Quite often we get customer inquires related to electric motor operating temperatures.  If a customer explains that the motor is hot to the touch, that in an of itself is not a proper or even helpful measurement.  Gauging temperatures by touch is never recommended, because depending on the motor, or application, proper or normal operating temperatures may be way above what could be considered safe to touch.  Measuring the temperature at the surface of the motor is not a useful indicator of the temperature of the windings or bearings.

Motor temperatures limits are mainly based on bearing, lubricant, and winding thermal capability.  The winding limit is given by the temperature class of the insulation system.  For example, 266 degrees F, for class B, and the bearing and lubricant upper limit is usually around 212 degrees F.   To determine if a motor is running hot, we need to measure the winding temperature.  If the temperature is under the insulation class system rating, the temperature is acceptable.  If the temperature is at the limit or above, the bearing temperatures should be checked.