, hence there will be electricity and current in the circuit.

Basics of Generators

Generators consist of a rotor (which spins!) and a stator which is fixed. In most a.c. generators, the stator is the coil of wire and the rotor provides the magnetic field. In d.c. generators and some a.c. generators, the coil in on the rotor and the magnet is stationary. The rotor is turned by some external force such as a shaft powered by turbines.

When the coil of wire is the stator, the rest of the circuit can simply be attached to the ends. When the coil of wire is the rotor, attaching the rest of the circuit becomes more complicated. The ends can not be attached to anything outside the generator because then the rotor would not be able to turn. The connection must be made using either slip rings for a.c. generators or commutators for d.c. generators.

Slip Rings

In slip rings, the two ends of the coil are connected rings that rotate with the rotor. The two ends of the circuit are attached to 'brushes' each touching a rotating ring. The result is an alternating current in the circuit.

Commutators

In commutators, the two ends of the coil are connected to two halves of a ring. The half rings both rotate with the rotor. The two ends of the circuit are attached to 'brushes' at opposite sides of the rotor. As the rotor turns, the brushes swap between contact with one half ring and the other and never touch the same half ring at the same time. The result is a direct current in the circuit.

Industrial Generators

Industrial a.c. Generators are powered by a shaft generally attached to one or more turbines. Most have stationary coils and rotating magnets

Of course its not really quite that simple so here are some of the more brain boggling bits - you have been warned!

Three Phase Current

In general, industrial generators have three separate coils evenly spaced around each rotating magnet. The alternating current induced in each coil has the same frequency, maximum voltage and maximum current but each will have its peak voltage/current at different times. In technical terms they are 'out of phase' with each other by exactly two-thirds pie radians (or 120°) which means that they each peak one third of a cycle apart.

All three phases are sent out in their own wire and are called red, blue and yellow phases. In domestic situations, each house is assigned one of the phases. A single wire is used to connect all three phases back to the generator.¹. If the power use is the same for each phase then the effect of adding all three phases together in the return wire would be that it would carry effectively no current. (The three phases of altnernating current cancel each other out.) However, it is fairly unlikely that the power use would be equal so there is usually current in the 'neutral' wire. In industry it is sometimes possible to balance the power use of each phase and so remove the need for a neutral wire.

Exciters

As was said earlier, generators contain a spinning magnet which induces current in the surrounding coils. Unfortunately, iron core magnets are not strong enough to induce the currents required so an electromagnet must be used. The current for the electromagnet is induced using a d.c. generator called an exciter (the 'main' exciter)which runs off the same shaft. The magnet for the main exciter also has to be stronger than an iron base one so its electricity is supplied by yet another exciter (the 'pony' exciter).