An analytical method for energy efficiency estimation in the case of large in-service low voltage three phase asynchronous motors is proposed, tested and discussed. Starting from the steady-state operating motor equations of the modified per phase equivalent circuit model of the machine, a robust symbolic software package was generated. The developed algorithm only requires the basic power quality parameters as measured at the induction motor input and its nameplate data. Thus, a rather accurate and less intrusive method for field efficiency evaluation of in-service motors is described. In order to validate the results, five induction motors were tested. The obtained values for both operating power factor and efficiency are compared with those reported by the manufacturer specifications.

Keywords: Analytical method, Induction motors, Operating efficiency, On-site evaluation