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Thread: Voltage Dip Estimation

  1. #1
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    Voltage Dip Estimation

    Downloaded attached formula around one year ago. Did not keep a copy or recall source. Tried finding again with no luck.

    Question, is this a valid calculation method for Voltage Dip/Sag ?

    If I downloaded from MH prior posts, please help me find it.
    Attached Images Attached Images

  2. #2
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    that will work
    I find this easier
    assume 1 mva 480/3 5% puZ

    ifault = 1000000/(1.732 480 0.05) = 24056
    xfmr Z in Ohms = 480/24056 = 0.02

    assume feeder is 0.01 Ohm (value from nec table x 1.732 x length/1000)
    total Z = 0.03
    motor starting i = fla x 6 or calculated from mtot code)
    assume 100 x 6 = 600 A

    vdrop = 0.03 x 600 = 18 vac or 3.75%

  3. #3
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    Ingenieur...Thank you

    Using your method on PDF problem yields 11.34%.
    Assume this method is more accurate ?

    Max VD at MOTOR of 10%...acceptable target value ?

    Max VD at Service Equipment (for a given motor start) value ?

  4. #4
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    imo theirs is wrong

    should be 618.55/5454.46 x 100 = 11.34%
    should not add the motor lr kva to the fault kva in the numerator

  5. #5
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    Quote Originally Posted by MyCleveland View Post
    Assume this method is more accurate ?

    Max VD at MOTOR of 10%...acceptable target value ?

    Max VD at Service Equipment (for a given motor start) value ?
    It is just a different way of going about it.

    10% depends on the target reasoning. Are you concerned about lights flickering?

    Close to 25-30% could be acceptable if you were just confirming the motor controller and/or BMS wouldn’t drop out.

  6. #6
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    Quote Originally Posted by Ingenieur View Post
    imo theirs is wrong

    should be 618.55/5454.46 x 100 = 11.34%
    should not add the motor lr kva to the fault kva in the numerator
    Faults are fed by all sources, both utility and motor contribution.

  7. #7
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    Quote Originally Posted by publicgood View Post
    Faults are fed by all sources, both utility and motor contribution.
    true
    but this not a fault calculation
    this is a voltage drop calc due to motor inrush
    the xfmr can't supply more than it's sc fault current
    the motor contributes nothing, it is a load in this case

    my calc uses xfmr internal Z plus conductor Z x starting current to give V at the motor terminals
    very simple, like any v drop calc
    matches the op formula if the motor inrush is not added to xfmr sc

  8. #8
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    Quote Originally Posted by Ingenieur View Post
    true
    but this not a fault calculation
    this is a voltage drop calc due to motor inrush
    the xfmr can't supply more than it's sc fault current
    the motor contributes nothing, it is a load in this case

    my calc uses xfmr internal Z plus conductor Z x starting current to give V at the motor terminals
    very simple, like any v drop calc
    matches the op formula if the motor inrush is not added to xfmr sc
    True I retract my statement

  9. #9
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    What is the consensus on MAX target values ?

    Max VD at MOTOR during start...acceptable target value ? Max %VD ?

    Max VD at Service Equipment (for a given motor start) value ? Max %VD to avoid flicker?

  10. #10
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    Anyone.....?
    before this slips off the page.

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