Abstract This paper explores the widespread application of variable frequency speed control and its technology in the sizing system, sectional drive, winding system, and auxiliary facilities of paper machine production lines, and elaborates on the specific control requirements and implementation strategies. 1 Introduction A paper machine is a combined machine composed of a series of supporting equipment, divided into two main parts: the wet end and the dry end. The wet end includes the sizing feed system, wire section, and press, while the dry end includes the drying, calendering, and winding sections. The general production process is as follows: the pulp is conveyed to the headbox at the front end of the paper machine production line through the sizing feed system, and then the pulp flows sequentially through the wire section, press, front dryer, rear press, rear dryer, calender, and winding machine, becoming the base paper; the base paper can then be further processed into the coating and rewinding machines to produce finished paper. Papermaking is a continuous production process; therefore, the continuous and orderly control of the production line has become a bottleneck restricting the quality and quantity of finished paper. Variable frequency speed control (VFD), as the most powerful control method since the mid-1990s, has entered the papermaking industry, which was originally dominated by DC speed control (suitable for large and medium-sized paper machines) and slip-ring motors (suitable for small and medium-sized paper machines), and has achieved good market results. AC VFD technology, with its superior speed control performance, significant energy savings, and wide applicability in the national economy, is widely recognized as the most promising AC speed control method. DC speed control systems occupy an important position in the history of paper machine development, but due to the difficulties in maintenance and poor environmental resistance of DC motors, by the 1990s, the performance-price ratio of papermaking machines had been severely limited. VFD technology can maximize the inherent advantages of AC motors (simple structure, robustness, durability, economy, reliability, good dynamic response, etc.), and with VFD theory having formed a relatively independent discipline, the era of comprehensive application of VFD technology in paper machine production lines has arrived. This article further elaborates on the application of VFD and its control technology in paper machine production lines. 2. Variable Frequency Speed ​​Control of the Sizing System of a Paper Machine The sizing system of a paper machine is responsible for transporting pulp to the headbox of the paper machine through equipment such as pulp pumps, flushing pumps, and multi-stage screens. Among them, the pulp pumps and flushing pumps are the most important equipment because they are related to the quality of the formed paper, especially the basis weight. Therefore, variable frequency control must be used to meet the design requirements of the paper machine. Practice has proven that installing a variable frequency device can adapt to different machine speeds and different types of pulp usage, so that the pulp pump group always operates in a high-efficiency state. In addition, when using variable frequency control, the speed of the motor and the pulp pump decreases, the wear of mechanical parts such as bearings is reduced, the pump end sealing system is less likely to be damaged, the failure rate of the machine and pump is greatly reduced, and the maintenance workload is also reduced. The sizing process must meet the following requirements: (1) The amount of pulp delivered to the wire of the paper machine must be stable, with an error not exceeding ±5%; (2) The pulp ratio and concentration must be stable and uniform; (3) The pulp must be purified and selected. Therefore, speed control of the pulp pump and the headbox is crucial to ensuring the achievement of items (1) and (2). The following uses the headbox as an example to illustrate the speed control process: The frequency converter adopts a dual closed-loop speed control method, with the outer loop being a speed closed loop (V-control) and the inner loop being a current or torque closed loop (I-control). The set value of the headbox speed is obtained from the change in the pulp speed and wire speed ratio (V-set) on the one hand, and from the pressure controller (P-control) of the headbox on the other hand. The former is the main adjustment, and the latter is the fine adjustment. The pulp speed and wire speed ratio of the paper machine are basically constant, so when the wire speed of the paper machine changes, the V-set also changes, and the speed of the headbox also changes accordingly; in order to improve the accuracy of the speed regulator and reflect the actual process of the headbox, it is usually necessary to take ±5% of the change in the pressure PID control output value of the headbox as the additional speed set value of the headbox. The actual speed value (V actual) is taken from the actual speed sampling of the drive motor, which can be obtained by detection devices such as rotary tachometer motors or photoelectric rotary encoders. The current setpoint (M_set) is taken from the output signal of the speed loop, and the actual current value (M_actual) is taken from the measured value of the current transformer at the output terminal of the AC frequency converter at each drive point. Therefore, for the variable frequency speed regulation of the slurry pump, PID control is required, and the speed feedback method and various parameters of the PID must be correctly selected. The acceleration time of 0-100% is generally 60 seconds, while the deceleration time is about 30 seconds. The frequency converter's setting input value must have two or more input terminals and be able to perform basic calculations. The feedback signal must have a port for receiving analog or pulse signals. Understanding this is very important for selecting the model of frequency converter. 