transformer - wikipedia, the free encyclopedia

6/26/2014 Transformer - Wikipedia, the free encyclopedia

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Pole-mounted distribution

transformer with center-

tapped secondary winding

used to provide 'split-phase'

power for residential and

light commercial service,

which in North America is

typically rated 120/240

volt.[1][2]

TransformerFrom Wikipedia, the free encyclopedia

A transformer is an electrical device that transfers energy between two or morecircuits through electromagnetic induction.

A varying current in the transformer's primary winding creates a varying magneticflux in the core and a varying magnetic field impinging on the secondary winding.This varying magnetic field at the secondary induces a varying electromotive force(emf) or voltage in the secondary winding. Making use of Faraday's Law inconjunction with high magnetic permeability core properties, transformers can thusbe designed to efficiently change AC voltages from one voltage level to anotherwithin power networks.

Transformers range in size from RF transformers a small cm3 fraction in volume tounits interconnecting the power grid weighing hundreds of tons. A wide range oftransformer designs are used in electronic and electric power applications. Sincethe invention in 1885 of the first constant potential transformer, transformers havebecome essential for the AC transmission, distribution, and utilization of electrical

energy.[3]

Contents

1 Basic principles

1.1 Ideal transformer

1.1.1 Polarity

1.2 Real transformer

1.2.1 Deviations from ideal

1.2.2 Leakage flux

1.2.3 Equivalent circuit

2 Basic transformer parameters and construction

2.1 Effect of frequency

2.2 Energy losses

2.3 Core form and shell form transformers

3 Construction

3.1 Cores

3.1.1 Laminated steel cores

3.1.2 Solid cores

3.1.3 Toroidal cores

3.1.4 Air cores



Ideal transformer connected with source VP on

primary and load impedance ZL on secondary, where 0

< ZL < .

Ideal transformer equations (eq.)

By Faraday's law of induction

3.2 Windings

3.3 Cooling

3.4 Insulation drying

3.5 Bushings

4 Classification parameters

5 Types

6 Applications

7 History

7.1 Discovery of induction

7.2 Induction coils

7.3 First alternating current transformers

7.4 Early series circuit transformer distribution

7.5 Closed-core transformers and parallel power distribution

7.6 Other early transformers

8 See also

9 Notes

10 References

11 Bibliography

12 External links

Basic principles

Ideal transformer

It is very common, for simplification or approximationpurposes, to analyze the transformer as an idealtransformer model as represented in the two images. Anideal transformer is a theoretical, linear transformer thatis lossless and perfectly coupled; that is, there are noenergy losses and flux is completely confined within themagnetic core. Perfect coupling implies infinitely highcore magnetic permeability and winding inductances and

zero net magnetomotive force.[5][d]

A varying current in the transformer's primary winding

creates a varying magnetic flux in the core and a varyingmagnetic field impinging on the secondary winding. Thisvarying magnetic field at the secondary induces a varyingelectromotive force (emf) or voltage in the secondarywinding. The primary and secondary windings are



. . . (1)[a]

. . . (2)

Combining ratio of (1) & (2)

Turns ratio . . . (3) where

for step-down transformers, a > 1

for step-up transformers, a < 1

By law of Conservation of Energy, apparent, realand reactive power are each conserved in theinput and output

. . . (4)

Combining (3) & (4) with this endnote[b] yieldsthe ideal transformer identity

. (5)

By Ohm's Law and ideal transformer identity

. . . (6)

Apparent load impedance Z'L (ZL referred to the

primary)

Z'L = . (7)

wrapped around a core of infinitely high magnetic

permeability[e] so that all of the magnetic flux passesthrough both the primary and secondary windings. Withvoltage source connected to the primary winding andload impedance connected to the secondary winding, thetransformer currents flow in the indicated directions. (Seealso Polarity.)

According to Faraday's law of induction, since the samemagnetic flux passes through both the primary and

secondary windings in an ideal transformer,[7] a voltageis induced in each winding, according to eq. (1) in thesecondary winding case, according to eq. (2) in the

primary winding case.[8] The primary emf is sometimes

termed counter emf.[9] This is in accordance with Lenz'slaw, which states that induction of emf always opposesdevelopment of any such change in magnetic field.

The transformer winding voltage ratio is thus shown to bedirectly proportion to the winding turns ratio according to

eq. (3).[10][11][f][g]

According to the law of Conservation of Energy, anyload impedance connected to the ideal transformer'ssecondary winding results in conservation of apparent,real and reactive power consistent with eq. (4).

The ideal transformer identity shown in eq. (5) is areasonable approximation for the typical commercialtransformer, with voltage ratio and winding turns ratioboth being inversely proportional to the correspondingcurrent ratio.

By Ohm's Law and the ideal transformer identity:

- the secondary circuit load impedance can be

expressed as eq. (6)

- the apparent load impedance referred to the

primary circuit is derived in eq. (7) to be equal to the turns ratio squared times the secondary circuit load

impedance.[7][13]

Polarity



Ideal transformer and induction law[c]

Instrument transformer, with

polarity dot and X1 markings

on LV side terminal

A dot convention is often used in transformer circuit diagrams, nameplates or terminal markings to define therelative polarity of transformer windings. Positively-increasing instantaneous current entering the primary winding's

dot end induces positive polarity voltage at the secondary winding's dot end.[14][15][16][h][i][j]

Real transformer

Deviations from ideal

The ideal transformer model neglects the following basic linear aspects in real transformers.



