Editing Electromagnetic induction (section)

==Eddy currents==
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Electrical conductors moving through a steady magnetic field, or stationary conductors within a changing magnetic field, will have circular currents induced within them by induction, called [[eddy current]]s.  Eddy currents flow in closed loops in planes perpendicular to the magnetic field. They have useful applications in [[eddy current brake]]s and [[induction heating]] systems. However eddy currents induced in the metal [[magnetic core]]s of transformers and AC motors and generators are undesirable since they dissipate energy (called [[core losses]]) as heat in the resistance of the metal. Cores for these devices use a number of methods to reduce eddy currents:   
* Cores of low frequency alternating current electromagnets and transformers, instead of being solid metal, are often made of stacks of metal sheets, called ''laminations'', separated by nonconductive coatings. These thin plates reduce the undesirable parasitic eddy currents, as described below.
* Inductors and transformers used at higher frequencies often have magnetic cores made of nonconductive magnetic materials such as [[ferrite (magnet)|ferrite]] or iron powder held together with a resin binder.

===Electromagnet laminations===
[[File:Hawkins Electrical Guide - Figure 292 - Eddy currents in a solid armature.jpg|thumb|upright=0.9|left]]
Eddy currents occur when a solid metallic mass is rotated in a magnetic field, because the outer portion of the metal cuts more [[magnetic lines of force]] than the inner portion; hence the induced electromotive force is not uniform; this tends to cause electric currents between the points of greatest and least potential. Eddy currents consume a considerable amount of energy and often cause a harmful rise in temperature.<ref name="Imagesand">Images and reference text are from the public domain book: ''[[Hawkins Electrical Guide]]'', Volume 1, Chapter 19: Theory of the Armature, pp. 270–273, Copyright 1917 by Theo. Audel & Co., Printed in the United States</ref>
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[[File:Hawkins Electrical Guide - Figure 293 - Armature core with a few laminations showing effect on eddy currents.jpg|thumb|upright=0.9|left]]
Only five laminations or plates are shown in this example, so as to show the subdivision of the eddy currents. In practical use, the number of laminations or punchings ranges from 40 to 66 per inch (16 to 26 per centimetre), and brings the eddy current loss down to about one percent. While the plates can be separated by insulation, the voltage is so low that the natural rust/oxide coating of the plates is enough to prevent current flow across the laminations.<ref name="Imagesand" />
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[[File:Small DC Motor pole laminations and overview.jpg|thumb|upright=0.9|left]]
This is a rotor approximately 20&nbsp;mm in diameter from a DC motor used in a {{nowrap|CD player.}} Note the laminations of the electromagnet pole pieces, used to limit parasitic inductive losses.
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===Parasitic induction within conductors===
[[File:Hawkins Electrical Guide - Figure 291 - Formation of eddy currents in a solid bar inductor.jpg|thumb|upright=0.9|left]]
In this illustration, a solid copper bar conductor on a rotating armature is just passing under the tip of the pole piece N of the field magnet. Note the uneven distribution of the lines of force across the copper bar. The magnetic field is more concentrated and thus stronger on the left edge of the copper bar (a,b) while the field is weaker on the right edge (c,d). Since the two edges of the bar move with the same velocity, this difference in field strength across the bar creates whorls or current eddies within the copper bar.<ref name="Imagesand"/>

High current power-frequency devices, such as electric motors, generators and transformers, use multiple small conductors in parallel to break up the eddy flows that can form within large solid conductors. The same principle is applied to transformers used at higher than power frequency, for example, those used in [[switch mode power supply|switch-mode power supplies]] and the [[intermediate frequency]] coupling transformers of radio receivers.
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