For the last thirty-plus years, aluminum wound transformers have proven to be as reliable as copper wound units. That is why aluminum has replaced copper as the dominant conductor in transformers today. In addition to aluminum’s excellent performance, the cost of aluminum has also been lower and more predictable than copper. These facts have led many organizations to use aluminum wound transformers, where copper was once the preferred choice.
While aluminum has proven itself as a reliable winding conductor in transformers, for many, the question still remains, “Are there good reasons for copper windings to be preferred over aluminum?” Below, we examine the possible reasons this question remains. If you want to brush up on transformer coils first, check out our guide to electrical transformers and our article on transformer cores.
All copper and aluminum distribution transformers are built to the same efficiency standards per the latest DOE regulations. Yes, copper’s thermal and electrical conductivity and tensile strength are higher than aluminum’s. And, yes, aluminum’s conductivity is 60% that of copper. However, the cross-sectional area of an aluminum transformer coil is simply increased to make up the difference. The result is an aluminum winding with similar energy losses to a copper one.
Since copper’s raw material cost is higher than aluminum, distribution transformers with copper windings can cost up to 100% more than those with aluminum windings.
Copper’s higher tensile strength has traditionally made it the preferred choice for power class I and II transformers with circular disk windings. However, in transformers with a rectangular design (usually below 10 MVA), axial force is reduced through sheet (strip) conductors and layering the windings on top of each other. This layering design distributes the axial force evenly across the entire winding surface, making the tensile strength of the conductor of less consequence.
Size & Weight Considerations
The tank size could be reduced with copper windings because the coil assembly is smaller than aluminum, but the size difference is generally mere inches. Most manufacturers standardize their enclosures, meaning aluminum and copper coils will come in enclosures that are the same size.
Copper’s high density makes the transformer roughly 20% heavier than an aluminum option. Copper’s heavier weight may incur additional costs from a civil engineering perspective (i.e. if the unit is designed for a rooftop or platform application).
A copper wound unit may make for an easier fit for very specific custom-built situations with strict size constraints. This point is perhaps the most compelling when considering the benefits of utilizing copper in distribution transformers.
Aluminum is softer and easier to handle due to its greater malleability. It also tends to have an easier time adhering to the thermoset insulation coating between windings. Welding aluminum is also easier than copper. Due to the higher temperatures required for welding copper, brazing is often used for copper connections. This brazing can result in a slight loss of conductivity at the joint itself. Because aluminum connections are easier to join at lower temperatures, connections are typically welded; this yields a better connection with no loss of conductivity at the joint.
Both copper and aluminum undergo oxidation, but unlike copper, aluminum oxide forms a protective coating around the bare conductor, inhibiting further oxidation. This is often a concern for electrical contractors terminating aluminum cables in the field, as the aluminum oxide coating can compromise the connection between the lug and cable. This same concern is often used as a reason to discredit the use of aluminum in transformers, but this is not necessarily the case.
Since aluminum joints are welded via inert gas, the aluminum oxide creates a protective coating around the joint rather than in between. Subsequently, this reduces further oxidation of the bare conductor around the connection. Arguably, this makes for a more reliable connection that is less subject to deterioration.
Oxidation is a non-issue for sealed liquid-filled transformers where the connections are sealed in a dielectric fluid (such as pad-mounted and substation units).
A transformer’s life expectancy is determined by its insulation system. There is little evidence to suggest that the material of the winding conductor (whether copper or aluminum) has a significant bearing on the service life of distribution transformers used in commercial and industrial applications.
More often than not, the decision to use copper or aluminum windings comes down to preference. Manufacturing practices have come a long way since the early 1960s and 1970s when aluminum first came into prevalent use in transformers, and, as a result, aluminum and copper provide equally reliable transformer winding conductors today for distribution class transformers. However, the ability to manufacture reliable, high-efficiency transformers at lower costs has led to the prolific use of aluminum for commercial and industrial applications.
At Maddox we stock thousands of copper and aluminum transformers, though we usually encourage customers to consider aluminum transformers for their projects due to their cost, availability, and comparable performance.