Heat is generated due to power device losses in the main circuit, which can affect the normal operation of electronic equipment. If the heat dissipation capability of the variable frequency system is not strong, the power dissipation will cause the active area temperature inside the power electronic devices to rise and the junction temperature to increase. The failure rate of power electronic devices is exponentially related to the junction temperature, meaning their performance decreases as the junction temperature rises. For every 10°C increase in the operating temperature of the device, the failure rate doubles. Therefore, as medium and high-power variable frequency systems are increasingly widely used, it is very necessary and urgent to adopt reasonable external heat dissipation measures in the variable frequency system to improve their working performance and reliability.
Currently, common heat dissipation techniques for variable frequency drive equipment include natural air cooling, forced air cooling, water cooling, and heat pipes. This article elaborates on the principles and characteristics of these commonly used heat dissipation techniques. Based on the actual needs of engineering sites, R&D designers can choose the appropriate heat dissipation technology.
Common Cooling Methods for Variable Frequency Drive Heat Sinks
1. Natural Air Cooling
The air natural heat dissipation method refers to achieving temperature control by dissipating heat from the heating elements of a variable frequency converter to the surrounding environment without using any external auxiliary energy. It typically includes three main heat transfer methods: conduction, convection, and radiation, with convection primarily occurring through natural convection. The air natural heat dissipation method is often suitable for low-power devices and components with power consumption below 50W, low temperature control requirements, and relatively low heat flux density, as well as in cases where sealed or densely assembled devices are not (or do not require) other cooling technologies. Additionally, variable frequency converters using this heat dissipation method require increasing the size and area of the heat sink to achieve natural cooling. A drawback of this heat dissipation method is that the thermal resistance of the heat sink during free convection is often greater than the internal thermal resistance of the power module.
2. Forced Air Cooling
The air-cooled radiator is divided into two parts: the fin radiator and the fan. The fin radiator, as shown in Figure 1, is the part that directly contacts the heat source, responsible for extracting the heat emitted by the heat source; the fan is used to force convection cooling and lower the temperature of the radiator. Its cooling effect is closely related to the structure of the radiator used. Current research focuses on the heat dissipation characteristics and the optimization of the structure and materials of the radiator. Another parameter affecting the forced convection cooling effect is the wind speed; the higher the wind speed, the lower the thermal resistance of the radiator, but the greater the flow resistance. Appropriately increasing the wind speed is beneficial to reducing the thermal resistance, but increasing the wind speed beyond a certain value has little significance.
This cooling method is primarily used for systems without special requirements and general power levels. Due to its simple structure, low cost, safety, and reliability, it has become one of the commonly used cooling methods. However, its drawbacks include the inability to lower the system temperature below room temperature, significant noise due to fan operation, and a limited lifespan for the fans. This cooling method requires good ventilation conditions and is not suitable for inverters placed in enclosed shells.
3. Water-Cooled Cooling
Despite the low cost of air-cooled heat exchangers, their limited heat dissipation capacity restricts their application. As heat flux density continues to rise, water-cooled systems with greater heat dissipation capabilities will become increasingly prevalent. According to reference [4], the approximate heat transfer coefficient for gas-forced convection ranges from 20 to 100 W/(m²·℃), while the heat transfer coefficient for water-forced convection is as high as 15,000 W/(m²·℃), which is over a hundred times greater than that of gas-forced convection.
Currently, many variable-frequency drive units utilize water-cooled systems for heat dissipation. The water-cooled heat dissipation system is an enclosed liquid circulation unit, as shown in Figure 2, which circulates the liquid within the sealed system using the power generated by a pump. It transfers the heat produced by the chips absorbed by the heat sink through the liquid circulation to a larger heat dissipation unit for cooling. The cooled liquid then returns to the heat-absorbing equipment, and this cycle repeats. Alternatively, another method of water-cooled dissipation involves continuously replenishing new cooling water to cool the unit, directly draining the heated water from the system. However, this method is water-intensive and suitable only for specific applications, thus the former water-cooling method is generally used. Since the water-cooled system lacks fans, it does not produce vibration, and the noise level is relatively low. Its drawback is the high cost, and water in an enclosed state is prone to scaling and degradation, with the need to completely prevent leaks and water interruptions during operation. Additionally, during operation, the flow of water within the system can cause changes in the electromagnetic field around electronic components, which may affect system stability.
4. Heatpipe Cooling
Heat pipes are highly efficient artificial heat transfer components that utilize the principle of "phase change" for heat transfer, differing significantly from conventional metal materials, solid materials, and natural heat transfer methods. The structure of heat pipes is flexible and diverse, with considerable differences among them. A typical heat pipe, as shown in Figure 3, is composed of a tube shell, absorbent core, working fluid, etc. After extracting a portion of the gas inside the tube, it is evacuated to a constant negative pressure and then filled with an appropriate amount of working fluid. This fills the capillary porous material of the absorbent core that is in close contact with the inner wall of the tube, after which the tube is sealed. One end of the tube is the evaporation section (heating section), and the other end is the condensation section (cooling section), with an insulating section placed between the two. The liquid medium absorbs heat from the heat source and vaporizes in the evaporation section. Under the action of a small pressure difference, it quickly flows towards the condensation section, condensing into a liquid by releasing latent heat to the cold source. The condensed liquid is then drawn back to the evaporation section by the capillary suction force of the absorbent core. This cycle repeats, continuously transferring heat from the evaporation section to the condensation section. The major advantage of heat pipes is their ability to transfer large amounts of heat with very small temperature differences; their relative thermal conductivity is several hundred times that of copper, and they are known as "near-superconducting thermal bodies."
The principle and characteristics of heat pipe cooling allow engineering designers to select the system cooling method based on system features and actual needs. They are not limited to a single cooling method and can opt to use two or more cooling methods simultaneously in special circumstances.
