This paper is based on findings resulting from ASHRAE Research Project RP-1395.

The objectives of this paper are to provide information on heat loss for solid state VFDs at both full and part-loads and to show how these losses could be predicted. In developing these predictions, use is made of manufacturer and independently measured data. Two analytical part-load VFD power loss models are presented, a linear model in which the VFD power losses vary linearly with the load and a quadratic model in which the VFD power losses vary as the square of the load. The performance of these two analytical models is compared with measured part-load VFD power loss data. An example of VFD power loss at part-load is presented and compared with the results of the two models. The performance of the VFD power loss quadratic model is shown to be superior to the linear model.