While dissipation of the heat (stator, rotor and bearing) is related to
ambient temperature, it is also directly linked to air flow and air density.
Just changing the ambient isn't enough. Sure, you might dissipate slightly
more with a lower ambient, but increasing the cooling aiflow (both
externally and internally) can have a greater effect. Most standard motors
are rated for minus 20 to plus 40 Deg C ambient, up to 1000 m elevation.
Also keep in mind that most industrial motors are wound with one class of
insulation, but to increase operating life, actually run at the next lower
insulation class's temperature rise (ie: F insulation with a B temp rise).
Sometimes this cushion is used to accomodate the extra elevation.
But the end result is still the same: hotter running motors have lower
If you are purchasing a new motor for the application, it is possible for
the manufacturer to take into consideration the elevation, and make a unit
that is not de-rated.
I'd been involved in many pieces of equipment going to the copper mines of
South America at 11,000 feet.
(Also, higher rotor and stator temperatures can cause premature bearing
failure since the heat reduces the effectiveness of the lubricant)