Author Topic: 3 Phase Brushed Generator AVR Problem  (Read 6517 times)

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dimuthudw

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3 Phase Brushed Generator AVR Problem
« on: August 17, 2014, 06:31:25 AM »
Hello,
I have a very old 35kVA generator, but unfortunately its AVR was broken. So I purchased a brushed AVR (model number: STL-F-1) online. The supplier sent me the wiring diagram also, which I have attache here, but when I connected it like that the generator still did not produce any output.
Basically I measure the DC voltage that went to the exciter but it was zero. Tried changing the "volt" and "stab" resistors but still nothing happened. I contacted the seller and he says the circuit diagram he has provided is correct and it should work. Is there something I'm doing wrong here or am I missing something. Any help on this would be greatly appreciated.

Thank you

Flux

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Re: 3 Phase Brushed Generator AVR Problem
« Reply #1 on: August 17, 2014, 09:43:26 AM »
You will be quite lucky to fit another AVR to a machine at random. If it is the same scheme and has similar field requirements you may get away with it. Without a lot more information of the alternator and the AVR it is difficult to help.

You mention exciter, so is F+ and F- the exciter field. Is the main alternator field on the Z connections, Is the main field supplied via brushes, or is it brushless.

What type of exciter is it?, dc with commutator or ac with rectifier.

I find it odd to have the AVR output connected partly to what I assume is the exciter field and partly to what seems to be the main field. What is the diode bridge with one diode not connected.

If it is a very old alternator I would have expected it to have a dc exciter with commutator and brushes which are connected to the main alternator field brushes. If it has an ac exciter with rectifier this is more often 3 phase.

Any information on the alternator would help but details of the AVR are unlikely to be forthcoming.

Flux

tanner0441

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Re: 3 Phase Brushed Generator AVR Problem
« Reply #2 on: August 17, 2014, 04:44:44 PM »
Hi

I puzzled on the diode bridge but if you look carefully at the drawing there is a dot on the lead from the cathode on the left hand pair I think there should be a horizontal line on the drawing connecting the top of the floating diode to the dot.

Brian

dimuthudw

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Re: 3 Phase Brushed Generator AVR Problem
« Reply #3 on: August 17, 2014, 11:18:54 PM »
The exciter is a brushed DC exciter and powered with the Z connections.
I also noticed the dot in the diagram, but when connected like that also it did not produce an output which can be controlled by the AVR.

Flux

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Re: 3 Phase Brushed Generator AVR Problem
« Reply #4 on: August 18, 2014, 06:48:10 AM »
I think most of that diagram relates to the ST alternator.

If you have a dc exciter its dc output brushes will go to the main alternator field brushes.

With its original AVR it may have had the exciter field fed as a shunt winding from the exciter dc brushes, in that case the avr would be a simple regulator controlling that shunt field, the ac line connections would just be for voltage sensing.

The other possibility was that the AVR rectified part of the ac voltage and supplied the exciter field directly, the ac connections then would be voltage sensing and also supplied the exciter field. If it was a fairly accurate AVR there would have been a stabilising signal taken from the main alternator field brushes.

To attempt to use the AVR you have, you need to feed its output to the exciter field, from your comment it may be ZZ but it will not be the main alternator field.  ( It will only be one field and no diodes are involved). You will have to flash the field with a battery to get the polarity correct.

If it works you will probably find that it won't stabilise, the other possibility is that you don't have enough residual for it to build up without flashing each time.

If it doesn't stabilise you may be able to use the AVR to feed the main alternator field without the exciter, but I suspect the AVR will not be man enough for a 35kVA alternator main field.

The more likely solution would be to use a Brushless AVR which would have the stabilising network for the exciter and main alternator time constants. The brushless alternator is basically what you have except that the dc commutator and brushes and the main field brushes are replaced by a rotating rectifier, the AVR will not know the difference.

As the main field is not available for stabilising in a brushless machine the stabilising is fudged within the AVR and will still work via a dc exciter.

Flux

dimuthudw

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Re: 3 Phase Brushed Generator AVR Problem
« Reply #5 on: August 20, 2014, 11:29:12 PM »
Thanks for the all inputs Flux, might well as try the brushless one and see what happens.