What factors do engineers consider when they specify aluminum or copper windings?
A high percentage of low voltage transformers rated 15 KVA or larger use aluminum windings. Osborne’s utility industry clients report that nearly all new service entrance connections at commercial industrial facilities use aluminum conductors. The technology has become commonplace over the past 30 years. Price is an important factor in the rise in popularity of aluminum.
Aluminum commodity prices have been lower than copper in recent years. The result is that aluminum windings have an economic advantage for many applications.
Engineers should not allow raw material costs to overshadow other important points of comparison such as circuit performance and energy efficiency.
More points to consider:
| Characteristic: | Aluminum | Copper |
|---|---|---|
| Thermal Conductivity: | 0.57 (Cal/s-cm-K) | 0.94 (Cal/s-cm-K) |
| Electrical Resistivity: | 2.69 (X 10^8, Ohm-m) | 1.673 (X 10^8, Ohm-m) |
| Coefficient of Expansion: | 23.5 (X 10^6, 1/Deg.C) | 17.0 (X 10^6, 1/Deg.C) |
| Tensile Strength | 12,000 (lb/in)^2 | 32,000 (lb/in)^2 |
Source: “Electronic Engineers Handbook”, Donald G. Fink, McGraw-Hill, 1997.
One additional and important characteristic to consider is size. Aluminum is less conductive than copper, and as a result, aluminum wound transformers need to be designed using larger wire sizes. Larger wire translates to larger diameter coils and greater volume cores. This means an aluminum wound transformer will be bigger than a comparably performing copper wound transformer.
The size advantage of copper wound transformers tends to be immaterial for units rated greater than 15 KVA. This is due to the performance sensitivity that size plays in smaller scale equipment. Machines that require higher power transformers generally have ample space to accommodate the relatively larger size of aluminum wound transformers.
Copper wound coils can deliver similar electromagnetic performance in a smaller physical footprint. This size advantage is beneficial when designing transformers rated below 15KVA.
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