Large currents are generated when induction motors are started. This is called the inrush current, and is much larger than the rated current even if it’s under the normal condition. On the other hand, the motor needs several levels of protection to guard against out-of-specification startup conditions by using the overcurrent relay or the impedance relay. Thus, the appropriate relay settings need to be determined by analyzing the transient trajectories and the impedance of the motor.

This application shows how to analyze the transient behavior of a motor with impedance and torque characteristics, obtained from an equivalent circuit model of an induction motor. The application also demonstrates a protection scheme study based on these result of analysis.