I. Introduction The AC wound-rotor motors used in mine hoists typically achieve speed regulation by switching their rotor resistance. However, the low speed obtained by the motor relying on rotor resistance has a relatively soft operating characteristic. When the hoisting container passes a given deceleration point, different loads will result in different decelerations, failing to achieve stable low-speed crawling and ultimately leading to inaccurate stopping positions and inability to load and unload properly. The required deceleration and low-speed crawling are achieved by simultaneously applying mechanical brakes, utilizing the combined characteristics of brake braking and motor drive. This method not only consumes a lot of electricity and causes significant brake shoe wear, but also places extremely high stress on the operators, resulting in poor safety and reliability. Thyristor cascade speed-regulating automated hoists can achieve better control characteristics. However, they require more and larger-capacity electrical control equipment. To obtain the braking torque during the deceleration phase, a dynamic braking device is also needed, thus complicating the system and increasing investment. Especially for wound-rotor motors above 500kW, the rotor voltage is approximately 700V, making the selection of thyristor devices difficult. When an AC hoist uses only dynamic braking, the deceleration and crawling phase requires multiple transitions between braking and electric braking to achieve an average, rather than a steady, crawling speed suitable for hoists with long crawling distances. This method requires the main reducer to have two main shafts and an added pneumatic clutch, increasing the complexity of the mechanical structure and manufacturing process. The biggest weakness of dynamic braking is its inability to provide positive torque. When the system requires low-speed positive force crawling, it must switch from dynamic braking to high-voltage operation, implementing pulse crawling with secondary power supply during the crawling phase. This method has relatively soft mechanical characteristics and is difficult to control. Low-frequency braking is employed, where the motor stator windings are disconnected from the three-phase power grid (6kV, 50Hz) and connected to a low-frequency power supply with the same voltage phase sequence. The hoist's low-frequency drive keeps the motor operating in the regenerative braking zone during the deceleration phase and in the electric braking zone during the crawling phase. Furthermore, the transition of the hoist motor from braking to electric braking is natural. AC-AC frequency converters, as an AC speed control solution widely used in high-power, low-speed ranges, offer easily adjustable frequency ranges and are suitable for various AC hoisting operations as low-frequency power supplies. Digitalization is the development trend of modern transmission technology, and achieving fully digital control is a new challenge for the automation of AC hoists. II. AC-AC Frequency Converters An AC-AC frequency converter speed control system is a variable frequency speed control system that directly converts a higher fixed-frequency voltage to a lower-frequency, variable-output voltage without an intermediate DC link. Each phase consists of two sets (positive and negative) of three-phase full-wave converters connected in anti-parallel. The output rectified voltage is: By changing the firing angle frequency of the positive and negative rectifiers, the frequency of the output voltage can be changed; by changing the output voltage ratio k, the output voltage value can be changed. AC-AC frequency converters generate a low-frequency AC voltage for the load by alternately operating two sets of anti-parallel thyristors, which presents a circulating current problem. The operating modes used in reversible DC drives (such as logic-free circulating current, misaligned circulating current, and controllable circulating current) are generally applicable to AC-AC frequency converters. The main circuit and basic control section of the AC-AC frequency converter can use the same components and technologies as those used in DC drives. III. Main Circuit Wiring and Characteristics The SIMOREG K6RA24 is a compact, three-phase AC-powered, fully digital DC speed control device manufactured by SIEMENS, with a design current range of 15A to 120A. Based on a high-performance 16-bit microprocessor, it uses a parameter configuration method to implement various control functions of the speed control drive system in software, demonstrating a high level of technology. This fully digital AC-AC frequency converter system consists of three 6RA24 units. The main circuit is a three-phase bridge type, with speed-current dual closed-loop control and a logic-free circulating current operating mode. The outer loop is the speed loop, achieving precise speed control, while the inner loop consists of three current loops to meet the balance and coordination of the three-phase currents. The three-phase currents are AC modulated to make the output