In electrical machines, the insulation system must withstand the high electrical stresses and severe mechanical stresses induced by the magnetic field of the moving rotor. In machines, multi-turn windings are put in narrow armature slots, which are usually grounded.
Rotating machine stator insulation construction is divided into principal types:
- Random wound conductor machines with small end round conductors.
- Form-wound conductor machines, relatively large rectangular conductors.
1. In random wound machine insulation, the principal insulation components are the enamel insulation on the wire and the insulation between the wire windings and grounded slot.
Enameling is done by passing the wire through a solution of polymerizable resin at high temperature giving a thin solid flexible coating.
In random wound machine, another aspect is slot and phase insulation, which is class A type. It uses the laminated sheet of cellulose paper and polyester film, or the film alone. The fused resin coating is sometimes applied in stators.
2. Form wound windings are usually employed in higher power machines. The conductors are rectangular in cross-section to fit into rectangular stator slot. The insulation on the coil turn is applied before winding into the slot. In most high voltage machines, the coils are completely impregnated with resin and are in a finished form prior to winding.
Modern machines employ mica paper in place of large mica splitting. The paper is formed in continuous sheets from tiny flakes of mica. The mica paper is weak in tension, so it must be supported during application to the conductor with a strong and thin glass cloth or polyester mat.