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About Aluminium . . .

30th November 1951
Page 84
Page 84, 30th November 1951 — About Aluminium . . .
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ELECTRICAL PROPERTIES

A LTHOUGH primarily a structural 1-1. material, aluminium has for many years been of some importance in electrical engineering, and in one branch at least, that of overhead transmission, it is almost exclusively used. Recently, large changes in the relative costs of aluminium and copper have admitted aluminium to other functions that copper has traditionally performed, and it can be assumed that thc role of aluminium in the electrical industry is by no means fully developed.

Electrical Conductivity

The aluminium normally used for electrical purposes is of 99,5% purity, which at zo C. has a specific resistance (volume resistivity) of about 2.7 microhm/cm. cube in the annealed state. This value may be compared with 1.7 for copper, 7.2 for brass, ii for iron, and 19 for steel.

As in other metals, electrical conductivity is highest in the pure form and is reduced by the presence of alloying elements. Work-hardening and solution treatment also tend to reduce conductivity, while annealing and precipitation treatment (artificial ageing) have the opposite effect. These effects have to be considered should it be necessary, from strength requirements, to employ a heat treatable alloy as a conductor. More relevant, however, is the effect on resistivity when the metal is converted from soft to hard temper by drawing into wire: in alumini urn of electrical purity the change is from 2.7 to 2,85 microhm/cm. cube.

The conductivity of aluminium is thus about 6o% of that of copper, the principal conductor material, In comparing conductors made of these two metals, there are several factors to be considered. If both are to have the same resistance, the aluminium conductor must have 70% more sectional area than the copper one and will then weigh about half as much.

As the aluminium conductor has a larger surface through which to lose heat to the air, its temperature rise will be less. It may be convenient to accept a larger voltage drop and to reduce the section until it carries the same current as the copper at the same temperature; in this case, the aluminium will have about 5o% more sectional area than the copper and will weigh 0.45 times as much.

Aluminium Conductors

One of the earliest uses of aluminium in the electrical industry was in the form of bus bars, uninsulated conductors carrying heavy currents for short distances, and to-day aluminium is the economic choice for this task. By adopting rectangular, channel, or tubular sections, a high strength/weight ratio and large cooling surfaces are obtained. Relatively few supports are needed, the bars are easily formed, and are joined by bolting or welding.

By far the largest single use of aluminium as a conductor is in overhead transmission lines, where a lightweight conductor means less Sag and fewer towers. For many years it has been the usual practice to employ lines comprising aluminium wires stranded over a reinforcing steel core. Current-carrying fittings are of electrical-purity aluminium, usually cast, while corrosion-resistant alloys are used for those subject to high mechanical loads. Advantages other than strength and low weight include freedom from deterioration, and low inductive and corona losses. Steel-cored aluminium conductor is employed for high-voltage transmission systems throughout the world, including the British Grid. Some recent European overhead transmission lines use cables consisting entirely of a heat treated aluminium alloy, which has sufficient strength to make a steel core unnecessary and yet has a conductivity equal to 54% of that of annealed copper.

Considering insulated conductors, aluminium is more economical than tinned copper despite the additional insulating material needed to cover its larger bulk. Covered aluminium wire has been introduced for the wiring of buildings and should eventually be widely adopted.

The oxide film that forms on aluminium in air has dielectric properties, and it is often thought that this makes the conductivity of mechanical joints uncertain. In practice, however, the film is easily removed by abrasion when the joint is being prepared and, with a coating of grease to exclude air and moisture from any areas where contact is imperfect, the film cannot reform, so the joint remains satisfactory.

Anodically thickened, the oxide film can stand pressures of the order of several hundred volts, and this form of insulation, which has negligible bulk and does not deteriorate when heated, is adopted for some special applications, as in electrolytic condensers, and in the windings of large lifting magnets, where reduced bulk permits a lighter iron casting and increased payload.

Non-Magnetic Property

Aluminium and its alloys arc very slightly paramagnetic, the susceptibility (magnetization / magnetizing force) . of 99,5% purity aluminium being o.0000006. For practical purposes they are regarded as non-magnetic, and this characteristic is of value in ships, where aluminium alloy is now frequently used for wheelhouses and other structures surrounding magnetic compasses or electronic equipment. It has the advantages of lightness and lower cost over other non-magnetic metals.

In radio and other electronic work, aluminium is very suitable for electrostatic screening purposes and for condensers of all types.

Other Applications

Properties other than conductivity recommend aluminium for various minor but important duties in the electrical industry.

Flexible cables, both for mining equipment and arc-welding sets, which have constantly to be dragged and lifted by the operator, can profitably be lightened by the use of aluminium.

Aluminium is also used for armouring tape in place of steel when weight is objectionable, as in mines, or when corrosion conditions are severe. For wiring conduit, extruded aluminium tubing offers many advantages, being easy to manipulate and install, and requiring painting only in severe conditions. Aluminium is also coming into use for sheathing of paperinsulated, oil-filled, and gas-filled cables, being drawn or extruded directly over the cable. Being lighter, stronger, and cheaper, aluminium may largely take the place of lead for this purpose.

Light alloys are often employed to reduce inertia forces in moving components of high-speed switchgear, a high-strength alloy being used for the mechanical parts and, in the most modern practice, braided aluminium wire for the conducting "tail." Equipment fitted with aluminium windings for the sake of their low mass includes certain rotary machines designed for high speed or rapid acceleration, and movingcoil instruments. When mobility is important in electrical machinery, as it is in mines, it is worthwhile to use aluminium for motor-casings and other static parts that are normally of cast iron.

After completion, this series of ten articles will be reprinted in collected form, and will be available on request.

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