1 Introduction
In recent years, the development of power electronics and automatic control technologies has also driven the upgrading of production equipment in coal mines. To meet the requirements of energy conservation and safe production in coal mines, the use of explosion-proof frequency converters in coal mines is increasing. These converters are mainly used in underground conveyors, hoisting winches, fans, water pumps, scraper conveyors, and other load types, requiring extremely high reliability. Several factors affect the reliability of frequency converters, and ventilation and heat dissipation are among the most crucial.
During normal operation, components that generate high heat in a frequency converter include the isolation transformer, electronic power components, reactors, and filters. Ordinary frequency converters are open to the outside environment, and their heat dissipation is easily solved through natural air exchange. However, explosion-proof frequency converters are kept in a closed cavity for extended periods, where heat circulates within a small, confined space. Heat exchange occurs through the explosion-proof enclosure, preventing timely heat dissipation. Consequently, their failure rate increases with rising cavity temperature, and their lifespan decreases exponentially with increasing temperature. The most direct solution is to increase the volume of the explosion-proof cavity, but this increases production costs and limits application scenarios, contradicting the optimization design goals of cost reduction, material conservation, and maximizing output. Especially with increased production volume, the demand for large-capacity frequency converters is growing, making heat dissipation a particularly prominent issue. This necessitates the rational selection of heat dissipation methods for explosion-proof frequency converters based on actual conditions.
2. General heat dissipation methods for frequency converters
The design of a frequency converter's cooling and heat dissipation system includes two aspects: the design of the radiator structure and the selection of the cooling medium. The selection of the radiator structure should consider the following factors: the energy consumption, size, and weight of auxiliary equipment; the complexity and ease of operation of the device; and the reliability, availability, and maintainability of the device. The selection of the cooling medium should consider electrical insulation, chemical stability, corrosiveness to materials, environmental impact, flammability, and the cost of the medium.
The heat dissipation methods of frequency converters can be divided into air cooling and liquid cooling, depending on the cooling medium.
2.1 Air cooling
Air cooling methods are divided into two types: "natural cooling" and "forced air cooling".
(1) Heat dissipation through natural cooling
It is mainly used in inverters with relatively low power, large heat dissipation area of components, and where the use of fans is not allowed. Cooling is mainly achieved by removing heat through natural convection and radiation of air. Due to its simple structure, no noise, maintenance-free operation, and high reliability, it has a wide range of applications, especially suitable for impact loads and intermittent duty loads. The disadvantage is that it cannot be used in inverters with high power and long-term operation.
(2) Forced air cooling
It is mainly used in applications without special requirements and with a slightly higher power rating. Its advantages are high heat dissipation efficiency, with a heat dissipation coefficient 2 to 4 times that of natural cooling. Its disadvantages are that it requires a separate fan, resulting in higher noise levels, the potential for coal dust to be blown in, and relatively lower reliability.
2.2 Liquid Cooling
Liquid cooling methods are further divided into two types: "water cooling" and "oil cooling".
(1) Water cooling
Water-cooled radiators offer extremely high heat dissipation efficiency, significantly increasing the capacity of power components. However, ordinary water has extremely poor electrical insulation properties, and impurities in the water can cause electro-corrosion and leakage under high voltage. Furthermore, due to the hardness of well water, scale easily forms in the waterways, hindering heat dissipation and potentially clogging the channels. Therefore, water-cooled radiators are generally used in low-voltage frequency converters. When used in high-voltage frequency converters, the two major issues of reliability and corrosion during operation must be considered and addressed.
(2) Oil cooling
Although oil-cooled radiators are not as efficient as water-cooled radiators, they were once widely used in ordinary high-power frequency converters due to their high electrical insulation and electromagnetic shielding effects. However, their cost and environmental requirements have led to their gradual decline in recent years. Furthermore, according to coal mine safety regulations, oil-filled electrical equipment is not allowed to be used in underground coal mines, making oil-cooled radiators unsuitable for use in underground coal mines.
3. Selection of Radiator for Explosion-proof Inverter
Since IGBTs and rectifier diodes are high-density heat-generating components, the heatsink must have sufficient instantaneous heat absorption capacity to absorb their heat as quickly as possible. Therefore, the material chosen for the heatsink is not only directly related to its size and weight but also affects its heat dissipation performance. Manufacturing process and production cost are crucial design considerations for heatsinks. Metal has become the sole choice for heatsinks today because, compared to other solid materials, metals possess advantages such as better thermal conductivity, high ductility, and relative stability at high temperatures.
For metals, thermal conductivity and specific heat are important factors in selecting them as heat dissipation materials.
The thermal conductivity coefficient represents a metal's ability to conduct heat, and its unit is W/mK. The higher the value, the stronger the thermal conductivity, that is, the faster the heat conduction speed. As can be seen from Table 1, the thermal conductivity coefficient of copper is about 1.8 times that of aluminum, which can quickly carry away heat. Although the thermal conductivity of silver is higher than that of copper, it is expensive and impractical to produce.
