Page 242 - Industrial Power Engineering and Applications Handbook

Winding insulation and its maintenance 9/221
9.1 Insulating materials and their the insulating properties or enhance the thermal ageing
properties of the insulating material. Typical materials in this class
are varnished cloth and oil-impregnated paper.
The common types of insulating materials in use for 9.1.2 Insulation class E
electric motors are E and B for small motors and F for
medium sized and large ones. General industrial practice, This includes wire enamels on a base of polyvinyl formal,
however, is to limit the temperature to class B limits, polyurethane or epoxy resins as well as moulding powder
even if class F insulation is used. plastics on phenol-formaldehyde and similar binders, with
Insulation class A, previously in use, has been cellulose fillers, laminated plastics on paper and cotton
discontinued in view of its low working temperature. cloth base, triacetate cellulose films, films and fibres of
Motor frames have also been standardized with class E polyethylene terephthalate.
insulation only by IEC recommendations, as in Table
1.2, to harmonize the interchangeability of electric motors. 9.1.3 Insulation class B
This decision was taken because class E insulation offers This includes inorganic materials such as mica, glass
a higher working temperature and a longer working life. fibre and asbestos etc., impregnated or glued together
These frames also ensure optimum utilization of active with varnishes or compositions comprising ordinary
materials such as copper and steel in a particular frame organic substances for heat resistance such as oil-modified
size. The classification of insulating materials is based synthetic resins, bitumen, shellac and Bakelite.
on their maximum continuous working temperature,
established for 20 years of working life. The recommended
temperature according to IEC 60034- 1 is, however, less 9.1.4 Insulation class F
than this, as shown in Table 9.1, to ensure an even longer This includes inorganic materials such as glass fibre and
life. Where the ambient temperature is likely to be high, mica impregnated or glued together with epoxy,
of the order of 60°C or so during operation, such as polyesterimide, polyurethane or other resins having
close to a furnace, class F and H insulations are normally superior thermal stability.
used, as they have higher working temperatures and
thermal resistivity. 9.1.5 Insulation class H
A brief description of the insulating materials in use
for different classes of insulation is given below to provide This comprises composite materials on mica, glass fibre
an introduction to the types of materials being used in and asbestos bases, impregnated or glued together with
the preparation of a particular class of insulation. The silicone resins or silicone elastomer. These materials must
actual ingredients may be an improvised version of these not contain any organic fibrous materials such as paper
materials, in view of continuous research and development or cloth backing, which is covered under class B and
in this field, to search for still better and more suitable even F insulation systems.
materials.
9.1.1 Insulation class A 9.2 Ageing of insulation
This includes organic fibrous materials on a cellulose
base such as paper, pressboard, cotton, cotton cloth and With time, the insulation becomes brittle and shrinks,
natural silk etc., impregnated with lacquers or immersed leading to cracks. The insulation at the point of cracks
in an insulating liquid. The impregnation or immersion weakens gradually as surrounding pollutants find their
ensures that the oxygen content of the air does not affect way through these cracks. The weakening of insulation
with time is called 'ageing'.
Table 9.1 Maxirnumlperrnissible working temperatures for The life of the insulation will also be affected by an
different insulating materials excessive operating temperature. It is halved for every
1 1 "C rise in temperature over its rated value and occurs
Class of Maximum Permissible operating temperature when a machine is occasionally overloaded. Sometimes
insulation attainable as in IEC 60034-1 by the resistance the size of the machine may be only marginal when it
temperature as method" was initially chosen and with the passage of time, it may
in IEC 60085 be required to perform duties that are too arduous. Every
Up to 5000 kW Above 5000 kW time the machine overheats, the insulation deteriorates,
"C "C "C
and this is called thermal ageing of insulation. Figure
Ab 105 100 100 9.1 illustrates an approximate reduction in life expectancy
Eb 120 115 110 with a rise in operating temperature.
B 130 120 120
F 155 145 140
H 180 165 165 9.3 Practices of insulation systems

a Using the thermometer method this temperature will be less by
10°C. Insulation of steel laminations
These insulations are generally not used, for large motors, due to (i) For smaller ratings: by steam bluing the steel
their low operating temperature. surfaces on both sides.