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Exciter Brush Arcing

Exciter Brush Arcing

Exciter Brush Arcing

(OP)
You guys have been very helpful in pointing me in a direction in the past.  I recieved this letter the other day and wanted to get any ideas that might be out there. Thanks in advance.

Gentlemen,

As you are all aware, we have had some unwanted burning of the Exciter Brushes on SPA #2 Unit. During the last outage, two holders were replaced . While rewiring the Gen winding OT device a few days ago, the crews reported seeing arcing from the Exciter brushes when we shut the unit down for the rewiring process.

Yesterday, we ran several tests on the unit to check its response:

Case #1 - Unit loaded to 50% gate, 2.1 MW. Tripped unit by opening generator BKR. Results:
               Max Gen Voltage went to 3.68 KV, Exciter max voltage to 150.8 VDC.
       Field BKR flashed a little as it opened, no appreciable arcing on the Exciter brushes.

Case #2 - Unit paralleled at SNL. Followed normal SD procedure - Tripped Governor, opened generator BKR,
       Opened Field BKR, C/I resistance on rheostat. Results:
       Max Gen Voltage went to 2.30 KV, Exciter max voltage to 194 VDC.
       Field BKR flashed a little as it opened, about 2 seconds later all brushed started arcing some, but on the two
       brushes in question, heavy arcing and sputtering for 2-3 seconds (looked like rosette fireworks, but stationary)

Case #3 - Unit paralleled at 0.1 MW. Modified SD proceedure - Open Gen BKR, C/I resistance on rheostat,
       opened FLD BKR, then tripped Governor (Note - Unit speed increased from 300 to 340 RPMs). Results:
       Max Gen Voltage went to 2.46 KV, Exciter max voltage to 79.2 VDC.
       No arcing on Field BKR when it opened, no arcing of the Exciter brushes.

There should not be arcing during the normal SD process, but when the governor is tripped, the unit is motoring. Obviously, we can take the unit to SNL, and use the modified SD procedure at least until the cause for the arcing is analyzed. I hope the above information helps. If we need to run some other test scenarios, let me know.
 

RE: Exciter Brush Arcing

When you sudden changes in load current, dc machines will spark at the commutator till the interpoles get their act together. Question is, is there sparking under steady load ? If yes, how vicious ? (since minor sparking under brushes is not a serious concern)

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