Basic Electrotechnology

The present advanced state of the pure and applied sciences could not have been achieved without experimentation and without measurements made with the utmost accuracy possible with existing resources. By measurement we mean comparison with something of the same physical nature which is regarded as a standard. Hence, in order to state the magnitude of any physical entity we make use of two concepts, a numeric and a unit. A numeric is a number and it may be integral or fractional, positive or negative. A unit is a physical entity of such magnitude that it has a numeric of unity. Thus if we say that an electric current is equal to 10 amperes, 10 is the numeric and ampere is the name given to a current of such magnitude that it is equal to the accepted unit of current.

Electricity is a constituent of all matter in its normal state. There are two kinds of electricity called positive and negative and if they are brought together they tend to neutralise one another’s effects in space between them. If two quantities of electricity, one positive the other negative, completely neutralise one another’s effects then the amounts of each are said to be equal. Since matter in its normal state does not exhibit any state of electrification it follows that the amounts of the two kinds of electricity in it are equal.

The word ‘work’ is restricted in the science of Physics to those cases in which a force has to be overcome. If a mass of 1 kg is lying on the ground it will be attracted to the earth by a force of 1 × 9.81=9.81 N.

Suppose that two separate points on a conductor such as a copper wire have a p.d. maintained between them; then an electric field will be set up inside the wire and it will act from the end at high potential V ₁ towards the end at lower potential V ₂ . In this case there can be no mechanism such as that of figure 3.2 to remove this internal field because the state of the conductor is dictated by an outside agency, namely the voltage source such as a battery, which maintains the p.d. (V₁−V₂) between the ends. Consequently all the free electrons, of charge − e will be acted on by a force F= Ee acting in an axial direction from right to left, as shown in figure 4.1a.

The number of amperes flowing in a circuit = the number of coulombs /second.

There are two basic ways in which electric circuits can be made up, namely the series arrangement and the parallel arrangement.

The appliances we call magnets are characterised by, among other properties which are considered later, several easily demonstrated properties:

The fact that an electric current always produces a magnetic field, no matter what the shape of the circuit may be, was discovered accidentally by Oersted in 1820. He was demonstrating what he believed to be the non-connection between electricity and magnetism. On arranging a current-carrying wire parallel to a compass needle he was surprised to find that the needle was deflected. On reversing the current the defection was reversed.

The path of an electric current is called the electric circuit.

If a conductor is moved in a magnetic field in such a way as to cut across the lines of force of the field, an e.m.f. will be induced. If the conductor lies in the plane of the field or if it is moved along the lines of force, then no e.m.f. will be induced. Figure 11.1 shows two important cases.

Figure 12.1 shows a reservoir of very large capacity, supplying a small vessel via a pipe. If the head of water in the vessel is less than that in the reservoir, a current of water will flow along the pipe to the vessel, but as soon as the head of water is the same as that in the reservoir, this flow of water ceases. The filling of the vessel is thus accomplished by a current of temporary nature.

When current flows in a metallic conductor, such as copper, there is no change in the apparent state of the conductor, apart, perhaps, from a rise in temperature. This is because the current is the result of an axial drift of electrons and as many electrons enter at one end as leave at the other. There are many liquids, mainly solutions of ionic compounds in which the passage of current is accompanied by a chemical change. Such liquids are called electrolytes.

Figure 14.1 shows a line OP which rotates with uniform angular velocity w about the end O.

The winding in which the e.m.f. is induced in a three-phase source consists essentially of three separate windings so disposed that there is a phase difference of 120° between each pair. Diagrammatically the arrangement is that of figure 15.1. Each separate winding is called a phase. In the diagram of induced e.m.f.s the arrows indicate the positive direction of the e.m.f. for each particular phase. If all three phases were kept separate a six-conductor line between source and load would be necessary. By interconnecting the phases the number can be reduced to three; in one special case four. There are two methods.

The transformer is a stationary appliance used to change the voltage of an a.c. supply without changing the frequency. For long distance transmission very high voltages are required; for distribution, much lower voltage; and for utilisation, say in the home, voltages of the order of 240 V. The transformer is essentially a ‘close-coupled’ mutual inductor, this meaning that almost of the whole of the flux set up by the primary m m f. links with the secondary winding, and almost of the whole of the flux set up by the secondary m.m.f. links with the primary winding. In other words, the leakage flux, the flux which links with one winding only, is very small. To ensure such close coupling the windings are arranged with their coils in close physical association and they are wound on a magnetic core.

The function of all rotating electric machines is the conversion of one form of energy to another: mechanical into electrical energy in the case of generators; electrical into mechanical energy in the case of motors. The fundamental facts which make this conversion possible are first that an e.m.f. is induced in a moving conductor when it cuts the lines of force of a magnetic field, or when the number of lines of force cutting a circuit changes; and second, that a conductor carrying current is acted on by a mechanical force if it distorts the magnetic field in which it is placed. We see from the first consideration that an electric machine must consist essentially of two parts, namely that which produces the magnetic field, and that which carries the conductors in which the e.m.f.s are induced. It is obvious that there must be relative motion between these two parts.

Imagine an enclosure provided with two electrodes, but completely evacuated. If a p.d. is applied no current can flow because of the absence of carriers. If the enclosure contains electrons these will move to the anode, enter it, and become part of an external current, but this current will cease when all the electrons have left the enclosure. For the current to be maintained there must be a continuous supply of electrons and, clearly, these must come from the cathode. The essential condition for a continued flow under steady conditions is that for each electron which enters the anode an electron must be liberated at the cathode. This necessary emission of electrons from the cathode can be accomplished in several ways, and we will consider two of these: field emission and thermionic emission. In the former, electrons at the cathode surface are pulled out by the attraction of an intense electric field, and we shall see that this necessitates positive as well as negative carriers and therefore does not apply when the carriers are electrons only. In the latter the cathode temperature is raised. This causes the ions of the metal to oscillate and collisions between ions and electrons at the surface will, if sufficiently violent, cause electron emission. Clearly the electron emission will increase as the temperature is increased.

There are three essential features: