light industry

   Variable frequency speed control technology is an important development direction of modern power transmission technology, and as the core of the variable frequency speed regulation system of the inverter performance is increasingly becoming the speed performance of the merits of the decision factors, in addition to the inverter itself manufacturing process of "innate" conditions, of converter production use of what kind of control method is also very important. Starting from the industrial practice, this paper summarizes the characteristics of various inverter control methods in recent years, and looks forward to the future development direction.

1, inverter profile
1.1 the basic structure of the inverter
Inverter is the power frequency (50Hz or 60Hz) transform into various frequency AC power and to realize the motor speed operation of equipment, which control circuit complete control of the main circuit, rectifier circuit will be converted into AC DC, DC intermediate circuit of the rectifier circuit output of smoothing filter, inverter circuit will DC and then reverse into the AC power. For such a vector control inverter this requires a large number of operations of the inverter, and sometimes also need a torque calculation of the CPU and some of the corresponding circuit.
1.2 classification of frequency converters
There are many types of frequency converter, according to the classification of the main circuit mode can be divided into voltage type inverter and current source inverter; in accordance with the switch mode classification, can be divided into for PAM inverter control, PWM control inverter and the high frequency PWM inverter control; according to the working principle of the classification can be divided for the V / F control transducer, a slip frequency control inverter and vector control inverter etc.; according to the use of classification can be divided into general inverter, high performance special frequency converter, frequency converter, a single-phase inverter and three-phase inverter.
2, frequency changer control mode
2.1 non intelligent control methods
The non intelligent control methods used in AC inverter are V/f coordinated control, slip frequency control, vector control, direct torque control, etc..
(1) V/f control
V / F control in order to get the ideal torque speed characteristic, proposed based on changes the power frequency speed at the same time, but also to ensure the motor flux constant thought, general inverter basically are using this control method. The V / F control converter structure is very simple, but the converter using open loop control mode, cannot achieve high control performance and at low frequencies, we must compensate the torque to change the low frequency torque characteristics.
(2) slip frequency control
The slip frequency control is a control mode of direct torque control (DTC), it is on the basis of V / F control, according to know power frequency corresponding to the actual rotational speed of asynchronous motor and according to the desired torque to regulate the output frequency of the converter, you can make the motor with the corresponding output torque. This control mode in the control system need to install the speed sensor, sometimes with current feedback, to control the frequency and current. Therefore, it is a closed loop control mode, so that the inverter has good stability, and the rapid acceleration and deceleration and load changes have good response characteristics.
(3) vector control
Vector control is by vector coordinate circuit control the size and phase of the stator current, in order to achieve control of the motor in D, Q, 0 coordinate axis magnetizing current and torque current separately, in order to control the torque of motor. By controlling the action sequence and time of each vector and the time of the time and the action time of the zero vector, all kinds of PWM waves can be formed. For example, the PWM wave with the least number of switches is formed to reduce the switching loss. At present, the vector control mode of the inverter is mainly based on the vector control mode and speed sensorless vector control mode, which is based on slip frequency control.
Based on slip frequency vector control method and the slip frequency control both the steady characteristics consistent, but based on slip frequency vector control but also through coordinate transformation phase of the motor stator current control to meet certain conditions, to eliminate torque electric flow transition process fluctuations. Therefore, the vector control mode based on the slip frequency is better than the slip frequency control method in terms of output characteristics. However, this kind of control mode belongs to the closed loop control mode, and it needs to install the speed sensor on the motor, so the application scope is limited.
Speed sensorless vector control is handled via coordinate transformation respectively on the excitation current and the torque current control, and then by controlling the motor's stator voltage and current speed identification in order to control the excitation current and torque current. This control way speed range wide, starting torque, reliable work, convenient operation, but the calculation is relatively complex, generally need to dedicated processor to calculate. Therefore, real-time is not too ideal, control accuracy is affected by the accuracy.
