Key Takeaways: Voltage imbalance means the three line-to-line voltages feeding a motor are not equal. It is one of the most damaging supply problems because a small voltage imbalance produces a much larger current imbalance, and that extra current becomes concentrated heat in one winding. Measuring it takes only a multimeter, and most cases come down to loose connections, an unequal spread of single-phase loads, or a weak supply leg.
The rule worth remembering: a percentage of voltage unbalance drives roughly six to ten times that percentage of current unbalance. So a supply that looks almost fine on a voltmeter can be quietly cooking a motor. This guide covers how to calculate percent voltage unbalance, why it does so much damage, the common causes, and how to diagnose and fix it.
On a healthy three-phase supply the three line-to-line voltages are nearly equal. Voltage imbalance is the spread between them. The standard way to put a number on it (the NEMA definition) is the percent voltage unbalance:
Percent voltage unbalance = (maximum deviation from the average voltage / average voltage) x 100.
A worked example. Say you measure the three line-to-line voltages as 400 V, 395 V, and 410 V.
This is live work. Measuring line voltage on a running 400 V or 480 V motor circuit must be done by a qualified person, with a meter rated for the circuit (CAT III 600 V or higher for plant distribution) and the arc-flash PPE your site requires for that panel. OSHA requires both: only qualified persons may test live circuits, and test instruments must be rated for the circuits they are connected to.
Measure all three line-to-line voltages under normal load, not at no load, at the starter or local disconnect feeding the motor, or at the motor terminals only under your site's energized-work permit, because the imbalance often only shows up when current is flowing. A true-RMS multimeter rated for the circuit is enough.
An unbalanced set of voltages can be split mathematically into a normal (positive-sequence) set that drives useful torque and a reverse-rotating (negative-sequence) set that does not. The motor has a very low impedance to that negative-sequence component, so even a small unbalanced voltage pushes a large unbalanced current through the windings. That is the origin of the six-to-ten-times rule of thumb.
The damage is heat. The extra current is not shared evenly; it piles into the phase with the lowest voltage, so one winding runs far hotter than the other two and ages first. NEMA guidance reflects this in a derating curve: a motor is derated to roughly 0.95 of its rating at 2 percent unbalance and about 0.88 at 3 percent, and operating above 5 percent unbalance is not recommended at all. In practice, a few percent of voltage imbalance can add tens of percent to the temperature rise in the hottest winding, which is why it shows up as premature failures. Voltage imbalance is one of the classic drivers of electric motor overheating, and the heat it creates is exactly what an insulation resistance test later reveals as broken-down insulation.
Voltage imbalance is almost always a wiring or loading problem, not the motor:
Beyond the overheating already described, voltage imbalance causes higher running current and energy cost, torque pulsations that add vibration and noise, and nuisance tripping of the overload protection as the hottest phase pushes the relay past its limit. Left alone it drives premature winding and bearing failure. The current signature of an unbalanced supply is also visible in motor current signature analysis, and the resulting thermal stress is what makes a well-set overload relay trip.
Work from the motor back toward the supply:
One important diagnostic fork: if the three voltages are balanced but the three currents are not, the problem is inside the motor (a winding fault), not the supply. Balanced voltage with unbalanced current points you at the motor; unbalanced voltage points you at the wiring and load.
Add voltage-balance checks to the periodic route on critical motors, fit phase-loss and phase-imbalance protection relays that trip before a winding cooks, and keep single-phase loads distributed evenly when circuits are added. The slow version is the dangerous one: an imbalance that sits at 3 percent for months gives no alarm, it just spends motor life. Trending voltage, current, and the downtime these failures cause turns that silent drift into something visible. Fabrico captures the stoppages unbalanced-supply failures create with computer-vision-verified OEE and turns the recurring ones into tracked maintenance actions, so a motor that keeps failing on one phase is flagged as the capacity loss it is instead of being reset and forgotten. For the wider set of machine-side references, see the machine troubleshooting guides index.
See how Fabrico turns equipment stoppages into tracked work orders. Book a demo.
NEMA's rule of thumb is that the extra temperature rise in the hottest winding, in percent, is about two times the square of the percent voltage unbalance. At 2 percent unbalance that is about 8 percent more heat, at 3 percent about 18 percent, and at 3.5 percent about 25 percent.
NEMA motors are designed to carry rated load at up to 1 percent voltage unbalance. Above that, derate the motor:
Beyond 5 percent, do not run the motor until the supply is fixed.
If the motor runs on a variable frequency drive, the drive builds its own output voltage, so check supply balance at the drive input rather than at the motor. A badly unbalanced or missing supply phase shows up as an input phase trip on the drive, such as PHF input phase loss on a Schneider Altivar or F003 Power Loss on a PowerFlex 525, which Rockwell describes as single-phase operation detected with excessive load.
What is an acceptable voltage imbalance for a motor?
Below 1 percent is ideal. Between 1 and 2 percent the motor should be derated slightly; above about 5 percent NEMA advises not running the motor at all until the cause is fixed, because the winding heating becomes severe.
How do I calculate percent voltage unbalance?
Take the three line-to-line voltages, find their average, find the largest deviation of any one reading from that average, then divide the largest deviation by the average and multiply by 100. For 400, 395, and 410 V the answer is about 2.1 percent.
Why does a small voltage imbalance overheat a motor?
The unbalanced voltage creates a reverse-rotating (negative-sequence) component that the motor barely resists, so a small voltage imbalance drives a much larger current imbalance. That extra current concentrates in one winding as heat.
Can voltage imbalance trip the overload relay?
Yes. The overload relay senses the elevated current in the hottest phase and trips. Repeated nuisance trips with no obvious mechanical overload are a strong hint to measure voltage balance.
Is current imbalance the same as voltage imbalance?
No. Unbalanced voltage causes unbalanced current, but if the voltages are balanced and the currents are not, the fault is inside the motor windings rather than in the supply.
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