Design of a Microcontroller-Based Three-Phase Motor Starter with Overload and Phase-Failure Protection for Small Industries in Imo State

Three-phase induction motors are the primary drivers of industrial machinery in small manufacturing enterprises across Imo State, South East Nigeria. Motor failures due to overload conditions and phase supply imbalances are frequent and result in costly motor replacements that disproportionately affect small business owners. This project presents the design of a microcontroller-based intelligent motor starter integrating overload protection, phase-failure detection, and phase-imbalance sensing. Current transformers on each of the three supply phases feed an ATmega328P microcontroller with continuous phase current data. A phase voltage monitoring circuit provides simultaneous voltage readings. The microcontroller implements protection logic to trip a motor contactor via a relay output when overload current, phase loss, or phase voltage imbalance exceeding 10 percent is detected. A soft-start function gradually increases supply voltage to the motor at startup to reduce mechanical and electrical starting stress. The system was bench-tested with a 5.5 kW three-phase motor subjected to simulated fault conditions. Overload tripping response time was 1.8 seconds at 150 percent rated current, and phase-failure detection operated within 0.5 seconds of a phase loss event. A seven-segment display and alarm buzzer communicate fault type to operators. The study benchmarks the prototype against three commercial motor protection relays in terms of functionality and cost, showing that the prototype delivers equivalent protection at 40 percent of the commercial relay cost. Keywords: motor protection, phase failure, overload detection, motor starter, Imo State