Motor Temperature: Insulation Class and Temperature Ratings
A frequently misunderstood subject related to electric motors is insulation class and temperature ratings. This paper tries to describe, in basic terms, the temperature relationships that are meaningful in standard AC induction motors. Some of the same information can be applied to DC motors but DC motors are more specialized and some of the ratings are slightly different.
Perhaps the best way to start is to define the commonly used terms.
MOTOR SURFACE TEMPERATURES
Motor surface temperature is frequently of concern. The motor surface temperature will never exceed the internal temperature of the motor. However, depending upon the design and cooling arrangements in the motor, motor surface temperature in modern motors can be high enough to be very uncomfortable to the touch. Surface temperatures of 75° to 95° C can be found on T frame motor designs. These temperatures do not necessarily indicate overload or impending motor failure.
Insulation life is affected by many factors aside from temperature. Moisture, chemicals, oil, vibration, fungus growth, abrasive particles, and mechanical abrasion created by frequent starts, all work to shorten insulation life. On some applications if the operating environment and motor load conditions can be properly defined, suitable means of winding protection can be provided to obtain reasonable motor life in spite of external disturbing factors.
OLD AND CURRENT STANDARDS
U frame 184 through 445U frames, were designed based on using Class A insulation. Temperature rise was not precisely defined by the resistance method. Temperature rise by thermometer for Class A, open drip proof motors was 40° C. This was generally thought to be equivalent to approximately 50° C by resistance. U frame motors were the industry standard from 1954 to 1965 and are still preferred in some industries and plants. T frame, 143T through 449T motors are generally designed based on using Class B insulation