Electrical Insulation and Wire Enamels

A winding is a long conductor packed tightly into a small space and asked to carry current without any part of it touching any other part. Every material in this category exists to keep that separation. The enamel on the wire is the primary insulation and is the thinnest layer in the machine: what stands between one turn and the turn wound against it is two enamel films in contact, and nothing else. The impregnating resin comes next. It displaces the air left in the slot, bonds the winding into a solid block and carries heat out of the copper into the iron. Casting and potting resins encapsulate whole assemblies, where the cured resin is the insulation and the mechanical body of the part at the same time. Flexible laminates line slots and separate phases, and in low voltage random wound machines the slot liner is also the ground wall, while high voltage stator bars use mica based tapes for that duty. Finishing varnishes close the surface of the completed winding against moisture, dust and oil. The line against Energy and Electronics Coatings is drawn by the object being coated: insulation is the winding, the motor and the transformer, and energy and electronics is the cell, the board and the enclosure. A traction motor is wound with materials from this range, and the controller board and its housing are finished from that one.

The defining constraint is that three properties have to hold together, not one. Electrical strength decides whether the turn survives the voltage. On mains frequency sine wave duty that comes down to the breakdown voltage of the film, but on an inverter drive the stress is different in kind: the fast rising pulse divides unevenly along the phase, so the first few turns take a disproportionate share of it, and it is repetitive partial discharge in the small air gaps between turns, rather than a single breakdown event, that erodes an ordinary organic film. Windings on that duty are conventionally specified with corona resistant enamel, in which an inorganic filler slows the erosion. Mechanical toughness decides whether the enamel reaches service intact, because the wire is drawn through dies, bent over formers and pulled into slots before it ever sees a volt, and a nick in the film during winding is the commonest route to a turn-to-turn short. Thermal endurance decides the life. IEC 60085 sets out the thermal classes used to describe insulating materials and systems, with class F at 155 °C and class H at 180 °C the two that most motor and transformer work in India is written against. Two points about that are worth stating plainly. Thermal class belongs to the combination of materials and not to any one of them: the thermal endurance of a single material is established to IEC 60216, while the class of a rotating machine insulation system is established on the system as built, to IEC 60034-18, so no enamel or varnish carries a class into a machine on its own. And conventional engineering practice treats insulation life as roughly halving for every 8 to 10 K of sustained overtemperature, which is why the weakest element in the stack sets the class of the whole.

Process matters as much as chemistry here, so it is worth being clear about what each part of the range needs on the customer’s side. Wire enamels are applied on a wire enamelling line: the bare conductor passes repeatedly through applicator dies or felts and a curing oven, building the film in many thin passes, with catalytic oxidation of the solvent vapour in the oven supplying part of the heat. That line belongs to the wire maker, and Arctic supplies the enamel formulated for it, not the wire. Impregnation is within reach of a general winding shop, at three levels of plant. Dip and bake needs a resin tank and a curing oven. Trickle needs a rotating fixture, preheat by current injection and a controlled dispensing head. Vacuum pressure impregnation needs the most: a vessel that can pull vacuum and then apply pressure, a separate storage tank to hold the resin between cycles, and an oven to cure the treated unit. Solventless impregnating resins cut oven emissions, and because nothing evaporates out of the film they fill a slot more completely for the same dip, but the word solventless is worth reading carefully. The conventional unsaturated polyester types cure through a reactive monomer, usually styrene, which is itself volatile and has to be extracted and handled, and monomer free grades exist where that is the constraint. Solventless resins also demand more discipline in the shop, since pot life, storage temperature and gel time govern the process rather than drying, and the resin polymerises where it sits. Casting work needs two component metering and mixing, degassing or casting under vacuum wherever voids cannot be tolerated, moulds, and an oven for cure and post cure. Electrical insulation is a new capability area for Arctic Materials. Grades are formulated against the customer’s process and the thermal class of the system they go into, and each is supplied with a Technical Data Sheet and a Safety Data Sheet.

Base enamels applied directly to the bare conductor on a wire enamelling line. The subfamily runs from phenolic-modified polyvinyl acetal for class 120 E oil-immersed transformer wire up to THEIC-modified polyesterimide for class 180 H motor wire. These grades are used alone where the winding duty allows it, and as the base coat under an Arcwire PA overcoat where it does not.

