Our country’s paper machine branch transmission equipment used SCR DC speed regulation in the past. Due to the existence of slip rings and carbon brushes, the reliability and accuracy of the paper machine are not high, which leads to the backwardness of the paper machine machinery and the speed is only about 200m/min, which is difficult. Compared with foreign high-speed paper machines of 1000m/min. From this point of view, the frequency conversion of the paper machine branch transmission machinery is the general trend.
Divisional transmission involves many transmission points such as the wire section, press section, front dryer, rear press, rear dryer, calender, etc. of the paper machine. Because the paper has the shortcomings of thinness and fragility, in order to prevent the paper from breaking , Curl, wrinkle, indentation, high-precision speed control must be carried out on each transmission point, and the paper must be stretched according to the stretch rate limited by the direction of the paper. In other words, in the whole process from sizing to winding, the paper web must be cascaded at a certain speed, so that there is tension.
It is the frequency conversion control based on the sub-drive that emphasizes the on-line stepless speed regulation and synchronous following performance of each transmission point. Therefore, the frequency converter used in the sub-drive must have the following characteristics:
1. The speed range is wide, and the efficiency must be above 90% in the full speed range.
2. The power factor is higher than 0.9.
3. The total distortion of the input harmonic current is less than 3%.
4. Adopt standard IGBT with high reliability and mature technology.
5. It can reduce the output harmonic components and effectively reduce the effect of dv/dt noise and torque ripple.
6. Use the communication function to realize high-speed serial transmission of data.
For example, the power of the 4 branch transmission points of a 1092 tipping paper machine in a paper mill is 11kw for pressing, 11kw for main cylinder, 7.5kw for group cylinder, and 5.5kw for roll paper. Another example is the 2820 kraft boxboard paper machine of a certain factory. The frequency converter adopts the American AB company 1336 type high-function frequency converter, the PLC adopts the AB company SLC500 series, and the operation interface adopts a touch screen. The upper computer controls the storage of operating parameters, the automatic generation of data tables, the automatic printing of production reports, and the analysis of fault alarm status. The network communication adopts the international PROFIBUS DP communication network. The motor adopts AC variable frequency motor, and uses magnetic flux vector closed-loop control. In addition, the load distribution of each transmission point can be controlled by calculating the actual power used by each transmission point, and in the production process, local tension and relaxation can be performed without affecting the speed chain by inching.
The above two examples are more typically applied to the control principle of the divisional frequency conversion control. The control principle of the divisional transmission is to maintain the speed cascade and high-speed transmission. The former uses the upper computer (PLC or industrial computer) to calculate the speed cascade. The latter is achieved through the high-speed serial communication capability of the inverter itself. In this way, the entire system has simple wiring, high degree of automation, rich information and compatible interfaces, which facilitates the integration of FA and management and control.
The result of the speed cascade is to form a series of speed chains for the main transmission points of the paper machine subsection 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 parameter load ratio control.
The so-called load ratio control means that in order to maintain the tightness and reasonableness of the contact between the two intermeshing rollers, the load distribution of the two rollers is required to achieve the best effect. In the control of the engagement rollers of the press section of the paper machine, the calender, etc., according to technological requirements, when the paper web breaks, the two engaged rollers are in the disengaged position, and the two rollers are in the closed position when the paper web passes. Among them, in the meshing roller closing, in order to maintain the close contact between the two rollers on the machine maple and meet the meshing requirements, a load ratio control method is required.
The main drive of the meshing roll of the paper machine still adopts the speed cascade control mode, and the secondary drive adopts the load ratio control mode. In this load ratio control, the speed closed-loop setting value is added to the main drive setting value (V setting), and the speed is increased by 5%. In this case, the output value of the speed loop is that of the current loop. The set input value (M setting) will increase, and then the output value of the main drive speed loop is multiplied by the load ratio percentage value to limit the set input value (M setting) of the secondary drive current loop, and it will be tight. Keeping up with the main drive makes the meshing degree of the two rollers closer and closer.