The Auxiliary Power Unit (APU) has its own AC generator which:
must have the same characteristics as the main AC generator so that it can be easily coupled with the latter
supplies the aircraft with three-phase 115-200 V, 400 Hz AC
is driven at constant speed through a Constant Speed Drive (CSD) in the same way as the main AC generator
is excited by its Generator Control Unit (GCU) as soon as the APU starts up