OT Opinions on electrical contact resurfacing needed

Could you guys offer any advice on contact resurfacing? I've come across a 90 amp magnetic starter that has some pretty seriously pitted contacts. There is no dome shape remaining. All are flat, or even slightly concave.
My questions are twofold

1. Can I just sand these things down on a surface plate by hand (to maybe a 400 grit paper finish?) until all pitting is gone and call it good? There is plenty of "material" left on each contact, but I dont know if the above will work, OR will problems arise if say, some contacts require more material removal than others, resulting in slightly varying heights. ??

2. OEM Furnas/Seimens replacements are not cheap, even on ebay. Does anyone have opinions on the aftermarket "YUCO" and also "REPCO" branded replacements I see that are much cheaper than OEM???

Don't attempt to remove all the pitting, just take them down till they are flat and smooth,removing the minimum amount possible. the doming is not neccasary. You have nothing to loose here, and cleaning and resurfacing relay and starter contacts used to be a common yask in electrical engineering. Good luck with it,
Phil

the best way to do anything positive to a set of burnt contacts is to clean them up with contact files. Order you a few of the files (they are cheap and at one time they were give away's with business names on them). With the world going to VFD's and soft starts and no one wanting to compromise a production line, cleaning contacts with a contact file has went the way of the doo-doo.

Once you get some files open the contactor up to where you can put the contact file between the contacts and then hold some pressure on the contacts and pull the file out and then repeat the process till they look smooth and clean. Depending on the damage and hardness of the contacts it might take awhile. As stated above you won't end up with new looking contacts just ones that arc less and are less prone to sticking together.

I remember a thread on PM some time back where this was discussed. Several replies said "don't sand" as the abrasive grit would embed in the surface and cause problems down the road. So file or scrape, don't sand seems like safe advice. As soon as your 90 Amp contacts open and close once they will pit again, so no point in trying for a smooth finish, rough shape to remove worst slag off the top and your good to go.


CarlBoyd

Ok thanks guys. IF these were 100% brand new contacts, approx how long would it even take them to get to this point? I realize it depends on their daily use, but are we talking months here? Years? Just trying to get a rough idea of how much use a set of 90amp pitted contacts have likely seen?

I don't know how old your contactor is, so there's no telling how long it took for the points to get pitted. That said, it might help to add something like a Quench Arc snubber across the A1 an A2 of the coil in the contactor. I got some arcing in my lathe contactors when I rebuilt the electrical panel in my lathe after the control transformer burned up. I added these on each contactor and it seems to help, at least visibly.

As I understand a Quench Arc, (That's a brand of snubber), it's a capacitor and a resistor in series placed across the points. I guess if you knew the values, you could make your own. The values would be different for each installation depending upon the voltage and currents being switched.

If you do look at replacement you might look locally for a retipping service. They can often replace the burnt points and get you going again at a lower cost than replacing the whole unit.

"Don't sand"? Well, I wouldn't use normal grades of sandpaper: 80, 100, 220, etc. But, as others have stated, the standard method of renewing relay contacts is to use a burnishing tool. They are thin (like 0.005") metal blades with a very fine abrasive on them, probably diamond. That is how it is done. The abrasive is well bonded to the metal blade and little to none of it will come off. And if a particle or two does come off and become embedded in the contact, SO WHAT. Worst case is it will make a scratch that is a few thousandths of an inch long due to the wiping action of the contacts, if they are designed that way. Totally harmless! I have cleaned many relay contacts, always with a burnishing tool made for that purpose, and have never had any bad effects from the process.

Besides, just how are you going to get a file or scraper between those closely spaced contacts let alone manipulate it in a manner to clean the contacts. And even something like a nail file is probably too thick and will probably remove way too much material. With a scraper you have no idea how much you would remove. But that may not be a problem because a scraping tool that is thin enough to get between the contacts will surely be too flexible to allow anything to be scraped off.

CarlBoyd said:

I remember a thread on PM some time back where this was discussed. Several replies said "don't sand" as the abrasive grit would embed in the surface and cause problems down the road. So file or scrape, don't sand seems like safe advice. As soon as your 90 Amp contacts open and close once they will pit again, so no point in trying for a smooth finish, rough shape to remove worst slag off the top and your good to go.


CarlBoyd

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Here we go again, advise from people who don't know what their are doing.

First, take a photo of the contacts and post .

Because of the normal switching action, the contacts will quickly loose the smooth surface. There are two types of wear. First is do to the the making of the motor currents a roughly 6 times full load current. The appearance is a grainy rough surface somewhat like very course sand paper. The second is arc interruption wear which looks smooth shiny where the arc initiated and may have a trail from the arc spot to the edge. Both are perfectly acceptable.

When is wear too much. If the silver has been eroded to the support, its time. If the erosion of the two contacts has caused mechanical locking, its time. If the erosion has thinned the silver to the point that contact is lifting off of the support due to bond failure, its time.

