3k st alternator just stopped mid work

[BruceM]

Yes, you got it right.  Both Z wires must be disconnected or cut, and insulated, as they will still have some voltage when running. 

[tyssniffen]

ok, so, did that...

the voltage jumped up to 110 for a second, then went back down to low 60s... the engine acted like it had a HUGE load on it, and the alternator sounds different.   and there was some 'hot electricity' smell.

I only ran it for 20 seconds or so.


I put it back the way it was, and only 3v came out. alternator sounds different.

now what?

[Tom]

Well if I were a bet'in man I'd say that the field windings are shorted.

[tyssniffen]

Well if I were a bet'in man I'd say that the field windings are shorted.

how do I check, and how do I fix?

[BruceM]

+1 for Tom's opinion.  Bad spot on your field windings...  it may be cheaper to replace your ST-3 than to have the field windings replaced.

[tyssniffen]

well, can you tell me this - what might a 'bad spot' look like?  I might just have some wearing out of a wire just inside the case, right?  if that was touching something it shouldn't, that'd be a short, no?

and if I pull the whole axle and look at it, what might a bad spot look like? could one see it with the naked eye, or is it not testable?

Tys

[Tom]

There 2 probable types of shorts it could be. One is a short between the wires in the winding. In this case the resistance of the winding will be low because instead of going through all the loops of wire, the circuit is functionally shorter. The other is  a short to the metal structure of the armature. In this case the resistance between the field winding's and the armature will not be infinite. To find the exact location of either short, you'll probably need to remove the winding's unless you can see the spot that got hot and smelly. So +1 on Bruce's recommendation on a new generator.

[BruceM]

You can pull apart the unit and check the rotor windings visually. By separating the 4 individual coils, you could locate the bad one by ohm meter, which could then be rewound, by unwinding it, counting turns, and then rewinding it and dipping in electrical varnish.  Quite a big job and not one for a novice, I think.

[tyssniffen]

so, what happened when I did the AC outs into the rectifier?

why am I only seeing 2 volts out now?

[dieselgman]

We would hope that you fed your AC output to your AVR voltage sensing circuitry, not directly to your rectifier.
The rectifier, (and in turn the exciter windings), would be fed from the AVR outputs.

dieselgman

[Tom]

What I would guess happened is that the z windings have limited amperage output. When the ac was connected to the rectifier, it's a high amperage output. Enough amps to briefly bring the voltage upto spec and then overheat at the short.

[BruceM]

+1 for Tom's analysis.  Most likely scenario, since you had no over voltage and AVR was briefly able to regulate at 110V before the field windings failed.  The field coils (4) can be disassembled and are not as difficult as the stator windings to rewind, but again, it's not a job for a novice. 

[tyssniffen]

So, go the thing apart today. oy.  what an ugly mess.  It would appear that if what Tom said, that it got up to high amperage and then overheated at the short, the short is in the outside windings, as the only melting I can see is there, on the bottom of the circle.    All the rust and rubbing are another ugly part. 

Looking at this picture, does that assessment seem accurate? does that explain everything else that has been going on?  how "gone" is this alternator?

(figured out how to post sort of decent sized photos here)

[BruceM]

Hard to tell from the photo of your stator wingings but they look burnt in the dark area.  If the windings have the electrical varnish burnt off, then that's the end of that set of windings.  You'll need to have it rewound or replace it, which is likely much cheaper. 

A failing stator winding would also fully account for your symptoms, including what happened when hooking the stator as the excitation source to the AVR.

Given your running environment is a bit corrosive, you might want to consider a better quality generator head for the replacement, one where the windings are sealed/dipped completely, unlike the ST heads.  The best thing about the ST head is the price; they are NOT a high quality alternator.   I use one for my off grid homestead primary AC power, where my typical max load is 2000 watts and my ST-3 has been reliable once I replaced failed bearings, failed diode bridge, added my own design AVR and got rid of the "doghouse".   But I am in Arizona, and my engine and ST-3 are indoors in a dedicated engine shed; corrosion is not an issue.  I also have a spare ST-3 on hand for parts or replacement.  I am using it again on my neighbor's similar setup.

Hopefully Gary of Diesel Electric Service could suggest an alternator that might fair better in your environment.  I'm just a hobbyist; his years and depth of experience/knowledge is awesome.  I'm just guessing about corrosion being a factor in this failure.

[dieselgman]

A one-time overload can cause an insulation failure between your stator windings, that failure can take time and circumstance to finally completely show up as an outright failure though. Your photo appears to show a fairly large area degraded by excess heat. If the bearings or bearing carriers allow the rotor to ever touch the stator, that will also destroy the integrity of the head very quickly. That area requires a specific air gap to be maintained at all times.

The Chinese ST heads are fine for most folks, but I would not consider them for any mission-critical applications for the reasons previously stated. It is not the Chinese manufacturing that is the primary issue here... just the fact that the STs are primarily built at absolute minimum costs. Marathon and other major brands are now made in China as well, and their reliability does not appear to be an issue. We have been using the Chinese Stamford (brushless) copies for some time now and not had any issues. Their quality is readily apparent when inspecting castings/bearings/windings and such.

dieselgman