变频器的控制方式

变频器的控制方式 Summary of control methods used in frequency converter 1 brief introduction of frequency converter [1] 1.1 basic structure of frequency converter Frequency converter is the power frequency power supply (50Hz or 60Hz) into various frequency AC power supply, in order to realize the motor speed change operation of equipment, The structure is shown in Figure 1, wherein the control circuit completes the control of the main circuit, and the rectifier circuit converts the alternating current into direct current, The DC intermediate circuit carries out smoothing filtering on the output of the rectifier circuit, and the inverting circuit converts the direct current into the alternating current. For inverters such as vector control inverters that require a great deal of computation, sometimes a CPU for torque calculation and some corresponding circuits are needed. 1.2 classification of frequency converters There are many kinds of frequency converters, classified according to the working modes of main circuits, and can be divided into voltage type inverters and current type inverters; According to the switch mode classification, can be divided into PAM control frequency converter, PWM control frequency converter and high carrier frequency PWM control frequency converter; According to the principle of classification, can be divided into V/f control, frequency converter, slip frequency control, frequency converter and vector control, frequency converter, etc. According to the use classification, can divide into the universal frequency changer, the high performance special-purpose frequency changer, the high frequency frequency changer, the single-phase frequency changer and the three-phase frequency changer and so on. 2 frequency converter commonly used control method 2.1 non intelligent control mode The non intelligent control modes used in AC frequency converter include V/f coordinated control, slip frequency control, vector control, direct torque control, etc.. (1) V/f control V/f control is to obtain the ideal torque speed characteristics, based on changing the frequency of the power supply speed control, but also to ensure that the magnetic flux of the motor unchanged, General purpose inverters basically use this mode of control. The V/f control frequency converter structure is very simple, but this kind of frequency changer uses the open-loop control way, can not achieve the higher control performance, Moreover, in low frequency, torque compensation must be carried out to change the low frequency torque characteristics. (2) slip frequency control Slip frequency control is a direct control of torque control, which is based on the V/f control, in accordance with the actual speed of the asynchronous motor corresponding to the power frequency, The output frequency of the inverter is adjusted according to the torque desired, so that the motor has a corresponding output torque. In this control system, a speed sensor is installed in the control system, and sometimes a current feedback is added to control the frequency and current, Therefore, this is a closed loop control mode, which can make the converter have good stability and good response to rapid acceleration and deceleration and load change. (3) vector control [2] The vector control is through the magnitude and phase of motor stator current vector coordinate control circuit, in order to achieve the motor in D, Q, 0 axes in the excitation current and torque current are controlled separately, thereby to control the motor torque to. By controlling the action sequence and time of each vector and the action time of the zero vector, a variety of PWM waves can be formed to achieve various control purposes. For example, the PWM wave with the least number of switches is formed to reduce switching losses [3]. At present, there are two kinds of vector control methods applied in frequency converter, which are vector control based on slip frequency control and vector control mode without speed sensor. The vector control method based on slip frequency is consistent with the slip frequency control method, But the vector control based on slip frequency is also controlled by coordinate transformation to control the phase of motor stator current so as to satisfy certain conditions In order to eliminate the fluctuation during torque current transition. Therefore, the vector control method based on slip frequency can greatly improve the output characteristics than the slip frequency control method. However, this control mode belongs to the closed loop control mode and needs to be equipped with speed sensors on the motor. Therefore, the range of application is limited. Sensorless vector control is to control the excitation current and torque current by coordinate transformation, Then, by controlling the voltage and current on the stator winding of the motor, the speed is identified so as to control the excitation current and torque current. This kind of control mode has wide speed range, big starting torque, easy operation, easy operation, [4], but the calculation is more complicated. Generally, a special processor is needed to do the calculation, Therefore, the real-time performance is not ideal, and the control accuracy is affected by the computational accuracy. (4) direct torque control Direct torque control (DTC) is a mathematical model of AC motor in the stator coordinate system by using the concept of space vector coordinate, The flux linkage and torque of the motor are controlled, and the stator flux is detected by detecting the stator resistance. Therefore, the complex transformation calculation, such as vector control, is avoided, The system is intuitive and simple, and the calculation speed and accuracy are better than the vector control method. Even in the open-loop state, you can also output 100% of the rated torque, for multi drag load balancing function [5]. (5) optimal control The