[Abstract] AC-AC frequency converter systems, especially non-circulating AC-AC frequency converter systems, have been applied to high-capacity motor speed control systems, playing a positive and important role in improving motor speed regulation. This paper mainly analyzes AC-AC frequency converter systems and discusses their application in mine hoists, hoping to help us understand AC-AC frequency converter systems and improve their application in mines. [Keywords] AC-AC frequency converter system; mine hoist; AC-AC frequency converter control theory; AC-AC frequency converter low-speed regulation device AC-AC frequency converter systems are a rapidly developing and widely used new technology, which plays a positive role in improving motor speed regulation systems. I. AC-AC Frequency Converter System This paper will introduce AC-AC frequency converter system control theory and AC-AC frequency converter system zero-voltage detection technology separately. 1. AC-AC Frequency Converter System Control Theory AC-AC frequency converter system control theory can be divided into single-variable phase control theory and two-variable phase control theory. Single-variable phase control theory is the core of traditional phase control theory. Its research mainly focuses on theoretical issues such as harmonic power factor and sinusoidal power, and its applications are even more prominent, such as commutation, protection, standard design, simulation and auxiliary design, new materials, and new devices. The shortcoming of single-variable (A-angle) control theory is that it only considers the conduction moment of the thyristor, without considering the various laws and controllability issues after the thyristor conducts. It relies entirely on the next A-angle control signal to adapt to various possible load and external conditions. Although there are double-pulse and wide-pulse theories, these are only for the purpose of forming a loop and starting up. In bivariate phase control theory, the two variables refer to the control angle A and the pulse width b. The control method for the first variable is the same as that for single-variable phase control. The second variable b has two functions: first, to block various possible circulating current conditions; and second, to guide the commutation of the current. This variable is related to factors such as the size and nature of the load, the output frequency, the angle A, and the instantaneous value of the power supply. Bivariate control theory introduces a conduction function to describe the mathematical expression of the output voltage. The conduction function determines the trailing edge of the modulation function pulse sequence. The introduction of this new function makes the expression more flexible and universal. 2. Zero-current detection technology in AC-AC frequency converter systems. Zero-current detection technology is a key technology in non-circulating current AC-AC frequency converter systems. The electronic zero-current detection circuit ensures reliable commutation of the thyristor, providing sufficient switching latency and meeting the requirement of a dead zone of less than 2ms, demonstrating its superior performance. See the figure below. II. Application of AC-AC frequency converter systems in mine hoists One of the most important application areas of AC-AC frequency converter systems is in mine hoists. my country is a major mineral resource country, and improvements in mine hoist technology will have a positive impact on the development of mineral resources and the safe production of mines. 1. Current Status of Mine Hoists in China: Currently, some domestically capable coal mines have already put into use synchronous machine drive systems powered by AC-AC frequency converters. However, the current equipment is still mainly imported, and domestic research and production are still insufficient. Nevertheless, continuous progress in theoretical and applied research in related fields has provided corresponding theoretical and technical support for domestic production. ① Significant progress has been made in the research on the control theory, system structure planning, and component selection calculation of AC-AC frequency converter control devices. ② New achievements and breakthroughs have been made in the research on vector control technology in synchronous machine drive systems powered by AC-AC frequency converters. ③ New achievements have been made in the research on hydraulic brake control, hoist container position control, multiple hoisting speed monitoring, hoisting signals with specific requirements, and some special protections. ④ New progress has been made in the design of motors to address the additional torque and losses caused by the positive arc waveform distortion of AC-AC frequency converters. ⑤ The auxiliary equipment required for producing AC-AC frequency converter systems, such as rectifier transformers, fast switches, and high and low voltage switchgear, are all domestically produced with standardized specifications and a complete range of products, and their performance and quality have been greatly improved. 2. Application of AC-AC Frequency Converter Systems in Mine Hoists Mine hoists are key equipment in coal mine production, affecting the safety of the entire operation. Therefore, the drive system of mine hoists must be safe, reliable, efficient, and accurately positioned. my country has over a thousand AC mine hoists, and AC-AC frequency converter systems are widely used in them. The application of AC-AC frequency converter systems in mine hoists can be summarized as the application of non-circulating current AC-AC frequency converters, fully digital AC-AC frequency converters, and sinusoidal AC-AC frequency converters in mine hoists. Based on the requirements of the hoist for braking and crawling, the circulating current AC-AC inverter system is designed as a power supply with a fixed and very low output frequency (2-5Hz) and a continuously variable output voltage. The main circuit is shown in the figure below. It is a three-phase low-frequency output circuit composed of three half-wave non-circulating current reversible rectifier circuits, also known as a three-phase half-wave AC-AC inverter. Based on relevant experience, the application design of AC-AC frequency converters in mine hoists mainly involves a variable frequency speed control system consisting of a main shaft assembly, power station, frequency converter cabinet, control console, and online monitoring computer. The main shaft assembly comprises a main shaft, casing, support rollers, manufacturing wheels, bearing housings, etc. The power station consists of a main motor, brake, reducer, and base. The frequency converter cabinet is the power supply unit of the power station, converting input power frequency energy into adjustable frequency energy to supply the AC motor. The control console is the core of the entire mine hoist transportation system. It allows setting the system's operating and control modes, issuing various control commands, and receiving feedback signals from the frequency converter cabinet and power station to achieve various control functions such as hoist start-up, acceleration, smooth operation, deceleration, stopping, and emergency braking. The development of AC-AC frequency converter systems and their application in mine hoists have not only saved energy but also improved speed and operational quality, achieved stepless speed regulation, and extended service life. We believe that the continued development of AC-AC frequency converter system theory and related supporting technologies will play a positive role in promoting the upgrading of mine hoist technology and performance improvement. References: 1. Du Qingnan, Tian Lidong, Wang Xin: "Research on the Two-Variable Control Theory of AC-AC Frequency Converters", Journal of Jiaozuo Institute of Technology, Vol. 17, No. 4; 2. Fang Bing: "Design and Application of AC-AC Low-Frequency Power Supply Device", Power Electronics Technology, February 1998; 3. Wang Yong: "New All-Digital AC-AC Frequency Converter Low-Frequency Power Supply and Its Application in AC Hoists", Coal Mining, No. 47; 4. Liu Chengyin, Ma Pengxiang, Xu Hongchun, Sun Baoren: "Research on All-Digital AC-AC Frequency Converter Low-Frequency Speed ​​Regulation Device", Mining Machinery, No. 5, 2002; 5. Tan Bo, Li Yanlin, Tan Guanzheng: "Application of Variable Frequency Speed ​​Regulation in Mine Hoists", *Mechanical and Electrical Product Development and Innovation*, March 2005. 6. Fan Xiaohong; He Fengyou; Ma Zhixun: "Research on Upper Computer Monitoring and Diagnostic System for AC-AC Variable Frequency Hoists", *Coal Engineering*, July 2007.