Aluminium encyclopaedia

Electrical engineering

Aluminium has been associated with electrical engineering for a long time: it was power generation on a large scale that first made extraction of aluminium using fused-salt electrolysis at all possible after 1886 and the first aluminium overhead cables were used in the USA as long ago as 1898.

The combination of several properties makes the metal attractive for use in electrical engineering: high electrical conductivity (the second-best of all industrially used metals with a conductivity around 60 per cent of that of copper), low density (2.7 grams per cubic centimetre compared with the 8.9 grams per cubic centimetre of copper), good corrosion resistance, non-magnetisability and, unlike copper, abundant deposits (bauxite). As the price of aluminium is also significantly lower, it is economical for many electrotechnical applications.

The greater the purity of a metal and the lower the temperature, the better the conductivity. The electrical conductivity of Al 99,5 (unalloyed aluminium) at room temperature is 36x106 siemens per metre, that of Al 99,9999 (refined aluminium) at room temperature is about ten per cent higher but at -260 °C it is almost 10,000 times greater. The alloying elements that only have a small detrimental effect on conductivity are copper, magnesium, nickel, zinc and tin. Chromium, lithium, manganese, silicon, titanium and vanadium have a strong effect, as does cold forming (whose effect on conductivity can be reversed, however, by heat treatment).

"Conductor-grade aluminium" is unalloyed aluminium or lowly alloyed aluminium alloys used for applications in electrical engineering. According to EN 573-3, the upper limits are: 0.5% iron, 0.1% copper, 0.9% silicon, 0.15% zinc, 0.03% chromium + manganese + titanium + vanadium, and 0.03% each of other elements. Unalloyed aluminium obtained by fused-salt electrolysis usually satisfies these requirements. If not, the addition of boron reduces the amounts of troublesome metals due to precipitation of metal borides. Lowly alloyed aluminium is also used worldwide; the presence of the alloying elements only lowers the conductivity slightly but strength and creep resistance are markedly higher. This is important to prevent any contacts from working loose.

Applications

  • When using electrical connections with aluminium, the insulating oxide layer has to be removed by means of pressure or heat. Suitable methods are soldering, screwed joints and welding, as well as press and crimp connectors for quick connections. The most important applications of conductor-grade aluminium (in alphabetical order):
  • Busbars for very large currents, for use as generator bus ducts, in electroplating, in fused-salt electrolysis, as switchgear, for underground railways and for high voltage applications
  • Conductors, sheathing, shielding and vapour barriers of cables for high- and low-voltages, high-voltage currents and telecommunications
  • Electric motors with windings made from anodised strip
  • Electrical wiring in the household (not in Germany) and in rail and road vehicles

 

Electronics

  • Aluminium cables or aluminium-steel cables (galvanised steel wires at the core accommodate the tensile forces, the aluminium wires conduct the electricity) have been used for power lines since the 1960s and are used almost exclusively for such applications today.
  • Variable capacitors made of sheet, paper capacitors made of foil and paper, electrolytic capacitors made of strip with an artificially strengthened oxide layer (using anodising) as dielectric.