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Inverters basic structure and control Profile
Industry News > Add time:2008-1-28
About an inverter
The basic structure of 1.1 Converter
The frequency converter is the power supply (50 Hz or 60 Hz) transform into various frequency AC power, in order to achieve the speed motor running equipment, control circuit complete control of the main circuit, rectifier circuit will be converted into AC DC, DC middle rectifier circuit to the output smoothing, DC inverter circuit will again reverse into AC. The vector control inverter such as the need for massive computing inverter, and sometimes also to the need for a torque of the CPU as well as the corresponding circuit.
Category 1.2 Inverter
Inverter classified a number of means, in accordance with the main circuit working methods classification, can be divided into current and voltage inverter-converter; switch means in accordance with the classification, can be divided into PAM control inverter control PWM carrier frequency and high-frequency converter PWM control inverter; accordance with the principle of classification, can be divided into V / f controlled inverter slip frequency control inverter and vector control inverter; use in accordance with the classification, can be divided into general inverter, high-performance dedicated inverter High-frequency converter, single-phase and three-phase inverter, and other inverter.
2 converter used in the control mode
2.1 Non-Intelligent Control
AC drives in the use of non-intelligent control methods are V / f coordination and control, slip frequency control, vector control, direct torque control.
(1) V / f control
V / f control to be ideal torque - velocity characteristics, based on changes in the frequency of power supply to the governor at the same time, but also ensure that the magnetic flux motor unchanged ideas proposed general-converter basically have adopted it control. V / f inverter control structure is very simple, but this converter using open loop control, the control can not reach a higher performance, but also in the low-frequency, torque compensation must be carried out to change the low torque characteristics.
(2) to the poor frequency control
Slip frequency control is a direct control of the torque control mode, it is in the V / f controlled basis, in accordance with know that the actual speed asynchronous motor power frequency response, and in accordance with the wish of the torque converter to adjust output frequency, we can make a corresponding motor torque. This control method, in the control system needs to install speed sensors, sometimes plus a current feedback, frequency and current control, therefore, it is a closed-loop control method will enable inverter has a good stability, and rapid acceleration and deceleration and load changes with good response characteristics.
(3) The vector control
Vector control through the vector motor control circuit coordinates the size and stator current phase of the motor in order to achieve d, q, 0 coordinates shaft in the excitation current and torque control current, respectively, and achieve the purpose of control of motor torque. By controlling the role of the vector sequence and time, and time is the role of vector, we can form various PWM wave to a variety of control purposes. For example, a PWM switching frequency of the wave at least to reduce the switching loss. Inverters in the practical application of vector control methods mainly based on the slip frequency control vector control mode and speed sensor vector control mode two.
Based on the slip frequency vector control and slip frequency control characteristics of the two unsteady consistency, but based on the slip frequency vector control to transform the coordinates of the motor stator current phase of the control, so as to meet certain conditions, to eliminate torque current fluctuations in the process of transition. Therefore, based on the slip frequency vector control to the poor than in the output frequency control characteristics can be greatly improved. However, this control method is closed-loop control, the need to install speed sensors on the motor, therefore, the scope of application is limited.
Speed sensorless vector control is handled through the coordinate transformation of the respective current excitation current and torque control, and then control the motor stator winding voltage and current to speed identification to control excitation current and torque current purposes. This wide range of speed control mode, the starting torque, reliable, convenient operation, but more complicated calculation, the general need for specialized processors for computing, and real-time, not too satisfactory, control precision by the accuracy of .
(4) Direct Torque Control
Direct Torque Control vector coordinates the use of space is the concept of the stator coordinate system analysis of the mathematical model AC motor control motor torque and flux through the resistance of the stator to the stator flux to the observation purposes, and therefore avoid the Vector control transform complex calculation, the system intuitive, simple, calculation speed and accuracy than vector control has improved. Even in the open-loop state, but also to 100% of the rated output torque, with more than drag load balance function.
(5) the optimal control
Optimal control in the application in accordance with the actual requirements vary, according to the theory of optimal control of the control requirements of a particular individual parameters are optimized. For example, in the high voltage inverter control applications, the success of the introduction of the Sub-time control and phase shift control of two strategies, under certain conditions, in order to achieve the optimal voltage waveform.
(6) other non-intelligent control mode
In practical application, there are a number of non-intelligent control methods in the control of inverter can be achieved, for example, adaptive control, sliding variable structure control, poor frequency control, circulation control, frequency control.
Intelligent Control 2.2
Intelligent control major neural network control, fuzzy control, expert system, learning control. Inverter control in the use of intelligent control methods in the application of some specific examples of success.
(1) Neural Network Control
Application of neural networks in control of the inverter control, the general is a more complicated control system, then the system model is poorly understood, therefore it is necessary to complete neural network system identification of the function, but also control. And neural network control method can simultaneously control multiple converter, inverter cascade in a number of control when more appropriate. But too many layers of the neural network algorithm too complicated or in specific applications will bring a lot of practical difficulties.
(2) Fuzzy Control
Fuzzy control algorithm is used to control the voltage and frequency inverter, the motor speed or time to gain control, in order to avoid excessive speed of the motor or life, as well as the impact of slow or speed affect efficiency. Fuzzy Control is the key domain, as well as fuzzy-level membership, the demarcation of this control method particularly applicable to multi-input single-output control system.
(3) Expert System
Expert System is the use of so-called "experts" to control the experience of a controlled manner, therefore, expert system in general to establish a library experts, some experts stored information, and will have reasoning mechanisms, in order to seek information on the basis of known the ideal control the outcome. Experts with the reasoning mechanism design is particularly important, the relationship between the pros and cons of expert system control. Application Expert System can control the voltage inverter, and will control the current.
(4) Learning Control
Learning control is mainly used for the importation of repeatability, and the rules of the PWM signal (such as PWM modulation Centre) just to meet this condition, it can also be used for learning control of the inverter control. Learning control system do not need to know too much information, but it should be 1 to 2 learning cycles, rapid relatively poor, and, learning control algorithm sometimes need to achieve ahead of links, which use analog devices can not be achieved and at the same time , learning control also involves a question of the stability of the application, should pay special attention to.
Inverter control of the three prospects
With the power electronics technology, microelectronic technology, computer networks, and other high-tech development, the converter will control the future development of the following aspects.
(1) the realization of digital control inverter
Now, the inverter control can be achieved using digital processor complex computing, digital converter will be an important direction of development, the ongoing digital converter used mainly SCM MCS51 or 80 C196MC, supplemented by SLE4520 LCD or EPLD display to achieve improved performance.
(2) the combination of a variety of control
A single control method has the advantages and disadvantages of each, and there is no "panacea" control, control, in some occasions, the need to combine some control, for example, will learn to control and integration of neural network control, adaptive control and fuzzy control combination of direct torque control and neural network control combination, or as a "hybrid control," and such each other, control would be more effective.
(3) the realization of remote control
The development of computer networks, "a distant right close by home" and rely on computer networks for remote control of the inverter is a development direction. Through the RS485 interface and some network protocol for remote control of the converter, so that some human beings are not suited to the operation of the scene occasions, it is easy to achieve the control objectives.
(4) Green Inverter
With the strategy of sustainable development proposed for the protection of the environment are attracting increasing attention. Inverters the high harmonics of the power grids will bring pollution, reduce working hours converter noise and enhance the reliability of its work, security, and so on these issues, trying to take control of the appropriate means to resolve the design Green converter.
4 Concluding remarks
Inverter control is a problem worthy of study, relying on the work of insight to the joint efforts, China-made inverter to the world market as soon as possible and become a first-class products.

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