Voltage imbalance occurs when two or more phases differ in voltage. Motors are especially vulnerable to voltage imbalance. You can find published recommendations, such as the oft-quoted 3%. The context of this number is that of preventing motor failure and 3% is dangerous territory. At 5%, you shouldn’t even operate the motor.
Motor failure isn’t the only result of voltage imbalance. Any imbalance means energy waste plus additional heat in the motor. Ideally, you will reduce voltage imbalance to zero.
Two other problems that result from voltage imbalance are difficult starts and nuisance tripping.
In the latter case, the motor may draw just enough excess current to open its protective overloads. Increasing to the next size often “fixes” this symptom, but not the underlying cause (and you’re still left with an energy-wasting motor). This symptom might manifest only periodically, and thus appear to be intermittent. What’s really going on is a normally benign voltage sag is enough to cause the trip because current is already high at normal voltage.
