Unbalance by the NEMA/IEEE max-deviation-from-average definition, with an interpolated approximation of the published NEMA MG-1 derating curve.
Standard method — stated scope limits apply
Measured Line Voltages
Enter the three line voltages.
About This Calculation
A three-phase induction motor is highly sensitive to voltage unbalance between its supply phases — even a modest 2-3% voltage unbalance (well within many utility supply tolerances) produces a disproportionately large 6-9x negative-sequence current in the motor, since motor negative-sequence impedance is much lower than its positive-sequence (normal running) impedance. That negative-sequence current creates a counter-rotating magnetic field that induces significant additional heating in the rotor specifically, which NEMA MG-1 addresses with a derating curve: the same motor must be run at reduced load as supply voltage unbalance increases, to keep total heating within the motor's thermal design.
Frequently Asked Questions
Why does a small voltage unbalance cause such disproportionately large current unbalance?
A motor’s impedance to negative-sequence current is roughly equal to its locked-rotor (starting) impedance, which is much lower than its normal positive-sequence running impedance — a small negative-sequence VOLTAGE therefore drives a much larger negative-sequence CURRENT than the same percentage would produce if impedances were equal, which is exactly the 6-9x multiplication NEMA’s guidance describes.
How is voltage unbalance actually calculated?
The NEMA definition is the maximum deviation of any one line voltage from the average of the three line voltages, divided by that average, expressed as a percentage — not simply the difference between the highest and lowest voltage, which is a common calculation mistake that overstates the actual unbalance.
What causes voltage unbalance on an otherwise healthy supply?
Unequal single-phase loading distributed across the three phases upstream is the most common cause on an otherwise sound supply — a large single-phase load (or several, poorly balanced) draws more current from one or two phases than the others, causing unequal voltage drop across the supply impedance and showing up as unbalance at the motor terminals.
At what unbalance level should a motor be taken out of service rather than just derated?
NEMA MG-1 explicitly recommends against operating a motor above about 5% voltage unbalance at all, regardless of derating, since the derating curve itself is only defined and validated up to that point — above it, the root cause (usually an upstream supply or loading problem) needs to be fixed rather than compensated for at the motor.