Abstract
Induction motors are widely used in industry because of their rigidity and speed-control flexibility. Therefore, the problem of induction motor protection attracted many researchers. The digital protection techniques that are used in digital relays provide better performance and higher accuracy than the conventional electromagnetic and solid-state relays. This paper tackles the problem of three-phase induction-motor overall protection using digital protection algorithms. A stand-alone, microcontroller-based digital protector is designed and implemented. Many algorithms are developed to realize the various functions of the protector. Mc-8031 assembly-language procedures are coded to activate the hardware of the protector. Moreover, the electrical and thermal behavior of the induction motor is studied. The implemented protection system can be programmed to suit a wide range of induction motor sizes, and to monitor the motor parameters during its operation and after a fault occurrence through seven-segment display units. The relay protection functions that are considered in this implementation are overload, over-temperature, supply under-voltage, supply over-voltage, unbalance of supply-voltages, phase reversal, phase loss, heavy over-current, excess ground-current, unbalance of supply-currents, and repeated startings.