Abstract : The air compressor system adopts variable frequency speed control technology, and the motor can be smoothly adjusted within a wide range. In the past, the discharge capacity of the air compressor was determined by the opening of the intake valve. If the discharge capacity is determined by the motor speed adjusted by the frequency converter, not only can energy be saved, but the air network can also be kept constant. The key to adopting variable frequency speed control technology is that the motor speed is adjustable and controllable. The variable frequency speed control system of the motor is controlled by the air pressure detector and PID transmitter. Keywords: frequency converter, air compressor. The air compression station of Mudanjiang Petrochemical Plant is an important department of the plant. There are four 20m³ air compressors with a discharge capacity of 22m³/min produced by Nanjing Compressor Factory in the air compressor station. Each compressor is driven by a 132kW/380V three-phase asynchronous motor with a rated current of 262A. Due to the small transformer capacity, each motor start-up requires a self-priming reduced-voltage start, and the three compressors run continuously 24 hours a day. Since frequent starts of the motor are not permitted, the motors still run under no-load when air consumption is low, resulting in very high power consumption and significant energy waste. Frequent unloading and loading cause frequent pressure fluctuations in the entire air network, making it impossible to maintain a constant pressure. Therefore, the factory decided to modify the air compressors to reduce power consumption, improve air network pressure quality, and extend equipment lifespan. 1. System Principle The system principle is shown in the figure. It mainly consists of a power control circuit, oil pressure sensor, water pressure sensor, exhaust pressure sensor, frequency converter, 132kW/380V asynchronous motor, air compressor, pressure transmitter, and air pressure detector. Operation is controlled via buttons and indicators on the control panel (see Figure 1). 2. Control Principle The system has two control loops: frequency converter and mains frequency, which are independent and interlocked. Variable frequency drive (VFD) control starts the motor at low speed by changing the frequency, and after a certain time, it switches to the set speed. During system operation, the VFD uses the pressure value (0-10V analog signal) from the pressure detector of the air tank via a pressure transmitter as feedback for PID constant pressure regulation. This value is compared with the set pressure value within the VFD, and after calculation, the VFD adjusts the frequency to the required value, automatically adjusting the motor speed to achieve the desired pressure. When the air tank pressure is lower than the set pressure, the VFD output frequency continuously increases, reaching its maximum at 50Hz, at which point the motor speed and air compressor discharge volume also reach their maximum. When the air tank pressure is higher than the set pressure, the VFD output frequency decreases, the motor speed decreases, and the air compressor discharge volume also decreases, thus achieving the constant pressure target. [b][align=center]For more details, please click: Application of VFDs in Air Compressor Retrofitting[/align][/b]