Leakage flux of a transformer

Core losses, collectively called magnetizing current losses, consist of[20]

Hysteresis losses due to nonlinear application of the voltage applied in the transformer core, and

Eddy current losses due to joule heating in the core that are proportional to the square of the transformer's

applied voltage.

Whereas windings in the ideal model have no resistances and infinite inductances, the windings in a real transformerhave finite non-zero resistances and inductances associated with:

Joule losses due to resistance in the primary and secondary windings[20]

Leakage flux that escapes from the core and passes through one winding only resulting in primary and

secondary reactive impedance.

Leakage flux

The ideal transformer model assumes that all flux generatedby the primary winding links all the turns of every winding,including itself. In practice, some flux traverses paths that

take it outside the windings.[21] Such flux is termed leakageflux, and results in leakage inductance in series with the

mutually coupled transformer windings.[9] Leakage fluxresults in energy being alternately stored in and dischargedfrom the magnetic fields with each cycle of the powersupply. It is not directly a power loss, but results in inferiorvoltage regulation, causing the secondary voltage not to bedirectly proportional to the primary voltage, particularly

under heavy load.[21] Transformers are therefore normallydesigned to have very low leakage inductance.

In some applications increased leakage is desired, and long magnetic paths, air gaps, or magnetic bypass shunts

may deliberately be introduced in a transformer design to limit the short-circuit current it will supply.[9] Leakytransformers may be used to supply loads that exhibit negative resistance, such as electric arcs, mercury vaporlamps, and neon signs or for safely handling loads that become periodically short-circuited such as electric arc

welders.[22]

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

that have a DC component flowing in the windings.[23]

Knowledge of leakage inductance is also useful when transformers are operated in parallel. It can be shown that ifthe percent impedance (Z) and associated winding leakage reactance-to-resistance (X/R) ratio of two transformerswere hypothetically exactly the same, the transformers would share power in proportion to their respective volt-ampere ratings (e.g. 500 kVA unit in parallel with 1,000 kVA unit, the larger unit would carry twice the current).



Real transformer equivalent circuit

However, the impedance tolerances of commercial transformers are significant. Also, the Z impedance and X/Rratio of different capacity transformers tends to vary, corresponding 1,000 kVA and 500 kVA units' values being,

to illustrate, respectively, Z ~ 5.75%, X/R ~ 3.75 and Z ~ 5%, X/R ~ 4.75.[24][25]

Equivalent circuit

Referring to the diagram, a practical transformer's physical behavior may be represented by an equivalent circuit

model, which can incorporate an ideal transformer.[26]

Winding joule losses and leakage reactances are represented by the following series loop impedances of the model:

Primary winding: RP, XP

Secondary winding: RS, XS.

In normal course of circuit equivalence transformation, RS and XS are in practice usually referred to the primary side

by multiplying these impedances by the turns ratio squared, (NP/NS) 2 = a2.

Core loss and reactance isrepresented by thefollowing shunt legimpedances of the model:

Core or iron losses:

RC

Magnetizing

reactance: XM.

RC and XM are

collectively termed themagnetizing branch of the model.

Core 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.[27] The finite permeability core requires a magnetizing current IM

to maintain mutual flux in the core. Magnetizing current is in phase with the flux, the relationship between the twobeing non-linear due to saturation effects. However, all impedances of the equivalent circuit shown are by definition

linear and such non-linearity effects are not typically reflected in transformer equivalent circuits.[27] With sinusoidalsupply, core flux lags the induced emf by 90. With open-circuited secondary winding, magnetizing branch current

I0 equals transformer no-load current.[26]

The resulting model, though sometimes termed 'exact' equivalent circuit based on linearity assumptions, retains a

number of approximations.[26] Analysis may be simplified by assuming that magnetizing branch impedance isrelatively high and relocating the branch to the left of the primary impedances. This introduces error but allows



Power transformer over-excitation

condition caused by decreased

frequency; flux (green), iron core's

magnetic characteristics (red) and

magnetizing current (blue).

Transformer universal emf equation

If the flux in the core is purely sinusoidal, therelationship for either winding between its rmsvoltage Erms of the winding, and the supply

frequency f, number of turns N, core cross-

sectional area a in m2 and peak magnetic flux

density Bpeak in Wb/m2 or T (tesla) is given by

the universal emf equation:[20]

If the flux does not contain even harmonics thefollowing equation can be used for half-cycleaverage voltage Eavg of any waveshape:

combination of primary and referred secondary resistances and reactances by simple summation as two seriesimpedances.

Transformer equivalent circuit impedance and transformer ratio parameters can be derived from the following tests:

open-circuit test,[k] short-circuit test, winding resistance test, and transformer ratio test.

Basic transformer parameters and construction

Effect of frequency

The time-derivative termin Faraday'sLaw showsthat the flux inthe core is theintegral withrespect to timeof the applied

voltage.[29]

The idealtransformer'score fluxincreasinglinearly withtime for anynon-zero

frequency.[6][30] The flux in a real transformer's coreoperates non-linearity as instantaneous current decreasesbeyond a finite range resulting in magnetic saturationassociated with increasingly large magnetizing current,which can eventually lead to transformer overheating.

