COPPER VS. ALUMINUM
The windings of a Low-Voltage, Dry-Type Transformer can be made from copper or aluminum. Like other power transformers, Dry-Type Transformers typically have two windings — a primary winding to draw power from a source and a secondary winding that feeds the same power to a connected load. These windings are coils of insulated conducting (copper/aluminum) wire, and regardless of whether the windings are made from copper or aluminum, the function of these windings remains the same.
In North America, Dry-type, Low-voltage Transformers rated for 15 kilovolt-amperes (kVA) or more typically use aluminum as their winding material. In most other parts of the world, copper is the most predominant winding material. There are two reasons for choosing aluminum windings, and the primary reason is a lower initial cost. Historically, copper metal has had a much more volatile cost than aluminum, which has made procuring copper conductors more expensive. The second reason for choosing aluminum windings is that aluminum is extremely malleable; thus, it is easier to weld aluminum into windings, reducing manufacturing costs. However, copper is much easier to connect reliably, while the expertise of transformer installers determines the reliability of aluminum connections.
Both aluminum and copper have technical qualities that make each of them preferable for certain applications. The choice of winding material depends on the specific requirements and needs of the purchaser. This comprehensive article assists purchasers in making an informed decision on which winding material they should choose. For this purpose, aluminum and copper windings are compared through different criteria.
Coefficient of Expansion
When heated to the same temperature, aluminum expands more than copper by nearly one-third. The ductile nature and higher expansion coefficient of aluminum can cause issues, like joint loosening, if the bolted connections are not installed correctly. To avoid these issues, some kind of compensation is required for aluminum windings. These compensation methods include using spring pressured connections and either split or cupped washers to provide the required elasticity at the joint without causing the aluminum to compress. Proper hardware can raise the quality of the aluminum joints to copper joint levels.
Copper has superior thermal conductivity compared to aluminum, of an identical design, geometry, and wire size. Thus, copper has a higher capability to quickly reduce the hot-spot temperature rise in transformer windings, keeping them much cooler and ready for more extensive loading. To get the same current-carrying capacity from an aluminum coil, the cross-sectional area of the aluminum coil must be larger by about 66%. However, cooling the transformer coils is not an important criterion for choosing the coil material. This is because cooling of a transformer is an important aspect of transformer design itself, with special systems and devices added to the transformer to cool it down. A manufacturer utilizes a combination of conductor shaping, air ducting, cooling oils, coil geometry, and cooling surface area to produce satisfactory hot-spot gradients regardless of the coil material.
In terms of conductivity, a piece of aluminum is only 61% as conductive as a similar-sized piece of copper. Thus, aluminum transformers are perceived to have higher energy losses. However, aluminum transformers are always designed with large aluminum conductors to keep their temperature below the insulation rating. The higher cross-sectional area of the aluminum windings compared to copper windings means that, on average, both transformers have the same energy losses regardless of the conductor material.
In reality, the cost of copper means that transformer manufacturers hesitate to stock copper in different conductor sizes. This limitation translates into the fact that the copper winding size is limited, and improvements in energy loss minimization through conductor sizing cannot be made. In contrast, aluminum windings are cheaper; thus, their sizing can be adjusted to lower energy losses, even lower than copper transformers in some cases. In terms of core losses, the frames of low-voltage dry-type transformers are identical regardless of the winding material; thus, core losses remain the same for aluminum and copper transformers.
The unavailability of copper wire in different sizes and the similar core losses of the two transformers are two factors that can contribute to the design of both transformers – copper and aluminum – such that they have equal efficiency. Additionally, the much lower cost of aluminum can facilitate the design of a much more efficient aluminum transformer through conductor sizing at the same or lower cost than it would take to design a less efficient copper-wound transformer.
When certain loads, that draw high peaks of current such as ac drives with SCR controllers, are connected to the transformer, the high peak currents flow through the transformer and produce strong electromagnetic forces that can move the coil and conductor leads and even deform them. In such scenarios, the lower tensile strength of aluminum, equal to only about 38% of that of a copper piece of the same design and cross-sectional area, has driven concerns about its use in cyclic load applications. However, this concern is a presumption on the part of purchasers and individuals. As mentioned before, aluminum transformers have about 66% larger aluminum windings compared to the cross-sectional area of copper windings in copper-wound transformers. The use of larger-sized aluminum conductors significantly improves the winding strength of the aluminum windings to a level almost equivalent to that of copper windings. The material’s tensile strength plays a smaller role in determining the transformer’s ability to withstand the long-term mechanical stresses caused by high impact loads. The more prominent factors are lead support and coil balance.
Connectivity is the principal concern when using aluminum-wound transformers. Continuous exposure to the atmosphere can cause chemical changes through the oxidation of copper and aluminum. The oxidation of aluminum and copper creates aluminum oxide and copper oxide, respectively. These new compounds must be regularly cleaned, especially aluminum oxide, which has insulating properties and can hamper bolted connections between metal plates. A quality joint compound is needed to brush and clean the joint to prevent oxidation, regardless of the conductor material.
The process of making bolted aluminum connections is a proven and well-established practice. Generally, a bolted connection should never be made between an unplated piece of aluminum and a piece of copper. While several reliable methods exist and can be used for this purpose, e.g., welding and explosive bonding, modern transformer manufacturers do not use any of them. Most connections between copper and aluminum conductors are made by tin-plating or silver-plating the two conductors within the bolted connection. For instance, most connections between copper cables and aluminum transformers are assisted by tin-plated aluminum lugs. These lugs are precisely rated (Al/Cu) for connecting the copper wire to either metal, copper, or aluminum. This method of connecting the two metals has great reliability and a proven track record of more than 30 years.
Ultimately, the choice of transformer winding material boils down to personal preference. Copper is a more expensive choice with a premium price tag that requires justification through application demands. Low-voltage transformers are generally made with aluminum windings due to their significant cost advantage. This is because all of the technical and performance benefits copper windings have over aluminum windings can be overcome by implementing different compensations with aluminum windings. And while the expertise and compensation requirements for making reliable aluminum connections are entirely unavoidable, they are quickly becoming a relatively minor detail in the face of the extensive list of benefits afforded by aluminum. Therefore, the buyer should examine the reasons for preferring copper in terms of specification, cost, and alternatives before investing in the additional cost of copper winding transformers.