The paper machine is a combined machine composed of a series of supporting equipment, which is divided into two parts: the wet part and the cadre. The wet part includes the sizing flow system, the wire part and the pressing part, and the dry part includes the drying, calendering and winding. The production process is generally: the slurry is transferred to the front headbox of the paper machine production line through the sizing flow delivery system, and then the pulp flow is sequentially passed through the wire section, the press, the front drying cylinder, the rear pressing, the rear drying cylinder, and calendering. The branch equipment including the machine and the reeling machine becomes the base paper; the base paper can also enter the off-machine coating and rewinding machine to produce the finished paper.
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 the finished paper. Frequency conversion speed regulation as the most powerful control method after the mid-1990s entered the field of papermaking that originally belonged to DC speed regulation (applicable to large and medium paper machines) and slip motors (applicable to small and medium paper machines), and has achieved good results recently. Market effect.
AC variable frequency speed regulation technology is recognized as one of the most promising AC speed regulation methods due to its excellent speed regulation performance, significant power saving and wide applicability in the field of national economy. The DC speed control system occupies an important position in the development history of paper machines. However, due to the difficulty of DC motor maintenance and poor environmental resistance, the performance and price ratio of the paper machine has been severely restricted by the 1990s. The variable frequency speed regulation technology can maximize the inherent advantages of the AC motor itself (simple structure, ruggedness, economic reliability, good dynamic response, etc.). In addition, the variable frequency speed regulation theory has formed a relatively independent discipline. The time for speed regulation technology to be fully applied to the paper machine production line has arrived. This article makes further elaboration on the application of frequency conversion speed regulation and its control technology in the paper machine production line.
2 Frequency control of the paper machine sizing system
The sizing system of the paper machine is responsible for the process of transporting the pulp to the headbox of the paper machine net through the pulp pump, flushing pump, multi-stage sieve and other equipment. The pulp pump and flushing pump are the most important equipment because they are related to forming The quality of the paper, especially the grammage, must be controlled by frequency conversion to meet the design requirements of the paper machine. Practice has proved that the installation of frequency conversion device can adapt to different vehicle speeds and different varieties of dosage changes, so that the operation of the slurry pump group is always in a highly efficient state. In addition, when the frequency conversion control is adopted, the rotation speed of the motor and the slurry pump is reduced, the wear of the mechanical components such as bearings is reduced, the pump end sealing system is not easily damaged, the failure rate of the machine pump is greatly reduced, and the maintenance workload is also reduced.
The sizing process must meet the following requirements: (1) The amount of slurry delivered to the net section of the paper machine must be stable, and the error does not exceed Â± 5%; (2) The ratio and concentration of the slurry must be stable and uniform; (3) Purify and select the slurry. Therefore, the speed control of the slurry pump and the flushing pump is the most important thing to ensure the realization of items (1) and (2).
The following uses the flushing pump as an example to illustrate the flow of speed control: the frequency conversion control adopts the speed control mode of the double closed-loop system (as shown in Figure 1), the outer loop is the speed closed loop (V-control), and the inner loop is the current or rotation Moment closed loop (I-control). The setting value of the speed of the flushing pump is on the one hand the V setting obtained by the change of the ratio of the pulp speed and the net speed, and on the other hand is the pressure controller (P-control) from the headbox. The former is the main theme, and the latter is the fine-tuning. The ratio of pulp speed and net speed of the paper machine is basically constant, so when the net speed of the paper machine changes, the V setting also changes, and the speed of the pulp pump also changes; in order to improve the accuracy of the speed regulator and Reflecting the actual process of the headbox, it is usually necessary to take the change of Â± 5% of the output value of the PID control of the headbox as the additional speed setting value of the flushing pump. The actual value of speed (Vactual) is taken from the actual speed sampling of the drive motor, which can be obtained by a detection device such as a rotating speed motor or a photoelectric rotary encoder. The current setting value (M setting) is taken from the output signal taken from the speed loop, and the current actual value (M actual) is taken from the measurement value of the current transformer at the output end of the AC drive at each transmission point.
Therefore, for the variable frequency speed regulation of the flushing pump, PID control is required, and the speed feedback method and various parameters of PID must be correctly selected. The set value of acceleration time 0 ï½ž 100% is generally 60 seconds, while the deceleration time is about 30 seconds. The set input value of the inverter needs to have two to many ends of input, and can perform basic operations. The feedback signal must have a port to receive an analog signal or pulse signal. Knowing this is very important for choosing the model of the inverter.
