The discussion about the choice of winding material—copper versus aluminum—has long been a topic in the field of dry-type cast resin transformers. From the perspective of physics, mechanics, and long-term structural reliability, aluminum is a material whose properties align exceptionally well with the specific requirements of cast resin technology used in medium power transformers. In European practice, aluminum is used in more than 90% of cast resin transformers. This is not a coincidence nor an economic compromise, but a result of material optimization.
A Material that Matches the Physics of Casting Technology
In dry type transformers, the conductor and the epoxy insulation form one mechanically bonded unit. The behavior of this system under temperature changes is therefore a critical design parameter. Aluminum has a linear thermal expansion coefficient (CTE) of approximately 22 × 10⁻⁶ K⁻¹, while copper reaches approximately 17 × 10⁻⁶ K⁻¹. Epoxy resin filled with microsilica exhibits mechanical characteristics that complement the behavior of aluminum conductors extremely well. This enables full utilization of the expansion compatibility between aluminum and the insulation system, resulting in favorable mechanical conditions throughout the winding. In practice, this material compatibility leads to:
✔ reduced mechanical stress within the cast coil
✔ higher resistance to repeated thermal cycling
✔ long-term stability of the insulation system
Reliability of Electrical Connections
Modern connection technologies and methods for implementing internal joints ensure long term stable performance of aluminum windings—and thus the entire transformer. Extensive industrial experience confirms that the quality of electrical connections does not limit transformer lifetime.
In applications where the transformer is connected to copper cables, we offer a technically proven solution using bi-metallic cupal washers. These washers are installed on the aluminum terminals of the transformer and ensure stable, safe, and long-term reliable connections between dissimilar materials.
Electrical Parameters Without Compromise
Copper has an electrical conductivity of approximately 58 MS/m, while aluminum has about 35 MS/m. The lower conductivity of aluminum is routinely compensated by an optimized conductor cross-section. The resulting electrical behavior of the winding is therefore fully comparable.
In a well designed transformer, losses, winding temperature rise, and overall efficiency are equivalent. International standards therefore do not define different loss limits based on conductor material.
Lower Weight, Real Advantages
The density of copper is approximately 8.9 g/cm³, while aluminum has a density of about 2.7 g/cm³. Even with a larger conductor cross-section, the resulting transformer design is typically 20–25% lighter.
Lower weight directly impacts practical project aspects. Key benefits include:
✔ easier handling and installation
✔ lower load on supporting structures
✔ optimized transport costs — a factor that can be economically significant at higher power ratings
Technical and Economic Reasons in Balance
Aluminum offers long term price stability and significantly lower market volatility than copper. For investors and designers, this means better cost predictability and more stable project planning.
Design Optimization Over Material Preference
The performance, losses, and temperature rise of a transformer are the result of the design concept—not merely the choice of conductor metal. Aluminum enables the achievement of identical electrical parameters while optimizing the mechanical behavior of the coil and supporting an efficient structural design.
Summary
The use of aluminum in dry type transformers produced by SGB Czech Trafo is not a compromise, but the result of technical optimization of the entire conductor–insulation system. The combination of material compatibility, equivalent electrical performance, lower structural weight, and stable economic factors makes aluminum a rational and proven long term choice.
