Self-excitation

An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation.

For a machine using field coils, which is most large generators, the field current must be supplied, otherwise the generator will be useless. Thus it is important to have a reliable supply. Although the output of a generator can be used once it starts up, it is also critical to be able to start the generators reliably. In any case, it is important to be able to control the field since this will maintain the system voltage.

Except for permanent magnet generators, a generator produces output voltage proportional to the magnetic field, which is proportional to the excitation current; if there is no excitation current there is zero voltage. A small amount of (electric) power may control a large amount of power. This principle is very useful for voltage control: if the system voltage is low, excitation can be increased; if the system voltage is high, excitation can be decreased. A synchronous condenser operates on the same principle, but there is no "prime mover" power input; however, the "flywheel effect" means that it can send or receive power over short periods of time. To avoid damage to the machine through erratic current changes, a ramp generator is often used.A generator can thus be considered as an amplifier:

For large, or older, generators, it is usual for a separate exciter dynamo to be operated in conjunction with the main power generator. This is a small permanent-magnet or battery-excited dynamo that produces the field current for the larger generator.

Modern generators with field coils are self-excited, where some of the power output from the rotor is used to power the field coils. The rotor iron retains a magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field creates a weak voltage in the stator coils, which in turn increases the field current, until the machine "builds up" to full voltage.

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Wikipedia