Copper Foil in Foil-Wound Transformers
Transformer copper foil is a wide, thin, accurately slit copper strip that is wound edgewise to build a winding layer by layer. Each turn is one conductor, so a high current winding can be produced with a large cross section without the layer stacking and transposition work that round wire demands. The foil is manufactured from high conductivity copper, usually electrolytic tough pitch copper, and is supplied in coils of long continuous length so that the winding has no intermediate joints.
Foil windings are used in dry type and cast resin transformers, in distribution transformers, in reactors and inductors, and in welding and furnace transformers. The same strip also appears in communication and power cables where a flat conductor is required.
Material Grades and Electrical Properties
| Grade | Copper content | Conductivity | Comment |
|---|---|---|---|
| C11000 | 99.90% min | 100% IACS min annealed | Standard electrical grade, most common for foil windings |
| C10200 | 99.95% min | About 101% IACS | Oxygen-free, used where reducing atmosphere processing is required |
Electrical conductivity of transformer foil is normally quoted between 97% and 102% IACS depending on grade and temper, and it is verified on the coil by a resistivity measurement. Strip for electrical use is ordered to ASTM B152 for copper sheet, strip, plate and rolled bar, to EN 13599 for copper sheet, plate and strip for electrical purposes, or to the equivalent national strip standard.
Thickness, Width and Edge Condition
Foil thickness for transformer windings is generally in the range of about 0.1 mm to 2.0 mm, selected from the current, the number of turns and the allowable eddy current loss. Widths extend beyond 600 mm for large distribution and power transformers, and coil lengths reach about 2000 m so that a complete winding can be produced from one coil.
Edges are supplied trimmed, rounded or rolled. Rounded and rolled edges are preferred for thin foil, because they reduce the risk of cutting the interleaved insulation during winding.
Burr height is agreed with the customer. As a working guide, foil of 0.4 mm and above is supplied with burrs under 0.05 mm, and foil thinner than 0.4 mm with burrs under 0.03 mm.
Thickness is measured across the width, and the edge to centre variation must stay inside the tolerance agreed in the order.
Flatness, Camber and Surface Quality
Foil must be flat because camber and waviness make the edges of the winding step out of line. A camber limit of no more than 2 mm per metre is a common requirement, together with a slight uniform waviness that does not affect winding. Surface condition matters as much as shape: the foil must be free from rolled in oxide, rolling oil residues, stains, scratches, dents and inclusions, because any of these can reduce the effective conductor section or damage the insulation. Hardness and temper should be uniform along the coil so that the winding tension stays constant from the first turn to the last.
Winding Practice and Benefits
Foil and interleaved insulating paper or film are wound together with controlled tension, so the edges remain aligned and the winding stays compact.
The high space factor of a foil winding leaves more room for copper and less for voids, which lowers the winding temperature and cuts the size of the transformer for a given rating.
Because each turn is a full width conductor, the winding withstands the mechanical forces produced by short circuit currents better than a stranded winding.
Eddy current loss is controlled by the layer thickness, so the foil thickness is chosen as a compromise between loss and winding cost.
Fewer joints and simple layer insulation reduce winding labour and the number of possible fault locations.
Quality Control and Packing
Each coil is inspected for thickness, width, camber, burr height, surface condition and conductivity before packing. The coil is wound on a suitable core, wrapped in a moisture barrier and protected against edge damage in transit, and it is marked with grade, temper, thickness, width, coil number and weight so that the material can be identified when it reaches the winding line. Store foil dry and indoors, and do not remove the wrapping until the coil is mounted on the winding machine, because edge damage and surface staining are the two faults that cause most rejections at the transformer works.
FAQ
Q: Which copper grade is used for transformer foil?
High conductivity electrolytic tough pitch copper, C11000, is the standard choice. Oxygen-free C10200 is used where the foil will be processed in a reducing atmosphere or where very high conductivity is required.
Q: What conductivity should the foil reach?
Transformer foil is normally specified between 97% and 102% IACS, with the exact figure depending on grade and temper, and it is confirmed by a resistivity test on the coil.
Q: Why is camber limited?
Camber makes the strip run crooked during winding, so the layer edges step out and the winding becomes loose. A limit of about 2 mm per metre keeps the edges aligned.
Q: Why are rolled or rounded edges preferred?
A rolled edge has no sharp corner, so it does not cut through the interleaved insulation or mark adjacent turns during winding, which matters most with thin foil.
Q: How is the thickness of the foil selected?
Thickness controls eddy current loss and winding space. Thin foil reduces loss, thick foil reduces the number of turns and winding time, so the value is chosen from the loss budget and the current.
Q: How should transformer foil be stored and handled?
Keep it indoors and dry in the original moisture barrier wrapping, lift it on a proper mandrel, and leave the wrapping in place until the coil is on the winding machine.
