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The efficiency of variable speed drives with induction motors and permanent magnet synchronous motors has received little attention so far in international standards. However the number of such applications is increasing rapidly and the potential energy savings are large. This paper is based on a measurement campaign of three collaborating research institutes and reports on the efficiency of motors up to 15 kW. The efficiency values are represented by means of iso efficiency contours. From these contours, the efficiency for IE1, IE2 and IE3 induction motors is compared with that of a permanent magnet machine for the entire torque -speed operation region. Also the impact of flux optimization with induction motors is analyzed. The required number of measurement points to construct accurate iso efficiency contours with minimum measurement effort is also discussed.

Increasing awareness of ecological problems forces machine manufacturers to design greener machines. This implies amongst other things the selection of the most efficient electric motor system for their specific application. On the other hand, machine building applications evolve more and more from constant speed and load characteristics to varying speed and load applications. Therefore, the motor system that is used evolves more and more from direct online (DOL) to motors fed by a variable speed drive (VSD). However, current efficiency standardization focuses on DOL applications, and can by consequence not offer assistance to the machine builder to select the most efficient motor-VSD combination for his particular varying load application. The goal of this paper is to present a methodology that allows to predict the energy consumption for a specific motor-VSD combination and a specific varying speed-load application, using the fairly new concept of iso efficiency contours. By comparing the predicted energy consumption for a number of selected combinations, the most efficient one is revealed.