Specifications

3
In the above classification Non-impregnated, moisture absorbing materials of Y- class are
not generally used for motor winding insulation purposes. Since they easily absorb moisture,
their quality quickly degrades. C-class materials are generally brittle, so they too are not suited
for motors. Insulation materials of A and B class are being used for a long time for winding
insulation purposes. In recent times F and H class are being increasingly used for winding
insulation.
1.4 CHARACTERISTICS OF A GOOD INSULATING MATERIAL : A good insulating material
should possess the following characteristics.
I. Very high insulation resistance.
II. High dielectric strength.
III. Low thermal expansion.
IV. Non-inflammable when exposed to arcing.
V. Resistant to oils or liquids, gas fumes, acids and alkalies.
VI. Should have no deteriorating effect on the material, in contact with it.
VII. Good thermal conductivity.
Fig.1.2
VIII. High mechanical strength
IX. High thermal strength.
X. Should be resistant to thermal and chemical deterioration.
XI. Should be resistant to moisture absorption.
1.5 THERE ARE FOUR PRINCIPAL AREAS WHERE INSULATION
MUST BE APPLIED. They are
a) between conductor /coils and earth (phase-to-earth),
b) between conductor /coils of different phases (phase-to-phase),
c) between turns in a coil (inter-turn) and
d) between the coils of the same phase (inter-coil).
1.6 INSULATING MATERIALS - FORMS : Insulating materials are
available in different shapes and sizes. Insulating materials are available
as Tapes, rolls, sleeves, paper and cloth.
Insulation Tapes and sleeves : Insulation tapes are used cover the
windings(coils) on the overhang side. Shellac or varnish are applied over
this covering to prevent it from absorbing moisture and improve insulation
strength. Tapes are sold as rolls in required lengths. Different types of
Insulation tapes available are: Cotton tape, PVC tape, Silk tape, Polyester
tape, Asbestos tape, Glass Fiber tape, Empire cloth tape, Mica tape.