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3k Chinese alternator just stopped mid run

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tyssniffen:
Thanks for all the help. Do I need to do that reading when the thing is spinning? Or will it always have that resistance?

ajaffa1:
No,  do it stationary. the resistance will be the same. I would expect to find resistances of between 5 and 20 ohm. This will confirm if the windings are still intact.

Bob

ajaffa1:
Having thought about your problem It seems to me that you have suffered a catastrophic failure of the bridge rectifier, unusual but not unheard of. This caused a short circuit through the rectifier and burned out one wire.
Having replaced the bridge rectifier it generated power for a short time before failing again. I suspect that the burned wire was badly over heated and has failed. There are two short wires that connect AC voltage to the rectifier, replace them both as they have been overheated. Do not be tempted to replace them with lager wire, as a repeat failure with larger wires could well burn out the windings in the generator head. You might consider fitting an inline fuse in the circuit maybe 1 amp.
Let us know how you go.

Bob

tyssniffen:
This is some high quality advice, thanks!

more details to see if this matches your thought:
- this ST 3k is only 7 years old; thus it doesn't have more than 700 hours on it. 

- I'm sure my habits are harder on this machine than ideal, but the thing has been inside it's shed and bolted down it's whole life. I spill enough diesel around that I don't think there's mice doing anything horrible in there either.

- when I noticed the loose wire, the attachment/terminal, which I believe was a little square with a hole, with the screw holding it down was just ... disintegrated.  it looked like failed solder, or aluminum foil flakes.  I have now reconnected it (as I said before) and it doesn't seem bad... just went and yanked on it, I can't see anything wrong with it. 

Are you suggesting that both that wire and the rectifier are bad?    Either of those would be a great problem over the windings obviously. 

WHY does something like this happen?

For some reason, I had that other rectifier - that other one I put in - but I don't know if it was a good spare or one I'd replaced.  If it was 'new' (sat on the shelf for 7 years) would it have failed like that so quickly?   

and, if your theory is correct, you would advise:
1. get another rectifier
2. get another wire where that one failed, put a 1amp fuse inline in it.






 

ajaffa1:
Yes, I would recommend getting a new bridge rectifier, better still buy two so you have a spare, they are cheap. As for the wiring I haven`t seen it so I was just speculating on the worst case scenario. Some ST heads are wired with aluminum rather than copper ( it`s cheaper), while aluminum is a great conductor it does corrode and cause problems.
The problems with rodents, mud wasps and ants are global problems and we suffer just as much in Australia as elsewhere, vigilance and regular maintenance are the only solutions.
Why do rectifiers break down? Rectifiers get hot when in use, they should have a large heatsink to dissipate that heat. The heating/cooling cycle causes microscopic cracks that eventually lead to failure. If you buy a new TV plug it in and leave it on it is probably good for 10 years, the more often you turn it on and off the quicker it will fail.
I am guessing at the value of the inline fuse required, without knowing the resistance of the field winding I can`t do the calculations. Based on Ohms law: V=IxR  where V is volts, I is current in Amps and R is the resistance of the field windings in ohms. So dividing the voltage by the resistance should give you the required fuse amperage.
Silicon rectifiers have an almost unlimited shelf life, if it was one you swapped out because it was breaking down or was of poor quality  it could fail very quickly.

Let us know how you go

Bob



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