9 - Motor protection

Abstract

Motor protection is essential to safeguard motors and their cables from damage caused by overheating. Most motor protection relays detect the conditions of overloading, stalling, and single-phasing that cause overheating. There has been a complete change-over from thermal to electronic and microprocessor-based relays for motor protection, ranging from simple overload relays to comprehensive relays, which not only detect all fault and abnormal conditions but can also report, to a remote location, the state of the motor while running. This chapter describes a large number of thermal motor protection relays that are also in use. Before discussing the protection, the chapter provides a short description of the operation of the squirrel-cage induction motor. One of the most important relays for the detection of abnormal conditions is the overload relay, which is applied to the protection of motors. Compared to the overcurrent relay, which under fault conditions is required to detect a current that is many times the normal current, the overload relay must be capable of accurately measuring the current that is only slightly greater than the nominal full-load current. The function of the overload relay is to prevent the overheating of equipment. The operating time of a typical motor overload relay is of the order of two minutes at twice full-load current.