Have a look at this.
http://www.anotherpower.com/board/index.php/topic,781.0.html There are some circuits I drew as well, which will give you a sense of how it works.... and maybe why it does not.
No-one can be that unlucky, and if you look at the circuits, we can see that the things that may cause you to over extend the AVR will be .
1.. too ,much load for too long with rpm starved. In this case the AVR tries to wind up more amp turns in the rotor... the more amps is the bit that affetcs it.
2. Some shorted turns in the rotor will cause the same problem, If we need to develop X amount of flux, we need Y amps for a set number of turns..... if we loose turns for some reason, internal short for instance, then more current again..... short life in the AVR
3. RPM too low.... in which case the AVR will try to get the volts up, but the speed is a little low, so the average amp turns needs to be higher to achieve the same output volts under your load.... the frequency will suffer, but he emf may be ok in this case.
So I expect there is some operational foible that is causing the AVR to work too hard ( rpm and ;loads for long periods perhaps),... or a nasty little short in the rotor or the stator.... something not terminal, but enough to cause over current in the rotor while it tries to compensate.
Failure mode will depend on if the diodes went first, or the switches went. If the switches go, then probably fail short... so max voltage will hit the brushes, if the bridge fails, then we have no DC emf to drive the rotor...... so I don't know for certain, but thats my guess.
I am not an elec engineer, but am marooned on an island in the middle of nowhere....so we fend for ourselves a bit.
I have rewound them, fiddled the AVR, built AVR's for temporary use... etc..... but too much coincidence for my money.
Here is another 7k5 unit that had problems with the AVR and stator sense winding I did some time ago. "
http://www.fieldlines.com/index.php/topic,147246.msg1015700.html#msg1015700"
I has to be something causing it.. thats my take anyway.
..........oztules