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Author Topic: Piggybacked mystery DC generator  (Read 6412 times)
fourfootfarm
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« on: October 02, 2016, 07:12:29 PM »

Hello all,

Lurking ominously in a shed and very seldom started we have an ancient 30kva three phase generator.

Tacked onto the back of the 3 phase is a DC generator. Rated about 87 volts and roughly 8 amps.

Any idea what on earth it would be for?

Thanks!
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Outback FM60. EPsolar 30a MPPT and a bunch of Tristar 45's. Hodge Podge of solar ~ 4500w. Various generators and 1000ah 24v forklift battery.

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fourfootfarm
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« Reply #1 on: October 02, 2016, 07:18:58 PM »

My only thought is maybe it provides power to engerise to coil for big generator?
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guydewdney
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« Reply #2 on: October 02, 2016, 11:29:59 PM »

Welder?
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Fionn
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« Reply #3 on: October 03, 2016, 12:07:48 AM »

FFF you are correct, it's an most likely an excitation current generator, probably a permanent magnet generator of some sort. It's output is probably rectified and passed through the AVR before being fed to the rotor via a pair of slip rings.
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Philip R
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« Reply #4 on: October 03, 2016, 01:00:18 AM »

Yes definitely a PMG for the excitation, input to the AVR, The 87 volt output will counteract the L/R time constant of the rotor field circuit. the 8 amps should easily excite the salient pole machine to provide peak kVA on its generator operating characteristic.

8 Amps is not suitable not for welding,  IMHO, it would hardly punch a hole through the skin of a rice pudding.

Philip R
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Tombo
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« Reply #5 on: October 03, 2016, 05:41:20 AM »

Yes it is for excitation. It suggests the genset is more than 30 years old. 
The system has its benefits and drawbacks.  I know people who have bought old gen sets that didn't work,  where an expensive rewind had been diagnosed. These were brought back to life by "flashing" the excitation coils. ie running them backwards to operate as an electromagnet to recover lost magnetism in the magnets themselves.  Or the more crude method of hitting them hard with a hammer, which I assume puts tiny cracks in the magnets rejuvenating them enough to work for a bit longer.  I don't think either method is text book or desirable, unless you really need to get your generator running .
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eabadger
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« Reply #6 on: October 03, 2016, 08:37:36 AM »

flashing the coil is done by applying fixed dc, normally a battery, the retained magnetism then takes over and allows self excitation, never heard of hitting with a hammer, would think broken magnets would just not work, not better.

yes the piggy back will be the excitor, i have an old ww2 set with a 36v 15a dynamo on top of main alternator, belt driven to the same pully as cooling fan.
on mine the 36v originally went to a carbon pile regulator, i never had much success with that so i replace with a modern avr (Datakom 20a) this removed need for the piggyback dynamo and the carbon pile, worked well until big ends died.
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fourfootfarm
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« Reply #7 on: October 03, 2016, 10:56:09 AM »

This is certainly an old beast. It was made by Meadows of Wolverhampton who got merged in the early 60s.

As far as I know it works fine. It even has a hand crank to start it though I don't fancy that very much. Getting the Petter going is difficult enough.

Thanks for the info all.
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Outback FM60. EPsolar 30a MPPT and a bunch of Tristar 45's. Hodge Podge of solar ~ 4500w. Various generators and 1000ah 24v forklift battery.

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