INDUCTION MOTORS - PROTECTION and STARTING

It is well understood that induction motors draw higher currents during their starting operations than is the case under full load running conditions. Since the early days of induction motor availability, starting methods other than Direct-on-Line have been used, and in some cases mandated by Utilities, to reduce the effect of these high starting currents on the electrical distribution network. What is generally not recognised, is the existence of short duration inrush currents, that greatly exceed these starting currents. Furthermore, the introduction of complex starting methods to reduce starting currents is often compromised by other unanticipated inrush currents introduced by the starting system itself, unless special precautions are taken. This paper examines the sources and consequences of these inrush currents on the distribution system, as well as on the motor protection components. Recommendations are given in regard to the selection of both overcurrent and earth leakage protection of induction motors that consider not only the motor protection requirements, but also the limitations on the protection devices that may result from the abnormal circuit disturbances that could be introduced through the starter.