The 1980s witnessed rapid development in my country's textile and chemical fiber industries, with many enterprises importing large quantities of textile and chemical fiber equipment from developed countries such as Germany, Italy, and Japan. This period also coincided with the early stages of general-purpose AC variable frequency speed control technology. Variable frequency drives (VFDs) using silicon controlled rectifiers (SCRs) as switching elements were gradually being adopted, while VFDs using high-power transistors (GTRs) were just emerging from the laboratory and not yet widely used. Therefore, the electrical speed control systems of the textile and chemical fiber equipment imported into my country at that time primarily used SCR VFDs. I. Overview We know that SCRs, as power electronic devices, are still widely used in DC speed control systems today due to their high voltage and current withstand characteristics and simple and reliable conduction control. However, the turn-off of a SCR requires a high reverse voltage, resulting in numerous additional devices and a large size in VFDs. Furthermore, their switching frequency is inevitably limited, with carrier frequencies typically only a few hundred Hz, and even lower for high-power VFDs, leading to poor output waveforms that affect motor operation. Given the limitations of electronic technology at the time, the control circuits of frequency converters could only utilize a large number of discrete components. Various regulators and signal generators were analog circuits, resulting in low control accuracy, poor stability, high wet drift, and low reliability. As the equipment aged, frequency converter failures increased year by year, leading to a rise in the number of burnt-out motors. The high-speed motors used in chemical fiber equipment were specialized motors, with repair costs reaching 20,000 RMB per unit, and imported control boards costing tens of thousands of RMB. Clearly, the operating costs of existing equipment were very high. With the development of modern microelectronics and power electronics technology, fully digital, high-switching-frequency, vector-control-featured frequency converters have been widely introduced to the market, significantly reducing their cost. Upgrading the original thyristor-controlled frequency converter system with advanced frequency converters is an efficient and economical option. In 1998, we upgraded two German Barmag POY and FDY high-speed spinning machines using the then-advanced Japanese Sanken LF and IF series frequency converters, achieving excellent results. II. Characteristics and Current Status of the Original Speed ​​Control System Changsha Nylon Factory has three production lines in its POY and FDY spinning lines, all manufactured by Barmag GmbH of Germany. Their electrical speed control system uses Siemens SIMOVET-V frequency converters for centralized control. Each production line's drive is handled by four frequency converters, with two additional frequency converters driving the metering pump motor and the oil pump motor. Each production line has 16 positions (winding machines), each containing one friction roller motor (2.0KW) and one grooved roller motor (0.9KW). The 16 friction roller motors are driven by one "running" frequency converter, operating at a frequency of 25-212HZ. The 16 grooved roller motors are driven by another "running" frequency converter, operating at a frequency of 50-325HZ. Each winding machine starts independently. Due to the large mechanical inertia of the friction roller and grooved roller, each requires a separate "starting" frequency converter. When the output frequency of the "starting" frequency converter equals the output frequency of the "running" frequency converter, the two motors of the winding machine switch to the "running" frequency converter for drive. III. Proposed Modification Scheme There are generally two schemes for the frequency conversion speed regulation technology modification of high-speed spinning machines. One is to change from "centralized" speed regulation to "distributed" control. That is, each position (winding machine) uses two frequency converters to drive the friction roller and grooved roller motors respectively. In this way, each position can be controlled independently, without any electrical control connection between them. This avoids the situation where a failure of any of the original four frequency converters causes a complete line shutdown. It also facilitates the development and research of new products and the production of small batches of multiple varieties, and can also replace chemical fiber testing machines. The spinning machines newly released by the German company Barmag in the 1990s adopted this method. Some domestic manufacturers have also used this method for frequency converter retrofitting, but this approach involves a huge investment. The entire machine requires 32 frequency converters, and with the metering pump motor and oil pump motor, a total of 64 are needed. It is reported that a filament mill in northern China invested over one million yuan in frequency converters alone for this solution. Considering that each position does not necessarily operate at completely different speeds, and that it is impossible to simultaneously trial-produce 16 varieties, we adopted an alternative solution based on the principles of "high efficiency and economy." Its characteristics are as follows: The original system is retained, and a switching circuit is added, allowing the newly added Sanken