The break arc has the tendency to run from the initial point of contact to an edge where it stay until interruption. With this type of wear, the arc will move to the edge of a contact and will sit there chewing of silver. Small balls or streamers of silver can occur on the edge of the contact. Once the edge of the silver is worn down to the support then the arc will chew on the support. There will be a smooth transition from the silver to the support. As long as there is sufficient contact material available, the contact will function as designed. This type of wear is most often seen when interruption of heavy currents and is not normal for switching a motor on and then shutting down the machine normally.

The color of the contacts has no effect on the operation of the starter. It is the color of silver oxide. Films of oxidation or silver sulfide are not a problem on heavy current motor starters for two reasons. first is that on closure there is considerable impact and sliding that will mechanically break through any films. Secondly, when operating on voltages in excess of 90 volts peak, there is sufficient voltage to puncture the films.

Blow the dirt off the starter. Make sure the magnet faces are clean of dirt and oil, that the motion is free of mechanical binding, that the contact springs haven't taken a set or annealed and all the screws are tight and use it.

Doing any of what has been said will only lead to lose of contact life and potential problems from contact contamination.

Tom

I borrowed a large man lift years ago. Wasn’t working right I found contacts in relay burnt real bad. Needing it to work and living in a small town there was no 24 volt continues duty relay to be had. I went to the house and found a pre 1964 dime and cut it into strips and built the contact back up filed it down and I guess its still working.

true temper said:

I borrowed a large man lift years ago. Wasn’t working right I found contacts in relay burnt real bad. Needing it to work and living in a small town there was no 24 volt continues duty relay to be had. I went to the house and found a pre 1964 dime and cut it into strips and built the contact back up filed it down and I guess its still working.

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Whatever it takes!

Tom

true temper said:

I borrowed a large man lift years ago. Wasn’t working right I found contacts in relay burnt real bad. Needing it to work and living in a small town there was no 24 volt continues duty relay to be had. I went to the house and found a pre 1964 dime and cut it into strips and built the contact back up filed it down and I guess its still working.

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TDegenhart said:

Whatever it takes!

Tom

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Exactly, I have used sand paper, files, emery boards (or whatever women file their finger nails with) to clean up contacts. A low budget quick and dirty job will usually keep them going for years or maybe decades. When I was a rat on a budget in High School and College trying to keep $300 junkers alive that had points in the distributor, I always had sand paper and an emery board in the tool box in the trunk for emergency roadside repairs.

One thing to keep in mind, the last two posts deal with an entirely different issue, that of contacts used in low voltage and in DC applications. In the case of automotive, the DC current transfers material from one side to the other leading to cup and cone problems. The contact materials are often tungsten. I have worked briefly on battery truck applications, the type True Temper mentioned but do not have enough background about contact wear.

Tom

My father told me some old Dodges had polarity reversing on the starters to reverse metal deposition and eroding on the relay contacts. Theory was it would make contact life almost infinite. In fact I think it added more to go wrong.
Bill D.

Tom, I am glad you are on here, keeps me from feeling like a voice crying in the wilderness. For those of you who do not know his background, he designed many of the contactors you use, working with far better resources than any of us on the outside have available.

Re grit embedment, Hans Fischer, who was in the honing lab at Sunnen, did experiments on the subject. He found that to get embedment you had to almost deliberately put pressure on it to force it into the metal. In any normal sanding operation, all you had to do was make sure there is no loose grit left on the surface.

There were distributors with polarity reversing but the points eroded anyway.

Re the use of dimes, coin silver works well if the contact is not being worked too hard. In more demanding applications silver will weld shut, causing no end of grief. I resurfaced thousands of locomotive braking and reversing switch contacts with 3% copper silver alloy but they were not switched under load. The control system turns off the generator field, switches, then brings the generator up again. In that service the criteria were contact area and pressure. There was no arcing and most of the wear was from "wipe", the slight motion sliding the contacts to keep a clean surface. When the GE dash 8 locomotive went on line, a glitch in the computer program would switch under load and we would get to resurface the contacts. Business was great while it lasted but they got the program fixed and business went back to normal.

Bill

Uhm okay....then why do some have "blow out coils" ?

digger doug said:

Uhm okay....then why do some have "blow out coils" ?

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Blow out coils are used on DC contactors. An electric arc is subject to the same forces as a wire carrying a current in a magnetic field. The current in the blowout coil pushes the arc into the arc chute, stretching it until it breaks.

Bill

9100 said:

Blow out coils are used on DC contactors. An electric arc is subject to the same forces as a wire carrying a current in a magnetic field. The current in the blowout coil pushes the arc into the arc chute, stretching it until it breaks.

Bill

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Yes I know that.

Why are they on a loco then ?

You stated that they drop the field off the alternator, so switching
is done "cold", yet there are contactors with blow out coils in there.

9100 said:

...When the GE dash 8 locomotive went on line, a glitch in the computer program would switch under load and we would get to resurface the contacts. Business was great while it lasted but they got the program fixed and business went back to normal.

Bill

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Were the contactors switching AC or DC?

Is digger doug referring to the old DC generator loco's?

Tom

TDegenhart said:

Were the contactors switching AC or DC?

Is digger doug referring to the old DC generator loco's?

Tom

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no, i'd be referring to a dash 8 and newer.