Sparking brushes of Synchronous Motors

[JatTiw]

I have two identical 4000hp Synchronous Motors operating reciprocating compressors. The ‘–ve’ terminal brushes on one machine tend to develop spark. The excitation control system (GE-Stat-x-ator) has almost the same volts/amps/pf for each machine, as the loading is identical. Need help to find root-cause for the carbon-brush spark?

 

[electricpete]

First thoughts:
rough slip ring causes bouncing
not enough spring tension
brush face condition

Is it a condition that developed over time?

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[dpc]

Lack of maintenance?

 

[JatTiw]

No mechanical/maintenance type issues. New brushes/slip-rings/holders installed 3-4 weeks back. Enclosed & instrument air for cooling. Each time set of brushes on ‘-ve’ terminal develop spark.

 

[GTstartup]

Change the polarity periodically. Then both sets of brushes will wear more evenly

 

[waross]

Do you have a bad diode in the field supply?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JatTiw]

Like I mentioned in 1st thread, Only one machine has the problem, Other work fine.

 

[edison123]

what epete said + out of roundness of that slipring + slipring loose on the hub

Muthu

http://www.edison.co.in

 

[oldfieldguy]

A former employer used to have problems with some sliprings arcing. The sliprings were exposed to a somewhat corrosive atmosphere and had tiny little surface pitting. They started re-facing the sliprings with a different metal.

Another client had problems with only one brush of the two on one slipring arcing. The problem was that the installer had not installed a jumper to equally share the current between the two brush-holders.

Another problem surfaced when we found that an ambitious electrician had "re-sized some off the shelf brushes from generator exciter service to make them fit the synchronous motor application. After all, it was "all black carbon, right?" Wrong composition=arcing.



old field guy