Impregnation

After a coil is wound the best practice is to impregnate it in some sort of insulating liquid which hardens after filling. This is done for several reasons. First, it protects the wire from movement and possible mechanical damage. Second, it prevents the entrance of moisture and foreign matter which might corrode the wire or cause insulation deterioration. Third, it increases the dielectric strength of fibrous insulating materials. Fourth, it assists in heat dissipation from the coil. Single-layer coils may be dipped in the liquid, drained, and dried, but deeper, thicker coils require the use of vacuum to remove air from the coil and admit the liquid to all parts of the interior. The best mechanical result is obtained when coils are assembled with cores before treatment.

Insulation is considered to be impregnated when a suitable substance replaces the air between its fibers, even if this substance does not completely fill the spaces between the insulated conductors.

Coils having little or no temperature rise in normal use are impregnated with chemically neutral mineral wax. The wax is melted in a sealed tank and is drawn into another tank in which preheated coils have been placed, and a vacuum is maintained. Coils are removed from the tank, drained, and allowed to cool. Wax treatment provides good dielectric qualities and moisture protection. It is a quick, simple process.

Transformers having operating temperatures of 65°C or higher are impregnated with varnish. Varnish of good grade and close control is essential to achieve thorough filling and dry coils after impregnation. Oleoresinous varnishes, which polymerize to a hard state by baking, are notably useful for the purpose. A high degree of vacuum, fresh varnish, and accurate baking temperature control are necessary for good results. Plasticizers are sometimes added to the varnish to prevent brittleness in finished coils. Varnish may attack wire enamel (which itself is a kind of varnish), and so the soaking and baking time periods must be regulated carefully.

Varnishes for impregnation of electrical coils have until lately been diluted by solvents to lower the viscosity so as to permit full penetration of the windings. When the coils are baked, the varnish dries and the solvent is driven off. The drying leaves very small holes through which moisture can penetrate and in which corona may form. Eventually, the insulation deteriorates. It is, therefore, necessary to allow large clearances for high voltages or to immerse the coils in oil. Either of these alternatives increases the size of a high-voltage transformer in relation to that of a low-voltage transformer. For this reason, solventless resins have come into use as filling compounds for dry-type coils. They are known by trade names such as Fosterite, Para-plex, and Stypol. These resins have the advantage of changing from a liquid to a solid state by heat polymerization, so that small holes formed by drying of the solvent are eliminated. Filling of the coil may be accomplished by casting the transformer in a mold, or by encapsulation. Encapsulation is readily adapted to irregular coil surfaces and is accomplished by a leak-proof coat before filling. In either process, a good vacuum is necessary to insure complete filling.

Silicone materials are moisture-resistant. Basic insulation should be inorganic, or silicone-treated cloth, tape, laminated sheets, and tubes. Through the use of silicones, some transformers may be designed to have very small dimensions for their ratings. This may be achieved most successfully if the coil insulation comprises only silicone or inorganic materials, including impregnation with silicone varnish. Dielectric strength of silicones is about the same as class A materials. Hence the thickness of silicone coil insulation is similar to that for organic materials.

Continual development improves all classes of insulation; present A, B, and H insulation classes may be superseded eventually by new classes based entirely on functional evaluation. Life tests have been proposed⁽¹⁾ which classify a transformer according to its ability to withstand the effects of voltage, moisture, and vibration, as well as temperature.

In encased high-voltage units, air around the coils, bushings, and leads is especially subject to the formation of corona. To reduce this tendency, the containers are filled with asphaltic compound which replaces the air with solid, non-ionizing material. A similar compound is often used to fill containers of low-voltage transformers to avoid the need for mechanically fastening the core to the case. This is a permissible practice if the melting point of the compound is higher than the highest operating temperature and if its cracking point is below the lowest operating temperature.

(1)	See "Functional Evaluation of Insulation for Small Dry-Type Transformers Used in Electronic Equipment," by R. L. Hamilton and H. B. Harms, AIEE Tech. Paper 54-121.