Permissible Temperatures for Insulation

Author: E.E. Kimberly

The full-load rating of a motor or generator is that load which it can deliver without exceeding the safe temperature of the insulation on the windings. Many insulating materials are used in various combinations, and the temperature permissible for any combination is that permissible for the material in it which has the lowest safe rating. The National Electrical Manufacturers Association (NEMA), which is an organization of electrical manufacturers formed for the purpose of setting manufacturing standards for all types of electrical apparatus, and the American Institute of Electrical Engineers have grouped the various materials into classes according to permissible operating temperatures. These classes are 0, A, B, C, and H.

Class 0: This class of insulation consists of cotton, silk, paper, and similar organic materials when neither impregnated nor immersed in liquid dielectric. The term organic is defined as a chemical compound containing carbon in some form. The maximum safe temperature for this material is 90 C. The 90 degrees are generally thought of as being composed of 40 C ambient, 40 degrees average rise over ambient, and 10 degrees hot spot over average rise. The standard ambient temperature of 40 C has been selected because it is near the maximum encountered in most parts of the United States. Should the ambient be at some temperature lower than 40 C, the average rise may be increased by the difference between the actual ambient temperature and 40 C. Machines with Class-0 insulation are called 40-degree machines.

Class A: This classification includes: (1) all insulation materials listed under Class 0 when impregnated or immersed in a liquid dielectric; (2) molded and laminated materials with cellulose filler, phenolic resins, and other resins with similar properties; (3) films and sheets of cellulose acetate and other cellulose derivatives having similar properties; (4) varnishes (enamel) as applied to conductors. By a "rule of thumb" it is considered that the life of this type of insulation is halved with every increase in temperature of 8 degrees C. The limiting hot-spot temperature for this class is 105 C. Machines with Class-A insulation are called 55-degree machines.

Class B: Class-B insulation consists of inorganic materials with organic substances as binders and a small percentage of Class-A materials for structural purposes. Mica, asbestos, fiberglass, and similar inorganic materials are included in this class. The binder is generally recognized as being the temperature-limiting part in most Class-B insulation. The limiting hot-spot temperature for this class is 105 C. The average life of this class is said to be halved for every increase in temperature of 10 degrees C. Machines with Class-B insulation are called 80-degree machines.

Class C: This class of insulation includes only inorganic materials, such as porcelain, quartz, glass, and mica. The temperature stability of these materials is so great that no limiting hot-spot temperature is specified. The performance of the machine, rather than its temperature, usually serves to limit the maximum operating load.

Class H: This class includes almost all Class-B materials, but the binding substance is composed of silicone compounds or materials of equivalent properties. The recommended limiting hot-spot temperature for this class is 180 C.

The temperature specified on the name plate of power machinery is the temperature rise over ambient permissible for the class of insulation used. The temperature is measured on the outside of the insulation by a mercury-in-glass thermometer.

A number of books known as the NEMA Standards are published by the National Electrical Manufacturers Association. In addition to the permissible temperature rises, NEMA has established horsepower ratings of motors and many other standards of performance and design.