high-performance frequency converter to control the original 16 positions, or 7, 8, or 1 position. This allows one position to be used for trial production of new products, or the Sanken frequency converter to control 7 or 8 positions for small-batch production, while the other 9 or 8 positions use the original SCR frequency converter to produce another variety, achieving two varieties on one machine to meet the requirements of the sales department and users. The electrical drive circuit of each winding machine unit after the retrofit is shown in Figure 2. When the output frequency of the starting inverter equals the output frequency of the original running inverter, switching the motor to the original running inverter via K22 and K24 allows the winding machine to produce the original product; switching the motor to the new running inverter via KXM and KXC allows the winding machine to produce the new product. The product structure can be adjusted through different combinations of these two sets of contactors. IV. System Debugging For the debugging of this technical upgrade, besides ensuring correct wiring of the two motors and their contactors in each unit winding machine, the most critical aspects are setting the swing frequency of the winding machine and switching between the starting inverter and the new running inverter. Sanken LF and IF series inverters from Japan contain a disturbance operation mode specifically designed for high-speed chemical fiber spinning machines, which can easily complete the swing frequency setting of the winding machine. Due to the large mechanical inertia of the friction rollers and grooved rollers, switching when the output frequency of the starting inverter equals the output frequency of the running inverter will not generate electrical shock. If the output frequency of the newly added running inverter in the upgraded system differs from the original running inverter, and is higher, a large current will be generated during switching. Switching at a lower voltage level will cause overvoltage (the motor is in generator mode during switching). Therefore, the capacity of the newly added operating frequency converter should have a certain margin, and a braking unit and braking resistor should be provided. When the braking torque TB > 0.2Te, that is, when the braking torque is less than 20% of the rated torque, there is no need to set a braking resistor, that is, there is no need to consider a braking scheme. The voltage of the intermediate DC circuit can be limited below the overvoltage protection action value by the active power loss inside the motor. Since only one unit winding machine is engaged when the motor switches from the starting frequency converter to the operating frequency converter at any time, the resulting inrush current will not be large. After computer analysis, 37KW and 22KW frequency converters were selected and equipped with two sets of BU-430 braking units and DB-430 braking resistors. No overcurrent or overvoltage tripping occurred during actual operation. V. Conclusion 1. This technical improvement overcomes the drawbacks of the original centralized control of the equipment. After the improvement, 16 positions can be controlled in groups, and they can be switched between each other when necessary without interference, which is convenient for trial production of new products and small-batch production of various varieties. 2. High reliability. If the inverter in the original speed control system fails, all 16 bits can be switched to the new speed control system, and the switching process can be completed in just a few minutes. 3. The new speed control system uses the LF and IF series inverters, new products from the late 1990s by Sanken Corporation of Japan. These use quasi-32-bit and 32-bit CPUs respectively, with a frequency setting resolution of 0.01Hz, 20 times more accurate than the original inverters. The switching modules use IGBT and IPM modules respectively, with a maximum carrier frequency of 14kHz, quiet operation, low operating current, and comprehensive protection functions. The LF and IF inverters have a disturbance operation mode, specifically designed for high-speed chemical fiber spinning. Process parameters for chemical fiber spinning can be set digitally directly on the inverter. The LF series inverters can also read or modify disturbance parameters (swing frequency parameters) directly from the panel display during operation and display the actual operating speed, which is highly welcomed by process engineers. 4. Low investment. 5. Short renovation period, convenient installation and commissioning; commissioning and operation can be completed in just one hour, with a small footprint. 6. The investment yields results within the same month. The upgraded production line produces new products that were previously impossible to manufacture, with market prices approximately 2000 yuan/ton higher than the old products. This profit alone allows the investment to be recovered within three months. 7. Energy saving. The motor operating current is approximately 80% of the original system's operating current. 8. Since the equipment was put into operation, no faults have been found in the new frequency converter, and not a single motor has burned out. Based on statistics from previous years, annual maintenance costs are saved by approximately 150,000 yuan. It is evident that the "grouping" frequency conversion speed regulation technology upgrade of high-speed spinning machines is an efficient and economical solution.