The emf of a transformer at a given flux density increases with frequency.[20] By operating at higher frequencies,transformers can be physically more compact because a given core is able to transfer more power without reachingsaturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss andconductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies

which reduce core and winding weight.[31] Conversely, frequencies used for some railway electrification systemswere much lower (e.g. 16.7 Hz and 25 Hz) than normal utility frequencies (50 60 Hz) for historical reasonsconcerned mainly with the limitations of early electric traction motors. As such, the transformers used to step-downthe high over-head line voltages (e.g. 15 kV) were much heavier for the same power rating than those designedonly for the higher frequencies.

Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reducedmagnetizing current. At a lower frequency, the magnetizing current will increase. Operation of a transformer at otherthan 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-excitation relays toprotect the transformer from overvoltage at higher than rated frequency.

One example of state-of-the-art design is traction transformers used for electric multiple unit and high-speed trainservice operating across the country border and using different electrical standards, such transformers' beingrestricted to be positioned below the passenger compartment. The power supply to, and converter equipment beingsupply by, such traction transformers have to accommodate different input frequencies and voltage (ranging from ashigh as 50 Hz down to 16.7 Hz and rated up to 25 kV) while being suitable for multiple AC asynchronous motorand DC converters & motors with varying harmonics mitigation filtering requirements.

Large power transformers are vulnerable to insulation failure due to transient voltages with high-frequency

components, such as caused in switching or by lightning.[32]

Energy losses

Real transformer energy losses are dominated by winding resistance joule and core losses. Transformers' efficiencytends to improve with increasing transformer capacity, the efficient of typical distribution transformers being about

98%.[33][l]

As transformer losses vary with load, it is often useful to express these losses in terms of no-load loss, full-load loss,half-load loss, and so on. Hysteresis and eddy current losses are constant at all load levels and dominateoverwhelmingly without load, while variable winding joule losses dominating increasingly as load increases. The no-load loss can be significant, so that even an idle transformer constitutes a drain on the electrical supply. Designingenergy efficient transformers for lower loss requires a larger core, good-quality silicon steel, or even amorphoussteel for the core and thicker wire, increasing initial cost. The choice of construction represents a trade-off between

initial cost and operating cost.[35]

Transformer losses arise from:

Winding joule losses

Current flowing through winding conductors causes joule heating. As frequency increases, skin effect and

proximity effect causes winding resistance and, hence, losses to increase.

Core losses

Hysteresis losses

Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the

core. According to Steinmetz's formula, the heat energy due to hysteresis is given by

, and,

hysteresis loss is thus given by

where, f is the frequency, is the hysteresis coefficient and max is the maximum flux density, the



empirical exponent of which varies from about 1.4 to 1 .8 but is often given as 1.6 for iron.[35][36][37]

Eddy current losses

Ferromagnetic materials are also good conductors and 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 heating of the core

material. The eddy current loss is a complex function of the square of supply frequency and inverse

square of the material thickness.[35] 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 use laminated or similar cores.

Magnetostriction related transformer hum

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, the frictional energy of

which produces an audible noise known as mains hum or transformer hum.[10][38] This transformer hum is

especially objectionable in transformers supplied at power frequencies[m] and in high-frequency flyback

transformers associated with PAL system CRTs.

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.[39] There are

also radiative losses due to the oscillating magnetic field but these are usually small.

Mechanical vibration and audible noise transmission

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

and secondary windings. This energy incites vibration transmission in interconnected metalwork, thus

amplifying audible transformer hum.[40]

Core form and shell form transformers

Closed-core transformers are constructed in 'core form' or 'shell form'. When windings surround the core, thetransformer is core form; when windings are surrounded by the core, the transformer is shell form. Shell form designmay be more prevalent than core form design for distribution transformer applications due to the relative ease in

stacking the core around winding coils.[41] Core form design tends to, as a general rule, be more economical, andtherefore more prevalent, than shell form design for high voltage power transformer applications at the lower end oftheir 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.[41][42][43][44] Shell form design tends to bepreferred 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.[44]



Core form = core type; shell form =

shell type

Laminated core transformer showing

edge of laminations at top of photo

Power transformer inrush current

caused by residual flux at switching

instant; flux (green), iron core's

magnetic characteristics (red) and

magnetizing current (blue).

Laminating the core greatly

reduces eddy-current losses

Construction

Cores

Laminated steel cores

Transformers for use at power or audio frequencies typically have cores

made of high permeability silicon steel.[45] The steel has a permeabilitymany times that of free space and the core thus serves to greatly reducethe magnetizing current and confine the flux to a path which closely

couples the windings.[46] Early transformer developers soon realized thatcores constructed from solid iron resulted in prohibitive eddy currentlosses, and their designs mitigated this effect with cores consisting of

bundles of insulated iron wires.[47] Later designs constructed the core bystacking layers of thin steel laminations, a principle that has remained inuse. Each lamination is insulated from its neighbors by a thin non-

conducting layer of insulation.[48] The universal transformer equationindicates a minimum cross-sectional area for the core to avoid saturation.

The effect of laminations is to confine eddy currents to highly ellipticalpaths that enclose little flux, and so reduce their magnitude. Thinner

laminations reduce losses,[49] but are more laborious and expensive to

construct.[50] Thin laminations are generally used on high-frequencytransformers, with some of very thin steel laminations able to operate upto 10 kHz.

