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Hi all, I have a 90kva 3 phase 440volt/220volt stepdown

  1. Feb 4, 2012 #1
    Hi all, I have a 90kva 3 phase 440volt/220volt stepdown transformer.
    When i test on no load ie secondary not connected to anything but the power is on,
    i get 220v between phases but to earth or ground i get
    1st phase 150volts
    2nd phase 104volts
    3rd phase 150volts
    Does this mean my transformer is U/S due to phase unbalance
     
  2. jcsd
  3. Feb 4, 2012 #2

    Averagesupernova

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    Re: Transformers

    Well if the secondary truly is not connected to anything at all then how can you expect any meaningful reading between a 'phase', and I think you mean 'leg', and ground? So tell us exactly what is hooked up.
     
  4. Feb 5, 2012 #3
    Re: Transformers

    sorry yes one leg and ground. Ok i have a 3phase 440v/220v tranny that when connected to mulitple switchboards i get a reading of 155v red to ground,4v white to ground and 153v blue to ground. When i disconnect main switchboard and have nothing connected to the tranny my voltages from each leg to ground are as above
     
  5. Feb 5, 2012 #4

    jim hardy

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    Re: Transformers

    i bet you're using a pretty good meter, maybe a DMM.

    Try a real cheap analog one that's just a couple thousand ohms/volt.
    sometimes low-tech works best.
     
  6. Feb 6, 2012 #5

    NascentOxygen

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    Re: Transformers

    Does the transformer have a star point that you connected to ground? Otherwise, what Averagesupernova said applies. The voltages are floating, your measurements are meaningless. Don't throw the transformer away. :smile:
     
  7. Feb 7, 2012 #6

    jim hardy

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    Re: Transformers


    Draw yourself an imaginary circle around the transformer, like a fence..
    That fence represents the insulation of the transformer.

    When you connect a voltmeter from one transformer phase to earth, you let a trickle of current flow out of transformer, through meter and over your fence into earth.

    Apparently one of your switchboards connects white phase to earth.
    So you get solid readings then, the meter current returns to transformer from earh via white phase.

    But -How does that meter current get back into transformer when you've disconnected white phase from earth? How can it jump the fence?

    It can't.
    That's why you get flakey unbalanced readings, only a fraction of your meter current finds its way back over the fence. Insulation is imperfect and your meter is too sensitive.

    Use a less sensitive meter and you'll read nearly zero on all three phases.

    Or use a lightbulb instead of a meter.

    it's a common problem ever since high impedance DMM's became so popular.
    Typical DMM is 10 meg input.
    Simpson 260 is i think 5Kohm/volt on AC, so on 250 volt scale = 1.25 meg.

    A 1 kohm per volt el cheapo meter is what you need for that measurement.
    Check the little yellow $ 9.95 GE analog meter at Walmart.


    old jim
     
    Last edited: Feb 7, 2012
  8. Feb 7, 2012 #7

    Averagesupernova

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    Re: Transformers

    Jim Hardy: Have you experienced the strange looks from people that I have when you tell them a low-tech cheap meter is still necessary?
     
  9. Feb 7, 2012 #8

    jim hardy

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    Re: Transformers

    indeed.

    I do 95% of my work with the $9.95 meter, even have an external resistor to make a 15 volt scale for automobile work. I soldered nice long test leads with alligator clips to it(nice Meuller copper ones) , because the cheapies are too short and fall out.

    My high accuracy Fluke DMM is wonderful on rare occasions one needs precise resolution, maybe three times a year around the house.

    When i retired the guys gave me a caricature drawn by the office wag portraying me in my old hat carrying a Simpson 260 and analog oscilloscope. With those two instruments you can fix anything.
     
  10. Feb 7, 2012 #9

    Averagesupernova

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    Re: Transformers

    I have something similar to a 260 except it has a 3000 volt or higher range I believe. I don't have any test leads that fit it though. At least they didn't draw you with a pair of dry-cells taped around a door-bell.
    -
    Edit:
    CORRECTION: It is a Triplett with a 6000 volt range.
     
    Last edited: Feb 7, 2012
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