Generator Suddenly No Output

[ExpressoLogic]

I have an old 25kva three phase PTO generator with a Newage Stamford C20B alternator. Rarely used, but very useful to run the big mig whenever I have heavy welding to do (I don't have three phase mains). I've been using it on and off today for several hours with no problems, then turned it off as normal (breaker off, tractor to idle, PTO off). When I turned it back on 20 minutes later there was no output. Checking with a multimeter showed about 4VAC phase to phase and 0.5VAC phase to neutral. No smells / smoke / noises, just no output after restarting. All windings look good, no signs of burning etc.

Google would seem to suggest it's probably lost its magnetism, although I'm dubious about that in the space of 20 minutes. Does anyone have any suggestions? Also can anyone identify the field windings in the attached pictures as nothing is clearly labelled? I have checked the fuse for the AVR and cleaned the contacts. Any help will be much appreciated.

 

[Bornfree]

It's probably the avr. You should have 50-70 v ac coming to it and around 12v dc coming from it.

 

[ExpressoLogic]

Thanks for your message @Bornfree , I think based on the below you are correct.

I connected a 12vdc supply across the field terminals (x+ and xx-, red and black at the top of the third picture) (with them disconnected from the AVR). The generator produced approximately 500VAC phase to phase (no load). While the field coil was externally energised I measured just under 200vdc across the field coil terminals of the AVR (I guess it must rely on the load of the field coil to achieve regulation). I presume doing this would also have restored magnetism to the coil (if it had been lost which I doubt).

I then stopped the PTO and reconnected the field coil to the AVR. I'm now back to 4VAC phase to phase and negligible voltage across the field coil. I think this points at the AVR being dead (although I was struggling to work out which pair of terminals was the input supply, so there could still be a question here.

Do you agree that based on the above experiment it looks like the AVR at fault? And if so do I have any options (as the modules are potted and the alternator is likely older than I am so I doubt Newage Stamford (now I believe Cummins Generators) will carry spares anymore)? Or do I now just have a 300kg paperweight?

 

[Robotstar5]

Try contacting Generator Guru for advice on AVR replacement.

 

[Bornfree]

Thanks for your message @Bornfree , I think based on the below you are correct.

I connected a 12vdc supply across the field terminals (x+ and xx-, red and black at the top of the third picture) (with them disconnected from the AVR). The generator produced approximately 500VAC phase to phase (no load). While the field coil was externally energised I measured just under 200vdc across the field coil terminals of the AVR (I guess it must rely on the load of the field coil to achieve regulation). I presume doing this would also have restored magnetism to the coil (if it had been lost which I doubt).

I then stopped the PTO and reconnected the field coil to the AVR. I'm now back to 4VAC phase to phase and negligible voltage across the field coil. I think this points at the AVR being dead (although I was struggling to work out which pair of terminals was the input supply, so there could still be a question here.

Do you agree that based on the above experiment it looks like the AVR at fault? And if so do I have any options (as the modules are potted and the alternator is likely older than I am so I doubt Newage Stamford (now I believe Cummins Generators) will carry spares anymore)? Or do I now just have a 300kg paperweight?

It does point to the avr. I got one for an old Stamford alternator from tarplet generator services.