In this article we will discuss about the applications of low and high resistivity materials in electrical engineering.
Applications of Low Resistivity Materials:
i. Copper:
It is most widely used metal because soft its high conductivity (i.e., low resistivity), Silver has the lowest resistivity but due to its high cost it is not used as conducting material. Copper is available in two forms, viz. annealed copper and hard drawn copper, for use as conducting material. Hard drawn copper is about 4% less conducting than annealed copper, but has more tensile strength and is used in transmission and distribution lines where conductors have to be stretched.
Annealed copper is used at place where flexibility is required e.g., wiring of buildings, winding wires and electrical machines and transformers. Hard drawn copper is obtained by drawing copper bars or rods in cold condition. Annealed copper is obtained by heating at specific temperature and then cooling.
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For windings of electrical machines and transformers, copper is preferred to aluminium. This is because aluminium wires have a tensile strength much lower than that of copper. This results in tendency in the breaking of the aluminium wire under tension as well as developing kinks. Moreover, the resistivity of aluminium is higher as compared to copper and, therefore, aluminium wire has to have a thicker cross- section to keep I2R losses low.
The winding, thus, occupies more space and the machines size increase. However, aluminium wound machines have less weight because of low density of aluminium as compared to copper. Hence for winding of electrical machines, copper is preferred as compared to aluminium due to higher tensile strength and lower resistivity of copper in comparison with aluminium.
Copper and aluminium are low resistivity material and if they are used for making the elements for electric heaters, the length of the wire would be too large which would increase the overall size of the equipment to a large extent. Hence copper and aluminium are not suitable for this purpose.
ii. Aluminium:
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Its electrical conductivity is next to copper. Aluminium occurs in abundance on earth’s surface, it is available in various forms such as oxides, sulphates, silicates, phosphates, etc.
Aluminum is a white metal with bluish tinge and is very light in weight (Cu = 3.5 Al), which is its chief asset. Pure aluminium is softer than copper, so can be rolled into thin sheets or foils. Due to low mechanical strength, it cannot be drawn into very thin wire. The resistivity is about 1.65 times greater than copper.
Keeping in view the resistivity, mechanical strength and density, for the same resistance and length of a wire, an aluminium conductor should have a cross-sectional area 1.6 times that of the copper conductor; the weight of the aluminium will equal to 0.48 times the weight of copper.
Use of aluminium as an electrical material, particularly in the aircraft industry, has considerable advantages because of the saving in weight involved. Again electrochemical plants are enormous users of aluminium busbars, because electrolytic cell operators with heavy currents at low voltages and to carry these current, massive bars are required. Aluminium, because of its lightness, is being used more and more for such busbars.
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The current carrying capacity of aluminium being 75% that of copper, and its density being approximately one-third that of copper, an aluminium busbar is only about half the weight of copper busbar of equal current carrying capacity. Since aluminium costs much less than copper, an aluminium busbar will cost only about half as much as its counterpart copper.
Aluminium wires have greater resistance to corrosion. In open air, thin film of aluminium oxide protects the metal from further oxidation. Oxide film insulation is extensively used in electrolytic capacitors. The steel reinforced aluminium conductor is being extensively used for long span transmission lines. Aluminium is not easily solderable, but fluxed have been devised to make soldering easy. Mechanical clamping and screwing methods have also been developed.
iii. Steel:
Steel is not very often used as a conducting material because of its low electrical conductivity, in spite of the fact that it has higher mechanical properties and low cost. When alternating current flows in steel wire, its resistivity increases and power losses are much higher than when direct current flows, due to the magnetic properties of steel. Steel is easily corroded by moisture and heat. Overhead steel conductors are galvanized to prevent corrosion.
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In this method, steel is thoroughly cleaned and then it is dipped in a bath of molten zinc. The layer of zinc protects steel from rusting. Steel overhead conductors, due to less electrical conductivity, are used only to transfer small amount of power. Aluminium conductors used for high voltage systems, long spans, are usually reinforced with steel, which gives high tensile strength to overhead lines.
Applications of High Resistivity Materials:
Alloys of copper, nickel, chromium, iron and manganese are extensively used as resistance materials. A resistor is a device which is used to introduce resistance in an electric circuit for the various purposes such as-
1. As heater elements to produce heat energy from electrical energy
2. As meter elements such as ammeter shunts and voltmeter multipliers
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3. For controlling of current such as rheostats.
For resistor applications, the primary requirements are uniform resistivity (achieved in a homogenous alloy), stable resistance (achieved by avoiding metallurgical changes such as ageing and relaxation of residual stresses), small temperature coefficient of resistance (α) and low thermoelectric potential with respect to copper. A small minimized the error in measurements due to variations in the ambient temperature, is defined as-
α = 1/R dR/dT
where R is the resistance of the alloy at temperature T. For manganin alloy (87% Cu and 12% Mn), α is only 20 × 10-6 K-1 as against 4000 × 10-6 K-1 for pure copper. Constantan (60% Cu and 40% Ni) is another such alloy. These alloys have also good resistance to atmosphere corrosion, another desirable property in a resistor.
Manganin and constantan are usually used for making wire-wound precision resistance for measuring instruments, shunts for electrical measuring instruments, resistance boxes, standard resistance coils and coils for precision electrical measuring instruments. Constantan is used in making thermocouples and in field regulators for regulating the generated voltage of a generator.
A low thermoelectric potential with respect to copper, to which the resistor is commonly connected, reduces errors due to temperature differences between junctions. For high precision, dissimilar junctions should be maintained at the same temperatures so that no thermoelectric potential develops.
Ballast resistors are used to maintain constant current in some industrial circuits. If the flow of current increases, the temperature increases and the resistance of the ballast increases. This in turn decreases the current in the circuit towards the initial value. An iron-nickel alloy (71% Fe, 29% Ni) with excellent oxidation resistance and a high α of 4500 × 10-6 K-1 is used for this application.
For heating elements, the primary requirements are high melting point, high electrical resistance, good oxidation resistance, good creep strength, low elastic modulus and low thermal expansion. The last two requirements help in reducing thermal fatigue due to repeated heating and cooling. The heating elements should be designed in a way as to allow unhindered expansion and contractions, for example, in the form of a coil of wire.
Nichrome (80% Ni and 20% Cr) and Kanthal (69% Fe, 23% Cr, 6% Al and 2% Co) are used for heating elements (for electric heaters, electric ovens, electric iron, and room heaters) up to 1300°C. SiC and MoSi2 can be used at higher temperatures up to 1700°C. Graphite, by virtue of its high sublimation temperature and good fabrication properties, is also widely used upto 1800°C.
Molybdenum and pantalum need protective atmospheres at high temperatures, as their oxidation resistance is poor. By virtue of its very high melting point (3410°C), tungsten is used for filaments in incandescent lamps. Its creep resistance at white heat (above 1500°C) can be improved by dispersion hardening with thoria (ThO2).
Resistance thermometers should have a high temperature coefficient of resistance for good sensitivity. A pure metal is obviously the choice for this application. Platinum obtainable in very pure form is used.
Table 6.7 indicates the properties of the some of the conductor and resistor materials.
