Abstract : Currently, various enterprises are carrying out energy-saving renovations on their equipment to reduce energy consumption. Among these, frequency conversion retrofitting of air compressors is a relatively mature measure. Our factory has also tried this approach and achieved success. Keywords : Air compressor, frequency converter 1. Equipment status before retrofitting 1.1 Equipment working principle Our factory has two 02h/min screw air compressors and two 10h/min screw air compressors. Their working principle is that a pair of parallel meshing male and female rotors rotate in the cylinder, causing the air between the rotor teeth to continuously produce periodic volume changes. The air is then transported from the intake side to the output side along the rotor axis, realizing the entire process of intake, compression, and exhaust of the screw air compressor. The air compressor's inlet and outlet are located at opposite ends of the casing. The slots of the female rotor and the teeth of the male rotor are driven to rotate by the main motor. 1.2 Equipment Control Method Our 20N13/N screw air compressor has a main motor rated power of 132kW, and the 101113/N screw air compressor has a main motor rated power of 75kW. Both operate using a star-delta reduced-pressure start method. The specific operation process is as follows: Pressing the start button activates the starter coil and opens the oil shut-off valve, starting the air compressor in no-load mode. At this time, the intake valve (i.e., the loading valve) is closed. When the system oil temperature has not reached the required temperature (our equipment's oil temperature is set at 50℃), a portion of the lubricating oil bypasses the oil cooler and enters the compressor directly to quickly raise the oil temperature. When the specified oil temperature is reached, the loading valve opens, and the equipment begins to operate under load, causing the system pressure to rise. If the system pressure rises to the upper limit of the pressure switch (i.e., the trip pressure), the controller closes the intake valve, the oil-gas separator releases gas, and the compressor runs unloaded until the system pressure drops to the lower limit of the pressure switch (i.e., the return pressure). Then, the controller opens the intake valve, closes the oil-gas separator release valve, starts the compressor, closes the oil-gas separator release valve, and the compressor starts running again. 1.3 Problems with the original system: 1) Although the main motor uses star-delta reduced pressure starting, the starting current is still very large, which will affect the stability of the power grid and the operational safety of other electrical equipment; 2) The air compressor often runs unloaded, which is uneconomical and wastes a lot of electricity; 3) The air compressor operates at the power frequency, resulting in a lot of noise during operation; 4) The power frequency starting of the air compressor has a large impact on the equipment, and the operating frequency of various components such as solenoid valves is high, so the equipment maintenance cost is relatively high. 2 Feasibility Analysis of Variable Frequency Retrofit 2.1 Energy Saving: Since the gas load of our plant fluctuates greatly, there is great potential for variable frequency energy saving retrofit of the air compressor. Compared to fixed-frequency controlled air compressors, variable frequency (VFD) air compressors offer significant energy savings. VFD control systems can accurately adjust the compressor output based on the specific needs of different air-consuming equipment, achieving energy savings. This is primarily due to the soft-start function of the VFD, which reduces the compressor's starting current and saves energy. Secondly, the VFD adjusts the motor speed according to changes in air supply during compressor operation, resulting in energy savings. Finally, when air consumption is extremely low, the compressor operates under unload conditions, significantly reducing motor energy consumption (below 30kW for a VFD-controlled compressor). According to the VFD curve table (provided by the air compressor manufacturer as test data), the average power consumption of the air compressor under load when using VFD control is approximately 80kW. Based on our actual data from one year, the annual operating time was over 10 hours (sllX), the load ratio was 45%, and the average power under power frequency load was 113kW. Compared with power frequency, the annual electricity saving under variable frequency control load was 118,800 kWh, with a saving amount of 77,220 yuan (calculated at 0.65 yuan per kWh). The annual electricity saving under no-load was 88,000 kWh (power frequency no-load power is approximately 50kW, while variable frequency no-load power is approximately 30kW), with a saving amount of 57,200 yuan. The total annual savings amounted to 134,420 yuan. The cost of retrofitting a 20m³/min screw air compressor with variable frequency is approximately 150,000 yuan, therefore, theoretically, this retrofit is feasible. 