Microcontroller based three-phase induction motor protection relay with operator selectable thermal it curve feature

Abstract

This paper presents a Microcontroller based three phase induction motor protection relay. The scheme employs the Hall effect current transducers for sensing motor line currents, and an Auto ranging feature, in order to acquire motor line currents accurately from no load to blocked rotor current. The flexibility of Operator selectable thermal current versus time (I-T) curves feature is incorporated in the relay software, in which using the operator fed cold curve points and the thermal time constant of the motor, the corresponding I-T cold and hot curves can be fitted by the relay Software. The relay performance, with the selected I-T curves has been studied in the laboratory by testing on a three phase, 5 h.p. induction motor and it has been observed that the relay to follow the selected I-T characteristics. 1998 Taylor & Francis.