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Thread: Three Phase Motor Potential Single Phasing

  1. #11
    Join Date
    Dec 2007
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    NE Nebraska
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    No continuity on B phase lead to motor means it could be open between motor and controller. If it were faulted to ground on motor end of this run it would still run normally, though it would trip motor overload if it has phase loss detection. If it were faulted to ground on motor end of that grounded conductor and windings are good you would still fail ground fault megger test.

    Exact details of how some of your testing was done is important to know what actually was measured in those tests. Tests you done so far appear include the conductors between controller and motor, so is possible motor is good and there is an issue with conductors or a termination.
    I live for today, I'm just a day behind.

  2. #12
    Join Date
    Jan 2019
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    Quote Originally Posted by zbang View Post
    (actually, check the currents on all 3 leads).
    Yep. That's the first thing I would do. That'll give a quick and easy test to see if the motor is single phasing.

  3. #13
    Join Date
    Aug 2016
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    North Augusta, SC USA
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    So we went back in today with different mechanics, and they noticed that the motor leads were degraded. The connection was made with pink lady's, old wire nuts from the 80's. They stripped back the motor leads, and performed the megger test and continuity test again. All testing results passed, so the problem was more likely a bad connection at the motor. This explains the single-phasing causing the heaters to trip. I had a hard time being an open winding on the motor as ya'll believe. So I'm glad they found a bad connection. We plan to fix the motor leads, and repair some other issues and restart the motor.

  4. #14
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    Today we completed the repairs, and restarted the motor. It has now been running for over three hours, I don't expect any other problems. I do appreciate the replies to the post. Thanks.

  5. #15
    Join Date
    May 2005
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    Richmond, Virginia
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    That's why we're here!
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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