One common design of laminated coreis made from interleaved stacks of E-shaped steel sheets capped with I-shaped pieces, leading to its name of

'E-I transformer'.[50] Such a designtends to exhibit more losses, but is veryeconomical to manufacture. The cut-core or C-core type is made bywinding a steel strip around arectangular form and then bonding thelayers together. It is then cut in two,forming two C shapes, and the coreassembled by binding the two C halves

together with a steel strap.[50] Theyhave the advantage that the flux is

always oriented parallel to the metal grains, reducing reluctance.



Small toroidal core transformer

A steel core's remanence means that it retains a static magnetic field when power is removed. When power is thenreapplied, the residual field will cause a high inrush current until the effect of the remaining magnetism is reduced,

usually after a few cycles of the applied AC waveform.[51] Overcurrent protection devices such as fuses must beselected to allow this harmless inrush to pass. On transformers connected to long, overhead power transmissionlines, induced currents due to geomagnetic disturbances during solar storms can cause saturation of the core and

operation of transformer protection devices.[52]

Distribution transformers can achieve low no-load losses by using cores made with low-loss high-permeabilitysilicon steel or amorphous (non-crystalline) metal alloy. The higher initial cost of the core material is offset over the

life of the transformer by its lower losses at light load.[53]

Solid cores

Powdered iron cores are used in circuits such as switch-mode power supplies that operate above mainsfrequencies and up to a few tens of kilohertz. These materials combine high magnetic permeability with high bulkelectrical resistivity. For frequencies extending beyond the VHF band, cores made from non-conductive magnetic

ceramic materials called ferrites are common.[50] Some radio-frequency transformers also have movable cores(sometimes called 'slugs') which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

Toroidal cores

Toroidal transformers are built around a ring-shaped core, which,depending on operating frequency, is made from a long strip of silicon

steel or permalloy wound into a coil, powdered iron, or ferrite.[54] A stripconstruction ensures that the grain boundaries are optimally aligned,improving the transformer's efficiency by reducing the core's reluctance.The closed ring shape eliminates air gaps inherent in the construction of

an E-I core.[22] The cross-section of the ring is usually square orrectangular, but more expensive cores with circular cross-sections arealso available. The primary and secondary coils are often woundconcentrically to cover the entire surface of the core. This minimizes thelength of wire needed, and also provides screening to minimize the core'smagnetic field from generating electromagnetic interference.

Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Otheradvantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum(making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses(making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes. The maindisadvantages are higher cost and limited power capacity (see Classification parameters below). Because of thelack of a residual gap in the magnetic path, toroidal transformers also tend to exhibit higher inrush current,compared to laminated E-I types.

Ferrite toroidal cores are used at higher frequencies, typically between a few tens of kilohertz to hundreds ofmegahertz, to reduce losses, physical size, and weight of inductive components. A drawback of toroidaltransformer construction is the higher labor cost of winding. This is because it is necessary to pass the entire length



Windings are usually arranged

concentrically to minimize flux leakage.

of a coil winding through the core aperture each time a single turn is added to the coil. As a consequence, toroidaltransformers rated more than a few kVA are uncommon. Small distribution transformers may achieve some of thebenefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary andsecondary windings.

Air cores

A physical core is not an absolute requisite and a functioning transformer can be produced simply by placing thewindings near each other, an arrangement termed an 'air-core' transformer. The air which comprises the magnetic

circuit is essentially lossless, and so an air-core transformer eliminates loss due to hysteresis in the core material.[9]

The leakage inductance is inevitably high, resulting in very poor regulation, and so such designs are unsuitable for

use in power distribution.[9] They have however very high bandwidth, and are frequently employed in radio-

frequency applications,[55] for which a satisfactory coupling coefficient is maintained by carefully overlapping theprimary and secondary windings. They're also used for resonant transformers such as Tesla coils where they canachieve reasonably low loss in spite of the high leakage inductance.

Windings

The conducting material used for the windings depends upon theapplication, but in all cases the individual turns must be electricallyinsulated from each other to ensure that the current travels

throughout every turn.[56] For small power and signal transformers,in which currents are low and the potential difference betweenadjacent turns is small, the coils are often wound from enamelledmagnet wire, such as Formvar wire. Larger power transformersoperating at high voltages may be wound with copper rectangularstrip conductors insulated by oil-impregnated paper and blocks of

pressboard.[57]

High-frequency transformers operating in the tens to hundreds ofkilohertz often have windings made of braided Litz wire to minimize

the skin-effect and proximity effect losses.[29] Large powertransformers use multiple-stranded conductors as well, since even atlow power frequencies non-uniform distribution of current would

otherwise exist in high-current windings.[57] Each strand is individually insulated, and the strands are arranged sothat at certain points in the winding, or throughout the whole winding, each portion occupies different relativepositions in the complete conductor. The transposition equalizes the current flowing in each strand of the conductor,and reduces eddy current losses in the winding itself. The stranded conductor is also more flexible than a solid

conductor of similar size, aiding manufacture.[57]

The windings of signal transformers minimize leakage inductance and stray capacitance to improve high-frequencyresponse. Coils are split into sections, and those sections interleaved between the sections of the other winding.

Power-frequency transformers may have taps at intermediate points on the winding, usually on the higher voltagewinding side, for voltage adjustment. Taps may be manually reconnected, or a manual or automatic switch may beprovided for changing taps. Automatic on-load tap changers are used in electric power transmission or distribution,



Cut view through transformer windings.

White: insulator. Green spiral: Grain

oriented silicon steel. Black: Primary

winding made of oxygen-free copper. Red:

Secondary winding. Top left: Toroidal

transformer. Right: C-core, but E-core

would be similar. The black windings are

made of film. Top: Equally low capacitance

between all ends of both windings. Since

most cores are at least moderately

conductive they also need insulation.

