Motor torque ratings change when motors operate under ambient temperature conditions different than those listed in the specifications. The approximate change is calculated from the equation:

where TNEW = new torque rating; TSPEC = specified torque rating; ta = actual temperature rise; and tr = rated temperature rise.

For example, consider a servomotor rated at 24.8 Nm when operated at 40°C ambient temperature. The rated maximum temperature rise of the motor is 100°C. Ambient temperature in the area of operation was specified to be 65°C.

First, calculate the maximum temperature of the motor by adding the rated motor temperature rise to its ambient specification: 100 + 40 = 140°C maximum motor temperature.

Subtract the specified ambient temperature from the maximum motor temperature to determine the actual temperature rise permitted for the motor in this application: 140 - 65 = 75°C actual temperature rise.

Plug the numbers into the equation to calculate the new torque rating of the motor:

In this example, motor torque dropped 13%. The current rating of the motor was also reduced by the same percentage.