The main induced draft fan is one of the key pieces of equipment in sintering production. Changes in its air pressure and volume have a significant impact on sintering production; furthermore, its power consumption generally accounts for about one-third of the entire production line. Therefore, rationally controlling the operation of the main induced draft fan is of great importance for the rational control of sintering production and for significantly reducing power consumption. 1. The Importance of Applying Variable Frequency Speed ​​Regulation to Sintering Main Fans Sintering plants in China's steel enterprises widely experience changes in raw materials and operating conditions. To adapt to production, currently, the only solution is to change the damper opening of the main induced draft fan to meet the needs of the sintering process. This has become almost a uniform model in the past, but it has many drawbacks. First, it is impossible to dynamically track the process and adjust the air volume in real time to meet the optimal process requirements. Second, according to our investigation of the main induced draft fans in sintering plants such as Baosteel and Shougang, the damper opening is generally between 40% and 60%, resulting in significant energy loss and waste. Third, the fan is difficult to start, has a high failure rate during startup, and has a significant impact on the power grid. To overcome these drawbacks, we considered applying variable frequency speed regulation technology. Based on our factory's production experience, the airflow requirements during sintering involve a dynamic balance process; the more advanced the technical and economic indicators, the higher the balance requirements. Therefore, we conceived the idea of ​​retrofitting the main induced draft fan with variable frequency speed control. Utilizing advanced modern tracking methods, we aim to achieve large and small circulation of raw materials, coordinated operation of the three machines (fan, ignition temperature, and main sintering negative pressure), and integrated control of the ignition temperature and main sintering negative pressure into a large closed-loop system. The most important and challenging aspect of realizing these ideas is the speed control of the main induced draft fan. With the rapid development of high technology, the conditions are ripe for using high-voltage frequency converters to achieve the aforementioned continuous and precise control. Implementing variable frequency control of the main induced draft fan allows for reasonable, real-time, and dynamic adjustment of the airflow and negative pressure, ensuring relatively constant airflow and pressure, providing favorable conditions for homogeneous sintering production. Conversely, without proper adjustment methods, relying on manual adjustment of the sintering airflow is impossible to apply effectively. This could lead to a large amount of raw material, making it difficult to achieve satisfactory yield and process energy consumption. After the main induced draft fan adopted frequency conversion speed regulation, the air volume, which was originally controlled by the damper, is now controlled by the speed. From the relationship between output power P and fan speed p=KW³ (where K is the fan constant and W is the motor's angular frequency), it can be seen that when the air volume needs to be reduced, the speed decreases, and P decreases with the cube of the speed, overcoming the problem of large power loss when adjusting the damper. Furthermore, the motor starting has changed from full-voltage or reduced-voltage starting to frequency conversion starting, greatly improving starting performance and solving problems such as difficulty starting large motors, large impact, and easy damage. 2. Feasibility of Applying Variable Frequency Speed ​​Regulation to Sintering Fans Currently, the benefits of applying variable frequency speed regulation to many small-power fans have been proven. However, there is no precedent in China for applying variable frequency speed regulation to sintering main induced draft fans. Can variable frequency speed regulation be applied to sintering fans? While variable frequency speed regulation of large asynchronous motors is common, there is no precedent in China for applying it to large synchronous motors. How effective is variable frequency speed regulation for synchronous motors? How is the oil replenished after the coaxial main oil pump achieves fan speed regulation? These are all questions that need to be addressed before applying variable frequency speed regulation. 1) To verify the feasibility of applying variable frequency speed regulation to sintering fans, we conducted variable frequency speed regulation experiments in sintering cup tests and obtained a set of satisfactory data as shown in Table 1. The test data shows that before installing the frequency converter, the air volume remained basically unchanged from 0 to 90 degrees, while the air pressure varied from 3.24 to 20.77 kPa. Throughout the test, the current remained approximately constant at 3.4 A. After installing the frequency converter, with the damper fully open and a 600 mm material layer, the measured air volume increased from 9800 to 20000 m³/h, the negative pressure increased from 3.04 to 19.65 kPa, and the current increased from 0.6 to 2.6 A, demonstrating a significant energy-saving effect. In January 2001, with the assistance of Rockwell Automation and Guangxi Nanning Yide Development Co., Ltd. Gezhou Branch (Liuzhou Yide Anhao Technology Development Co., Ltd.), we officially applied the AB high-voltage frequency converter to a 2000 kW synchronous motor, further verifying the actual effect of frequency conversion speed regulation on the sintering main fan, as shown in Table 2. [align=center] [/align] The parameters in the table show that under normal production conditions... The negative pressure decreased by nearly 4 kPa, and power was saved by nearly 600 kW, which played an important role in production processes and energy conservation. Tests and practice have fully demonstrated that the application of variable frequency speed control to the sintering main fan is entirely feasible and highly successful. 2) Is the application of variable frequency speed control to large synchronous motors feasible? Through our practice, it has been fully affirmed. After applying variable frequency speed control to the synchronous machine, the original asynchronous starting method was abandoned. With the AB high-voltage variable frequency system, a synchronous starting method was adopted. As the motor torque and speed changed, the operating parameters were automatically adjusted, resulting in a very smooth process and minimal impact on the power grid. The AB high-voltage variable frequency converter can achieve optimal matching of various parameters such as the stator and rotor currents and power factor of the synchronous machine, thereby achieving the best operating effect. 