Threephase Voltage Unbalance Calculator
This calculator uses the formula as defined by NEMA to determine Voltage Unbalance. Voltage Unbalance = 100 x Maximum Deviation from Average Voltage / Average Voltage.
Did you know that experts in the field of electrical energy estimate that more than half of the worlds electricity may be consumed by threephase AC Induction Motors? That's right, more than half! Because of that, Voltage Unbalance in threephase motors is an important subject. Here's why!
When there are unequal incoming voltages between the three legs of a motor, the motor runs hotter. The reason why an unbalanced motor runs hotter is because as the voltage is out of balance so is the motor winding currents. A small voltage unbalance causes a larger current unbalance, which in turn causes the motor to run hotter because the unbalanced magnetic fields created by the windings are working against each other. This rise in temperature causes two main problems.
Problem 1. Unbalanced voltage reduces motor life by shortening the lifespan of it's insulation. Using this calculator's formula, a voltage imbalance of greater than 1% causes the motor to run hotter. The temperature of the windings (in degrees Celsius) will increase by twice the percent of voltage unbalance squared. Every 10 degrees Celsius cuts the insulation life in half, a 3% voltage unbalance reduces the life of the windings to almost onefourth it's expected lifespan. 2x(3x3)=18 degrees Celsius
Problem 2. Unbalanced voltage wastes energy by reducing the efficiency of the motor. An unbalanced AC Induction Motor's Rotor will not run at full rated RPM, this is because the rise in temperature creates what is called "slip". An example of this would be an unbalanced motor running a pump, that combination will take longer to pump the same amount of volume over time than a pump powered by a balanced motor. The end result, energy is wasted!
One leaking faucet dripping at a rate of 30 drops a minute wastes 1,388 gallons of water a year, how much electricity would you estimate is wasted in one year in just the U.S. due to the voltage unbalance of all those motors? The U.S. Department of Energy published an article on June 29, 2007 estimating that it may be as much as $28 Billion a year!
By Bruce A. Johanningmeier
1. Measure the voltage across each of the three phases from L1 to L2, L1 to L3, and L2 to L3.
2. Enter the voltages in the below designated textboxes.
3. Click the Calculate button to get the results.

