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1. Development of control methods for AC motors
(1) Constant voltage-to-frequency ratio control mode, which performs frequency conversion speed regulation according to the asynchronous motor equivalent circuit. Its characteristics are: the control circuit has a simple structure and low cost. Voltage refers to the effective value of the fundamental wave. Changing the voltage can only adjust the steady-state flux and torque of the motor, but cannot be dynamically controlled. The control curve will change with the load, the torque response is slow, and the motor torque utilization is not high.
(2) Vector control mode. The theory and practice of AC drive control finally made a breakthrough in the 1970s, that is, the emergence of vector control technology. The essence is that the AC motor is equivalent to a DC motor, and the two components of speed and magnetic field are independently controlled. By controlling the rotor flux linkage, the rotor flux is oriented, and then the stator current is decomposed to obtain two components of torque and magnetic field, and coordinate transformation is used to realize orthogonal or decoupling control. Thus, the motor model reconstructed by coordinate transformation can be equivalent to a DC motor. The vector control method realizes the decoupling control of the magnetic flux and torque of the asynchronous motor, which makes the dynamic characteristics of the AC drive system significantly improved, and opens up a new era of AC drive. However, in practical systems, due to the difficulty of accurate observation of the rotor flux linkage and the complexity of vector rotation transformation, the actual control effect is not as good as theoretical analysis.
This is a shortcoming in the practice of vector control technology. Experts and scholars in the field of AC drive have also done a lot of research on the defects in the research and development of vector pure electric vehicle AC asynchronous motor and vehicle assembly controller, such as parameter identification and modern control theory using state observer. However, the introduction of these solutions makes the system more complicated, and the real-time and reliability of control is reduced.
(3) Direct torque control mode. Direct torque control is a new type of high-performance AC variable frequency speed control technology developed after vector control frequency conversion speed regulation technology in recent years. Professor Depenbrock of Ruhr University in Germany first proposed the theory of direct torque control in 1985, and then extended it to the field of weak magnetic speed regulation in 1987. This control method is to make the stator flux linkage move according to the regular hexagonal trajectory. Since the six sides of the regular hexagon correspond to the six non-zero voltage space vectors respectively, the inverter can be simply switched by three Schmitt triggers. The six working states directly implement flux linkage control through six non-zero voltage space vectors. Compared with other methods, this control method has a simple structure, and the number of component switching is the least when the same frequency is output, and the switching loss is also small, so it is widely used in a high-power occasion where the component switching frequency is not too high. Since the stator flux linkage moves according to the hexagonal trajectory in this method, the voltage and current waveform distortion is severe, and the torque ripple is large at low speed, which limits the performance of the direct torque control to a certain extent. Another form of direct torque control is proposed by Japanese scholar Takahashi, which is a control scheme in which the stator flux linkage trajectory is approximately circular. This method selects the corresponding switch vector by calculating the motor torque and flux linkage error in real time, combined with the spatial position of the motor stator flux linkage. Since the magnetic chain motion trajectory is approximately circular, the harmonic content in voltage and current is reduced to some extent, but the control system is complicated. This control method can fully exploit the switching frequency advantage of the new power electronic device. Medium and small power applications are widely used.
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