Maintenance techniques on medium voltage contactors. 2

[Thedroid]

During a recent shutdown we removed an old GE limit-amp starter from service in order to check the condition of everything. The contacts looked to be in good condition, and we used Scotch-brite to shine them up a bit.

Is Scotch-brite recommended for cleaning any contacts? Low voltage; ie.

Also the stabs for the 4160/120 control transformer had a buildup of black crud on them and were not making a good connection.

Was this crud whats referred to as carbon tracking?

We were unsuccessful in cleaning this off using delicat methods, and had to resort to using a wire wheel to get it off. The silver plating is now worn off, and there are a whole bunch of pits in the stabs where the black crud was. They are making far better contact than before, but I question the "techniques" we used to clean the contacts and the stabs.

When I asked for replacement parts, I was informed that they were obsolete and to do the best we can until a new starter is in the budget. This could be several years. We have not brought this motor online yet, and I'm concerned that we will have problems down the line.

I thoroughly enjoy having the opportunity to work on this large equipment, but finding guidance on some of the inside tricks is difficult. I try to read as much as possible, but for every useful book i find there are 5 useless one.

 

[Zogzog]

Scotch brite is fine, the wire wheel is not (As I think you found out). Cant tell you much about the "Crud" without pictures. The silver plating being worn off is going to give you problems, maybe not tommorow, but eventually it will. There is a lot more to MV stater maintenence than shining everything up, shiny dosent mean the contacts are good, you need to check contact pressures and contact resistance using a microhmeter. There is a long list of tests to be done to these, I suggest you hire a qualified testing company to do the testing and maintenence or get the right equipment and the right training to do this yourself. You will wnat to get the NETA MTS for guidance on what tests should be done and the specs that apply.

We rebuild these everyday in our shop, buying a new one is a waste of money.

BTW, I stock replacement parts for those. Not a problem, contact me if you still need them.

Scott

http://www.cbsnuclear.com

 

[LaffeyDD724]

You did not state what type of industrial plant you are maintaining. I will just make this statement. In the pulp and paper mills where I worked, believe it or not, the hydrogen sulphide gas generated in the pulp mill section was all over the mill area and the hydrogen sulphide gas will cause silver to form a black coating which is not good. There is a product available packaged in a can which is a silver compound that can be rubbed on a clean silvered surface to resurface with silver, it is called something like Silver Plating Rubbing Compound, I think McMaster Carr company carries it. Copper is really bad about forming the black stuff it will get so heavy that it forms scabs that will flake off and cause shorts between horizonal buss bars in switch boards (MCC). It was so bad in our mill that we changed the specs on the MCC, we began specifying that all buss bars be TIN plated since TIN is impervious to hydrogen sulphide. This has worked out very well and has solved the problem with MCC buss bars, it would also be wise to tin the control wiring copper wire ends to avoid this condition. I have also had the large MCC fuse blades tin plated along with the fuse holder connections and this worked very well and eliminated the frequent cleaning that was previously being performed. Exercise caution you would not want tin on the contactor movable contacts. I would not recomend it for that service since I have not tried it. But the tin plating works very well in static conditions. Please state what industry you are with.

Tom


None of us is as smart as all of us.

 

[Thedroid]

Cement plant. The equipment is old, but generally in good condition. These starter are only cycled about 6 times a year. The stabs for the control power are what concerned me the most. The carbon that flaked off left deep pits in the stabs, and we were not able to obtain new ones.

 

[Zogzog]

Laffey,

That paint on silver plating stuff is a temp fix that wont last long. You are on the right track with the tin plating, here are some other solutions.

 

[LaffeyDD724]

Is your cement plant close to any other industry? Do you think the atmosphere in the cement plant is the culprit? If so what do you think is present in the plant that would attack the silver coatings. Do you have the same problem with the copper items also? Do you have cement dust in the MCC Rooms.
Tom

None of us is as smart as all of us.

