Comparative study of unsymmetrical voltage sag effects on adjustable speed induction motor drives

This study analyses unsymmetrical voltage sag effects on the torque and speed deviation in induction motor (IM) adjustable speed drives. The following three general types of IM drives control are analysed: scalar controlled (V/Hz), rotor-field oriented and direct-torque controlled. The analytical expressions for dc-link voltage variations incorporated into the corresponding drive models as well as formulas for assessment of current/torque deviation are derived depending on the applied control algorithm. The analytically predicted theoretical results for the IM drive performance deterioration as a result of voltage sags have been validated experimentally. Measurements of sag-caused current/torque ripple and mechanical vibrations have been used for additional verification of the obtained results.