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  1. #1
    Join Date
    Apr 2005
    Location
    Richmond, Virginia
    Posts
    365
    Did a service call last night on an old Trane heatpump. The circuit breaker would trip after the outdoor unit ran for an hour. I checked the temp and pressures everything was in order. But, when I checked the run amps 10.4 A from the nameplate, one leg was was at 10.3 the other was at 6.4 A. The unit is 240V/1 phase.

    Being it was getting close to 11 PM I couldn't stay to see the breaker trip. I believe the compressor is on its last leg. Is this correct?
    Sean Cantrell

  2. #2
    Join Date
    Feb 2004
    Location
    Maine
    Posts
    483
    Use a megohmeter and check the resistance of the start and run windings to a bare spot on the compressor shell. This will tell you the condition of the start and run winding insulation. Maybe the breaker is bad? Don't condem that compressor until you meg out the windings, and make sure that the start cap and all start components are working properly. Who knows maybe someone tapped into the electric line that feeds the heat pump and now that circuit is overloaded. Just some thoughts to ponder.

  3. #3
    Join Date
    Apr 2005
    Location
    Richmond, Virginia
    Posts
    365
    I did check the breaker and it was ok, lugs were tight, no cracks. Ohmed out the compressor, I got 3, 3.5 and 1 ohms for start run common (it's a start run cap), it's a 1.5 ton heat pump, but I don't have any info compressor winding ohms. It had a hard start kit on it already.

    I'm going back this afternoon to watch it cycle through and trip the breaker.
    Sean Cantrell

  4. #4
    Join Date
    Feb 2004
    Location
    Maine
    Posts
    483
    You could have weak winding insulation in the motor that might not trip the breaker until she gets good and hot. You need to use a megohmeter to check the winding insulation. A megohmeter uses 600 volts to measure the breakdown resistance of the windings. The minumum before a dead short is 1000 ohms per volt. So for a 208 volt unit you would need at least 208,000 ohms. Most motors when new are 100 million ohms or more. You can't use a regular ohmeter to check winding insulation, they just don't put out enough voltage to work. I would be suspect of anything less than 20 million ohms. Replace the breaker anyway, they can and do fail internally with no outward signs. Breakers are cheaper than compressors, wouldn't want to see you replace the compressor only to have the same thing happen with the new one. Also be sure that the wire feeding juice to the unit is the proper size for what you are drawing for amps. When you check the amperage use a clamp-on style meter which I'm pretty sure that is what you are using. Check the amperage on the common, that is what the compressor is drawing for total amps. Does this unit have heat strips on it? Could they be tied into the same line or breaker Could the defrost on the outside unit be shorted? Remember to check more than just the compressor, other electical devices can short and trip the breaker as well. Hope this helps and keep us posted.

  5. #5
    Join Date
    Nov 2000
    Location
    Waco, Texas, USA
    Posts
    6,153
    Well I see a couple mistakes you made.

    #1 You didn't check the start winding amps. The should be about half of the run winding amps. If not then your run cap is bad.
    #2 You didn't short cycle the unit to see if starting under load is tripping the breaker.
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

  6. #6
    Join Date
    Feb 2004
    Location
    Maine
    Posts
    483
    Originally posted by Steve Wiggins
    Well I see a couple mistakes you made.

    #1 You didn't check the start winding amps. The should be about half of the run winding amps. If not then your run cap is bad.
    #2 You didn't short cycle the unit to see if starting under load is tripping the breaker.
    Very good points Steve.

  7. #7
    Join Date
    Apr 2005
    Location
    Richmond, Virginia
    Posts
    365
    Thanks Steve, I didn't short cycle it. It was the last job of the day ending at 11 PM and I was dead tired. I'm headed over there now to check it out. I'll keep you all posted.

    jdenyer how much is a megometer and what brand do you recommend?

    Thanks
    Sean Cantrell

  8. #8
    Join Date
    Apr 2005
    Location
    Richmond, Virginia
    Posts
    365
    Went back to job yesterday evening, turned on unit started up fine.

    Cycled the unit-the unit started no problems.

    Checked start amps - they were within specs.

    SO I waited, waited, waited, for about an hour then "pop", the breaker trips. I ohmed from the run to the compressor case and sure enough there was the dead short.

    The outdoor unit is 17 yrs old so it's had a good life.

    Thanks for all your help.
    Sean Cantrell

  9. #9
    Join Date
    Feb 2004
    Location
    Maine
    Posts
    483
    Originally posted by sean88
    Went back to job yesterday evening, turned on unit started up fine.

    Cycled the unit-the unit started no problems.

    Checked start amps - they were within specs.

    SO I waited, waited, waited, for about an hour then "pop", the breaker trips. I ohmed from the run to the compressor case and sure enough there was the dead short.

    The outdoor unit is 17 yrs old so it's had a good life.

    Thanks for all your help.
    No problem, glad we could all be of help. You can get a decent megohmeter for 2-3 hundred. Did you disconnect all the leads going to the comp and just check the run to comp case? The run is hooked to more than just the comp, so if you left the common or run attached to the comp and there was a short somewhere else, ie a capacitor you may think your comp is shorted when in fact its not. Caps can short out intermitently when they get hot. Comps with dead shorts won't start and run for an hour before they trip.

  10. #10
    Join Date
    Apr 2005
    Location
    Richmond, Virginia
    Posts
    365
    I pulled all the wires from the capacitor. And checked run, start and common to compressor case. Checked cap, it was good it had been replaced last year along with a hard start kit. I
    Sean Cantrell

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