application of optimal control in practice is different according to the requirements. According to the theory of optimal control, the individual parameters can be optimized for a certain control requirement. For example, in the control application of high voltage frequency converter, two strategies, time division control and phase shift control, are successfully adopted to achieve the optimal voltage waveform [6] under certain conditions. (6) other non intelligent control modes; In practice, there are some non intelligent control methods in the control of frequency converter to achieve, For example, adaptive control [7][8][9], sliding mode variable structure control [10][11][12], difference frequency control [13], circulation control [14], frequency control [15][16][17] and so on, are limited to space, This is no longer the case. Interested readers may refer to the relevant documents themselves. 2.2 intelligent control mode The main intelligent control methods include neural network control, fuzzy control, expert system, learning control, etc.. There are some successful examples in the application of intelligent control in the control of frequency converter. (1) neural network control The application of neural network control method in the control of frequency converter is generally complicated system control. At this time, the model of the system is very little understood, Therefore, the neural network not only completes the function of system identification, but also controls it. Moreover, the neural network control mode can control multiple frequency converters [18][19] simultaneously, so it is suitable to control multiple inverters in cascade. But there are too many layers in the neural network, or the algorithm is too complex, which will bring many practical difficulties in the concrete application. (2) fuzzy control The fuzzy control algorithm is used to control the voltage and frequency of the converter, so that the speed of the motor is controlled In order to avoid speeding up too fast, the impact of motor life and speed up too slow, affecting the efficiency of [20][21]. The key of fuzzy control lies in the division of domain, membership degree and fuzzy level. This control method is especially suitable for multi input and single output control system. (3) expert system Expert system is a kind of control method controlled by the experience of the so-called expert. Therefore, an expert library should be established in the expert system to store a certain amount of expert information, There is also a reasoning mechanism, so that the ideal control results can be obtained according to the known information. The design of expert database and reasoning mechanism is very important, which is related to the pros and cons of expert system control. The application of expert system can control the voltage of converter and control its current [22][23]. (4) learning control Learning control is mainly for repetitive input, while regular PWM signals (such as central modulation PWM) just meet this condition, so learning control can also be used in the control of frequency converters, [24]. Learning control does not require much information about the system, but it takes 1~2 learning cycles, so the speed is relatively poor, Moreover, the algorithm of learning control sometimes needs to realize the advance link, which can not be realized by the simulator, At the same time, learning control involves a problem of stability, which should be paid special attention to in application. 3 outlook of frequency converter control With the development of high power technology, such as power electronics, micro electronics and computer network, the control mode of frequency converter will develop in the following aspects. (1) the realization of digital control frequency converter Now, the control mode of frequency converter can realize complex operation with digital processor. Digital converter will be an important direction of development, At present, the frequency converter digitization mainly uses the monolithic integrated circuit, MCS51[25] or 80C196MC[3][20] and so on, Auxiliary to SLE4520[25][28] or EPLD liquid crystal display, [29], etc. to achieve more perfect control performance. (2) combination of multiple control methods A single control system has its own advantages and disadvantages, and there is no "universal" control mode. In some control situations, some control methods need to be combined, For example, combining learning control with neural network control, [30] combines adaptive control with fuzzy control, [31][32] combines direct torque control with neural network control, [33], Or "hybrid control," [34], so that the effect of control will be better. (3) the realization of remote control The development of computer network, the "end of the world if so, rely on computer networks for remote control of the inverter is also a development direction. The remote control of the frequency converter is carried out through the RS485[35][36] interface and some network protocols, so as to be suitable for some occasions where human beings are not allowed to perform on-site operations, It is also easy to achieve control goals [37][38]. (4) green frequency converter With the proposal of sustainable development strategy, people pay more and more attention to the protection of environment. The higher harmonics generated by the inverter will cause pollution to the power grid, and reduce the noise of the inverter work, and enhance its work * * * * * * * * * * * * * safety issues, They all try to solve the problem by adopting the proper control method, and design the green frequency converter. 4 Conclusion The control mode of frequency converter is a problem worthy of study, which is based on the joint efforts of the people of insight who are committed to this work, Make domestic inverter to the world market as soon as possible, and become a first-class product.