A frequency converter is a commonly used electrical energy control device with excellent energy-saving properties, and it is used in many electronic devices. Frequency converters have two control modes: non-intelligent control and intelligent control. Users need to understand both control modes to make future use of the frequency converter more convenient. Today, we will introduce these two control modes of frequency converters in detail.
Non-intelligent control method
Non-intelligent control methods used in AC frequency converters include V/f coordinated control, slip frequency control, vector control, and direct torque control.
(1) V/f control
V/f control is proposed to achieve ideal torque-speed characteristics. It is based on the idea of adjusting the speed by changing the power supply frequency while keeping the motor's magnetic flux constant. Most general-purpose frequency converters use this control method. V/f control frequency converters have a very simple structure, but they use an open-loop control method, which cannot achieve high control performance. Furthermore, at low frequencies, torque compensation is necessary to change the low-frequency torque characteristics.
(2) Slip frequency control
Slip frequency control is a direct torque control method. Based on V/f control, it adjusts the inverter's output frequency according to the power supply frequency corresponding to the actual speed of the asynchronous motor and the desired torque, thus enabling the motor to achieve the corresponding output torque. This control method requires a speed sensor in the control system, and sometimes current feedback is added to control both frequency and current. Therefore, it is a closed-loop control method, which gives the inverter good stability and excellent response characteristics to rapid acceleration/deceleration and load changes.
(3) Vector control
Vector control uses vector coordinate circuits to control the magnitude and phase of the motor's stator current, thereby controlling the excitation current and torque current of the motor in the d, q, and 0 coordinate systems, and ultimately controlling the motor's torque. By controlling the sequence and timing of the action of each vector, as well as the duration of the zero vector, various PWM waves can be generated to achieve different control objectives.
(4) Direct Torque Control
Direct torque control (DTC) utilizes the concept of spatial vector coordinates to analyze the mathematical model of an AC motor in the stator coordinate system, controlling the motor's flux linkage and torque. It achieves this by detecting the stator resistance, thus eliminating the complex transformation calculations required by vector control. The system is intuitive and simple, with improved calculation speed and accuracy compared to vector control. Even in open-loop operation, it can output 100% of the rated torque and provides load balancing for multi-drive applications.
(5) Optimal control
The practical applications of optimal control vary depending on the requirements. Based on optimal control theory, individual parameters can be optimized for a specific control requirement. For example, in the control applications of high-voltage frequency converters, time-segmented control and phase-shift control strategies have been successfully employed to achieve the optimal voltage waveform under certain conditions.
Intelligent control method
Intelligent control methods mainly include neural network control, fuzzy control, expert systems, and learning control. There are some successful examples of using intelligent control methods in the control of frequency converters.
(1) Neural network control
Neural network control is typically used in inverter control for complex systems where little is known about the system model. Therefore, the neural network must perform both system identification and control. Furthermore, neural network control can control multiple inverters simultaneously, making it suitable for cascaded inverter configurations. However, an excessively large number of neural network layers or overly complex algorithms can introduce practical difficulties in real-world applications.
(2) Fuzzy control
Fuzzy control algorithms are used to control the voltage and frequency of frequency converters, thereby controlling the acceleration time of the motor to avoid the impact of excessive acceleration on motor lifespan and excessive acceleration on work efficiency. The key to fuzzy control lies in the universe of discourse, membership degree, and fuzzy level division. This control method is particularly suitable for multi-input single-output control systems.
(3) Expert System
Expert systems are a control method that utilizes the experience of so-called "experts." Therefore, an expert system typically requires an expert database to store expert information, as well as an inference mechanism to seek the ideal control result based on known information. The design of the expert database and the inference mechanism is particularly important, as they determine the quality of the expert system's control. Expert systems can control both the voltage and current of frequency converters.
(4) Learning control
Learning control is primarily used for repetitive inputs, and regular PWM signals (such as center-modulated PWM) perfectly meet this condition. Therefore, learning control can also be used in inverter control. Learning control doesn't require much system information, but it does need 1-2 learning cycles, resulting in relatively poor speed. Furthermore, the algorithm sometimes requires a lead element, which is impossible with analog devices. Additionally, learning control involves stability issues, requiring special attention during application.
