Energy demand is significantly impacting global economic development. China also faces pressure from economic growth on energy demand. In the 1990s, the energy consumption of high-energy-consuming products in China was 12-55% higher than in developed countries, and the overall energy utilization efficiency was only 32%. China urgently needs to improve energy efficiency. Electric motors are among the largest energy consumers. China's total installed capacity of electric motors has reached 400 million kilowatts, with an annual electricity consumption of 600 billion kilowatt-hours, accounting for 80% of industrial electricity consumption. However, even now, over 80% of the various types of motors in use in China are still small and medium-sized asynchronous motors, indicating a huge potential for energy conservation in the field of electric motors. Variable frequency drive (VFD) technology is the most prominent energy-saving technology for electric motors. However, although research on VFD technology in China is very active, industrialization is still far from ideal, with foreign products occupying almost 60% of the Chinese VFD technology market. The current state of VFD technology: The 20th century was the era of the birth and development of VFD technology. Especially since the 1990s, the development of new power electronic devices such as IGBTs and IGCTs (Integrated Gate Commutated Thyristors), the rapid development of DSPs (Digital Signal Processors) and ASICs (Application-Specific Integrated Circuits), and the improvement of novel control theories and technologies (such as field-oriented vector control and direct torque control) have enabled variable frequency speed control systems to surpass DC speed control systems in performance indicators such as speed range, speed accuracy, dynamic response, power factor, operating efficiency, and ease of use. They have reached the point of replacing DC speed control, gaining popularity across various industries and achieving significant economic benefits. The development trend of variable frequency speed control technology: 1. High-voltage, high-power variable frequency speed control systems: In China, low-voltage variable frequency speed control devices have gained user recognition, with a total market size reaching approximately 4 billion RMB in 2000, demonstrating their energy-saving effects. Statistics show that the number of low-voltage (below 690V) motors in China is dozens of times that of high-voltage motors, but their energy consumption is only one-eighth that of high-voltage motors. The recent emergence of high-voltage, high-current power devices and the development of parallel and series technologies have enabled high-voltage, high-power variable frequency speed control, achieving an average energy saving of up to 30%, demonstrating significant energy efficiency. However, my country is still in its initial stages, and many technologies, such as multi-level voltage inverters and transformer-coupled multi-pulse inverters, need to catch up rapidly. Both new projects like the West-to-East Gas Transfer Project and the South-to-North Water Transfer Project, as well as technological upgrading projects, represent a huge potential market for high-voltage, high-power variable frequency speed control systems. 2. The development of permanent magnet synchronous motors and their control systems: The development of variable frequency devices with fast current tracking systems, DSP signal processors, and high-performance neodymium iron boron permanent magnet materials has brought vitality to the development of various permanent magnet synchronous motors and their control systems. Permanent magnet synchronous wheelless motors and their control systems are a new generation of green elevator drive devices. Foreign elevator-specific variable frequency devices have very comprehensive software support, can accept feedback signals from sensors at any position, have self-learning functions, automatically identify motor parameters, and automatically perform initial positioning when achieving magnetic field-oriented servo control, exhibiting excellent linear torque control characteristics similar to DC motors. Its small size, high efficiency, high power factor, low vibration, low noise, and good leveling accuracy make it widely used in high-rise buildings, machine-room-less elevators, and small home elevators. However, the motor needs to directly output large torque and reduce low-speed torque fluctuations, which presents certain design challenges. Permanent magnet synchronous motors are also the preferred choice for drive units in electric vehicles, hybrid electric vehicles, and electric ships. In such applications, special attention needs to be paid to the magnetic circuit structure, seeking a large Xp/Xd value to obtain a large constant power speed regulation range and large dynamic torque. It has fast dynamic response, hard mechanical characteristics, and an extremely wide speed regulation range. The fully digital permanent magnet synchronous servo system with good low-speed stability and precise position and trajectory control is the most important actuator in modern automated equipment and can be widely used in high-precision CNC machine tools, robots, etc. At present, the domestic market is still dominated by imported products. 3. Development of PWM technology in variable frequency speed regulation systems PWM control is the core of variable frequency speed regulation systems, and almost any control algorithm is implemented using various PWM control methods. Since the 1990s, sinusoidal PWM (SPWM) modulation has been gradually replaced by the following methods: Fast Current Tracking PWM Technology: Fast current tracking PWM inverters are current-controlled voltage source inverters, generally employing hysteresis current control to enable the three-phase current to quickly track the command current. This inverter has simple hardware, fast current control response, and combines the advantages of both voltage and current control inverters, and is widely used in PMSM servo systems and asynchronous motor vector conversion control systems. Flux Linkage Tracking PWM Technology: This method treats the inverter and motor as a single unit, using the ideal circular magnetic field of the AC motor under three-phase symmetrical sinusoidal voltage supply as a reference. The actual flux linkage vector generated by different switching modes of the inverter tracks the reference flux linkage circle, and the tracking result determines the inverter's switching mode, forming a PWM wave. Because the flux linkage trajectory is achieved through the selection of space vectors, it is also called the voltage space vector method. Intelligent Direct Torque Control PWM Technology: Conventional direct torque PWM technology cannot distinguish between very large and relatively small deviations in torque and flux linkage, which will cause system stagnation during motor startup. By employing fuzzy control in intelligent control, the switching mode of the inverter can be derived from the spatial position of the stator flux linkage using a series of fuzzy rules (positive and negative values) based on the deviations, thus improving system performance. Dual PWM control technology, specifically AC-DC-AC voltage-type inverters, is currently the most widely used type, but it often causes harmonic pollution to the power grid. Research on dual PWM control technology is currently very active; a dual PWM inverter, composed of a PWM rectifier and a PWM inverter, can make the input current on the grid side approach a sine wave without any additional circuitry, achieving a power factor of approximately 1, completely eliminating harmonic pollution on the grid side, and realizing four-quadrant operation. 4. Development of Vector Control and Direct Torque Control Technologies. Since the establishment of vector transformation control theory in 1971, vector transformation control technology, with rotor magnetic field orientation, uses vector transformation to achieve complete decoupling of asynchronous motor speed and flux linkage control. This gives asynchronous motors control performance as good as DC motors. This technology has been widely applied. Sensorless vector transformation control technology is a hot research topic, attracting significant attention from both academia and industry. The performance of vector transformation control systems at low speeds, especially at zero speed, and the installation and maintenance of speed sensors significantly impact the system's performance, reliability, price, and ease of use. The key to this technology is acquiring speed signals. Common methods include: deriving speed equations from the motor's basic equations for calculation; selecting a suitable reference model based on adaptive control theory and using an adaptive method to identify speed; and the rotor spatial information method—using high-frequency injected current to identify the rotor's position and speed. Related products already exist abroad, with speed ranges reaching 1:75. The establishment of a low-speed model for direct torque control presents several challenges, particularly in identifying stator resistance. Further research and refinement are needed on: accurate estimation and observation of flux linkage; sensorless implementation; online identification of motor parameters; torque control at low and zero speeds; and high-performance control strategies for multi-level inverters.