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  1. #1

    Single phase voltage imbalance/

    i just replaced a 208v/single phase copeland scroll compressor in a 6 month old rooftop unit. the compressor was shorted to ground. i installed the new compressor and it ran backwards. i moved the run cap from L1 to L2 and the compressor draws locked rotor and trips the breaker. i found the voltage to be out of balance coming from the panel L1 is 110v and L2 is 120v could this be the problem?

  2. #2
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    I in all my years I have never had a single phase comp run backwards on me. The run cap should be wired into the run and start windings with your common going back to L1 through a contactor.

    If you started switching wires around its possible you switched the run and the start windings on the run cap accidentally.

    That will kill a single phase compressor real quick. How did you confirm rotation ?

  3. #3
    Join Date
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    What's the voltage between L1 & L2 ?

    What was your symptoms of the comp running backwards?

    By moving the wire, you took out one of the winding from the circuit thus drawing LRA tripping the CB trying to start a comp on only the start or run winding.

  4. #4
    Join Date
    Sep 2007
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    If L1 and L2 to neutral are not exactly the same then you have a corroded connection somewhere, could be anywhere, incl L1 or L2, but most likely the neutral either in the panel, meter trough, or even on the pole. But for a 208 compressor the neutral is not used so I doubt this is your problem

  5. #5
    The compressor runningbackwards is diagnosed with no pressure differential and a noisy operation. The wiring has l1 and l2 coming from the disconnect to a 2 pole contractor l1 goes to common on the compressor l2 goes to start on the can and has another wire coming from the same lug on the contactor thru a run cap and to the run winding.

  6. #6
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    Huh ? That's wrong.

    L1 Should go from this disconnect to the line side of one of the NO contacts.
    The common winding should connect to the T1 side of that contact.

    L2 should go from the disconnect to the line side of the other NO contact. The run winding should connect to the T2 side of that contact.

    The cap should have a wire connected to the L2 side of the contactor and and the start winding of the comp should land on the other side of the cap.

  7. #7
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  8. #8
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    Quote Originally Posted by cheesy name View Post
    i just replaced a 208v/single phase copeland scroll compressor in a 6 month old rooftop unit. the compressor was shorted to ground. i installed the new compressor and it ran backwards. i moved the run cap from L1 to L2 and the compressor draws locked rotor and trips the breaker. i found the voltage to be out of balance coming from the panel L1 is 110v and L2 is 120v could this be the problem?

    The two main possibilities are that one pole of the two-pole breaker is frying on its buss bar, OR, one that I have seen a couple of times on older service drops, the connector at the drip loop at the top of the weatherhead (where it connects to the triplex) has gone bad and needs to be replaced. Sometimes they are called "burndys" or "bugs."

    Of course, you looked at the disconnect for the unit first, right?

    However, your problem with the copressor running backward is likely something else.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  9. #9
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    Never mind
    Last edited by hvac69; 07-09-2012 at 06:51 AM. Reason: Never mind

  10. #10
    The voltage difference exists at the feed into the panel. I have a feeling that I brain cramped in the heat. I seem to remember checking off possibilities and saying " the run cap is connected to the run winding. Check." on my way back now to double check. Thanks to Six for the assist.

  11. #11
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    Quote Originally Posted by cheesy name View Post
    The voltage difference exists at the feed into the panel. I have a feeling that I brain cramped in the heat. I seem to remember checking off possibilities and saying " the run cap is connected to the run winding. Check." on my way back now to double check. Thanks to Six for the assist.
    Doh.

    Looks like you found your issue.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  12. #12
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    Quote Originally Posted by cheesy name View Post
    The voltage difference exists at the feed into the panel. I have a feeling that I brain cramped in the heat. I seem to remember checking off possibilities and saying " the run cap is connected to the run winding. Check." on my way back now to double check. Thanks to Six for the assist.
    No problem. I know how it is.

    The heat can definitely affect your train of thought and concentration.

  13. #13
    Yup. Cap on the run winding was the culprit. Embarrassing. Reminds me of some great advice "the faster I work the slower the job gets done."

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