When generating AC power there needs to be DC supplied to the field winding on the rotor. To do this, a brushless exciter is mounted on the rotor and generates AC on its windings. The AC is then taken to rotating rectifiers, now in the form of DC, to the generator’s armature. When doing this, it’s typically to keep the output voltage of the generator as near as possible to a constant, regardless of the variations of the load. To determine the amount of DC which flows to the main rotor, it’s regulated by altering the DC which is supplied to the field winding on the stator.