Contact cleaning

A surface grinder was single phasing because of an intermittent in the switch. The switch is an old manually operated Cutler Hammer with buttons for on and off. The contact leaves are about 5/8" long with about 3/16" diameter silver contacts on each end. They are spring loaded closed and a Bakelite bar actuated by a cam pushes all three out of contact. The leaves were held in place and the fixed contacts were riveted in place, preventing removal for service. I did the 400 wet or dry routine that everyone tells me will have some terrible effect and got a lot of black smut on the paper, which I had to keep cutting off because it loaded with the crud. Anyway, the switch works now. I would like to get a new one, but the part number comes up dry and the only switches with that basic configuration seem to be single pole and I need at least two (I can get away with leaving the grounded B phase connected}.

I guess this is a question for Tom D who keeps telling us not to sandpaper contacts. What should I have done instead?

Bill

Did you try something like this: https://www.amazon.com/gp/product/B000BXOGNI/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 ?

The only reasons I know of not to sandpaper contacts are:

1) some sandpaper leaves conductive grit. The "wet-or-dry" does not.

2( for contacts with a plating, the paper will wear it away. But plated power contacts lose plating right away anyhow.

3) You can change the contact shape, which may mess up the pressure per unit area and lead to problems. But generally, you can reshape to the original shape and that is generally OK... Have never had an issue if I made an effort to keep the original shape.

9100 said:

What should I have done instead?

Bill

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When... I've lost track of where my proper "ignition point" files and/or diamond-coated relay-contact burnisher kit has walked-off to?

Just go and rob the pretty-lady goods rack at the local dollar store for a fine carbide grit or even diamond-dust coated "nail file".

Reshape with one of those, finish-off with Crocus. The bespoke tools are faster, but you don't have a Strowger switch to service, so BFD.

The moving contacts are captive in wells in the Bakelite housing. They can't be removed without drilling rivets out, in fact, I couldn't see their surfaces. Any rigid file or whatever will not work because the housing interferes. I had to form the sandpaper strip into a curve to slide it into the gap, then let the contact closed and pull it out. The space was so small that I couldn't even fold the strip double and had to sand each side individually.

I would like to find a direct replacement but probably will have to adapt a modern one. This grinder is a little Harig, probably from the 60s and I doubt that these switches are still made.

Bill

I saw this only by accident. The contacts I recommend not sanding are power contacts. Small contacts with light spring forces I would recommend just rubbing with a contact cleaner such as alcohol, clean solvent, brake cleaner or any of the other materials used to clean metals. The contacts themselves are usually not damaged, just coated with contamination. A common goo is lub oil mixed with dirt, plastic residue or whatever.

Tom

Buy one of those fat lead pencils. Stick it in a drill and use the eraser end. Works great.

I'm thinking instead of looking for some specialized switch, why not just get a 3 phase contactor with overloads, and a couple of cheap buttons? Just hook it up like any 3 phase motor control. AutomationDirect has contactors and overloads and buttons and even enclosures for good prices. I'd get the overloads that do not use replaceable heaters, but an adjustable knob. You'll need to know the approx. amp draw before ordering this stuff. Then you'll be protected from single phasing damage,

greggv said:

I'm thinking instead of looking for some specialized switch, why not just get a 3 phase contactor with overloads, and a couple of cheap buttons? Just hook it up like any 3 phase motor control. AutomationDirect has contactors and overloads and buttons and even enclosures for good prices. I'd get the overloads that do not use replaceable heaters, but an adjustable knob. You'll need to know the approx. amp draw before ordering this stuff. Then you'll be protected from single phasing damage,

Click to expand...

This is only a 2 hp rated switch, mounted in a hole in the base casting. I can graft some other switch onto it but I would like to find one that will fit in the original opening. When the spindle failed to come up to speed, the operator had sense enough to turn it off. Right now it is working and may for a long time since the contacts still have a reasonable amount of silver left. Because there is no rush, I will keep looking for a drop in replacement.

BTW, this reminds me of my favorite single phasing story. A salesman visiting a maintenance electrician's shop noticed some disconnect boxes that looked like they had been blown up. When he asked about them, the electrician explained that they had a pump house in the middle of a lake that attracted lightning strikes, which sometimes blew one fuse and the motor would burn up single phasing. When the pump came off the line he wanted it all the way off so he used changeable link fuses filled with gunpowder. When one blew, it dismantled the whole box, saving the motor.

I make no claim of authenticity, just repeating the story I heard.

Bill

How this be a multiphase motor if it needs a start switch?
Tom

It may not be proper but I have used A/B 800T selector switches with 3 contact blocks on 3HP motors. Never had one fail........

From an engineering standpoint there are valid reasons not to sand or file contacts, the first three of which are what JST brought up. The other big issue is that what some people see as "pitting" is actually not a bad thing, it is what they are DESIGNED to do. From a microscopic standpoint when contacts separate and form an arc, that arc (the temperature of the surface of the sun) is melting the contact material and as they separate the "peaks" on one side will match up to "valleys" on the other side. So as they deform like this with use, they actually are INCREASING the surface areas that are making contact, thus reducing the resistance and voltage drop across the connection. Sanding / filing the contacts starts the process all over again, but begins with less material to work with and it diminishes with each subsequent "cleaning", plus the issue of getting the sanding / polishing medium into those peaks and valleys as a contaminants. Eventually it all works to INCREASE the resistance across the contacts, which becomes heat that builds up until the entire assembly deforms the armature / arc chutes to the point of failure.

That knowledge has however never stopped people from doing it.

Just clean them see how they work. If deemed unsatisfactory then a points job may be ok to quiet down a 12V relay. A higher voltage relay will eventually get no better, maybe even worse. In the meantime the height of the contact has been lost needlessly.

