Why are Electrical Power Transformers / generators de-rated for operations on altitutude?
The “efficiency” of a device may drop with rising temperatures, so many mechanical devices are air-cooled to reduce their operating temperatures. As the air thins out with increasing altitude its ability to carry away heat decreases, so it does not cool the device as well, so the device’s temperature rises, and so its efficiency falls. When its efficiency falls, the amount of power it can put out decreases, and so they derate it. The reason a generator, for instance, might loose efficiency as its temperature increased, is because the copper wire coils become more resistive and lossy as their temp rises, so more of the generated power is lost in the internal windings, thus reducing the efficiency.
