Donuts > Fried Eggs
Idle time, especially if the unit is spun with no field, significant temperature changes, certain types of faults can apparently do it.What are the reasons that cause a self excited generator to lose it's residual magnetism?
Many-self excited generators do exactly that. Often there's a small PMG that feeds the exciter stator....Wouldn't it be better to manufacture the alternators with a small permanent magnets on the armature to avoid the excitation loss problem?
Large generators have difficulty self-exciting for some reason, even with good residual they need a flashing circuit. If that fails, you'll see them get to about 40% stator voltage and just hang, I don't know if the AVR design limits them or what.