Efficiency degradation detection for VFD-motor-pump systems

Abstract

Electric motor-driven fan, pump, and compressor systems accounted for 27% of the electricity usage in commercial sectors in United States in 2013. Therefore, even small efficiency improvements will produce very large energy savings. In fact, the efficiency of these systems deteriorates over their lifetime. The efficiency of motors could degrade by up to 5% while the efficiency of pumps could degrade by up to 25%. Since variable frequency drives (VFDs) are widely applied on electric motors, the efficiency degradation is more often concealed by the significant energy reduction introduced by VFDs. The purpose of this paper is to develop and demonstrate an approach, which can effectively detect the efficiency degradation of VFD-motor-pump systems by simply measuring the pump water flow rate and head as well as the system input power. First, the ideal system efficiency was modeled by combining the pump, motor, and VFD efficiencies and applied to correlate the pump water flow rate and head to the ideal system input power. Then, an experiment was conducted on a ten-year-old VFD-motor-pump system to quantify the efficiency degradation by comparing the measured and ideal system input power. The results of the experiment reveal that the efficiency of the studied system degraded by 19%.