A Novel Protecting Method for Induction Motor Against Faults Due to Voltage Unbalance and Single Phasing

A system is designed and implemented to make protection for faults due to single phasing, voltage unbalance and under voltage in a 3-ph induction motor. In the system, three potential transformers with transformation ratio 400/5V are connected to each phase of induction motor. The sampling circuit is realized such that the low AC signals taken from transformer´s secondary windings are converted into DC values. Control process is implemented using Microsoft PIC 16F877 microcontroller. Sampled DC values are transmitted to microcontroller using analog to digital converter unit. Measured values are continuously compared with reference values by means of software. When a voltage unbalance, under voltage or single phasing are sensed, the system opens the normally closed contactor by activating the 12 V, 10 A, DC relay and thus cuts the power supply to the induction motor. The corresponding fault is displayed in the seven segment LED display unit and alerts the operator. Trip and reset delays prevent nuisance tripping due to rapidly fluctuating power line conditions.