this article is about the electrical device

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This article is about the electrical device. For the toy line franchise, seeTransformers. For other uses,

seeTransformer (disambiguation).

Pole-mounteddistribution transformerwithcenter-tappedsecondary winding. This type of transformer

is commonly used in theUnited Statesto provide 120/240 volt "split-phase" power for residential and

light commercial use. Note that the center "neutral" terminal is grounded to the transformer "tank",

and agroundedconductor(right) is used for one leg of the primary feeder.

A transformer is a device that transferselectrical energyfrom onecircuitto another throughinductively

coupledconductorsthe transformer's coils. A varyingcurrentin the first orprimarywinding creates a

varyingmagnetic fluxin the transformer's core and thus a varyingmagnetic fieldthrough the secondary

winding. This varying magnetic fieldinducesa varyingelectromotive force (EMF), or "voltage", in the

secondary winding. This effect is calledinductive coupling.

If aloadis connected to the secondary, current will flow in the secondary winding, and electrical energy

will be transferred from the primary circuit through the transformer to the load. In an ideal transformer,

the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp) and is

given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary

(Np) as follows:

By appropriate selection of the ratio of turns, a transformer thus enables analternating current (AC)voltage to be "stepped up" by making Ns greater than Np, or "stepped down" by making Ns less than

Np.

In the vast majority of transformers, the windings are coils wound around aferromagnetic core,air-core

transformers being a notable exception.

Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage

microphoneto huge units weighing hundreds of tons used to interconnect portions ofpower grids. All

operate on the same basic principles, although the range of designs is wide. While new technologies

have eliminated the need for transformers in some electronic circuits, transformers are still found in
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nearly all electronic devices designed forhousehold ("mains") voltage. Transformers are essential for

high-voltageelectric power transmission, which makes long-distance transmission economically

practical.

Contents[hide]

1 Historyo 1.1 Discoveryo 1.2 Induction coils

2 Closed-core transformers and parallel power distributiono 2.1 Other early transformers

3 Basic principleso 3.1 Induction lawo 3.2 Ideal power equationo 3.3 Detailed operation

4 Practical considerationso 4.1 Leakage fluxo

4.2 Effect of frequencyo 4.3 Energy losseso 4.4 Dot conventiono 4.5 Core form and shell form transformers

5 Equivalent circuit 6 Types

o 6.1 Autotransformero 6.2 Polyphase transformerso 6.3 Leakage transformers

6.3.1 Uses 6.3.2 Resonant transformers

o 6.4 Audio transformerso 6.5 Instrument transformers

7 Classification 8 Construction

o 8.1 Cores 8.1.1 Laminated steel cores 8.1.2 Solid cores 8.1.3 Toroidal cores 8.1.4 Air cores

o 8.2 Windingso 8.3 Coolingo 8.4 Insulation dryingo 8.5 Terminals

9 Applications 10 See also 11 Notes 12 References 13 External links

[edit] History
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[edit] Discovery

Faraday's experiment with induction between coils of wire[1]

The phenomenon ofelectromagnetic inductionwas discovered independently byMichael Faradayand

Joseph Henryin 1831. However, Faraday was the first to publish the results of his experiments and thusreceive credit for the discovery.[2]The relationship betweenelectromotive force(EMF) or "voltage" and

magnetic fluxwas formalized in anequationnow referred to as "Faraday's law of induction":

.

where

is the magnitude of the EMF in volts and B is themagnetic fluxthrough the circuit inwebers.[3]

Faraday performed the first experiments on induction between coils of wire, including winding a pair of

coils around an iron ring, thus creating the firsttoroidalclosed-core transformer.[4]

[edit] Induction coilsThe first type of transformer to see wide use was theinduction coil, invented by Rev.Nicholas Callanof

Maynooth College, Ireland in 1836. He was one of the first researchers to realize that the more turns

the secondary winding has in relation to the primary winding, the larger is the increase in EMF.

