Insulation Classes of Windings

The common types of insulating materials in use for electric motors are E and B for small motors and F for medium sized and large ones. General industrial practice, however, is to limit the temperature to class B limits, even if class F insulation is used.

Insulation class A, previously in use, has been discontinued in view of its low working temperature. Motor frames have also been standardized with class E insulation only by IEC recommendations, to harmonize the interchangeability of electric motors. This decision was taken because class E insulation offers a higher working temperature and a longer working life.
 

These frames also ensure optimum utilization of active materials such as copper and steel in a particular frame size. The classification of insulating materials is based on their maximum continuous working temperature, established for 20 years of working life. The recommended temperature according to IEC 60034- 1 is, however, less than this, as shown in table,  to ensure an even longer life.

Maxmum permissible working temperatures for different insulating materials

Class of Insulation	Maximum attainable temperature as in IEC 60085 oC	Permissible operating temperature as in IEC 60034-1 by the resistance method in oC
		Up to 5000 kW	Above 5000 kW
A	105	100	100
E	120	115	110
B	130	120	120
F	155	145	140
H	180	165	165

Notes: 1. For large motors, Class A is not used due to its low operating temperature 2. Using the thermometer method, these temperatures will be less by 10^oC.

A simple acronym will be useful in remembering the temperature values. Consider the phrase, "Japanese Manufacturers Normally Serve West". The first letters of all words are JMNSW. Take the coordinate numbers of each letter. They are, 10, 13, 14, 19 and 23. Multiply each by 5 and add 50 to it. You will get the maximum temperature limit for classes A, E, B, F and H in that order.

Where the ambient temperature is likely to be high, of the order of 60°C or so during operation, such as close to a furnace, class F and H insulations are normally used, as they have higher working temperatures and thermal resistivity.
 

A brief description of the insulating materials in use for different classes of insulation is given below to provide an introduction to the types of materials being used in the preparation of a particular class of insulation. The actual ingredients may be an improvised version of these materials, in view of continuous research and development in this field, to search for still better and more suitable materials.

Insulation Class A                  

This includes organic fibrous materials on a cellulose base such as paper, pressboard, cotton, cotton cloth and natural silk etc., impregnated with lacquers or immersed in an insulating liquid. The impregnation or immersion ensures that the oxygen content of the air does not affect the insulating properties or enhance the thermal ageing of the insulating material. Typical materials in this class are varnished cloth and oil-impregnated paper.

Insulation Class E              

This includes wire enamels on a base of polyvinyl formal, polyurethane or epoxy resins as well as moulding powder plastics on phenol-formaldehyde and similar binders, with cellulose fillers, laminated plastics on paper and cotton cloth base, triacetate cellulose films, films and fibres of polyethylene terephthalate.

Insulation Class B                

This includes inorganic materials such as mica, glass fibre and asbestos etc., impregnated or glued together with varnishes or compositions comprising ordinary organic substances for heat resistance such as oil-modified synthetic resins, bitumen, shellac and Bakelite.

Insulation Class F         

This includes inorganic materials such as glass fibre and mica impregnated or glued together with epoxy, polyesterimide, polyurethane or other resins having superior thermal stability.

Insulation Class H

This comprises composite materials on mica, glass fibre and asbestos bases, impregnated or glued together with silicone resins or silicone elastomer. These materials must not contain any organic fibrous materials such as paper or cloth backing, which is covered under class B and even F insulation systems.