Abstract: Based on the practical application of the SIMOREG K6RA24 fully digital DC speed control device, and considering the characteristics of the AC-AC frequency converter's main circuit and basic control, this paper proposes a method for constructing a fully digital drive control system for AC hoists, and discusses its main circuit wiring and control section. Keywords: Low-frequency braking, AC-AC frequency converter, fully digital control 1 Introduction The AC wound-rotor motors used in mine hoists are usually speed-regulated by switching their rotor resistance. However, the low speed obtained by the motor relying on rotor resistance has relatively soft operating characteristics. When the hoisting container passes through a given deceleration point, different decelerations will be obtained due to different loads, failing to achieve stable low-speed crawling, ultimately leading to inaccurate stopping positions and inability to load and unload normally. By having the operator simultaneously apply mechanical brakes, the required deceleration and low-speed crawling can be obtained by utilizing the combined characteristics of brake braking and motor drive. This method not only consumes a lot of electricity and causes significant wear on the brake shoes, but also puts the operator under great stress, resulting in poor safety and reliability. Thyristor cascade speed regulation automated hoists can achieve better control characteristics. However, they require more electrical control equipment and have a larger capacity. To obtain braking torque during deceleration, a dynamic braking device is required, thus complicating the system and increasing investment. This is especially true for wound-rotor motors above 500kW, where the rotor voltage is approximately 700V, making the selection of thyristor devices difficult. When AC hoists only use dynamic braking, the deceleration and crawling phase requires multiple switching between braking and electric power, and between electric power and braking, to obtain 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 low-frequency drive of the hoist keeps the motor running in the regenerative braking zone during the deceleration phase and in the electric zone during the crawling phase. Furthermore, the transition from braking to electric state is natural. AC-AC frequency converters, as an AC speed control solution widely used in high-power, low-speed ranges, have easily adjustable frequency ranges and are suitable for various AC hoist operations as low-frequency power supplies. Digitalization is the development trend of modern transmission technology, and achieving full digital control is a new challenge for the automation of AC hoists. 2. AC-AC Frequency Converter 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: [align=center] [/align] By changing the frequency of the firing angle 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 inverters generate a single-phase low-frequency AC voltage to supply the load by alternately operating two sets of anti-parallel thyristors, resulting in circulating current issues. Operating modes used in reversible DC drives (such as logic-free circulating current, offset-free circulating current, and controllable circulating current) are generally applicable to AC-AC inverters. The main circuit and basic control section of the AC-AC inverter can use the same components and technologies as DC drives. 3. Main Circuit Wiring and Characteristics The SIMOREG K6RA24 is a compact, three-phase AC directly 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 parameter configuration to implement various control functions of the speed control drive system in software, demonstrating a high level of technology. This fully digital AC-AC inverter system consists of three 6RA24 units, with a three-phase bridge main circuit, speed-current dual closed-loop control, and logic-free circulating current operation. The outer loop is a speed loop for precise speed control, while the inner loop consists of three current loops to ensure the balance and coordination of the three-phase currents. AC modulation of the three-phase currents ensures a sinusoidal output current waveform. The main circuit wiring is shown in Figure 1. As can be seen from Figure 1, this 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 inverter's input power factor. If the three phase voltages contain 3rd, 6th, or 9th harmonics, these harmonics are in phase and cancel each other out in this wiring (Y-connected output), preventing them from being reflected in the load or line voltage. In other words, the third harmonic in the output phase voltage will not be transmitted to the motor. Therefore, the system has high output power, low high-order harmonics, good output waveform, and reliable operation. 4 The control system is configured with low-frequency braking mode, so that the hoist can convert some mechanical energy into electrical energy and feed it back to the grid in the deceleration section, and naturally transition to the crawling stage to achieve stable low-speed crawling. By adopting digital control technology, its control performance has been greatly improved. This system is a dual closed-loop control of speed and current, which makes full use of the basic control functions of SIMOREGK6RA24. It mainly consists of the following parts: (1) Main board. The core is a 16-bit microcontroller, which is used to complete the automatic adjustment, logic operation, fault diagnosis, automatic optimization and operation status and fault display of the system; (2) Signal board. It completes the control power distribution, current feedback signal transmission, pulse signal isolation amplification, operation interlock, etc. (3) Opto-isolated switch input and output board; (4) Intelligent A/D, D/A board; (5) Bus board. Each function control is connected to an external bus to form a control system and jointly complete various control tasks. Its digital control structure block diagram is shown in Figure 2. In Figure 2, 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. Digital trigger pulse signals are output from the six high-speed channels of the microcontroller and sent together with high-frequency modulation signals to the logic gate array circuit, 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 regulation and control are fully digital, ensuring the system's regulation accuracy. The working status of the transmission device is selected by a switch. In "internal control", parameters are set and the device is debugged by 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 each data and the PC. 5 Application Example This AC-AC frequency converter fully digital drive control system was used in the main shaft of a mine. The hoist model is JKMD-2.25×4E, AC6kV, 800kW. Its stator and rotor circuits adopt vacuum contactor commutation. The entire operation process is controlled by PLC with CRT monitoring. It was manufactured and installed by CITIC Heavy Machinery Automation Engineering Co., Ltd. and put into use in December 1998. The technical performance fully meets the design requirements, the operation effect is good, and the safe production of the hoisting equipment is guaranteed. 6 Conclusion (1) The use of a fully digital control AC-AC frequency converter as a low-frequency power supply for the drive control of the AC hoist improves the safety of the hoist operation and has a significant energy-saving effect. It is beneficial to improve the control performance of the hoist braking and crawling stages, reduce the crawling distance, shorten the hoisting cycle, and improve production capacity. (2) According to the system operation results, this device can be applied to various transportation forms of mine hoists. Since the controller hardware adopts the advanced standard products of SIEMENS, it has high reliability. Combined with the self-developed software and main circuit control system, the whole device has a high performance-price ratio.