A typical brushless ac generator as used on commercial aircraft work like this:
Mounted on a single rotating shaft are a permanent magnet (PMG), an exciter coil and a main field electromagnet around which are a PMG stator, and exciter field winding and a main field stator. Rotation of the PMG induces an alternating current in the 3-phase windings of the PMG stator. This alternating current is supplied to the generator control unit (GCU) where it is rectified into direct current. The GCU voltage regulator uses this rectified DC to control the current in the windings of the generator’s exciter field. The magnetic field produced in the exciter field induces a current in the rotating windings of the exciter armature (rotor). This current is converted to DC by the rotating rectifier assembly on the rotor. The resultant current is supplied directly to the windings of the main field to produce a rotating magnetic field. This rotating magnetic field induces an alternating current (AC) in the windings of the main generator stator. This is fed via appropriate contactors the aircraft busbars. In most cases the rotation speed of the armature is controlled by a constant speed drive (CSD) to give a constant frequency (400Hz) but latest design has binned the CSD to save weight so output is "frequency wild" as we used to call it or "variable frequency as they say these days.