AvK alternators use two types of excitation systems
The Auxiliary winding system uses the main field to generate power required for the AVR through a separate winding embedded in the main stator winding.Two permanent magnets are installed in the exciter stator for initiation of the excitation.
This PMG method places an additional small permanent magnet generator at the non-drive end of the alternator to produce power for the AVR.
This permanent magnet generator output is isolated from the disturbances on the main output from the alternator and produces a clean, uninterrupted 3 phase waveform as long as the alternator shaft is turning, thereby preventing any disturbances to the input power of the AVR. The PMG provides reliable isolated power input to the AVR where significant voltage waveform distortion may have been a problem in a self-excited system.