Electronic Applications: Part One

Abstrakti:

Copper was the metal first used for conductors during the great development of electrical engineering in the early 1880’s. However, the simultaneous rapid growth of the aluminum industry soon reduced the price of aluminum so that it could compete in electrical conductor applications.
For overhead transmission lines, aluminum is now used almost to the exclusion of copper. Although the increasing use of aluminum bus conductor is credited largely to economics, advances in joining techniques and general experience have prompted its use in many manufacturing, chemical and utility installations.

Copper was the metal first used for conductors during the great development of electrical engineering in the early 1880’s. However, the simultaneous rapid growth of the aluminum industry soon reduced the price of aluminum so that it could compete in electrical conductor applications.

For overhead transmission lines, aluminum is now used almost to the exclusion of copper. Although the increasing use of aluminum bus conductor is credited largely to economics, advances in joining techniques and general experience have prompted its use in many manufacturing, chemical and utility installations. The most recent aluminum conductor development, magnet strip, owes its success to savings in coil manufacturing costs as well as to the lower initial cost of aluminum.

Many other electrical, mechanical and structural applications of aluminum in electrical industries developed along with the growth of aluminum wire and cable. Non-current-carrying applications of aluminum are numerous in transformers, capacitors, motors and other types of electrical equipment.

Wire and Cable

The favorable attributes of aluminum for electrical applications are: relatively high electrical and thermal conductivities, low density, nonmagnetic properties, ease of drawing down to the smaller wire sizes and high resistance to weathering.

Because of technological advances and favorable price as compared to copper, there has been continuous significant growth in the volume, sizes and varieties of aluminum conductors.

Aluminum Conductor Materials. It was early learned that the electrical resistivity of aluminum is markedly increased by impurities; electrical conductor grade (EC) metal, containing approximately 99.5% Al, was established for most conductor uses. At present, aluminum producers offer EC with a minimum of 99.6% Al and conductivity of 62.0% IACS on a volume basis, although ASTM specifications permit 99.45% Al minimum and 61.0% conductivity. EC wire has a tensile strength range of (83-200 MPa), depending on temper.

Transmission Conductor. All-aluminum stranded conductors, although having the advantage of light weight, had to be strung with large sags, because of their low yield strength compared to stranded copper.

Experiments were conducted with several alloys of aluminum, and with combinations of aluminum with other metals, to obtain higher mechanical strength combined with suitable electrical characteristics.

Aluminized and aluminum-clad core wires recently have been employed to reinforce aluminum conductors. For many years, it was believed that a superior acsr could be produced if the zinc coating of the steel wire were replaced with aluminum, in 1957, hot-dipped aluminum-coated (aluminized) steel wire was produced as an acceptable core wire for acsr. The excellent resistance to atmospheric corrosion of aluminized wire resulted in its acceptance for service in coastal and severely corrosive industrial atmospheres. Aluminized steel core wire is covered by ASTM B 341.

Another method of covering steel wire incorporates an aluminum cladding that is about 10% of the composite diameter, representing approximately 25% of the total cross section, bonded to a high-strength steel core. It is utilized as a highly corrosion-resistant core for acsr, and as an overhead ground wire for transmission and distribution lines.

In recent years, both heat treatable alloy 6201-T81 (0.7% Si, 0.75% Mg) and non-heat-treatable 5005-H19 (0.8% Mg) have been employed as stranded aluminum conductors for transmission and distribution lines. Material selection involves comparing the estimated installed cost of these and other conductors with expected service requirements, considering current-carrying capacity, thermal overload characteristics, line electric-energy losses, and atmospheric exposure characteristics.

Distribution Conductor. The use of aluminum conductors for urban distribution has increased rapidly since World War II, stimulated by the change in the aluminum-copper price relationship. Aluminum has over twice the conductivity of copper on a weight basis, and aluminum conductor is used for virtually all overhead subtransmission and distribution lines.

For many years, predating even the introduction of acsr, weatherproof distribution conductors were covered with cotton braid and impregnated with asphalt. After World War II, the cotton braided coverings were displaced by neoprene and, later, by polyethylene. In addition to excellent weathering characteristics, neoprene and (especially) polyethylene have high electrical insulating properties.

Utilizing these properties, multiplex cable, a new secondary and service-drop multiple conductor, was developed. These cables are either duplex, triplex, or quadruplex, consisting of one, two or three insulated conductors wrapped around a bare aluminum or acsr neutral. The bare neutral act as the messenger, supporting the entire cable when strung is an aerial line.

The aluminum alloys employed for conductor accessories, including drawn, extruded, and cast products, vary with the specific application and with the preferences of the individual manufacturer. However, the alloys are generally selected to provide suitable conductivity, high resistance to atmospheric corrosion, galvanic compatibility with conductor grade (EC) aluminum, and satisfactory mechanical properties. Typical suitable alloys are 6061-TG for wrought forms and 356-T6 for castings.

Aluminum clamps, frequently 356-T6, are preferred to other metals for supporting stranded aluminum conductors. They eliminate heating and power waste from hysteresis losses that occur in ferrous materials. Because the use of dissimilar metals is avoided, the possibility of galvanic corrosion is minimized. The body and keeper components of suspension clamps and dead-end clamps are aluminum castings or forgings.

Aluminum armor rods are spiraled around the conductors at points of support. They provide some vibration damping and reinforce the conductors against the effect of vibration. Two general types of armor rods are available. One is straight rods that are applied by winding them on the conductor with special wrenches. The other is pre-formed during manufacture, and usually can be worked into place on the conductor by hand. The pre-formed type is generally easier to install; however, the straight rods applied with wrenches have considerably better damping characteristics.

Insulated Power Cable. Today the choice between aluminum and copper for the metal in insulated conductors is based primarily on cost considerations for the particular application. The cost comparison should be based on the final installed cost of the circuits involved. For a specific application, the construction of insulated cable with aluminum conductor is essentially the same as that with copper conductor. However, there is usually a difference in conductor size, compensating for aluminum’s lower electrical conductivity.

The minimum conductivity of EC grade aluminum is 62.0% that of copper. A convenient and reasonably accurate guide derived from this relationship is that, for equal voltage drop, an aluminum conductor should be two average sizes larger than a given copper conductor.

If, however, circuit design is based primarily on ampacity (current-carrying capacity), conductor size is determined by the temperature rating of the cable and its rate of heat dissipation. The ampacity of a cable of specific size depends on the details of construction and on the metal used for the conductor. For insulated cables of identical size and construction, those with aluminum conductors have 78 to 84% the ampacity of those with copper conductors.

The aluminum conductors normally used in insulated cable are EC grade, in either the H24 or H26 temper. Where unusually high strength is required, 5005 or 6201 alloy, or sometimes acsr, is used. The most common specifications for conductors in insulated cables are B 323 for solid conductors and B 231 for stranded conductors.

Specialty Wire and Cable. There are several specialty wire and cable constructions where aluminum may be advantageous for one or more components.

For example, portable power cables and welding cables having aluminum conductors usually cost appreciably less than equivalent copper-conductor cables, Conductor weight is reduced almost half by using aluminum instead of copper. The aluminum cable is larger in diameter when the lower conductivity of aluminum makes this necessary.

Applications of aluminum for wire and cable exist where the diameter difference is small or unimportant and the lighter weight of the aluminum cable is of major significance. If ultimate fatigue failure of the cable is expected, as a result of repeated flexing, somewhat shorter life is anticipated.

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