How do I know when contacts in a motor starter are too worn?

I bought a size 2 motor starter on ebay. It is Allen Bradley and looks like 500 series. (the label does not say what series).
I want to use it for my 10 HP compressor, which is the kind of service where it would restart the motor many times per day.
Anyhow, I opened up the cover to look at contacts and it appears that they have some wear. I cannot really tell if it is too much or not, because I am not qualified to tell.
So, I have a couple of questions.
1) How can I know if the contacts are too worn?
2) If I buy an "aftermarket contact kit", are they really as good as the originals? Do the original contacts have silver plates?
Thanks
i
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You will see voltage drop and excessive temperature rise during use.
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At 29 amps of FLA, a voltage drop of even 1 volt will melt the contacts in short order. I do not think that voltage drop will be a good indicator.
i
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Do the math, Mr. Algebra.com - any contact big enough to call itself a motor starter will take 29 measly Watts all day long without melting unless it's made of Wood's metal.
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I think that 29 watts per contact (three poles makes 87 watts) will quickly destroy the contactor.
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Ignoramus2083 wrote:

I'm with Iggy on this one. No time to do the math, but I have had experience trying to remove a couple of watts of heat from a small semiconductor footprint.
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Jim Stewart wrote:

Sigh. The contacts are a hell of a lot larger than the die in a semiconductor, which may only be .001 square inch.
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They're also ignor(amus)ing (gotcha, Iggy) that the contacts are not just floating alone in still air. They're welded or brazed to larger metal pieces which form the armature or fixed contacts in the starter. Those metal parts also dissipate heat (and conduct it away from the contacts very well, by design).
Someone, probably a long time ago, kept the heat dissipation needs of those parts in mind when they designed the contactor part of that starter.
When it's working properly, I'd expect to see a larger drop across the heaters than across the contacts.
LLoyd
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"Lloyd E. Sponenburgh" wrote:

Likely in Edison's day, when they had to manually adjust the field current of their DC generators to maintain the proper output voltage. there is little difference in very early designs and today's, other than better insulation. Contactors I used in the '60s are almost identical to today's. The older units were US made, but all bets are off for new contactors.

All he needs to do is check & log the temperature of the cabinet & the contacts with a non contact IR thermometer once in a while. If he notes any change in the differential, then it's time to either dress or replace the contacts and retest.
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Michael A. Terrell wrote:

Sigh. By footprint I meant a TO-220 package.
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Jim Stewart wrote:

The poackage is a lot larger than the die. It's the junction temperature that's critical.
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On Wed, 12 Oct 2011 14:17:18 -0500, Ignoramus2083

Iggy, this is one of those good things to do on a new install. Even if they aren't that bad. if you make it a policy to get your equipment in top shape when you do work on it, you'll have way less downtime in the future.
Karl
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Karl, I agree and I will do just that. But what do you think about aftermarket contact kits? Are they OK?
i
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On Wed, 12 Oct 2011 19:53:27 -0500, Ignoramus2083

I'm sure they are fine
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It may be helpful to go to ab.com and look in their literature library, it's great to get instructions (for those who use instructions) on older equipment.
We have an indexing festoon at work that cycles every few seconds 24/7, the contactor welded itself closed, made a bit of a mess but out of 17 such indexing festoons, we don't replace contactors very often. This was an AB 100 series IIRC.
RogerN
"Ignoramus2083" wrote in message
I bought a size 2 motor starter on ebay. It is Allen Bradley and looks like 500 series. (the label does not say what series).
I want to use it for my 10 HP compressor, which is the kind of service where it would restart the motor many times per day.
Anyhow, I opened up the cover to look at contacts and it appears that they have some wear. I cannot really tell if it is too much or not, because I am not qualified to tell.
So, I have a couple of questions.
1) How can I know if the contacts are too worn?
2) If I buy an "aftermarket contact kit", are they really as good as the originals? Do the original contacts have silver plates?
Thanks
i
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wrote:

