Transformer geometry

Chapter Transformer geometry

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Laminated ironIn transformers for 60-Hz utility ac, and also at low audio frequencies, sheets of siliconsteel, glued together in layers, are often employed as transformer cores. The siliconsteel is sometimes called transformer iron, or simply iron.The reason layering is used, rather than making the core from a single mass ofmetal, is that the magnetic fields from the coils cause currents to flow in a solid core.These eddy currents go in circles, heating up the core and wasting energy that wouldotherwise be transferred from the primary to the secondary. Eddy currents are chokedoff by breaking up the core into layers, so that currents cannot flow very well in circles.Another type of loss, called hysteresis loss, occurs in any ferromagnetic trans-former core. Hysteresis is the tendency for a core material to be “sluggish” in acceptinga fluctuating magnetic field. Laminated cores exhibit high hysteresis loss above audiofrequencies, and are therefore not good above a few kilohertz.Ferrite and powdered ironAt frequencies up to several megahertz, ferrite works well for radio-frequency (RF)transformers. This material has high permeability and concentrates the flux efficiently.High permeability reduces the number of turns needed in the coils. But at frequencieshigher than a few megahertz, ferrite begins to show loss, and is no longer effective.For work well into the very-high-frequency (VHF) range, or up to 100 MHz ormore, powdered iron cores work well. The permeability of powdered iron is less thanthat of ferrite, but at high frequencies, it is not necessary to have high magnetic perme-ability. In fact, at radio frequencies above a few megahertz, air core coils are often pre-ferred, especially in transmitting amplifiers. At frequencies above several hundredmegahertz, ferromagnetic cores can be dispensed with entirely.Transformer geometryThe shape of a transformer depends on the shape of its core. There are several differ-ent core geometries commonly used with transformers.Utility transformersA common core for a power transformer is the E core, so named because it is shapedlike the capital letter E. A bar, placed at the open end of the E, completes the core oncethe coils have been wound on the E-shaped section (Fig. 18-4A).The primary and secondary windings can be placed on an E core in either of twoways.The simpler winding method is to put both the primary and the secondary aroundthe middle bar of the E (Fig. 18-4B). This is called the shell method of transformerwinding. It provides maximum coupling between the windings. However, the shell-winding scheme results in a considerable capacitance between the primary and thesecondary. This capacitance can sometimes be tolerated; sometimes it cannot. Anotherdisadvantage of the shell geometry is that, when windings are placed one atop the other,the transformer cannot handle very much voltage.330 Transformers and impedance matching