principles: firstly, that an electric current can produce a magnetic field and secondly

8/14/2019 Principles: Firstly, That An electric Current can Produce a magnetic Field and Secondly

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pr nc p es: rst y, t atan electric current canproduce a magneticfield and secondly that

a changing magneticfield within a coil of wire induces a voltageacross the ends of thecoil.By changing thecurrent in the primarycoil, it changes thestrength of itsmagnetic field; sincethe changing magneticfield extends into thesecondary coil, a

voltage is inducedacross the secondary.



A current passing through the primarycoil creates a magnetic field.

The primary and secondary coils arewrapped around a core of veryhigh magnetic permeability, suchas iron; this ensures that most of themagnetic field lines produced by the

primary current are within the ironand pass through the secondary coilas well as the primary coil.



The voltage induced across thesecondary coil may be calculatedfromFaraday's law of induction, whichstates that:

Where V S is the

instantaneous voltage, N S is thenumber of turns in the secondary coiland Φ equals the magneticflux through one turn of the coil. If theturns of the coil are orientedperpendicular to the magnetic fieldlines, the flux is the product of the magnetic field strength B and thearea A through which it cuts. The areais constant, being equal to the cross-sectional area of the transformer core,whereas the magnetic field varies withtime according to the excitation of theprimary. Since the same magnetic fluxpasses through both the primary and

secondary coils in an ideal transformer,

the instantaneous voltage across therimar windin e uals:



A transformer usually consists of 1 primary coiland 1 or more secondary coils wound around acommon metalic core (iron for low frequency,and ferrite for high frequency). Some very highfrequency transformers are air cored only.

A transformer works by inducing a voltage fromthe primary coil to the secondary coil.(See Faraday's Law) When an alternatingvoltage is applied to the primary coil, analternating magnetic field is created aroundthis coil since it acts as an electromagnet. Sincethe secondary coil is in the alternating magneticfield, a voltage is induced into it in much thesame way it does when moving a magnet(alternating magnetic field) in and out of a coil.

The amount of voltage on the secondary coildepends on several factors: the ratio of primaryto secondary turns (often just called the turnsratio), the core material, the driving frequencyand coupling.



The most important utility of transformers is to convert voltages.

With AC (which is supplied by theelectricity grid), the voltage isconverted several times betweenthe large electrical generators andyour house. At this point is usuallyaround 120 V AC or 240 V ACdepending on where you live. Whenyou plug in an electrical appliance,

it may require a different voltage orvoltages to operate, theseappliances will usually use atransformer to convert the voltagewhich can later be rectified toDC later if required. (note:transformers do not operate on DC,which is much more complex toconvert)

To obtain several voltages,transformers can either haveseveral secondaries with differentwinding ratios or a single tappedsecondary (output wires are

connected to several places alongthe secondary coil, allowing thenumber of turns to be selected)



When primary windings areless than the secondarywindings, the emf produced inthe secondary windings ismore than the originalvoltage.

The transformer having such

a setup is called STEP-UP TRANSFORMER.

Vs>V P



When primary windingsare more than thesecondary windings, the

emf produced in thesecondary windings is lessthan the original voltage.

The transformer havingsuch a setup is calledSTEP-DOWN

TRANSFORMER.

Vs< V P



Properties of an ideal transformerMagnetizing current is as low aseconomically possible through highinductanceLeakage inductance is as low aseconomically possible through near 100%couplingOperation does not take the core intosaturation

A transformer having these ideal

properties is called an IDEAL TRANSFORMER

Examples of ideal transformers"ordinary" transformers, from the wall-wartto the distribution sub-station

Auto-transformerVariac Potential Transformer



Losses in the transformer arise

from :Winding resistance Current flowingthrough the windings causes resistiveheating of the conductors. At higherfrequencies, skin effect and proximityeffect create additional windingresistance and losses.

Hysteresis losses E ach time themagnetic field is reversed, a smallamount of energy is lost dueto hysteresis within the core. For a givencore material, the loss is proportional tothe frequency, and is a function of thepeak flux density to which it is subjected.

Eddy currents Ferromagnetic materialsare also good conductors, and a solidcore made from such a material alsoconstitutes a single short-circuited turnthroughout its entire length. Eddycurrents therefore circulate within thecore in a plane normal to the flux, andare responsible for resistive heating of the core material. The eddy current lossis a complex function of the square of



Magnetostriction Magnetic flux in aferromagnetic material, such as the core,causes it to physically expand andcontract slightly with each cycle of themagnetic field, an effect knownas magnetostriction. This produces thebuzzing sound commonly associated withtransformers, and in turn causes lossesdue to frictional heating in susceptiblecores.Mechanical losses In addition to

magnetostriction, the alternatingmagnetic field causes fluctuatingelectromagnetic forces between theprimary and secondary windings. Theseincite vibrations within nearbymetalwork, adding to the buzzing noise,and consuming a small amount of power.Stray losses Leakage inductance is by

itself lossless, since energy supplied to itsmagnetic fields is returned to the supplywith the next half-cycle. However, anyleakage flux that intercepts nearbyconductive materials such as thetransformer's support structure will giverise to eddy currents and be converted toheat.



A transformer cannot operateon dc supply.Flux produced in thetransformer core will not varybut remain constant.No emf will be produced in thesecondary wingding except atthe moment of switching on.

Thus incapable of raising orlowering the dc voltage.No self induced emf in theprimary winding.

Thus heavy current can flowwhich can damage the primarywinding

Dc is never applied to atransformer

principles: firstly, that an electric current can produce a magnetic field and secondly

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