1. Introduction The main shaft winch of Gaozha Coal Mine, Ningxia Hui Autonomous Region, is responsible for the heavy work of lifting coal and transporting materials throughout the mine. Once a fault occurs, it will affect the production of the entire mine. Therefore, its normal operation plays a vital role. The shaft is a single-hook operation with a slope of 45° and a slope length of 300m. Originally, a 280kw/380v wound-rotor motor with series resistance was used for speed regulation, and an AC contactor was used to realize speed segment switching. This formed a working process of low-speed voltage reduction start, gear switching acceleration, full-speed operation, gear switching deceleration, and low-speed voltage reduction stop. This operating mode has obvious disadvantages, mainly reflected in the following aspects: (1) A large amount of electrical energy is consumed in the rotor resistance, resulting in serious energy waste. When the material car is lowered empty, the motor speed exceeds the synchronous speed, and the motor is in the generator state. Since there is no energy processing link, a large amount of electrical energy is consumed in the rotor resistance, which increases the energy consumption of the motor. This not only wastes a large amount of electrical energy, but also increases the copper loss and iron loss of the motor, and increases the maintenance cost of the motor. From the on-site situation, the motor current during descent is basically the same as that during elution, both around 500A, which is equivalent to the rated current of the motor. However, the motor current during no-load operation should be about 60% of the rated current. From this point of view, about 30% of the energy should be consumed. (2) The original control system uses the winding motor rotor series resistance for speed regulation, which not only consumes a large amount of slip power on the rotor resistance, wasting a large amount of electrical energy, but also requires a large space for the installation of the resistor, increasing the construction cost of the machine room. (3) This control system for speed regulation by switching rotor resistance is complex, resulting in a high failure rate of the system. Contactors, resistors, and carbon brushes of the winding motor are easily damaged, and the maintenance workload is large, which directly affects the production efficiency of the hoist. (4) During the low speed and crawling stages, speed control is achieved by relying on the friction of the brake pads against the drum. Especially when the load changes, it is difficult to achieve constant deceleration control, resulting in discontinuous speed regulation and poor speed control performance. (5) The starting and shifting impact current is large, causing a large mechanical impact, which greatly reduces the service life of the motor and makes it very easy to "derail". (6) The low level of automation increases mining costs and affects mine output. (7) The starting torque is small at low voltage and low speed, the mechanical characteristics are relatively soft, the load-carrying capacity is poor, and constant torque cannot be achieved. In response to these problems, the coal mine decided to upgrade the original system. The more advanced variable frequency speed control was adopted to replace the original wound-rotor motor series resistance speed control, which is the ideal solution. Variable frequency speed control realizes soft start and soft stop of the motor, and continuous smooth speed regulation. In particular, the four-quadrant operation frequency converter with energy feedback can send the regenerated electrical energy generated by the motor in the generator state back to the grid, reduce energy consumption, and save a lot of energy. 2. Frequency Conversion Control Scheme 2.1 Features of Frequency Conversion Control (1) The frequency conversion system eliminates the AC contactor and speed control resistor used in the original electric control speed regulation, improves the reliability of the system, and improves the working environment of the operators; (2) It realizes soft start and soft stop of low frequency and low voltage, making the operation more stable and the mechanical shock less; (3) The impact current during the start and acceleration process is small. The maximum starting current during the acceleration process does not exceed 1.3 times the rated current. The hoist can smoothly and steplessly rise from low speed to the highest speed under heavy load without large current, which greatly reduces the impact on the power grid; (4) The DC braking function is added, making the stop of the heavy vehicle more stable; (5) The torque compensation meets the standard requirements, and the start of the heavy vehicle is normal; (6) The energy saving effect is significant. According to actual measurement, the energy saving in the low speed section is obvious, generally reaching about 20%. The energy saving effect is more obvious when regenerative braking is adopted; (7) After the frequency conversion control is adopted, the rotor of the original wound-rotor motor is short-circuited, which avoids the burning and maintenance of the rotor carbon brush in terms of motor maintenance; (8) The combination of unified CPU control and PLC external circuit interface makes the speed regulation system highly reliable and increases the anti-interference capability of the system. At the same time, the powerful control function of PLC is used to realize flexible control mode; (9) The machine is equipped with a feedback unit, and the feedback energy can be directly transmitted to the power grid without being limited by the size of the feedback energy. It has a wide range of applications and obvious energy saving effect. The system can realize four-quadrant operation; (10) The safety protection functions are complete. In addition to self-protection such as overvoltage, undervoltage, overload, overheating and short circuit, there are also interlock protection of external control, including interlocking of brake signal with forward and reverse signals, interlocking of inverter fault signal with system safety circuit, and automatic deceleration program in the machine. 