Compressor Contactor with shorted coil

[facilityguy]

Hello, I have a 20 ton water source heat pump that is two stage. Today is the second day in a row that I've come in to find the second stage contactor has a shorted coil. The unit tries to run and the fan will come on, the first stage compressor will come on, and after the one minute time delay when the second stage compressor tries to run, the direct short momentarily cuts out the transformer and then the Honeywell stat goes into it's three minute time delay and then the cycle starts all over again.

I replaced the contactor yesterday and everything seemed to run good for the rest of the day but i had the same problem today when I came in. Any ideas or advice?

Thanks

[hvacmike85]

That sucks everytime a new part fails, it could be the part again, don't assume its not, most likely its a wiring issue. Have you checked voltage on the coil side, amps etc? Now when you say shorter is the contactor coil open, blowing a fuse, what? Could be a bad wire i guess need more info..

[Six]

You mean your second stage input is shorted to ground through the unit.

Its a compressor circuit so it will have safeties ahead of that coil.

Isolate the subcircuits in that compressor circuit and check for shorts.

I doubt seriously that its the coil.

It could be the t-stat wire cut as it runs through a bulkhead.

[facilityguy]

I'm not getting any grounded control wires, nothing has continuity with the housing. The two coil tabs on the contactor are reading 0.2 ohms across them and on a new contactor the 24volt coil is reading 7.9 ohms. Also I don't have fuses in the control circuit to blow, it looks like they were never installed, I'll be buying some this afternoon when I head to the parts house to stock up on a few more contactors.

[facilityguy]

Also my control voltage at the contactor is reading 23.4 volts so I believe that's not a problem. I get the same voltage at the first stage compressor and also fan contactor with no problem.

[hvacmike85]

Is start with another contactor fused like you said and see.. let us know, if it happened again you need to keep a meter on the low voltage tabs for awhile while it runs and see, I've seen em drop out after a few min from a broken wire heating up

[Six]

With control wires pulled your resistance through the coil is only 200 millohms ?

Sounds like its getting hot and the insulating material they coat the coil wire with is breaking down causing coductors to make contact.

That coil for some reason is pulling excessive amps.


When you install a new coil check your amp draw through that circuit.

Again you might have a weak short causing excessive load on that coil.

If amps are high disconnect tstat wire and jump from unit 24 volt to see if your amps drop.

That will isolate either unit or t-stat wire as the problem.

[jpsmith1cm]

moved to tech to tech commercial

[hvacmike85]

[QUOTE=Six;13805601]With control wires pulled your resistance through the coil is only 200 millohms ?

Sounds like its getting hot and the insulating material they coat the coil wire with is breaking down causing coductors to make contact.

That coil for some reason is pulling excessive amps.


When you install a new coil check your amp draw through that circuit.

Again you might have a weak short causing excessive load on that coil.

If amps are high disconnect tstat wire and jump from unit 24 volt to see if your amps drop.


what he said, I;m rooting for a t-stat wire, if theres a spare try it first unless you'll have to pull new

[facilityguy]

Thanks for the input guys but I guess there's something to be said for brand recognition. When I put the new contactor on there I did take a current reading and I was only drawing .25 amps on the Y2 leg of the control circuit that was going to that contactor. The two contactors that had burnt control coils were Packard brand contactors that I got from Johnstone, the one that I have on there now is a Furnace brand contactor and has been holding up for two days now without a problem.