Protecting Asynchronous Motors

48 DESIGN IDEAS The contents of this column are based solely on information supplied by the author and do not imply practical experience by Elektor Electronics. PROTECTING ASYNCHRONOUS MOTORS by Mehrdad Rostami, University of Tehran, Iran The circuit described here was designed for protecting heavy-duty asynchronous motors during the start-up period. As is well-known, without protection such motors may easily get damaged by poor starting. The circuit may also be used for other applications where a trip circuit needs to be triggered, such as, for instance, in the monitoring of liquid levels. Every motor has a time-speed character- istic that shows how, or otherwise, it starts and reaches its normal speed. A number of such curves are illustrated in Fig. 1. If the characteristic of a particular motor is similar to the lower (bold) one, any attempt at start- ing the motor should be stopped immediate- ly and the motor in-spected thoroughly. The dashed curve in-dicates the lower limits ...