Electric Motor Efficiency under Variable Frequencies and Loads

Charles M. Burt, California Polytechnic State University - San Luis Obispo
Xianshu Piao, California Polytechnic State University - San Luis Obispo
Franklin Gaudi, California Polytechnic State University - San Luis Obispo
Bryan Busch, California Polytechnic State University - San Luis Obispo
N. F. N. Taufik, California Polytechnic State University - San Luis Obispo

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© 2008 American Society of Civil Engineers. The definitive version can be found online at http://dx.doi.org/10.1061/(ASCE)0733-9437(2008)134:2(129).

Abstract

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.