In choosing a winding conductor, the decision between copper and aluminium often starts as a cost discussion and ends as a design discussion. Because changing the material changes not only the price but also the cross-section, winding volume, weight and connection details.
The Numbers First
| Property (20 °C) | Copper | Aluminium |
|---|---|---|
| Electrical conductivity | About 58 MS/m (100% IACS) | About 35 MS/m (61% IACS) |
| Resistivity | About 0.0172 Ω·mm²/m | About 0.0282 Ω·mm²/m |
| Density | 8.89 g/cm³ | 2.70 g/cm³ |
| Section needed for equal resistance | 1.00 (reference) | About 1.6 times |
| Relative weight for equal resistance | 1.00 (reference) | About 0.5 times |
| Coefficient of thermal expansion | Lower | About 1.4 times higher |
Two rows in the table sum up the decision: aluminium needs about 60% more section to give the same resistance, yet even when it does, it stays at about half the weight of copper.
Criterion 1 — The Winding Window
The decisive question is: do you have room? When the section grows 60%, the winding height — and therefore the core window — grows too. If the window is fixed, moving to aluminium forces a change in the number of turns or the current density. In compact machines copper is therefore often the only option.
Criterion 2 — Weight
Where weight directly affects cost, aluminium is a strong option: wind generators lifted to the top of a tower, portable equipment, the transport of large distribution transformers. The reduction in winding weight also reflects on the supporting-structure and assembly costs.
Criterion 3 — Thermal Behaviour and Cooling
An aluminium winding dissipates the same loss from a larger surface; at first sight this looks like an advantage. But aluminium's thermal conductivity is lower than copper's and hot-spot management within the winding must be done more carefully. The difference in thermal expansion also matters: in thermal cycles aluminium moves more, which stresses the insulation and connection points.
Criterion 4 — Connection and Termination
Aluminium's best-known difficulty is here. The oxide layer that forms quickly on the surface is insulating and raises the contact resistance; moreover, aluminium creeps over time under pressure, which leads to loosening at bolted joints. The solution is known: a suitable welding/soldering method, bimetallic transition pieces, the correct tightening torque and re-tightening when needed. None of these details arise with copper.
Criterion 5 — Short-Circuit Strength
Very large mechanical forces arise on the winding at the moment of a short circuit. Since copper's mechanical strength is higher, in power transformers where short-circuit stress is critical the choice usually falls to copper. If the design is to use aluminium, the winding compression and support structure must be sized accordingly.
Criterion 6 — Cost and Supply
The unit-mass price favours aluminium; moreover, the mass needed for equal resistance is also lower. Material cost therefore comes out markedly lower for aluminium in most scenarios. But the comparison must not be made on raw material alone: when a larger core, more insulation material and more complex connection details are counted, the gap can narrow.
What Changes in a Conversion Scenario?
Converting an existing design from copper to aluminium is not a matter of changing a line in the catalogue. In a real conversion study the following topics are handled in order:
- Section recalculation: The new section is determined over the target resistance or target current density; whether a switch from round to flat is needed becomes clear here.
- Window check: The winding height and number of layers are calculated with the new section; whether it fits the core window is verified.
- Thermal verification: The winding temperature rise is recalculated; cooling channels are resized if needed.
- Connection design: The termination method, bimetallic transition and tightening-torque values are set.
- Mechanical verification: The winding support structure is reviewed under short-circuit forces.
- Trial production: Feasibility is tested with a prototype on the winding line.
Hybrid solutions are also seen in practice: the main winding may be aluminium and the lead-outs and connection areas copper. Thus the cost advantage is kept while the termination risk is limited.
How Should the Decision Be Made?
- Copper: Applications where the winding window is limited, short-circuit stress is high, compactness is critical and the number of connections is large.
- Aluminium: Applications where weight is decisive, volume is free, cost pressure is high and the connection details are under control.
The decision must always be made together with the winding calculation; a material change is not a purchasing decision on its own. In conversion scenarios we can work through the section, weight and window comparison together.
One final caution: comparing materials only on the current exchange price is misleading. Copper and aluminium prices move independently; the gap that looks in aluminium's favour today may not stay the same over the life of the design. A sound comparison must be made over the total cost of ownership, covering production, assembly and operating costs as well as the material price.
