Further Electrical and Electronic Principles

C R Robertson

• Publisher: Routledge
• Year: 2010
• ISBN:
  9781136401503
• Edition: 3
• Topic: Technik & Maschinenbau
• Subtopic: Elektrotechnik & Telekommunikation

Further Electrical and Electronic Principles is a core text for pre-degree courses in electrical and electronic engineering courses. The coverage of this new edition has been brought in line with the specialist unit 'Further Electrical Principles' of the 2007 BTEC National Engineering specification from Edexcel. As the book follows a logical topic progression rather than a particular syllabus, it is also suitable for other Level 3 students on vocational courses such as Vocational AS/A Level, City & Guilds courses and NVQs.More advanced material has also been included, making this text also suitable for HNC/HND and foundation degree courses.Each chapter starts with learning outcomes tied to the syllabus. All theory is explained in detail and backed up with numerous worked examples. Students can test their understanding with end of chapter assignment questions for which answers are provided. The book also includes suggested practical assignments and handy summaries of equations. In this new edition, the layout has been improved and colour has been added to make the book more accessible for students.The textbook is supported with a free companion website featuring supplementary worked examples and additional chapters.http://books.elsevier.com/companions/9780750687478

Chapter 1

Single-Phase Series A.C. Circuits

Learning Outcomes

This chapter concerns the effect of resistors, inductors and capacitors when connected to an a.c. supply. It also deals with the methods used to analyse simple series a.c. circuits. At the end of the chapter, the concept of series resonance is introduced.

On completion of this chapter, you should be able to:

1 Draw the relevant phasor diagrams and waveform diagrams of voltage and current, for pure resistance, inductance and capacitance.

2 Understand and use the concepts of reactance and impedance to analyse simple a.c. series circuits.

3 Derive and use impedance and power triangles.

4 Calculate the power dissipation of an a.c. circuit, and understand the concept of power factor.

5 Explain the effect of series resonance, and its implications for practical circuits.

1.1 Pure Resistance

A pure resistor is one which exhibits only electrical resistance. This means that it has no inductance or capacitance. In practice, a carbon or metal film resistor is virtually perfect in these respects. Large wire-wound resistors can have a certain inductive and capacitive effect.

Consider a perfect (pure) resistor, connected to an a.c. supply, as shown in Fig. 1.1. The current flowing at any instant is directly proportional to the instantaneous applied voltage, and inversely proportional to the resistance value. The voltage is varying sinusoidally, and the resistance is a constant value. Thus the current flow will also be sinusoidal, and will be in phase with the applied voltage. This can be written as follows

Fig. 1.1

Thus, the current is a sinewave, of maximum value V_m/R, is of the same frequency as the voltage, and is in phase with it.

Hence,

The relevant waveform and phasor diagrams are shown in Figs. 1.2 and 1.3 respectively.

Fig. 1.2

Fig. 1.3

The instantaneous power (p) is given by the product of the instantaneous values of voltage and current. Thus p = vi. The waveform diagram is shown in Fig. 1.4. From this diagram, it is obvious that the power reaches its maximum and minimum values at the same time as both voltage and current. Therefore

Note: When calculating the power, the r.m.s. values must be used.

Fig. 1.4

From these results, we can conclude that a pure resistor, in an a.c. circuit, behaves in exactly the same way as in the equivalent d.c. circuit.

Worked Example 1.1

Q Calculate the power dissipated by a 560 Ω resistor, when connected to a v = 35 sin 314 t volt supply.

A

R = 560 Ω; V_m = 35 V

1.2 Pure Inductance

A pure inductor is one which possesses only inductance. It therefore has no electrical resistance or capacitance. Such a device is not practically possible. Since the inductor consists of a coil of wire, then it must possess a finite value of resistance in addition to a very small amount of capacitance. However, let us a...