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Quatrième année Power quality · tool

Calculateur de déséquilibre de tension

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.

Type de mesure
  • Moyenne—
  • Écart maximal par rapport à la moyenne—
  • Déséquilibre (écart max. ÷ moyenne)—
  • Déséquilibre (composante inverse)—
Voir le calcul détaillé

    Arithmétique sur vos trois mesures. Le déséquilibre qu’un moteur peut supporter, et de combien le déclasser, relève des données du fabricant du moteur.

    Garder ces résultats

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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 fréquentes

    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.

    Voir aussi

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