current waveform a sine wave. The main circuit wiring is shown in Figure 1. As shown in Figure 1, the system is configured as a three-phase bridge-type 6-pulse AC-AC inverter. The phase voltages are 120° out of phase, serving as the three-phase voltage output. This connection allows for increased output voltage even when the selected thyristors have relatively low withstand voltages. If the three phase voltages contain the same DC component, the line voltage will not contain a DC component due to the star connection, and the voltage waveform output from the inverter to the load will also not contain a DC component. This improves the input power factor of the inverter. If the three phase voltages contain 3rd, 6th, and 9th harmonics, these harmonics are in phase and cancel each other out in this connection (Y-connected output), and are not reflected in the load or line voltage. That is, the third harmonic in the output phase voltage will not be transmitted to the motor. Therefore, the system has high output power, few high-order harmonics, good output waveform, and reliable operation. IV. Control System Configuration The low-frequency braking method allows the hoist to convert some mechanical energy into electrical energy and feed it back to the grid during the deceleration phase, naturally transitioning to the crawling phase and achieving stable low-speed crawling. By adopting digital control technology, its control performance is greatly improved. This system is a dual closed-loop control system for speed and current, making full use of the basic control functions of the SIMOREGK6RA24. It mainly consists of the following parts: 1. Mainboard. The core is a 16-bit microcontroller, used to complete the system's automatic adjustment, logic operation, fault diagnosis, automatic optimization, and operation status and fault display; 2. Signal board. Completes control power distribution, current feedback signal transmission, pulse signal isolation and amplification, operation interlocking, etc.; 3. Opto-isolated switch input/output board; 4. Intelligent A/D and D/A boards; 5. Bus board. All functional controls form a control system through an external bus, jointly completing various control tasks. Its digital control structure block diagram is shown in Figure 2. In Figure 2, the three current feedback signals are detected by current transformers and shaped by two pairs of sampling switches before being sent to the microcontroller; the speed setpoint and speed feedback signals are transformed by filtering circuits and absolute value circuits before being sent to each group of microcontrollers. Current and speed regulation are both completed by computer software. The digital trigger pulse signal is output from the six high-speed channels of the microcontroller and sent to the logic gate array circuit along with the high-frequency modulation signal. It is transformed into a double pulse train with a 60° phase difference, and then amplified and isolated to trigger the power components of each phase. All adjustments and controls are fully digital, ensuring the system's adjustment accuracy. The working status of the transmission device is selected by a switch. In "internal control," parameters are set and the device is debugged through the buttons on the host panel; in "external control," the transmission device is connected by the operating console, and the main setpoint is applied through the host serial interface RS232 (485) to automate the low-frequency braking process of the AC hoist. At the same time, the status word of 6RA24 can be used to observe the thyristor working status feedback signal, read the actual value and write and store the parameter group, and complete the communication between the data and the PC. V. Application Example This AC-AC frequency converter fully digital drive control system is used in the main shaft of a mine. The hoist model is JKMD-2.25×4E, AC6kV, 800kW. Its stator and rotor circuits use vacuum contactors for commutation, and the entire operation process is controlled by PLC with CRT monitoring. Manufactured and installed by CITIC Heavy Machinery Automation Engineering Co., Ltd., and put into use in December 1998. Its technical performance fully meets design requirements, and its operation is excellent, ensuring the safe production of the hoisting equipment. VI. Conclusion 1. The use of a fully digital control AC-AC frequency converter as a low-frequency power supply for the AC hoist's drive control improves the safety of the hoist's operation and has significant energy-saving effects. It is beneficial for improving the control performance of the hoist's braking and crawling stages, reducing crawling distance, shortening the hoisting cycle, and increasing production capacity. 2. Based on the system's operating results, this device is applicable to various transportation modes of mine hoists. Because the controller hardware uses advanced standard products from SIEMENS, it has high reliability. Combined with self-developed software and the main circuit control system, the entire device has a high performance-price ratio.