Electric Motor Efficiency under Variable Frequencies and Loads

This paper details a study performed by the Irrigation Training and Research Center to determine motor performances under varying speeds [induced by a variable frequency drives (VFD) controller] and loads. A further goal of the study was to provide sufficient information to designers so that they could estimate total pumping plant power usage with a VFD-controlled installation. Motors were tested with a VFD as well as across-the-line. On average, the relative efficiency of the electrical system with a VFD may be approximately 8% lower than the relative efficiency of a properly designed, full-load across-the-line system. If one considers actual field operating conditions this 8% is misleading because overall energy savings can be obtained with VFDs due to their ability to properly adjust speeds to meet actual field conditions.