Bill:

I have a couple of boxes full of old switch, relay, and contactor parts. Post a picture of what you need and I'll see if I have anything that would work for you.

TDegenhart said:

How this be a multiphase motor if it needs a start switch?
Tom

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I don't know the horsepower of the motor and can't look right now but it is probably 1 hp or so. As I said earlier, it is on a 6X12 Harig grinder. The motor is 3450 RPM three phase and the switch is simply three poles single throw. It doesn't need anything fancy, just an on-off switch.

Bill

So this is not a switch in the motor, part of the on-off switch?

Tom

Jraef said:

From an engineering standpoint there are valid reasons not to sand or file contacts, the first three of which are what JST brought up. The other big issue is that what some people see as "pitting" is actually not a bad thing, it is what they are DESIGNED to do. From a microscopic standpoint when contacts separate and form an arc, that arc (the temperature of the surface of the sun) is melting the contact material and as they separate the "peaks" on one side will match up to "valleys" on the other side. So as they deform like this with use, they actually are INCREASING the surface areas that are making contact, thus reducing the resistance and voltage drop across the connection. Sanding / filing the contacts starts the process all over again, but begins with less material to work with and it diminishes with each subsequent "cleaning", plus the issue of getting the sanding / polishing medium into those peaks and valleys as a contaminants. Eventually it all works to INCREASE the resistance across the contacts, which becomes heat that builds up until the entire assembly deforms the armature / arc chutes to the point of failure.

That knowledge has however never stopped people from doing it.

Click to expand...

This is fine if the contacts are positively located so they always mate up. In this case, the moving contacts are little links sitting in pockets in the Bakelite housing. There is no restraint on them only a spring pushing them closed and they have at least 1/32" movement available in each direction. There is no guarantee that they will close the same from one time to the next and it is not possible to clean them without moving them.

Besides, many contactors are designed to move the contact after it touches. I have remanufactured thousands of locomotive contacts that supply power to the traction motors. The manual has specifications for "wipe", the amount they move after initial contact. Admittedly, they are switched with the power off, done by shutting off the generator field, switching the reversing or braking switch, then bringing the power back up. The contacts wound up with deep grooves in the wipe direction. There was no arcing but look at regular magnetic motor starters. Many of them have contact wipe so the points of contact when they are completely closed are not the ones of initial contact.

Another place for our resident contactor expert to comment.

Bill

awander said:

Bill:

I have a couple of boxes full of old switch, relay, and contactor parts. Post a picture of what you need and I'll see if I have anything that would work for you.

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Here is an ebay listing on a similar switch but it appears to only be single pole. The present one is working fine at the moment and there is plenty of silver left so we don't need another used one. We would like to find a new one which we would keep as a spare in case the present one fails. It may not for a long time.

Anyway, thanks for the offer.

Bill

CUTLER HAMMER 10250H3829A PUSH BUTTON | eBay

TDegenhart said:

So this is not a switch in the motor, part of the on-off switch?

Tom

Click to expand...

It is mounted on the base casting, just a simple switch. Any three pole one that could handle the current would work; it is just that I would prefer to find a direct replacement.

Someone called?

Contacts are not designed to pit. This a phenomenon that occurs when contacts close and open under arcing conditions. Arcing conditions are those where there is sufficient energy in the circuit to allow an arc to occur. An example of non-arcing contacts are those used in logic circuits.

I will restrict the discussion to inductive circuits.

First thing to keep in mind is that all mechanical systems have a tendency to bounce on closure. Contacts are no different. When the contacts come together, the spring-mass characteristics will cause the moving contact member to rebound. The stationary contact assembly will also move, but very little and will be considered immobile. The bouncing time period will be in the neighborhood of 1 to 5 msec. The open time of the contacts will be in the older of 100 micro-sec to 1 msec.


To demonstrate, connect a switch contact assembly through a 9 volt battery and through a 1000 ohm or so resistor. Connect a storage oscilloscope across the resistor and the contacts. Non-storage will work but you have to be fast to see the bounce. When the contacts are closed, there will be string of openings and closings of the contacts. The pulses will be well defined. Now substitute an inductor such as a high voltage (460 volt) coil for the resistor. Now the pulses will no longer be shapely defined but will show the classic charge discharge of an inductor.

Now as the contacts bounce open there will be a short arc. If the values are correctly chosen for the voltage and the inductor, the voltage across the contacts will not shoot up to the battery voltage as with the resistor, but will rise to the arc voltage of 10-20 volts and will be more or less constant. This is an arc between the contacts. This occurs during the bouncing period and settles out to zero.

The same phenomenon occurs during switching the contacts off accept that voltage across the switch will go from closed to open circuit voltage with an arc occurring at the moment of opening and extinguishing when the energy in the coil is expended.

That's the basics.

As you know arcs liberate an intense amount of energy in the arc. During the contact closure, it these short arcs that melt a tiny amount of silver and create a solid bridge (weld) between the two surfaces. Fortunately, the weld is very week and is easily broken by the impact of contact carrier. If you look at the weld spot with a microscope the surface is rough and ragged. This is the peaks and valleys that are observed. This is not desired as bits of broken silver will fall out of the contacts and be lost. Another reason this not desired is that when the contacts close on these tiny peaks not so much energy is needed to melt the silver and can result in solidly welded contacts.

Under normal power switching conditions, the break arc which is higher energy than the make arcs have a tendency to melt a large area and smooths the surface. Another feature of the break arc is that it acts to clean the surface of the silver of sulfides, dirt and such.

I wish I had pictures and videos of these happenings but I do not.

Tom