Frequency speed control technology is an important development direction of modern electric drive technology. As the core of inverter speed control system, the performance of inverter has increasingly become the decisive factor of speed performance. In addition to the inherent "Conditions, the inverter used what kind of control is also very important.
Based on the industrial reality, this article summarizes the characteristics of various inverter control methods in recent years and looks forward to the future development direction.
1 Inverter Introduction
1.1 The basic structure of the inverter Inverter is the power frequency power (50Hz or 60Hz) into a variety of frequency AC power to achieve the motor variable speed operation of the device, the structure shown in Figure 1, in which the control circuit to complete the main Circuit control, rectifier circuit will be converted into DC AC, DC intermediate circuit smoothing the output of the rectifier circuit smoothing, the inverter will be inverse DC into AC. For a frequency converter such as a vector control inverter, which requires a large number of operations, it is sometimes necessary to have a CPU for torque calculation and some corresponding circuits.
1.2 Classification of Inverter There are many kinds of classification methods for inverters. According to the working mode of the main circuit, they can be divided into voltage type inverter and current type inverter. According to the switch mode, they can be divided into PAM control inverter, PWM control inverter According to the working principle of classification, can be divided into V / f control inverter, slip frequency control inverter and vector control inverter; According to the use of classification, can be divided into general-purpose inverter, high Performance-specific inverter, high-frequency converter, single-phase inverter and three-phase inverter.
2 Inverter commonly used control methods
2.1 non-intelligent control
Non-intelligent control methods used in the AC drive are V / f coordination control, slip frequency control, vector control, direct torque control.
(1) V / f control
V / f control in order to get the ideal torque - speed characteristics, based on changing the frequency of the power supply speed at the same time, but also to ensure that the motor flux unchanged idea put forward, the universal inverter basically use this Kind of control method. V / f control inverter structure is very simple, but this drive uses open-loop control, can not achieve higher control performance, and, at low frequencies, 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 method, which is based on the V / f control, according to know the actual speed of the asynchronous motor corresponding to the power supply frequency, and based on the desired torque to adjust the inverter Output frequency, you can make the motor has a corresponding output torque. This control method, the need to install the speed sensor in the control system, and sometimes also added current feedback, the frequency and current control, therefore, this is a closed-loop control, the inverter can have good stability, and Rapid acceleration and deceleration and load changes have good response characteristics.
(3) vector control
Vector control is through the vector coordinate circuit to control the size and phase of the motor stator current, in order to achieve the motor in the d, q, 0 axis of the excitation current and torque current control, respectively, to achieve the purpose of controlling the motor torque. By controlling the order 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, the formation of the PWM wave with the least number of switching to reduce switching losses [3]. At present, the actual vector control methods used in frequency converters mainly include two kinds of vector control methods based on slip frequency control and vector control methods without speed sensor.
However, the vector control based on the slip frequency also needs coordinate transformation to control the phase of the motor stator current so as to satisfy the certain conditions. The vector control method based on the slip frequency is the same as the slip frequency control method. To eliminate the fluctuations in the torque current transition. Therefore, the vector control method based on the slip frequency can greatly improve the output characteristics compared with the slip frequency control method. However, this control method belongs to the closed-loop control method, 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 by coordinate transformation processing, and then to control the excitation current and torque current by controlling the voltage and current on the stator winding of the motor. This control mode has a wide range of speed control, large starting torque, reliable operation and convenient operation [4]. However, the calculation is rather complicated and generally requires a special processor for calculation. Therefore, the real-time performance is not so good and the control accuracy is calculated The impact of precision.
(4) direct torque control
Direct torque control is to use the concept of space vector coordinate to analyze the mathematical model of AC motor in stator coordinate system, control motor flux linkage and torque, and detect the stator resistance to achieve the purpose of observing stator flux. Therefore, Vector control and other complex transformation calculation, the system is intuitive, concise, the calculation speed and accuracy than the vector control method has been improved. Even in the open-loop state, it can output 100% of the rated torque, with multi-drag load balancing [5].
(5) Optimal control
Optimal control in the actual application of the different requirements are different, according to the theory of optimal control of a particular control requirements for the optimization of individual parameters. For example, in high-voltage inverter control applications, the successful use of the time-phased control and phase-shift control of two strategies to achieve the optimal voltage under certain conditions waveform.
(6) other non-intelligent control methods
In practice, there are some non-intelligent control methods can be achieved in the inverter control, such as adaptive control, sliding mode variable structure control, frequency control, circulation control, frequency control, due to space limitations, no longer tired here Stated, interested readers can refer to the relevant literature.
2.2 intelligent control method
Intelligent control methods are mainly neural network control, fuzzy control, expert system, learning control. In the control of the inverter using intelligent control methods in the specific application has some successful examples.
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