Insulating varnishes and impregnating resins are used to ensure electrical devices, including motors, generators, transformers, sensors and other devices that function by electromagnetic induction, have the necessary electrical insulation and structural integrity for operation.
Transformers are comprised of coil windings where the electric wires are protected by a thermosetting resin insulating varnish (also called a wire enamel or primary insulation) to prevent electrical shorting. The completed transformer, comprised of an assembly of coil windings and insulating paper, is encapsulated with a thermosetting resin to provide environmental protection (especially from moisture and dust), to eliminate air in the transformer that can conduct electricity and heat, and to provide strength. This encapsulating material is also called an impregnating resin (as it impregnates the insulating paper) or secondary insulation.
Insulating varnishes and impregnating resins are almost always thermosetting resins such as epoxies or phenolics. These materials crosslink when applied and are therefore inherently strong, environmentally stable, impervious and durable.
Insulating varnishes and impregnating resins must be superior electrical insulators with long life. The products must not degrade with exposure to heat, cold, water, chemicals or other environmental factors that might reduce the insulating characteristics of the products. If these products become electrically conductive or degraded such that electricity can flow through the wire, transformer or other electrical device to the surrounding area, system failure and damage will likely occur.