Three-Phase Voltage Imbalance Checker

NEMA MG-1 expects ≤1% for full motor rating, with explicit derating to ~75% at 5% (and operation above 5% not recommended). ANSI C84.1 suggests utilities keep service voltage imbalance under 3% — but motor-protective practice is stricter because rotor heating rises with the square of imbalance. If you're above 1% at the motor terminals under load, there's economic damage accruing.

The motor's negative-sequence impedance is roughly its locked-rotor impedance — 5–7× smaller than normal running impedance — so each percent of negative-sequence voltage drives 6–10% of negative-sequence current. The physics is unforgiving: that current produces a counter-rotating field heating the rotor without producing useful torque. This amplification is why voltage is the number to police even though current is the number that trips.

You'll get the supply's open-circuit balance, which usually looks misleadingly good: imbalance largely appears under load as unequal currents drag unequal voltage drops through supply impedance. Measure under representative plant load, line-to-line (phase-to-neutral mixes in neutral-shift effects), and at several times of day — single-phase load patterns move with shifts and HVAC cycles.

Largely yes: open-delta (V-V) banks inherently produce voltage imbalance under load (commonly 2%+), which is one reason they're a temporary/economy configuration. Mitigations: keep loading well below the bank's reduced rating (57.7% of the equivalent closed delta), distribute single-phase load carefully, and prioritise conversion to a full three-transformer bank where three-phase motor load matters. Until then, derate motors per the NEMA curve.