Understanding the differing insulation classes, and limitations for each, is an essential part of an effective reliability program. Operation of motors within the designed parameters of their insulation class can yield significantly increased reliability.
There are basically four classes of insulation - A, B, F and H - class F being the most commonly 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, the 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 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.
Note: Abnormal deterioration of insulation may be expected for regular operation at ambient temperatures above 40˚C (104˚F). (Source MG-1 12.43)
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 being installed in a particular application has insulation designed to withstand the temperatures developed under full load and the effects of the ambient. Failure to do so may significantly reduce motor longevity.