How to Select Electrical Engineering Materials?

An important fundamental factor on the basis of which selection of electrical engineering materials is made consists of its physical, chemical, mechanical and electrical properties. Considering this factor alone is an over simplification of the problem involved in selecting a material.

There are other factors also which must be considered in order to select the most appropriate material and they are listed below:

(i) Ease of fabrication,

(ii) Availability,

(iii) Cost.

The ease of the manufacturing operations possible with a particular material should claim due consideration. In the present age where the speed of production is very important many materials, inspite of having suitable properties, have restricted application in electrical field because of their limited fabrication possibilities. Consider ceramic insulators as an example.

From the point of view of their properties they compare favourably with other insulators. Having served for quite a good their place is now being threatened by certain plastic materials because of the ease with which they can lend themselves to high speed, automatic and better controlled methods of production. Marble stone cannot be used in industry because of its fabrication limitations.

Easy availability of materials and their continuity of supply are factors which may weigh more in favour of selecting a material even though its other properties may be comparatively inferior. India is the biggest exporter of Mica for electrical industry.

But the export of mica is dwindling year after year because the other user countries have already started substituting this by insulating materials which are indigenously available. Rapid development of plastic industry is attributed to Second World War. War caused cut off in trade and thus supply of many insulating materials to involved nations. That forced the industry to seek and develop insulating materials which were easily available.

Another classic example is that of copper as conducting material. Although it is expensive, yet it is preferred by the user as ‘it is the best conducting material easily available today. However, uncertain production and changing political interest among nations have compelled many countries to seek a readily available substitute material i.e. aluminium.

The cost of a material is an equally important factor when considering its selecting for application in industry. As an example we may take silver which although the best known conductor of electricity is not used as such because of prohibitive cost.

Let us take a few examples to explain the above factors- Consider a two pin plug required for connecting to the supply mains domestic appliances like table fan, electric iron etc. The two pins are made of brass to work as conductors. The pins are embedded in a plastic body.

It is required that there should not be any flow of current between the two pins when a voltage is applied. The distance that normally separates the two pins is more than is necessary to provide sufficient insulation between the two pins. Then why not reduce distant between the pins?

This is because mechanical requirements would not permit it. In practice the demand on a plug may be to the extent of thousands of insertions in its life time. The plug should have sufficient mechanical strength to withstand this duty without breaking down.

Moreover, insulating material should be capable of retaining its properties even after getting wet or greasy i.e., it should not absorb moisture or oils. Also the plugs should be designed to suit aesthetic requirements. Next, it is also required that the insulating materials should withstand the effects of climate and its variations.

Humid, acidic or alkaline atmosphere associated with temperature variations from severe winter to extreme summer should be taken into account when selecting insulating materials. Ceramic materials meet these requirements to a great extent, but do not meet the requirements of production with ease and economy.

It is difficult to give appropriate shape of a plug to ceramic materials. This analysis clearly shows the importance of the various factors to be considered while selecting insulating materials for making plugs.

These factors may be listed below in order to priority:

(i) The material should be able to acquire different shapes, textures and colours with ease.

(ii) It should be cheap.

(iii) It should be readily available from local sources; otherwise the product cannot compete in the market.

(iv) The material should possess required mechanical strength.

(v) It should be chemically stable and should not lose its mechanical strength and electrical property under the influence of varying atmospheric and handling conditions.

(vi) It should possess good electrical insulating property to prevent the flow of any appreciable leakage current.

The above was a particular example to illustrate how to select an insulating material for making a plug. This logic can be extended for selecting materials for any other application and a list of factors affecting this selection can similarly be drawn. Electrical engineering is a vast field in which the number and types of materials used are very large.

This was done by first listing the functions of electrical engineering materials which are as follows:

(i) To carry electricity from one place to another,

(ii) To obstruct the flow of electricity except in a particular direction,

(iii) To store electrical energy,

(iv) (a) to transfer electrical energy from one circuit to another circuit,

(b) To change electrical energy to another form of energy and vice versa.

(v) To change electrical signals from one form to another form.

The above functions are accomplished by means of the following which are written in the same order as the functions written above:

(i) Electrical overhead conductors, underground cables, flexible wires etc.,

(ii) Insulators for overhead conductors and insulation covering for cables and flexible wires, transformer oil, air between two conductors etc.,

(iii) Condensers using dielectrics like air, pipes etc.,

(iv) (a) Transformers,

(b) Rotating electrical machines, lamps, heaters etc.,

(v) Transistors, rectifiers etc.

Analysing the above, we can now make a functional classification of electrical engineering materials as follows:

(a) Materials are required which will allow current to pass through them for such varied purposes as a lamp whose filament produces extremely high temperatures, a heater wire whose temperature does not reach so high but is high enough to produce heat and a cable in which the power loss and, therefore, the temperature rise is minimal.

It is obvious that for all these requirements the materials needed must be conductors of electricity. This type of materials is classified as conducting materials. Conducting materials are available in a large variety having diverse properties. We must have on the one hand conducting materials which can withstand high temperatures and on the other, materials which should waste minimum power even when large currents are passing through them.

(b) Materials which obstruct the flow of current without any appreciable power loss are classified as insulating materials. Here again this class of materials is available in a large variety from a plastic material used for a plug to porcelain insulators used in overhead transmission lines, from insulating oils used in transformers to air used between conductors in overhead lines, from plastic insulation used in power cables to mica or asbestos used in high temperature applications like electrical iron, etc.

(c) Materials which store electrical energy are classified as dielectric materials and are used in condensers used for power factor correction in tube lights, in single phase motors etc. For these applications condensers work on mains frequency. Condensers are also required for radio frequency and ultra-high frequency in various electronic applications.

(d) Electro-mechanical energy conversion in rotating electrical machines as well as transformation of energy in a transformer is accomplished through the agency of magnetic field. Materials which provide a path, to the magnetic flux are classified as magnetic materials.

(e) Electrical signals are changed from one form to another by means of devices like transistors, rectifiers etc. The materials used for these purpose are classified as semi-conductor materials.

It must, however, be appreciated that apart from the primary materials which must be used in order that a given function is performed, it is often necessary also to use certain other materials not directly connected with the given function but to assist in performing the function efficiently.

For example, steel used for the tank of a transformer is not used to conduct electricity but to hold insulating liquid in the case of oil cooled transformers. Similarly, lead covering on a paper insulated cable does not perform its function as conductor but protects the paper insulation from the surrounding effects.