Unbalanced Voltage
The measured line-to-line voltage values of the three phase supply are not equal. Per NEMA Standard MG1 14:35, voltages should be as evenly balanced as practical. Voltage unbalance results in a current unbalance that is 6 to 10 times the voltage unbalance. This causes the overheating that deteriorates the winding insulation. If an unbalanced condition exists, the nameplate horsepower rating must be derated to prevent overheating of the windings. A three phase motor operating in an unbalanced circuit, cannot deliver its rated horsepower. With 3% voltage unbalance, a motor can produce 90% of its rated horsepower without overheating. Operation of a motor when the unbalance exceeds 5% is not recommended. When unbalanced voltages exceed 5%, the temperature rise is so fast that protection by derating is not practical.
Example:
3% Unbalance = 18% Overheating
5% Unbalance = 50%Overheating
7% Unbalance = 98% Overheating
