Contactor service and trouble shooting

[zerodegreec]

I had a really good manual quite a few years ago but it has grown legs of its own (typical). Now I don't remember where I got it.

What I need is documentation explaining how and why contactors fail, and in particular explaining when to change contacts. I have quite a few guys that cant seem to grasp the concept that just because you have some pitting does not mean we need to bend the customer over with repairs they don't need.

Thanks

[pactor]

One thing that might be worth doing is to measure the voltage across the closed contactor when is on load, if is starting to get bad contacts it might show up as a measurable voltage as the resistance increases across the contacts. If there only a small volt drop its probably taking the load ok.

[Electrocoolman]

How about using a point and measure infra-red thermometer as a means of checking for heat difference at contacts.?
Any body tried this?
In the past I've used a thermal imaging camera to pick up poor contacts generating heat.

The best maintenance practice is to check wiring connections are tight, and to measure contact resistance with system power isolated, and pressing contactor in whilst measuring across contacts.

[Jockie Gee]

Try using a diamond spatula or commutator sticks to clean the contacts up. You should be able to see how much metal is left on the contacts, if its to low then you need to think about changing them. Its your call once you have cleaned them up you will see what level of arcing you have on the contacts that will make your mind up for you.

[chillyblue]

Why are all contactors rated as KW rather than amps???

CB

[paul_h]

Amps mean nothing without voltage. kW tells you the amps and voltage sum.
No point getting a 10A switch if it's rated at 10A @ 12V and you plan on putting 240V through it. Rate the switch in kW however and that mistake won't happen.

[Electrocoolman]

You also need to understand the difference between AC1 and AC3 ratings.

AC1 = resistive load
AC3 = Inductive (ie. motors) load.

[nike123]

Amps mean nothing without voltage. kW tells you the amps and voltage sum.
No point getting a 10A switch if it's rated at 10A @ 12V and you plan on putting 240V through it. Rate the switch in kW however and that mistake won't happen.

That doesn't work like that. Contactor contacts must be rated in amperes because that is what are dimensioned for. It is not same 2kW at 20V and 2kW at 200V.
You will see at every contactor that has noted amperage rating.
Also every rating in kW has voltage and AC class noted for that rating.

[chillyblue]

So what would be the correct procedure for selecting a contactor??

CB

[nike123]

So what would be the correct procedure for selecting a contactor??

CB

You should first know what AC category is your motor (AC3 for compressors) then find contactor who has amperage rating in that category at least as FLA of compressor. Also, you could find AC3 rated contactor for your operating voltage by selecting matching kW rating for your voltage.
Then, find if main and signal contacts configuration is for your requirement. And last, find one with control voltage matching to your control circuit.

http://www.pacificcoastbreaker.com/f...%20catalog.pdf

Mostly contactor type label has in it amperage rating for AC3 category contacts.

[casstrig]

It seems we are slightly off track the question was how do we know if a contactor has had it,the best way is to measure resistance unpowered and voltage drop running especially on startup.Remember time is money and I dont think dismantling small contactors(less than 15kw)is worth the effort or as reliable as a new one.

[walden]

What kills contactors quicker than anything else is the frequency at which they have to switch on and off. This especially applies to refrigeration compressors where the lack of gas makes the LP switch open and close quickly. A limit start timer to prevent the compressor starting more that 12 times an hour is common.

Too frequent switching means that the contactor tips get too hot and melt, regardless of the current passing through them.

Don't try to mend burnt-out contactors: throw them away and replace with new ones rated at the current for the compressor motor.

And don't think that putting over sized contactors will stop them being destroyed if they are switching too frequently. The tips still get too hot and melt. It's not the current destroying them: it's the heat building up every time the tips open.

Look for the "Ie" rating of the contactor for the amps.

[chillyblue]

Thanks

Thats a great help

CB

[walden]

In my haste to write a reply, I left out a section! I did not mention what is happening at the other end of the contactor, the compressor motor!

Open compressor motors are not so bad but the same thing happens to them as happens to semi-hermetic compressor motors when they are switched on and off too frequently, they just take longer to die.

Both of them get hot and the winding insulation begins to fall apart!

This is especially true of semi hermetic compressor motors where the refrigerant gas is used to cool the motor windings. You have not got much gas anyway, that’s why the auto LP switch is going in and out like a fiddler’s elbow, and what volume there is, is insufficient to cool the windings adequately and on top of that the motor is being switched off after a few seconds as the system pumps down and whatever flow there is, stops.

So now, the compressor motor is getting too hot: the insulation fails and the windings short out. The contactor which has been going in and out too now tries to switch off a large fault current and fails as the tips melt. Often, one or more of the contactor tips weld together and the contactor cannot open and continues to supply all or some of the motor windings. The compressor motor is now rapidly destroyed and it is only when the fuses or MCBs operate that the smoking beast is laid to rest!