Induction coils evolved from scientists' and inventors' efforts to get higher voltages from batteries. Since

batteries producedirect current(DC) rather thanalternating current(AC), induction coils relied upon

vibratingelectrical contactsthat regularly interrupted the current in the primary to create the flux

changes necessary for induction. Between the 1830s and the 1870s, efforts to build better induction

coils, mostly by trial and error, slowly revealed the basic principles of transformers.
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Faraday's ring transformer

By the 1870s, efficientgeneratorsthat producedalternating current(alternators) were available, and it

was found that alternating current could power an induction coil directly, without aninterrupter. In1876, Russian engineerPavel Yablochkovinvented a lighting system based on a set of induction coils

where the primary windings were connected to a source of alternating current and the secondary

windings could be connected to several"electric candles"(arc lamps) of his own design.[5][6]The coils

Yablochkov employed functioned essentially as transformers.[5]

In 1878, theGanzCompany inHungarybegan manufacturing equipment for electric lighting and, by

1883, had installed over fifty systems inAustria-Hungary. Their systems used alternating current

exclusively and included those comprising botharcandincandescentlamps, along withgeneratorsand

other equipment.[7]

Lucien GaulardandJohn Dixon Gibbsfirst exhibited a device with an open iron core called a "secondary

generator" inLondonin 1882, then sold the idea to theWestinghousecompany in theUnited States.[8]

They also exhibited the invention inTurin, Italyin 1884, where it was adopted for an electric lightingsystem.[9]However, the efficiency of their open-core bipolar apparatus remained very low.[9]

Induction coils with open magnetic circuits are inefficient for transfer of power toloads. Until about

1880, the paradigm for AC power transmission from a high voltage supply to a low voltageloadwas a

series circuit. Open-core transformers with a ratio near 1:1 were connected with their primaries in

series to allow use of a high voltage for transmission while presenting a low voltage to the lamps. The

inherent flaw in this method was that turning off a single lamp affected the voltage supplied to all

others on the same circuit. Many adjustable transformer designs were introduced to compensate for

this problematic characteristic of the series circuit, including those employing methods of adjusting the

core or bypassing the magnetic flux around part of a coil.[9]Efficient, practical transformer designs did

not appear until the 1880s, but within a decade, the transformer would be instrumental in the "War of

Currents", and in seeing AC distribution systems triumph over their DC counterparts, a position in which

they have remained dominant ever since.[10]

[edit] Closed-core transformers and parallel power

distribution
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Shell-form transformer. Sketch used by Uppenborn to describe Z.B.D. engineers' 1885 patents and

earliest articles.[9]

Core-form, front; shell-form, back. Earliest specimens of Z.B.D.-designed high-efficiency constant-

potential transformers manufactured at theGanz factoryin 1885.

Stanley's 1886 design for adjustable gap open-core induction coils[11]

In the autumn of 1884,Kroly Zipernowsky,Ott BlthyandMiksa Dri(Z.B.D.), three engineers

associated with theGanz factory, had determined that open-core devices were impracticable, as they

were incapable of reliably regulating voltage.[12]In their joint 1885 patent applications for novel

transformers (later called Z.B.D. transformers), they described two designs with closed magnetic circuits

where copper windings were either a) wound around iron wire ring core or b) surrounded by iron wire
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core.[9]The two designs were the first application of the two basic transformer construction types in

common use to this day which can as a class all be termed as either core-form or shell-form (or

alternatively, core-type or shell-type), as in a) or b), respectively (see images).[13][14][15][16]TheGanz

factoryhad also in the autumn of 1884 made delivery of the world's first five high-efficiency AC

transformers, the first of these units having been shipped on September 16, 1884.[17]This first unit had

been manufactured to the following specifications: 1,400 W, 40 Hz, 120:72 V, 11.6:19.4 A, ratio 1.67:1,

one-phase, shell-form.[17]In both designs, the magnetic flux linking the primary and secondary

windings traveled almost entirely within the confines of the iron core, with no intentional path through

air (see 'Toroidal cores' below). The new transformers were 3.4 times more efficient than the open core

bipolar devices of Gaulard and Gibbs.[18]Their patents included two other major interrelated

innovations: one concerning the use of parallel connected, instead of series connected, utilization loads,

