Crepe Paper and Pressboard Used in a Transformer
The primary insulating material in most motors and transformers is the varnish that coats the copper wire that is used for the coils. In addition, coated paper is often used as an insulating material between windings. Generally, most small transformers are “potted” with an epoxy resin after construction to eliminate the vibration (humming) and loosening of coils, and to provide protection against moisture & corrosion.
Majority of power transformers are oil filled transformers and they work with “class A” insulation with maximum permissible temperature of 105 degrees centigrade. The insulation consists mainly of (1) cellulose (wood) based insulation viz. paper and pressboard; and (2) mineral insulating liquid called transformer oil which works both as an insulating medium and as a coolant. The overall insulation can be termed as oil impregnated paper and pressboard.
Note that transformer insulation is quite different from the insulation used in motors and generators. Though transformer insulation has lower temperature class (unlike class F insulation used in motors and generators), its dielectric strength is very high which suits high electrical stresses involved in power transformers.