2.2 Scheme Determination: During the design phase, since the winch system generally operates at full load and requires a large starting torque, the frequency converter should generally be one specification higher than the required capacity (380V/280kW) and load requirements. Therefore, the JD-BP32-315T model (315kW/380V) hoist-specific frequency converter with four-quadrant operation, manufactured by Shandong Xinfengguang Electronic Technology Development Co., Ltd., was adopted. To prevent production disruptions in case of frequency converter failure and ensure production continuity, the original system and the frequency converter coexist. The original system serves as a backup for the frequency converter, and is put into production when the frequency converter fails or is under maintenance, ensuring production continuity and avoiding delays. 2.3 System Composition: The control system consists of an electrical control cabinet (provided by the mine) (controlling the start/stop of the entire system); a 380V/315kW mine hoist frequency converter; and a remote control device. The system's control signals and protection devices still use the original control system, and the mechanical brake is also retained. The frequency converter is used only as a speed control device. 2.4 Control Principle The core control unit is the JD-BP32-315T mine hoist-specific frequency converter produced by Shandong Xinfengguang Electronic Technology Development Co., Ltd. It utilizes the original control system to interface with the frequency converter hoist system to control the winch's start, stop, acceleration, deceleration, and mechanical brake system. Simultaneously, the frequency converter adjusts the frequency, enabling effective and precise control of the winch motor's speed. The remote control box displays its operating frequency, forward/reverse rotation, speed range, and other information. The system composition is shown in Figure 1. Figure 1 Principle of frequency conversion speed regulation of control system for mine hoist driven by frequency converter (1) Main circuit diagram as shown in Figure 2: Figure 2 Main circuit of hoist frequency converter Main circuit working process: Three-phase AC power is rectified and filtered into DC power to provide power for inverter. The function of inverter is to convert the rectified DC power into AC power with frequency and voltage regulation to drive the motor. Electrical energy is converted into mechanical energy to realize hoisting operation. When the motor decelerates at high speed or the single hook winch is lowered, the actual speed of the motor will exceed its synchronous speed due to the inertia of the load. Mechanical energy is converted into electrical energy, and the motor running state becomes the generator running state. The AC power generated is rectified into DC by the freewheeling diode of the inverter section, which increases the bus voltage and directly endangers the power devices. This part of the energy must be released. Therefore, we made a feedback and braking unit to process this part of the energy. Part of it is sent to the power grid and part of it is consumed through the resistor. (2) The control loop adopts a combination of unified CPU control and PLC external circuit interface, which makes the speed regulation system highly reliable. At the same time, the powerful control capability of PLC is used to realize flexible control mode and electrical isolation. (3) As a booster type frequency converter, the most fundamental problem is the handling of feedback energy. When the load is lowering heavy objects, decelerating rapidly and stopping suddenly, a large amount of energy will be fed back to the frequency converter. Therefore, this frequency converter system adopts energy consumption braking and feedback braking technology to effectively solve this problem. In particular, feedback braking directly feeds energy back to the grid, which not only ensures the safe operation of the equipment, but also saves electrical energy and makes regenerative energy recoverable. (4) The frequency converter is a typical AC-DC-AC voltage source type frequency converter. Its power module is an imported Siemens new IGBT device. It adopts 16-bit full digital single-chip microcomputer control technology, which can realize stepless smooth speed regulation of AC motors over a wide range. During operation, it can detect the load of the motor at any time and automatically adjust the power output so that the motor always runs in the best state, and the energy saving effect is obvious. (5) The control block diagram of the system is shown in Figure 3. [align=left] Figure 3 Control System Principle 3. Application Effect (1) When the winch is lowered, the regenerative energy of the frequency converter can be recovered and utilized due to the regenerative braking, resulting in significant energy saving. According to calculations, the current during frequency conversion is 220A, which is half of the original. This is because the frequency converter improves the power factor, which can generally be increased to above 0.95, reducing reactive power loss; on the other hand, a portion of the electrical energy is fed back to the grid, with a feedback current of about 200A, which also reduces the operating current. (2) After adopting frequency conversion control, due to the DC braking, the brake is fully open during operation, reducing the wear and tear under the original power frequency control. The brake is only used as an auxiliary facility, used for quick braking after the motor stops or during emergency stop. According to calculations, this loss is greatly reduced, and 20,000 to 30,000 yuan can be saved every year. (3) The original power frequency control uses AC contactors for speed segment switching and speed regulation resistors for speed regulation, while frequency conversion control eliminates all of these, increasing reliability and reducing equipment maintenance costs. In summary, its comprehensive economic benefits are quite significant. In terms of energy saving and production reliability, the cost savings are approximately 30%. 4. Conclusion The application of frequency converter control in coal mine hoists is a major feature of Shandong Xinfengguang Electronic Technology Development Co., Ltd.'s frequency converter development. Its products have been widely used in coal mines in many provinces across China, including Shandong, Shanxi, Henan, Hebei, Sichuan, Guizhou, Yunnan, Ningxia, and Inner Mongolia. They demonstrate significant advantages in terms of ease of operation, reliable operation, improved working environment for workers, and significant energy savings. Especially in recent years, product optimization and upgrades, along with the application of new technologies (including PLCs), have greatly improved its reliability. The hoist frequency converter has implemented new technologies such as four-quadrant operation and energy feedback, and its application prospects are very broad.