the other concerning the ability to have high turns ratio transformers such that the supply network

voltage could be much higher (initially 1,400 to 2,000 V) than the voltage of utilization loads (100 V

initially preferred).[19][20]When they employed them in parallel connectedelectric distribution

systems, closed-core transformers finally made it technically and economically feasible to provide

electric power for lighting in homes, businesses and public spaces.[21][22]Blthy had suggested the use

of closed-cores, Zipernowsky the use ofparallel shunt connections, and Dri had performed the

experiments;[23]The vast majority of transformers in use today are based on principles discovered bythe three engineers. They also popularized the word "transformer" to describe a device for altering the

EMF of an electric current,[21][24]although the term had already been in use by 1882.[25][26]In 1886,

the Z.B.D. engineers designed, and theGanz factorysupplied electrical equipment for, the world's first

power stationthat used ACgeneratorsto power a parallel-connected common electrical network, the

steam-poweredRome-Cerchipower plant.[27]

AlthoughGeorge Westinghousehad bought Gaulard and Gibbs' patents in 1885, theEdison Electric

Light Companyheld an option on the U.S. rights for the Z.B.D. transformers, requiring Westinghouse to

pursue alternative designs on the same principles. He assigned toWilliam Stanleythe task of developing

a device for commercial use in United States.[28]Stanley's first patented design was for induction coils

with single cores of soft iron and adjustable gaps to regulate the EMF present in the secondary winding(see image).[11]This design[29]was first used commercially in the U.S. in 1886[10]but Westinghouse

was intent on improving the Stanley design to make it (unlike the Z.B.D. type) easy and cheap to

produce.[29]Westinghouse, Stanley and a few other associates had soon developed a core consisting of

a stack of thin "E-shaped" iron plates, separated individually or in pairs by thin sheets of paper or other

insulating material. Prewound copper coils could then be slid into place, and straight iron plates laid in

to create a closed magnetic circuit. Westinghouse applied for a patent for the new design in December

1886; it was granted in July 1887.[23][30][edit] Other early transformersIn 1889, Russian-born engineerMikhail Dolivo-Dobrovolskydeveloped the firstthree-phasetransformer

at theAllgemeine Elektricitts-Gesellschaft("General Electricity Company") in Germany.[31]

In 1891,Nikola Teslainvented theTesla coil, an air-cored, dual-tuned resonant transformer forgenerating veryhigh voltagesat high frequency.[32][33]

Audio frequencytransformers ("repeating coils") were used by early experimenters in the development

of thetelephone.[citation needed]

[edit] Basic principles
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An ideal transformer. The secondary current arises from the action of the secondary EMF on the (notshown) load impedance.

The transformer is based on two principles: first, that anelectric currentcan produce amagnetic field

(electromagnetism) and second that a changing magnetic field within a coil of wire induces a voltage

across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes

the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a

magnetic field. The primary and secondary coils are wrapped around acoreof very highmagnetic

permeability, such asiron, so that most of the magnetic flux passes through both the primary and

secondary coils. If a load is connected to the secondary winding, the load current and voltage will be in

the directions indicated, given the primary current and voltage in the directions indicated (each will be

alternating currentin practice).

[edit] Induction lawThe voltage induced across the secondary coil may be calculated fromFaraday's law of induction, which

states that:

where Vs is the instantaneousvoltage,Ns is the number of turns in the secondary coil and is the

magnetic fluxthrough one turn of the coil. If the turns of the coil are oriented perpendicularly to the

magnetic field lines, the flux is the product of themagnetic flux densityB and the areaA through whichit cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas

the magnetic field varies with time according to the excitation of the primary. Since the same magnetic

flux passes through both the primary and secondary coils in an ideal transformer,[34]

the instantaneous

voltage across the primary winding equals
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Taking the ratio of the two equations for Vs and Vp gives the basic equation[35]

for stepping up or

stepping down the voltage

Np/Ns is known as the turns ratio, and is the primary functional characteristic of any transformer. In the

case of step-up transformers, this may sometimes be stated as the reciprocal, Ns/Np. Turns ratio iscommonly expressed as anirreducible fractionor ratio: for example, a transformer with primary and

secondary windings of, respectively, 100 and 150 turns is said to have a turns ratio of 2:3 rather than

0.667 or 100:150.

