TRAINING DEPARTMENT

College / Polytechnic

Syllabus Year

Branch Sem (x) IPT Session ID (xx)

Training Ref Number (xxx)

C 10 E 3 0 2 0 0 5

INSULATING VARNISHES:

Varnish coating, also called Secondary Insulation, is an important component of the

insulation system of an electrical machine. Varnishes, of different types are used in the

insulation system of electrical machines for impregnation and finishing applications.

Advantages of these coatings are: Increased mechanical bonding to the winding wires

Improved dielectric properties Improved thermal conductivity Protection to the winding

against moisture and chemically corrosive environment.

Varnishes are classified based on:

1. Applications of varnish. 2. Type of (varnish) curing method. 3. Based on main raw material used in varnish.

Insulating varnish based on applications:

Impregnating varnish Finishing varnishes Core plate varnishes Bonding varnishes Special purpose varnishes

Insulating varnish based onCuring method : 1. Air drying type 2. Oven baking type

Main raw material used:

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TRAINING DEPARTMENT

Alkyd Phenolic, Alkyd, Polyurethane, Isophthalic Alkyd, Modifiedpolyester, Epoxyester

Melamine, Polyestermide, Epoxy, Phenolic, Phenolic Melamine - based.The above

varnishes come in Solvent based and Solvent-less based.

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 ofcurrent-carrying

conductors. but to fill the empty spaces in and around windings and to

providemechanical reinforcement of the loose grouping of conductors, even at high

temperatures. Thefilling 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 resistanceto chemical attack, to moisture, thus

extending its service life. They are applied by dipping thecomponent in the varnish, or

less often by trickling process. These type of varnishes needs tobe 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 toprotect the component

from external attack by environment conditions. They are applied purelyas a surface

coating, and are characterised by outstanding film forming properties. Often appliedby

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 :

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TRAINING DEPARTMENT

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

C-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 ofabout 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 afterunder going required curing process at appropriate

temperature forms into a uniform film on thematerials. 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 varnishesthe methods

generally used are Vacuum impregnation, Hot dipping. Finishing varnishes areusually

applied by brush or spray. Mica sticking varnishes are applied by brush or sometimes

bymachine[ by passing a roller which dips in the varnish]. Synthetic varnishes are

frequently usedfor impregnation by dipping and require baking to develop their

properties fully.

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TRAINING DEPARTMENT

TYPES OF INSULATING VARNISHES FOR VARIOUS APPLICATIONS

1. Clear baking varnishArmatures, field coils and instruments.

2. Black baking varnishes Armatures, field coils and transformers when higherelectric strength and resistance to moisture, acids, and alkalies are wanted. They haveless resistance than those of clear.

3. Sticking varnishCementing cloth, paper, mica etc

4. Core-plate varnish[air drying, baking and flashing]Insulating armature andtransformer laminations. The air drying is not suitable for oil-immersed operation.

5. Epoxy resin varnish [baking].All coil impregnation, internal curing, where superior durability and chemical and moisture resistance are required.

6. Silicone resin varnish [air drying and baking] Motor stators and rotors,transformers, coils, for high temperatures and high-humidity service.

7. Polyester resin varnishes [baking] Motor stators and rotors, transformers, coils, for high temperature service not so severe as to require silicones.

8. Phenolic varnishes [baking ] Hermetic motor coils and bonding of form wound

coils.

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