Excessive wear of carbon brushes on slip rings

[sngpl]

We have two hydroelectric generators of 100 MW each. Unit 2 has more wear of carbon brushes on the slip ring than unit 1. Under the same operating conditions (load, field volts, field amps) the temperature of the slip ring of unit 2 is 25 C higher than unit 1. The brushes are of the same make and material. The only thing that is different about unit 2 is there is a minor oil leak from an oil reservoir above the slip rings. This oil reservoir is for the hydraulic oil (Tresso 46) supply to the runner blades of the Kaplan turbine.

In what way will a thin film of Tresso 46 between the slip ring face and the characteristics of the brushes etc?

What could be the cause(s) of the excessive carbon brush wear? I am confident that the brush springs are set to the appropriate tension.

 

[sngpl]

Correction:

In what way will a thin film of Tresso 46 between the slip ring face change the characteristics of the brushes etc?

 

[edison123]

sngpl,

1. Is this brush wear a new phenomenon or was there since commissioning of the units ?

2. Is the slipring material same in both the units ? If not, then using the same grade brushes for both the machines may not be advisable. Better to get the opinion form the OEM or brush manufacturer about correct grade of brushes to be used.

 

[sngpl]

edison123

1. Actually the brush wear was there but not to this extent. This excessive wear has been noted after the annual maintenance of the units last spring.

2. The slip rings on the both the units are identical. Brushes were replaced during the annual maintenance. It’s the same kind and make.

What would be the effect of oil (Tresso 46) generally on the performance of the over all collector housing and especially on the brushes?

As I mentioned earlier there definitely is some oil present on slip rings of unit 2, which has the most wear. This oil is due to an oil leakage from the oil reservoir above the collector housing and can only be fixed when an outage can be availed........... Not in the near future.

 

[edison123]

sngpl,

while oil contamination may lower the IR of the sliprings, I don't think that would affect the brush wearing rate. May be you could contact the oil company about its effect on carbon brushes.

 

[jbartos]

Suggestion: A thin film of oil will increase resistance between the brush and the slip ring. There will be some sparking developed under the brush that will produce EDM (Electrical Discharge Machining) that will in turn be wearing out the brush and slip ring more than at the other unit.

 

[edison123]

jbartos,

The slipring - brush contact ia a continous one (as against commutator-brush contact, which is intermittent). So, I fail to see how sparking can develop in slipring arrangement.

 

[jbartos]

Suggestion to the previous post. I am aware the difference between the slip rings and commutators. Apparently, the voltage level or electrical field between the brush and slip ring, over the oil film dropping from the leaking oil reservoir above the slip rings, will cause the electrical discharges or oil film breakdown via sparks or high heat of the high electrical field density, similarly as it is the case over the thin layers in semiconductors, until the layer breaks down thermally. The sparking or electrical discharge machining is very destructive to the surfaces in general. Visit

http://www.sme.org/cgi-bin/storefront/prodgrp/V/V10000/SME/

for fundamentals of EDM, tutorial for $225

http://www.ee.washington.edu/conselec/CE/kuhn/manufact/95x2.htm

for: Electrical discharge machining: Electrical discharge machining removes materials by application of an electric spark which vaporizes the material. EDM finds its most common application in the making of dies for die casting or forging. These dies are manufactured from hard refractory materials which are difficult to machine conventionally. EDM also allows for detailed stress free machining of many materials. EDM is also used for machining of very small burr free holes such as jets in fuel injection nozzles.
Visit

http://fusion.gat.com/icf/highlights/10-98.pdf

for EDM info

 

[edison123]

jbartos,

edm occurs when there is very small air gaps. The brushes are always in contact with the slipring under spring pressure and I don't see any air gaps being developed there. More over, sngpl has mentioned only a minor oil leakage which I am sure is not flowing over the slipring surface. So, sorry, I don't see any edm activity in this particular case.

sngpl,

do you acutally see oil film on the sliprings ?

 

[kannanR]

The thin film of sticky oil might have attracted dust particles over the slip ring, which might increase brush friction. Increse in brush friction will increse the brush wear rate. The temperature increase might be due to the following,
1. The thin film of the oil between the brush and slip ring might have increased the contact resistance.
2. The thin film of oil layer over the entire slip ring might have reduced the heat tranfer rate, and the temperature might have stabilised at +25 deg than normal.

