Abstract: The basic principle of variable frequency drive (VFD) vector control is to measure and control the stator current vector of an asynchronous motor, and control the excitation current and torque current of the asynchronous motor according to the field orientation principle, thereby achieving the purpose of controlling the torque of the asynchronous motor.
Vector control specifically decomposes the stator current vector of an asynchronous motor into a current component that generates the magnetic field (excitation current) and a current component that generates torque (torque current), and controls them separately, while simultaneously controlling the amplitude and phase between the two components. In other words, it controls the stator current vector, hence the name vector control. Vector control methods include slip frequency-based vector control, sensorless vector control, and vector control with a speed sensor.
Vector control based on slip frequency control, also based on constant U/f control, detects the actual speed n of the asynchronous motor to obtain the corresponding control frequency f, and then controls the stator current vector and the phase between the two components according to the desired torque, thereby controlling the output frequency f of the general-purpose frequency inverter. The biggest advantage of vector control based on slip frequency control is that it can eliminate torque current fluctuations during dynamic processes, thus improving the dynamic performance of the general-purpose frequency inverter. Early vector control general-purpose frequency inverters basically adopted vector control based on slip frequency control.
Sensorless vector control is based on the theory of field-oriented control. Achieving precise field-oriented vector control requires installing a flux detection device within the asynchronous motor. However, installing such a device is difficult. It was discovered that even without directly installing the device within the motor, a quantity corresponding to the flux can be obtained within a general-purpose frequency converter, leading to the so-called sensorless vector control method. Its basic control idea is to detect the excitation current (or flux) and torque current, which are the basic control quantities, according to certain formulas based on the input motor nameplate parameters. By controlling the frequency of the voltage on the motor stator windings, the commanded values of the excitation current (or flux) and torque current are made consistent with the detected values, and torque is output, thus achieving vector control.
General-purpose frequency converters employing vector control can not only match the speed range of DC motors but also control the torque generated by asynchronous motors. Because vector control relies on accurate parameters of the controlled asynchronous motor, some general-purpose frequency converters require precise input of these parameters, while others require speed sensors and encoders, and must use manufacturer-specified motors for control; otherwise, achieving ideal control results is difficult. Currently, newer vector control general-purpose frequency converters possess automatic asynchronous motor parameter identification and adaptive functions. These converters automatically identify the asynchronous motor parameters before driving it to normal operation and adjust relevant parameters in the control algorithm based on the identification results, thus achieving effective vector control of ordinary asynchronous motors. Besides sensorless vector control and torque vector control, which improve asynchronous motor torque control performance, current new technologies also include adjusting the asynchronous motor control constant and adaptive control to match the mechanical system, further enhancing the application performance of asynchronous motors. To prevent speed deviations in asynchronous motors and to achieve a smoother speed in the low-speed range, a control method using large-scale integrated circuits and dedicated digital automatic voltage regulation (AVR) control technology has been put into practical use and has achieved good results.
