Motor protection relays

This chapter presents motor protection relays. Electric motors are the workhorses of industry and are extensively used to convert electrical energy into rotational mechanical energy. Squirrel cage induction motors, particularly the TEFC type (totally enclosed, fan cooled), have become extremely popular mainly because of their simple, rugged construction, and good starting and running torque characteristics. For example, in a small country such as South Africa, about 100 thousand of this type of motor above 1 kW are sold annually, mainly for new applications but also as replacements for worked out or “burned out” motors. The TEFC design improves the mechanical life of the motor because dust and moisture are excluded from the bearings and windings. Some of the early designs of motor protection relays have a single function whose purpose was to protect the motor against overloading by ensuring that it never draws in excess of the rated current. This was done by continuously monitoring the electrical current drawn by the motor and arranging for the motor to be disconnected when the current exceeded the rated current and remains so for a certain period of time. Higher the overload current, shorter is the permissible time before disconnection. This time delay is achieved in various ways that have been discussed in the chapter.