Role of Ambient Temperature on the Motor Performance

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Further to my post I agree with JMUELLER"s view.Lower ambient also creates problems in addition to nessecity of special low temp grease for the bearings.In case of temp rise every deg rise beyond permissible temp. limit life of insulation is halved.While very low ambients are posible , higher ambient beyong 50 is rare.I have not come across but I don't rule out the possibility.

The insulation used in windings has a temp. limit.Normally motors to day have class B or F Class Insulation and in special cases class H.Mostly it is class F and in some cases it is class B as far as LT motors are concerned.All HT motors invariably have class F Insulation.Let us take the case of a Class F Insulation motor.The max.temp it can withstand is 155Deg C .a hot spot allowance of 15Deg C as per design practice and std is allowed(10Deg For class B)this reduces the max temp limit to140Deg C.If the ambient is 40Deg you can allow temp rise of 100Deg.If it is 50 it is 90 deg.so out put gets reduced with increased ambient.In short a deration is required.in addition many times it is customary to limit temp.rise to class B even in class F motors.The temp rise and reduction in out put can be determined more accurately for an individual motor by looking at the max temp it has reached during temp.rise test.based on this suitable reduction in out put is possible.Mostly motors have lot of cushion by the way of margins on HP,Hot spot allowance,Limitation on temp rise and motor designers inherent safety margins.Things are better with computerised deign where on can go to the brink.

See Ohm's & Watt's Laws. Resistance is related to temperature. Current is related to resistance. Wattage is related to current. The NEC has excellent charts on the current carrying capacity of a conductor vs. temperature ratings. This is in addition to friction and other variables that change with temperature, all of which effect performance.

Lightning.arrester,

In addition to Nesubra's good comments, let me stress that heat is the "enemy" of insulation. The closer the motor insulation is to its maximum rated temperature, the shorter its life span will be. If the temperature is above its rated working temperature, the fall-off in life span can be rapid as the insulation carbonizes and becomes a conductor. Besides the problems with insulation, there are other problems with bearing lubricants and (to a lesser extent) changing tolerances between parts due to differential thermal expansion rates, etc.

Excessive cold can also cause problems in addition to the well-known problems of internal condensation when the motor is in a humid environment and is colder than the air temperature (like a warming-up day).

--JMM