Variable frequency technology upgrade for papermaking machinery

1. Introduction The paper industry is a vital sector of the national economy. According to an industry report from the Economic Forecasting Department of the State Information Center, since March of this year, with adjustments in the prices of major resources such as water, electricity, and gas, the cost of paper manufacturing has increased again, further intensifying cost pressures on profit growth. Industry losses have increased, with losses of loss-making enterprises rising by 67.34% year-on-year. The cost increase is due to both rising prices of raw materials and energy sources such as water, electricity, and gas, and is also closely related to energy waste. China's overall energy consumption in the paper industry is twice that of developed countries, and its water consumption per ton of pulp is nearly 10 times higher than the world's advanced levels. Electricity is the primary energy source for the paper industry, and electricity costs are one of the three major costs, making high annual electricity bills a major burden for paper companies. For Chinese paper companies to increase competitiveness, achieve sustainable development, and win in fierce competition, they must reduce costs, and energy conservation is a key factor in cost reduction. Advances in energy-saving technologies, especially the development of frequency conversion technology, have provided ample opportunities for paper companies to improve energy efficiency, reduce electricity costs, and increase profits. 2 Necessity of Frequency Conversion Retrofit for Paper Machines In China, most paper machine transmission equipment uses SCR DC speed regulation (high power) and slip motor (low power) transmission. During production, speed matching is often mismatched due to factors such as mechanical wear and belt slippage, resulting in paper breakage and uneven thickness. (1) DC speed regulation system occupies an important position in the development history of paper machine. However, due to the difficulty in maintenance and poor environmental resistance of DC motor, the main problems are as follows: ● Severe wear of commutator, failure of commutator to burn out, resulting in long downtime; ● Difficult maintenance of DC motor, high requirements, and high repair costs; ● Easy wear of tachogenerator, resulting in low transmission system accuracy; ● DC speed regulation control system is complex and difficult to debug. It is difficult for general technicians to debug a good machine. (2) AC slip motor consists of two parts: a regular AC motor and a slip clutch. The output speed of the motor is adjusted by adjusting the voltage of the clutch excitation coil. It has the problems of high power consumption, low transmission efficiency, poor speed accuracy, and unstable speed, which affects the quality of the product. 3 Introduction to the Variable Frequency Control System of Papermaking Machinery The structure of papermaking machinery is generally composed of headbox, wire section, press section, drying section, calendering section, and winding section. The transmission system is a speed chain coordination system composed of multiple transmission points. In order to reduce energy consumption, optimize product quality, and improve labor productivity, modern papermaking equipment mostly adopts multi-motor transmission, that is, an AC motor is installed in each transmission section and equipped with a corresponding frequency converter, requiring that each section can achieve synchronous control and speed adjustment within a certain range. Papermaking is a continuous production process, so the continuous and orderly control of the production line has become a bottleneck restricting the quality and output of finished paper. Using a variable frequency control system can enable the transmission system to have very convenient and precise speed adjustment functions: (1) The transmission ratio between each transmission point can be kept constant, so that the linear speed of each transmission point is consistent. It is convenient for the equipment to speed up and decelerate, and avoids paper breakage due to large differences in linear speed between each transmission point; (2) It can continuously and smoothly drive the paper machine; (3) It has smooth acceleration and deceleration functions. Currently, the frequency converters used in the transmission of paper machine sections must simultaneously possess the following characteristics: (1) wide speed range, with an efficiency of over 90% across the entire speed range; (2) power factor higher than 0.9; (3) total input harmonic current distortion less than 3%; (4) use of reliable and technologically mature standard IGBT devices; (5) ability to reduce output harmonic components and effectively reduce dv/dt noise and torque pulsation. 4 Case Analysis For example, the production line of Dalian XX Paper Mill originally used DC speed regulation and slip differential motor speed regulation for its paper production process, including the post-drying, calender, and winding machine. Now, Anbangxin G9 frequency converters are used to modify and control the multi-motor transmission of the paper machine sections. The basic components of papermaking machinery are divided into wire section, press, pre-drying, post-press, post-drying, calender, winding machine, etc., according to the order of paper formation. The process is as follows: the pulp output from the headbox is dewatered and formed in the wire section, compressed in the press section to make the paper layer uniform, dried in the pre-drying section, then sized in the post-pressing section, then dried in the post-dryer, then smoothed by the calender, and finally formed into a mother paper roll by the winding machine. The papermaking process flow chart is shown in Figure 1. [align=center] Figure 1 Papermaking process flow chart[/align] 4.1 The modification scheme adopts the G9 series frequency converter, and its electrical diagram is shown in Figure 2. [align=center] Figure 2 Electrical diagram[/align] 4.2 Function introduction (l) The overall adjustment function adopts a master-slave structure, and its speed can be given by the overall adjustment potentiometer. One G9 frequency converter is used as the main drive output, and the slave drives all adopt G9 series products. Multiple slave drives can share one main drive. (2) l Fine adjustment function The operating frequency of the main drive is output through the analog port of the overall adjustment potentiometer as the initial synchronous speed of the slave drive frequency converter. Its deviation can be corrected by the fine adjustment potentiometer of the slave drive. (3) Due to the strong overload capacity and low-frequency rated torque of the frequency converter, the speed is guaranteed not to change with the load. At the same time, it overcomes the defect of unstable speed at low speed. 4.3 System advantages (1) This system adopts advanced frequency conversion control technology, which enables the motor to start softly, reduces mechanical shock and noise, extends the service life of mechanical equipment, and reduces mechanical maintenance costs. (2) It is easy to operate, maintain and operate, and improves the quality and output of products. (3) Since the frequency converter also reduces the voltage when adjusting the speed, while the output voltage of the motor is constant when adjusting the electromagnetic slip speed, the frequency conversion speed regulation saves a lot of energy. 4.4 Analysis of energy saving effect Dalian XX Paper Mill is a large paper mill. The 1760 whiteboard paper machine that our company has modified has 5 transmission points, all of which use electromagnetic speed regulation. The speed of each transmission is intermittent and very unstable, and paper breaks often occur during production. After the modification by adopting the frequency conversion synchronous control system designed by our company, a good energy saving effect has been achieved. (1) Before the modification, it took 242.5 kWh to produce 1 ton of paper; (2) After the frequency conversion modification, it took 127 kWh to produce 1 ton of paper (6-month average); (3) The savings per ton is 242.5-157=85.5 kWh; (4) The power saving rate reaches: (85.5÷242.5)×100%=35.2%. In other words, after the paper machine is modified, it can save the factory 42,750 kWh of electricity per month. At 0.7 yuan per kWh, it can save 29,925 yuan per month. It can save 359,000 yuan per year. Moreover, the operation is still quite stable from 45m/min to 75m/min, with excellent synchronization effect, which increases the paper output and increases the company's benefits. 5. Conclusion After the overall frequency conversion control and transformation of the production line, the required speed of the production line motors is directly provided by the motors, avoiding torque transmission losses. This allows for both coordinated motor control and individual speed control, enabling the entire production line to operate under unified command and in an orderly manner. We believe that with the development and improvement of AC frequency conversion control systems and communication technologies, AC frequency conversion drive systems, with their advanced technology, wide speed range, good speed regulation performance, energy saving, and convenient operation, will be increasingly widely used in new papermaking machines!