Author Topic: 125vdc generator - no voltage  (Read 2085 times)

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rustkolector

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125vdc generator - no voltage
« on: September 25, 2014, 05:10:23 PM »
I have an 85 year old 350 watt 125vdc generator that will not self excite. It has run fine up until it was disassembled for cleaning. The last time it ran the engine quit abruptly with about 50% load on the generator. I was thinking it had lost its residual magnetism. As a generator it is turned clockwise facing the generator. When I got no output, I applied voltage to the leads and the armature turned counter-clockwise even with reversing the leads on the battery. Figuring this was wrong, the brush leads were reversed to get clockwise motoring rotation. Applying 12v to the generator leads when running excites the generator to about 150-160v and it will maintain that voltage when battery is removed, but it will not self excite again after stopping. I must somehow have incorrectly polarized the field winding and messed up rotation, but don't know how to correct it. I can't determine the brush connected lead with the field winding from resistance readings. Both leads have near the same readings but readings are reversed. I thought motoring it in the correct rotation would do it. Guess not. Judging from past voltage/load readings, I would say the generator is compound wound, but that is all I know about it. Any help is appreciated. Thanks.

Jeff

wpowokal

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Re: 125vdc generator - no voltage
« Reply #1 on: October 04, 2014, 02:11:05 AM »
Jeff you seem to have not had any reply's so far, maybe due to the confusing post, PM me and we will try to sort it out before posting again.

regards Allan
A gentleman is man who can disagree without being disagreeable.

Flux

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Re: 125vdc generator - no voltage
« Reply #2 on: October 04, 2014, 03:43:38 AM »
Without more information this is slightly tricky.

Assuming this is a shunt dynamo, it will motor in the direction that it charges. Reversing the supply polarity will not affect the direction of rotation, it will only affect the polarity when it builds up as a generator.

If you dismantled it then it seems almost certain you crossed the field leads. Now you have put that right you seem to be part way back to normal. The most likely reason for not building up after you flashed it and got it running is that you have disturbed the brush bedding in the dismantling process.

Not knowing the state of the commutator it is difficult to comment, it is old and the original brushes could be hard carbon, which is a bit mixed in its properties.

Check that the commutator has not worn down to the point where the mica is level with the copper bars. If the mica is still below the surface, the comm is smooth and without flats on some bars you may be able to just re-bed the brushes with glass paper wrapped round the comm. 

At that age you may have to resort to a skim of the commutator and undercutting the micas, quite a skilled job, if you do it, make sure the commutator is skimmed dead true with the shaft, under cut it carefully, removing all the mica at the sides of the cut and remove any flash from the edges of the slots as this will "flip" the brushes just as badly as any projecting mica.

If it has the original hard carbon brushes and they are well worn you may be better off changing them for modern electro graphite ( these look like black carbon, not shiny graphite).

Now you have got the field connected correctly and made it build up by flashing it will have residual as long as you haven't dismantled it. Try running it up to speed and press on the brushes with bits of wood, quite likely it will build up unless the brush bedding is really bad.

Not seen this post until Allan replied. If you are in trouble after trying this then take up his offer  it may need a bit of backwards and forwards correspondence especially if it does not turn out to be a plain shunt dynamo.

Flux

Flux

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Re: 125vdc generator - no voltage
« Reply #3 on: October 04, 2014, 03:56:28 AM »
I have just noticed your comment that you suspect it is compound wound. This may complicate the issue depending on the level and direction of compounding. With slight compounding it will motor in the direction it generates just as a shunt machine.

If it is heavily compounded the series field may dominate when you motor it and it could motor either way depending whether it is cumulative or differential compounded. The fact that you have got it to keep running after flashing it makes me suspect the compounding is not enough to confuse things, if it is running unloaded you must have the shunt field polarity correct now so my comments should still apply.

Flux

rustkolector

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Re: 125vdc generator - no voltage
« Reply #4 on: December 18, 2014, 12:48:48 PM »
After getting back to this project, I wish I could say what was wrong with it, but I can't. I took it apart again, checked everything mentioned more than once, then flashed the field, and it started working again. No actual repair work needed. I must have screwed up somewhere along the line. I tested it at near its rated 300 watt capacity and the voltage holds well. Have started it many times since, and it always works properly. I hope I work as well when I am as old as this generator. Thanks to all for the help.

Jeff