1. Introduction With the development of power electronics technology, frequency converters are increasingly widely used in speed regulation. They offer advantages such as stability, ease of operation, and significant energy savings, making them increasingly important to engineering and management personnel. TBEA Shandong Luneng Taishan Cable Co., Ltd. is a large-scale wire and cable manufacturing enterprise. Most of its production equipment uses compressed air, previously supplied by individual small air compressors. This supply method suffered from unstable pressure, high failure rate, and high maintenance costs. In 2002, a technical upgrade was carried out, establishing an air compressor station and installing two new Wuxi Atlas Copco compressors GA132-8.5 screw air compressors. However, high power consumption was discovered during actual operation. After investigation and demonstration, a frequency converter upgrade was implemented in February 2005, selecting the Comron sensorless vector frequency converter KV2000-G1320-4T. After more than a year of actual operation, it has been working normally, with stable pressure, and significant energy savings, achieving good economic and social benefits. 2. Operating Status of the Air Compressors Before Modification Before the modification, the two air compressors operated in a fixed-frequency mode, one in use and one on standby. The starting method was a star-delta reduced-pressure start, and pressure control used a two-point control system (upper and lower limit control). When the pressure in the compressor cylinder reached the set upper limit, the compressor closed the intake valve using its own hydraulic pressure; when the pressure dropped to the set lower limit, the compressor opened the intake valve. Due to fluctuations in air consumption during production, the air compressors frequently loaded and unloaded, often for 2 minutes followed by 4 minutes of unloading. During periods of low air consumption, they sometimes stopped for 10 minutes before restarting, putting significant stress on the air compressors, motors, and the power grid. Furthermore, the air compressors did not produce compressed air during no-load operation, leaving the motor idle and wasting electrical energy. Therefore, frequency conversion modification was essential. 3. Air Compressor Variable Frequency Drive Retrofit Implementation Plan System Parameters: Air Compressor Model: GA132-8 Main Motor Model: YXLA315M-4 Rated Power: 132 kW Rated Voltage: 380V Rated Current: 250A 1 L: 485 r/min Remote Pressure Gauge: VTP10-A-G10-B Variable Frequency Drive: Comron sensorless vector inverter KV2000 G1320-4T Rated Current: 253A Based on the actual operating conditions of the air compressors, we use one variable frequency drive to control two air compressors. The variable frequency operation of the two air compressors is switched via electrical control. This way, when one air compressor fails, the system quickly switches to the other, without affecting production and saving on equipment investment. During the system retrofit, the original power frequency system is retained, and a variable frequency system is added, achieving power frequency/variable frequency interlocking switching. When the variable frequency drive fails, the original power frequency system is used for operation. This system adopts a closed-loop pressure regulation method. The air pressure setpoint is set via the inverter's control panel, and a pressure sensor is installed on the air tank. The air pressure signal is converted into a 0-5V electrical signal, which is fed back to the PID controller inside the inverter. The controller compares the signal with the pressure setpoint, performs calculations, and then outputs a control signal. The inverter then adjusts the motor speed according to this signal's output frequency, keeping the air pressure stable. This ensures the air compressor always operates in an energy-saving state. 4. Issues to be noted during system modification 4.1 Motor heat dissipation: Since the motor relies on its built-in fan for heat dissipation, operating below the power frequency will affect the heat dissipation effect. Therefore, after extensive observation, we finally set the lower limit of the inverter's frequency to 25Hz. 4.2 Air pressure: When setting the system pressure, under the condition of meeting production process requirements, the pressure should not be set too high, as this will result in a more significant energy-saving effect. After extensive experimentation, we set the air pressure to 0.6MPa. 5. Energy Saving Effect and Economic Benefit Analysis Before the modification, the air compressor's operating current was 240A for 2 minutes under load, and 110A for 4 minutes under unload. After the modification, the air compressor's frequency is basically between 30 and 45Hz, with an average operating current of 120A. There is no unloading time, and it basically does not stop. Power consumption analysis: 24 hours a day, 30 days a month. Before the modification, W = 1.732•I•U•24x30/1000 = 72661 kW•h. After the modification, W = 1.732•I•U•24x30/1000 = 56865 kW•h. From the above, it can be calculated that the monthly power saving after the modification is W = 15796 kW•h. Calculated at 0.58 yuan per kilowatt-hour, the monthly power saving is 9161.68 yuan. The total system modification cost is about 90,000 yuan, and the investment can be recovered in 10 months. 6. Several major advantages after the modification: 1) Energy saving and consumption reduction. 2) The motor starts at low speed, resulting in a small inrush current and smooth operation of both the motor and air compressor. 3) The inverter's robust protection features reduce equipment failure rates. 4) System pressure is more stable, improving product quality.