Polyamide-imide topcoats applied over a polyesterimide base in the final passes of the same enamelling line, giving the dual coat construction specified where the wire is wound hard or run hot. Polyamide-imide is the conventional choice for abrasion and cut-through resistance, hydrolysis resistance and refrigerant resistance. A filled, corona resistant grade covers inverter-fed and hermetic duty.

Polyimide enamel for the highest thermal class in the range, used where the continuous conductor temperature, short-term overload or chemical exposure is beyond what polyesterimide and polyamide-imide will hold. The film is built from a single chemistry through the whole pass sequence rather than as a base and an overcoat.

Bondcoat enamels applied over a cured base enamel, so that a wound coil can be fused into a self-supporting body by hot air, solvent or resistance heating instead of by a former, a tape or an impregnating resin. Used for air-cored coils, voice coils, deflection and sensor coils and small ignition coils.

Resins that displace the air left in the slot after winding, bond the conductors into a solid block and carry heat out of the copper into the iron. The subfamily covers a solvent-borne dip and bake varnish for shops with a tank and an oven, and a solventless resin formulated for trickle and for vacuum pressure impregnation.

Two-component systems for encapsulating coils, sensors and control assemblies, where the cured resin is the insulation, the mechanical body of the part and the heat path out of it at the same time. Selection is governed by flow and gel profile against the cavity being filled and by thermal expansion against the conductor and the housing.

Finishing and touch-up varnishes that close the surface of a completed or rewound winding against moisture, dust and oil. They are applied after impregnation rather than in place of it, and are matched to the thermal class of the enamel and the impregnating resin below them.

The figures below are the typical operating window for this range, not a measured result for any one batch. Confirmed values are issued with the Technical Data Sheet and Certificate of Analysis by our laboratory.

The four parts of this range are applied on four different kinds of plant, and the grade has to be formulated for the plant rather than picked off a list. Wire enamels run on the wire maker’s enamelling line, where the bare conductor passes repeatedly through applicator dies or felts and back through a curing oven. The film is never laid down in one thick coat. It is built in six to sixteen thin passes, each of them a few micrometres and the number set by the chemistry and the build, because a thick wet layer cannot release its solvent from the underside before the surface skins over, and the result is blistering, entrapped bubbles and pinholes that show up directly in the continuity test of IEC 60851-5. Thin passes also let each layer cross-link fully, which is what gives the finished film its flexibility and its adherence after elongation under IEC 60851-3, and they let the build be trimmed to grade 1 or grade 2 by adding or dropping a pass. What decides whether a line runs well is wire speed against oven length, the temperature gradient between the evaporation zone and the curing zone, die or felt condition, and the heat returned by catalytic oxidation of the solvent vapour, which supplies a substantial part of the oven’s energy and shifts the whole profile as the catalyst bed ages. Degree of cure is followed by the dissipation factor transition of IEC 60851-5 rather than by appearance. A dual coat construction adds one more variable, the pass at which the polyamide-imide overcoat starts, because the base has to be cured enough to carry the topcoat and not so far cured that the two layers fail to bond.

Impregnation is done in the winding shop, at three levels of plant. Dip and bake needs only a tank and an oven, penetrates a cold winding well and suits varied one-off work such as rewinds, but the solvent leaves during bake and the film shrinks as it goes, so full slot fill takes two or three cycles. Trickle needs a rotating fixture, preheat by current injection through the winding itself and a controlled dispensing head, and it is the high-volume route: resin is metered onto a turning, already hot stator, drawn into the slot by capillary action and gravity, and gelled before it can run out, which keeps cycle time short and waste low. Vacuum pressure impregnation needs the most plant, a vessel that pulls vacuum and then applies pressure, a separate storage tank to hold the resin between cycles and a cure oven, and it is what is specified when the air has to come out of the slot completely, for high voltage windings, traction machines and anything whose partial discharge behaviour is part of the specification. Whichever route is used, the result is judged on bond strength by the helical coil test of IEC 61033, on how much resin is actually in the slot, and on whether the cured block is free of voids.

