Commutator Brush Arcing
Commutator Brush Arcing
(OP)
I have a small DC generator (motor driven) that provides field voltage and current to a hydroelectric generator. The brushes started arcing and we changed brushes and seated the brushes, however there is still arcing. Anyone have additional hints for why it's still arcing and what's the fix?
There have been no load changes, it's been running at full load for the past several weeks.
Cheers in Advance
There have been no load changes, it's been running at full load for the past several weeks.
Cheers in Advance
RE: Commutator Brush Arcing
1. Try to shave insulation (maybe mica) between commutator lamellae, if the insulation is higher than the commutator lamella's copper.
2. If 1. is o.k. then there is a higher current being drawn. This may indicate a possible inter-turn shorts and it may need a dc machine repairer to examine/test it.
RE: Commutator Brush Arcing
RE: Commutator Brush Arcing
If you have changed the brushes and seated them as best as your mounts will allow, you need to look very closely at the commutator. There are three main possibilities:
(1) Damage to the surface of the commutator. Check for pitting and/or mechanical wear.
(2) The commutator is "out of round". What I mean by this is that if you were to mount your generator shaft on a lathe, you could determine that the commutator is not exactly rounded any more, but has worn in a slightly (or sometimes not so slightly) elliptical pattern. When this happens the distances can exceed the tolerances of the mounts and springs enough to cause arcing.
(3) A combination of (1) and (2).
Unless the surface damage to the commutator is extremely slight, or extremely severe, the best solution is to turn the commutator on a lathe. This will remove most surface irregularities and pitting, and will bring the commutator back into "true". This will ensure that the brushes are making consistant, constant contact throughout the rotation. All this assumes that the commutator is of a design that CAN be turned: not all are. If not, the shaft assembly will have to be reworked or replaced.
Hope this helps!
RE: Commutator Brush Arcing
RE: Commutator Brush Arcing
http://www.joliet-equipment.com/dc_motors-generators_maintenance_(3).htm
[Good article]
RE: Commutator Brush Arcing
P.S.: electricpete, this is a good site to add to your web page.
RE: Commutator Brush Arcing
The commutator was stoned in place to a good clean surface and then undercut. A dial indicator was placed on it and the runout is only 0.010 (per the website recommended by Huko, max allowable is 0.030). Brushes seated and unit run with 160A @ 125Vdc and no arcing.
However, when unit comes down for annual PM inspection, I intend to send out exciter for inspection, test and cleaning. The commutator is worn about 1/4" from new but it is about 80+ years old.
Thanks for all the help :)
RE: Commutator Brush Arcing
Once the old film is removed a new film must be developed. Burnishing the comm with hard wood will begin the process. While the motor is running apply a film of hydrogen peroxide to the comm with a piece of canvas rolled up and attachted to a stick. This begins the process and allows better operation of the brushes.
All of the above information is good, but usually when a motor is already running good and all you have done is changed out the brushes the film is a good place to look and how well the brushes are seated.
RE: Commutator Brush Arcing
Did you use the RIGHT brushes? NOT the ones that were in there before you changed them, but the ones the manufacturer recommends?
Seen this one too many times to discount it completely.
Don, the guy who still doesn't know what he's doing
RE: Commutator Brush Arcing