Motor Derating Due To High Ambient Temperatures

January 12, 2017, by Hurley Gill

Block and TackleBlock and Tackle Series Volume 2 – Motor Derating Due To High Ambient Temperatures


How do I calculate a motor’s continuous torque when it is operating in an environment above its rated temperature? Since the motor’s continuous torque (Tc) is rated in a 40°C ambient, how can I estimate the motor’s continuous torque during my worst-case ambient temperature of 55°C? 


Motor Specification SheetFirst you will need to know the maximum winding temperature of the motor based on the published ratings data (e.g. 40°C ambient).

If not directly given within the motor’s catalog data, it can be determined by adding the 40°C to the published delta (Δ) Temperature of the winding (assuming it is given; if not, you will need to contact the motor’s manufacturer.  Assuming it is given to be 100°C, the maximum winding temperature = 140°C).

Temperature  derating equation

where ΔTemp Rated = Temp(maximum winding) – Temp(rated ambient)


So the new continuous torque Tc(new) would be:
    Example Equation for Temperature Derating


Performance CurveTorque required, Tr(new) at a specific RPM, can also be estimated by graphically finding  the maximum continuous capability with the specified ambient temperature at the desired RPM, from the motor’s performance curve (PC), if not available from the motor’s data sheet.

This is most often done by graphically determining Tr(40°C) at the desired RPM from the motor’s PC and using it in place of Tc(40°C), in the above formula.


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