Power measurements of variable speed motors

Accurate measurements of voltage, current and power under nonsinusoidal conditions are essential for determining the efficiency of a motor drive system and performing separation of losses between an adjustable speed drive (ASD) and an electric motor. That information is invaluable for equipment designers, manufacturers and users. Three modern power meters and analyzers were tested to determine their accuracy with various nonsinusoidal waveforms applied. The meters were subjected to waveforms that are characteristic to the three most common fractional and integral horsepower ASD technologies, namely, PWM induction, brushless DC and switched-reluctance drives. The tests were performed under field conditions and in a computer-controlled laboratory environment. The obtained results show that some meters are able to measure accurately electric power at the input to ASD´s. However, the output power of ASD´s for brushless DC and switched-reluctance technology was not measured with acceptable accuracy by any of the tested meters. Possible reasons of meters inaccuracies and suggestions for performance improvement are presented. Conclusions on the present state of ASD electric power measurements and recommendations for further research are given