Block 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?
First you will need to know the maximum winding temperature
of the motor based on the published ratings data (e.g. 40°C ambient).
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).
where ΔTemp Rated = Temp(maximum winding) – Temp(rated
So the new continuous torque Tc(new) would
Torque 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.
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.