Nonsinusoidal electrical measurement accuracy in adjustable-speed motors and drives

Accurate measurements of voltage, current and power under nonsinusoidal conditions are essential for determining the efficiency of a 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, pulsewidth modulation 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 ASDs. However, the output power of ASDs 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.