Insulating 'varnish'--what it is, what it does

DIP AND BAKE-WE ALL KNOW WHAT THAT means. The process is an essential step during manufacture or repair of motors, generators, many transformers, and magnet coils. But there are exceptions, and many variations, some of them controversial. Do we fully understand why it's done, and what governs the selection of methods and materials?

We need not examine this question from the standpoint of any specific type of electrical apparatus. Just consider a coil of some kind, wound upon or within a core structure (usually steel), and necessarily insulated electrically from that core or other nearby conducting material.

Two basic kinds of insulation are necessary. We call the most important category primary insulation. That term doesn't refer to voltage level, or to what's implied by transformer primary. Rather, it means the principal dielectric barrier. That material must retain its integrity if a breakdown-a fault, or short-circuit-is to be avoided.

Within the coil, an obvious item of primary insulation is the covering on the wire itself. That covering ranks first on the list because insulation breakdown most often begins with a short-circuit between coil turns. Surprisingly, during the infancy of electromagnetic research 160 years ago, bare wire was commonly used. Only after pioneer American physicist Joseph Henry began using wire wrapped with silk did powerful magnets become possible.

The second item of primary insulation is commonly termed ground insulation. That separates coil conductors from the core. In a motor, it would be the slot liners, or overall coil taping. In a transformer, it would be the sheet or tube barriers between high- and low-voltage windings, or between winding and core.

Depending upon the apparatus and its size, several other materials form secondary insulation. Besides dielectric stress, caused by potential differences between conductors and other components, any current-carrying winding undergoes a variety of other stresses-mainly mechanical and thermal. In any alternating current coil, vibration at the operating frequency is constantly present. In a motor, transformer, or electromagnet, switching the power on and off adds the high stress of inrush current, tending to drive each wire away from the one next to it. Current flow generates heat, and heat produces thermal expansion.

To help restrain movement, which in turn creates mechanical forces tending to bend and crush primary insulation, coils must be tied, wedged, or wrapped in position. That may be done with layers of tape, or with ties of tape or cord-all considered secondary insulation. Although those materials are not dielectric barriers, they must not be electrically conductive.

The purpose of impregnating varnish

So where does the impregnant, the "varnish," fit in? This is the matrix encasing the coil and its other insulation. Said G.B. Helman in a 1985 technical paper: ". . . impregnation is not done for insulation purposes." If taken out of context, that quote is misleading, especially in view of what we know today about partial discharge behavior.

The text of a 1996 manufacturer's seminar on electrical insulation systems stated that varnish was indeed primary insulation. The reason was the 1982 issuance of UL Standard 1446 (for evaluation of insulation systems), which required a re-assessment of the role played by an impregnant. From that time on, such a liquid has been considered "major" or "primary" insulation. (Yet even the manufacturer seemed unsure, declaring in another presentation that "VPI resin is not the primary electrical insulation in a motor.")

An impregnant serves three purposes:

* To provide the principal physical restraint of wires in the coil, by making the coil a solid unit-like the cement in a concrete structure.

* To facilitate heat transfer away from the conductors. Without something to fill the unavoidable air spaces between individual wires and layers of turns, heat dissipation would depend upon the low thermal conductivity of those air voids.

* By wrapping a solid blanket of material around and within the coil to resist the entrance of moisture and of corrosive or conductive contaminants, the impregnant acts as protection for the primary insulation.

Another often-stated characteristic of the impregnant in a winding made of film-coated "magnet wire" is that the varnish or resin will coat the individual wires so as to fill in or smooth over defects in the film itself. (NEMA magnet wire standards allow manufacturing defects as a specific number per 100 feet of wire, and further wire damage can occur later in handling or in coil winding.) Many service personnel have observed "crazing" or cracking of wire enamel in sharply bent conductors.

The role of impregnating varnish in "mending" such defects is somewhat controversial. Although conceding that good bonding of coil turns requires some degree of fusion, some engineers are uncomfortable with what they call the "invasion" of wire enamel by heated varnish during the impregnation process. However, if damage to the wire covering renders it unable to sustain operating or test voltages, whatever fills in such a defect does become primary insulation.