Variable frequency speed control technology is an important development direction of modern electric drive technology. As the core of the variable frequency speed control system, the performance of the frequency converter is increasingly becoming the decisive factor in the speed control performance. Besides the inherent conditions of the frequency converter's manufacturing process, the control method adopted is also crucial. This article, based on industrial practice, reviews the characteristics of various frequency converter control methods in recent years and looks forward to future development directions. 1. Introduction to Frequency Converters 1.1 Basic Structure of Frequency Converters A frequency converter is a device that converts industrial frequency power (50Hz or 60Hz) into AC power of various frequencies to achieve variable speed operation of a motor. The control circuit controls the main circuit, the rectifier circuit converts AC to DC, the DC intermediate circuit smooths and filters the output of the rectifier circuit, and the inverter circuit converts DC back into AC. For frequency converters requiring a large amount of computation, such as vector control frequency converters, a CPU for torque calculation and some corresponding circuits are sometimes also needed. 1.2 Classification of frequency converters There are many ways to classify frequency converters. According to the main circuit working mode, they can be divided into voltage-type frequency converters and current-type frequency converters; according to the switching mode, they can be divided into PAM control frequency converters, PWM control frequency converters and high-carrier frequency PWM control frequency converters; according to the working principle, they can be divided into V/f control frequency converters, slip frequency control frequency converters and vector control frequency converters; according to the application, they can be divided into general-purpose frequency converters, high-performance special frequency converters, high-frequency frequency converters, single-phase frequency converters and three-phase frequency converters. 2. Commonly used control methods in frequency converters 2.1 Non-intelligent control methods Non-intelligent control methods used in AC frequency converters include V/f coordinated control, slip frequency control, vector control, direct torque control, etc. (1) V/f control V/f control is proposed to obtain ideal torque-speed characteristics. It is based on the idea of ​​changing the power supply frequency to adjust the speed while ensuring that the magnetic flux of the motor remains unchanged. General-purpose frequency converters basically adopt this control method. V/f control inverter has a very simple structure, but this type of inverter adopts an open-loop control method, which cannot achieve high control performance. Moreover, at low frequencies, torque compensation must be performed to change the low-frequency torque characteristics. (2) Slip frequency control Slip frequency control is a direct torque control method. It is based on V/f control. According to the power supply frequency corresponding to the actual speed of the asynchronous motor, and according to the desired torque, the output frequency of the inverter is adjusted so that the motor has the corresponding output torque. This control method requires the installation of a speed sensor in the control system. Sometimes current feedback is also added to control the frequency and current. Therefore, this is a closed-loop control method, which can make the inverter have good stability and good response characteristics to rapid acceleration and deceleration and load changes. (3) Vector control Vector control controls the magnitude and phase of the stator current of the motor through a vector coordinate circuit to control the excitation current and torque current of the motor in the d, q, and 0 coordinate system, thereby achieving the purpose of controlling the motor torque. By controlling the action sequence and time of each vector and the action time of the zero vector, various PWM waves can be formed to achieve various different control purposes. For example, generating a PWM wave with the fewest switching operations can reduce switching losses. Currently, the vector control methods practically used in frequency converters mainly include slip frequency-based vector control and sensorless vector control. Slip frequency-based vector control and slip frequency control have the same steady-state characteristics, but slip frequency-based vector control requires coordinate transformation to control the phase of the motor stator current, ensuring it meets certain conditions to eliminate fluctuations during torque current transition. Therefore, slip frequency-based vector control offers significantly improved output characteristics compared to slip frequency control. However, this control method is a closed-loop control, requiring a speed sensor on the motor, thus limiting its application. Sensorless vector control controls the excitation current and torque current separately through coordinate transformation, and then identifies the speed by controlling the voltage and current on the motor stator windings to achieve the purpose of controlling the excitation current and torque current. This control method has a wide speed range, large starting torque, reliable operation, and convenient operation, but the calculation is relatively complex and generally requires a special processor for calculation. Therefore, the real-time performance is not ideal, and the control accuracy is affected by the calculation accuracy. (4) Direct Torque Control Direct torque control uses the concept of spatial vector coordinates to analyze the mathematical model of the AC motor in the stator coordinate system, control the motor flux linkage and torque, and achieve the purpose of observing the stator flux linkage by detecting the stator resistance. Therefore, it eliminates the complex transformation calculations of