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  1. #1
    Join Date
    Jun 2012
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    35

    3 phase reverse rotation - Why/how

    Hi Guys,

    I was just wondering why 3 phase compressors/fans run backwards sometimes?

    I know reversing any 2 leads will fix the issue and it always seems too but I was curious as to if there is a way to test the phasing to know its right or is it always start it up if it runs backward swap some wires?

    I was sort of thinking the wild leg (P, 240 to ground) must be in the wrong spot in relation to the L's (120 to ground, 240 between). If that's right then what is the correct configuration L1-L L2-P L3-L or something like that?

    Thanks,

  2. #2
    Join Date
    Jan 2011
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    1,171
    There is a way to test and it's done with a meter like this one by Ideal or this one by Extech. I've owned both of these and they both do the job. It's probably the best $100 one can spend if they are required to swap larger phases.

    When you swap wires I highly recommend you swap the incoming power wiring. That way all wiring inside the unit conforms to the wiring diagram.

  3. #3
    Join Date
    Nov 2006
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    Southeastern Pa
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    When a space is reconfigured for a new store, the electricians are not tasked with knowing or maintaining proper phase relationships. When a find a unit spinning the wrong way, I reverse two leads in the disconnect.

    A phase box is a BIG help.
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    2 Tim 3:16-17

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  4. #4
    Join Date
    Feb 2009
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    Tallahassee, FL
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    Could you potentially switch the disconnect leads to reverse a fan motor and then cause compressor to reverse or vise versa?

  5. #5
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    Oct 2010
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    Anderson, South Carolina, United States
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    Yep

  6. #6
    Join Date
    Feb 2009
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    Tallahassee, FL
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    So sometimes you have to change it from the diagram

  7. #7
    Join Date
    Jan 2004
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    Lancaster PA
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    Quote Originally Posted by SBKold View Post
    Could you potentially switch the disconnect leads to reverse a fan motor and then cause compressor to reverse or vise versa?
    On a RTU. If the fan is spinning backwards, and its 3 phase. Then the compressor should also be spinning backwards. unless you just put in a new fan.
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  8. #8
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    Nov 2006
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    Quote Originally Posted by SBKold View Post
    Could you potentially switch the disconnect leads to reverse a fan motor and then cause compressor to reverse or vise versa?
    Potentially? Yes.

    The reality is asking yourself (and investigating) the "why" of the reversed fan. A three phase blower going the wrong way, and a noisy scroll (confirmed by a pressure check) and it's time to go to the main feed for the lead switch.
    [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
    Join Date
    Nov 2006
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    Southeastern Pa
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    Quote Originally Posted by SBKold View Post
    So sometimes you have to change it from the diagram
    If it was properly installed, and running correctly, you should not have to vary from the diagram. The leads going to the blower and the compressors should be correct if the power is correct. They aren't going to change their phase relationship without some "help."

    In other words, something had to be changed previously by other persons for you to need to change it again, now.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  10. #10
    Join Date
    Jan 2011
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    1,171
    Quote Originally Posted by SBKold
    So sometimes you have to change it from the diagram
    Everything from the manufacturer should be phased properly and according to the diagram. In the hundreds of startups I performed I never had a situation where the wiring had to be changed from the diagram to get something to turn in the right direction. In other words, if one was right, they were all right.

  11. #11
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    Nov 2006
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    Quote Originally Posted by KnewYork View Post
    Everything from the manufacturer should be phased properly and according to the diagram. In the hundreds of startups I performed I never had a situation where the wiring had to be changed from the diagram to get something to turn in the right direction. In other words, if one was right, they were all right.
    In fact, I installed two new York Coleman-style units a couple of years ago, and they have a module that will not even let the unit run unless the phasing for the incoming power is correct.
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  12. #12
    Join Date
    Jun 2008
    Posts
    35
    The link below explains it very thoroughly.

    http://www.modular.org/Magazine/comfort7_00.aspx

  13. #13
    Join Date
    Jun 2007
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    Greenwood Indiana (Indianapolis)
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    420

    Red face testimony

    Quote Originally Posted by KnewYork View Post
    Everything from the manufacturer should be phased properly and according to the diagram. In the hundreds of startups I performed I never had a situation where the wiring had to be changed from the diagram to get something to turn in the right direction. In other words, if one was right, they were all right.
    A few years ago the company I used to work for installed a rtu 3 phase.

    Installers good with install, not good with service.

    They called out our top tech and the compressor going backwards. he switchs 2 legs on the contactor.

    Next day, I go out for no cool. Frozen up because the blower motor now going backward.

    Point is,
    change the wires in the dis-connect like previously mentioned, GOOD
    As iron sharpens iron so one man sharpens another Proverbs 27:17 NIV84

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