Excessive brush wear 2

[THooper]

We are working on a GE 300 hp DC motor. New brushes were installed in April and they showed 60% wear in a months time. Another set of brushes were installed and they had 50% wear in the last month. Concentricity has been checked on the commutator and there is about .001" run out. All of the mica segments have been undercut to .032" below the surface of the copper. Brush spring tension has been verfied at 5 =6 lbs per square inch and the springs are constant pressure springs. The commutator has been seasoned with a good film developed for lubrication and conductivity.

Each set of brushes that were installed were CPO brushes and ordered by the OEM part number so that the correct grade was installed. The brushes are sand seated to 100% contact with the commutator.

We have repaired a numerous amount of these motors over the past years and never experienced this type of problem.

Can a failure in the SCR package cause premature wear?

 

[teco]

motorhead1:-rapid brush wear can be caused by a) overload b)underload (wrong currend density of brush) c) high ambiant temp d) contamination (ie gasses or dust) or wrong brush grade.
you have not indicated if there is any sparking at the brush face or elivated temp. a faulty s.c.r would result in sparking (normally orange in colour) and high running temp of the machine.
you could try contacting a carbon brush supplier and ask them to select a brush grade to match the load.
kind regds.

 

[electricpete]

I don't have any experience with DC motors, but it sounds like TECO has it covered.

One thing I vaguely remember is that dc motors have interpole windings which function to define the neutral plane in a manner that minimizes sparking. Not sure if that type winding can fail to perform it's function due to miswiring or open circuit.

 

[THooper]

Teco thanks for the good input. The motor runs a belt drive and is in tandem with another motor through a gear drive, there is a motor control that keeps them at the same load. The other motor is not seeing the same failure. The belt drive is underground and the ambient temperature is mild all year. The commutator surface is in excellent condition and does not show signs of the pitting usually associated with arcing. There is high humidity in the area. The commutator does show some signs of slot bar burning.

The brush grade is the same that we have used for years and is specified by the Manufacture. The motor that is in tandem has the same brush in it and has had 35 mm of wear in the last year.

We have not had the opprotunity to watch the motor run on their application. Therefore we have not been able to check out the arcing.

I will call Carbon Products engineers, they have always been helpful.

All input is appreciated and welcome.

 

[rhatcher]

Hi motorhead,
I have two questions to start. First, is this a new problem (ie. one day without other changes the brush wear unexpectedly increased)? Second, 5-6 psi seems a little high, are both motors the same in that respect?

Note that GE did have some DC motors with high pressure and (unrelated to brush pressure) some motors that "as designed" have brush wear problems.

 

[THooper]

Hi rhatcher,

The motor was installed in April and went through two sets of brushes in two months. We have been doing these motors for years and the brush wear is usually 30 mm to 40 mm per year, on a 78 mm brush. This wear has been unique to this motor, we can not find anything wrong with the motor or the application of the brushes.

The formula that we use for brush spring tension is 4-5psi for industrial applications and the bottom angle is more than 25 degrees so you add another 1\2-1 psi. This is the pressure per square inch of brush surface not the measured force.

The springs are the constant pressure type and can not be adjusted. However measureing the force applied it does fall on the parameters set by the above formula. This is also the force that we have recorded for years.

Thanks for the reply, I will contact CPO engineers today for their input and let all interested know what they say.

 

[teco]

motorhead1:- just a few thoughts:- are the pig tails on the brushers discoloured in any way?
we had a simular problem with dc motors running in tandem to a common shaft,after service one motor went through brushers. the pig tails were discolourd and shown sighns of heating. on furher inspection we found the serviced motor was running slightly faster even though it was on a closed loop system the drive did not respond quick enough to share the load, hence the re serviced motor was going through brushers. adjustments were made to the drive feed back card and the problem went away.
might be worth a look!
kind regds.
p.s. the problem with the motor was later found to be an air gap variance. when the machine was serviced the main coils had been removed and shimms had been left out.

 

[THooper]

I contacted an Engineer at Carbon Products, he was very helpful. He felt that we had a silicone problem, given all the facts that we had.

We went back to the site and the motor is fed with fresh air. The customer does a wash down of the area and had sealed the duct work to this motor with a RTV silicone sealant.

One interesting spot that corralates with TECO. The tach was out of calibration and the motor was running 100 rpm's faster.

Thanks for all the input Motorhead1.

 

[electricpete]

Sorry, I'm a little slow in understanding this one. Are you saying the airflow to the motor was restricted?

 

[THooper]

Hi electricpete,

The air flow isn't being restricted. The seams of the duct work were sealed with a silicone sealant. The silicone sealant realeases silicone in the air that effects the operation of the brush.

Silicone materials have long been known to cause accelerated brush wear. This has been a problem in industries where silicone vapors are in the ambient atmosphere.

It is known that the presence of silicone will cause very rapid wear of carbon brushes. The wear rate can be many times the rate that would occur without the silicone present. The concentration necessary to cause the rapid wear is extremly small. It is less than a part per billion.

The exact mechanism of the wear is not completely understood. Silicone vapor collects in the brush and in the commutator film. The silicone is abrasive and results in faster wear.

