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Thread: phase to phase voltage is erratic

  1. #1
    Join Date
    Feb 2018
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    phase to phase voltage is erratic

    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

  2. #2
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    If things are running ok why are you checking voltages?
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

  3. #3
    Join Date
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    And exactly what kind of meter are you using to make the measurement?

    I would be very much interested in seeing the three line currents over the same time period. Either at the feed panel where you measured the voltages or on any single large motor that is continuously connected and running.

  4. #4
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    Started taking the reading to locate the cause of an Overvoltage Fault.
    Meter is an Extech 3-phase power analyzer/datalogger. Model# 382100

    I am happy to send the Excel file I have if you supply an email address.
    total size is 134Mb

  5. #5
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    Readings were taken at the load side of the main breaker for this panel.
    When the system is not running (all motors Idle) amps = near 0, and the phase to phase voltage is steady at 480v. There are no large motors, but 15 <3HP motors on Powerflex525 drives. These motors are conveyors and start/stop independently of each other.

  6. #6
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    Quote Originally Posted by Belnald View Post
    Readings were taken at the load side of the main breaker for this panel.
    When the system is not running (all motors Idle) amps = near 0, and the phase to phase voltage is steady at 480v. There are no large motors, but 15 <3HP motors on Powerflex525 drives. These motors are conveyors and start/stop independently of each other.
    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.

  7. #7
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    So I'm assuming the 480 readings are with the meter set on averaging, and the erratic readings are reading on instantaneous? If so, and all of the loads are VFDs, I'd suspect this is the result of your meter not being able to deal with the harmonics caused by the VFDs. Do you at least have line reactors ahead of them? If not, that's the first thing I'd recommend.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  8. #8
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    what is the period?
    time to swing from min to max?
    your supply should be 'stiffer' than that

    either bad readings or bad connections imo
    The difference between genius and stupidity is that genius has its limits.

  9. #9
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    Wild readings tend to make me check the meter and the leads first.
    Tom
    TBLO

  10. #10
    Join Date
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    Boca Raton, Fl, USA
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    Bel‚Äčnald...

    Where did you insert (connect) 4th wire... neutral or ground?

    Regards, Phi Corso

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