3 Frequency conversion control of paper machine branch drive
China's paper machine branch transmission equipment used SCR DC speed control method in the past. Due to the existence of slip rings and carbon brushes, the reliability and accuracy are not high, which leads to the backwardness of the machine of the paper machine. Compared with foreign high-speed paper machines of 1000m / min. From this point of view, the frequency conversion of paper machine segment transmission machinery has become the general trend.
The partial transmission involves many transmission points such as the wire part, the press part, the front dryer, the rear press, the rear dryer, and the calender of the paper machine. Because the paper has the disadvantages of thinness and fragility, in order to prevent the paper from breaking , Curling, wrinkling, creasing, high-precision speed control must be performed on each transmission point, and the paper must be stretched according to the stretch rate defined in the paper forming direction. In other words, the whole process from sizing to winding must keep the paper web at a certain speed cascade so that there is tension. It is precisely based on the frequency conversion control of the partial drive that emphasizes the online stepless speed regulation and synchronous follow-up performance of each transmission point, so the frequency converter used in the partial drive must have the following characteristics: (1) The speed range is wide, and the Within the speed range, the efficiency must be above 90%; (2) The power factor is higher than 0.9; (3) The total distortion of the input harmonic current is less than 3%; (4) The use of high reliability, mature technology standard device IGBT; ( 5) Can reduce the output harmonic component and effectively reduce the effect of dv / dt noise and torque ripple; (6) Use the communication function to achieve high-speed serial transmission of data.
For example, the power of the four branch transmission points of a 1092 tipping paper machine in a paper mill are 11kW for pressing, 11kW for main cylinder, 7.5kW for group cylinder, and 5.5kW for roll paper. According to the actual situation and requirements of the manufacturer, the inverter adopts the newly launched MM420 series from Siemens, and the PLC uses the Siemens S7-200 series. The network communication adopts RS485 interface, and the protocol adopts Siemens USS protocol. The speed adjustment of the host machine and the single adjustment of the single transmission point adopt the button speed adjustment, and can adjust the speed in front of the console and the equipment. Another example is the 2820 kraft paperboard paper machine of a certain factory. The frequency converter adopts the 1336 type high-performance frequency converter of the American AB company, the PLC uses the SLC500 series of the AB company, and the operation interface uses the touch screen. It is controlled by the host computer to store operating parameters, automatically generate data tables, automatically print production reports, and analyze fault alarm status. The network communication adopts the international general PROFIBUS DP communication network. The motor adopts AC variable frequency motor, and the flux vector closed loop control is applied. In addition, the load distribution of each transmission point can be controlled by calculating the actual power used at each transmission point, and in the production process, local tension and relaxation can be performed by inching without affecting the speed chain.
The above two examples are more typically applied to the control principle of sub-frequency conversion control. The control principle of sub-transmission is to maintain speed cascade and high-speed transmission. The former uses a host computer (PLC or industrial control computer) to calculate the speed cascade. The latter is achieved by the high-speed serial communication capability of the inverter itself. How to come, the whole system is simple wiring, high degree of automation, rich information and compatible interface, which is convenient for the integration of FA and management.
The result of speed cascading is to form a series of speed chains for the main transmission points of the paper machine branch transmission. As long as the speed of one of the points is adjusted, each subsequent transmission point is also adjusted quickly. In the actual paper machine control, it is often necessary to add a specific transmission control method according to a specific position, especially the important load ratio control.
The so-called load ratio control means that in the two meshing rollers, in order to maintain the tightness and rationality of the contact, the two rollers need to be distributed to achieve the best results. In the control of intermeshing rollers such as the press section and calender of the paper machine, according to technological requirements, when the paper web breaks, the two intermeshing rollers are in the disengaged position, and the two webs are in the closed position when the paper web passes. Among them, when the meshing rollers are closed, in order to keep the two rollers in close mechanical contact and meet the requirements of meshing, the load ratio control method needs to be adopted.
