Shell Electrical Engineer Handbook

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  10.06.24 - 020 1 Theory Electrical engineering Electromagnetism, colour coding, direct and alternatingcurrent, and transformers Following a brief introduction, drawings are used to explain how anelectromagnet is made by passing an electric current through a coil.An application of this principle is also described. Next, the terms direct currentand alternating current are then explained. This knowledge is then used inexplaining the transformer and a few applications. Finally, colour coding andvarious wall sockets are discussed. Contents of the lesson 1Magnets2Electromagnets3Direct current, alternating current and three-phase current4Transformers and their applications5Colour coding electric cables6Wall sockets7Colour coding wall sockets The copyright in this material is vested in Shell Global Solutions International B.V., The Hague, The Netherlands and Shell Netherlands Raffinaderij B.V. All rightsreserved. Neither the whole or any part of this document may be reproduced, stored in any retrieval system or transmitted in any form by any means (electronic,mechanical, reprographic, recording or otherwise) without the prior written consent of the copyright owner.  Theory / 10.06.24 - 020 2 Lesson 1. Magnets We are all familiar with magnetism - the peculiar way in which pieces of a particular kind of iron either attract or repel each other. Magnets can be madefrom the special ore which possesses these properties naturally. These are knownas natural magnets (see figure 1). It is also possible, however, to make magnetsfrom ordinary steel by placing it in a magnetic field. If the steel is relatively soft,the magnetic properties will soon disappear again. If it is hard, however, thesteel will remain magnetic for a very long time; we then have a permanentmagnet 5578-020-001  Figure 1(Natural magnets) 2. Electromagnets There is another way of making a magnet, by using an electric current. If acurrent passes through a straight wire, a (weak) magnetic field is created aroundthis wire. The field lines run in circles around the wire (see figure 2a). If wewind the wire to form a coil (also known as a solenoid), however, we cangenerate a much stronger magnetic field with the same current. The separatefields around the wire windings reinforce one another, as indicated in figure 2b.The coil now has the same properties as a permanent magnet. The strength of themagnetic field is determined by the current and by the number of windings of the coil. And we can obtain an even stronger magnetic field in this situation byinserting a soft iron core in the coil. The core “bundles” the field lines. In thisway, the coil with core can function as an electromagnet.- natural magnets - permanent magnet - field lines - coil- solenoid  Theory / 10.06.24 - 020 3 Many electrical appliances work by means of these electromagnets, such as agenerator, motor, magnetic switch and the solenoid valve. In the case of thesolenoid valve, the core is fitted so that it can move in the coil. If the core is thenattached to a valve, for example (see figure 3), the valve can be opened byswitching on the current (the core is then drawn into the coil) and closed again by switching off the current (a spring pulls the core out of the coil again). 5578-020-002  Figure 2a. Weak magnetic field around a live conductor b. Strong magnetic field in a live coil c. Very strong magnetic field in a live coil with iron core 5578-020-003  Figure 3Solenoid valve a. not energised (I = 0), b. energised  - electromagnet - solenoid valve  Theory / 10.06.24 - 020 4 3. Direct current, alternating current and three-phasecurrent In the above, we have not yet discussed the type of voltage or current. There aredifferent kinds, such as:-   direct voltage or current, indicated by =-   alternating voltage or current, indicated by ~-   three-phase voltage or current, indicated by ≡  or 3~The latter is often also referred to as a rotary current.Up to now, a direct voltage source has always been used. With such a directvoltage source, one terminal is always positive and the other terminal negative.Examples include batteries for a flashlight or transistor radio and car batteries. If we connect a load resistance to these voltage sources, the current will flow inone direction only. Question 1Give a practical example where it does not matter whether the positive and negative terminals are reversed and an example where this definitely must not happen. With an alternating current source, there are no fixed positive and negativeterminals. In fact, the two terminals alternate between being positive andnegative (see figure 4). Therefore, the current is continuously changingdirection. 5578-020-004  Figure 4 AC circuit  If we connect a load resistance R to the alternating voltage source, the currentwill also continuously change direction. It is therefore called an alternatingcurrent. The number of times per second the current performs one completecycle (a period) is referred to as the frequency (see figure 5).The frequency is expressed in hertz (Hz). 1 Hz is one period per second. Themains frequency used is either 50 or 60 Hz depending on the country where youlive.- direct voltage - alternating voltage- period - frequency - hertz - mains
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