Motor tip: Resistance imbalance change from baseline

It is not unusual to see a large three-phase induction motor with phase resistance perfectly or closely balanced with an imbalance calculating well below 0.1 percent. With this kind of manufacturing precision, the balance of resistance measurement can be a very sensitive indicator of a changing condition in the motor windings or circuit. Although we direct your attention more to inductance and impedance for identifying winding degradation, strong consideration should be given to establishing a change from baseline caution and alarm set point for balance of resistance. The recommended change from baseline alarm set points are caution at a 50 percent increase and alarm at a 100 percent increase.

Here is an example of the importance of establishing these alarm levels. A motor tests at baseline with a .1 percent balance of resistance. Two years later the balance of resistance test in the field was 1.5 percent. A balance of resistance of 1.5 percent is not in excess of previous alarm set points and may not seem too high. However, when compared to the baseline it is over a 1000 percent increase and immediately needs to be investigated further.