A comparison of two energy efficient motors

Limited testing provides evidence that a particular energy efficient motor (EEM) appears to have a unique susceptibility to reduced negative sequence impedance. The effect of the reduction in negative sequence impedance is to allow a higher negative sequence current flow under conditions of voltage unbalance or negative sequence harmonic distortion in the bus voltage. This higher current flow results in additional heating of the rotor and 120 Hz vibration, and may combine with other conditions to cause premature failure. In addition, energy efficient motors operate at slightly higher speeds, and may sometimes cause the driven loads to require more horsepower from the motor causing further overheating. Existing industry guidance for the application of three phase motors in poor power quality environments may be inadequate because this guidance does not consider the combined effect of such conditions as voltage unbalance, harmonic distortion and overvoltage. A specific application of an EEM is studied in this paper; the motor may be failing due to a combination of overvoltage, voltage unbalance, harmonic distortion and loading the motor to full rated load, while the motor has a service factor of 1.0.