[edit] Ideal power equation

The ideal transformer as a circuit element

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted

from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient. All the

incoming energy is transformed from the primary circuit to themagnetic fieldand into the secondary

circuit. If this condition is met, the inputelectric powermust equal the output power:

giving the ideal transformer equation

This formula is a reasonable approximation for most commercial built transformers today.

If the voltage is increased, then the current is decreased by the same factor. The impedance in one

circuit is transformed by the square of the turns ratio.[34]For example, if an impedanceZs is attached

across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of

(Np/Ns)

2

Zs. This relationship is reciprocal, so that the impedanceZp of the primary circuit appears to thesecondary to be (Ns/Np)

2Zp.

[edit] Detailed operationThe simplified description above neglects several practical factors, in particular, the primary current

required to establish a magnetic field in the core, and the contribution to the field due to current in the

secondary circuit.

Models of an ideal transformer typically assume a core of negligiblereluctancewith two windings of

zeroresistance.[36]When a voltage is applied to the primary winding, a small current flows, drivingflux
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around themagnetic circuitof the core.:[36]The current required to create the flux is termed the

magnetizing current. Since the ideal core has been assumed to have near-zero reluctance, the

magnetizing current is negligible, although still required, to create the magnetic field.

The changing magnetic field induces anelectromotive force(EMF) across each winding.[37]

Since the

ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and VS

measured at the terminals of the transformer, are equal to the corresponding EMFs. The primary EMF,

acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF".[38]This is in

accordance withLenz's law, which states that induction of EMF always opposes development of any

such change in magnetic field.

[edit] Practical considerations

[edit] Leakage fluxMain article:Leakage inductance

Leakage flux of a transformer

The ideal transformer model assumes that all flux generated by the primary winding links all the turns of

every winding, including itself. In practice, some flux traverses paths that take it outside the

windings.[39]Such flux is termed leakage flux, and results inleakage inductanceinserieswith the

mutually coupled transformer windings.[38]Leakage results in energy being alternately stored in and

discharged from themagnetic fieldswith each cycle of the power supply. It is not directly a power loss

(see"Stray losses"below), but results in inferiorvoltage regulation, causing the secondary voltage to

not be directly proportional to the primary voltage, particularly under heavy load.[39]Transformers are

therefore normally designed to have very lowleakage inductance. Nevertheless, it is impossible to

eliminate all leakage flux because it plays an essential part in the operation of the transformer. The

combined effect of the leakage flux and the electric field around the windings is what transfers energy

from the primary to the secondary.[40]

In some applications increased leakage is desired, and long magnetic paths, air gaps, or magnetic bypassshunts may deliberately be introduced in a transformer design to limit theshort-circuitcurrent it will

supply.[38]Leaky transformers may be used to supply loads that exhibitnegative resistance, such as

electric arcs,mercury vapor lamps, andneon signsor for safely handling loads that become periodically

short-circuited such aselectric arc welders.[41]

Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers in

circuits that have a direct current component flowing through the windings.[42]
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kipedia.org/wiki/Transformer#strayhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Transformer#cite_note-calvert-37http://en.wikipedia.org/wiki/Series_and_parallel_circuitshttp://en.wikipedia.org/wiki/Leakage_inductancehttp://en.wikipedia.org/wiki/Transformer#cite_note-mclaren-38http://en.wikipedia.org/wiki/Leakage_inductancehttp://en.wikipedia.org/w/index.php?title=Transformer&action=edit&section=11http://en.wikipedia.org/w/index.php?title=Transformer&action=edit&section=10http://en.wikipedia.org/wiki/Lenz%27s_lawhttp://en.wikipedia.org/wiki/Transformer#cite_note-calvert-37http://en.wikipedia.org/wiki/Counter-electromotive_forcehttp://en.wikipedia.org/wiki/Transformer#cite_note-Heathcote-36http://en.wikipedia.org/wiki/Electromotive_forcehttp://en.wikipedia.org/wiki/Transformer#cite_note-Say-35http://en.wikipedia.org/wiki/Magnetic_circuit


