Mastering Electrical Engineering

Frontmatter

Chapter 1. Principles of Electricity

Abstract

About 100 basic substances or chemical elements are known to man, each element consisting of a number of smaller parts known as atoms. Each atom comprises several much smaller particles, the principle ones being electrons, protons and neutrons.

Noel M. Morris

Chapter 2. Electrochemistry, Batteries and Other Sources of e.m.f.

Abstract

The chemical effect of an electric current is the basis of the electroplating industry; the flow of electric current between two electrodes (one being known as the anode and the other as the cathode) in a liquid (the electrolyte) causes material to be lost from one of the electrodes and deposited on the other.

Noel M. Morris

Chapter 3. Resistors and Electrical Circuits

Abstract

A resistor is an element whose primary function is to limit the flow of electrical current in a circuit. A resistor is manufactured either in the form of a fixed resistor or a variable resistor, the resistance of the latter being alterable either manually or electrically. Many methods are employed for the construction of both fixed and variable resistors, the more important types being described in this chapter.

Noel M. Morris

Chapter 4. Measuring Instruments and Electrical Measurements

Abstract

So far we have discussed values of voltage, current, resistance, etc, without mentioning the way in which they are masured. In general, the operating principles of instruments are beyond the technical level reached at this point in the book and we are in the ‘chicken and the egg’ dilemma, insomuch that it is difficult to explain how an instrument works before its principle can be fully comprehended.

Noel M. Morris

Chapter 5. Electrical Energy and Electrical Tariffs

Abstract

The amount of heat energy, W joules or watt seconds, produced by a current of I amperes flowing in a resistor R ohms for t seconds is

$$ W = {I^2}Rt\,{\text{joules (J) or watt seconds}} $$

Thus, if 1 A flows for 1 second in a resistance of 1 Ω, the energy dissipated is

$$ W = {I^2} \times 1 \times 1 = 1\,{\text{J}} $$

However, if 100 A flows for the same length of time in the same resistor, the energy dissipated is

$$ W = {100^2} \times 1 \times 1 = 1{\text{0}}\,{\text{000}}\,{\text{J}} $$

The electrical power rating of an item of electrical plant is related to its ability to dissipate the energy which is created within the apparatus. Since the heat energy is related to I², the rating of an electrical machine depends on its ability to dissipate the heat generated (I²R) within it; the rating therefore depends on the current which the apparatus consumes.

Noel M. Morris

Chapter 6. Electrostatics

Abstract

Many hundreds of years BC it was discovered that when amber was rubbed with fur, the amber acquired the property of being able to attract other objects. The reason for this was not understood until man knew more about the structure of matter. What happens when, for example, amber and fur are rubbed together is that electrons transfer from one to the other, with the result that the charge neutrality of the two substances is upset. The substance which gains electrons acquires a negative charge, and the one which loses electrons acquires a positive charge.

Noel M. Morris

Chapter 7. Electromagnetism

Abstract

Purely ‘electrical’ effects manifest themselves as voltage and current in a circuit. It was discovered early in the nineteenth century that electrical effects also produce magnetic effects, and it is to this that we direct studies in this chapter.

Noel M. Morris

Chapter 8. Electrical Generators and Power Distribution

Abstract

The early activities of scientific pioneers led to the idea that magnetism and electricity were interrelated with one another. It was found that when a permanent magnet was moved towards a coil of wire — see Figure 8.1 — an e.m.f. was induced in the coil. That is, the e.m.f. is induced by the relative movement between the magnetic field and the coil of wire. Lenz’s law allows us to predict the polarity of the induced e.m.f.

Noel M. Morris

Chapter 9. Direct Current Motors

Abstract

The motor effect can be regarded as the opposite of the generator effect as follows. In a generator, when a conductor is moved through a magnetic field, a current is induced in the conductor (more correctly, an e.m.f. is induced in the conductor, but the outcome is usually a current in the conductor). In a motor, a current-carrying conductor which is situated in a magnetic field experiences a force which results in the conductor moving (strictly speaking, the force is on the current and not on the conductor, but the current and the conductor are inseparable).

Noel M. Morris

Chapter 10. Alternating Current

Abstract

As mentioned earlier, an alternating quantity is one which reverses its direction periodically, being in one direction (say the ‘positive’direction) at one moment and in the opposite direction (the ‘negative’ direction) the next moment. The frequency of alternations can be as low as once every few seconds or as high as once every few nanoseconds \( (1\,{\text{ns = 1}}{{\text{0}}^{\text{ - }}}^9{\text{ s or }}\frac{1}{{1\,000\,000\,000}}\,{\text{s)}} \) .

