Motor derating due to ambient temp

You need to determine the rated rise of the motor. Most modern motors are rated as class F insulation equipped. If the rated rise is 95 Degrees C, I would not worry about derating unless you are operating in the service factor. If it is a two pole running at approximately 3000 RPM or faster, I would pay attention to the greasing interval.

Hello Pop,

The most electric motors are designed for an altitude Up to 1000 meters."The environment of the motor is taken to include its entire conditions of working. These include: the maximum ambient temperature of the cooling air: temperatures above 40 C reduce the nominal rating of the motor. As an approximate guide an increase of 5 C in the ambient air temperature reduces the power rating by 5%.

The altitude of installation: air density is reduced at significant altitudes above sea level and with it the capacity of the air to cool the motor. The following table may be used as a guide:

Altitude (m) 2000 3000 4000
Reduction in Power Rating (%) 8 15 25

The combination of these two factors will affect the class of insulation to be specified for the motor since both affect the cooling rate. The following table gives a guide to the allowable temperature rise for the most common insulation classes"


Max. Temp Rise (C) 75 90 140
Insulation Class (BS 4999) E B F

Regards
Petronila

For Ambient temperature increase to 50°C as a rule of thumb, derate the motor capacity 10%.
Exact calculation is complex since part of the motor losses are constant (core loss, friction and windage) and the remaining losses are proportional to the motor current squared (stator I^2*R, rotor I^2*R and Stray load loss). The reduction of load will attempt to reduce the motor current and then the temperature rise by 10°C, this will allow the extra 10°C on ambient.

pop
remember it is current you are derating not kilowatts and also be aware of other equipment either controlling or starting the motor may also need looking into if the ambient is increasing to 50°C, especially a VSD/VFD.