Casting and potting call for two-component metering and mixing, degassing of the mix or casting under vacuum, moulds, and an oven for cure and post cure. The failure to design against is mechanical rather than electrical: the cured resin, the copper and the housing expand at different rates, and thermal cycling puts the encapsulation into tension against the stiffest part it is bonded to. Filler loading, cure schedule and post cure are the levers on that, and gel profile against cavity geometry decides whether the resin reaches the bottom of the part and releases its air before it sets. Finishing varnish is the last step, applied by brush, dip or spray over an impregnated winding, and it seals the surface rather than filling anything.

On an inverter drive the electrical duty of the whole stack changes, because the fast-rising pulse divides unevenly along the phase and the first turns take a disproportionate share of it. Type I insulation for converter-fed machines is qualified on the system as built, to IEC 60034-18-41, and that qualification belongs to the machine and not to any single material in it. The same logic governs thermal class: IEC 60085 designates the class of a material, IEC 60216-1 establishes the endurance of one material, and IEC 60034-18-21 establishes the thermal class of a wire-wound rotating machine insulation system, so no enamel, resin or varnish carries a class into a machine on its own. The enamel, the impregnating resin, the slot liner and the finishing varnish are therefore specified together and to the same class, because the system is limited by whichever element sits lowest.

Grades in this range are formulated against the customer’s process and against the thermal class of the system they go into, so the specification starts with questions about the machine and the plant rather than with a product code. Arctic needs the following before a grade is proposed.

• Thermal class of the finished system. Which IEC 60085 class the machine is written against, 120 E, 155 F, 180 H, 200 N, 220 R or 240, what the expected hot-spot temperature is, and whether short-term overload above the class is part of the duty cycle.
• Supply and waveform. Mains sine wave or inverter drive. For an inverter, the DC link voltage, the pulse rise time and the switching frequency, and whether the winding has to be built as corona resistant.
• Which part of the range is in scope. Wire enamel, impregnating resin, casting compound, finishing varnish, or a matched set. Materials at different classes in the same stack are limited by the lowest.
• For enamels, the conductor and the build. Round or rectangular, copper or aluminium, conductor diameter or section, grade 1 or grade 2 build, and single coat or dual coat.
• For enamels, the enamelling line. Vertical or horizontal, oven length and zone temperatures, maximum line speed, die or felt application, number of passes available, whether catalytic solvent oxidation is fitted, and the solvent system the line is already set up to handle.
• Winding and termination method. Hand wound, machine wound, needle wound or pull-in, how tight the slot fill is, and whether the ends are stripped and soldered, welded or crimped. None of the enamels in this range is directly solderable, so a joint that has to be tinned through the film calls for a different chemistry and a lower thermal class.
• Chemical and environmental exposure. Mineral or synthetic transformer oil, refrigerant and lubricant for a hermetic motor, coolant, fuel, humidity and condensation, and any cleaning solvent used in assembly.
• For impregnation, the plant available. Dip and bake, trickle or vacuum pressure impregnation, the vessel and tank sizes, fixture and preheat arrangement, oven capacity and temperature, target cycle time, and whether solvent emissions or reactive monomer extraction is a constraint on site.
• For casting and potting, the part. Cavity geometry and wall section, the smallest gap the resin has to reach, mixing and metering equipment, whether vacuum casting is available, acceptable pot life and demould time, and the housing material the resin is bonded to.
• For self-bonding wire, the activation route. Hot air, solvent or resistance heating, the coil geometry, and the temperature the bonded coil has to hold in service, which the bondcoat sets and not the base enamel.
• Qualification and documentation. Which test series the material will be judged on, typically the IEC 60851 parts for winding wire and IEC 60455-2 for reactive resins, whether a system evaluation is planned to IEC 60034-18-21 for thermal class or to IEC 60034-18-41 for converter-fed duty, and what documentation is to accompany each delivery. A Technical Data Sheet and a Safety Data Sheet are supplied with every grade, and a compliance dossier is issued on request.
• Volume and supply. Annual offtake, batch size, pack format, shelf life needed at the customer’s storage temperature, and whether cold storage is available for reactive systems.

The Technical Data Sheet, Safety Data Sheet and regulatory declaration for any grade in this range are issued by our laboratory against the substrate and process you are running. Tell us the job and we will specify against it.