Summary of common control methods of the hottest i

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Summary of common control methods of frequency converter

Abstract: This paper summarizes the common control methods and their respective characteristics in frequency converter control in recent years, and looks forward to some development directions of frequency converter control methods in the future

key words: inverter control

abstract: the cont rol methods and characteristics of inverters are summarized in this paper and the developments in the future of the inverters control methods are proposed, too

keywords: inverter control

variable frequency speed regulation technology is an important development direction of modern power transmission technology. As the core of variable frequency speed regulation system, the performance of frequency converter has increasingly become a decisive factor for the performance of speed regulation. In addition to the "inborn" conditions of the manufacturing process of the frequency converter itself, it is also very important to adopt what kind of control mode for the frequency converter. Based on the industrial practice, this paper summarizes the characteristics of various inverter control modes in recent years, and looks forward to the future development direction

1 introduction to frequency converter [1]

1.1 basic structure of frequency converter frequency converter is a device that converts power frequency power supply (50Hz or 60Hz) into AC power supply of various frequencies to realize variable speed operation of motor. Its structure is shown in Figure 1. The control circuit controls the main circuit, the rectifier circuit converts AC power into DC power, and the DC intermediate circuit smoothly filters the output of the rectifier circuit, The inverter circuit inverts the direct current into alternating current. For a frequency converter that needs a lot of calculation, such as vector control frequency converter, sometimes a CPU for torque calculation and some corresponding circuits are required

1.2 classification of frequency converter there are many classification methods of frequency converter. According to the working mode of main circuit, it can be divided into voltage type frequency converter and current type frequency converter; According to the classification of switching mode, it can be divided into PAM control frequency converter, PWM control frequency converter and high carrier frequency PWM control frequency converter; According to the working principle, it can be divided into v/f control frequency converter, slip frequency control frequency converter and vector control frequency converter; According to the use classification, it can be divided into general frequency converter, high-performance special frequency converter, high-frequency frequency converter, single-phase frequency converter and three-phase frequency converter

2 control modes commonly used in frequency converters

2.1 non intelligent control modes

non intelligent control modes used in AC frequency converters include v/f coordinated control, slip frequency control, vector control, direct torque control, etc

(1) v/f control

v/f control is proposed to obtain ideal torque speed characteristics, which is based on the idea of changing the power frequency for speed regulation and ensuring that the magnetic flux of the motor remains unchanged. This control mode is basically adopted by general-purpose frequency converters. The structure of v/f control inverter is very simple, but this inverter adopts open-loop control mode, which can not achieve high control performance. Moreover, at low frequency, torque compensation must be carried out to change the low-frequency torque characteristics

(2) slip frequency control

slip frequency control is a control method that directly controls the torque. On the basis of v/f control, it can make the motor have the corresponding output torque by adjusting the output frequency of the frequency converter according to the actual speed of the asynchronous motor and the desired torque. In this control mode, the hydrogen chloride recycled by Bayer materials technology needs to be installed with a speed sensor 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 frequency converter have good stability and good response characteristics to rapid acceleration, deceleration and load changes

(3) vector control [2]

vector control is to control the magnitude and phase of the motor stator current through the vector coordinate circuit, so as to control the excitation current and torque current of the motor in the D, Q and 0 coordinate axis system respectively, so as to control the motor torque. By controlling the action sequence and time of each vector and the action time of zero vector, various PWM waves can be formed to achieve various control purposes. For example, a PWM wave with the least switching times is formed to reduce switching loss [3]. At present, there are two vector control methods in practical application in frequency converter, which are based on slip frequency control and speed sensorless vector control

the steady characteristics of the slip frequency based vector control mode and the slip frequency control mode are the same, but the slip frequency based vector control also controls the phase of the motor stator current through coordinate transformation to meet certain conditions, so as to eliminate the fluctuation in the torque current transition process. 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 the speed sensor needs to be installed on the motor. Therefore, the application range is limited. Speed sensorless vector control is to control the excitation current and torque current respectively through coordinate transformation, and then identify the speed by controlling the voltage on the stator winding of the motor ASTM G105 SAE Recommended Practice (Falex dry sand/rubber wheel apparatus) and other standards and current to achieve the purpose of controlling the excitation current and torque current. This control mode has wide speed regulation range, large starting torque, reliable operation and convenient operation [4]. However, the calculation is complex and usually requires a special processor for calculation. Therefore, the real-time performance is not ideal, and the control accuracy is affected by the calculation accuracy

