The overall system efficiency (wire to water efficiency) of a variable speed pumping system (VSPS) is determined by drive, motor and pump efficiencies. All three component efficiencies vary with the pump operating conditions. A 15 hp experimental VSPS was developed to measure the component and system efficiencies over a wide range of operating conditions. The test results demonstrate the significant energy savings of a VSPS but at partial load condition the system efficiency is significantly lower than the efficiency at full load or near full load conditions. Especially, when the actual flow rate is less than 60% of the full load flow rate, the system efficiency decreases more rapidly with decreasing flow rate. The system efficiency also displays a strong dependency on system flow rate. Based on test results, a correlation was developed between the non-dimensional system efficiency and the nondimensional flow rate. This correlation provides a simple method for calculating VSPS efficiency for a wide range of part-load operating conditions and is recommended as an alternative to assuming full load efficiencies remain constant.

Units: Dual