Brushes

I know this isn't PLC related but I know I will get some good answers from what i've seen of this site. We have 2 DC motors that wear the brushes out about every three week. The commutator "looks ok". What else might cause this. They are located in a pretty stable enviroment where there is no vibration and they are hardly under any load.
Suggestion are greatly appreciated. Thanks

If the com is good and by good I mean perfect, no abrasions, black score marks or copper splashes then it could be the wrong type of brush fitted.
Brushes are graded by compound and this makes them harder or softer. Too soft a brush will wear out prematurely.
Take a brush out and examine the part that touches the commutator. It should be slightly concave and very shiny with no score marks on it at all. If a brush is allowed to get too low, the 'wick' - 'tail' will score the comm causing quicker wearing of the new brushes.

Other checks

I would agree with Goody. I would also check the brushes for chipping. This could be indicative of poor spring tension. Make sure your brushes are free in the brush holder. You do not say what size of motor, are you seating the brushes prior o use. Also check the comm for raised bars.

Regards,

Ken R.

Two things come to mind. First, old DC motors designed to run on the nice smooth DC that comes from an M-G set will often eat brushes like this when driven by modern SCR chopper DC drives. The motors will also heat up quicker on SCR DC and need to be derated in torque. There is no fix that I know of short of replacing the motor with a modern design.

Second, when DC motors are repaired, it is very important that the motor is reassembled and aligned properly. Motor shop people talk of magnetic center alignment and other critical coil alignment procedures to reduce brush wear and arcing. This is something for a good motor shop to attend to. I'm not sure if this can be done on site or not. Just use a good reputable shop. There are a lot of AC repair shop people that don't really have the DC skills to get this right.

I know you have probably heard all this before, but with the cost of inverters being so low, it is often cheaper in the long run to convert all DC motors to AC and eliminate maintenance costs and downtime.

I agree with the above but it wasnt mentioned if you were cleaning the ccmmutator when changing the brushes. If the commutator has carbon buildup then new brushes will never seat and arc extensively which in turn will cause them to burn out faster. When the motor is running there should be a black film buildup on the commutator and little or no arcing.

There could be other issues but the first is to determine if you are using the right brushes with correct springs and that the holders havent been moved out of position. Clean the commutator while its running with a brush seater/commutator cleaner..Example:http://www.grainger.com/Grainger/productdetail.jsp?xi=xi&ItemId=1611677850

I would monitor the motors carefully to determine where the issues are, its possible they may need sent out for rebuild, resurfacing and/or recut.

rsdoran said:

There could be other issues but the first is to determine if you are using the right brushes with correct springs and that the holders havent been moved out of position. xi=xi&ItemId=1611677850[/URL]

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I am agree with this one. Thats always our problem here specially during summer season. Thats the always recommend for us by our consultant here, always check the right specifications of the brushes and check the tension of the springs or the holders. So out of 32 dc motors we have, even now we already one by one minimize this kind of problem

Excessive brush wear

The excessive wearing of the brushes can be caused by a dust getting into the motor. I have had a situation where fine plastic dust was being sucked into the motor and this caused the exact same scenario you describe. The brushes were wearing out every 3 weeks. The solution was to provide extra filtering and in fact the cooling air was being supplied by a duct who's intake was outside of the production area. Once we were able to cool the motor with the clean air we stopped having problems.

Sorry if not related again to the PLC...

I remember when I am first year here in our company, I personally see the explosion happen to our 1000hp motor. It totally damaged and cannot be used anymore. The only thing that our maintenance engineer confusing here, is the coupling of the motor to the gearbox is also totally damaged. According to him, for 15 years working in as maintenance engineer, he had never experienced like this? Takenote, there is no damage on the gearbox, only the motor and the coupling.
So what do you think is the reason behind this incident?

By the way, our company during that time is almost operate to failure because of the wrong system they are doing... thats why every year we are always experience explosion to the motor. But now, Thanks God it won't happen again after our Manager from India, was resigned...

DickDV said:

Motor shop people talk of magnetic center alignment and other critical coil alignment procedures to reduce brush wear and arcing. This is something for a good motor shop to attend to. I'm not sure if this can be done on site or not.

