DON'T JUDGE A MOTOR BY ITS SURFACE TEMPERATURE

Because heat eventually destroys electrical insulation, a motor (like any other electrical device) has a limiting temperature rating. That's normally stated as an allowable rise or increase above a certain ambient or surroundings temperature. How high a rise can be permitted will depend upon the particular insulation materials used; the motor enclosure, which determines how ventilation removes internal heat; and whether or not a "service factor" overload rating applies. But the ambient is standardized for all motors at a single value: 40 degrees C (104 degrees F). Industrial or commercial ambients seldom exceed 25- 30 degrees C, but occasionally can hit 50-55 (which will require a special motor design.)

Although less sensitive than insulation, bearings and lubricant are also affected by heat. Separated by some distance from the hot motor winding, bearings are seldom damaged. But different lubricants, or more frequent lubrication, may still be needed in hot surroundings.

Whatever nameplate temperature rating applies to a motor governs only the winding temperature. How hot the motor may become on its exposed outside surface is in no way standardized (except for explosion-proof motors, where too hot a surface would present a fire hazard). Many an old-time electrician or plant maintenance worker has claimed to be able to judge motor safety or quality by feel by placing a hand on the motor for a few seconds. Some rules of thumb even relate actual temperature to the length of time the average person can maintain hand contact. But this is at best a highly subjective, inaccurate practice.

Fully-loaded internal winding temperature of a standard motor, whatever its efficiency, will be as much as 90 degrees C above ambient. In a typical 30 degree ambient, that's a total temperature of 120 degrees C — far above the boiling point of water. How hot will the motor surface be? No design or "typical" values apply, but 80 to 100 degrees C is easily possible. No one can stand skin contact with that for more than an instant. Any meaningful distinction between one temperature and another at such levels, in that way, is clearly impossible.

A thermometer or thermocouple would, of course, give accurate values. But with what is the reading to be compared? How shall it be judged? Some users presume that a more efficient motor will run at a lower temperature. When comparing external surface temperature of two motors, they expect the cooler one to be more efficient. However, such a comparison is pointless. Heat transfer paths differ from one motor to the next. A motor having higher internal loss may display lower surface temperature. Conversely, some high efficiency motors, with low internal loss, may run hotter than a standard efficiency unit because smaller ventilating fans have been used to reduce windage loss. The larger fan isn't necessary because there's less heat to carry away from the lower-loss motor.

Regardless of how it's measured, then, motor external surface temperature is no reliable guide to either performance or reliability of the machine.

Richard L. Nailen, P. E.

Temperature Rise and Class of Insulation System