Stuck 3 phase compressor

[Birmingham]

we have changed this compressor 3 times in 2 years, i would expect this compressor to read less than 1 ohm between legs and show nothing to ground, this compressor pulls massive amps, my meter jumps and does its im thinking thing then goes to 0. the internal overload trips almost instantly. i have never seen one do this. i am worried that the breaker is bad that feeds the unit, slow blow fuses in the disconnect so i see why they don't go but i wouldn't think the breaker would take this.

this also poses another question unit running great when i change compressor:
phase monitor says were all good
its a trane 10 ton can style reciprocating
not burned last time it was pulling high amps (6 more) on one leg normal on other 2 (Im thinking loose wire connection somewhere) so changed all the wiring we could get access to.
no voltage imbalance on the contactor load side between legs or to ground.
new refrigerant every time new driers never any acid so no suction drier expansion valves working like they should checked in heat and cool working well. Package unit weigh in the charge.

[Mike19]

i came across one today that was at 0 ohms all 3 legs on compressor, scary part was it didnt trip the breaker, the thermal overload in the compressor was catching it before it flipped the breaker, i have never seen a compressor shorted on all 3 legs not flip the breaker, before i replace the compressor im going to check the breaker.

How do you check to see if a breaker will flip? i really cant think of a safe way.

I had a unit with two 3 phase bristols that did that. One was about 5 ohms to ground on all 3 legs. The overload in the compressor tripped before the 60 amp breaker in the building did.

[Jimmie Winer]

you said it was 3phase and was drawing 65A. was that LRA, did you check each legs amp draw. i just went through this myself with a breaker back in a mcc was not making good contact on one of buss bars. it was an intermittent issue so of course when i checked the unit amp draws everything was fine till the breaker warmed up.

I'm thinking one leg on the breaker is opening under load as well. If your luck is like mine it will pick the leg thats not carrying the transformer primary or the condenser motor cuz that would just be too easy...


When your the guy giving the second opinion you always dig a little deeper to make sure, plus you have nothing to lose.

I diagnosed one just like that, up sold to a new condenser and when I powered it up I had a knockdown drag out fight with my conscience before I walked into my customers office and told him I just bought him a new unit because I mis-diagnosed in the first place. Luckily his mom was there, (True Story) and insisted he pay me, we negotiated a discount, still hurts...

Ive reversed em and unstuck them more than once. You can do it with a single phase if you know how. pull up a PDF on an old ANNIE. I always considered it a temporary fix until I could get in a compressor. I lived in a small town, compressors were overnight on a good day...

[Mrak]

Before you change that compressor with 0 ohm readings on the windings, make sure it's not the internal overloads.

I was about to comment to the thread until I read everyone else's comments. That's when I realized that all my ideas were covered. It really could have been a bad breaker single phasing. Perhaps a loose connection at the breaker, contactor or peckerhead. Who knows, perhaps a box-elder bug on the contacts which I've seen two or three times where I'm at. If it's running now and wasn't before I would 'tend to believe what the guy told you which was a bad breaker. Or talk to the service guy who checked it out and diagnosed it. I'm guessing it was probably the breaker like they told you.

[steve wright]

How about the one that was growling and tripping breaker, is it still running?

[atmosphere]

Single phasing. Nothing unstuck.

You have lost credibility.

You appear to be a humble man and should bounce right back and smarter too.

[ICanHas]

One thing that is overlooked is they see voltage at all 3 legs but when the system starts the voltage drop from a leg, bad wiring loose connections corrosion weak breaker etc.. Always pull the Breaker if its hot to the touch.
Many compressors are replaced when it was wiring all the time that was the problem.
Bruce

This is what you use to diagnose it. Those round things are light bulbs. Our eyes are incredibly good at picking up flickers. The bulbs need to be 240v rated for this setup. You can expect that they will flicker, but they all need to dim the same amount. If one remains brightly lit, but the other two dims, you can figure out which contactor is bad. You have to use some common sense to use this hack job.

If you hook it up right below disconnect then, AB dims on compressor start, you know phase 1 is weak.
After turning off the breaker feeding it, place this above the disconnect but below the breaker and you still experience the same pattern, its upstream of the disconnect.


Build it on a plywood but label it clearly so the user doesn't get confused.
1 bad = AB dims
2 bad = AC dims
3 bad = BC dims

[atmosphere]

Thanks for this diagnostic tool. Brings me back to building my first vacuum pump from an old GM Fridgidaire refrigerator compressor (they ran in a vacuum normally). And the old Simpson 260, then when the Beckett meters came out before the head designer left to start Fieldpiece.

In the days of Moncrief, Luxaire, borg and Stewart Warner, and on and on.


Enjoyed the blast from the past. After 34 years I still
like them.

Thanks