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Thread: Voltage Imbalance Calculation Method with AVG/MIN/MAX Data

  1. #1
    Join Date
    Mar 2004
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    Voltage Imbalance Calculation Method with AVG/MIN/MAX Data

    Good Morning
    I have a question on how to calculate voltage imbalance.

    The data that have contains AVERAGE MINIMUM and MAXIMUM values for each of three phases, with records recorded one time per second.

    I know the voltage imbalance percentage formula, and repeat it here:

    Formula stating Voltage unbalance=100 x Max. deviation from average voltage/Average voltage

    For example, if measured line voltages were 455, 460, and 492, the average would be 469 volts (455 + 460 + 492 = 1407 / 3 = 469). The maximum deviation from that average is 23 volts (492 - 469 = 23). To find the voltage unbalance, solve the equation for the average voltage and the maximum
    voltage deviation: Formula stating Voltage unbalance=100 x (23/469) = 4.9%

    My question is which voltage values should be used in the calculation.

    I think that I should use the AVERAGE of all (AVGab +AVGbc + AVGca)/3, and the maximum deviation being the largest of the AVERAGE from each phase AVG. correct?


    I should have mentioned that the average value recorded is not a simple average of the MIN and MAX values, but must be calculated during the one second before recording the value.
    Last edited by natfuelbill; 10-18-06 at 02:30 PM.

  2. #2
    Join Date
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    Quote Originally Posted by natfuelbill
    Good Morning
    Formula stating Voltage unbalance=100 x Max. deviation from average voltage/Average voltage
    My question is which voltage values should be used in the calculation.

    I think that I should use the AVERAGE of all (AVGab +AVGbc + AVGca)/3, and the maximum deviation being the largest of the AVERAGE from each phase AVG. correct?
    I have an old motor hand book by Allis-Chalmers that computes the
    %voltage unbalance the way you have indicated. They use the average of the 3 phase to phase voltages as you suggest. Another interesting point brought out is the the increase in temperature rise in the motor is about
    2 times the (% unbalanced)².
    Example: For an unbalance of 4%, the temp rise = 2(4)² = 32% increase in temperature for the phase affected by the unbalance.

  3. #3
    Join Date
    Oct 2006
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    Is there a chart, table or formula that shows expected motor CURRENT imbalance based on the measured and calculated voltage imbalance?

    I have heard that the current versus voltage imbalance can be around 5 to 1.

    So if I had a 0.5% voltage imbalance that I might see a 2.5% current imbalance.

    Correct?

  4. #4
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    Nice to see someone else thinks about this stuff. I always just figured that I should use the same min, max or avg reading for all three. Now I may have to rethink. Kind of depends on what you are looking for or what you want to prove.

  5. #5
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    Quote Originally Posted by Natfuelbilll
    Is there a chart, table or formula that shows expected motor CURRENT imbalance based on the measured and calculated voltage imbalance?

    I have heard that the current versus voltage imbalance can be around 5 to 1.

    So if I had a 0.5% voltage imbalance that I might see a 2.5% current imbalance.

    Correct?
    Voltage unbalance causes extremely high current unbalance. The magnitude of current unbalance may be 6 to 10 times as large as the voltage unbalance. For a motor, the line currents (at full-load with 2.5% voltage
    unbalance) may be unbalanced by 25%.

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