The ultimate aim of any engineering work is practical application of material in the manufacture of machines for use keeping in view the highest degree of economy that is possible. Selection of the right material is the first and certainly the most important step to achieve this goal. Selection of right material for the right job requires a deep study of the various factors which we shall discuss in the following chapters. The story of the development of any branch of technology is obviously also the story of development of raw materials involved. The development of technology creates new requirements which create the necessity of new engineering materials. Therefore, any new development calls for corresponding development of suitable materials.
An important fundamental factor on the basis of which selection of an electrical engineering material is made consist of its physical, chemical, mechanical and electrical properties. The study of this factor will be made separately for each material while dealing with the respective electrical engineering materials in the chapters that follow. However, 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:
- Ease of fabrication
No matter what the function of the material to be selected is going to be, these factors must be considered.
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 view of their properties they compare favourably with other insulators. Having served for quiet a good period 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 protection. Marble stone cannot be used in industry because of its fabrication limitations.
Ease 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 material 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 selection for application in industry. As an example we make take silver which, although the best known conductor 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 conducting to the supply mains domestic application 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 is more than is necessary to provide sufficient insulator between the two pins. Then why not reduce distance 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 insertion in its life time. The plug should have sufficient mechanical strength to withstand this duty without breaking down. Further, plug are handled by electrical engineers as well as by lay housewives. The latter may use wet or greasy hands. Therefore, insulating material should be capable of rationing its properties even after getting wet or greasy i.e., it should not absorb moisture of oils. Also the plug should be designed to suit aesthetic requirements. Next, it is also required that the insulating materials should withstand the effects of climate and it variation. Humid, acidic and 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 of priority:
- The material should be able to acquire different shapes, textures and colours with ease.
- It should be cheap.
- It should be readily available from local sources, otherwise the product cannot complete in the market.
- The material should possess required mechanical strength.
- It should be chemically stable and should not lose its mechanical strength and electrical properties under the influence of varying atmospheric and handling conditions.
- It should possess good electrical insulating properties 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 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.
Functional classification of electrical engineering materials as follows:
- Materials are required which will allow current to pass through them for such varied purposes as a lamp whose filament produces extremely high temperature, 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 conductor of electricity. This type of materials is classified as conducting materials. Conducting material 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.
- 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 insulator used in overheard transmission lines, from insulating oils used in transformers to air used between conductors in overheard lines, from plastic insulation used in power cables to mica or asbestos used in high temperature application like electrical iron, etc.
- 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 condenser work on mains frequency. Condensers are also required for radio frequency and ultra high frequency in various electronic applications.
- Electro-mechanical energy conversion in rotating electrical machines as well as transformation of energy in a transform are accomplished through the agency of magnetic field. Materials which provide a path, to the magnetic flux are classified as magnetic materials.
- Electrical signal are charged from one form to another by means of devices like transistor, rectifiers etc. The materials used for these purposes 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 to assist in performing the function efficiently. For example, steel used for the tank of a transform 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 a conductor but protects the paper insulation from the surrounding effects.
The study of the behaviour materials has its foundation in chemistry and physics. The understand how materials behave as conductor, semi-conductor, insulator or magnetic materials, it is necessary to refer to atomic structure of materials. In the following section it has been shown how internal bonding of atoms in a material causes it to behave as conductor, semi-conductor or insulator.