3) Before direct start-up, the lubrication of the main induced draft fan system is handled by the auxiliary oil pump. After start-up, the main oil pump, coaxial with the synchronous machine, supplies oil. Thus, after the fan's frequency conversion speed regulation, the main oil pump may operate at a low speed. Therefore, the oil supply system must consider the replenishment of oil. When completing this task, Yide Company adopted a variable frequency PID control method for the auxiliary oil pump, ensuring normal oil supply to the lubrication system when the main fan is at low speed with minimal investment. This also played a crucial role in ensuring the implementation of the main fan's frequency conversion speed regulation. 4) Since the synchronous motor's cooling is self-circulating air cooling, the cooling effect decreases as the motor speed decreases. However, due to the characteristics of the fan, the motor load decreases cubically at low speeds, thus greatly reducing heat generation. Practice has shown that the motor temperature actually decreases when operating at low speeds. 3. Specific Scheme Selection and Design Block Diagram High-power, high-voltage frequency converters are not yet widely used in China. Therefore, such a large investment requires careful selection of a reasonable scheme. Medium and high voltage frequency converters can currently be divided into two types based on the main circuit structure: AC-DC-AC and AC-AC. AC-AC inverters, due to their control method, can only output frequencies ranging from 1/3 to 182 of the original frequency, thus failing to meet our requirements. AC-DC-AC inverters, on the other hand, are not limited by the power supply frequency, have a wide frequency adjustment range, fewer components, and higher utilization rates, meeting our needs. AC-DC-AC inverters can be further categorized into voltage-type and current-type inverters depending on the DC section. Voltage-type inverters output a square wave voltage, which is prone to overcurrent when a short circuit occurs or when a load is applied while the inverter is running. They also have a slow dynamic response and significant harmonic components. Current-type inverters, with their superior current control and ability to limit overcurrent caused by inverter commutation failure or load short circuits, offer high operational reliability. When the load is a motor, the voltage approximates a sine wave. They can also achieve regenerative braking, converting mechanical energy into electrical energy to feed back to the grid. They also feature a simple circuit structure and fast dynamic response. Through analysis and comparison, we selected the AB 1557 current-type high-voltage frequency converter. It can guarantee normal operation when the grid voltage varies within the range of 5.9–6.8 kV, with current harmonic distortion ≤3%, cos@≥0.98, and n≥97%. The converter can interface with a large PLC to achieve closed-loop control of the sintering process. The system block diagram is shown in Figure 1. Through actual operation, the system has the following characteristics: 1) The AB 1557 current-type high-voltage frequency converter, used in the sintering main fan speed control system, has a large speed range, balanced operation, and good control accuracy. 2) Low synchronous starting current, with no electrical or mechanical impact. 3) Automatic tracking of load and speed changes by the excitation current, optimizing the system operating point. 4) Strong automation communication capabilities, facilitating network control. 5) Convenient system debugging, reliable operation, low motor noise, and a reduction in heat generation of over 30%. 6) Significant energy-saving effect. 4. Operational Performance Analysis 1) The operation of the main fan after the inverter was put into operation showed a difference between variable frequency speed control and operation at mains frequency. Before the application of variable frequency speed control, the motor speed was 1500 r/min, and the damper could not be fully opened, otherwise the motor would be overloaded. During normal production, the damper opening is generally between 80% and 90%, so a significant portion of the electrical energy is consumed by the damper. The 2000kW motor frequently operates at full load. Simultaneously, due to the relatively high voltage of our power grid (generally 6.3–6.7 kV), the motor's operating magnetic flux is close to the saturation region, increasing excitation losses and raising the operating current. According to the operation records, after the inverter was put into operation, the motor only needs to operate at 42 Hz to meet production requirements. Typical production parameters are shown in Table 3. Under normal operating conditions, the annual electricity savings for the motor are 685.9 kW × 24 h × 360 d × 0.904 = 5,357,263 kW·h (m904 is a coefficient for other factors). At an electricity cost of 0.46 yuan/kW·h, this translates to annual savings of 2.46 million yuan. 2) Due to the ideal starting performance of the frequency converter system, the large motor can be started and stopped at will. Short-term production shutdowns can stop the fan, saving electricity and reducing mechanical and motor losses, resulting in annual savings of approximately 200,000 yuan. 3) From a process perspective, after frequency conversion speed regulation, the fan negative pressure drops from 14 kPa to around 10 kPa, perfectly meeting the low negative pressure requirements of sintering production. Simultaneously, with the dampers fully open, damper losses are reduced, and this energy, without replacing the motor, can relatively increase the airflow. Furthermore, since variable frequency speed control can be easily linked to process reference values ​​for adjustment, it creates more favorable conditions for homogeneous sintering technology and improved sinter production quality, resulting in significant benefits. 5. Conclusion Based on the application results of the 2000 kW synchronous motor of the main induced draft fan in the Gegang sintering plant using the American AB 1557 voltage variable frequency speed control system, the system design is reasonable, installation and commissioning are convenient, operation is highly reliable, and energy-saving effects are significant. It has a good promoting effect on the production process and is a successful example.