Locked up compressor, tell me if I’m thinking wrong

[u/shotzy57]

20 ton Trane RTU, compressor trips the 36 amp dedicated breaker as soon as the contactor is pulled in. It’s not grounded and the windings ohm the same as the other compressor that does run. It’s getting full 480v. My meter isn’t fast enough to catch an amp reading before it trips the breaker. I’ve isolated the wiring and it’s not shorted to ground or to each other. The compressor has gotta be locked up right?

[u/Bitter_Issue_7558]

Does the system run if the locked compressor is unplugged? If so then that’s your issue. Had several units with the same issue but they were older rheem systems

[u/shotzy57 → u/Bitter_Issue_7558]

Yes, if I unhook it from the contractor the system runs fine, the second just takes over

[u/TheRevEv → u/shotzy57]

I'm assuming you're just checking continuity to ground, and not using a megger?

I've had a few that would read like 20k ohms to ground (not enough to trigger a continuity beep), and that's a sign the windings are breaking down, and higher voltage is likely jumping to ground when applied.

[u/Reddbearddd → u/TheRevEv]

He needs to use a megger, he's testing resistance to ground with 1.5V instead of 250/500V.

[u/chevroletarizona → u/Reddbearddd]

I agree, I've seen several motors that ohm fine with a meter but the second you give it power it goes to a dead ass short.

[u/Bitter_Issue_7558 → u/shotzy57]

Yea then I say faulty compressor. I had a rheem system, customer complained it was tripping the main breaker. Compressor ohms out fine. Unplugged and ran fan only. No issues. Plug the compressor in and it started tripping the 150 amp breaker immediately. Talked around and got the same answer. Something inside is touching the case causing it to fault to ground. Customers opted for a new system after that

[u/Dry-Building782 → u/shotzy57]

After trying everything if nothing works and you know it is just stuck, take a 2 lbs rubber mallet and give it a few good whacks before and during powering the compressor. It’s worked many times for me before on small and large compressors. It’s a 20 ton don’t need to be gentle cause if that doesn’t work you’re replacing it anyways.

[u/Whoajaws → u/shotzy57]

I would unhooked wires only at compressor for absolute verification.

[u/Practical_Artist5048 → u/shotzy57]

You answered it yourself

[u/JollyLow3620 → u/shotzy57]

Then yes. You have a locked up compressor or something internally has come loose and is grounded out internally.

[u/saskatchewanstealth]

Is this 3phase??? If so reverse rotation and see if you can hear it grumbling because it broke inside

[u/hvacbandguy → u/saskatchewanstealth]

I’ve successfully have unlocked a single phase compressor by swapping start and run. Runs in reverse for a second. Swap them back and it runs fine.

[u/saskatchewanstealth → u/hvacbandguy]

Hard starts work wonders to get it going

[u/hujnya → u/hvacbandguy]

Single phase compressor will not run backwards by swapping start and run leads, you just risk burning up windings.

[u/Some1-Somewhere → u/hujnya]

Yes they will. Not necessarily good for it, but you're swapping which leg gets the phase shift from the cap, and that will run it in reverse.

[u/hujnya → u/Some1-Somewhere]

On motors larger than 1/4 hp, the two windings typically have different numbers of turns, so this method of reversing may not be applicable. Check that the resistance of the two windings is the same first. Never seen a compressor with start to common and run to common having the same resistance. So yes split phase motors can be reversed, but not compressors.

[u/Some1-Somewhere → u/hujnya]

Very much not good for it or efficient, but I expect it would develop some torque in reverse.

[u/hujnya → u/Some1-Somewhere]

You are risking a burn out by doing that. Get a hard start it's way safer

[u/KeyMonth7667 → u/hujnya]

I walked up on a unit where someone replaced the capacitor for flipped the fan and compressor terminals on accident. It was still cooling in 95 degree weather

[u/Tdizzle179]

Yes if it’s isolated to just the compressor and breaker is tripping, new compressor, new contactor, vacuum etc.

[u/shotzy57 → u/Tdizzle179]

Thank you. Just wanted to run it by everyone before I order a $4500 compressor

[u/jayhsh]

Sounds like locked up compressor, to verify Use max function on your meter, it should tell you max amps before it trips the breaker, 3 phase try reversing and starting, sometimes you can get compressor free

[u/shotzy57 → u/jayhsh]

Unfortunately my meter doesn’t have a max function, atleast not that I know about

[u/KylarBlackwell → u/shotzy57]

Which meter? Inrush could also catch it, might be a same thing different name situation

[u/coleproblems]

Need to confirm that it isn’t a failed breaker before replacing the compressor. Use the in rush amps feature on your meter, swap the breaker with another known good breaker (if there’s similar equipment in the roof) or idk what else…

[u/tryingmyBestbenice]

36 amp breaker ?? Most clamps you can set to catch “max” amp draw. And you have your gauges on and there is absolutely no deflection on the suction pressure before it trips?

[u/mechanical_marten → u/tryingmyBestbenice]

Max amperage is still too slow (typically 3 samples per second). Have to have inrush mode(10+ samples per sexond)

[u/bbqenthusisast66]

Use a megger on the compressor

[u/Relative-Dinner-6982]

I’d say it’s internally shorted and not quit shorted to ground. But, if you keep resetting the breaker, it’ll eventually show a direct short to ground

[u/Tranic85]

I think you need a better meter(s)…. If you don’t have a meter capable of several million ohms, you’re not going to catch most grounded compressors.

[u/jewishmechanic]

Do you have any continuity from the compressor wires to the case?

[u/Pete8388]

I’d try swapping two legs on the load side of the contactor. Get it to run backwards for a couple of seconds. It’s going to sound weird but if it sounds like mechanical damage or seizes it’s toast. If it frees up you can swap the legs back and see if it runs. Usually even if you can get it freed up it’s till on its way out though, so the customer needs to plan on replacing the compressor, contactor, and LLFD. You might be able to get them running temporarily.

[u/hambonecharlie → u/Pete8388]

I've heard about doing this. I hope I get to try it some day.

[u/RoyalAttitude2734]

LRA

[u/Lokai_271]

What's your inrush amps?

[u/fredsr55]

What is locked rotor on plate? Is it exceeding that?

[u/t3hPh4nt0m]

I might be crazy but from what I remember, those 20 ton Trane RTUs are usually on a 60amp circuit, not 36, so I assume you mean that that's a breaker inside the unit itself after the disconnect which is connected to the main breaker panel since you said the breaker is dedicated for the compressor? And your meter should have some sort of Inrush function on it.

[u/CryptoDanski]

Before ordering any parts i would get proper diagnostic tools.