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Friday, 4 October 2019

Wire Insulation using Varnish


For coil windings of instruments and AC and DC machines wires are used which have to be insulated such that there is no turn-to-turn short. The materials generally used are :

(a) Enamel or varnish
(b) Rayon
(c) Fibrous glass
(d) Cotton
(e) Silk

The type of insulation varnishes for various applications are given below. For treating coils, windings and insulating parts with insulating varnishes. The following two methods are generally followed:

(a) Vacuum impregnation
(b) Hot dipping

Finishing varnishes are usually by brush or spray. Synthetic varnishes are frequently used for impregnation by dipping and require baking to about 100 – 110o C to develop their properties fully. Baking ovens of the continuous type are used for baking varnish treated coils. For armature field coils, transformers when higher electric strength and resistance to moisture, acid, alkalies are required black baking are done. In internal curing type the varnish thermosets throughout the depth of the coil, during the baking operation and assist bonding the components of the coil. In the black finishing varnish usually shellac or synthetic resins are used for producing a harden surface as a protection against oil, moisture, dirt and metal bush. Epoxy resin varnish is also used for coil impregnation, internal curing. Some of the applications of varnish is given in Table.

Clear or black baking varnish
Armature, field coils, instruments, transformers
Epoxy resin varnish
All coils
Silicon resin varnish
Motor starter, rotors, transformer coils
Phenolic varnish
Hermaetically sealed motors


Varnishing is a method to insulate the coils of transformers, motors and instruments such that they increase the performance of the equipments. In general they are used to :

(a) Improve insulating properties
(b) Protect and brighten the surface
(c) Improve appearance
(d) Increase mechanical strength
(e) Give a fire retarding finish
(f) Protect from insects
(g) Protect from atmospheric corrosion
(h) Protect from moisture, rain etc.
(i) Decrease mechanical vibration.

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