This is one brushless 3-stage method - there are others.

1st stage. - pilot exiter

rotor = permanent magnet
stator = static coils

Rotate the rotor and AC power is induced in stator coils.
Output from stator coils is fed to second stage within the
stator.

2nd stage. - main exciter

stator = coils fed from 1st stage
rotor = coils

Output from 1st stage stator is rectified and fed to second
stage stator coils - regulation can be imposed here.
Rotate 2nd stage coils in 2nd stage magnetic field
- AC power is produced in rotor and output is fed to 3rd stage.

3rd stage. - output

rotor = coils fed from 2nd stage.
stator = coils

Output from 2nd stage rotor is rectified within the rotor body
and fed to 3rd stage rotor coils.
Rotating electromagnets cause induction of output in 3rd stage
stator coils



That's most of what I found online, this is different though, it has
only one stage, unless other parts are cleverly buried within the
armature. It looks like a giant two pole DC motor armature except it has
no commutation, the free ends of the winding connect to each other
through a diode which is mounted to the rotating assembly. The only
external component is a single capacitor.