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Thread: phase to phase voltage is erratic

  1. #21
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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.
    It must be a very high current spiking since the voltages was shaken! We will not notice these if the source is very stiff, IMHO.

  2. #22
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    Quote Originally Posted by ptonsparky View Post
    Open question.

    I would think that the stiffness of the POCO power supply would be more than adequate for a plant of any size at all.
    I do work at a cement plant that has to go on generator power in the winter when the temperature dips to around 30. POCO's phase imbalance causes phase monitors to shut it down. POCO says problem will be fixed by the end of 2019.

  3. #23
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    phase to phase voltage is erratic

    Sounds like an open phase on the supply, check the fuses on the upstream supply transformer. If the fuses are ok open the lead and check winding resistance.


    Sent from my iPad using Tapatalk

  4. #24
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    Ground Fault?

    I would check your voltage to ground per phase.
    Have seen this in systems where the fault isn't enough to trip OCPD but bled off voltage to ground

  5. #25
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    Quote Originally Posted by Shortcircuit79 View Post
    I would check your voltage to ground per phase.
    Have seen this in systems where the fault isn't enough to trip OCPD but bled off voltage to ground
    I don't quite see "bleeding off voltage" to ground so much as I maybe see (and have seen happen before) where a lead faults to ground, burns completely open, the supply side is "isolated" and the side connected to the load is still faulted - and draws abnormal current because the voltage to ground isn't the same as it would have been to the proper supply conductor.

    Had this happen somewhat recently on a single phase motor, 240 volts. One of the "thru bolts" that holds the ends on the motor had one lead to the main winding fault to it. Burned clear from the 120 volt input lead, motor side of that burned conductor was still contacting the "thru bolt" and therefore the main winding was operating from the other lead to this fault - only seeing 120 volts instead of 240. Motor still worked, but they kept blowing start capacitors. I finally figured it all out after replacing a few capacitors - I stayed around to watch them use it (they never were using it other times I had been there, and to run it without a truck there to take on the grain it was moving would have left it piled on the ground) and when it got loaded enough it began to surge as the rotor slowed enough to kick in the start capacitors, sped up, kicked capacitors out, then slowed down again - lather, rinse, repeat - eventually took a toll on start capacitors.

    "bleeding off voltage" would have to do so through some resistance - which would also cause heating and a hot spot, or it eventually burns itself clear or turns into low enough resistance to get fault level current that overcurent device can respond to.

  6. #26
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    Is your supply grounded properly?

    Quote Originally Posted by Shortcircuit79 View Post
    I would check your voltage to ground per phase.
    Have seen this in systems where the fault isn't enough to trip OCPD but bled off voltage to ground
    I too would like to see your phase to ground voltages? You don't have a floating ground on this system do you?

  7. #27
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    I did check all drives. Before and after changing out the defective one. No voltage swings after changing the drive.

  8. #28
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    Thank you for the follow up.
    Tom
    TBLO

  9. #29
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    Quote Originally Posted by Phil Corso View Post
    Bel‚Äčnald...

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

    Regards, Phi Corso
    What fourth wire?
    Si hoc legere scis nimium eruditionis habes.

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