 

[jbartos]

Suggestion to edison123 (Electrical) Aug 30, 2003 marked ///\\edm occurs when there is very small air gaps.
///Not necessarily, depending on the electrical field stregth, it can develop over thin layer of a material, e.g. paint.\\ The brushes are always in contact with the slipring under spring pressure and I don't see any air gaps being developed there.
///So are bearings under heavy pressure of weight they support, e.g. many tons. However, the oil film develops even under the ball or cylinder type of bearings. I do not see any reason why the oil film would not develop between the brushes and slip rings.\\ More over, sngpl has mentioned only a minor oil leakage
///A minor oil leakage may be enough to cause a marginally higher brush wear-out. It was not mentioned that there was rapid wear-out of the brushes.\\which I am sure is not flowing over the slipring surface. So, sorry, I don't see any edm activity in this particular case.
///There is no need to appologize. This Forum accepts differences in electrical engineering, design and experiences.\\sngpl,

do you acutally see oil film on the sliprings ?
///Do you actually see oil under balls of bearings?\\\

 

[edison123]

jbartos,

I said " no air gaps developing " and nothing about oil films under the brushes.

If it is edm, what about sliprings ? Do they not suffer erosion too about which the poster hasn't mentioned anything.

When I said sorry, I was just being polite not apologetic.

I asked sngpl about oil film over sliprings to get some more info. It was not about edm.

 

[edison123]

sngpl,

another thing to check - you said brushes were replaced during annual maintenance. may be their seating was not full (so more current density under the brushes leading to increased heat and wear). anyway, keep us posted of further developments.

 

[edison123]

sngpl,

Another thought, since both machines are identical, can you try interchanging brushes from one machine to the other ? That way, if the problem travels with the brushes, then brushes are suspect. If the problem stays with the machine, then problem could be oil contamination, slipring material and surface finish, field current, spring pressure, etc.

 

[sparky1976]

Hey guys,

Is that a thread thats explain exessive wear of bruch caused by silicon coating or something,the guys that wrote the thread found out that one of the duct to brush just newly resealed by silicon, forgive If I'm wrong but I think there a thread like that in this forum.

 

[jbartos]

Suggestion to edison123 (Electrical) Aug 31, 2003 marked ///\\jbartos,

I said " no air gaps developing " and nothing about oil films under the brushes.
///Actually, I was addressing "The brushes are always in contact with the slipring under spring pressure and I don't see any air gaps being developed there." by pointing out the thin oil film development that might eventually have some air voids here and there.\\
If it is edm, what about sliprings ? Do they not suffer erosion too about which the poster hasn't mentioned anything.
///If there is an electrical arc developed (it may have about 3000degC) it will affect the brush as well as the slip ring.\\
When I said sorry, I was just being polite not apologetic.
///It is better to say "so, sorry for myself, ... which is clear. It does not imply "so, sorry for you, ...\\I asked sngpl about oil film over sliprings to get some more info. It was not about edm.

 

[edison123]

jbartos,

This is a technical forum where everybody has got the freedom to express their opinions even if they difeer with yours. Ultimately, if the original poster or others benefit from these opinions, that is what matters.

If you want to argue about semantics, this is not the forum.

 

[JamesMDavis]

I think that this question has been answered already, but to summarise depending on the insulating strength vs. temp(easily checked with OEM) the hydraulic oil may be acting as the dielectric medium between the carbon brush and the slip ring, and then rapidly breaking down to become a conductor, hence allowing a rapid discharge, easy way to find out is to fix the reservoir.

 

[jbartos]

Suggestion to edison123 (Electrical) Sep 1, 2003 marked ///\\jbartos,
This is a technical forum where everybody has got the freedom to express their opinions even if they difeer with yours.
///It appears that you were asking me questions and started challenging me and now I am the one to take all the blame.\\
///See the sequence of postings:
I volunteer my posting:
jbartos (Electrical) Aug 30, 2003
Suggestion: A thin film of oil will increase resistance between the brush and the slip ring. There will be some sparking developed under the brush that will produce EDM (Electrical Discharge Machining) that will in turn be wearing out the brush and slip ring more than at the other unit.

This was followed by:
edison123 (Electrical) Aug 30, 2003
jbartos,
The slipring - brush contact ia a continous one (as against commutator-brush contact, which is intermittent). So, I fail to see how sparking can develop in slipring arrangement.\\\

So, I volunteered my answers to you.

Ultimately, if the original poster or others benefit from these opinions, that is what matters.
///I am well familiar with this. I volunteered my answers to posed questions. My answers are based on my experience and education.\\
If you want to argue about semantics, this is not the forum.
///You started arguing semantics. I just volunteer answers to your questions and comments. If you do not like my answers, do not ask me questions.\\

 

[sngpl]

edison123

1-There is a thin film of oil on the sliprings.

2- The oil leaksge cannot be stopped at this point in time, because its needs some modification and for that the unit has to be stopped. An outage cannot be afforded now.

I really appreciate and respect edison123 and jbartos because I know both of you have good in depth engineering knowledge and experience and your suggestions/comments/answers are highly appreciated through out the forum. So please don't get involved in pitty arguments because we need you ...... your positive comments !!!!