DC commutation

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joshgarcia

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I have a DC generator on the end of an large generator that is used for excitation purposes. The brushes were arcing badly so the unit was taken down. After performing an inspection it appeared that most of the arcing was on two commutator bars next to each other and exactly 180 degrees to the opposite side there were two joining bars with the same appearance. Is it safe to say that maybe the neutral plane is out and needs adjusted?

Thanks a lot
 
joshgarcia said:
I have a DC generator on the end of an large generator that is used for excitation purposes. The brushes were arcing badly so the unit was taken down. After performing an inspection it appeared that most of the arcing was on two commutator bars next to each other and exactly 180 degrees to the opposite side there were two joining bars with the same appearance. Is it safe to say that maybe the neutral plane is out and needs adjusted?

Thanks a lot
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I think a bad commutator bar or connection to it more probable than a shift in brush position from the magnetic neutral axis.
 
did you try a sand stick to clean the com and a blade to undercut the mica segments?

You might try turning it on a lath. if you clean the copper and clear the mica insulation and still get a ring its probably a bad armature.
 
Does the commutator look like the attached graphic:

Pitch Bar Burning
Results in commutator bars being eroded in a pattern related to one-half the number of brush arms, progressing into a pattern equal to the number of brush arms.
This condition is caused by a cyclic mechanical or electrical disturbance such as:
unbalanced armature
misaligned shafts
bent shaft
bad bearings
weak foundation
foiled equalizers
poor riser connection
If not corrected, this condition will result in a flashover.
 
Sorry guys I should have mentioned earlier that this is a reaccuring problem. The first time we grooved the mica, cleaned the com, and shaped the brushes. The second time we replaced the armature and commutator as well as the brushes. Now we are looking at the same issues all of which seem to arise about every 6 months or so. I do not think it could be a bad field coil or interpole because each time it seems to run for awhile. I am new to the plant but was told by the operators that this has been an issue with this particular generator for YEARS.

Thanks
 
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