Understanding the differing insulation classes, and limitations for each is an essential part of an effective reliability program. The operation of motors within the designed parameters of their insulation class can yield significantly increased reliability and motor longevity.

There are four classes of insulation - A, B, F, and H - class F being the most used. These insulation classes were established to meet motor temperature requirements found in industrial applications. Class A is an older form of insulation utilizing organic varnishes and resins available many years ago. You also see class A insulation on a lot of older generators. Today it would only be seen in very old motors and would not be used for new or remanufactured motors.

The motor’s ambient temperature, internal temperature rise, altitude (>3300 Feet), and the service factor will be the determining factors in choosing the class of insulation. The ambient temperature factor is determined by the measurement of ambient air surrounding the motor. The higher the temperature rise permitted, the higher the insulation class. The higher the insulation class, the greater temperature the insulation can withstand without degradation. Temperature charts of the insulation classes are used to select the proper insulation to ensure long dependable insulation life.

Motor ambient temperature is assumed to be 40˚C (104˚F), unless otherwise specified, altitude 3300 feet or less, service factor of one, and there is a margin allowed for the center of the motor windings “Hot Spot.” These numbers, and the allowable temperature rise set, must not exceed the maximum operating temperature for the class of insulation selected.

Cautions from NEMA MG-1

Note: Abnormal deterioration of insulation may be expected for regular operation at ambient temperatures above 40˚C (104˚F). (Source MG-1

Note: Do not confuse the NEMA insulation classes with the NEMA motor design codes which are also given by letters.

Note: For successful operation of induction machines in ambient temperatures higher than 40˚C, the temperature rises of the machines given for the selected insulation class shall be reduced by the number of degrees that the ambient temperature exceeds 40˚C. (Source MG-1 12.43.1)

Understanding motor insulation class is essential. You must ensure that a motor that is being installed in a particular application has insulation designed to withstand the temperatures developed under full load and the effects of the ambient conditions. Failure to do so may significantly reduce motor longevity. A good “thumb rule” would be for every 10OC above the maximum insulation temperature, you cut motor longevity by half. Say for example you have a class F motor that reaches 165OC, if it would have lasted for 20 years, it will now last 10.

Another good point of general guidance for determining motor internal temperature, is to add 20OC, 36OF to the acquired external motor temperature. This will closely approximate the maximum internal temperature.