Abstract: This paper briefly introduces the renovation of the low-voltage braking power supply of the hoisting winch in the main shaft of Shuoli Coal Mine, Huaibei Mining Group. It analyzes the necessity of the renovation from several aspects, including the generation, working principle, existing problems, renovation methods, and energy-saving effects after the renovation. Keywords: Thyristor; Frequency Conversion; Power Supply [b]1 Overview[/b] In Shuoli Coal Mine, Huaibei Mining Group, Anhui Province, all coal production underground relies on a single vertical shaft, namely the main shaft. This shaft is equipped with an 800kW hoisting winch. During normal operation, especially near the top and bottom, the hoisting winch relies entirely on an additional low-frequency, low-voltage braking power supply. Without a dedicated power supply, a low-frequency generator set is used to obtain the low-voltage, low-frequency braking power. This energy conversion is inefficient, wasteful, and makes the system bulky and unreliable. Therefore, it is necessary to technically renovate this cumbersome method of obtaining and braking power. [b]2 Working Principle of Low-Frequency Braking Power Supply[/b] The main shaft's electrical braking power supply uses a low-frequency generator set. The power supply unit consists of five motors: two 130kW and 7.5kW drives, a synchronous generator, a commutator frequency converter, and a low-frequency generator. The 7.5kW motor drives both the synchronous generator and the commutator frequency converter, while the 130kW motor drives the low-frequency generator. The excitation amplifier compares the speed measurement signal with the given signal and outputs a control quantity to control the input of the pulse transformer. The output of the pulse transformer controls the conduction angle of the thyristors in the single-phase full-wave semi-controlled rectifier circuit, thereby controlling the rectified output. This rectified output supplies excitation to the synchronous generator. The synchronous generator amplifies the DC signal into an AC synchronization signal to excite the commutator frequency converter. The commutator frequency converter amplifies the AC synchronization signal into a 2.5-3Hz low-frequency signal, which then excites the low-frequency generator. After passing through the low-frequency generator's power amplifier, the signal is supplied to the main motor for low-frequency braking and low-frequency drive. The control logic is shown in Figure 1. [b]3. Defects of Low-Frequency Generator Sets[/b] The original low-frequency generator set had the following defects: ① Large footprint, high noise, and a very high failure rate; ② Due to the use of a large number of carbon brushes in the generator, poor contact between the carbon brushes and slip rings easily leads to poor low-frequency power quality, causing mechanical system shocks, unstable operation, and significantly shortening the equipment's service life, increasing maintenance workload; ③ Although the lifting is intermittent, the entire generator set is only used to ensure a brief deceleration period of approximately 25 seconds during each lifting operation. However, due to the performance limitations of the equipment's own electrical control system, it cannot be frequently started. Therefore, the entire generator set must maintain continuous operation for 23 hours a day (excluding 1 hour of maintenance), resulting in a huge waste of electrical energy. It wastes more electrical energy than a thyristor power supply. 4. Technical Transformation Methods Referring to relevant materials and learning from good experiences abroad, after careful research, it was decided to transform the original low-frequency generator set power supply into a thyristor AC-AC frequency converter. The main circuit of the thyristor power supply adopts a three-phase zero-type structure, and its working principle is shown in Figure 2. The new thyristor-controlled low-frequency power supply unit adopts a fully digital, non-circulating current operating mode. The main control trigger circuit uses world-leading German Siemens technology. It consists of a frequency converter with a 16-bit single-chip microcomputer 80C196KC as the control core, coupled with auxiliary circuits. It boasts advantages such as high control precision, fast processing speed, noiseless operation, comprehensive and reliable protection, small size, light weight, significant energy saving, and high production efficiency. 5. Energy Saving Effect After Modification The newly modified thyristor-controlled low-frequency power supply unit was put into use after maintenance in October 2006. The system operates safely and reliably, with a low failure rate, and significantly reduced maintenance workload and costs. The most obvious benefit of the improved control technology is energy saving. Compared with the original generator-electric braking power supply, the thyristor-controlled low-frequency power supply unit shows significant energy savings. Consider two drives with power ratings of 130KW and 7.5KW, running for 23 hours a day. Considering that each hoisting operation takes only about 25 seconds (the time for light and heavy hooks differs), and taking into account factors such as the no-load current being less than the rated current, and multiplying by a coefficient of 0.5, and calculating based on 355 effective operating days per year, the annual energy savings w is: W = (130 + 7.5) × 23 × 355 × 0.5 = 561343.75 (kWh). If calculated based on the current average electricity price of 0.58 yuan/kWh at the mine, the annual electricity cost savings would be 325,500 yuan. Clearly, the energy-saving benefits are considerable. 6. Conclusion The modification of the low-voltage electrical control system of the main shaft hoisting winch is limited to the modification of the low-frequency generator set braking power supply and has not yet directly entered the high-voltage section. Therefore, if high-voltage frequency conversion technology is further adopted for modification, it can replace part of the control system, at which time the energy savings will be significantly increased. Due to the reduction in system facilities, the failure rate will also be greatly reduced, and the maintenance workload and costs will be significantly reduced. After the upgrade using high-voltage frequency conversion technology, the system's automation level has been greatly improved, and its operation is safer and more reliable. At the same time, it can also significantly increase the mine's production capacity. Editor: Chen Dong