Bottom: Lowest capacitance for one end of

the secondary winding needed for low-

power high-voltage transformers. Bottom

left: Reduction of leakage inductance

would lead to increase of capacitance.

on equipment such as arc furnace transformers, or for automaticvoltage regulators for sensitive loads. Audio-frequency transformers,used for the distribution of audio to public address loudspeakers,have taps to allow adjustment of impedance to each speaker. Acenter-tapped transformer is often used in the output stage of anaudio power amplifier in a push-pull circuit. Modulationtransformers in AM transmitters are very similar.

Dry-type transformer winding insulation systems can be either ofstandard open-wound 'dip-and-bake' construction or of higherquality designs that include vacuum pressure impregnation (VPI),vacuum pressure encapsulation (VPE), and cast coil encapsulation

processes.[58] In the VPI process, a combination of heat, vacuumand pressure is used to thoroughly seal, bind, and eliminateentrained air voids in the winding polyester resin insulation coatlayer, thus increasing resistance to corona. VPE windings are similarto VPI windings but provide more protection against environmentaleffects, such as from water, dirt or corrosive ambients, by multiple

dips including typically in terms of final epoxy coat.[59]

Cooling

To place the cooling problem in perspective, the accepted rule of thumb is that the life expectancy of insulation in allelectric machines including all transformers is halved for about every 7 C to 10 C increase in operatingtemperature, this life expectancy halving rule holding more narrowly when the increase is between about 7 C to

8 C in the case of transformer winding cellulose insulation.[60][61][62]

Small dry-type and liquid-immersed transformers are often self-cooled by natural convection and radiation heatdissipation. As power ratings increase, transformers are often cooled by forced-air cooling, forced-oil cooling,

water-cooling, or combinations of these.[63] Large transformers are filled with transformer oil that both cools and

insulates the windings.[64] Transformer oil is a highly refined mineral oil that cools the windings and insulation bycirculating within the transformer tank. The mineral oil and paper insulation system has been extensively studied andused for more than 100 years. It is estimated that 50% of power transformers will survive 50 years of use, that theaverage age of failure of power transformers is about 10 to 15 years, and that about 30% of power transformer

failures are due to insulation and overloading failures.[65][66] Prolonged operation at elevated temperature degradesinsulating properties of winding insulation and dielectric coolant, which not only shortens transformer life but can



Cutaway view of liquid-

immersed construction

transformer. The conservator

(reservoir) at top provides

liquid-to-atmosphere isolation

as coolant level and

temperature changes. The

walls and fins provide required

heat dissipation balance.

ultimately lead to catastrophic transformer failure.[60] With a great body of empirical study as a guide, transformeroil testing including dissolved gas analysis provides valuable maintenance information. This underlines the need tomonitor, model, forecast and manage oil and winding conductor insulation temperature conditions under varying,

possibly difficult, power loading conditions.[67][68]

Building regulations in many jurisdictions require indoor liquid-filled transformers to either use dielectric fluids that

are less flammable than oil, or be installed in fire-resistant rooms.[69] Air-cooled dry transformers can be moreeconomical where they eliminate the cost of a fire-resistant transformer room.

The tank of liquid filled transformers often has radiators through which the liquid coolant circulates by naturalconvection or fins. Some large transformers employ electric fans for forced-air cooling, pumps for forced-liquid

cooling, or have heat exchangers for water-cooling.[64] An oil-immersed transformer may be equipped with aBuchholz relay, which, depending on severity of gas accumulation due to internal arcing, is used to either alarm or

de-energize the transformer.[51] Oil-immersed transformer installations usually include fire protection measures suchas walls, oil containment, and fire-suppression sprinkler systems.

Polychlorinated biphenyls have properties that once favored their use as adielectric coolant, though concerns over their environmental persistence led to a

widespread ban on their use.[70] Today, non-toxic, stable silicone-based oils, orfluorinated hydrocarbons may be used where the expense of a fire-resistant

liquid offsets additional building cost for a transformer vault.[69][71] PCBs fornew equipment was banned in 1981 and in 2000 for use in existing equipment in

United Kingdom[72] Legislation enacted in Canada between 1977 and 1985essentially bans PCB use in transformers manufactured in or imported into thecountry after 1980, the maximum allowable level of PCB contamination in

existing mineral oil transformers being 50 ppm.[73]

Some transformers, instead of being liquid-filled, have their windings enclosed in

sealed, pressurized tanks and cooled by nitrogen or sulfur hexafluoride gas.[71]

Experimental power transformers in the 500-to-1,000 kVA range have beenbuilt with liquid nitrogen or helium cooled superconducting windings, which

eliminates winding losses without affecting core losses.[74][75]

Insulation drying

Construction of oil-filled transformers requires that the insulation covering thewindings be thoroughly dried of residual moisture before the oil is introduced.Drying is carried out at the factory, and may also be required as a field service.Drying may be done by circulating hot air around the core, or by vapor-phasedrying (VPD) where an evaporated solvent transfers heat by condensation on the coil and core.

