1. Introduction AC asynchronous motors have been widely used in industrial and agricultural production due to their simple structure, robustness, good performance, and low manufacturing cost. With the rapid development of variable frequency speed control technology, AC speed control technology has made significant progress, making the application prospects of AC asynchronous motors even broader, and they are poised to replace DC drive systems. This paper introduces a design method for accurately stopping a large-inertia AC drive system using the DC braking function of a frequency converter. Compared with the general method, this scheme is not only stable and reliable in operation and has high control accuracy, but also eliminates the need for a dedicated braking unit/braking resistor for the frequency converter, which costs tens of thousands of yuan, effectively reducing equipment modification costs. To date, several converter systems modified according to this model have been operating stably for several years, providing a mature technical modification scheme for traditional AC drive systems with the main objectives of energy saving, consumption reduction, and improved automation. 2. Composition of the Original System and Main Problems Steelmaking converters are an important production equipment in steel plants. The production process places high demands on the stability and stopping positioning accuracy of the drive system controlling the converter's rotation. A converter in a steel plant was originally driven by a 30kW AC asynchronous motor, with a gear reducer for speed reduction and a brake for accurate stopping control. During operation, the system ran at low speeds, resulting in significant energy loss. Furthermore, the furnace had considerable inertia, and the harsh working conditions meant that the gearbox gears were frequently damaged during start-up and shutdown, causing system failure. If not addressed promptly, the molten steel would solidify in the furnace, causing a major "furnace malfunction" and resulting in substantial economic losses. Due to limitations, the mechanical reduction system was difficult to maintain, and the stopping positioning control accuracy was low. These problems had become a bottleneck hindering further production development, and the manufacturer urgently needed solutions. 3. Discussion of Key Technical Issues in the Modification Plan To address the main problems in the system, we propose using a frequency converter to modify the existing AC drive system. Because the system operates at low speed, and there is a cubic relationship between motor speed and energy consumption (i.e., when the motor speed drops to n% of the rated speed, the energy consumed by the motor is PN * (n%)³ of the original rated power PN; for example, when the speed drops to 80% of the rated speed, the power consumption is only 51.2% of the rated power), replacing the original mechanical speed control system with a frequency converter not only greatly improves the stability of system operation, but also achieves significant energy-saving and consumption-reducing benefits; at the same time, it eliminates the large amount of maintenance work caused by the mechanical reduction system. The key technical problem to be solved is how to ensure and improve the parking positioning accuracy of the entire system. Generally speaking, the braking methods provided by general-purpose frequency converters mainly include: energy consumption (DC) braking, regenerative braking (braking unit/braking resistor, rectifier feedback), etc. The applicable scope, occasions, and limitations of these schemes differ, and their effects also vary. Selecting an economical and effective braking method and function based on actual conditions is one of the key issues in successfully designing a variable frequency speed control system. When modifying the first converter, considering the system's large inertia and the required high braking torque, we chose an external braking resistor and braking unit, recommended by the frequency converter manufacturer and successfully tested at a sister steel plant, combined with DC braking. The overall control effect was very satisfactory. In further discussion, we considered the following: the converter drive system has slow operating speed and long start-stop intervals; could we omit the external braking unit/braking resistor and simply use DC braking? Since the frequency converter manufacturer lists the external braking resistor and braking unit as optional components, requiring separate purchase and incurring high prices, if this solution is feasible, it will effectively reduce the overall system modification cost by nearly 20%, which is considerable in the long run. Therefore, we conducted a detailed analysis of the system's operating characteristics. "DC braking" generally refers to the process where, when the frequency converter's output frequency approaches zero and the motor speed decreases to a certain value, the frequency converter redirects DC current into the stator windings of the asynchronous motor, forming a stationary magnetic field. At this time, the motor is in an energy-consumption braking state, and the rotating rotor cuts this stationary magnetic field, generating braking torque and causing the motor to stop quickly. Since the kinetic energy stored in the rotating system is converted into electrical energy and consumed as heat in the rotor circuit of the asynchronous motor, a braking unit/braking resistor needs to be connected in series to prevent the motor from overheating during regenerative braking and DC braking. The converter drive system has its own characteristics: firstly, the inverter's output frequency is generally around 35-38Hz during operation; secondly, the converter system does not frequently start and stop. Figure 1 shows the