brushes wearing out

I have 8 DC motors with about 3000 hours on them that are starting to use up the brushes alot faster than they did during the first 2500 hours. Each motor has 4 brushes and when they start arcing and sparking real bad I'll take it apart and one brush will be longer than the other 3. The shorter brushes still have 60% of their length but they start arcing bad after about a week.
I use a piece of emory cloth held against the commutator ring while spinning by hand to smooth out the burn spots but it doesnt last very long, a week at most, before it starts the arcing again.
Is it absolutely necessary to turn the commutator with a lathe to get it perfectly smooth? Am I wasting time with the emory cloth?
These motors drive the wheels on automated forklifts (AGVs) so they constantly vary the RPMs all day as it drives around.

Motors: Ohio Electric model:A-182299X9166 FF 1.0, HP: SPEC, frame: A180, RPM: 2400, volt: 48vdc, type: PM, ser. No: K0443157

Brushes: some motors have #84 and some have #149, whatever that means. i get the brushes from the OEM vendor so I'm not sure what brand they are or any other specs on them.

Controller: Advanced Motion Controls 120A10, www.a-m-c.com, brush-type, PWM servo amplifier, 0-10 VDC control input

Thanks for the help

Roy

You can check 'round' by using an indicator gauge. Place a metal bar in the brush socket against the commutator, put the indicator gauge on the end of the bar. Turn the motor while holding the bar against the commutator and watch the gauge.

More likely, the brushes are hanging in the brush socket. A hung brush will start to arc vey fast and burn away. Clean the soskets really well and check for any burrs. Plus. make sure your brushes are not slightly over-sized.

Last, it could be weak springs. They can cause the brush to skip and arc.

Thanks Star I'll do that..taking it apart now

ascot01 said:

I use a piece of emory cloth held against the commutator ring while spinning by hand to smooth out the burn spots but it doesnt last very long, a week at most, before it starts the arcing again.

Click to expand...

You did not mention "undercutting" the insulators between the commutator segments. So I will.

The commutator has insulating material between the segments. If these insulators are not cut down below the segments they can wear the brushes very fast.

ascot01 said:

Is it absolutely necessary to turn the commutator with a lathe to get it perfectly smooth? Am I wasting time with the emory cloth?

Click to expand...

Some people have the skill to do armature dressing while still in the motor. I have never seen it done successfully with emory cloth though. What I have seen used with some success is a very soft "jointing" stone or "dressing" stone.

Is there any evidence of solder missing at the junction of the segments and the armature wires?

TheStarr is correct, a dial indicator will show any "out of round"

Good point milldrone, I learn something every day here!

P.S. If the brushes are wearing fast, that dust goes somewhere. I put a rag over the end of a shopvac, to prevent small parts from disappearing, and put it up against the brush well. You would be amazed how much dust will come out of the motor.

The problem with emory cloth is that it's hard to get a flat surface for the brushes to ride on. The right way is to use commutator stones that are shaped for the size commutator in the motor.

As Milldrone mentioned you have to undercut the insulators. If they are not undercut they will chip and wear the brush very agressively.

I never do a DC motor service without putting in new brush springs. Using old weak springs is a sure way to end up overhauling it again in a short time.

A proper overhaul should include dressing the commutator with the right stone, new brushes, springs, and a a thorough cleaning of the brush sockets and motor housing to remove all of the brush dust. I usually take the motor apart to get to everything. Brush dust is conductive and it leads to arcing and sticking of motor parts.

If the problem is arcing, I suggest an overhaul instead of just a brush service. You can get by with a brush service if the motor is in good condition and there is no arcing present, otherwise you might as well take it apart and do an overhaul.

Based on experience with 300 Kw MGs on submarine where we could not get armature out.

Yes you can use stones to cut the armature. Takes a special rig up to get a true cut. IF you can put armature in lathe do it ALWAYS.

If you want brush wear and BUNCHES of arcin n sparkin dont bother to undercut the mica. ALWAYS make sure the mica is undercut.

Clean motor as much as possible when in place and completely on bench teardown. If real dirty use soap and hot water, then wash with water then final with isopropyl alcohol. Dry in warm oven ie 120 F or so.

Meggar everything and while it is out growler the armature.

Repair nicked dinged varnish.

If a large motor that takes a day to pull and put back I would go ahead and replace bearings. IF small would probably do it while I have it out anyway.

I fully agree that the brush springs should be replaced on a major teardown. Also check tension using spring scale on brush pigtail.

New brushes should be seated. Do NOT use conductive sand paper.

Check neutral plane setting.

Dan Bentler

Thanks much that's all great info. One of the leads that jumpers one brush over to the opposing brush was shorting to the case. I put new heatshrink on the entire length of the wire.
The mica in between the segments is very clogged and am cleaning it all out now. i will order new brush springs as well.
thanks again

Really makes me appreciate AC motors!

DickDV said:

Really makes me appreciate AC motors!

Click to expand...

Half as much as I

Dan Bentler

"growler the armature" never seen that before, but Google never fails!

http://www.4king.com/01_english/growler.htm

So that is two new things I learned today!

I was taught how to use a growler in 1959 - my first week in a job as an apprentice Electrical Fitter.
The old growler is absolutely essential in a motor shop for finding shorts in armatures.
Brings back a memory or two.
By the way the growler we used was a coil wound around electrical steel. The end was open and was placed on the armature surface. A feeler guage was then passed over the armature slots between the electrical steel open end. If the feeler guage vibrated you were usually right on the money. The one attached to the link in this treead is a bit fancy.

Emery is a coarse rock of corundum and magnetite or haematite used for polishing metal & hard materials. Bonded onto a flexible cloth strip it's ideal for removing flash in contours on models & castings and for smoothing filler

DO NOT USE EMERY CLOTH...conductive as Dan said. Purchase a dressing stone which even comes with a plastic handle and you can clean up comm. while motor is running and will also help seat brushes.

The only thing we used as a sandpaper on motor work was garnet I believe which I recall was supposedly non conductive. We used that to seat the brushes total of 98 in a 300 Kw if I recall. Now I find that for a lot of this work Scotch green pads work pretty well.

Dan Bentler

while the motor is running..that's great..I'll see if I can find one.
Is it safe to blow out carbon dust while the motor is running or will it be like July 4?