For small transformers, resistance heating by injection of current into the windings is used. The heating can becontrolled very well, and it is energy efficient. The method is called low-frequency heating (LFH) since the currentused is at a much lower frequency than that of the power grid, which is normally 50 or 60 Hz. A lower frequency



reduces the effect of inductance, so the voltage required can be reduced.[76] The LFH drying method is also used

for service of older transformers.[77]

Bushings

Larger transformers are provided with high-voltage insulated bushings made of polymers or porcelain. A largebushing can be a complex structure since it must provide careful control of the electric field gradient without letting

the transformer leak oil.[78]

Classification parameters

Transformers can be classified in many ways, such as the following:

Power capacity: From a fraction of a volt-ampere (VA) to over a thousand MVA.

Duty of a transformer: Continuous, short-time, intermittent, periodic, varying.

Frequency range: Power-frequency, audio-frequency, or radio-frequency.

Voltage class: From a few volts to hundreds of kilovolts.

Cooling type: Dry and liquid-immersed - self-cooled, forced air-cooled; liquid-immersed - forced oil-

cooled, water-cooled.

Circuit application: Such as power supply, impedance matching, output voltage and current stabilizer or

circuit isolation.

Utilization: Pulse, power, distribution, rectifier, arc furnace, amplifier output, etc..

Basic magnetic form: Core form, shell form.

Constant-potential transformer descriptor: Step-up, step-down, isolation.

General winding configuration: By EIC vector group - various possible two-winding combinations of the

phase designations delta, wye or star, and zigzag or interconnected star;[n] other - autotransformer, Scott-T,

zigzag grounding transformer winding.[79][80][81][82]

Rectifier phase-shift winding configuration: 2-winding, 6-pulse; 3-winding, 12-pulse; . . . n-winding, [n-

1]*6-pulse; polygon; etc..

Types

Various specific electrical application designs require a variety of transformer types. Although they all share thebasic characteristic transformer principles, they are customize in construction or electrical properties for certaininstallation requirements or circuit conditions.

Autotransformer: Transformer in which part of the winding is common to both primary and secondary

circuits.[83]

Capacitor voltage transformer: Transformer in which capacitor divider is used to reduce high voltage



An electrical substation in Melbourne, Australia

showing three of five 220 kV 66 kV transformers,

each with a capacity of 150 MVA[86]Transformer at the Limestone

Generating Station in Manitoba,

Canada

before application to the primary winding.

Distribution transformer, power transformer: International standards make a distinction in terms of

distribution transformers being used to distribute energy from transmission lines and networks for local

consumption and power transformers being used to transfer electric energy between the generator and

distribution primary circuits.[83][84][o]

Phase angle regulating transformer: A specialised transformer used to control the flow of real power on

three-phase electricity transmission networks.

Scott-T transformer: Transformer used for phase transformation from three-phase to two-phase and vice

versa.[83]

Polyphase transformer: Any transformer with more than one phase.

Grounding transformer: Transformer used for grounding three-phase circuits to create a neutral in a three

wire system, using a wye-delta transformer,[80][85] or more commonly, a zigzag grounding winding.[80][82][83]

Leakage transformer: Transformer that has loosely coupled windings.

Resonant transformer: Transformer that uses resonance to generate a high secondary voltage.

Audio transformer: Transformer used in audio equipment.

Output transformer: Transformer used to match the output of a valve amplifier to its load.

Instrument transformer: Potential or current transformer used to accurately and safely represent voltage,

current or phase position of high voltage or high power circuits.[83]

Applications

Transformers areused to increasevoltage beforetransmittingelectrical energyover longdistances throughwires. Wires haveresistance whichloses energythrough jouleheating at a ratecorresponding tosquare of thecurrent. By

transforming power to a higher voltage transformers enable economicaltransmission of power and distribution. Consequently, transformers have shaped the electricity supply industry,

permitting generation to be located remotely from points of demand.[87] All but a tiny fraction of the world's

electrical power has passed through a series of transformers by the time it reaches the consumer.[39]



Faraday's experiment with induction between

coils of wire[88]

Induction coil, 1900, Bremerhavn,

Germany

Transformers are also used extensively in electronic products to step-down the supply voltage to a level suitable forthe low voltage circuits they contain. The transformer also electrically isolates the end user from contact with thesupply voltage.

Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphonesand record players to the input of amplifiers. Audio transformers allowed telephone circuits to carry on a two-wayconversation over a single pair of wires. A balun transformer converts a signal that is referenced to ground to asignal that has balanced voltages to ground, such as between external cables and internal circuits.

History

Discovery of induction

Electromagnetic induction, the principle of the operation of thetransformer, was discovered independently and almostsimultaneously by Joseph Henry and Michael Faraday in 1831.Although Henry's work likely having preceded Faraday's workby a few months, Faraday was the first to publish the results of

his experiments and thus receive credit for the discovery.[89] Therelationship between emf and magnetic flux is an equation nowknown as Faraday's law of induction:

.

where is the magnitude of the emf in volts and B is the

magnetic flux through the circuit in webers.[90]

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

an iron ring, thus creating the first toroidal closed-core transformer.[91] However he only applied individual pulses ofcurrent to his transformer, and never discovered the relation between the turns ratio and emf in the windings.

Induction coils

The first type of transformer to see wide use was the induction coil,invented by Rev. Nicholas Callan of Maynooth College, Ireland in 1836.He was one of the first researchers to realize the more turns thesecondary winding has in relation to the primary winding, the larger theinduced secondary emf will be. Induction coils evolved from scientists'and inventors' efforts to get higher voltages from batteries. Since batteriesproduce direct current (DC) rather than AC, induction coils relied uponvibrating electrical contacts that regularly interrupted the current in theprimary to create the flux changes necessary for induction. Between the1830s and the 1870s, efforts to build better induction coils, mostly by

trial and error, slowly revealed the basic principles of transformers.