3 Variable Frequency Control of Paper Machine Division Drives In the past, China's paper machine division drive equipment used SCR DC speed regulation. Due to the existence of slip rings and carbon brushes, the reliability and accuracy were not high, resulting in the mechanical backwardness of the paper machine, and the machine speed was only about 200m/min, which was difficult to compare with the high-speed paper machines of 1000m/min abroad. Therefore, the trend towards frequency conversion in the sectional drive mechanisms of papermaking machines is inevitable. Sectional drives involve numerous drive points in the paper machine, including the wire section, press section, front dryer, rear press, rear dryer, and calender. Due to the thinness and fragility of paper, high-precision speed control of each drive point is necessary to prevent breakage, curling, wrinkling, and dents, ensuring the paper extends according to the stretch rate defined in the paper forming direction. In other words, a certain speed cascade must be maintained throughout the entire process from sizing to winding to maintain tension. The frequency converter based on the sectional drive emphasizes the online stepless speed regulation and synchronous following performance of each transmission point. Therefore, the frequency converter used in the sectional drive must have the following characteristics: (1) wide speed range, and the efficiency must be above 90% in the full speed range; (2) power factor above 0.9; (3) total distortion of input harmonic current less than 3%; (4) use of high reliability and mature standard device IGBT; (5) can reduce output harmonic components and effectively reduce dv/dt noise and torque pulsation; (6) use communication function to realize high-speed serial data transmission. For example, the power of the four sectional drive transmission points of a 1092 type cork paper making machine in a paper mill is 11kW for pressing, 11kW for main cylinder, 7.5kW for group cylinder, and 5.5kW for winding paper. According to the actual situation and requirements of the user, the frequency converter adopts the newly launched MM420 series of Siemens, and the PLC adopts the Siemens S7-200 series. The network communication adopts RS485 interface, and the protocol adopts Siemens USS protocol. The main unit speed regulation and individual transmission point fine-tuning are achieved using button speed control, and speed adjustment can be performed from both the control panel and the machine itself. Another example is a 2820 kraft linerboard papermaking machine in a certain factory. The frequency converter used is an AB 1336 high-performance frequency converter, the PLC is an AB SLC500 series, and the operating interface is a touch screen. The host computer controls the storage of operating parameters, automatic generation of data tables, automatic printing of production reports, and fault alarm status analysis. Network communication uses the internationally recognized PROFIBUS DP communication network. The motor is an AC variable frequency motor, and flux vector closed-loop control is applied. Furthermore, the load distribution of each transmission point can be controlled by calculating the actual power used at each transmission point, and local tensioning and relaxation can be performed during production without affecting the speed chain through jogging. The above two examples are typical applications of the control principle of sectional frequency conversion control. The control principle of sectional drive is to maintain speed cascading and high-speed transmission. The former utilizes the host computer (PLC or industrial computer) to calculate speed cascading, while the latter is achieved through the high-speed serial communication capability of the frequency converter itself. This results in a simple wiring system, high automation, rich information content, and compatible interfaces, facilitating integrated FA and control. Speed ​​cascading creates a speed chain between the main drive points of the paper machine's various drives; adjusting the speed of one point rapidly adjusts the speed of every subsequent drive point. In actual paper machine control, specific drive control methods are often added based on specific locations, with load-ratio control being particularly important. Load-ratio control refers to the distribution of load between two meshing rolls to maintain tight and optimal contact. In the control of meshing rolls in the press section and calender of a paper machine, depending on process requirements, the meshing rolls are disengaged when the paper web breaks and engaged when the paper web passes through. During the engaged position, load-ratio control is used to maintain tight mechanical contact and achieve the required meshing. The main drive of the meshing rolls still uses speed cascading control, while the driven drive uses load-ratio control. In this load ratio control, the speed closed-loop setpoint is increased by 5% based on the main drive setpoint (V setting). This increases the output value of the speed loop, i.e., the set input value of the current loop (M setting). Then, the output value of the main drive speed loop, multiplied by the load ratio percentage, limits the set input value of the slave drive current loop (M setting), ensuring it closely follows the main drive and resulting in a tighter and more consistent meshing between the two rollers. 