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In an AC generator, the exciter is a device that provides the necessary DC current to the field windings of the generator, which is necessary to create the magnetic field required for the generation of AC electricity. The voltage regulator controls this DC current to ensure the generator produces a stable output.

In AC generators, there are generally two types of exciters used:

1. Brushless Exciters: These are commonly used in modern AC generators. They consist of a small AC generator mounted on the same shaft as the main generator. The AC output from this exciter is rectified to DC and used to supply the field windings of the main generator. This setup eliminates the need for brushes and slip rings, reducing maintenance and increasing reliability.

2. Static Exciters: These use power electronics to provide the excitation current. They draw power from the generator's output or an external source, convert it to DC, and supply it to the field windings. Static exciters offer precise control over the excitation current and are often used in large power plants.

These exciters ensure that the generator produces a stable and controlled output by maintaining the necessary magnetic field.