Polyimide Wire Insulation. Polyimide coatings are extensively used for
high temperature wire insulation and for insulation that must withstand
high radiation fluxes. Pyre-ML (tradename of Summit Precision Polymers;
available from Rea Magnet Wire Co.) polyimide varnish has been in use
for many years as a magnet wire insulation for dry-type transformers,
dc field coils, and submersible pump motors. It is rated for continuous
use at 220°C. The measured thermal endurance for PD Wire & Cable’s
polyimide Imideze® is given in Fig. 4.33. As an overcoating, polyimide
provides an extremely tough abrasion-resistant film that also exhibits
high resistance to radiation and heat. Polyimide-coated round wire is
specified in NEMA MW-16. Polyimide coatings are also used as insulation
and impregnants for motor, coils, and relays. Because of their low
weight loss at elevated temperatures and their high chemical
resistance, polyimide varnishes are useful in encapsulated windings and
hermetically-sealed components. Polyimide wire coatings are compatible
with numerous phenolic, polyester, and epoxy varnishes and encapsulants
and are resistant to most solvents and chemicals. The exceptional
toughness, thermal stability, and chemical resistance of cured
polyimide, however, becomes a drawback in its ability to be easily
stripped from wire in order to perform rework or make electrical
connections. Other limitations include its hydrolytic instability at
temperatures above 105°C in a sealed system containing moisture.
Polyimides under the DuPont tradename of Kapton® may also be used as a
film to insulate wire.
Impregnating Varnishes. After coils and their cores are assembled, the part (for example, a transformer or toroidal coil) is impregnated with varnish and encapsulated or otherwise embedded in one of a variety of materials (Fig. 4.34). Complete impregnation serves four main functions:
• Additional electrical insulation for the wires
• Better heat dissipation
• Protection from moisture and other contaminants
• Better shock and vibration resistance
The exclusion of air and oxygen by thorough impregnation with organic coatings results in substantial improvements in thermal life. Epoxy impregnation, for example, may upgrade a Class A insulation to a Class B. The cooler operation of coils and transformers has permitted a degree of miniaturization never before possible.