Today, humanity enjoys a highly developed modern material civilization. The crucial role of electricity in the word "modern" is self-evident. We cannot, and dare not, imagine what modern society would be like without electricity. Of course, we don't need to make such assumptions or worries, because the enormous production and transmission capacity of the power industry is reliable and safe. However, we are also keenly aware that energy and resources on Earth are not inexhaustible. The Earth's ecological environment also does not allow humanity to squander these energy and resources recklessly. There seems to be a consensus that energy, resources, and the environment are the three major problems plaguing humanity in the 21st century. Faced with these challenges, modern people will not remain inactive. Variable frequency drive (VFD) technology is becoming one of the powerful tools for solving these three major problems. VFD technology originated in the 1950s with the invention of the thyristor, which led to research on using weak current to control strong current. After decades of effort, a new discipline emerged—power electronics technology. It is a discipline that studies how to use power electronic devices to control, transform, and transmit electrical energy. The core, or rather the most important achievement, of this discipline is VFD technology. It is the culmination of the integration and mutual permeation of multiple disciplines such as microelectronics, modern control technology, and computer technology. It enables the speed regulation performance of inexpensive AC induction motors to be as perfect as that of DC motors. Power plants generate electricity, the grid transmits it, and the electricity reaches users—this is the 50 Hz mains power. To convert mains power into a power source with a wide range of continuously adjustable frequencies, users need to use frequency converters. The output frequency of a frequency converter can be arbitrarily selected or automatically adjusted within a range of a few hertz, even a fraction of a hertz, to several hundred hertz. AC asynchronous induction motors were invented in the 1880s. In this type of motor, the rotor current is not introduced from the outside but generated by electromagnetic induction, resulting in a simple and robust structure, small size, light weight, low price, and easy maintenance. Currently, this type of motor accounts for about 80% of the total capacity of electrical drives. However, it has a fatal flaw: it is difficult to adjust the speed. Without speed adjustment, this type of motor has been operating for 120 years. In all applications requiring speed adjustment, DC motors reign supreme. Until the 1980s, frequency conversion technology reached a practical stage in developed countries. In our country, the application of frequency conversion technology is only a matter of the last ten years or so. The AC motor is still the original AC induction motor; since the advent of the frequency converter, it has gained the freedom to speed up or slow down. As the power source for the AC induction motor, since the motor speed is approximately proportional to the power supply frequency, and the frequency is continuously adjustable, the speed is also continuously adjustable. The full name of the frequency converter should be AC ​​induction motor variable frequency speed controller. II. Frequency Conversion and Energy Saving Vigorously promoting the application of frequency converters is of great significance. Its social and economic benefits are considerable. In applications such as hoists, cranes, machine tools, food machinery, extruders, and automated warehouses, it can significantly improve productivity. In applications such as fans, blowers, various pumps, machine tools, food machinery, papermaking machinery, film production lines, steel plate processing production lines, printed circuit board substrate drilling machines, and high-speed engraving machines, it can significantly improve product quality. In applications such as material handling machinery, metal processing machinery, fiber machinery, papermaking production lines, film production lines, and steel plate processing production lines, frequency converters can reduce maintenance, lower costs, simplify operation, or automate operations. Using frequency converters significantly saves energy in most situations. The energy-saving principle is briefly described below. Fans and blowers are common in mines, oil fields, chemical plants, and power plants. Their power can reach hundreds or even thousands of kilowatts. There are two methods to adjust airflow and pressure: one is the traditional method, using dampers. When reducing airflow is required, the damper is closed. Obviously, a considerable amount of energy is consumed by pipe resistance. The other method is to change the equipment's rotational speed, without using dampers, or with the dampers always open to the maximum position. Therefore, the system's pipe resistance is very small, and naturally, the energy consumed by the piping system is less. From aerodynamics, we know that airflow is directly proportional to rotational speed, torque is directly proportional to the square of rotational speed, and shaft power is directly proportional to the cube of rotational speed. For example, if the maximum airflow is 1, the corresponding rotational speed and shaft power are also at their maximum values, and are also set to 1. When the required air volume is reduced to 0.5, the corresponding rotational speed is also 0.5. At this point, the shaft head power is: (0.5)³ = 0.125 = 1/8. This means that if the air volume is reduced by half, only 1/8 of the required power is needed. For a fan with several hundred kilowatts, the energy-saving effect will be very significant. For example, in 1999, the main kiln fan of the Feixian Cement Plant in Shandong Province used the JD-BP4245 frequency converter produced in Wenshang, Shandong, and the power saving effect was ≥30%. The investment of over 100,000 yuan in the frequency converter was recovered in 5 months. Another example is the Shandong Sangdun Group's medium-density fiberboard factory, which used the JD-BP430 frequency converter produced in Wenshang on deep well water pumps. The Shandong Energy Utilization Testing Center tested it on October 31, 1997. The test report stated that the power saving rate was 58% for domestic water use only, and 40% for normal production and domestic water use. For example, after a paper mill in Binzhou, Shandong Province, installed frequency converters on its 30-kilowatt fans, the energy saving rate reached 31%. There are thousands of successful examples of frequency converter use; please don't consider saving on electricity bills as the only benefit. There are actually greater social benefits. For instance, when boiler fans use frequency converters, in addition to saving on electricity costs, they save fuel due to more complete combustion, and the black smoke from the chimneys turns into white smoke. Doesn't this connect to the three major problems mentioned earlier? In the future, the widespread application of frequency converter technology in transportation will make a greater contribution to environmental protection. It's remarkable that a single technology can benefit from solving three major problems. III. Improvement and Promotion Compared with foreign products from multinational corporations, domestically produced machines still lag behind. However, in most suitable applications such as fans, water pumps, oil pumps, air compressors, carding machines in the textile industry, wire feeding machines in steel mills, extruders in the plastics industry, wire drawing machines in the building materials industry, various conveyor belts, and mining hoists, domestically produced machines are fully capable, and successful examples abound. In the areas of high-voltage frequency converters, vector control, and direct torque control frequency converters, domestic manufacturers are making efforts and it is estimated that large-scale production capacity can be achieved within 3-5 years. Regarding promotion and application, I previously proposed an idea in *Frequency Variable Frequency Drive World*: a leasing model. Users could pay a leasing fee using the monthly electricity savings, and after six months, the manufacturer would receive full payment and then give the frequency converter to the user. I find this approach very attractive. The market prospects for frequency converters are very promising, and large-scale promotion will undoubtedly bring substantial economic and social benefits.