The polyester staple fiber unit is a major unit in the western area of ​​the polyester division of Shanghai Petrochemical Corporation, with six production lines and a designed single-line output of 15,000 tons/year. However, due to its inability to meet market demands, a new 30,000-ton/year polyester staple fiber production line (referred to as the new production line) was introduced and upgraded at the original No. 1 line location. The electrical drive uses German Siemens 6SE70 series frequency converters and permanent magnet synchronous motors (for front spinning) and asynchronous motors (for back spinning). This article discusses the application of a shared DC bus multi-inverter speed control system on the spinning line. 1 Basic Requirements of the Electrical Control System for the Polyester Staple Fiber Spinning Unit and the Shortcomings of the Existing Drive System 1.1 Basic Requirements of the Electrical Control System for the Polyester Staple Fiber Spinning Unit The requirements of the spinning machine for electrical drive can be summarized as "four highs" and "one low". The four highs are: high synchronization (the motor speeds at different spinning positions on a spinning machine must be consistent laterally and proportionally synchronized longitudinally); high precision (stable speed with an accuracy of 0.1% to 0.01%); high or very high speed (motor speeds can reach 8000 to 9000 r/min without a speed-boosting gearbox); and high reliability (guaranteeing at least 8000 hours of safe and continuous operation per year). The one low is: minimal or no maintenance, requiring no supervision. While a high-precision variable frequency drive and permanent magnet synchronous motor speed control system can achieve high synchronization, high precision, high speed, and minimal maintenance, high reliability is still not achievable, affecting the stable, long-term, and high-quality production of the spinning unit. Taking a 30,000-ton/year staple fiber production line as an example, its daily output is 100 tons of staple fiber. If the external power grid experiences a momentary low voltage (or momentary power failure), causing the metering pump frequency converter to stop and the motor to stop, it will increase the polyester melt pressure, forcing the polyester unit's melt booster pump to stop, thus affecting the normal production of the polyester unit. 1.2 Disadvantages of the original electric drive system The original 15,000 tons/year short filament direct spinning unit was driven by one frequency converter to drive multiple permanent magnet synchronous motors. The frequency converter still has the following shortcomings in operation: (1) Since multiple motors in the short filament unit share one frequency converter, soft start cannot be achieved. Therefore, when selecting the frequency converter, it is necessary to consider both direct start at the highest frequency and the fact that when several motors are running at high speed, a certain spinning position will continue to be put into operation after the fault is cleared. Therefore, the capacity of the frequency converter must be too large. (2) The failure rate of the spinning machine is too high. It cannot withstand the instantaneous low voltage of the power grid (including instantaneous power loss). Due to lightning, cable grounding faults and switch switching operations, instantaneous low voltage will definitely occur, causing the frequency converter to stop working, resulting in the entire spinning machine to stop production and causing huge losses. 2 Analysis of the characteristics of the frequency conversion speed regulation system of the front spinning unit (powered by UPS, small inverter permanent magnet synchronous motor open-loop synchronous drive system) The frequency conversion speed regulation system of the front spinning part of the new production line is shown in Figure 1. [align=center]Figure 1. Variable Frequency Speed ​​Control System Diagram of the Pre-spinning Section[/align] The main improvement of the pre-spinning unit is the adoption of a UPS (Siemens System 4233, 330kVA) power supply. Output: 380V, 480A; 450Ah battery. Under normal circumstances, it is powered by the mains. If the mains experiences a momentary power failure or low voltage, the electronic switch automatically switches to battery power to ensure that the inverter is not affected. Compared with the original production line, although the initial investment is larger, it ensures that the metering pump can work normally during momentary low voltage (including momentary power failure), improving economic efficiency. 