Computes NEMA-style percentage voltage unbalance from three line-line voltage readings, and the associated motor derating factor by linear interpolation of the published NEMA MG1 / ANSI C84.1 Figure D1 curve.
Standards followed
NEMA MG1 / ANSI C84.1 Figure D1 (voltage unbalance derating curve for motors)
What it can do
- Percentage voltage unbalance = max deviation from average ÷ average × 100.
- Derating factor by linear interpolation between the published curve points (1%→0.98pu, 2%→0.95pu, 3%→0.88pu, 4%→0.82pu, 5%→0.75pu).
- Flags unbalance above 5%, where motors should not be operated without manufacturer consultation.
Example problem
Vab=415V, Vbc=408V, Vca=420V.
- Average = (415+408+420)/3 = 414.33V.
- Max deviation = |408−414.33| = 6.33V.
- Unbalance% = 6.33/414.33 × 100 ≈ 1.53%.
- Interpolating between 1%→0.98pu and 2%→0.95pu at 1.53%: derating ≈ 0.964pu.
≈1.53% unbalance, ≈0.964pu derating.
Want the full step-by-step version?
The curve is published only up to 5% unbalance — NEMA MG1 states motors should not be operated above 5% unbalance without consulting the manufacturer, so this tool does not extrapolate a derating factor beyond that point.