Design and application of DC braking function of t

2022-08-20
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Design and application of DC braking function of frequency converter introduction AC asynchronous motor has been widely used in industrial and agricultural production because of its simple structure, durability, good performance and low manufacturing cost. With the rapid development of variable frequency speed regulation technology, AC speed regulation technology has made great progress, which makes the application prospect of AC asynchronous motor more extensive and has the potential to replace DC drive system

this paper introduces the design method of using the DC braking function of the frequency converter to realize the accurate shutdown of the large inertia AC drive system. Compared with the general mode, this scheme not only works stably and reliably, but also has high control accuracy, and saves the special braking unit/braking resistance of the frequency converter worth tens of thousands of yuan, effectively reducing the equipment transformation cost. Up to now, several converter systems reconstructed according to this mode have been running steadily for several years, providing a mature technical transformation scheme for the traditional AC drive system with the main purpose of saving energy, reducing consumption and improving automation level

2 composition and main problems of the original system

steelmaking converter is an important production equipment in iron and steel plants. The production process requires high stability and parking positioning accuracy of the drive system that controls the rotation of the converter. A converter in a steel plant was originally driven by a 30kW AC asynchronous motor, which was decelerated by a gear reducer and coordinated with the planing gate to achieve accurate shutdown control. When working, the system operates in the low-speed section, and the energy loss is large. Moreover, the inertia of the whole furnace body is large, and the on-site working conditions are poor. The gears of the reduction gearbox are often damaged during startup and shutdown, resulting in the paralysis of the whole system. If the treatment is not timely, the molten steel will solidify in the furnace Speed selection: 0.05 ~ 500mm/min (sent to the fast experimental conditioning manual control box) will cause the big fault of "squatting furnace" and bring huge economic losses. Due to the limited conditions, the maintenance of mechanical deceleration system is very inconvenient, and the accuracy of parking positioning control is low. The above problems have become the "bottleneck" problems that hinder the further development of production, and the manufacturers urgently need to solve these problems

That is, the discussion of key technical problems in the transformation scheme of tensile strength

3

in view of the main problems existing in the above system, we propose to use the frequency converter to transform the original AC drive system. Because the system has been running at low speed, and there is a cubic relationship between the motor speed and the energy consumption (that is, when the motor speed drops to n% of the rated speed, the energy consumed by the motor is PN * (n%) 3 of the original rated power PN, and when the speed drops to 80% of the rated speed, the power consumed is only 51.2% of the rated power), so after using the frequency converter to replace the original mechanical speed regulation system, Not only the stability of the system operation has been greatly improved, but also the benefits in energy saving and consumption reduction will be very significant; At the same time, it gets rid of a lot of maintenance work brought by the mechanical deceleration system. The key technical problem to be solved is how to ensure and improve the parking positioning accuracy of the whole system. Generally speaking, the braking methods provided by the general frequency converter mainly include: energy consumption (DC) braking, regenerative braking (braking unit/braking resistance, rectification feedback), etc. The scope of application, occasions and use restrictions of several schemes are different, and the use effect also has its own advantages and disadvantages. Choosing an economical and effective braking method and braking function according to the actual situation is one of the key problems in the successful design of variable frequency speed regulation control system

during the transformation of the first converter, considering the large inertia of the system and the high required braking torque, we chose the braking scheme of external braking resistance and braking unit, which is recommended by the frequency converter manufacturer and has been successfully tested by brother steel works, and cooperates with DC braking. The overall control effect is very ideal. In further discussion, we consider that the converter drive system has the characteristics of slow running speed and long start-up and shutdown interval; Can the external braking unit/braking resistor be omitted and DC braking be used only? Since the frequency converter manufacturer takes the external braking resistor and braking unit as optional components, it needs to be purchased separately and the price is high. If this scheme is feasible, the transformation cost of the whole system will be effectively reduced by nearly 20%, which is very considerable in total. Therefore, we analyze the working characteristics of the system in detail

the so-called "DC braking" generally refers to that when the output frequency of the frequency converter is close to zero and the speed of the motor is reduced to a certain value, the frequency converter changes to direct current into the stator winding of the asynchronous motor to form a static magnetic field. At this time, the motor is in the state of energy consumption braking, and the rotating rotor cuts the static magnetic field to produce braking torque, which makes the motor stop quickly

