Effects of unsymmetrical voltage sag types E, F and G on induction motors

This paper analyzes the effects caused by voltage sag types E, F and G on the induction machine supply system: current and torque peaks, and mechanical speed loss. These effects depend on many elements, such as sag magnitude and duration, type of sag, and fault and recovery voltage instants. It is shown that the most severe transient occurs for specific instants. Two examples of the transformation of the sag to lower voltage level (through a transformer) are shown in this paper. The second example also includes a capacitor bank on the secondary side. Extensive ranges of voltage drops have been analyzed (for different magnitude and duration and for the most severe fault and recovery voltage instants), and machine sensitivity is graphically shown in CBEMA curves. These curves can be applied to protective relay coordination, especially for large machines. The unsymmetrical sags can be grouped to express machine sensitivity in CBEMA curves, using positive-sequence voltage for ordinates.