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Leakage inductance is also helpful when transformers are operated in parallel. It can be shown that if

the "per-unit" inductance of two transformers is the same (a typical value is 5%), they will automatically

split power "correctly" (e.g. 500kVAunit in parallel with 1,000 kVA unit, the larger one will carry twice

the current).[citation needed]

[edit] Effect of frequency

Transformer universal EMF equationIf the flux in the core is purelysinusoidal, the relationship for either winding between itsrmsvoltage

Erms of the winding, and the supply frequencyf, number of turns N, core cross-sectional area a and peak

magnetic flux densityB is given by the universal EMF equation:[36]

If the flux does not contain evenharmonicsthe following equation can be used for half-cycle average

voltageEavg of any waveshape:

The time-derivative term inFaraday's Lawshows that the flux in the core is theintegralwith respect to

time of the applied voltage.[43]Hypothetically an ideal transformer would work with direct-current

excitation, with the core flux increasing linearly with time.[44]In practice, the flux rises to the point

wheremagnetic saturationof the core occurs, causing a large increase in the magnetizing current and

overheating the transformer. All practical transformers must therefore operate with alternating (or

pulsed direct) current.[44]

The EMF of a transformer at a given flux density increases with frequency.[36]By operating at higher

frequencies, transformers can be physically more compact because a given core is able to transfer more

power without reaching saturation and fewer turns are needed to achieve the same impedance.

However, properties such as core loss and conductorskin effectalso increase with frequency. Aircraft

and military equipment employ 400 Hz power supplies which reduce core and winding weight.[45]

Conversely, frequencies used for somerailway electrification systemswere much lower (e.g. 16.7 Hz

and 25 Hz) than normal utility frequencies (50 60 Hz) for historical reasons concerned mainly with the

limitations of earlyelectric traction motors. As such, the transformers used to step down the high over-head line voltages (e.g. 15 kV) were much heavier for the same power rating than those designed only

for the higher frequencies.

Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to

reduced magnetizing current. At a lower frequency, the magnetizing current will increase. Operation of

a transformer at other than its design frequency may require assessment of voltages, losses, and cooling

to establish if safe operation is practical. For example, transformers may need to be equipped with

"volts per hertz" over-excitationrelaysto protect the transformer from overvoltage at higher than rated

frequency.

One example of state-of-the-art design is transformers used forelectric multiple unithigh speed trains,

particularly those required to operate across the borders of countries using different electrical

standards. The position of such transformers is restricted to being hung below the passenger

compartment. They have to function at different frequencies (down to 16.7 Hz) and voltages (up to 25

kV) whilst handling the enhanced power requirements needed for operating the trains at high speed.

Knowledge of natural frequencies of transformer windings is necessary for the determination of winding

transient response and switching surge voltages.

[edit] Energy lossesAn ideal transformer would have no energy losses, and would be 100% efficient. In practical

transformers, energy is dissipated in the windings, core, and surrounding structures. Larger
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transformers are generally more efficient, and those rated for electricity distribution usually perform

better than 98%.[46]

Experimental transformers usingsuperconductingwindings achieve efficiencies of 99.85%.[47]The

increase in efficiency can save considerable energy, and hence money, in a large heavily loaded

transformer; the trade-off is in the additional initial and running cost of the superconducting design.

Losses in transformers (excluding associated circuitry) vary with load current, and may be expressed as

"no-load" or "full-load" loss. Windingresistancedominates load losses, whereashysteresisandeddy

currentslosses contribute to over 99% of the no-load loss. The no-load loss can be significant, so that

even an idle transformer constitutes a drain on the electrical supply and a running cost. Designing

transformers for lower loss requires a larger core, good-qualitysilicon steel, or evenamorphous steel

for the core and thicker wire, increasing initial cost so that there is atrade-offbetween initial cost and

running cost (also seeenergy efficient transformer).[48]

Transformer losses are divided into losses in the windings, termedcopper loss, and those in the

magnetic circuit, termediron loss. Losses in the transformer arise from:

Winding resistance

Current flowing through the windings causesresistive heatingof the conductors. At higher

frequencies,skin effectandproximity effectcreate additional winding resistance and losses.

