Squirrel-Cage Induction Motors: Understanding Starting Transients

In this article, the transients associated with line starting of squirrel cage induction motors and the potential difficulties they pose are examined. These difficulties include transient torques significantly in excess of the rated motor torque, large inrush currents, and corresponding large dips in voltage. Test results that illustrate these problems are presented. Various techniques to ameliorate the effects of the starting process, including reduced voltage and soft starting, are discussed, and the benefits of these techniques are illustrated through simulation. The oscillatory transient torques discussed in this article are not new, but have often been ignored.