 

[Thedroid]

I don't think that it is corrossion, because all of the other surfaces look just fine. The connection was loose on the stabs, and it looks like some sort of arc damage. I was wondering if this is what is reffered to as carbon tracking? I'm definately not happy with the way we clean up the stabs, but I'm told that we are not getting any new parts for these starters because they are 50 yrs old. It's the bosses way of forcing an update to the equipment.

Corrosion is definately not a problem at the plant. the small amounts of cement dust present in the substations doesn't seem to bother anything. We clean all the equipment thoroughly quite often.

 

[LaffeyDD724]

It sounds like to me from what you are describing that the receiver contacts for the stabs do not have enough tension on them to maintain adequate contact pressure on the stabs and this has caused hot spots to develop at times and has eroded away some of the stab contact surface by pitting the surface. Carbon on the contact surface is not good and could cause further overheating by forming an irregular surface thereby reducing the contact area. I believe that the term carbon tracking would more than likely refer to an insulating material that has carbon tracking (burnt insulation) and could contribute to a fault condition. If I were you I would get in touch with Zogzog because he is correct in his assessment of the situation.
Tom

None of us is as smart as all of us.

 

[Zogzog]

"I'm told that we are not getting any new parts for these starters because they are 50 yrs old"

I have parts for those

 

[LaffeyDD724]

You are going to have a big surprise with those starters someday and it's going to be a burn out or a blow out which can be quite nasty and then it will cost an arm and a leg to get those motors operating again. If it were to be taken care of sooner rather than later it would be pocket change compared to what it will cost after a blow out. Not to mention the unwanted down time on the plant which is quite costly in itself and lost business with a lot of unhappy customers because maybe you cannot fill the orders.

Tom

None of us is as smart as all of us.

 

[mcgyvr]

The pitting and black crud(carbon) is definately from arching at the contacts. Good thing you found it now before you had any damage/fire,etc..

 

[Thedroid]

Zogzog, could you send me some contact info I would like to be able to order some parts if possible. I sure don't want to be on duty when one of these starters blows, It's fed from a 5 MVA transformer through a 1200 amp breaker, I'm sure the destruction would be ugly. Getting any info on the MV equipment in my plant is hard. I'm trying to pursue a more active program in getting these old starters in better working condition.

 

[Zogzog]

Forum rules are touchy on this, I think I am allowed to do this.

Scott

http://www.cbsnuclear.com

 

[whitehendrix]

i would think that the crud used to be the grease that resided on those stabs and contact area... heat cycling and eventually arcing have just fried it up... let alone contamination with the coal dust i'm sure is there and the other foreign particulate matter and nasties floating around.. cement is does unholy things to electrical stuff..

scotch brite pads and (if you're good..) needle files are a good way to dress contact buttons and remove scale, tho.. good call on that.

 

[Thedroid]

Thanks. I wouldn't be suprised if this crud was old electrical grease. It was tough as nails. a hand held wire brush wouldn't even scratch it. I had to use the angle grinder with a wire wheel to take it off.

 

[ScottyUK]

If it is old grease then look for a solvent called dichloromethane. If it is an organic grease (as opposed to a silicone) it will dissolve it given a bit of time and some gentle heat. It's a nasty and very aggressive solvent so if you do use it be careful and keep it away from any polymers or organic materials. Paint stripper basically dilutes it down with methanol and some gelling agents.


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If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

We have a 180 Degree, 280 PSI parts washer that takes all that old grease right off, in the field we use a Zep product called I.D.Red. Wish we could get pictures of the "black crud"

 

[ScottyUK]

Now that is a proper parts washer!


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If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

http://www.karchercuda.com/whyKarcherCuda.aspx

We have the large one the guy is standing next to.

When we do something we do it right, we also have a 24 foot Big Ass Fan

http://www.bigassfans.com/

to cool the shop

 

[Zogzog]

And our moblie field service trailers are not shabby either

http://photos.gcbsinventory.com/trailers/