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.
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