Energy saving versus purchase price.

Contrary to the energy-saving concepts of all major countries, a trend is beginning to emerge in the market for inductive winding goods – windings made of aluminum instead of copper. The big advantage, apart from the lower weight, is the comparatively much lower purchase price for aluminum compared to copper.

Especially in times of a strong franc, many machine and plant manufacturers only have their eyes on the prize. Energy efficiency must not be of interest under price pressure, or savings will be made at the wrong end. In many sectors, such as private media and household appliances, the battle is on for high efficiency and minimal standby losses. In the industrial sector, however, “energy management” is too often ignored. Examples include wiring with too small a cross-section, cheap switches/contacts or “cheap” inductive winding goods such as transformers, chokes and motors. It is often worth taking a closer look at the losses here.

Lower heat losses reduce the design
This is because the direct energy savings often compensate for the higher initial price within 12 to 24 months. Indirect energy savings are also possible in rooms or switch cabinets if they need to be ventilated or air-conditioned. Thanks to lower heat losses, additional building volume and cost savings are possible. In principle, the losses of “cheap” inductive winding goods can now be halved by economically “normal” means without having to double the price.

With the exception of silver, copper has the best conductivity value with γ = 56, while aluminum only has γ = 36. Aluminum follows with a gap of around 35 percent. Copper is therefore the best precious metal and aluminum “only” the second best of the technically and economically usable conductor materials. Nothing comes after that. All other metals are out of the question as conductors, and alloys generally have a considerably lower conductivity than pure metals. Silver or gold are ruled out completely due to their high price. Aluminum is a light metal with only about 35 percent of the density of copper.

Aluminum requires lower current densities
In order to wind a transformer with aluminum that is equivalent in terms of efficiency, the current densities must be reduced by around 35 percent compared to copper. This can be achieved by increasing the conductor cross-sections accordingly. This means that the laminated cores and all mechanical components have to be enlarged. The volume, weight and material usage of the entire transformer increase accordingly. This situation can also result in a price increase. The savings on the conductor material are partially offset by this. However, in order to save costs, transformers are built with many cooling channels to keep the temperature under control. Many aluminum transformers today must have insulation class F, H or higher because heat is a problem. Just as if the higher the insulation class, the better the transformer.

Conductor material aluminum compared to copper
Aluminum is quite ductile, but not as ductile as copper. The magneto-mechanical load on the individual turns of a wound coil increases enormously with the current density and amount of current. Imagine that the coil is loaded with a 100 Hz cycle in a 50 Hz alternating voltage network. This causes the winding to literally blow up. The maximum compressive stress on the conductor can be several N/mm2 even in small transformers. This deforms the cross-section of the wire and, in the worst case, can even cause it to break in the event of a prolonged short circuit. This is often not taken into account when dimensioning transformers. This problem increases with aluminum compared to copper. Furthermore, the individual windings start to rub against each other at 100 Hz, which damages the insulation. The fault remains unnoticed until the winding suffers a short circuit, 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 aluminum, not to mention the tendency to form such constrictions, and an arc is formed, which means an acute fire hazard. In terms of volume, the heat capacity is also lower. At α = 23.1, aluminum has a thermally dependent coefficient of expansion that is around 30 percent higher than copper at α = 16.5. This means that aluminum expands more when heated, which means that the winding can lose more strength and mechanical friction has a greater influence. However, there is a small advantage here: in fully encapsulated epoxy resin transformers, aluminum has approximately the same coefficient of expansion as the epoxy resin itself. As a result, enormous temperature and load fluctuations in the F or H insulation system can result in less internal winding stress. When exposed to air, aluminum very quickly becomes coated with a hard, resistant oxide layer that does not conduct electricity and therefore makes contacting more difficult. Contact resistance can occur, which in turn can result in a fire risk.

“Mechanically”, aluminum is problematic
Aluminum tends to flow over time. The material yields over time under high pressure. This means that initially tight connections can gradually loosen. For this reason, aluminum conductor ends should always be contacted with tightly tightened screw contacts and spring washers, but these are often not permanent anyway. In principle, spring contacts provide a remedy, but then the oxide layers are a problem again. In both cases, there is a slow increase in contact resistance and therefore a risk of fire. Only large-scale soldering or welding can help here. However, electrochemical contact corrosion between aluminum and copper should not be neglected. Aluminum transformers are often incorrectly connected to copper cables out of ignorance. The only purely technical domain of aluminum would otherwise be its weight with a density of ~2.7 g/cm³ in contrast to copper with ~8.93 g/cm³, where the space requirement is not a criterion, but the weight is. This is relevant for overhead lines or transformers, for example, which have to be very light.

Conclusion
Anyone who wants to consider not only the all-dominant price, but also the lifetime costs, should pay more attention to this topic. The efficiency of the entire system with regard to the choice of materials will be decisive for energy efficiency in the future, if special solutions are not only required due to economic, geometric or functional constraints. ”

Author:
Frank Hanisch, electrical engineering graduate,
Head of the Technology and Development department at Bächli AG