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:
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.
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.
Hot dip: This is quite suitable for small coils and windings. The winding is heated up to a temperature of 800to 1000C 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 1000 to 1100C for at least four hours, by which time the varnish will get completely baked and hard set.
For best results, and also for large windings and armatures, transformer coils, etc. vacuum impregnation should be used.
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.