DISTRIBUTED ELECTRIC PROPULSION [CONTINUED]
general industrial applications may be successfully employed in eVTOL air taxi applications, Glenair’s expectation is that the FAA and other qualifying agencies will take a dim view of interconnect technology that has not been tested and approved for its proven resistance to common airborne operational stress factors. Current Rating When electrical current travels across a conductor, inefficiencies and resistance in the conductor cause some of the electrical power to convert into heat. This can be seen in the glowing filament of an incandescent light bulb or the red-hot coils in a toaster oven. The total heat produced is dependent on the current moving across the conductor (measured in amps, “A”) and the inefficiency of the conductor, or resistance (measured in Ohms, Ω). For any given conductor, as more current is applied, more energy is converted into heat. The heat generated causes the conductor to reach temperatures above ambient, a process known as “temperature rise.” For any given current, less efficient (or more resistive) conductors will lose more energy to heat, and experience greater temperature rise. Large conductors (measured in “gauge,” typically American Wire Gauge or “AWG” in North America) have more material to conduct the current, and therefore lower resistances. Similarly, different conductor materials have inherently lower resistances (copper versus aluminum, for example). The lower the resistance, the lower the temperature rise. By extension, the lower the resistance, the greater the allowable current for a specified allowable temperature rise. This is the foundation for current rating. For any given conductor and current, the temperature rise is a balance between heat produced and heat lost through conduction or convection. For eVTOL airborne applications, low- density, high-altitude air is a very poor coolant, keeping more heat in the conductor and leading to higher temperature rise. As a result, current ratings for eVTOL interconnect products are generally lower
than current ratings for land and sea interconnect products, even if the underlying interconnect is essentially identical. Here’s another handy metaphor: it is common knowledge that water feels colder than air. 60°F air would feel like a nice fall afternoon. However, 60°F water would lead to hypothermia within an hour. Compared to air, water has a high heat-capacity and conductivity. Water can pull heat from the human body faster than it is metabolically produced. Similarly, water effectively pulls heat from conductors, allowing higher currents to be obtained while maintaining a low temperature rise. In eVTOL airborne power distribution systems, there are practical limitations to the maximum temperature rise. Most electrical interconnects are only rated to 200°C, some a bit higher (230-260°C), some a bit lower (150-175°C). Often the electrical equipment utilizing the interconnect could have an even lower temperature rating. Also, ambient temperature will vary between applications, depending on location within the aircraft and proximity to other equipment. The difference between the maximum permissible temperature of the interconnect and the ambient temperature is the maximum allowable temperature rise . Application conditions dictate maximum allowable temperature rise, and by extension the maximum current permissible on any given conductor. Current Derating With baseline current rating established by allowable temperature rise under ideal conditions, conductor performance can then be derated to conform with estimated performance under actual application. As just discussed, the first current derating element is allowable temperature rise , determined by the difference between ambient temperature and maximum allowable temperature of the interconnect. Since temperature rise under
QwikConnect • July 2021
12
Powered by FlippingBook