Abstract : This paper describes a method of controlling an air compressor by using a frequency converter instead of a traditional unloading valve. Keywords : Frequency converter; air compressor; pressure; unloading valve; PID controller. Frequency conversion technology is often used for motor speed control, but can it be applied to air compressor control? How does it achieve air compressor control? Let's explore this below. Commonly used air compressors, especially reciprocating air compressors, use unloading valves to control the air compressor pressure and reduce power consumption. Examples include reciprocating air compressors produced by Liuzhou Air Compressor Factory in China, and screw air compressors produced by KAESER and Ingersoll Rand abroad. The advantages of this method are: simple control, no need for an additional logic control system, and therefore lower cost; the disadvantages are: large pressure fluctuation range, low safety factor, and frequent occurrences of unloading failure or inability to pressurize after unloading. In response to this situation, in 2000 our company decided to improve the four piston air compressors on the PMI paper machine, changing the original unloading valve control to variable frequency drive (VFD) control of the motors, which yielded significant results. Subsequently, in 2002, we improved the two KEASER air compressors on the PM2 paper machine, also achieving significant results. The improved air compressor system maintained stable pressure while greatly saving power consumption. Our solution is as follows: In an air compressor station system, we eliminate the unloading valves at the inlet of all air compressors. Then, based on the air compressor's air production capacity and the factory's compressed air consumption, we determine the number of air compressors to use VFD control (e.g., one air compressor on the PM1 paper machine and two air compressors on the PM2 paper machine), while other air compressors use fixed frequency control (and use soft starters instead of contactors). Whether the air compressors operate on VFD or fixed frequency, they are all controlled by a programmable control system. Additionally, a pressure sensor is added to the main output pipeline to provide a feedback signal to the PID controller, forming a closed-loop control system. The operating frequency of the frequency converter is controlled by the output of the PID controller. When the difference between the actual value and the set value is large, the PID controller outputs a larger value, causing the variable frequency air compressor to run rapidly (possibly close to the mains frequency) to bring the actual pressure value close to the set value as quickly as possible. When the actual pressure value is close to the set value and the pressure change Ap is small, the frequency converter gradually decreases until the actual pressure value is approximately equal to the set value. At this point, the frequency converter basically stabilizes and changes with the actual pressure. Furthermore, regardless of whether it is a piston air compressor or a screw air compressor, a minimum frequency should be set to ensure good lubrication; otherwise, the air compressor may alarm or be damaged due to lubrication issues. [b][align=center]For more details, please click: Application of Variable Frequency Technology in Air Compressors[/align][/b]