Refer to figure.
Modern brushless AC generators contain an exciter generator mounted on the same shaft as the main generator. The exciter generator consisting of a set of permanent magnets will provide an initial excitation current for the main generator rotating field. When the generator is running, the voltage regulator will send a DC field current to the excitor stator to supplement the magnetic field of the permanent magnets, and allow the generator's output to be controlled. The AC produced in the exciter armature will be converted into DC for the main rotor field supply in order to keep the polarity in the spinning rotor.
This allows the motor to run without the need for brushes, thereby increasing lifespan. It also has a higher power to weight ratio than older methods of AC generation.
There is a further addition that can be made to this system, where a third smaller generator exists on the same shaft, which has a set of spinning permanent magnets and a stationary armature. The current from the armature is rectified to provide power to the generator control systems, and to the stationary field coils of the excitation generator, and effectively takes the permanent magnets from the older style brushless AC generator, and puts them in a different place (a small thrid generator), serving a similar purpose. This can improve efficiency, and these more modern types of AC generators are also called Permanent Magnet Generators.
Whichever type of AC generator that is being asked about, they both use permanent magnets for their intial excitation.
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