The main transmission of the meshing roller still adopts the speed cascade control mode, and the slave transmission adopts the load ratio control mode. In this load ratio control, the speed closed-loop setting value is increased by 5% on the basis of the main drive setting value (V setting). In this case, the output value of the speed loop is the current loop setting. The fixed input value (M setting) will increase, and then the output value of the main drive speed loop is multiplied by the load ratio percentage to limit the set input value (M setting) of the slave drive current loop, and it will be tight Keeping up with the main drive makes the meshing degree of the two rollers more tight and fit.
4 Tension control of rewinding and unwinding systems such as reel, rewinder and off-machine coating machine
There are two types of paper forming methods: flat paper and roll paper. Due to the large number of high-speed printing machines, the demand for roll paper is increasing day by day. Compared with flat paper, web paper can increase the speed of paper machine. However, the processing of the roll paper by the paper machine is a relatively complicated process, especially in the process of central winding, with the continuous increase of the roll diameter, the speed of the roll roller must be continuously reduced, and at the same time, the paper width must be ensured. The tension is relatively stable. Therefore, for the unwinding and unwinding system, the tension control is the core technology and the difficulty of frequency conversion speed regulation.
For such a complicated tension control system, the frequency conversion technology must be able to overcome the following four technical problems: (1) simplify the mathematical model of the complex AC asynchronous motor; (2) consider the delay and overshoot of the tension feedback signal; 3) Describe the dynamic parameters of the winding tension control process as a time-varying function; (4) Ensure that the tension closed loop has high anti-fluctuation capability and good instantaneous adjustability.
At present, the most commonly used in winding is the following two control methods: (1) Speed â€‹â€‹controlled winding (SPW), which uses PID to feedback the tension of the load cell or adjust the position of the roller to correct the speed reference; (2) Current-controlled winding (CPW), which uses PID to adjust the tension setting, can be applied to the central winding method. This type of control is generally open-loop.
The frequency conversion control system with tension control is generally composed of modules such as diameter, compensation, torque and speed calculation. Figure 3 shows the basic software structure of its winding system:
5 Frequency conversion control of auxiliary part of paper machine
The auxiliary parts of the paper machine usually include white water system, vacuum system, compressed air system, chilled water system, chemical dosing system, broken paper recovery system, ventilation system, etc. In the actual design of the paper machine, in order to make the paper machine run continuously and evenly, its auxiliary parts generally exceed the maximum production capacity of the paper machine by 15% to 35%. Therefore, the use of frequency conversion speed regulation and its control technology can effectively reduce the auxiliary energy consumption in actual operation, such as the fan and water pump that were originally controlled by adjusting the opening of the valve, and the belt conveyor at fixed speed. The following are some typical aspects to illustrate the wide range of applications of frequency conversion control.
5.1 Frequency control of chilled water
In the auxiliary equipment for papermaking, coating and sizing often need to use cold water to treat the paint to maintain the stability of the paint's chemical properties. The method often used now is the variable frequency speed control system of the chilled water pump. Its control principle is to use temperature or temperature difference control, because in a strict sense, the difference between the return water temperature and the outlet water temperature of the freezing host indicates the heat taken by the chilled water from the paint tank, so the temperature difference can be used as the basis for control. At the same time, since the outlet water temperature of the freezing host is generally relatively stable, in fact, it only needs to be controlled according to the return water temperature in the design.
The variable frequency speed regulation used in the chilled water system should be designed for PID control, because most inverters have PID functions, so the control loop is very simple, using the return water temperature as the feedback signal, and the target temperature can be passed through the inverter Panel setting or analog signal output from the host computer.
5.2 Frequency conversion speed regulation of broken paper belt conveyor
The broken belt conveyor is responsible for transporting the broken or processed paper of the cadre to the dry damage system, because the speed of the belt is usually designed according to the maximum speed of the paper machine, and the speed of the paper machine is usually 50% to 80% of the maximum speed In between, the belt running at constant speed will waste a lot of energy and also generate too much noise. At this time, frequency conversion is used to adjust the speed of the belt conveyor to the speed of the paper machine, and the starting frequency of the belt conveyor is 20. ï½ž 30Hz, so that the belt conveyor can respond quickly to avoid paper jams and reduce energy consumption.
With the development and maturity of frequency conversion technology, the frequency converter with its excellent speed control performance, simple operation, perfect function, easy to realize automatic adjustment and many other advantages, will certainly replace the traditional DC speed control system on the paper machine, and become the paper machine electrical The leader in transmission.
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