2.2 Reduced Maintenance Costs: The use of a variable frequency drive (VFD) control system reduces the impact on electrical and mechanical components due to the soft start of the air compressor. During normal operation, the rotational speed is mostly below the rated speed, and the operating frequency of various solenoid valves and pneumatic components is also reduced, extending the service life of parts. This reduces maintenance costs. 2.3 Precise Pressure Control: The VFD control system can precisely control the air compressor's output volume according to the actual needs of the pipeline air system. The VFD control system maintains stable pressure in the pipeline system by adjusting the speed of the main motor. The comprehensive protection measures of the VFD control system make compressed air more reliable. 2.4 Reduced Noise: When the exhaust volume decreases, the motor speed slows down, and the noise emitted by the air compressor is significantly reduced compared to full-speed operation, improving the operator's working environment and contributing to environmental protection. 3. Modification Plan and Equipment Operation 3.1 Variable Frequency Drive (VFD) Modification Requirements Based on the existing problems with the original equipment, the air compressor after VFD modification should meet the following requirements: 1) The motor should maintain a relatively stable outlet pressure of the air tank during VFD operation, with fluctuations of 0.1 MPa; 2) For system reliability, the system should have both VFD and power frequency control; 3) The VFD control system should adopt a closed-loop control method; 4) For system stability, one 2000mm screw air compressor should operate in VFD mode, but the two air compressors can switch between each other; 5) Based on the torque characteristics of the air compressor, a constant torque VFD should be selected; 6) When the electrical load is low, the VFD should operate at low frequency to ensure normal lubrication of the equipment. 3.2 Modification Plan Based on the above modification requirements, we adopt a VFD control system (ABBASC X type general-purpose VFD), using a switching device to control two 2000mm screw air compressors (one operating at VFD, the other at power frequency). The control loop still uses the Siemens 57-2 type PLC from the original air compressor system. A pressure transmitter is installed on the main compressed air pipeline, outputting a 4-20fnA signal to the frequency converter. This signal is compared with the pressure setpoint, and after PID calculation, a control signal is output to the frequency converter's drive loop to adjust the motor speed and the air compressor's loading and unloading, maintaining the balance of the set pressure to achieve constant pressure air supply (see Figure 1). [align=center] Figure 1[/align] After several months of actual operation and multiple parameter modifications, satisfactory control results were finally achieved. 3.3 Operational Effect Analysis We continuously compared the power consumption data for 6 months, and the power saving rate reached over 28%, saving over 200,000 yuan in electricity costs annually. The investment can be recovered in less than a year. 3.4 Problems and Solutions The air compressor underwent frequency conversion modification, but after more than two years of operation, other negative issues arose: 3.4.1 Frequent Breakage of the Motor's Cooling Fan After multiple investigations, it was found that because our air compressor motor has a power of 132kW and the cooling fan is made of metal, the compressor speed frequently changed after the frequency conversion modification. Although the acceleration and deceleration times of the frequency converter were extended within the allowable range, the fan still frequently broke. Finally, it was decided to adopt an independent cooling method, installing a separate fan to cool the motor, which fundamentally solved the problem. 3.4.2 Premature Carbon Deposits in the Air Compressor's Oil System The air compressor mainly cools compressed air through its heat exchanger. Due to the frequency conversion modification, the average speed of the air compressor decreased. After a certain period of use, carbon deposits formed on the cooling coils and the inner walls of the oil separator, significantly reducing the heat exchanger's efficiency. This caused the air compressor to trigger high exhaust temperature alarms, resulting in frequent shutdowns and affecting normal production. We used the carbon deposit cleaner recommended by the air compressor manufacturer. During cleaning, the cooling coils were removed, and the carbon deposit cleaner was added and soaked for 2 hours (or circulated using a dedicated pump for 50 minutes). After rinsing with clean water, it was dried with compressed air. The carbon deposit cleaner discharged from the cooling coils after cleaning turned dark brown. Upon opening the top cover of the oil separator, we found that the inner wall and bottom were also dark brown, especially the bottom. We poured in the carbon deposit cleaner and repeatedly scrubbed with a brush, then rinsed with clean water, dried with compressed air, replaced the oil separator core, and added new lubricating oil. After starting the machine, the "high exhaust temperature" problem was eliminated.