Three-Phase Voltage Unbalance (NEMA)

The induction motor in the MCC burns out for the third time in two years and nobody finds a fault in the machine. The maintenance electrician turns up with a clamp meter, reads the three currents and finds one sitting well above the other two — the classic sign that the supply voltage, not the motor, is at fault. Measuring all three line voltages and boiling the spread down to a single figure is what turns that hunch into something you can log, compare against a standard and take to the utility or to the electrical designer.

NEMA MG-1 defines unbalance as the largest deviation between any line voltage and the average of the three, divided by that average. With the values on screen — 380, 372 and 385 V — the average is 379 V, the widest gap is the 7 V on phase BC, and the answer comes out at 1.847%, shown to three decimals because the band that matters is narrow. From 1% upward the standard already calls for derating; at 2% the factor is around 0.95 and at 5% operation is forbidden. The companion rule of thumb bites hard: every 1% of voltage unbalance turns into 6 to 10% of current unbalance, and temperature rise climbs by roughly 2 × (percent)², near 6.8% here.

The method uses only the magnitudes of the three voltages, which is why it was chosen here: the IEC and IEEE factor, a ratio of negative to positive sequence components, needs full phasors with their angles, and an ordinary multimeter will not deliver those. The trade-off is that a system with equal magnitudes and shifted angles reads 0% on this screen. Take the three readings almost simultaneously, with one true RMS instrument and with the load running — unbalance shifts through the day and vanishes while the motor sits idle. And read line voltage, the quantity the standard was written around.