Commutator wear pattern

[aquilajohn]

I am hoping to find help with understanding ( and perhaps eliminating )a problem with unusual ( to me anyways ) wear on small AC motor cxommutators. The motors in question are rated at 120VAC 0.9A (95 watts) 50/60 cycle 3500RPM. What has me confused is that the commutator pads wear unevenly with every other one becoming pitted and rough. Thus high arcing / heat and the need to clean and resurface the commutator and reseat the brushes. I have checked for runout of the commutator assembly / shaft, brush bounce / tensioning springs / holder tolerences, etc. Then became aware the problem is consistent from unit to unit.

I am at the point that I suspect the design may have something to do with it. Is it possible, by design, that every other pad would be subjected to different forces (electrical) than the alternating pads. The pads that are not quickly becoming rough look perfect. The brushes seat cleanly at first but within 3-5hrs become rough and groved. At that point the pads reveal this every other pad pattern. A smooth polished pad, then a rough one, and so on. The motors application requires low torque loads that are even and consistant over time so current draw remains smooth and even, at below 60% of rating.

Any help offered would be most appreciated as I know little to nothing about AC motors.

 

[jbartos]

Suggestion: It appears that the proper way to pinpoint the root cause would be to bring one motor to an ac motor repair shop for a thorough analysis.
Causes may be:
1. Inadequately grooved mica between commutator lamellae.
2. More accurate adjustment of angles of the brushes on the commutator via brush braces.
3. Defective brush material.
4. AC supply power quality. This should be checked first before the motor is brought to the motor repair shop.
5. Etc.

 

[UKpete]

Universal motors are notorious for sparking and aren't really suited for long periods of operation. Commutators suffer a bit from a runaway effect, once the condition of a commutator bar starts to deteriorate, it will only get worse rather than stabilize. Running on dc may improve the performance by reducing the sparking, but basically they have high volts/bar and no interpoles so they are going to spark.

Marking on alternate bars suggests an electrical problem rather than a mechanical one. But it isn't obvious to me what it might be. I agree that it appears to be a design problem, as you are saying that it occurs consistently on all the motors, however many that may be. You might get a specialist to look at it, but I suspect that the best way out is to look for a replacement type more suited to your application.

 

[aquilajohn]

jbartos & UKpete,
Thank you for your help. I will keep on it until I find an answer. I would rather not "respecify" to an alternate motor, but that may be the ultimate solution.

Thanks again,

Aquilajohn

 

[RichLeimbach]

aquilajohn,
Check out the Morgan Carbon literature in this link:

http://www.morgancarbon.com.au/commutator leaflet-A4.pdf

Looks to me like you have what they call "slot bar burning". Not sure exactly what the best way for you to fix it is though...

- Rich