3. Analysis of the Characteristics of the Variable Frequency Speed ​​Control System of the Post-processing Unit The variable frequency speed control system of the post-spinning unit is shown in Figure 2. [align=center]Figure 2. Variable Frequency Speed ​​Control System Diagram of the Post-spinning Unit[/align] The driving of the drafting, tension heat setting, yarn stacking, and crimping processes in the post-processing unit adopts a shared DC multi-inverter variable frequency speed control system, with the inverters connected to the same DC bus. The motors are high-power asynchronous motors. The shared DC bus is powered by rectifiers #1 and #2. The superposition of the two rectifiers can both increase the capacity and reduce ripple and harmonics, and stabilize the DC voltage. Compared with the original production line, it has the following advantages: (1) The shared DC bus can adaptively adjust the braking torque of the driven motor under different draw ratios. For example, for a certain set draw ratio, the speeds of the first, second and third draw machines are n1, n2 and n3, respectively. Due to the tension of the wire, when there is no braking function, the first draw roller will be dragged by the subsequent draw roller. Now, with the frequency conversion speed regulation of the shared DC bus, once the value of n1 exceeds the set value, the motor enters the regenerative braking state. On the one hand, the driven motor becomes a generator, and the generated electrical energy is rectified by the freewheeling diode and fed back to the DC bus. The motor not only does not need to absorb energy from the grid, but can also supply braking energy to other inverters. This can stabilize the DC bus voltage. Also, since the motor capacity is large (e.g., the second draw machine motor is 400kw), the energy saving is also considerable. On the other hand, the driven motor is in a braking state. As long as the corresponding frequency ratio is set, the speed ratio can be controlled, ensuring the accuracy of the draw ratio control. (2) The polyester staple fiber post-processing drawing and heat setting combined unit is a key process in the production of polyester staple fiber. It is mainly responsible for stretching and setting the raw yarn at a certain draw ratio. The new production line adopts AC variable frequency speed regulation. Each drawing roller has an independent variable frequency drive. The draw ratio required by the process can be easily adjusted by simply changing the frequency of each inverter. The stacking machine and the crimping machine also adopt a shared DC bus multi-inverter speed regulation scheme, which will not be discussed further. The cutting machine is an independent inverter, and the principle is the same as that of general variable frequency speed regulation, which will not be elaborated here. 4 Conclusion (1) As mentioned above, the shared DC bus frequency conversion speed regulation technology is reliable. Although the initial investment is high, it can reduce the number of shutdowns by 2 to 3 times per year. Based on a production line with a capacity of 30,000 tons/year, it can reduce PET discharge by 8 to 12 tons. At the same time, it can also avoid fiber quality fluctuations caused by shutdowns (a single shutdown will affect the quality stability of 144 to 216 tons of fiber). According to this calculation, the cost of the renovation can be recovered in just a few years. (2) Due to the adoption of the shared DC bus frequency conversion speed regulation technology, the overall production conditions are in a stable state, which greatly facilitates the change of product specifications and the adjustment of process parameters. The transition time is short, the waste yarn is less, and the process adjustment is precise. (3) From the actual operation of the new production line, the shared DC multi-inverter speed regulation system has outstanding advantages in the production of polyester staple fiber and represents the development direction of spinning machine drive. However, the impact of power grid failure on the frequency converter cannot be completely eliminated in the post-spinning section. For example, once the frequency converter stops working, a section of polyester yarn (about 100m) that is being drawn will be scrapped. In addition, the installed capacity of the pre-spinning winding spinning machine is 196kw, and the output capacity of the UPS is 330kw, so the actual capacity used is relatively small. References [1] Liu Liangxi. Variable frequency speed regulation system of chemical fiber spinning machine [2] Zhang Renfei. Analysis of the current situation and transformation plan of variable frequency speed regulation of short fiber pre-spinning in polyester No. 2 plant. Jinshan Oil Chemical Fiber [3] Fan Shiqiang. Application of system/UPS in 30,000 tons/year short fiber unit. 2002 Technical paper Author Introduction Li Yiran (1964—), Shanghai Petroleum Industry School, senior lecturer. Graduated from Shanghai University in 1985 with a bachelor’s degree in automation. He has long been engaged in teaching professional courses such as power electronics converter technology and electric drive.