mechanical characteristic curves of a typical AC motor during braking. In Figure 1: ① is the curve during normal operation, and ② is the mechanical characteristic during DC braking. Let point A be the normal operating point. During the braking process typically set in variable frequency speed control, the motor decelerates first. At this time, the synchronous magnetic field speed is lower than the rotor speed. The operating point jumps from point A on curve ① to point B on curve ② at the same speed, that is, from the first quadrant to the second quadrant. This is usually called the characteristic jump at the same speed. The motor then receives a braking torque Tb in the opposite direction and enters the regenerative braking state. The drive system rapidly decelerates along curve ② in Figure 1. When it falls below a certain speed, the inverter outputs DC, forming a fixed magnetic field and generating braking torque. During this process, the motor will eventually stop after a brief period of regenerative braking and energy-consuming braking. Therefore, a braking unit/braking resistor needs to be connected to prevent the motor from overheating. [align=center] Figure 1: Mechanical Characteristics of DC Braking Figure 2: Setting of DC Braking[/align] Theoretically, if the speed of the synchronous magnetic field of the motor can be controlled to decrease slowly, when the motor's characteristic curve jumps at the same speed, it will remain in the first quadrant, as shown by the dashed line group ③ in Figure 1, gradually reducing speed without jumping to the second quadrant. In this way, the drive system can effectively avoid the regenerative braking process during the deceleration process. Next, when the motor speed is less than the critical speed nk, DC braking is connected, and the magnitude and time of DC braking are controlled accordingly. Theoretically, the motor will only experience a limited energy-consuming braking stage and will not overheat. The good internal and external characteristics of the frequency converter ensure that the above conditions are met. Figure 2 shows the output frequency of the frequency converter and the law of motor speed changing with time during DC braking. Under the control of the operating signal, the frequency converter first slowly and continuously reduces the frequency until it reaches fdb, and then starts DC braking. At this time, the output frequency is zero. In the system parameter settings, the system deceleration time tz, DC braking start frequency fdb, braking current Idb, and braking time tdb are crucial, directly affecting the accurate positioning of the production machinery and the normal operation of the motor. We have conducted experiments with frequency converters from different manufacturers and models, such as ABB, Siemens, and Sanken, and all of them met the operating requirements. Taking the Siemens 6SE21 series frequency converter used in the converter system as an example, we will now explain the parameter settings in detail: P372=1: Enable DC braking function. P373 (Idb): DC braking current setting, which directly affects the braking torque; the greater the system inertia, the larger the value should be. The selectable range is 20%-400% of the motor's rated current; our empirical value is around 60%, which should be repeatedly adjusted. P374 (tdb): Input DC time. It should not be too long, otherwise the motor will overheat; however, it should be slightly longer than the actual shutdown time, otherwise the motor will enter a free-slip state. In the converter system, the empirical value is around 5.5 seconds, which should be repeatedly adjusted according to the actual situation. Selectable range: 0.1-99.9S. P375 (fdb): DC braking start frequency. As mentioned earlier, this parameter should be as small as possible, and must be below the critical speed nk in curve ② of Figure 1; otherwise, the motor will overheat. The empirical value is around 10Hz. Improper selection of P373, P374, and P375 will all cause motor overheating, requiring repeated adjustments and tests on-site. With proper adjustment, the production machinery will accurately stop at the predetermined position. Special attention should be paid to the time tz for the inverter output frequency to drop from fx during normal operation to fdb, although it is not set in the DC braking parameter group. However, its setting is crucial. If the time is too short, the motor will run to the second quadrant, resulting in regenerative braking and causing motor overheating. In addition, it must be pointed out that this method is not suitable for operating conditions requiring frequent braking. After setting the parameters according to this scheme, the system was continuously and repeatedly tested. The motor temperature rise was normal, and the system stop positioning was accurate. It can be seen that in the frequency conversion transformation of large inertia drive systems such as converter drive systems, the use of DC braking alone is entirely feasible. 4. Conclusion In summary, it is entirely feasible to retrofit large and medium-power AC motor drive systems, such as those used in steelmaking converters, that require speed regulation and accurate stopping positioning. Especially in certain situations, the inherent capabilities of the frequency converter can be fully utilized, effectively reducing retrofit costs and improving economic efficiency. This solution has been implemented in the retrofitting of four 18.5–55 kW AC motor converter drive systems, saving nearly 60,000 yuan in retrofit costs by eliminating the need to purchase braking units alone. In recent years of operation, there have been no shutdowns due to drive system failures, generating significant economic benefits for the company. Therefore, this design scheme is a superior option with a high performance-to-price ratio.