What is Eddy Currents

Chapter 5.15 Eddy Currents

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • 172Fundamental Electrical and Electronic Principles The current coil is connected to a circuit so as to allow the circuit current to fl ow through it. The voltage coil is connected in parallel with the load, in the same way as a voltmeter. Both coils produce magnetic fi elds, which interact to produce a defl ecting torque on the voltage coil. Restoring torque is provided by contrawound spiral springs, as in the moving coil meter. The interacting fi elds are proportional to the circuit current and voltage respectively. The defl ecting torque is therefore proportional to the product of these two quantities; i.e. VI which is the circuit power. The voltage coil may be connected either on the supply side or the load side of the current coil. The choice of connection depends on other factors concerning the load. This aspect is dealt with in FurtherElectrical and Electronic Principles. The manner of connecting the wattmeter into a circuit is shown in Fig. 5.35 . currentcoilvoltagecoilsupplyload Fig. 5.35 For d.c. circuits, a wattmeter is not strictly necessary. Since power P VI watt, then V and I may be measured separately by a multimeter. The power can then be calculated simply by multiplying together these two meter readings. However, in an a.c. circuit, this simple technique does not yield the correct value for the true power. Thus a wattmeter is required for the measurement of power in a.c. circuits. 5.15 Eddy Currents Consider an iron-cored solenoid, as shown in cross-section in Fig. 5.36 . Let the coil be connected to a source of emf via a switch. When the switch is closed, the coil current will increase rapidly to some steady value. This steady value will depend upon the resistance of the coil. The coil current will, in turn, produce a magnetic fi eld. Thus, this fl ux pattern will increase from zero to some steady value. This changing fl ux therefore expands outwards from the centre of the iron core. This movement of the fl ux pattern is shown by the arrowed lines pointing outwards from the core. Since there is a changing fl ux linking with the core, then an emf will be induced in the core. As the core is a conductor of electricity, then