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Motor Derating Due To High Ambient Temperatures

09 Nov 2021
Hurley Gill

Question:

How do I calculate a motor’s continuous torque when it is operating in an environment above its rated temperature? Our selected 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?

Answer:

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.

About the Author

Hurley Gill

Hurley Gill

Hurley Gill is a senior application engineer at Kollmorgen. With over 40 years of expertise in the motion control and automation industry, he is often called upon to solve the toughest of application challenges for Kollmorgen customers. Hurley holds a B.S. degree in electrical and electronic engineering technology from Virginia Tech. When not focusing on motion control, Hurley enjoys data analysis, harmonics and building/rebuilding projects.

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