Design of a Phase Failure and Overvoltage Protection Relay for Three-Phase Industrial Motors in Aba, Abia State

This project designs a phase failure and overvoltage protection relay for three-phase induction motors used in industrial workshops in Aba, Abia State, South East Nigeria. Three-phase induction motors are the workhorses of Aba's manufacturing sector, driving machinery in metalworking, plastic moulding, and food processing plants. Phase failure, caused by a blown fuse, a disconnected supply conductor, or a distribution company feeder fault, forces motors to run on the remaining two phases, generating excess heat and eventually burning out the motor windings. The protection relay continuously monitors the voltage magnitude of all three supply phases using a precision rectifier and voltage comparator circuit. When any phase voltage deviates more than 15 percent from the nominal level, or when any phase is entirely absent, the relay activates a contactor to disconnect the motor from the supply within 100 milliseconds. Overvoltage above 265V on any phase also triggers disconnection. An automatic restart timer allows the motor to reconnect after a configurable delay once supply conditions normalise, suitable for pump and ventilation applications. Visual indicators show which phase condition triggered the protection event. The prototype relay was tested on a 5.5kW three-phase induction motor at a workshop in Aba using a variac to simulate phase voltage variations. The relay responded correctly to all tested fault conditions within the specified response time. Component cost is estimated at under 8,000 naira, making the relay economically accessible for small workshop operators. Keywords: phase failure protection, three-phase motor, overvoltage relay, industrial protection, Aba.