Specifications

175
tank, are easily removed periodically from a drain outlet.
15.15 Applying insulating varnish on electrical coils and windings
Coils are made up of insulated wires, the covering consisting of cotton, silk or enamel, etc.
which are hygroscopic, i.e. they tend to absorb and retain moisture. The insulation provided
between layers as well as the space between turns in the interior of the coil contain considerable
amount of air spaces. If the coils are not covered by any insulating varnish, moisture tends to
accumulate in these air spaces. Not only will it lower the insulation strength, but may ultimately
lead to a breakdown and internal short-circuit. By covering the coils with a good insulating varnish,
the air spaces are filled up and sealed; the windings get protected against ingress of moisture
and thereby they will give much better service. This process of coating electrical windings with
insulating varnish is called
‘
impregnation
’
. The right method of doing this is described below:
a) Before any insulating varnish is applied, it is absolutely necessary to ensure that the
surfaces of all coils, armatures etc. are perfectly clean and free of all dust, oily matter
and moisture.
b) Apply varnish with a brush, using air drying varnish. This is a quick method of protecting
the surface of coils during periodical overhauls, if they have already been properly
impregnated previously. The varnish is then allowed to dry up.
c) Hot dip: This is quite suitable for small coils and windings. The winding is heated up to
a temperature of 80
0
to 100
0
C in an electrically heated chamber for one to two hours.
The hot winding is then completely immersed in insulating varnish (baking type) and
kept immersed for one to two hours, until all air bubbles cease. The coil is then removed
and after draining out all surplus varnish, the coil is again put back in the heating chamber
and kept there at a temperature of 100
0
to 110
0
C for at least four hours, by which time
the varnish will get completely baked and hard set.
d) For best results, and also for large windings and armatures, transformer coils, etc.
vacuum impregnation should be used.
15.16 The Process of vacuum impregnation
Vacuum impregnation is very similar to the hot dip
method but is much more efficient. In the hot dip method,
one cannot be sure if all the air spaces inside the winding
are fully impregnated with the varnish. In vacuum
impregnation, all air is first removed out, which ensures
that the insulating varnish gets sucked in into the
innermost recesses. If the subsequent baking is
thorough, there will be no possibility of entry of humid
air from outside into the winding.
Fig.15.3 - Exploded view of Ball Bearing