4. Tension Control of Unwinding and Rewinding Systems in Paper Machines, Rewinders, and Off-Machine Coating Machines Paper has two forming methods: sheet paper and roll paper. Due to the proliferation of high-speed printing presses, the demand for roll paper is increasing daily. Moreover, roll paper allows for higher paper machine speeds compared to sheet paper. However, the handling of roll paper by the paper machine is a relatively complex process, especially during center winding. As the roll diameter increases, the speed of the roll rollers must continuously decrease while maintaining relatively stable tension in the paper web. Therefore, tension control is the core technology and the difficulty of variable frequency speed regulation for winding and unwinding systems. For such a complex tension control system, variable frequency technology must overcome the following four technical problems: (1) simplifying the mathematical model of the complex AC asynchronous motor; (2) taking into account the delay and overshoot of the tension feedback signal; (3) describing the dynamic parameters of the winding tension control process as a time-varying function; (4) ensuring high anti-fluctuation capability and good real-time adjustability of the tension closed loop. At present, the two most commonly used control methods in winding are: (1) speed control winding (SPW), which uses PID to correct the speed setpoint through the tension feedback of the force sensor or the position feedback of the adjusting roller; (2) current control winding (CPW), which uses PID to adjust the tension setpoint and can be applied to the center winding method. This type of control is generally open-loop. A variable frequency control system with tension control is generally composed of modules such as diameter, compensation, torque and speed calculation. 5. Variable Frequency Control of Auxiliary Components in Paper Machines The auxiliary components of a paper machine typically include a white water system, vacuum system, compressed air system, chilled water system, chemical dosing system, waste paper recycling system, and ventilation system. In actual paper machine design, to ensure continuous and balanced operation, the auxiliary components generally exceed 15% to 35% of the maximum production capacity. Therefore, using variable frequency speed regulation and its control technology can effectively reduce auxiliary energy consumption during actual operation, such as fans and water pumps previously controlled by adjusting valve openings, and belt conveyors operating at fixed speeds. The following describes the wide range of applications of variable frequency control in several typical aspects. 5.1 Variable Frequency Speed ​​Regulation of Chilled Water In papermaking auxiliary equipment, coating and sizing often require chilled water to treat the coatings to maintain the stability of their chemical properties. Currently, a commonly used method is a variable frequency speed regulation system for chilled water pumps. Its control principle utilizes temperature or temperature difference control. Strictly speaking, the difference between the return water temperature and the outlet water temperature of the chiller indicates the heat removed by the chilled water from the paint tank, so the temperature difference can be used as the control basis. Furthermore, since the outlet water temperature of the chiller is generally quite stable, in practice, control only needs to be based on the return water temperature in the design. The variable frequency drive selected for the chilled water system should be designed as a PID controller, as most frequency converters have PID functionality. Therefore, the control loop is very simple, using the return water temperature as the feedback signal, while the target temperature can be set through the frequency converter's panel or by outputting an analog signal from the host computer. 5.2 Variable Frequency Speed ​​Control of Waste Paper Conveyor The waste paper conveyor is responsible for transporting broken or processed paper to the waste paper system. Since the belt speed is usually designed according to the maximum speed of the paper machine, and the paper machine typically operates at 50% to 80% of its maximum speed, constant belt speed operation wastes considerable energy and generates excessive noise. Variable frequency speed control (VFD) is used to make the conveyor speed follow the paper machine speed, and the starting frequency of the conveyor is set to 20-30Hz. This allows the conveyor to react quickly to avoid paper blockage and reduces energy consumption. 6 Conclusion With the development and maturity of variable frequency technology, VFDs, with their excellent speed regulation performance, simple operation, complete functions, and ease of automatic adjustment, will inevitably replace the traditional DC speed control system on paper machines and become the leader in paper machine electrical drives.