Faraday's ring transformer

First alternating current transformers

By the 1870s, efficient generators producing alternating current (AC) were available, and it was found AC couldpower an induction coil directly, without an interrupter.

In 1876, Russian engineer Pavel Yablochkov invented a lighting system based on a set of induction coils where theprimary windings were connected to a source of AC. The secondary windings could be connected to several

'electric candles' (arc lamps) of his own design.[92] [93] The coils Yablochkov employed functioned essentially as

transformers.[92]

In 1878, the Ganz factory, Budapest, Hungary, began manufacturing equipment for electric lighting and, by 1883,had installed over fifty systems in Austria-Hungary. Their AC systems

used arc and incandescent lamps, generators, and other equipment.[94]

Lucien Gaulard and John Dixon Gibbs first exhibited a device with anopen iron core called a 'secondary generator' in London in 1882, then

sold the idea to the Westinghouse company in the United States.[47] Theyalso exhibited the invention in Turin, Italy in 1884, where it was adopted

for an electric lighting system.[95] However, the efficiency of their open-

core bipolar apparatus remained very low.[95]

Early series circuit transformer distribution

Induction coils with open magnetic circuits are inefficient at transferringpower to loads. Until about 1880, the paradigm for AC powertransmission from a high voltage supply to a low voltage load was aseries circuit. Open-core transformers with a ratio near 1:1 were connected with their primaries in series to allowuse of a high voltage for transmission while presenting a low voltage to the lamps. The inherent flaw in this methodwas that turning off a single lamp (or other electric device) affected the voltage supplied to all others on the samecircuit. Many adjustable transformer designs were introduced to compensate for this problematic characteristic ofthe series circuit, including those employing methods of adjusting the core or bypassing the magnetic flux around

part of a coil.[95] Efficient, practical transformer designs did not appear until the 1880s, but within a decade, thetransformer 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.[96]

Closed-core transformers and parallel power distribution

In the autumn of 1884, Kroly Zipernowsky, Ott Blthy and Miksa Dri (ZBD), three engineers associated withthe Ganz factory, had determined that open-core devices were impracticable, as they were incapable of reliably

regulating voltage.[98] In their joint 1885 patent applications for novel transformers (later called ZBD 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 core.[95] The two designs were the first application of the two basictransformer constructions 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).[41][42][99][100] The Ganzfactory had also in the autumn of 1884 made delivery of the world's first five high-efficiency AC transformers, the



Shell form transformer. Sketch used

by Uppenborn to describe ZBD

engineers' 1885 patents and earliest

articles.[95]

Core form, front; shell form, back.

Earliest specimens of ZBD-designed

high-efficiency constant-potential

transformers manufactured at the

Ganz factory in 1885.

The ZBD team consisted of Kroly

Zipernowsky, Ott Blthy and Miksa

Dri

Stanley's 1886 design for adjustable

gap open-core induction coils[97]

first of these units havingbeen shipped on September

16, 1884.[101] This first unithad been manufactured to thefollowing specifications:1,400 W, 40 Hz, 120:72 V,11.6:19.4 A, ratio 1.67:1,

one-phase, shell form.[101]

In both designs, the magneticflux linking the primary andsecondary windings traveledalmost entirely within theconfines of the iron core, withno 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.[102] The ZBD patents includedtwo other major interrelated innovations: one concerning the use ofparallel connected, instead of series connected, utilization loads, the otherconcerning the ability to have high turns ratio transformers such that thesupply network voltage could be much higher (initially 1,400 to 2,000 V)

than the voltage of utilization loads (100 V initially preferred).[103][104]

When employed in parallel connected electric distribution systems,closed-core transformers finally made it technically and economicallyfeasible to provide electric power for lighting in homes, businesses and

public spaces.[105][106] Blthy had suggested the use of closed cores,Zipernowsky had suggested the use of parallel shunt connections, and

Dri had performed the experiments;[107]

Transformers today are designed on the principles discovered by thethree engineers. They also popularized the word 'transformer' to

describe a device for altering the emf of an electric current,[105][108]

although the term had already been in use by 1882.[109][110] In 1886,the ZBD engineers designed, and the Ganz factory supplied electricalequipment for, the world's first power station that used AC generatorsto power a parallel connected common electrical network, the steam-

powered Rome-Cerchi power plant.[111]

Although George Westinghouse had bought Gaulard and Gibbs' patentsin 1885, the Edison Electric Light Company held an option on the USrights for the ZBD transformers, requiring Westinghouse to pursue alternative designs on the same principles. He

assigned to William Stanley the task of developing a device for commercial use in United States.[112] Stanley's firstpatented 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).[97] This design[113] was first used commercially in the US in 1886[96] butWestinghouse was intent on improving the Stanley design to make it (unlike the ZBD type) easy and cheap to

produce.[113]

Westinghouse, Stanley and associates soon developed an easier to manufacture core, consisting of a stack of thin'Eshaped' iron plates, insulated by thin sheets of paper or other insulating material. Prewound copper coils couldthen be slid into place, and straight iron plates laid in to create a closed magnetic circuit. Westinghouse applied for a

patent for the new low-cost design in December 1886; it was granted in July 1887.[107][114]

