1. Introduction In our coal mines, explosion-proof AC squirrel-cage motors are widely used in the electrical control of belt conveyors, scraper conveyors, fans, pumps, and winches. Due to the simple, direct control methods, this causes significant damage to the motors and mechanical transmission devices. In recent years, although thyristor-controlled AC step-down speed regulation technology has seen some application, its poor starting characteristics, large impact on the power grid, and high capacity requirements for mobile substations limit its application. With significant technological changes in the field of electrical drives, AC speed regulation technology has developed rapidly. Variable frequency drive (VFD) control overcomes the above problems and offers excellent speed regulation characteristics and energy-saving effects. From the initial variable voltage variable frequency (VVVF) speed regulation scheme to the current vector and direct torque control scheme, the frequency converter not only has steady-state control characteristics, but also has good dynamic performance, which can be compared with DC speed regulation system. It not only solves the driving of loads such as fans and pumps, but also solves the control of low-speed and high-torque occasions such as belt conveyors, scraper conveyors, winches, and hoists. Due to the special environment of explosive gas in coal mines, general frequency converters are not allowed to be used directly in the mine. Therefore, the design and development of explosion-proof frequency converters is very important. 2 Key technology considerations (1) Voltage level Low voltage general frequency converters, whether domestic (including domestically assembled) or imported, are generally 220V and 380V. Some manufacturers, such as Siemens, produce frequency converters that can be used for 660V. Currently, the voltage levels commonly used in my country's coal mines are 660V and 1140V. With the continuous improvement of coal mine production capacity and the continuous increase of the power of single production equipment, explosion-proof frequency converters not only need high power, but also require a 1140V working voltage. (2) Explosion-proof heat dissipation: The heat dissipation of general-purpose frequency converters is generally carried out by air cooling or water cooling. Due to explosion-proof requirements, all electronic components of the frequency converter are sealed in the main cavity of the explosion-proof housing, and air cooling cannot be achieved; water cooling requires a water circulation system and a radiator, which is large in size and inconvenient to install and maintain. Especially for the working environment conditions in coal mines, this heat dissipation method is not suitable. Therefore, the larger the power of the explosion-proof frequency converter, the more prominent the explosion-proof heat dissipation problem becomes. If it is not solved well, it will directly affect the service life and performance stability of the frequency converter. (3) Electromagnetic compatibility. Most frequency converters operate in harsh electromagnetic environments. As power electronic devices, they are composed of power devices, electronic components and computer chips, etc., and are susceptible to external electrical interference. The voltage and current on the input and output sides contain different high-order harmonics. When put into operation, it is necessary to prevent external interference and prevent it from interfering with the outside world, which is the so-called electromagnetic compatibility. The success of solving the electromagnetic compatibility problem of explosion-proof frequency converters largely depends on the reliability of the variable frequency speed control transmission system and the operation of peripheral equipment. 3 Technical countermeasures (1) Selection of power devices. The main factor determining the voltage level of the frequency converter is the power device of the main circuit inverter circuit. The main problem is to solve the power device problem of the voltage level of 660V and 1140V. In the transmission control of low-voltage AC motors, the most commonly used power devices are GTO, GTR, IGBT and intelligent module IPM. The latter two are the most widely used mainstream power devices in general frequency converters. In particular, IGBTs have a collector-emitter voltage Vce < 3V and a frequency of up to 20kHz. The ultra-high speed diode Trr between the collector and emitter contained in the IGBT can reach 150ns. Now, the fourth generation of IGBTs, which adopts channel gate technology and non-punch-through technology to significantly reduce the collector-emitter saturation voltage, has been launched. Its switching device heat generation is reduced, reducing the device heat generation that used to account for 50% to 70% of the main circuit heat generation by 30%. At the same time, it is high carrier control, which significantly improves the output current waveform. The drive power and size are smaller than before. The application of this power device has greatly improved the performance of explosion-proof frequency converters. Currently, there are IGBTs with a withstand voltage of 5000V and a current of up to 1200A available abroad, which can be used in 1140V systems in coal mines. Therefore, as long as we seriously tackle the technical problems of the relevant drive control circuit, we can solve the voltage level problem of explosion-proof frequency converters. (2) Solving the problem of explosion-proof heat dissipation Heat dissipation of high-power devices in explosion-proof products for coal mines is a major problem. The main