When electric machinery runs, it generates heat, which in turn requires regulation through a cooling system. In aircraft, machinery like generators are frequently managed with oil spray cooling; however, this technique demands large amounts of oil and causes excessive rotor friction, that of which lessens efficiency. For this reason, this blog will explore alternative cooling approaches and their benefits for aviation applications. A major concern when seeking cooling technologies for high power density machines in aircraft is the weight they will add to the system, so low-weight methods like direct slot cooling (DSC) have become promising approaches.
There are certain holistic requirements to consider when operating machinery in certain environments, and ambient temperature is a major concern for aviation machines. Ambient temperature refers to the temperature of the environment in which equipment, in this case aircraft equipment, operates. Ground temperatures around the world can vary from -40 to +40 degrees Celsius, so the task of maintaining ideal operating temperatures can prove difficult. For example, if the ambient temperature is 40 degrees Celsius, take-off weight must be reduced to account for heat produced by the generator.
In addition to your aircraft’s environment, the design of your machine, specifically its weight and density, contribute to its ability to function without generating too much heat. For example, your machine’s current density and its flux density must be balanced in a manner that allows for electromagnetic conductivity without adding too much weight to your aircraft. A wise solution is to utilize small pole pitches and an iron cobalt material with saturation flux density of 2.4 T, though this can be costly. Another factor to consider is the occurrence of loss from copper, iron, or other metal materials. Loss produces heat, so it is ideal to use materials that can operate at low ambient temperatures. Using low temperatures and dry air at high altitudes for the cooling of the drives allows the design of a lightweight cooling system like conventional fluid cooling jackets; however, when ground temperatures are high, machinery may require dimensioned cooling systems and additional boost systems.
One approach for additional cooling is to cool your system with cold gas, either from a compressor or from a compressor expander unit. The reservoir storage system adds a great amount of weight and is therefore less suitable for aviation; however, an improved approach could use an on-board compressor, isobaric cooling, and expansion by a turbine to achieve a low temperature for the cooling air. Theoretically, this design could work beyond providing boosts, but the added weight of this system’s piping, air dryer, and other components makes it harder to justify.
Another approach to cooling is to utilize systems that rely on phase change enthalpy. These systems remove loss through phase changes like boiling and evaporation. Systems that rely on water reservoirs and the Rankin cycle are appealing for aviation applications as they could function as lightweight boost systems in a thermal critical environment. Again, this requires additional piping, so experts have turned to focus instead on improvements that can be made directly to the machine.
Within the machine, the thermal conductivity path can be improved by the addition of cooling channels placed between the copper conductors and the slots. This approach to better cooling systems may increase the weight of the teeth and yoke, but the losses are then removed where they arise and high current densities under overload and steady state operations can be achieved with a decreased temperature rise in the winding. As this method exhibits potential to optimize the current sheet, force density, temperature distribution, and the weight of the machine, slots should be further considered for your high power density aircraft machinery.
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