since the kinetic energy stored in the rotating system is converted into electrical energy and consumed in the rotor circuit of the asynchronous motor in the form of heat loss, in order to prevent the regenerative power generation braking formed in the motor deceleration process and the motor heating in the DC braking process, it is necessary to string into the braking unit/braking resistor. The converter drive system has its own characteristics: first, the output frequency of the converter is basically about Hz in the working state; Secondly, the converter system will not start and stop frequently. Figure 1 shows the mechanical characteristic curve of general AC motor during braking. In Figure 1: ① is the curve during normal operation, and ② is the mechanical characteristics during DC braking. Set point a as the normal working point. In the braking process usually set for variable frequency speed regulation, the motor decelerates first. At this time, the speed of the synchronous magnetic field is lower than the rotor speed. The working point jumps from point a of curve ① to point B of curve ② at the same speed, that is, it transitions from the first quadrant to the second quadrant, which is usually called import 28312 in the form of special customs supervision area Adopt standard database to manage experimental data 480000 tons is the jump of characteristics at the same speed, then the motor gets the braking torque TB in the opposite direction, enters the power generation braking state, and the driving system rapidly slows down along curve ② in Figure 1. When it is lower than a certain speed, the frequency converter outputs DC, forms a fixed magnetic field, and generates braking torque. In this process, the motor will finally stop after a short period of regenerative power generation braking and energy consumption braking, so it is necessary to connect the braking unit/braking resistance to prevent the motor from heating

theoretically, if the speed of the synchronous magnetic field of the motor can be controlled to slow down, and the characteristic curve is maintained in the first quadrant when the characteristic jump of the motor occurs at the same speed, as shown in the dotted line group ③ in Figure 1, slow down the speed without jumping to the second quadrant, then the driving system can effectively avoid the regenerative braking process in the process of speed reduction. Next, when the motor speed is less than the critical speed NK, connect the DC braking, and control the size and time of connecting the DC accordingly. Theoretically, it is analyzed that the motor only experiences the limited energy consumption braking stage and will not overheat. The good internal and external characteristics of the frequency converter ensure that the above conditions are met. Figure 2 shows the output frequency of the frequency converter and the law of motor speed changing with time in DC braking. Under the control of the operation signal, the frequency converter first slowly and continuously reduces the frequency, and then starts DC braking after reaching FDB. At this time, the output frequency is zero. In the setting of system parameters, the setting of system deceleration time TZ, DC braking starting frequency FDB, braking current IDB and braking time TDB is very important, which is directly related to the accurate positioning of production machinery and the normal operation of the motor. We have conducted experiments with frequency converters of different manufacturers and models, such as abb, Siemens, Sanken, etc., which can meet the requirements of working conditions. Now take the Siemens 6se21 series inverter used in the converter system as an example to explain the parameter setting in detail:

p372=1: enable the DC braking function

p373 (IDB): the setting of the DC braking current is directly related to the braking torque. The greater the system inertia is, the greater the value should be. The optional range is 20% - 400% of the rated current of the motor, and our experience value is about 60% for repeated adjustment

p374 (TDB): input DC time. It should not be too long, otherwise the motor will overheat; However, it should be slightly longer than the actual shutdown time, otherwise the motor will enter the free sliding state. The empirical value in the converter system is about 5.5s, which should be adjusted repeatedly in combination with the actual situation. Optional range: 0 9S。

p375 (FDB): DC braking start frequency. As mentioned above, this parameter should be as small as possible and must be below the critical speed NK in curve ② in Figure 1, otherwise the motor will overheat. The empirical value is about 10Hz

improper selection of p373, p374 and p375 will cause motor overheating, which needs repeated adjustment and testing on site. If adjusted properly, the production machinery will stop at the predetermined position accurately. It should be noted that although the time TZ when the output frequency of the frequency converter decreases from FX to FDB in normal operation is not set in the DC braking parameter group, its setting is very critical. If the time is too short, the motor will run to the second quadrant, regenerative braking will occur, causing the motor to overheat. In addition, it must be pointed out that this method is not suitable for working conditions requiring frequent braking

after the parameter setting is completed according to this scheme, the system is continuously and repeatedly tested, the motor temperature rises normally, and the system parking positioning is accurate

it can be seen that in the frequency conversion transformation of large inertia drive system similar to converter drive system, it is completely feasible to apply DC braking alone

4 conclusion

to sum up, it is completely feasible to use frequency converters to transform large and medium power AC motor drive systems that need to complete speed regulation, accurate parking positioning and other functions, such as steel-making converters. Especially in some cases, we can make full use of the functions of the frequency converter, effectively reduce the transformation cost and improve economic benefits. This scheme has been successively implemented in the transformation of the drive system of four 18.5 ~ 55kW AC motor converters. The transformation cost of nearly 60000 yuan can be saved only by eliminating the purchase of brake units. In recent years, the boiler has not been shut down due to the failure of the drive system, which has created huge economic benefits for the enterprise. It can be seen that this design scheme is a preferred scheme with relatively high performance and price

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