Other early transformers

In 1889, Russian-born engineer Mikhail Dolivo-Dobrovolsky developed the first three-phase transformer at the

Allgemeine Elektricitts-Gesellschaft ('General Electricity Company') in Germany.[115]

In 1891, Nikola Tesla invented the Tesla coil, an air-cored, dual-tuned resonant transformer for generating very

high voltages at high frequency.[116][117]

See also

Transformer types#Resonant transformer

Compensation winding

Electronic symbol

Geomagnetic storm

Inductor

Load profile

Magnetic core

Magnetization

Paraformer

Polyphase system

Rectiformer

Switched-mode power supply

Notes

a. ^ Where Vs is the instantaneous voltage, Ns is the number of turns in the secondary winding, and d/dt is the

derivative of the magnetic flux through one turn of the winding. With turns of the winding oriented

perpendicularly to the magnetic field lines, the flux is the product of the magnetic flux density and the core area,

the magnetic field varying with time according to the excitation of the primary. The expression d/dt, defined as

the derivative of magnetic flux with time t, provides a measure of rate of magnetic flux in the core and hence of



emf induced in the respective winding.

b. ^ Although ideal transformer's winding inductances are each infinitely high, the square root of winding

inductances' ratio is equal to the turns ratio.[4]

c. ^ Direction of transformer currents is according to Right-hand rule.

d. ^ This also implies the following: Input impedance is infinite when secondary is open and zero when secondary is

shorted; there is zero phase-shift through an ideal transformer; input and output power and reactive volt-ampere

are each conserved; these three statements apply for any frequency above zero and periodic waveforms are

conserved.[6]

e. ^ Windings of real transformers are usually wound around very high permeability ferromagnetic cores but can also

be air-core wound.

f. ^ "The turn ratio of a transformer is the ratio of the number of turns in the high-voltage winding to that in the low-

voltage winding",[12] common usage having evolved over time from 'turn ratio' to 'turns ratio',

g. ^ A step-down transformer converts a high voltage to a lower voltage while a step-up transformer converts a low

voltage to a higher voltage, an isolation transformer having 1:1 turns ratio with output voltage the same as input

voltage.

h. ^ ANSI/IEEE Standard C57.13 defines polarity in terms of the relative instantaneous directions of the currents

entering the primary terminals and leaving the secondary terminals during most of each half cycle, the word

'instantaneous' differentiating from say phasor current.[17][18]

i. ^ Transformer polarity can also be identified by terminal markings H0,H1,H2... on primary terminals and X1,X2,

(and Y1,Y2, Z1,Z2,Z3... if windings are available) on secondary terminals. Each letter prefix designates a different

winding and each numeral designates a termination or tap on each winding. The designated terminals H1,X1, (and

Y1, Z1 if available) indicate same instantaneous polarities for each winding as in the dot convention.[19]

j. ^ When a voltage transformer is operated with sinusoidal voltages in its normal frequency range and power level

the voltage polarity at the output dot is the same (plus minus a few degrees) as the voltage polarity at the input dot.

k. ^ A standardized open-circuit or unloaded transformer test called the Epstein frame can also be used for the

characterization of magnetic properties of soft magnetic materials including especially electrical steels.[28]

l. ^ Experimental transformers using superconducting windings achieve efficiencies of 99.85%.[34]

m. ^ Transformer hum's fundamental noise frequency is two times that of the power frequency as there is an

extension and a contraction of core laminations for every cycle of the AC wave and a transformer's audible hum

noise level is dominated by the fundamental noise frequency and its first triplen harmonic, i.e., by the 100 & 300

Hz, or 120 & 360 Hz, frequencies.[38]

n. ^ For example, the delta-wye transformer, by far the most common commercial three-phase transformer, is

known as the Dyn11 vector group configuration, Dyn11 denoting D for delta primary winding, y for wye

secondary winding, n for neutral of the wye winding, and 11 for relative phase position on the clock by which the

secondary winding leads the primary winding, namely, 30 leading.

o. ^ While the above formal definition, derived from standards such as IEEE C57.12.80, applies to large

transformers, it is not uncommon in colloquial, or even trade, parlance for small general-purpose transformers to

be referred to as 'power' transformers, for distribution transformers to be referred to as 'power distribution'

transformers, and so on.



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External links

Inside Transformers, composed by J. B. Calvert, from Denver University

(http://www.du.edu/~jcalvert/tech/transfor.htm)

Substation and Transmission

(http://www.dmoz.org/Business/Electronics_and_Electrical/Substation_and_Transmission/) at DMOZ



Introduction to Current Transformers (http://www.elkor.net/pdfs/AN0305-Current_Transformers.pdf)

Transformer (Interactive Java applet) (http://www.phy.hk/wiki/englishhtm/Transformer.htm), 'Physics is fun'

by Chui-king Ng

HD video tutorial on transformers (http://www.afrotechmods.com/videos/transformer_tutorial.htm)

Three-phase transformer circuits (http://www.allaboutcircuits.com/vol_2/chpt_10/6.html) from All About

Circuits

Bibliography of Transformer Books (http://www.transformerscommittee.org/info/Bibliographybooks.pdf) by

P.M. Balma, from IEEE Transformer Committee

Einschalten des Transformators. German Wikipedia article about transformer inrush current at switch on (in

German).

Retrieved from "http://en.wikipedia.org/w/index.php?title=Transformer&oldid=614522160"

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