The fundamental principle of vector control is to control the excitation current and torque current of the asynchronous motor according to the principle of magnetic field orientation through the measurement and control of the stator current vector of the asynchronous motor, and then to achieve the purpose of controlling the torque of the asynchronous motor. Specifically, the stator current vector of the asynchronous motor is decomposed into the current weight of the magnetic field (excitation current) and the current weight of the torque (torque current), and the amplitude and phase between the two weights are controlled separately, that is, the stator current vector is controlled, so this control method is called the vector control method. Vector control forms include vector control forms based on slip frequency, vector control forms without speed sensors, and vector control forms with speed sensors.
The vector control method according to the slip frequency control is also based on the stable control of U/f=, after detecting the practical speed N of the asynchronous motor, the corresponding control frequency F is obtained, and then the phase between the stator current vector and the two weights is controlled according to the expected torque, and then the output frequency F of the general-purpose inverter (Variable-frequency Drive, VFD) is controlled. The most important feature of the vector control method according to the slip frequency control method is that it can eliminate the fluctuation of torque and current in the dynamic process, and then improve the dynamic function of the general-purpose type. In the early stage, the vector manipulation general-purpose type fundamentally adopted the vector manipulation method based on the slip frequency manipulation.
The velocity sensorless vector control is based on the magnetic field-oriented control theory. In order to accomplish the accurate vector manipulation of the magnetic field orientation, it is necessary to install a magnetic flux detector in the asynchronous motor. It is difficult to install a magnetic flux detector in an asynchronous motor. However, it has been found that even if the magnetic flux detector is not directly installed in the asynchronous motor, it is possible to obtain a quantity corresponding to the magnetic flux in the general-purpose type, and then obtain the so-called vector control form without a speed sensor. The fundamental control idea is to detect the excitation current (or magnetic flux) and torque current as the fundamental control variables according to the input motor nameplate parameters according to the certain connection. By manipulating the frequency of the voltage on the stator winding of the motor, the instruction value and the detection value of the excitation current (or magnetic flux) and torque current are used together to output the torque, and then the vector control is completed.
The universal vector control system not only matches the DC motor in the speed range, but also controls the torque generated by the asynchronous motor. Because the vector control method is based on the accurate parameters of the controlled asynchronous motor, some general-purpose motor VFDs need to accurately input the parameters of the asynchronous motor when using, and some general-purpose motor VFDs need to use speed sensors and encoders, and use the VFD special motor designated by the inverter manufacturer for control, otherwise it is difficult to achieve the desired control role. Now, the new vector control general-purpose model has the functions of asynchronous motor parameter active recognition and self-adaptation. The VFD with this function can actively identify the parameters of the asynchronous motor before it is driven to normal operation, adjust the relevant parameters in the control algorithm according to the recognition results, and then perform useful vector manipulation on the ordinary asynchronous motor. In addition to the above-mentioned sensorless vector control and torque vectoring that can improve the torque control function of asynchronous motors, new technologies include the adjustment of the control constants of asynchronous motors and adaptive control to match the mechanical system. , and then improve the use function of the asynchronous motor. In order to avoid speed deviations in asynchronous motors and achieve the desired smooth speed in the low-speed region, the use of LSI and dedicated digital active voltage regulation (AVR) manipulation technology has now been put into practice and has achieved good results.
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