Generator problems

I'm putting a 3 phase generator to work for the workshop. It's 13.5KVA so not too big. It's from the 1960's but looks in good condition. The
previous user says it worked his shop machines. I know him and accept that. Well I ran it on load here today for the first time. A simple 4 core cable feeding 3, 2KW heaters, one on each phase to neutral. (I know they werent earthed). At first it all worked beautifully. The 3 ammeters and voltmeter all read just what I wanted. I increased one heater to 3kW which is the max I could achieve. I know its an unbalanced load but it should have been OK. I left it running for about 20 mins to warm her up and blow a few cobwebs away. When I checked again, both volts and amps had dropped by about 50%. I turned all the elecs off and checked for hot joints etc but everything seemed OK. Theres now no output voltage, even with the load disconnected.
By now it was dark so I gave up. Tomorrow I shall have another look. The alternator has 3 wires out, one for each phase. I assume the other is a neutral and have earthed it to a ground spike. As the output fell slowly and equally over all 3 phases, I dont think its a winding problem.
Anyone any idea where I should start? Can alternators lose their excitation?
John
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"John" wrote

Its most likely not to be anything related to the main windings, or that you had an unbalanced load or that the neutral wasn't earthed.
I don't know if this applies to a 13.5kVA alternator, but certainly on larger alternators of the 25kVA+ size I would normally expect to see separate windings for the excitation which can include some rectifiers which are to be found actually on the rotor. Its possible that an excitation winding could be open or one or more diodes could have gone but this would be my last line of attack.
The two things I would first check is that the AVR (Automatic Voltage Regulator) is getting the requisite excitation from the alternator and then check the AVR before going any further.
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Thanks for the help. It does look like theres a problem in that area.
I've opened up the end of the alternator. There are 4 slip rings. All the brushes look good, are free to move with good spring pressure. Theres what looks like a 9 diode full wave rectifier bridge. Im reluctant to test it as that means disconnecting old crimped wires that look rather fragile. Theres 4 cans that I assume are capacitors and a couple of wire wound resistors in series with an adjustable pickup clamp. There are 6 coils in 2 sets of 3 each. Theyre not mounted near the rotor so I assume theyre transformers. One set has thick wire; the other has very fine wire. That set looks like its got hot at some time.
Oh yes, I also opened up the alternator terminal cover. There are 7 terminals. The 3 phases and neutral I can understand. I dont have a megger but a mutimeter says insulation and continuity are OK. The other 3 arent used and Ive no idea what they do.
Im still looking at the problem but suspect I may need professional help from a rewind company.
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"John" wrote

Forget my previous post - I was thinking of a brushless alternator which yours is definitely not. That's what happens when you try to reply to a posting at 6am!
Are all phases the same - i.e. zero output?
Even without excitation an alternator should provide some output voltage. If there is nothing then something is either open or short circuit.
If the windings appear too have continuity then I would check the insulation with a reasonable voltage.
Elf 'n Safety would hit the roof over what I am about to suggest and it would need some care in use. If you have an isolation transformer use it.
A very simple and crude insulation tester can be made from a low wattage 240V bulb (the old fashioned filament type) in series with the mains. Disconnect the neutral star point on the alternator from ground, and put the mains neutral to the alternator frame. For safety make sure the frame is grounded. Connect the bulb to the alternator's neutral star point. If the bulb doesn't light then the insulation is most probably OK.
If you're at all concerned about doing what I've suggested, which is potentially dangerous - then don't. Beg, borrow or steal a megger.
A similar technique can be used for the excitation winding - but be very careful - from what I remember, the excitation voltage for the alternators I've dealt with in the past was something in the order of 50V, so if you can find a step down transformer then use it.
If it passes the insulation test, then I would start looking at the diodes, resistors and associated components. I don't know what the cans are - my guess is probably not capacitors. The output from a full wave three phase bridge is actually a reasonably smooth DC supply, and you wouldn't normally need capacitors to provide additional smoothing when used for excitation.
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Gentlemen,
Is John talking to himself and loosing the plot or is there another John because all I can see is posts from John.
Martin P

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"campingstoveman" wrote

There are two of us. John and John. Good init?
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Shouldn't that be Aywee?
John
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SNIP
If you do get your hands on a megger beware of using it on higher voltage test ranges. Diodes along with most semiconductors have a high death rate from applying high voltage tests. A 3 range model will probably have 250, 500, and 1000 volt ranges. For a 240 volt phase to neutral circuit stick to the lower range as the higher ones might pop the diodes.
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Thanks John.
When I said zero output I was reading the rather crude meter on the gen set and it starts at about 100V. I tested it with a DMM and there's about 25V on 2 phases and about 35V on the other. However, I'm working on memory here but one was definately slightly higher than the other two.
I fairly certain there's capacitors somewhere in the system. When I do a test with the DMM, I get a reading that increases to >2M ohms over a couple of seconds. Being a digital meter, it's not easy to read as the numbers climb so I've no real feel for it. I've looked at the cans again. One side of each is connected to the frame.
I know what you mean about the insulation test, it's a good idea though I suspect my RCD will trip if I ground the neutral and apply any load. The machine's outside and it's too dark now for playing with mains like that. I can probably borrow a megger from work tomorrow. It's a lot safer as well.
I've been looking the 6 coils. Could the 3 with thinner wire be a LV output? Battery charging perhaps? Would that explain the 3 unused terminals in the alternator?
BTW Martin, I NEVER talk to myself. Occasionally I'm caught thinking aloud. Someone who does that is a highly intelligent superior being. Folks who talk to themselves are plain mad.
Anyway it's late now and my head is beginning to hurt. G'night all.
John M
John
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"John" wrote

To me that would indicate that the windings are probably OK and I would start to look at the diodes, resistors etc which comprise of the excitation circuit. 25V is certainly enough to kick start the voltage build up when the machine starts to turn.
BTW, I know you said in a previous post that the slip ring brushes appear to be OK - but it may be worthwhile having another look just in case.

I assume that the output from the capacitors goes to the slip rings and the excitation winding, and this load should be something in the region of, at a guess, perhaps a hundred ohms, and certainly not as high as 2M ohms.
Have you checked the resistance of the excitation winding?
At this point I'm feeling that I really need to see the machine myself. I've got a picture in my mind of what I think your alternator should be which is probably nothing like it really is, and I could be leading you up the creek.
Nevertheless I would still start by looking at the diodes and other components of the excitation circuit.

Dunno. They could be part of a battery charging circuit, but then they could have been used for something else.
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Well I've been fiddling again and there's a set of 3 transformer coils that look decidedly dark. Since they're fairly easy to remove, I shall have a go at repairing them. It will have to wait until the weekend though. It's too dark to see what I'm doing in the evenings. I will let you know how things go.
John
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If you start the Genny you will be able to see what your doing with lights on, Oh sorry your Genny don't work :-))
Martin P

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