Don't judge motor status by surface temperature


Question: Is motor reliability at risk if the unit's surface hits 200 ° F? We know that we're not suppose to use the frame surface temperature as a reliable indication of motor loading. But, it's easy to check, and while we don't use it to compare efficiencies, for instance, there must be some point at which we should become concerned about motor reliability. Some motors in our facility are reaching 200 ° F on the outside. What is a convincing argument as to what reasonable limit may apply?

Answer: One of the reasons surface temperature is a poor guide to motor condition is that it varies so widely with specific design and enclosure. All we can say is that the winding of a Class B insulated motor can safely operate indefinitely at 130 ° C (266 ° F) while the external surface temperature typically is 80 ° C to 85 ° C (177 ° F to 186 ° F). For a Class F version, add 45 ° F to all these numbers. If the motor is a small non-ventilated unit, add another 45 ° F. Clearly 200 ° F is no cause for alarm, and no specific upper limit will suit all motors.

Source: Dayton HVAC Motors and Grainger