Vector Control

 

The basic principle of vector control is to measure and control the stator current vector of asynchronous motor. According to the principle of field orientation, the excitation current and the torque current of asynchronous motor are controlled respectively, so as to control the torque of asynchronous motor. Specifically, the stator current vector of asynchronous motor is decomposed into the current component (excitation current) which generates magnetic field and the current component (torque current) which generates torque, and the amplitude and phase between the two components are controlled simultaneously, that is, the stator current vector is controlled, so this control method is called vector control method. Vector control mode includes vector control mode based on slip frequency control, vector control mode without speed sensor and vector control mode with speed sensor.

 

1. Vector Control Based on Slip Frequency Control

 

The vector control method based on slip frequency control is also based on U/f=constant control. By detecting the actual speed n of the asynchronous motor, the corresponding control frequency f is obtained. Then, according to the desired torque, the stator current vector and the phase between the two components are respectively controlled to transmit the universal frequency converter. The output frequency f is controlled. The most important feature of vector control 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 universal frequency converter. Early vector control universal converters were basically based on slip frequency control vector control mode.

 

2. Vector Control without Speed Sensor

 

Vector control without speed sensor is based on the theory of field oriented control. Accurate field-oriented vector control requires the installation of flux detection devices in asynchronous motors. It is very difficult to install flux detection devices in asynchronous motors. However, it has been found that even if the flux detection devices are not directly installed in asynchronous motors, the corresponding flux can be obtained in general frequency converters. The so-called speed sensorless vector control method is obtained. Its basic control idea is to detect the excitation current (or flux) and the torque current as the basic control variables according to the nameplate parameters of the input motor and the calculation formula of the torque, and to make the excitation current (or flux) and the torque current indicate by controlling the frequency of the voltage on the stator winding of the motor. The vector control is realized by making the values consistent with the detection values and outputting the torque.

 

The general frequency converter with vector control mode can not only match the DC motor in the speed range, but also control the torque produced by the asynchronous motor. Because the vector control method is based on the accurate parameters of the controlled asynchronous motor, some general inverters need to input the parameters of the asynchronous motor accurately when they are in use, some general inverters need to use speed sensors and coders, and need to use the special motor of the inverters designated by the manufacturer to control, otherwise. It is difficult to achieve ideal control effect. At present, the new vector control universal frequency converter has the functions of automatic parameter detection, automatic discrimination and self-adaptive. The universal frequency converter with this function can automatically identify the parameters of the asynchronous motor before driving the asynchronous motor to run normally, and adjust the control according to the identification results. The parameters in the algorithm can be used to control the general asynchronous motor effectively.

 

In addition to the above sensorless vector control and torque vector control technologies, which can improve the performance of asynchronous motor torque control, the current new technologies also include the regulation of the control constants of asynchronous motor and the adaptive control matching with the mechanical system, so as to improve the application performance of asynchronous motor. In order to prevent the speed deviation of asynchronous motors and obtain a better smooth speed in the low speed area, the control method of large-scale integrated circuit and special digital automatic voltage adjustment (AVR) control technology has been applied and achieved good results.