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Thread: phase to phase voltage is erratic

  1. #11
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    Quote Originally Posted by Belnald View Post
    System: 480v, 3 phase control panel. feeding numerous VFDs and 24vdc x-formers.
    Issue:
    Phase-1 to Phase-2 is steady at 480v with a deviation of 450 to 550
    Phase-2 to Phase-3 is erratic at 480v with a deviation of 300 to 475
    Phase-3 to Phase-1 is erratic at 480v with a deviation of 50 to 475
    Amps is the same on all 3 phases. it varies with load as expected.

    The system as a whole runs ok, but this voltage imbalance worries me.
    Any ideas what the cause is?

    thanks
    Highly unlikely that your supply is doing this.
    Try another meter.
    Si hoc legere scis nimium eruditionis habes.

  2. #12
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    Define your terms:
    Steady.
    Erratic
    Deviation

    How can you get steady with a deviation of almost 100 volts?
    Tom
    TBLO

  3. #13
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    Quote Originally Posted by GoldDigger View Post
    Once you rule out a problem with noise on the pickup leads of the data logger which might be causing false readings, the most straightforward problem would be the intermittent loss of or very high resistance in one of the two phase conductors (specifically the one which is common to both erratic voltage readings.

    It is possible that you could lose a line conductor intermittently and the single phased motors already running could produce a back EMF on the open or high resistance lead. This would prevent you from seeing total loss of voltage as the running motors act as rotary three phase converters.
    That is one reason why I would like to see current readings, at least on the phase wire common to the two bad readings. If it goes to zero and the motors keep running, you have a partial or complete single phase condition.
    I kind of agree that the partial or complete loss of a "phase conductor" is the direction I would be investigating first. If possible maybe bring in some temporary load and connect it and see how things respond. 10 or 15 kVA three phase space heater would be a great test load, if you still have similar issues you know it isn't harmonics or other solid state controls issues when the test load is simple resistance.

  4. #14
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    dummy load it like kwired said, then monitor only your dummy loads to see if the problem remains.

    my gut tells me ive seen this before where my service source was Wye 3ph 4w yet "some" of my motor loads were 3ph delta! and some of the motor loads were using two phases and a ground only.

    is your source panel / service wye?
    direct quote from a certified appliance repair tech dispatched to my house to repair my under warrenty water tank.
    "I dont know how many of them ohms your talking about is on a heating element, I just set my old meter to rx1 and if the needle pegs out then I know its good":rolleyes:

  5. #15
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    Quote Originally Posted by jcassity View Post
    dummy load it like kwired said, then monitor only your dummy loads to see if the problem remains.

    my gut tells me ive seen this before where my service source was Wye yet "some" of my loads were delta!

    is your source panel / service wye?
    That itself shouldn't really matter.

    Supply three single phase line to line loads and balance them across all three phases and you essentially have the same thing.

    Supply a three phase wye load but with a floating neutral point in that load (like a typical 9 lead motor, or a three phase heater wye connected but with no connection to the supply neutral) and you still sort of have the same thing when it comes to how the source sees the load.

  6. #16
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    Erratic Voltage Issue Solved.

    I thank all and everyone for their information. with this information I have narrowed down the problem to one of the drives is going bad, and causing the voltage to spike-dip every few seconds.
    By monitoring the system while it was idle, the voltage was steady and stable at 480v phase to phase.
    By running one motor (VFD) at a time, the voltage stayed right until the defective motor (VFD) was ran. Then the voltage went erratic on two phases. So the culprit was a defective VFD. But not a bad VFD. It was still running the motor and communicating properly.

    Thanks everyone for all your help.

  7. #17
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    Quote Originally Posted by Belnald View Post
    I thank all and everyone for their information. with this information I have narrowed down the problem to one of the drives is going bad, and causing the voltage to spike-dip every few seconds.
    By monitoring the system while it was idle, the voltage was steady and stable at 480v phase to phase.
    By running one motor (VFD) at a time, the voltage stayed right until the defective motor (VFD) was ran. Then the voltage went erratic on two phases. So the culprit was a defective VFD. But not a bad VFD. It was still running the motor and communicating properly.

    Thanks everyone for all your help.
    FWIW the defective drive almost certainly caused current spikes corresponding to the drops in phase voltage. But likely the current measurements did not have the same short time excursion catching that the voltage measurements did.

  8. #18
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    Open question.

    What would be causing this? The current draw on L3 must be fairly high even briefly to cause that much voltage swing. I would think that the stiffness of the POCO power supply would be more than adequate for a plant of any size at all.
    Tom
    TBLO

  9. #19
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    Quote Originally Posted by ptonsparky View Post
    Open question.

    What would be causing this? The current draw on L3 must be fairly high even briefly to cause that much voltage swing. I would think that the stiffness of the POCO power supply would be more than adequate for a plant of any size at all.
    My thoughts also.

    Isolated individual load like an irrigation machine- you might see this sort of thing easier then in a facility that has larger supply. If there were a single large drive in a larger facility doing this it would draw the entire plant down when it does it. If it only effects an individual feeder - he may still have problems on that feeder more so then a drive problem.

  10. #20
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    OP. Was this the last drive you tested or did you go on and check them all? A second time?
    Tom
    TBLO

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