Stuck 3 phase compressor
Two days ago I was called to a York 3 Ton 3 phase RTU. Thermostat off when I arrive. Turn thermostat on for cooling call. Go up on roof and the compressor is not running. Turn off the breaker. Ohm out the compressor and check for short to ground. All good. Flip the breaker back on and the compressor makes a grinding noise for about 5 seconds and kicks off. Shut the breaker off again and put my meter on compressor wire. Flip the breaker back on and its pulling 65 amps, making grinding sound, kicks off after 5 seconds. All three legs are getting power. Diagnose as a stuck compressor.
Give the owner of the business a quote to replace the compressor. The next day the owner calls me and says to cancel the compressor replacement. The property manager sent his a/c company out to verify it was indeed a bad compressor. Low and behold they tell the owner that it was a bad breaker and all is well now.
My question is would it be possible for a compressor to become unstuck? If so, what could I have done with a three phase compressor to make it run again???
Fix it right, get paid, sleep good.
You can try switching legs of the compressor (reversing the motor rotation). Sometimes, if you are persistent enough, this will free it. You can also try reversing the rotation and energizing the compressor while striking it with a rubber mallet. If it frees up, it is likely only a matter of when (as opposed to if) it's going to get "stuck" again.
Ive "un-stuck"(got going) a few 3 phase compressors by reversing any 2 legs of power going to it. Again this would work in a reciprocating application, and sometimes it works sometimes not. Even hitting the side of it with a tool/foot on start-up worked once, and this isn't a guarantee it will last long after either. Why not try right?
Did it pull 65A on all the phases? If it did it would show each winding was getting power and was trying to work. If not then it could be a break in the power anywhere from inside the motor to all the way back to the power companies supply!
I have unstuck a compressor by swapping phases ove but it only lasted a week so i expect if ones worn enough to seize its only going to get worse.
I have a small video camera that i film stuff like this with just incase there are any disputes over my diagnosis, can be useful to be able to show the custome how something up on the roof is behaving when they don't want to climb up and see themselves.
No mention of measuring 3 phase voltage.
A single phase could also cause that and a bad breaker would make more sense for a single phase.
I agree, but I assumed he checked all legs to compressor, checked amp draw etc before recommended a replacement.
Originally Posted by jpsmith1cm
Easier to see a blower motor single phasing on start up as it just shakes and rotates back and fourth until trips.
Last edited by Mikey83; 07-19-2012 at 12:06 PM.
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.
Sounds like single phase situation... Would look like locked but need to make sure all 3 legs present.
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.
Also no mention of checking wires in peckerhead. I have come behind other techs that said compressor bad only to find a burnt wire causing single phase. .02
I am going to guess the compressor was single phasing.
I had one about a month ago with bad breaker. the blower acted as if it were locked up. checked all phases and found a phase with no power.
it was the main breaker that fed the disconnect.
always check phase to phase and make sure contactors are good.
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.
Originally Posted by Birmingham
Did the motor run?
Typically, 3 phase motors have a very low line to line resistance.
Most that I've measured are under 1 ohm.