Introduction to Functions of Varnish
The impregnating varnish's major function is not electrical insulation of current-carrying conductors. But to fill the unfilled spaces in and around windings and to give mechanical reinforcement of the loose grouping of conductors, even at very high temperatures. The filling of unfilled spaces not only provides mechanical strength, but as well hinders or avoids penetration of not wanted substances from the environment. This provides the component enhanced resistance to chemical attack, to moisture, so extending the service life of it. They are applied through dipping the component in the varnish, or less frequently through trickling process. These kind of varnishes requires to be cured (heated in a oven) at temperatures ranging from 100oc to 160oc for 2 to 12 hours time.
Finishing or coating varnish:
Finishing varnish is employed not to strengthen the windings, but to guard the component from external attack through environment conditions. They are applied just like a surface coating, and are characterised through outstanding film forming properties. Frequently applied through paint brush or sprayed, in repair shops after rewinding works. They are mainly air drying type. It takes approximately a day to entirely cure.
Core plate varnish:
Core plate varnish is applied to electrical laminations employed in electrical machines. This works like insulating layer among successive laminations. Core plate varnish is baked at high temperatures, 350o-450o c for about 5 min.
Binder varnish is employed as bonding agent among two insulating materials. The materials that are mechanically while bonded depict good rigidity. Binder varnish is baked at temperatures of about 120oc to 450oc for duration of 3min. to 60 min, relies on the grade of the varnish.
Insulating varnish coating's properties after curing:
Varnish after application and after undergoing needed curing process at suitable temperature forms into a uniform film on the materials. The elastic varnish film comprises extremely good mechanical properties like flexibility, penetration, hardness, good adhesion and Bonding strength. The cured film is resistant to dilute acid, alkalis, moisture, chemicals such as benzene & Toluene, oils and tropical climate from 00C to 550C. It includes good dielectric behaviour and dielectric strength.
For treating windings, coils, and insulating parts with insulating varnishes the methods usually employed are Vacuum impregnation, hot dipping. Finishing varnishes are generally applied through brush or spray. Mica sticking varnishes are applied through brush or occasionally through machine [by passing a roller that dips in the varnish]. Synthetic varnishes are often employed for impregnation through dipping and need baking to develop their properties fully.
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