Abstract : This system uses a programmable logic controller (PLC) to control a frequency converter to achieve stepless speed regulation of the induced draft fan, maintaining a constant negative pressure in the furnace. Compared with traditional control methods, the system has high reliability and saves a significant amount of energy. The paper details the software and hardware design process of this system. Keywords : Frequency converter; Wiring diagram; Sequential function chart; Constant pressure 0 Introduction The boiler furnace needs to maintain a constant negative pressure, but excessive negative pressure will cause excessive heat loss. Therefore, it is necessary to dynamically control the induced draft fan's airflow to achieve a constant negative pressure in the furnace. The traditional approach is to control the induced draft fan motor to rotate at a fixed speed, and then adjust the airflow by adjusting the fan's damper. While adjusting the damper is simple, it is uneconomical because a considerable amount of energy is lost through the damper. Since motor energy consumption is proportional to the cube of the motor speed, using frequency converters to regulate the fan motor and adjusting the airflow through changes in its speed can save a significant amount of energy. Data shows that adjusting the motor speed saves up to 40%-50% more energy than adjusting the damper. Although this renovation is more expensive, the annual energy savings can essentially recoup the investment. 1. Scheme Selection This system uses a programmable logic controller (PLC) and a frequency converter (VDC) to achieve a constant negative pressure in the boiler furnace. The furnace pressure signal is converted into a standard 4-20mA current signal by a pressure transmitter. If this current signal were sent to the PLC for processing, the PLC would have to use an analog signal processing module, because the CPU of the basic PLC unit can only directly process digital signals. However, the analog input/output modules of PLCs are relatively expensive, which may be unacceptable to some users. Therefore, the current signal reflecting the negative pressure is directly input to the analog input terminals of the VDC, and after processing by a PI regulator, the VDC achieves stepless speed regulation. Compared with the multi-stage speed regulation of the VDC, this control method has the advantages of simple hardware wiring and PLC programming. The PLC is only responsible for controlling the operation and shutdown of the VDC and processing the VDC output signal. In addition, when the VDC fails, the PLC should be able to switch the VDC operation mode to the mains frequency power supply operation mode in a timely manner to ensure uninterrupted boiler operation. [b][align=center]For details, please click: Application of Frequency Converters and Programmable Controllers in Boiler Exhaust Fans[/align][/b]