Hysteresis lossesEach time the magnetic field is reversed, a small amount of energy is lost due tohysteresis

within the core. For a given core material, the loss is proportional to the frequency, and is a

function of the peak flux density to which it is subjected.[49]

Eddy currents

Ferromagneticmaterials are also goodconductorsand a core made from such a material

also constitutes a single short-circuited turn throughout its entire length.Eddy currents

therefore circulate within the core in a plane normal to the flux, and are responsible for

resistive heatingof the core material. The eddy current loss is a complex function of the

square of supply frequency and inverse square of the material thickness.[50]Eddy current

losses can be reduced by making the core of a stack of plates electrically insulated from

each other, rather than a solid block; all transformers operating at low frequencies uselaminated or similar cores.

Magnetostriction

Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand

and contract slightly with each cycle of the magnetic field, an effect known as

magnetostriction. This produces the buzzing sound commonly associated with

transformers[35]that can cause losses due to frictional heating. This buzzing is particularly

familiar from low-frequency (50 Hz or 60 Hz)mains hum, and high-frequency (15,734 Hz

(NTSC) or 15,625 Hz (PAL))CRT noise.

Mechanical losses

In addition to magnetostriction, the alternating magnetic field causes fluctuating forces

between the primary and secondary windings. These incite vibrations within nearby

metalwork, adding to thebuzzing noiseand consuming a small amount of power.[51]

Stray losses

Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is

returned to the supply with the next half-cycle. However, any leakage flux that intercepts

nearby conductive materials such as the transformer's support structure will give rise to

eddy currents and be converted to heat.[52]There are also radiative losses due to the

oscillating magnetic field but these are usually small.
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[edit] Dot conventionMain article:Dot convention

It is common in transformer schematic symbols for there to be a dot at the end of each coil within a

transformer, particularly for transformers with multiple primary and secondary windings. The dots

indicate the direction of each winding relative to the others. Voltages at the dot end of each winding are

in phase; current flowing into the dot end of a primary coil will result in current flowing out of the dotend of a secondary coil.

[edit] Core form and shell form transformers

Core form = core type; shell form = shell type

As first mentioned in regard to earliest ZBD closed-core transformers, transformers are generally

considered to be either core form or shell form in design depending on the type of magnetic circuit used

in winding construction (see image). That is, when winding coils are wound around the core,

transformers are termed as being of core form design; when winding coils are surrounded by the core,

transformers are termed as being of shell form design. Shell form design may be more prevalent than

core form design for distribution transformer applications due to the relative ease in stacking the corearound winding coils.[13]Core form design tends to, as a general rule, be more economical, and

therefore more prevalent, than shell form design for high voltage power transformer applications at the

lower end of their voltage and power rating ranges (less than or equal to, nominally, 230 kV or 75 MVA).

At higher voltage and power ratings, shell form transformers tend to be more prevalent.[16][13][53][54]

Shell form design tends to be preferred for extra high voltage and higher MVA applications because,

though more labor intensive to manufacture, shell form transformers are characterized as having

inherently better kVA-to-weight ratio, better short-circuit strength characteristics and higher immunity

to transit damage.[54]

[edit] Equivalent circuitRefer to the diagram below

The physical limitations of the practical transformer may be brought together as an equivalent circuit

model (shown below) built around an ideal lossless transformer.[55]Power loss in the windings is

current-dependent and is represented as in-series resistances Rp and Rs. Flux leakage results in a

fraction of the applied voltage dropped without contributing to the mutual coupling, and thus can be

modeled as reactances of eachleakage inductanceXp andXs in series with the perfectly coupled region.

Iron losses are caused mostly by hysteresis and eddy current effects in the core, and are proportional to

the square of the core flux for operation at a given frequency.[56]

Since the core flux is proportional to
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