(4) direct torque control
Direct torque control is using the concept of vector space, the analysis of the mathematical model of AC motor in the stator coordinate and control motor flux and torque, the stator resistance testing to achieve the purpose of observation of stator flux linkage is, thereby eliminating the need for the calculation of vector control of complex transformation, system intuitive, concise, calculation speed and precision are compared to vector control method improved. Even in the open loop state, can also be output 100% of the rated torque, for more than a drag with a load balancing function.
(5) optimal control
The application of optimal control in practical application is different according to the requirements, and can be optimized according to the theory of optimal control for a particular control requirement. For example, in the control application of high voltage frequency converter, the successful use of time segmentation and phase shift control two strategies to achieve certain conditions of the voltage optimal waveform.
(6) other non intelligent control methods
In practical applications, there are some non intelligent control methods in the inverter control can be realized, such as adaptive control, sliding mode variable structure control, differential frequency control, loop control, frequency control, etc..
2.2 intelligent control methods
Intelligent control methods include neural network control, fuzzy control, expert system, learning control and so on. There are some successful examples in the application of intelligent control in the control of frequency converter.
(1) neural network control
Neural network control application in inverter control, is generally more complicated system control, then their understanding of the model system is little. Therefore neural network both to complete the function of the system identification, but also control. And the neural network control method can control multiple frequency converters at the same time, so it is suitable for the control of a plurality of frequency converters. But the number of layers of neural network is too much or the algorithm is too complex will bring a lot of practical difficulties in the specific application.
(2) fuzzy control
The fuzzy control algorithm is used to control the voltage and frequency of the frequency converter to control the speed of the motor, so as to avoid the influence of the rising speed too fast on the service life of the motor and the influence of the speed rise too slow. The key of fuzzy control is the division of the domain, the membership degree and the fuzzy level, which is especially suitable for the control system of the multi input and single output.
(3) expert system
Expert system is the so-called "experts" experience to control a control mode, therefore, expert system in general to establish a database of experts, expert information storage, also have a reasoning mechanism, in order to seek for the ideal control result according to the known information. Expert database and reasoning mechanism design is particularly important, the relationship between the pros and cons of expert system control. The application of expert system can not only control the voltage of the inverter, but also can control the current.
(4) learning control
Learning control is mainly used for repetitive input, and the rules of the PWM signal (such as central modulation PWM) exactly meet this condition, so learning control can also be used in the control of the inverter. Learning control does not need to know about too much system information, but need 1-2 learning cycle, so fast and relatively poor and learning control algorithm is sometimes necessary to achieve the advanced unit, which by analog devices is can not be achieved, at the same time, learning control also involves a stability problem. In the application to pay special attention to.
3, inverter control prospects
With the development of power electronics technology, microelectronics technology, computer network and other high-tech development, inverter control mode in the future will be developed in the following aspects.
(1) the realization of the digital control frequency converter
Now, control mode of converter with digital processor can operations to achieve more complex, inverter digital is an important development direction is inverter digital mainly uses MCS51 or 80C196MC and, assisted with sle4520 or EPLD LCD to achieve better control performance.
(2) the combination of a variety of control methods
Single control method have their respective advantages and disadvantages, and there is no "universal" control mode, in some occasions and need to combine some of the ways to control, such as learning control and neural network control are combined, adaptive control and fuzzy control combined, direct torque control and neural network control combination, or called for "hybrid control", so learn from each other, the control effect will be better.
(3) the realization of remote control
The development of the computer network, the "Tianya Zhichi", relying on computer network remote control is a development direction of the inverter. Through the RS485 interface and a number of network protocols for remote control of the inverter, which is not suitable for people to conduct on-site operation of the occasion, it can be very easy to achieve control objectives.
(4) green frequency converter
With the development of the strategy of sustainable development, more and more attention has been paid to the environment protection. Noise generated by the frequency converter the grid harmonic pollution and reduce the converter work and enhance the reliability and safety of the work and so on these issues, trying to through the adoption of appropriate control method to solve design green converter.
4, the end of the language
Inverter control mode is a problem worthy of study, rely on the work of the people of the joint efforts of the people, so that the domestic inverter to the world market as soon as possible and become a first-class product.