sparkytools

Fourth year Power quality · tool

Voltage unbalance calculator

Enter three line-to-line voltage readings (or three line currents). The tool gives the average, the largest deviation from it, and the percent unbalance — and for voltages, the negative-sequence unbalance too.

Readings are
  • Average—
  • Largest deviation from average—
  • Unbalance (max deviation ÷ average)—
  • Negative-sequence unbalance—
Show the working

    Arithmetic on your three readings. What unbalance a motor can take, and how much to derate it, comes from the motor maker’s data.

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    How it works

    • Average = (A + B + C) ÷ 3of the three readings
    • % = max |reading − avg| ÷ avg × 100the field definition
    • VUF = V negative ÷ V positive × 100sequence method, from line-to-line magnitudes

    Worked: 460, 467 and 450 V

    1. Average = (460 + 467 + 450) ÷ 3 = 459 V.
    2. Deviations: 1, 8 and 9 V. The largest is 9 V (the 450 reading).
    3. Unbalance = 9 ÷ 459 × 100 = 1.96%.
    4. On a motor, expect the current unbalance to be several times that — measure it and compare.

    Questions people ask

    Why does a small voltage unbalance matter to a motor?

    An unbalanced supply contains a small negative-sequence voltage — a component trying to turn the motor backwards. The motor’s impedance to it is low, so a few percent of voltage unbalance drives a much larger percentage of current unbalance, and that extra current is pure heat in the windings. Motor makers publish how much a motor must be derated for a given unbalance.

    Should I measure line-to-line or line-to-neutral?

    Line-to-line for a three-phase motor — that is what the motor sees, and it is what the negative-sequence figure here is built for. The percent-deviation method works with either, as long as all three readings are the same kind.

    What is the difference between the two percentages?

    The first is the common field definition: the largest difference from the average, divided by the average. The second is the "true" unbalance — the negative-sequence voltage as a percentage of the positive-sequence voltage — calculated from the three line-to-line readings. For small unbalances they come out close; the field figure is the one most motor data is written against.

    Where does unbalance come from?

    Single-phase loads spread unevenly across the phases, a blown fuse in a capacitor bank, a bad connection or a high-resistance contact on one leg, an open-delta supply, or unequal transformer taps. Find it by measuring at points going upstream until the unbalance disappears.

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