Energy saving versus purchase price.

Expensive copper or cheap aluminium for coils?

In the market for inductive winding goods, contrary to the energy-saving concepts of all major countries, a trend is beginning to emerge – coils made of aluminium instead of copper. In addition to the lower weight, the big advantage is the comparatively much lower purchase price for aluminium compared to copper.

Many machine and plant manufacturers only have the price in mind, especially in times of strong a Swiss franc. energy efficiency should not be of interest under price pressure, or you save at the wrong end. In many industries, such as private media and household appliances, the battle is for high efficiency and minimal standby losses. In the industrial sector, however, “energy management” is too often left out. Examples are too small cross-sections of the wiring, cheap switches/contacts or “cheap” inductive winding goods such as transformers, chokes and motors. Here it is often worth taking a close look at the losses.

Lower heat losses reduce the design
Within 12 to 24 months, direct energy savings often compensate for the higher initial price. Furthermore, indirect energy savings can be achieved in rooms or switch cabinets if they need to be ventilated or air-conditioned. Thanks to lower heat losses, additional construction volume and cost savings are possible. In principle, the losses of “cheap” inductive winding goods can be halved today by economically “normal” means without having to double the price.
Expect for silver, copper has the best conductance with γ = 56. Aluminium, on the other hand has only γ = 36. Aluminium thus follows with a distance of about 35 percent. Thus copper is the best precious metal and aluminium “only” the second best of the technically and economically usable conductor materials. Nothing comes after that. All other metals cannot be considered as conductors, and alloys generally have a much lower conductivity than pure metals. Silver or gold are eliminated because of the high price. Aluminium is a light metal with only about 35 percent of the density of copper.

Aluminium causes lower current densities
In order to wrap an equivalent transformer with aluminium in terms of efficiency, it is necessary to reduce the current densities with copper by about 35 percent. This can be achieved by amplifying the conductor cross-sections accordingly. This means that the lamination packages and all mechanical components must be enlarged. The volume, weight and material usage of the entire transformer increase accordingly. This situation may well result in a price increase. The savings in the conductor material are partly used up as a result again. However, in order to save costs, transformers are built with many cooling channels to keep the temperature under control. Many aluminium transformers today must have the insulation class F, H or more, because the heat is a problem. Just about as if a transformer is the better, the higher the insulation class.

Conductor material aluminium compared to copper
Aluminium is quite ductile, but not as ductile as copper. The magneto-mechanical load of the individual windings of a wound coil increases enormously with current density and current quantity. One has to imagine that the coil in the a 50 Hz AC network is loaded with a 100 Hz cycle. In the process, the winding literally inflates itself. The maximum compressive stress on the conductor can be several N/mm2 even in small transformers. The wire deforms in its cross section and, in the worst case, can even tear in the event of prolonged short circuit. This is often not taken into account when dimensioning the transformer. This problem increases with aluminium compared to copper. Furthermore, the individual windings begin to rub against each other in 100 Hz cycles, which damages the insulation. The fault remains unnoticed until the winding suffers a short circuit once, which can take years.

The mechanical friction has an influence on the winding
In the event of a short circuit, the constriction melts through, which in turn can occur much more easily than with copper due to the lower melting point and lower thermal conductivity of aluminium, not to mention the tendency to form such constrictions, and an arc is formed, which means acute fire hazard. In relation of the volume, the heat capacity is also lower. With α = 23.1, aluminium has a thermally dependent coefficient of expansion that is about 30 percent higher than with copper α = 16.5. This means that aluminium expands more when heated, which means that the winding can lose more strength and the mechanical friction has a greater influence. However, there is a small advantage: in fully encapsulated epoxy resin transformers, aluminium has about the same coefficient of expansion as the epoxy itself. This means that fewer internal winding voltages can occur in the F or H insulation system in the event of enormous temperature and load fluctuations. In the air, aluminium quickly becomes coated with a hard, resistant oxide layer that does not conduct electricity and therefore makes it difficult to contact. Transitional resistances can occur, which in turn can result in a fire hazard.

“Mechanically” aluminium is problematic
Aluminium tends to flow over a long period of time. The material gives way under strong pressure over time. Thus, the fixed connections can gradually become loose. For this reason, aluminium conductor ends should always be contacted with tightly tightened screw contacts and spring washers, but these are often not permanent. In principle, spring contacts constitute are a remedy, but then again the oxide layers are a problem. In both cases there is a slow increase in contact resistance, which in turn leads to fire hazards. Here only large-scale soldering or welding helps. However, the electrochemical contact corrosion between aluminium and copper is not be neglected. Often one connects aluminium transformers with copper lines wrong from ignorance. The only purely technical domain of aluminium would otherwise be its weight at a density of ~ 2.7 g/cm³, as opposed to copper at ~ 8.93 g/cm³, where space requirements are not a criterion, but the weight all the more so. This is relevant for overhead lines or e. g. transformers, which have to be very light.

Whoever wants to consider not only the all-dominant price, but also the lifelong costs, should deal with this issue more intensively. The efficiency of the entire system with regard to the selection of materials will be decisive for energy efficiency in the future, if not only economic, geometric or functional constraints do call for special solutions.

Frank Hanisch, graduate engineer in electrical engineering,
Head of the Technology and Development Department at Bächli AG