DC generator reverses polarity

[wbd]

Hello,
Has anyone seen or heard of this happening? I haven't gotten all the details but here is the main points. A motor driven dc generator is used to provide the excitation to 2 ac generators. Units are running fine until recent trip. Operator could not get unit back on line and calls an electrician to troubleshoot. Electrician tells me that the dc voltmeters are indicating reverse polarity.
Any thoughts on how this could happen or am I being yanked?

 

[Skogsgurra]

If there's a shunt field, it is very well possible. All that is needed is that you have a transient that happens to go - and die out - on the negative side.

Just flash the field in positive direction and you should be fine again.

And.. Er.. Drive carefully!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[wbd]

Thanks Gunnar. I got some more information that one set of brushes was hung up, not making contact. Could this cause a transient to cause the polarity swap?
Could you explain or direct me to a more detailed explaination of what happens and why?

 

[powerjunx]

wbd, the possibility of carbon brush hangs up while recently the unit were running is a loosen brush spring instantly hangs out the brush holder..at this point commutator and brush tip creates a choatic sparks and spurious arcing that might builds up transient or uncontrollable surges to the excitation system.
I'm just wandering why the electrician told you that a dc voltmeter indicates reverse polarity while the unit was never get back on-line..??

"Whenever you are asked if you can do a job, tell them, certainly I can! Then get busy and find out how to do it." Theodore Roosevelt.

 

[wbd]

billybry,
This unit is seperate motor driven dc generator to provide dc to 2 hydroelectric generators, so we able to run it for testing. The dc voltmeters on the board were pegged in the wrong direction and reverse polarity confirmed with a voltmeter.

 

[waross]

I have seen automotive type shunt type DC generators reverse polarized. The battery was connected backwards and when the cutout relay, (reverse power relay) pulled in and connected the generator to the battery there was a brief surge of current and the generator operated normally, but with reversed polarity. The ammeter indication was reversed of course. We reversed the battery connections and the generator corrected itself.
I would expect that a transient or reverse voltage must have enough energy to exceed the hysteresis of the magnetic circuit of the generator so as to be able to reverse the polarity.
respectfully

 

[nbucska]

I think it should be possible to prevent similar failures
in the future -- but can you supply the data of the DC
generator?
Is it shunt field? Can you tell- or measure -- the field
current and VTG, too ? Perhaps a diode could do the trick.




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[waross]

The resistance of the shunt field will be much less than the resistance of the armature circuit. It would take much less energy for a transient to reverse the field with a brush hung up. Depending on the cause and nature of the trip, a transient back fed from the main alternator is probably the cause.
A diode would work as long as the transient did not exceed the voltage rating, (or current rating, if it was a transient in the opposite direction.) During the event you may have had a low frequency AC fed back to the exciter generator from loss of load and resulting speed variations. A pole slip would probably give a much more aggressive transient.
Rather than add diodes I would repair the brush holders and spend my energy on preventive maintenance.
That said, I would not say that diode protection is wrong.
My point of view.
Your choice.
respectfully