(4) direct torque control

direct torque control uses the concept of space vector coordinates to analyze the mathematical model of the AC motor in the stator coordinate system, control the flux and torque of the motor, and achieve the purpose of observing the stator flux by detecting the stator resistance. As a result, complex transformation calculations such as vector control are omitted. The system is intuitive and concise, and the calculation speed and accuracy are improved compared with the vector control method. Even in the open-loop state, it can output 100% of the rated torque, and has the load balance function for multi drive [5]

(5) optimal control

the application of optimal control in practice varies according to different requirements. Individual parameters of a control requirement can be optimized according to the theory of optimal control. For example, in the control application of high-voltage inverter, two strategies of time segmented control and phase shift control are successfully adopted to achieve the optimal voltage waveform under certain conditions [6]

(6) other non intelligent control modes

in practical application, there are also some non intelligent control modes that can be realized in the control of frequency converter, such as adaptive control [7][8][9], sliding mode variable structure control [10][11][12], differential frequency control [13], circulating current control [14], frequency control [15][16][17], etc. due to space limitations, they will not be described here. Interested readers can refer to relevant literature by themselves

2.2 intelligent control modes

intelligent control modes mainly 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

neural network control mode is applied to the control of frequency converter. Generally, it is used for complex system control. At this time, little is known about the system model. Therefore, neural network should not only complete the function of system identification, but also control. Moreover, the neural network control mode can control multiple frequency converters at the same time [18][19], so it is more suitable to control when multiple frequency converters are cascaded. However, too many layers of neural networks or too complex algorithms will bring many practical difficulties in specific applications

(2) fuzzy control

fuzzy control algorithm is used to control the voltage and frequency of the frequency converter to control the speed rise time of the motor, so as to avoid the impact of too fast speed rise on the service life of the motor and too slow speed rise on the work efficiency [20][21]. The key of fuzzy control lies in the division of universe, membership degree and fuzzy level. This control method is especially suitable for multi input and single output control systems

(3) expert system

expert system is a control method that uses the experience of so-called "experts" to control. Therefore, an expert database should be established in the expert system to store certain expert information. In addition, there should be a reasoning mechanism to facilitate the search for ideal control results according to the known information. The design of expert database and reasoning mechanism is particularly important, which is related to the quality of expert system control. The application of expert system can control both the voltage and current of frequency converter [22][23]

(4) learning control

learning control is mainly used for repetitive input, and regular PWM signals (such as center modulation PWM) just meet this condition. Therefore, learning control can also be used in frequency converter control [24]. Learning control does not need to know too much system information, but it needs 1~2 learning cycles, so its rapidity is relatively poor. In addition, the learning control algorithm sometimes needs to realize the leading link, which can not be realized with the simulator. At the same time, learning control also involves a problem of stability, which should be paid special attention to in application

3 prospect of frequency converter control

with the development of power electronics technology, microelectronics technology, computer network and other industry leading high and new technologies represented by nanmeixin and beihelong, the control mode of frequency converter will develop in the following aspects in the future

(1) realization of digital control of frequency converter at present, the control mode of frequency converter can use digital processor to realize relatively complex calculation, and the digitization of frequency converter will be an important development direction. At present, the single-chip microcomputer mcs51[25] or 80c196mc[3] [20] is mainly used for the digitization of frequency converter, and sle4520[25][28] or EPLD liquid crystal display [29] are used to realize more perfect control performance

(2) combination of multiple control modes. A single control mode has its own advantages and disadvantages. There is no "universal" control mode. In some control occasions, some control modes need to be combined, such as combining learning control with neural network control [30], combining adaptive control with fuzzy control [31][32], and combining direct torque control with neural network control [33], It is also called "hybrid control" [34], so that we can learn from each other and achieve better control effect.

(3) the development of computer network for remote control, so that "the horizon is close at hand", and remote control of frequency converter based on computer network is also a development direction. Through rs485[35][36] The interface and some network protocols control the frequency converter remotely, so that the control objectives can be easily achieved in some occasions that are not suitable for human field operation [37][38]

(4) with the proposal of sustainable development strategy, people pay more and more attention to environmental protection. The high-order harmonic generated by the frequency converter will pollute the electricity, and reduce the noise of the frequency converter and enhance its reliability and safety. These problems are all trying to be solved by adopting appropriate control methods to design a green frequency converter

4 conclusion

the control mode of frequency converter is a problem worth studying. Relying on the joint efforts of people of insight committed to this work, domestic frequency converter

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