Originally Posted by
gearlever
Modern AC generators with field coils are usually self-excited.
Not so much on aircraft. This is true for automotive alternators. But the inability to disconnect the excitation (field) would leave a generator continually feeding into a short circuit. And for a generator capable of producing hundreds of kilowatts, that's a major fire hazard.
The overall system might be considered to be self excited due to the PMG (permanent magnet generator) source. But the output of that is routed through the GCU (generator control unit) which provides field control for voltage regulation of the main generator and a relay to disconnect it for fault clearing. The PMG itself is a low energy generator and designed so that if it shorts internally, the energy dissipated is tolerable.