On a brushed style generator, the increase in AC voltage from the is fed to the automatic voltage regulator, converted to DC voltage and fed into the rotor.

On a brushed style generator, the increase in AC voltage from the slip rings is fed to the automatic voltage regulator, converted to DC voltage and fed into the rotor.

Explanation:

In an AC electrical generator, in order to maintain a fixed output, the output voltage  is taken from the slip rings to the brushes (some generators don't have brushes) and converted to DC voltage (since it is an AC generator) and fed back to the rotor.

The rotor winding rotate inside a stationary magnetic field known as stater which induces electrical current in the winding. These winding are connected to slip rings which transfers the current to the outside circuit.

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