neddysaysyo

I have a 900HP, 600V DC shunt motor that is one of many that I am maintaining at a resort. Most of the motors on site have great patinas and seem to be commutating just great. With this one in particular I'm experiencing some markings on the commutator that I consider a bit of a concern. From what I can gather, what is happening is "threading", a very slight scoring of the patina which, if left, ends up scoring the commutator. This motor has been neglected and has started to get a couple of stripes all the way around. I believe the threading arose from the motor being run under a very light load and not putting enough current though the brushes. Another factor is the poor construction of the motor blowers air filter contraption that seems to have been engineered by a 5 year old, allowing airborn contaminants into the motor.

My question is..........the brushes now have a small stripe of extruded carbon, while the commutator now has a slight trench in its circumference. What is the best way to get rid of those stripes? I have removed a set of brushes to increase the brush current density and have fixed the air filters' problem. Can I grind off the extruding carbon on the brushes so they are evenly round again and do not continue the shape that they and the commutator have taken?

Any thoughts?

Dan

itsunclebill

The brushes shouldn't ever be hard enough to cut the commutator, especially like this. The grooves are probably due to some sort of contamination on one of the brushes, unless the brush rack has had parts come loose that are rubbing. It takes a while for a brush to wear into a groove so this has been going on a while - UNLESS the atmosphere is particularly gritty, in which case getting the filter setup working right is particularly important. A lighter colored patina would be a good sign grit is an issue as it would be polishing the patina off. I wouldn't expect light loads to be an issue either. Long periods of sitting so grit can accumulate would be much more likely a reason.

Removing the projection on the brush should be done to allow the brush to make full contact when reinstalled - remove the projection then re-seat the brushes. Garnet paper or a brush seating stone should be used to reseat the brushes as they won't be sitting exactly the same when reinstalled.

FWIW even power tools used in severe environments manage to have reasonably good looking commutators. Any "stripes" wind up being something stuck in the brushes the majority of the time.

I wouldn't advise running the motor without some of the brushes as under heavy load the commutator slugs may be become hot enough to delaminate.

MDShunk

I'd almost bet money on a piece of grit embedded in the brush.

__________________

InPhase277

Just out of curiosity, what at a resort uses a 900 HP DC motor? Sky tram?

KayJay

Ideal used to make a special commutator cement that was used for filling holes and pits in the bars like that. I have no idea if it is still made though.
They also had a pretty good line of resurfacers, etc… if you want to attempt it.

John Valdes

All brushes are not created equal. Some are softer or harder than others. Have you contacted the manufacturer for information on this issue?
If you let a brush get to short the wire inside the brush will be exposed and will cut a groove. Brush spring pressure can cause grooving. Do you have a brush tensioner?

What I would do is get the manufacturer or supplier involved. It could be an alignment issue with either the brush ring, armature or bearings. A good overhaul would be a great place to start.
Do not try to smooth the comm yourself. The comm needs to be machined, the armature balanced, the motor load tested and the grooves re-cut to manufacturers specifications.

When installing new brushes do you "seat" the brushes? If the new brush does not mate to the comm perfectly, this can cause premature brush failure and grooving.
To "seat" the brushes:
Remove all the brushes. Wrap the comm with emory cloth and tape it down good. re-install the brushes and hand turn the motor shaft until the brushes are seated just right. Loosen the brushes and remove the emory cloth. Vacuum it out real good. Emory cloth has metal particles impregnated in it.