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alternator residual magnetism lose

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Hi

What are the reasons that cause a self excited generator to lose it's residual magnetism ??





PS

wouldn't it be better to manufacture the alternators with a small permanent magnets on the armature to avoid the excitation loss problem ?
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What are the reasons that cause a self excited generator to lose it's residual magnetism?
Idle time, especially if the unit is spun with no field, significant temperature changes, certain types of faults can apparently do it.
...Wouldn't it be better to manufacture the alternators with a small permanent magnets on the armature to avoid the excitation loss problem?
Many-self excited generators do exactly that. Often there's a small PMG that feeds the exciter stator.

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.
 
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