Specifications

10
Method of applying varnish:
 Applying a coating with a paint brush  Dipping the specimen into varnish
 Vacuum pressure method  Conveyorised dip method.
Impregnating varnish: The main function of impregnating varnish is not electrical insulation of
current-carrying conductors. but to fill the empty spaces in and around windings and to provide
mechanical reinforcement of the loose grouping of conductors, even at high temperatures. The
filling of empty spaces not only gives mechanical strength, but also hinders or prevents penetration
of unwanted substances from the environment. This gives the component improved resistance
to chemical attack, to moisture, thus extending its service life. They are applied by dipping the
component in the varnish, or less often by trickling process. These type of varnishes needs to
be cured(heated in a oven) at temperatures ranging from 100
o
c to 160
o
c for 2 to 12 hours time.
Finishing (coating) varnish : Finishing varnishe is used not to strengthen the windings, but to
protect the component from external attack by environment conditions. They are applied purely
as a surface coating, and are characterised by outstanding film forming properties. Often applied
by paint brush or sprayed, in repair shops after rewinding works. They are mostly air drying type.
It takes almost a day to completely cure.
Core plate varnish : This varnish is applied to electrical laminations used in electrical machines.
This acts as insulating layer between successive laminations. It is baked at high temperatures,
350
o
-450
o
c for about 5 min.
Binder varnish : This type of varnish is used as bonding agent between two insulating materials.
Mechanically weak materials when bonded show good rigidity. It is baked at temperatures of
about 120
o
c to 450
o
c for a duration of 3min. to 60 min, depending on the grade of the varnish.
Properties of Insulating varnish coating after curing : Varnish after application and after
under going required curing process at appropriate temperature forms into a uniform film on the
materials. The elastic varnish film has very good mechanical properties such as hardness,
flexibility, penetration, good adhesion and Bonding strength. The cured film is resistant to moisture,
dilute acid, alkalis, chemicals like benzene & Toluene, oils and tropical climate from 0ºC to 55ºC.
It has good dielectric behaviour and dielectric strength.
Applying Varnishes: For treating coils, windings, and insulating parts with insulating varnishes
the methods generally used are Vacuum impregnation, Hot dipping. Finishing varnishes are
usually applied by brush or spray. Mica sticking varnishes are applied by brush or sometimes by
machine[ by passing a roller which dips in the varnish]. Synthetic varnishes are frequently used
for impregnation by dipping and require baking to develop their properties fully.
1.10 TYPES OF INSULATING VARNISHES FOR VARIOUS APPLICATIONS
1. Clear baking varnish…………Armatures, field coils and instruments.
2. Black baking varnishes…..…..Armatures, field coils and transformers when higher
electric strength and resistance to moisture, acids, and alkalies are wanted. They have
less resistance than those of clear.