Cooling air for ventalated motors must be clean and contaminant free with a relative humidity between 40% to 99% humidity and in a temperature range of 10 to 40 C. Water vapor lubricates the brush collector interface. Lack of water vapor is known to result in acclerated wear. Silicone repels water. Without the lubrication water vapor provides, wear is rapid. Years ago we used to see one dc motor mfg come in for repair with silicone lead wire that would have the same type of effect on the wear of the brushes. We would replace the lead wire to epdm and solved this acclerated brush wear. This mfg does not use silicone lead wire anymore on their dc motors. I should of thought of this sooner.

 

[Tamone]

Hey Motorhead1,

Try putting the brushes from the other motor on the drive to isolate the problem to that motor alone. After determining that the motor is the problem and not dirty power, look at the carbon content of the brushes, you may have to try a different brush manufacture. If it is in the motor check the shunt fields or connections on the commutator.

 

[rhatcher]

Tamone, I believe that motorhead1's post just prior to yours explained that he found the source of the problem. I would accept that what he says is correct as silicone and brushes do not mix well.

 

[Tamone]

Rhatcher, Thank you for your observation and my oversite. You are correct to say that silicone and brushes do not mix well.

 

Though this problem seems solved, one other consideration that was not mentioned directly is that many times the controls that insure equal load sharing are defeated by inexperienced (lazy) personnel that I'd like to throttle with the coupling belts/chains, or club over the head with the common shafts, whichever is applicable.

I think in most cases with similar symptoms that discolored pigtails will be found to be present, with unequal (over)loading being the cause. I'd like to hear a follow-up to this in another month or so, stating that without the silicon in the air the problem desisted. Also, was the RPM/load difference corrected at the same time as the silicon content was removed? If so, I would credit this with the correction before I would hang my hat on the silicon. However, this being said, I've never seen such excessive brushwear in so short a time. I've also never seen a system in a high-silicon environment, which may also explain my skepticism. Give me some regular-ol' H2S anytime. ;-)

Silicon-based sealants, though, I've seen applied to motor-junction boxes for years. I can't see how this would not contaminate the brushes at densities greater than 1 ppb for the duration of the curing, in the least. Does anyone have a source for information on this silicon-brush-wear connection? I would dearly love to see it.

This entire post, by the way, is not meant to imply that I disbelieve in the least anything that was stated. I'm only drawing on my personal experience as a guide, and trying to further my own knowledge.

Don, the guy who has to make it work in spite of the design and maintenance

 

[THooper]

Hi Donulus,

I would be more than happy to fax you information on silicone materials causing accelerated brush wear, Please advise no#. Also I will update brush wear condition after this motor is back in service and we perform our next quarterly inspection. Note: This motor is now their spare, so it will probably be sometime before I can follow-up.

This is good input Donulus, This application had the complete cooling hood assembly and duct work sealed to the motor with silicone. I don't think you are ever 100% sure you have corrected a problem like this until it has proven
to operate to specification in service for a good period of time with monitoring. I will let everyone know the outcome.

 

[THooper]

Donulus,

In re-evaluating your question I felt I should give you a better explanation. We have been on the maintenance side of these motors as well as the rebuild side for over ten years. The type of brush has always been the original maufacturere which is Carbon Products. They have been excellent in answering questions and their product is at the top of the line with Helwig.

The motors that is in question have a fresh air system that was sealed at the joints of the duct work with silicone, a latex sealant would have been a better choice. There was approximately 10 joints in the fresh air system that silicone was heavily used on. The fresh air supply blows right on the commutator and would place the silicone vapors directly to the commutator and brushes. With silicone being applied at a jb it would not be a intake but more than likely an exhaust. Therefore the silicone would be discharged from the motor.

The tach was repaired and is now within 20 rpm of other tach and in operation on the motor that replaced this problem child. One thing that we have been trying to understand is the varying of different brushwear from motor to motor. We are looking at 5 to 10 mm in a six month period. I thought TECO had some good input on this. If each tach is verying by 50 to 100 rpms then each motor would have slightly higher or lower wear due to the amount of revolutions. We are going to start checking the accuracy of the tachs, for a more precise understanding of the brush wear.

The motor that we have installed in the place of this one is running fine and all of the silicone has been removed from the cooling hood, There is a lot of documented cases of even more extreme cases of brush wear due to silicone. As I stated above if you would like to supply me with a fax number I would be happy to fax you this information.

 

[rhatcher]

I can always use more technical reading. Please fax me a copy if you don't mind. Thanks

Attn: Ray Hatcher, fax: (843)747-0137

 

[jbartos]

Suggestions:
1. The higher frequency content from SCRs or malfunction of SCRs could reduce the lifetime of brushes since the surrounding capacitances become more conductive. Try to install the same brushes elsewhere, where there is no SCR conversion.
2. If in doubt about the brush material from the existing brush manufacturer, ask about a substitute replacement or customize similar brushes to fit brushes there on a trial period.

 

[spmoha]

we have found brush fails prematurely if motor is very lightly loaded.
we removed some brushes from rocker to solve this problem.
s.p.mohanty