Wire must be electrically insulated and environmentally and physically protected. A variety of polymer coatings can and are being used for this purpose. They are classified according to their thermal and physical endurance and manner of application. Wires may be dip-coated in a liquid bath of the resin solution, then cured or dried, wrapped with fibrous coverings or resin-impregnated coverings, wrapped with tape, or covered with extruded polymers. Large-diameter wire, as used in cables, is generally insulated by the extrusion of solid thermoplastic resins.[57] Thermoplastic resins soften on heating then resolidify on cooling. During this process they can be shaped around a conductor wire by forcing the plastic material under pressure and heat through an orifice concurrently with the conductor. Extruded insulation such as polyvinyl chloride, polyethylene, or polytetrafluoroethylene (Teflon) is relatively thick compared to the diameter of the wire and is, therefore, used mainly for interconnect lead wires and cables. Tape wrapping involves the wrapping of thin tape or plastic film around the wire with a minimum of overlap. Several layers of tape can be used to build up the desired insulation thickness.
Finally, the insulation may be applied by dip coating or vacuum
impregnation. The wire is moved through the liquid coating solution,
then dried or baked. The greatest use of liquid polymer coatings is to
insulate and isolate thin magnet wire used in coils and windings for
transformers, inductors, hermetic motors, automotive alternator
stators, and solenoids. This section primarily addresses the liquid
resin coatings for magnet wire insulation, but some properties of
extruded types are mentioned for comparison. Further information on
wire and cable materials may be found in the
literature.[58]
Magnet Wire Classifications
Magnet wire may be classified as coated wire (film insulation), wire insulated with fibrous wrappings, or wire with an impregnated fibrous wrapping. Wire coated with only an organic coating is frequently referred to as enameled wire.
The requirements and standards for insulated magnet wire are thoroughly treated in a National Electrical Manufacturers Association specification, NEMA- MW-1000, Magnet Wire. This specification consists of three parts:
1. Covers the scope, definitions, and sub-tier standards.
2. Contains extensive tables of properties and requirements for various sizes and insulations of magnet wire.
3. Describes test methods, among which are adhesion, flexibility, elongation, heat shock, springback, dielectric strength, continuity, completion of cure, coverage, solderability, solubility, thermal endurance, scrape resistance, and hy-drolytic stability.
Insulated magnet wire is generally classified according to its thermal endurance and thermal index rating according to ASTM D2307, Standard Test Method for Thermal Endurance of Film-Insulated Round Magnet Wire. Thermal endurance is a measure of the stability of an electrical insulation material or combination of materials when maintained at elevated temperature for extended periods of time. The thermal index or class is a measure of the thermal stability of an insulation based on the temperature, in degrees Celsius, obtained by extrapolating the Arrhenius plot of life versus temperature to a specified time, usually 20,000 hours. Some major insulation types and their thermal classifications are given in Table 4.8.