Looking at specific heat, an inherent property of metals, it refers to the amount of heat absorbed by 1 kg of that metal when its temperature rises by 1°C. Table 2 shows the specific heat of copper and aluminum. Aluminum's specific heat appears to be significantly higher than copper's, better meeting the requirements for heat storage plates and heat-absorbing bases. However, copper's density is 3.3 times that of aluminum. This means that for the same volume, a copper heatsink weighs 3.3 times more than an aluminum heatsink. Therefore, a copper heatsink of the same volume can absorb 40% more heat than an aluminum heatsink, possessing greater heat storage capacity. Copper heatsink base plates have become the primary material for inverter heatsinks.
Depending on the power of the heat-generating element, to quickly dissipate the heat stored at the bottom of the heat sink, the heat must be conducted to every part of the fins to increase the heat exchange area. The thermal conductivity between the heat-absorbing bottom and the fins depends on the bonding method and connection area. For air cooling, more fins can be added to the heat sink to increase the heat exchange area. For high-capacity, high-power inverters, a better heat dissipation method—heat pipes—should be used. Heat pipe radiators use water or other hot fluids as the cooling medium, sealed within copper tubes with capillary structures. The heat generated by the power device is conducted to the fluid through the radiator. After vaporization, the fluid diffuses throughout the copper tube, is cooled by the heat sink fins, and then flows to the heat-absorbing surface. The heat transfer rate of a heat pipe is 10 times that of copper, so heat pipe radiators have advantages such as strong heat transfer capacity, excellent temperature uniformity, variable heat density, no external equipment required, reliable operation, simple structure, light weight, and no maintenance. When selecting the appropriate heat pipe size based on the inverter's power output, some considerations should be taken into account. The fins of heat pipe radiators are primarily made of aluminum. However, aluminum is a restricted material in underground coal mines. To prevent safety issues caused by collisions and friction sparks, and to prevent coal or other objects from damaging the radiator and affecting its cooling performance, the radiator must have protective devices. The design of the protective cover must fully consider the heat exchange capacity between the radiator and the cooling medium. Explosion-proof fans can also be installed for forced air cooling, further enhancing the cooling effect.
For applications where reliable clean water is available, water cooling can be used. Due to water's high specific heat capacity, it can quickly absorb heat from the heat sink. By circulating or discharging the hot water, a cooling effect is achieved. Therefore, water-cooled inverters can be used in situations where the explosion-proof cavity is small, allowing for the use of high-power inverters. This solves the volume requirement of coal mines. However, it also requires the availability of a clean water source at the application site. For coal mining machines and tunneling machines, water atomization dust suppression is required. The inverter part can be cooled with water first, followed by dust removal, making full use of resources. For inverters specifically designed for belt conveyors, water cooling can also be used if a water source is nearby.
Currently, common water-cooled explosion-proof frequency converters primarily mount high-power components on smooth aluminum or copper plates, which are then embedded within the explosion-proof enclosure. Heat is dissipated through water channels within the enclosure. Due to the materials and manufacturing process, heat is difficult to dissipate quickly. While this prevents rapid heat accumulation in small and medium-sized frequency converters, frequency converters with power ratings above 200kW are prone to localized overheating of the heat sink, leading to frequent malfunctions. To address this, high-power components can be mounted on aluminum or copper plates, with water directly introduced into the channels within the plates. This method allows the heat sink to be either completely within the explosion-proof enclosure or partially embedded in it. These two methods have different explosion-proof requirements and can be designed according to the actual production and operating environment. When using water cooling, the pressure requirements of the cooling circuit (water channels) must be considered. Setting appropriate water pressure and flow rates is crucial for the normal operation and explosion-proof requirements of the frequency converter. Simultaneously, condensation within the explosion-proof enclosure of water-cooled frequency converters must be considered from the initial design stage. This phenomenon is fatal to numerous power components and electronic circuit boards. In field use, there have been numerous instances of frequency converter malfunctions and shutdowns due to condensation. Therefore, to prevent condensation, heating elements are generally required within the water-cooled frequency converter's internal cavity to maintain the internal temperature when the frequency converter is not in use. However, the relationship between power and maintained temperature must be carefully verified when using heating elements to prevent excessively high local temperatures within the explosion-proof enclosure.
The heat generated by the frequency converter itself can now be calculated based on the losses of each heat-generating component, allowing for the selection of appropriate heat dissipation methods depending on the specific circumstances. Based on years of testing, we can roughly estimate that the losses of explosion-proof frequency converters are approximately 3-6% of their capacity. The main differences lie in the frequency converter's operating mode and parameter settings. Special consideration must be given to heat dissipation when DC or AC reactors are present within the same enclosure. For reactors with high heat generation, specific heat dissipation measures should be taken, or reactors made of low-loss, low-heat-generating microcrystalline materials should be selected.
4. Conclusion
Good heat dissipation is an important prerequisite for the normal operation of explosion-proof frequency converters in underground coal mines. Manufacturers can choose the appropriate heat dissipation method according to the specific usage conditions to ensure the normal use of explosion-proof frequency converters for mining.
