Assessment of Unsymmetrical Voltage Sag Effectson AC Adjustable Speed Drives

This paper analyses the influence of unsymmetrical voltage sags on the increase of torque and speed deviation induction motor adjustable speed drives. The three following general types of induction motor drives control are analysed: scalar-controlled (V/Hz), rotor-field-oriented (RFO) and direct-torque-controlled (DTC). The analytical expression for variations of dc link voltage incorporated into the corresponding drive models and formulas for assessment of current/torque deviation are derived depending on the applied control algorithm. Afterwards, the presented theoretical results for the deterioration of motor drive performance due to voltage sags are validated experimentally. Measurements of sag-caused mechanical vibrations are used for the additional verification of the obtained results.