Abstract: Motors with efficiency high values can became inefficient then standard motors when they are lightly loaded. The calculation procedure for the estimation of any part load efficiency when two load points are given is presented in paper. Efficiency dependence is expressed in the form of η=ηN± k(1-ηN), where is magnitude kP = f1(P0, PγN, P/PN), so that changes-deviations of efficiency values compared to rated value (γN) would be explicitly expressed. Based on those deviations, energy efficiency class changes of observed motors are directly identified with load change. Efficiency curve for several motors, with typical shapes, are considered, especially for loads P/PN=0.50-1.00. These results are shown justification claim that efficiency values at 50% and 75% load shall be stated in the documentation. The users can then select the motor best suited for their application.
Keywords: Induction motors, estimation, efficiency, energy efficiency class, partial load efficiency, testing.