1 Introduction Air compressors are widely used in industrial production. They are responsible for providing air sources for all pneumatic components, including various pneumatic valves. Therefore, the quality of their operation directly affects the production process. The main types of air compressors are piston type and screw type, but their air supply control methods are almost all based on loading and unloading control methods. 2 Motor principle and energy waste in air compressor loading and unloading air supply control methods 2 (1) The speed formula of AC asynchronous motor is: n=60f(1-s)/p where n—motor speed f—operating frequency; p—number of motor pole pairs s—slip rate; 2 (2) Problems in air compressor loading and unloading air supply control methods 2.1 Energy consumption analysis The loading and unloading control method causes the pressure of compressed gas to change back and forth between Pmin and Pmax. Pmin is the minimum pressure value, that is, the minimum pressure that can ensure the normal operation of the user. Generally, the relationship between Pmax and Pmin can be expressed by the following formula: Pmax = (1 + δ)Pmin δ is a percentage, and its value is approximately between 15% and 30%. In the air compressor under the air supply control mode of loading and unloading, the energy wasted is mainly in two parts: (1) Energy consumption during loading After the pressure reaches the minimum value, the original control mode determines that the pressure will continue to rise until the maximum pressure value. During the pressurization process, more heat must be released to the outside, resulting in energy loss. On the other hand, the pressure of the gas above the maximum pressure needs to be reduced by the pressure reducing valve before entering the pneumatic components, which is also an energy-consuming process. (2) Energy consumption during unloading When the pressure reaches the maximum pressure, the air compressor unloads by reducing the pressure in the following way: closing the intake valve to make the motor idle, and at the same time venting the excess compressed air in the separator tank through the vent valve. This adjustment method will cause a lot of energy waste. According to our calculations, the energy consumption of the air compressor during unloading accounts for about 10% to 25% of the energy consumption during full-load operation (this is under the condition that the unloading time accounts for a small proportion). In other words, the air compressor is in an unloaded state for 20% of the time, doing useless work. Obviously, under the loading and unloading air supply control mode, there is a lot of energy-saving potential for the air compressor motor. 2.2 Other shortcomings (1) The intake valve is adjusted mechanically, so the air supply cannot be continuously adjusted. When the air consumption changes continuously, the air supply pressure will inevitably fluctuate significantly. The air consumption accuracy does not meet the process requirements. In addition, frequent adjustment of the intake valve will accelerate the wear of the intake valve, increase the maintenance volume and maintenance cost. (2) Frequent opening and closing of the vent valve will not guarantee the durability of the vent valve. 3 Design of constant pressure air supply control scheme In view of the many problems existing in the original air supply control mode, the variable frequency speed regulation technology can be used for constant pressure air supply control. When adopting this scheme, we can take the pipeline pressure as the control object. The pressure transmitter YB converts the pressure P of the air tank into an electrical signal and sends it to the PID intelligent regulator. It compares the signal with the pressure setpoint P0 and calculates according to the established PID control mode based on the difference. The resulting control signal is sent to the variable frequency speed controller VVVF. The variable frequency controller controls the working frequency and speed of the motor, so that the actual pressure P is always close to the set pressure P0. At the same time, this scheme can add the function of switching between power frequency and variable frequency, and retain the original control and protection system. In addition, after adopting this scheme, the air compressor motor can be started by the variable frequency controller from standstill to rotation, realizing soft start and avoiding the starting inrush current and the mechanical shock to the air compressor. 4 Selection of system components and installation and commissioning of the system (1) Installation The control cabinet is installed in the air compressor room, separated from the original control cabinet, but the main wiring between it and the compressor should not exceed 30m. The wiring of the control circuit uses shielded twisted pair cable with a twisted pair pitch of less than 15m. In addition, the control cabinet is equipped with a ventilation device, and the inverter grounding terminal must not be mixed with the power grounding terminal. These measures enhance the stability and reliability of the system. (2) Debugging a) Inverter function settings (function settings may vary depending on the working conditions and control methods) 01-00=1 Set to 1: The running command is controlled by the external terminal 01-05=20 Acceleration time: Set to 20S, the specific value depends on the working conditions and production requirements 01-06=20 Deceleration time: Set to 20S 01-10=50 Maximum frequency: Set to 50Hz (equal to the rated frequency of the motor) 01-24=1 (Set as standard motor) 01-25=100-105 Motor overload protection level: The specific value depends on the working conditions and production requirements 05-00=1 PI control method 05-01=0 PI adjustment error polarity: Set to positive 05-02=0 PI setpoint signal selection: Set to 0 05-03=72% (setpoint pressure value) PI digital setpoint value (100% corresponds to 10KG) 05-04=1 or 2 PI feedback signal selection 05-07=5-10 Integral time 5-10 seconds 05-09=25 PI adjustment minimum operating frequency (3) Other matters during commissioning Adding a power frequency converter The entire set of modified devices does not change the original control principle of the air compressor, and the original air compressor system protection device remains effective. In addition, the power frequency/frequency conversion adopts electrical and mechanical double interlocking, which greatly improves the safety and reliability of the system. During the commissioning process, the lower limit frequency was adjusted to 40Hz, and then the temperature rise of the air compressor motor and the oil temperature of the pipeline were monitored for a long time and strictly using an infrared thermometer. The motor temperature rise was about 2-4℃, which is within the normal temperature rise range. The oil temperature remained basically unchanged, and the exhaust temperature dropped by 5℃. Therefore, the operation of the air compressor unit at the 40Hz lower limit frequency is very safe. (4) Energy Saving Data 5 Conclusion After a series of repeated adjustments, the system finally stabilized in the frequency range of 39.5-42.5Hz, the pipeline pressure was basically maintained at 0.69Mpa, and the gas supply quality was improved. The modified air compressor operates safely and reliably, and at the same time meets the process requirements of the factory's gas supply.