circuit diagram of the frequency converter is shown in Figure 1. The power consumption of a typical frequency converter is 4% to 5% of its capacity, with the inverter section accounting for about 50%, the rectification and DC circuits about 40%, and the control and protection circuits about 10%. Therefore, it is necessary to solve the problem of how to quickly and effectively dissipate the heat from the high-heat-generating components in the inverter and rectifier circuits through an explosion-proof enclosure to ensure the normal operating temperature of the power devices. Heat pipe radiators are a practical and reasonable technical solution. A heat pipe is an artificial component with excellent thermal conductivity, utilizing the principle of "phase change" heat transfer, which is completely different from the heat transfer methods of solid materials such as copper and aluminum and natural heat transfer methods. Its effective thermal conductivity is hundreds or even thousands of times that of non-ferrous metals such as copper and aluminum. Heat pipe radiators are new products made by improving radiators using heat pipe technology. For discrete power electronic devices with double-sided heat dissipation, the thermal resistance of air-cooled all-copper or all-aluminum radiators can only reach 0.04 ℃/W, while the thermal resistance of heat pipe radiators can reach 0.01 ℃/W. Under natural convection cooling conditions, heat pipe radiators can outperform solid radiators by more than 10 times. Heat pipe radiators are self-cooling, fanless, noiseless, maintenance-free, and safe and reliable. Figure 2 shows a water-copper heat pipe radiator (using water as the medium and copper as the shell material). Its evaporation section is press-fitted into a copper base, with the power device fixed to the base plane. The condensation section is press-fitted with aluminum heat sinks. This heat pipe radiator effectively conducts the heat accumulated in the explosion-proof cavity of the power device to the outside of the casing and dissipates it quickly through the heat sinks, thus solving the heat dissipation problem of large-power explosion-proof frequency converters. Figure 1 Main circuit block diagram of frequency converter 1—rectifier circuit; 2—inverter circuit Figure 2 Schematic diagram of water-copper heat pipe radiator 1—power device; 2—substrate; 3—explosion-proof housing; 4—heat sink; 5—heat pipe (3) Electromagnetic compatibility measures The input part of the frequency converter is a rectifier circuit and the output part is an inverter circuit. They are both composed of nonlinear components that act as switches. During the circuit switching process, high-order harmonics are generated. Due to the large power, they have a strong interference effect on other equipment in the system. The main interference paths are conduction, electromagnetic radiation and inductive coupling. To prevent interference, we can start from both hardware and software aspects. The principle of hardware anti-interference is to suppress and eliminate interference sources, cut off the coupling channel of interference and reduce the sensitivity of system interference signals. Therefore, isolation, filtering, shielding, grounding and other measures can be taken to suppress interference within the range allowed by relevant standards. 1) Connect series reactors or install harmonic filters: Connect a suitable reactor in series or install an LC-type harmonic filter at the power input terminal to absorb harmonics and increase the power supply or load impedance to suppress harmonics. 2) Separate grounding: Interference propagation through a shared grounding wire is the most common way interference propagates. Separating the grounding of power lines from the grounding of control lines is the fundamental way to cut off this path. Connect the grounding terminal of the power device to the ground wire, and connect the grounding terminal of the control device to the metal casing of the device. 3) Separate wiring: When signal lines are near conductors with interference source current, interference will be induced onto the signal lines, interfering with the signals. Wiring separation is effective in eliminating this interference. Separate power cables, control cables, and signal cables, maintaining a certain distance within a limited space, and maximizing the distance between the interference source and the affected circuit. Analog, low-level, and high-level signals should be connected using shielded twisted-pair cables and occupy separate cable trays. Control cables should ideally be routed perpendicular to their main return line. 4) Use transformer isolation: Use isolation transformers to isolate the power supply from the control circuit. Isolation transformers with isolation layers can be used. 5) Shielding: The computer control unit is shielded, and optical fiber is used for signal transmission between the IGBT drive unit and the control unit. 4 Conclusion (1) Through analysis and research, the overall structure of the mine low-voltage explosion-proof frequency converter needs to be redesigned. Under the premise of ensuring the performance of the frequency converter, the key technologies such as explosion-proof heat dissipation and electromagnetic compatibility need to be solved. (2) At present, explosion-proof frequency converters with rated voltage of 1140V and rated power of 200kVA and below have been widely used in the control systems of underground fans, belt conveyors and winches in coal mines. With the continuous increase of the single power of production equipment, it is necessary to develop mine explosion-proof frequency converters with higher power.