Protection of AC motors by means of microcontrollers

In this study, a system was designed and implemented to make protection for the current, voltage, winding temperature of induction motors. In the system, the voltage transformer with transformation ratio of 400/5 V and current transformer with transformation ratio of 25/5 A were connected to each phase of induction motors. The circuit was designed for measuring the winding temperature. The sampling circuit was realized such that low AC signals taken from transformer secondary windings and sensed temperature values convert into DC values. The control process was implemented by using PIC 16C84 microcontroller. To input reference values and to monitor these values, the LCDs and a keypad was added to the microcontroller. Sampled DC values were transmitted into the microcontroller by using an analog-digital-converter (ADC) unit. Measured data are compared with reference values by means of software. When a low voltage, high voltage, high current, voltage and current unbalance, high winding temperature or single phasing are sensed, the system opens its normally soft starter after a trip delay. Trip and reset delays prevent nuisance tripping due to rapidly fluctuating power line conditions. Besides, data such as current, voltage, temperature which are acquired by microprocessor are transmitted to the central PC by means of RS232C series communication line. In this way the motor data could be observed at any moment on the PC monitor. The PC then shows a message corresponding to power-system fault, and alerts the operator.