Voltage unbalance occurs when the line voltages on a three-phase supply are unequal. Voltages are seldom perfectly balanced, but when unbalance becomes excessive, it can create problems. Unbalanced three-phase circuits may cause damage to any three-phase equipment on the circuit if the unbalance is more than 3%.
Calculating motor unbalance
Unbalance is measured as the difference (positive or negative) of each phase to the average voltage of all three phases, expressed as a percentage. Assuming that you have three-phase conductors reading the following voltages phase-to-phase: 216, 201 and 207 volts. The average is 208 volts.
Average voltage = (216 +201+ 207)/3 = 624/3 = 208 volts
Next, subtract each of the voltage readings from the average and take the absolute value difference.
L1 u0394 = 208 u2013 216 = |-8| = 8
L2 u0394 = 208 u2013 201 = |7| = 7
L3 u0394 = 208 u2013 207 = |1| = 1
Now, divide the largest difference by the average to get the voltage unbalance number.
Voltage unbalance = 8/208 = 0.038 = 3.8%
Voltage unbalance and motor operation
The table on the right provides suggested deratings for various voltage unbalance conditions. For instance a voltage unbalance of 2% would require derating a 100 HP motor to 95 HP or less. A 3% unbalance would derate the motor to 88 HP. These are recommendations only. Check with your motor manufacturer for the derating that applies specifically to your situation.
Fortunately, voltage unbalance protection equipment is relatively inexpensive, typically ranging from $100 to $300, and simple to install. Check your three-phase motor voltages today and add protection if necessary.