Vector control, also known as magnetic field steering control, is a technique that uses a frequency converter to control a three-phase AC motor. The output frequency of the inverter, the magnitude and angle of the output voltage are used to control the output of the motor. Its characteristic is that it can individually control the magnetic field and torque of the motor, similar to the characteristics of its exciting DC motor. Vector control can be applied to AC induction motors and DC brushless motors. The purpose of early development is for high-performance motor applications. It can operate over the entire frequency range, output rated torque at zero speed of the motor, and fast acceleration and deceleration.
Because the dynamic mathematical model of asynchronous motor is a high-order, nonlinear, strongly coupled multivariable system. It was proposed by K. Hasse of TU Darmstadt in the late 1960s. In the early 1970s, the three-dimensional motor field oriented control method was proposed by the Siemens engineer F. Blaschke in the doctoral thesis published by the University of Braunschweig (TU Braunschweig). The asynchronous motor vector control theory was used to solve the AC motor torque control problem. The basic principle of vector control realization is to measure and control the stator current vector of the asynchronous motor, and control the excitation current and torque current of the asynchronous motor according to the principle of magnetic field orientation, so as to achieve the purpose of controlling the torque of the asynchronous motor.
The basic principle of vector control is to measure and control the stator current vector of the asynchronous motor, and control the excitation current and torque current of the asynchronous motor according to the principle of magnetic field orientation, so as to achieve the purpose of controlling the torque of the asynchronous motor. Specifically, the stator current vector of the asynchronous motor is decomposed into a current component (excitation current) that generates a magnetic field and a current component (torque current) that generates a torque, and simultaneously controls the amplitude and phase between the two components, that is, the control. The stator current vector, so this control method is called the vector control method. The vector control method has a vector control method based on slip frequency control, a speed sensorless vector control method, and a vector control method with a speed sensor.
Vector control system, coordinate transformation is the core idea. The basic idea of ​​vector control is to generate the same rotating magnetomotive force as the criterion, and the stator alternating current of the asynchronous motor in the stationary three-phase coordinate system is equivalent to the alternating current on the two-phase stationary coordinate system, which will be transformed by coordinate rotation. It is equivalent to the DC current on the synchronous rotating coordinate system, and the decoupling control of the magnetic flux and torque is realized in the equivalent process to achieve the control effect of the DC motor, and the control amount of the DC motor is obtained. The three-phase asynchronous motor can be controlled by a DC motor to obtain dynamic and static performance close to the DC speed control system.
The vector control method based on the slip frequency control is also based on the U / f = constant control, by detecting the actual speed n of the asynchronous motor, and obtaining the corresponding control frequency f, and then separately controlling according to the desired torque. The stator current vector and the phase between the two components control the output frequency f of the general-purpose inverter. The biggest feature of the vector control method based on slip frequency control is that it can eliminate the fluctuation of torque current in the dynamic process, thus improving the dynamic performance of the general-purpose inverter. The early vector control general-purpose inverters basically adopted the vector control method based on the slip frequency control.
The vector control method without speed sensor is based on the development of magnetic field oriented control theory. To achieve accurate magnetic field oriented vector control, it is necessary to install a magnetic flux detecting device in an asynchronous motor. It is difficult to install a magnetic flux detecting device in the asynchronous motor, but it has been found that even if the magnetic flux detecting device is not directly installed in the asynchronous motor, The amount corresponding to the magnetic flux can be obtained inside the general-purpose inverter, and thus a so-called vector control method without a speed sensor is obtained. Its basic control idea is to detect the excitation current (or flux) and torque current as the basic control amount according to the input name of the motor according to the nameplate parameters of the input motor, and by controlling the voltage on the stator winding of the motor. The frequency makes the excitation current (or magnetic flux) and the torque current command value and the detected value coincide, and outputs the torque, thereby implementing vector control.
The general-purpose inverter with vector control can not only match the DC motor in the speed regulation range, but also control the torque generated by the asynchronous motor. Since the vector control mode is based on the parameters of the accurately controlled asynchronous motor, some general-purpose inverters need to accurately input the parameters of the asynchronous motor when they are used. Some general-purpose inverters need to use the speed sensor and the encoder, and need to use The inverter-specific motor specified by the manufacturer is controlled, otherwise it is difficult to achieve the desired control effect.
At present, the new vector control general-purpose frequency converter has the automatic identification and adaptive function of asynchronous motor parameters. The general-purpose frequency converter with this function can automatically identify the parameters of the asynchronous motor before driving the asynchronous motor to perform normal operation, and according to The identification result adjusts the relevant parameters in the control algorithm to perform effective vector control on the ordinary asynchronous motor. In addition to the above-mentioned sensorless vector control and torque vector control, etc., which can improve the torque control performance of the asynchronous motor, the current new technology also includes the adjustment of the control constant of the asynchronous motor and the adaptive control matching with the mechanical system. Techniques for improving the performance of asynchronous motor applications. In order to prevent the asynchronous motor speed deviation and obtain a smoother speed in the low speed region, the application of a large-scale integrated circuit and a dedicated digital automatic voltage adjustment (AVR) control technology has been put into practical use and achieved good results.
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