During the operation of the converter, heat is generated due to the loss of power equipment in the main circuit, which affects the normal operation of the electronic equipment. If the heat dissipation of the converter system is not strong, the dissipation of power will cause the temperature rise in the active area of the chip and the junction temperature rise in the power electronic equipment. The power loss of power electronic equipment is exponentially related to its junction temperature, that is, its function decreases with the increase of junction temperature. When the operating temperature of the equipment rises by 10 C, its power loss increases by one time. Therefore, with the wide application of medium and high power converters, in order to improve their operational functions and reliability, it is necessary and urgent to adopt reasonable external cooling methods in the converter system.
At present, the commonly used heat dissipation skills of frequency converter equipment are natural air cooling, forced air cooling, water cooling and heat pipe, etc. This paper describes the principles and characteristics of these commonly used heat dissipation skills, and according to the actual needs of the project site, the development planners can select the appropriate heat dissipation skills.
2 Common methods of heat dissipation
2.1 Natural Heat Dissipation of Air
The natural air heat dissipation method of low-voltage frequency converter refers to the intention to complete the temperature control of the converter heating equipment to the surrounding environment without using any external auxiliary energy. Generally, there are three primary heat transfer methods: heat conduction, convection and radiation. Natural convection is the main method of convection. Natural air heat dissipation methods are often applicable to low-power equipment and components with less power consumption than 50w, low temperature control requirements, low heating current density, and inappropriate (or unnecessary) use of other cooling skills for sealed or densely packed equipment. In addition, the frequency converter that chooses this kind of heat dissipation method needs to increase the volume and area of the radiator to complete natural cooling. The disadvantage of this method is that the thermal resistance of the radiator is often greater than the internal thermal resistance of the power module when it is self-convection.
2.2 Forced Air Cooling Heat Dissipation
The air-cooled radiator is divided into fin radiator and fan. The fin radiator is the part that directly touches the heat source as shown in Figure 1. It acts as the heat source announcement. The electric fan is used to cool the radiator by forced convection cooling. Its cooling effect is closely related to the structure of the radiator used. Now the main focus of the research is on the optimization of the radiator's heat dissipation characteristics, structure and data. Another parameter affecting the effect of forced convection cooling is wind speed. The larger the wind speed, the smaller the thermal resistance of radiator. But the greater the active resistance, the more appropriate improvement of wind speed is conducive to the decrease of thermal resistance. However, it has little meaning to improve after the wind speed exceeds a certain value.
The heat dissipation method is mainly used in systems without special requirements and general power levels. Because of the advantages of simple structure, low price, safety and reliability, it has become one of the most commonly used methods of heat dissipation; its drawbacks are: the system temperature can not be reduced to below room temperature; and because of the rolling of the fan, there is a large noise and the number of life of the fan has time constraints. Choosing this heat dissipation method requires excellent ventilation conditions, and the frequency converter placed in the sealed shell is not applicable.
2.3 Water Cooling Heat Dissipation
Although the cost of air-cooled radiators is low, they are constrained by the ability of heat dissipation. With the continuous improvement of heat flux density, water-cooled equipment with greater ability of heat dissipation will be more and more widely used. According to the literature, the heat transfer coefficient of forced convection of gas is about 20 ~ 100w/(m2 C), and that of forced convection of water is as high as 15 000 w/(m2 C), which is more than 100 times of that of forced convection of gas.
Nowadays, many converter equipments use water-cooled equipments as heat dissipation system. Water-cooled heat dissipation system is a closed liquid circulating equipment. As shown in Figure 2, through the power generated by the pump, the liquid circulation in the closed system is promoted, and the heat generated by the chip absorbed by the heat absorber box is brought to a larger area of heat dissipation equipment through the liquid circulation for heat dissipation. The cooled liquid is refluxed to the endothermic device, so that it can be circulated back and forth. Another method of water cooling and heat dissipation is to make up for the new cooling water to cool the equipment and discharge the heat-absorbed water directly. However, this method of water cooling consumes a large amount of water only for some special occasions, so the former method is generally used. Because there is no electric fan in the water cooling system, it will not oscillate and the noise is relatively small. Its defect is that the price is more valuable, and water is prone to scale and metamorphosis in the closed state, and in the process of operation, it is necessary to completely eliminate leakage and water cut-off. In the process of operation, some changes in the electromagnetic field around the electronic components will be caused by the activity of water, which may affect the stability of the system.
2.4 Heat Pipe Heat Dissipation
Heat pipe is a kind of artificial component with excellent heat transfer performance. It uses the principle of "phase change" heat transfer, which is completely different from the physical data of general metal data and natural heat transfer methods. The structures of heat pipes are flexible and varied, and they differ greatly from each other. Typical heat pipes, as shown in Figure 3, are composed of shell, wick and working medium. The gas extraction part in the tube is changed into a constant negative pressure and filled with an appropriate amount of working liquid, so that the capillary porous material which is close to the inner wall of the tube can be filled with liquid and sealed. One end of the tube is the transpiration section (heating section), the other end is the condensation section (cooling section), and the insulation section is placed between the two sections. After vaporizing the heat generated by the heat source absorbed by the liquid medium from the transpiration section, under the effect of small pressure difference, it flows to the condensation section agilely and condenses into a liquid by releasing latent heat to the condensation source.