In this system, a D71 18.5KW frequency converter drives the 11KW main motor of the screw extruder. The frequency reference of the main motor inverter is given by the PLC through the RS485 interface. During operation, the speed can be arbitrarily modified on the touch screen to realize the accurate speed reference of the main motor; at the same time, the PLC reads the frequency and current of the inverter through MODBUS communication. .