Control methods commonly used in frequency converters - Solutions - Huaqiang Electronic Network

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1. Non-intelligent control mode The non-intelligent control methods used in the AC drive include V/f coordinated control, slip frequency control, vector control, and direct torque control.
(1) V/f control V/f control is to obtain the ideal torque-speed characteristic, based on the idea of ​​changing the power frequency to adjust the speed while ensuring that the magnetic flux of the motor is unchanged. The inverter basically adopts this control method. The V/f control inverter structure is very simple, but the inverter adopts open-loop control mode, which can not achieve high control performance. Moreover, at low frequencies, torque compensation must be performed to change the low-frequency torque characteristics.
(2) Slip frequency control slip frequency control is a direct control torque control method. It is based on the V/f control, according to the power frequency corresponding to the actual speed of the asynchronous motor, and according to the desired Torque to adjust the output frequency of the inverter, so that the motor has a corresponding output torque. This kind of control method requires the installation of a speed sensor in the control system, and sometimes a current feedback to control the frequency and current. Therefore, this is a closed-loop control mode, which can make the frequency converter have good stability and Rapid acceleration and deceleration and load changes have good response characteristics.
(3) Vector control vector control is to control the magnitude and phase of the stator current of the motor through the vector coordinate circuit to control the excitation current and torque current of the motor in the d, q, 0 coordinate system respectively, and then control the motor. The purpose of torque. By controlling the sequence and time of each vector and the action time of the zero vector, various PWM waves can be formed to achieve various control purposes. For example, a PWM wave with the fewest number of switching times is formed to reduce switching loss. At present, the vector control methods actually applied in the frequency converter mainly include two kinds of vector control methods based on slip frequency control and vector control methods without speed sensor.
The vector control method based on the slip frequency is consistent with the steady-state characteristics of the slip frequency control method, but the vector control based on the slip frequency also controls the phase of the motor stator current through coordinate transformation to satisfy certain conditions. To eliminate fluctuations in the torque current transition process. Therefore, the vector control method based on the slip frequency can be greatly improved in output characteristics compared to the slip frequency control method. However, this control method is a closed-loop control method, and a speed sensor needs to be mounted on the motor, so the application range is limited. The speed sensorless vector control controls the excitation current and the torque current by coordinate transformation processing respectively, and then controls the excitation current and the torque current by controlling the voltage and current on the stator winding of the motor to identify the excitation current and the torque current. This control mode has a wide speed range, large starting torque, reliable operation and convenient operation, but the calculation is complicated, and generally requires a special processor for calculation. Therefore, the real-time performance is not too ideal, and the control precision 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 linkage and torque of the motor, and detect the stator flux linkage by detecting the stator resistance. The purpose is to eliminate the complicated transformation calculations such as vector control. The system is intuitive and concise, and the calculation speed and precision are improved compared with the vector control method. Even in the open loop state, it can output 100% of rated torque, and has load balancing function for multi-drag [5].
(5) Optimal control The application of the optimal control in practice varies according to the requirements. The optimization of individual parameters can be optimized for a certain control requirement according to the theory of optimal control. For example, in the control application of high-voltage inverters, two strategies of time segmentation control and phase shift control are successfully adopted to achieve the optimal voltage waveform under certain conditions.
(6) Other non-intelligent control methods In practical applications, some non-intelligent control methods are implemented in the control of the inverter, such as adaptive control, sliding mode variable structure control, difference frequency control, circulating current control, frequency control, etc. Due to space limitations, it is not exhaustive here. Interested readers can refer to the relevant literature.
2. Intelligent control methods Intelligent control methods mainly include neural network control, fuzzy control, expert system, and learning control. The use of intelligent control in the control of the frequency converter has some successful examples in specific applications.
(1) Neural network control Neural network control mode is applied in the control of the frequency converter, generally it is to carry out relatively complicated system control. At this time, the model of the system is poorly understood. Therefore, the neural network must not only complete the function of system identification, but also Take control. Moreover, the neural network control mode can control multiple inverters at the same time, so it is more suitable to control when multiple inverters are cascaded. However, too many layers of neural networks or too complex algorithms will bring a lot of practical difficulties in specific applications.
(2) Fuzzy control The fuzzy control algorithm is used to control the voltage and frequency of the inverter, so that the speed increase time of the motor is controlled to avoid the impact of the too fast speed on the service life of the motor and the slow increase of the speed affects the working efficiency. The key to fuzzy control lies in the division of the domain, membership and fuzzy level. This control method is especially suitable for multi-input and single-output control systems.
(3) Expert system Expert system is a kind of control method that uses the experience of so-called “experts”. Therefore, an expert database is generally required to store a certain expert information, and there must be a reasoning mechanism to facilitate Seek ideal control results based on known information. The design of the expert database and the reasoning mechanism is particularly important, and it is related to the advantages and disadvantages of the expert system control. The application expert system can control both the voltage of the frequency converter and its current.
(4) Learning Control The learning control is mainly used for repetitive input, and the regular PWM signal (such as central modulation PWM) just meets this condition, so the learning control can also be used in the control of the frequency converter. Learning control does not need to know too much system information, but it takes 1~2 learning cycles, so the rapidity is relatively poor. Moreover, the learning control algorithm sometimes needs to implement the advanced link, which is impossible to achieve with analog devices. Learning control also involves a problem of stability, and special attention should be paid to the application.