Foil winding uses thin aluminum or copper foils instead of insulated wire conductors, processed into compact windings. This design offers particular advantages in short-circuit strength, mechanical stability, and heat dissipation – one reason it is widely used in modern distribution and cast-resin transformers. In the manufacture of modern distribution and cast-resin transformers, this design has therefore become the standard solution over decades and forms an essential basis for high security of supply. Foil windings have proven themselves for decades as a robust and reliable winding technology and form the basis of many modern distribution transformers.
How does a foil winding work?
A continuous, wide metal foil is wound spirally onto the winding former together with an insulating layer. The foil carries the entire current cross-section. Unlike classic wire windings, current distribution is very uniform, resulting in excellent mechanical stability during short circuits. Foil windings are especially common in the low-voltage windings of distribution transformers and in cast-resin transformers, where they integrate seamlessly into the casting process.
Key Characteristics at a Glance
- Aluminum or copper foils as current conductors
- Excellent short-circuit strength thanks to uniform force distribution
- Efficient heat dissipation via the large foil surface area
- Compact design – ideal for dry-type transformers
- High impulse voltage withstand when correctly designed
- Long service life of 30 to 40 years with proper design
- Low axial force loading due to homogeneous current distribution
- Easy adaptation to different winding voltages
Frequently Asked Questions
When is aluminum foil used instead of copper foil?
Aluminum foils are lighter and more cost-effective and are predominantly used in distribution transformers with standard loading. Copper foils offer higher electrical conductivity and are used where particularly compact dimensions or high current densities are required.
What advantages does foil winding offer over wire windings?
Foil windings are significantly more mechanically stable during short circuits, as axial forces act uniformly across the entire foil surface. Heat dissipation is also more effective. In addition, more compact designs can be achieved – an important advantage where installation space is limited.
Which applications benefit particularly from foil windings?
Applications with frequent load changes or high short-circuit currents – such as industrial transformers, transformers in wind turbines, or PV parks. Foil winding is also standard in cast-resin transformers, as it integrates well into the casting process.
What materials are used for the insulating layer?
Typical materials are high-grade films made of polyester, NOMEX, or comparable temperature-resistant materials. In cast-resin transformers, the winding is then completely encapsulated in epoxy resin, which provides additional mechanical and electrical strength.
Which tests are particularly important for foil windings?
In the factory, the turns ratio, winding resistance, insulation resistance, and impulse and withstand voltage strength are measured. For cast-resin transformers, partial discharge measurement is also essential, since even the smallest air inclusions in the cast resin would significantly impair service life.
Would you like to learn more about transformers with modern foil winding technology? Talk to our engineering experts – we will advise you on the right design for your application.