Noel M. Morris

Chapter 11. Introduction to Single-Phase a.c. Circuits

Abstract

In an a.c. circuit at normal power frequency, a resistance behaves in the same way as it does in a d.c. circuit. That is, any change in voltage across the resistor produces a proportional change in current through the resistor (Ohm’s law).

Noel M. Morris

Chapter 12. Single-Phase a.c. Calculations

Abstract

A single-phase circuit containing a resistor, an inductor and a capacitor is shown in Figure 12.1. You will recall that the phase relationship between the voltage and the current in a circuit element depends on the nature of the element, in other words, is it an R or an L or a C? This means that in an a.c. circuit you cannot simply add the numerical values of V _R , V _L and V _C together to get the value of the supply voltage V _S ; the reason for this is that the voltage phasors representing V _R , V _L and V _C ‘point’ in different directions relative to the current on the phasor diagram. To account for the differing ‘directions’ of the phasors, you have to calculate V _S as the phasor sum of the three component voltages in Figure 12.1. That is

$$ {\text{supply voltage, }}{V_S}{\text{ = }}phasor sum{\text{ of }}{V_R}{\text{ , }}{V_L}{\text{ and }}{V_C} $$

To illustrate how this is applied to the circuit in Figure 12.1, consider the case where the current, I, is 1.5 A, and the three voltages are

$$ {V_R}{\text{ = 150 V, }}{V_L}{\text{ = 200 V, }}{V_C}{\text{ = 100 V}} $$

We shall consider in turn the phasor diagram for each element, after which we shall combine them to form- the phasor diagram for the complete circuit.

×

Noel M. Morris

Chapter 13. Poly-Phase a.c. Circuits

Abstract

As its name implies, a poly-phase power supply or multi-phase supply provides the user with several power supply ‘phases’. The way in which these ‘phases’ are generated is described in sections 13.2 and 13.3, and we concentrate here on the advantages of the use of a poly-phase supply which are:

1.

For a given amount of power transmitted to the user, the volume of conductor material needed in the supply cable is less than in a single-phase system to supply the same amount of power. A poly-phase transmission system is therefore more economical than a single-phase supply system.

 

2.

Poly-phase motors and other electrical equipment are generally smaller and simpler than single-phase motors and equipment. For industry, poly-phase equipment is cheaper and easier to maintain.

 

A poly-phase supply system may have two, three, four, six, twelve or even twenty-four phases, with the three-phase system being the most popular. The National Grid distribution network is a three-phase system. An introduction to electrical power distribution systems was given in Chapter 8, where it was shown that power is distributed to industry using a three-phase system, a single-phase system being used for domestic power distribution.

Noel M. Morris

Chapter 14. The Transformer

Abstract

In Chapter 7 (Electromagnetism) it was shown that

1.

when a current flows in a coil, a magnetic flux is established;

 

2.

when a magnetic flux cuts a coil of wire, an e.m.f. is induced in the coil.

 

The above effects are involved in the process of mutual induction, illustrated in Figure 14.1, in which a changing alternating current in one coil of wire (the primary winding) induces an e.m.f. in the second coil (the secondary winding). The general principle of operation is described below.

Noel M. Morris

Chapter 15. a.c. Motors

Abstract

Imagine that you are looking at the end of the conductor in Figure 15.1(a) when the S-pole of a permanent magnet is suddenly moved from left to right across the conductor. By applying Fleming’s right-hand rule, you can determine the direction of the induced e.m.f. and current in the conductor. You need to be careful when applying Fleming’s rule in this case, because the rule assumes that the conductor moves relatively to the magnetic flux (in this case it is the flux that moves relatively to the conductor, so the direction of the induced e.m.f. is determined by saying that the flux is stationary and that the conductor effectively moves to the right). You will find that the induced current flows away from you.

Noel M. Morris

Chapter 16. Power Electronics

Abstract

A semiconductor is a material which not only has a resistivity which is mid-way between that of a good conductor and an insulator, but has properties which make it very useful in the field of electronics and power electronics. There are two principal categories of semiconductor, namely n-type semiconductors and p-type semiconductors.

Noel M. Morris

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