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Dickdv is quite right there, if the neutral point is not set correctly this will cause excessive arcing (bad communtation). There is a method of checking the neutral point on site, it is as follows:-

1. connect a millivolt meter across opposite (magnetically) brush studs.
2. apply 110v AC to field windings
3. Move brush ring position until least amount of meter deflection is noted. (if using a digital voltmeter then the voltage at the neutral point should be as close to zero as possible.)

Other problems that can cause excessive brush wear is commutator run out, bar to bar difference should only be a maximum of about .003". The distance that the brush holder is away from the commutator will also effect brush wear, this should be about 0.07" to 0.08" (allowing for commutator high and low spots), too much holder gap and the brushes will chatter to little and you risk damaging the commutator.

Having said all that the likely problem is incorrect brush grade,

Hope this helps

Paul

If you really want some detailed info on brushes

DC Motor Brush Life

Abstract-This paper looks at brush life, why brushes wear and what can be done to achieve longer brush life. It is meant to provide one with an understanding of the factors and conditions that contribute to brush wear in order that problem areas can be avoided and existing problems identified and resolved. This however, is not an instruction manual on how to fix problems. The motor manufacturer should be contacted for help in resolving brush and commutation problems. The following applies to medium and large horsepower industrial DC motors and generators.

I. Why Brushes Wear
DC brush wear is the result of mechanical friction and electrical erosion. Friction produces carbon dust; the result of electrical erosion is the vaporization of carbon with little physical residue.

A. Friction

Carbon rubbing on bare copper has a rather high coefficient of friction. A low coefficient of friction is achieved when the commutator has good film. With good film the coefficient of friction can be reduced to 10% of the original bare copper value.

Friction changes with commutator temperature. The coefficient of friction decreases to some point, with increases in commutator temperature and then increases again as the commutator temperature increases. For example, a given brush might have a coefficient of friction of 0.15 running on a commutator with a surface temperature of 140°F, 60°C. When running on a commutator with a surface temperature of 220°F, 104°C the coefficient of friction could be 0.08. Yet higher temperatures can result in an increase in the coefficient of friction. Standard brushes on warm commutators at medium speeds will typically have a coefficient of friction of 0.13 to 0.19. This is considered to be a low coefficient of friction. The coefficient of friction, to a large degree is a result of the film produced on the commutator which is dependent on commutator surface temperature and the other factors which influence film.

Click HERE to see more

Re: If you really want some detailed info on brushes

rsdoran said:

DC Motor Brush Life

Abstract-This paper looks at brush life, why brushes wear and what can be done to achieve longer brush life. It is meant to provide one with an understanding of the factors and conditions that contribute to brush wear in order that problem areas can be avoided and existing problems identified and resolved. This however, is not an instruction manual on how to fix problems. The motor manufacturer should be contacted for help in resolving brush and commutation problems. The following applies to medium and large horsepower industrial DC motors and generators.

I. Why Brushes Wear
DC brush wear is the result of mechanical friction and electrical erosion. Friction produces carbon dust; the result of electrical erosion is the vaporization of carbon with little physical residue.

A. Friction

Carbon rubbing on bare copper has a rather high coefficient of friction. A low coefficient of friction is achieved when the commutator has good film. With good film the coefficient of friction can be reduced to 10% of the original bare copper value.

Friction changes with commutator temperature. The coefficient of friction decreases to some point, with increases in commutator temperature and then increases again as the commutator temperature increases. For example, a given brush might have a coefficient of friction of 0.15 running on a commutator with a surface temperature of 140°F, 60°C. When running on a commutator with a surface temperature of 220°F, 104°C the coefficient of friction could be 0.08. Yet higher temperatures can result in an increase in the coefficient of friction. Standard brushes on warm commutators at medium speeds will typically have a coefficient of friction of 0.13 to 0.19. This is considered to be a low coefficient of friction. The coefficient of friction, to a large degree is a result of the film produced on the commutator which is dependent on commutator surface temperature and the other factors which influence film.

Click HERE to see more

Click to expand...

Very nice information...

By the way I want to post more question on your other forum, about maintenance and some technical stuff because here gonna be an off topic. Hope to see and read your answer there.

Good Luck

Thanks