You've got a different problem - with that kind of short-cycling service, the machines really should have been designed with a second fail-safe contactor to kill incoming power to the whole machine if this happens. Safety contactor, or a Shunt Trip Breaker.
The trick would be sensing the problem before the machine totally crashes and self-destructs. Is there a hard stop or an indexing pin at the station stop? Perhaps change it from a hard stop point (like an adjustable bolt on a solenoid drop-pawl) to a really stiff spring and the micro-switch to sense when it keeps pushing.
One other (far simpler) solution would be to change them out to an electronic starter - Soft Start or a full on Variable Speed Drive. Probably have to go way oversize on it because of the short-cycling heating issues.
The VSD should have a sanity check built in that drops out the safety contactor if the transistors(*) short - "Hey, I turned off but there's still power on the Load. Something's Wrong Here..."
--<< Bruce >>--
(* Transistors, SCR, SCS, Diac, Triac, etc. - whatever the hell they used. There are several ways to do it.)
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"Bruce L. Bergman (munged human readable)" wrote in message
wrote:

You've got a different problem - with that kind of short-cycling service, the machines really should have been designed with a second fail-safe contactor to kill incoming power to the whole machine if this happens. Safety contactor, or a Shunt Trip Breaker.
The trick would be sensing the problem before the machine totally crashes and self-destructs. Is there a hard stop or an indexing pin at the station stop? Perhaps change it from a hard stop point (like an adjustable bolt on a solenoid drop-pawl) to a really stiff spring and the micro-switch to sense when it keeps pushing.
One other (far simpler) solution would be to change them out to an electronic starter - Soft Start or a full on Variable Speed Drive. Probably have to go way oversize on it because of the short-cycling heating issues.
The VSD should have a sanity check built in that drops out the safety contactor if the transistors(*) short - "Hey, I turned off but there's still power on the Load. Something's Wrong Here..."
--<< Bruce >>--
(* Transistors, SCR, SCS, Diac, Triac, etc. - whatever the hell they used. There are several ways to do it.)
This uses a motor with brake and a prox switch to detect the index position. I have a timer in the program that if it doesn't complete the index in time, it alarms and shuts off the contactor, but that doesn't help when contacts are stuck. I agree we should have a contactor that kills the 460V AC to the starters when either the control detects a problem or when the E-Stop is pressed. We do have a disconnect between the starter and motor so things can be locked out.
I wouldn't have designed it the way it is but that's what the company has used for years and hasn't had enough failures for them to decide to change the circuit. Some of the new lines have VFD's for the festoon indexing, and power is killed to the drives on E-Stop conditions.
RogerN
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On Wed, 12 Oct 2011 14:17:18 -0500, Ignoramus2083

Open up the contactor and look at the fixed and moving contacts, then use your good judgment to assess them. Duh.
No, seriously. They are going to develop little pits and craters from the arcing of normal operation, but they tend to self-correct as they age (a bump on one side forms that fits into the crater of the mating contact) until they totally run out of metal.
As long as they are overall flat, and the little button has a good 1/16" or so left, and they are all even with each other, you're good.
You can file the old set down to see what the actual contact area is, but don't make a habit of it - you just kill them a lot faster.
The only time I really get crazy filing them is to get a totally bad set working JUST long enough to run and get replacements. And they have to start flat - because you swap the really bad contacts around so at least one side has a little contact left. Bad moving set to two halfway decent fixed points, and vice versa. Cross your fingers, and get on your horse.
The real way to tell is to buy a replacement set of contacts - that's the "before" that you want to remember how they look, and you want it to still maintain the same basic profile allowing for a little pitting.
You don't have to change the contacts now - just wrap the box up and stash them - I'd put them *inside* the contactor enclosure, and when it eventually fails on you... You open the box and see the 'present' you left for yourself. Ten minutes later, you're good to go.

The factory contacts are usually a Coin Silver alloy welded to a plated Copper based bar or screw lug base. When the Silver runs out, it'll start eating into the copper fairly fast - but they usually run out of spring overtravel by then and the gap grows too large.
The replacements are supposed to be the exact same thing, because there's often a UL Listing involved. But I would be really wary of aftermarket ones made overseas, only because they might cheap out on the contact buttons and they won't last nearly as long.
Heck, the replacements might be from the same factory that makes them for the OEM, they just decided to sell direct too. But you have no way to know. Caveat Emptor.
--<< Bruce >>--
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