vector control, and the system is intuitive and simple. The calculation speed and accuracy are improved compared with the vector control method. Even in the open-loop state, it can output 100% of the rated torque and has a load balancing function for multiple drives. (5) Optimal Control The application of optimal control in practice varies depending on the requirements. It can optimize individual parameters for a certain control requirement based on the theory of optimal control. For example, in the control application of high voltage frequency converters, time segmentation control and phase shift control strategies have been successfully adopted to achieve the optimal voltage waveform under certain conditions. (6) Other non-intelligent control methods In practical applications, there are also some non-intelligent control methods that can be implemented in the control of frequency converters, such as adaptive control, sliding mode variable structure control, differential frequency control, circulating current control, frequency control, etc. 2.2 Intelligent control methods Intelligent control methods mainly include neural network control, fuzzy control, expert system, learning control, etc. There are some successful examples of using intelligent control methods in the control of frequency converters. (1) Neural network control Neural network control is generally used in the control of frequency converters for relatively complex systems. At this time, the system model is little understood. Therefore, the neural network must perform both system identification and control. Moreover, the neural network control method can control multiple frequency converters at the same time, so it is more suitable for control when multiple frequency converters are cascaded. However, too many layers of the neural network or too complex algorithms will bring many practical difficulties in specific applications. (2) Fuzzy control Fuzzy control algorithms are used to control the voltage and frequency of the frequency converter, so that the acceleration time of the motor can be controlled to avoid the impact of excessive acceleration on the service life of the motor and excessive acceleration on the working efficiency. The key to fuzzy control lies in the division of the domain, membership degree, and fuzzy level. This control method is particularly suitable for multi-input single-output control systems. (3) Expert system Expert system is a control method that uses the experience of so-called "experts" for control. Therefore, an expert database is generally established in an expert system to store certain expert information. In addition, there is also an inference mechanism to seek the ideal control result based on the known information. The design of the expert database and the inference mechanism is particularly important and is related to the quality of expert system control. The application of expert system can control both the voltage and current of the frequency converter. (4) Learning control Learning control is mainly used for repetitive inputs, and regular PWM signals (such as center modulation PWM) just meet this condition. Therefore, learning control can also be used in the control of frequency converters. Learning control does not need to know too much system information, but it requires 1 to 2 learning cycles. Therefore, its speed is relatively poor. Moreover, the algorithm of learning control sometimes needs to implement a lead element, which cannot be achieved with analog devices. At the same time, learning control also involves a stability problem, which should be paid special attention to when applying it. 3. Prospects for Variable Frequency Drive Control With the development of high-tech technologies such as power electronics, microelectronics, and computer networks, the control methods of variable frequency drives will develop in the following aspects in the future. (1) Realization of digital control of variable frequency drives. Currently, variable frequency drive control can achieve relatively complex calculations using digital processors. Variable frequency drive digitization will be an important development direction. At present, variable frequency drive digitization mainly uses single-chip microcomputers such as MCS51 or 80C196MC, supplemented by SLE4520 or EPLD LCD displays to achieve more complete control performance. (2) Combination of multiple control methods. A single control method has its own advantages and disadvantages. There is no "universal" control method. In some control situations, it is necessary to combine some control methods, such as combining learning control with neural network control, adaptive control with fuzzy control, and direct torque control with neural network control, or what is called "hybrid control". This way, the advantages are combined and the control effect will be better. (3) Realization of remote control. The development of computer networks makes "the ends of the earth seem close". Relying on computer networks to remotely control variable frequency drives is also a development direction. Remote control of the frequency converter is achieved through RS485 interface and some network protocols, making it easy to achieve control objectives even in situations where human on-site operation is not suitable. (4) Green Frequency Converter With the introduction of the sustainable development strategy, environmental protection has received increasing attention. The high-order harmonics generated by the frequency converter will pollute the power grid. Reducing the noise of the frequency converter during operation and enhancing its reliability and safety are all issues that are being addressed by adopting appropriate control methods to design a green frequency converter. 4. Conclusion The control method of the frequency converter is a problem worth studying. With the joint efforts of insightful people dedicated to this work, domestic frequency converters can enter the world market as soon as possible and become first-class products.