[john d]

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 ?

 

[Big John]

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.

 

[john d]

Idle time

What exactly happens to the magnetism in this case ? there has to be some sort of effect that is causing the loss when the alternator is not moving.


What about turning the generator off while on load, i have heard of excitation lose after such cases but never experienced one personally ?

 

[dmxtothemax]

Overloading the genny will also cause residual magnatism to be lost.

 

[dmxtothemax]

What exactly happens to the magnetism in this case ? there has to be some sort of effect that is causing the loss when the alternator is not moving.

I would guess that the constant use of the genny in what keeps the residual magnatism going.
No use, means it just fades away.

 

[triden]

Too leading of a power factor