Converter-fed induction motor efficiency: Practical applicability of IEC methods

Electric motors consume a major proportion of the world's electric energy. In recent years, new regulations on the efficiency of electric motors have been introduced around the world. Reductions in the energy consumption of motors can be achieved not only by steering the industry to use more efficient motors but also by encouraging the application of frequency converters to supply motors. Related to this progress, the International Electrotechnical Commission (IEC) is preparing standards for converter-fed motor efficiency measurements. Existing efficiency measurement standards cover only direct-online (DOL) motors, and there is a strong demand for standardized classification measurements for converterfed motor efficiency. The current IEC methods for determining losses and efficiency of converter-fed induction motors (IMs) have been introduced as a technical specification while the standardization is still in process. Therefore, it is important to promote critical discussion about the issue in general. In this article, the IEC loss segregation methodology for converter-fed motors is described, executed in practice, and analyzed. Some features of the IEC main method, such as the strictly defined converter voltage waveform and the single motor's rated determination point, have been criticized in previous articles, and these issues are addressed in detail. Laboratory measurements are a technical specification while the standardization is still in process. Therefore, it is important to promote critical discussion about the issue in general. In this article, the IEC loss segregation methodology for converter-fed motors is described, executed in practice, and analyzed. Some features of the IEC main method, such as the strictly defined converter voltage waveform and the single motor's rated determination point, have been criticized in previous articles, and these issues are addressed in detail. Laboratory measurements are made with two frequency converters; the first converter is set to provide the IEC-defined voltage modulation, and the second converter represents a practical case, a commercial frequency converter for comparison. According to the results presented in this article, the IEC loss segregation for converter-fed motors is usable for motor loss comparison purposes, but the method itself is complex to apply. Further more, the method cannot be used to predict the losses of a variable speed drive (VSD) system in an end-user application.