Harmonic Distortion Indices for Experimental Characterization of Variable Frequency Drives

Frequency converters controlling electric motors are widely used as they increase the functionality of the equipment and improve energy efficiency. However, it is inevitable that the quality of the power delivered to the motor does not have the same characteristics as when it is supplied from the mains. Due to the impact of the power quality on the operation of the motor and even on its possible diagnosis, it is desirable to characterise the power quality, preferably by means of rates or indices that reflect the quality as completely as possible. However, the rates specified in the standards and in the literature are designed to characterise the quality delivered from the grid and are not well adjusted to the features of the signal at the output of the converter with a high interharmonic and harmonic content at high and low frequencies, and with variable fundamental frequencies. For this reason, this work proposes a set of distortion rates and the appropriate procedures for calculating them to reflect as reliably as possible the quality of the energy delivered by the converter to the motor. To verify the validity of the proposal, several practical examples are presented with induction motors fed from different frequency converters.