Unbalanced phase current is one of the largest contributors to reduced winding insulation life. Although the motor can be the culprit, unbalanced phase voltages and voltage distortion due to harmonics are the more typical causes.
Unbalanced phase voltages can be due to poor distribution by the utility or uneven single-phase loading on a three-phase system. A small voltage unbalance of only 2 percent can result in a current unbalance of 14 to 16 percent. This will result in an increase in temperature in the winding with the highest current load.
Figure 2. The effect of voltage unbalance on current unbalance
The percentage increase in temperature due to voltage unbalance is 2 x (% Voltage Unbalance)2. So, for a relatively small unbalance of 2 percent, winding temperature will increase by 8 percent. Current unbalance due to harmonics will not affect the phase voltages as measured by a volt meter, so the resulting current unbalance must be measured with an amp meter. Figure 2 shows the effect of voltage unbalance on current unbalance at various motor loads.
Current unbalance in motors that are operating at less than full load is usually not a problem as long as none of the legs exceed the nameplate amperage. For more information on diagnosing the sources of current unbalance and how to fix them, see "PumpEd 101," July 2008.
Although Class F and H insulations allow a temperature rise of 25 to 50 degrees more than Class B, motor manufacturers are using a portion of that increase to build higher horsepower motors on a smaller frame size. Therefore, we still have to pay attention to the factors that affect motor operating temperature if we want to achieve maximum winding life.
When Ed Cowern was with Baldor Motors, he wrote a 115 page white paper called the "Cowern Papers." It is, by far, the best and most comprehensive AC motor reference I have seen. It is also written in a simple and understandable "Pump Ed 101" manner.
Pumps & Systems, July 2011