Effects of unsymmetrical voltage sags on squirrel-cage induction motors

The consequences of unsymmetrical voltage sags on the three-phase squirrel-cage induction motor behaviour are analysed; namely torque and current peaks, and speed loss. These effects depend on several variables such as sag duration, depth and sag type. The study takes into account that the voltage recovery instant can take only discrete values since fault clearing occurs in the natural current zeroes. The torque and current peaks have a periodical dependence on the sag duration, and a linear dependence on depth. The study of the influence of sag type on speed loss shows that sag types C and D have the same behaviour, and the same is true for sag types E, F and G.