Although high-voltage variable frequency speed control systems are highly efficient speed control devices, they still experience losses of approximately 2%-4% during operation. These losses are converted into heat and ultimately dissipated into the atmosphere. How to effectively remove this heat from the frequency converter is a crucial issue in its design.
The heat-generating components of a high-voltage frequency converter mainly consist of two parts: the rectifier transformer and the power components. The heat dissipation method for the power components is crucial. Modern frequency converters generally use air cooling or water cooling. For lower power applications, air cooling is sufficient. For higher power applications, water needs to circulate through the radiator to remove heat. Because radiators typically have different electrical potentials, water with good insulation properties must be used; purified water, which has a lower ion content than ordinary distilled water, is generally preferred. An ion exchanger is usually added to the water circulation system because metal ions from the radiator continuously dissolve into the water, and these ions need to be adsorbed and removed.
From a heat dissipation perspective, water cooling is ideal. However, water circulation systems have high manufacturing requirements, complex installation, and require significant maintenance. Furthermore, leaks can pose safety hazards. Therefore, water cooling should be avoided in situations where air cooling can solve the problem.
Air cooling has its limits in terms of heat dissipation capacity, and these limits are related to the specific technology. For example, ABB's ACS1000 series three-level frequency converters require water cooling for capacities above 2000kW, while Robicon and AB in the US still use air cooling for their 3200kW/6KV frequency converters. Why is this?
It turns out that the amount of heat that air cooling can remove from equipment is related to the size of the effective heat dissipation area; the larger the heat dissipation area, the more heat can be removed. The more components there are, the larger the heat dissipation area, and the better the air cooling effect. For a 6KV frequency converter, there are more components than a 3KV frequency converter, and the current of each individual component is smaller, so there can be a larger heat dissipation area, which is equivalent to the heat being distributed more evenly.
Some might argue that simply increasing the surface area of the heat sink increases the heat dissipation area. However, our company's product development department's experiments have proven this to be a paradox. Heat from power electronic components is conducted as follows: it spreads along the surface of the heat sink, then transfers along the surface to the heatsink fins, and is carried away by the air. The area that spreads along the heat sink surface is very limited; at a distance from the component, the heat is barely perceptible. Therefore, increasing the surface area of the heat sink beyond a certain point has no significant impact on heat dissipation. The same applies to the heatsink fins; the temperature is higher at the root of the fins, while very little heat reaches the tips. Therefore, increasing the height of the fins beyond a certain point is also ineffective for heat dissipation.
Therefore, the only effective way to solve the air cooling problem of high-power products is to use many components to distribute the heat evenly and increase the effective heat dissipation area.
Of course, using new-generation components with lower power consumption or new heat sinks with lower thermal resistance can also increase the power of air-cooled frequency converters. For example, with the current IGBT packaging, we previously found that without parallel connection of devices, we could only achieve 1800KW/6KV. Now, due to the adoption of new-generation IGBT devices and new heat sinks, we can achieve 2300KW/6KV. This is another aspect of technological research and does not contradict the analysis above.
So why do we use IGBTs in parallel in frequency converters of 2500KW/6KV and above? It's not because we can't buy IGBTs with such high current, but because, through experiments, we found that, under current technological conditions, if we don't use parallel connection of components to increase the effective heat dissipation area, we cannot carry away the internal heat with air, and we cannot guarantee that the temperature rise of the components meets the requirements.
We are now researching and developing a 5000KW/6KV frequency converter. Why are we so confident? Because our original 3200KW/6KV frequency converter used 15 power units to remove heat. When we reached 5000KW, we increased the number of power units to 24, and the amount of heat removed by each power unit remained about the same.
Some people may ask: Why doesn't ABB use parallel connection of components? This is because, among all the components, only IGBTs and MOSFETs have a positive temperature coefficient and are suitable for parallel connection, while IGCTs are not suitable for parallel connection, so they must be water-cooled.
Another issue regarding inverter heat dissipation is how to dissipate the heat carried away from the inverter into the atmosphere. For water-cooled systems, a water-to-air cooling unit needs to be installed outdoors to cool the hot water. For air-cooled systems, if the heat dissipation is large, air ducts need to be installed to directly exhaust the hot air outdoors; otherwise, the hot air will accumulate indoors, causing the room temperature to rise. Some users previously considered using indoor air conditioning units for cooling, but this has proven uneconomical for high-power inverter applications, requiring large air conditioning configurations. If the user's factory has cooling water, we recommend using a water-to-air cooling unit. This unit is similar to the air conditioning system in our factory, with heat sinks embedded in water pipes. Cool water is circulated through the pipes, and the flow rate of the cool water must be large enough to ensure a low temperature for the heat sinks. The hot air dissipated from the inverter enters the heat sinks and is then cooled. This method allows the inverter to be placed in a sealed room, eliminating concerns about dust.
In short, there is a lot to learn about the heat dissipation of frequency converters, and structural designers have discovered many interesting phenomena during experiments. The structural design of frequency converters is often not as simple as just putting things in; it requires consideration of many factors.
