summary
Researchers Xiao Qinghao, Tang Ningping, and others from the School of Electrical Engineering and Automation at Fuzhou University published an article in the first issue of the journal "Electrical Technology" in 2019, proposing a hybrid control method for variable frequency speed regulation based on a combination of external control and automatic control for two different variable frequency speed regulation control methods for permanent magnet synchronous motors .
This method combines the high precision of manually controlled variable frequency speed regulation with the non-synchronous operation of automatically controlled variable frequency speed regulation, thus improving the dynamic and steady-state performance of the control system. A simulation model demonstrating automatic switching between the two was established in the Matlab/Simulink platform. Simulation results show that the proposed method is effective and feasible.
Permanent magnet synchronous motors (PMSMs) possess advantages such as simple structure, reliable operation, high power density, and high efficiency, making them easy to integrate into high-performance speed control systems. Furthermore, with the development of power electronics, motor control, and computer technologies, as well as the reduction in the price of permanent magnet materials, they are widely used in numerous fields including household appliances, transportation, industrial control, and aerospace, demonstrating significant application value in electric drive systems.
In variable frequency speed control systems, there are two different control methods: one is externally controlled variable frequency speed control, and the other is self-controlled variable frequency speed control. Each of the two control methods has its own advantages and disadvantages.
Externally controlled variable frequency speed regulation, also known as constant voltage-frequency ratio control, generates a rotating magnetic field in the air gap at a corresponding frequency by providing the frequency of the motor's stator current. The motor speed is strictly synchronized with the rotating magnetic field in the air gap, thus ensuring the high precision of externally controlled variable frequency speed regulation. In addition, externally controlled variable frequency speed regulation systems also have advantages such as simple control circuit structure and convenient adjustment.
However, this control method has an inherent drawback: the problem of losing synchronization. Automatic variable frequency speed regulation, on the other hand, uses a rotor position detector mounted on the motor shaft to control the output frequency, forming a closed-loop system where the stator current frequency automatically tracks the rotor position. This method fundamentally eliminates the risks of rotor oscillation and losing synchronization because the output frequency of the frequency converter supplying power to the motor stator windings is controlled by the rotor position detector, always maintaining synchronization. Therefore, it avoids loss of synchronization caused by sudden load changes, but the control system is relatively complex. Furthermore, a certain amount of static error will exist due to factors such as the accuracy of the position detector and the control parameters of the regulator.
Combining the advantages of the two control methods mentioned above, this paper proposes a hybrid control method for permanent magnet synchronous motors based on a combination of external control and automatic control [9-11], in order to better leverage the advantages of both variable frequency speed control methods. By establishing a corresponding system model on the Matlab/Simulink simulation platform and conducting simulation experiments, the simulation results demonstrate that the proposed method is effective and feasible.
Block diagram of a speed control system combining external and automatic control
in conclusion
This paper analyzes the characteristics of both external and automatic control methods and proposes a hybrid speed control method that combines external and automatic control with automatic switching. This speed control method employs a dual closed-loop control mode of speed and current during startup, enabling rapid motor startup, strong load-carrying capacity, avoiding the risk of loss of synchronism, and exhibiting excellent dynamic performance.
When the switching conditions are met, the system switches to external control mode. The switching process is smooth and rapid, without any transient inrush current. Under external control, the system has higher steady-state accuracy and a certain load-carrying capacity. When the load torque changes in the external control stable region, the system can return to steady state after a short period of self-adjustment by utilizing the motor's "torque-power angle self-balancing" characteristic. When the given speed changes, the system quickly switches to automatic control mode to avoid loss of synchronization. It then switches back to external control mode when the switching conditions are met, thus achieving automatic switching between external and automatic control.
This method allows for smooth switching between the two control modes, fully leveraging the advantages of high steady-state accuracy in external control and good dynamic speed and non-stepping characteristics in automatic control, thus improving the system's steady-state and dynamic performance. However, this method also has certain limitations; the automatic control mode requires a position sensor to enhance the speed signal, and it does not achieve completely sensorless control. This method has certain application value in situations where rapid speed adjustment and high speed accuracy are required during steady-state operation, such as in the textile, chemical fiber, and glass industries.
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