Generator fault finding.

supermalc

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14 Dec 2003
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539
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Lincolnshire.
Any electricians help with this fault finding. Non boaty subject, except it's to do with generators.

A friend has a 240v generator that won't charge. It is the brushless type, and all the usuall checks are ok. He now has a few generator sets he has bought, and now work, but he can't understand what to do next. There is approx 1v energising vault, but he has added a battery, which usually is enough to start them, but nothing.

I have said I will look, but can't readily find any information on brushless alternators.

Any help appreciated.
 

William_H

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28 Jul 2003
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13,500
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West Australia
Don't know the generator of course however most faults on small generators come from broken wires relay contacts and other crital metal pieces which fatigue with vibration so the first fault finding is to look hard at everything.
The brushless alternator starts with a small exciter alternator on the end of the shaft where the outside field coils are energised by a voltage (current) coming via the regulator from the output 240VAC. The rotating armature has rectifier diodes in the rotating part which feed their generated currrent to the main rotating field coil. The 240V AC is picked up off the outer coils of the main alternator. Now on start up presuming there is no battery in the system the exciter generator will rely on some residual magnetism to get some output to put to the main alternator to get an AC output which can then feed the regulated field coil. Obviously if nothing works it is all so interactive it is hard to know where the fault lies. A battery connected to the field outer coils of the exciter may give you a wild high AC output. If this is so then your problem lies in the rectifiers and regulator from the 240VAC side to the exciter. If it doesn't your problem lies in the coild and rectifier of the 2 rotating coils or in the connection from the pick up coils on the main alternator to the output socket. So an easy initial check with an ohmmeter (multimeter) should show quite low resistance (less than 10 ohms) from the active to the neutral connection on the output socket. If this is not apparent then look for a fuse or circuit breaker. Hope this helps will
 

supermalc

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14 Dec 2003
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Location
Lincolnshire.
Many thanks for your reply. I should see him later today, so will tell him.

He is good with alternators, and has got all the other ones going. He gets them from sales, when they come up, mostly it is the engine that won't go. I think it is just because he has not worked out quite how the brushless ones work, so this should help a lot.

As I had old cars, it was not until 1990 that I had a car with one on, and the easiest way when it didn't work, was to fit another second hand one, so I've never repaired one....but as long as the windings are intact, new bearings and diodes are usually all they need to recondition I think.
 

gorb

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9 Dec 2004
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Location
Troon, Scotland
Good advice from William_H. I used to work with electronic controls for big generators (5KVA up to about 5MVA) so know a little.

If this is a generator giving above about 5KVA it probably has an AVR (Automatic Voltage Regulator). This will be a board of electronics and could be duff. Best test (as suggested by Willaim_H) is to touch a DC source (a 12V battery is OK) briefly onto the generator field terminals (observe polarity!) while the generator is running. This may correct the whole thing if the residual magnetism which normally gets it going has disappeared. If output voltage collapses again, then it could be an AVR fault, or the rotating diodes, or the windings.

Good luck!
 
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