Report Dane Di

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A transformer is a device that transfers electrical energyfrom one circuit to another through inductively coupledconductorsthe transformer's coils. A varying current in thefirst orprimarywinding creates a varying magnetic flux in

the transformer's core and thus a varying magnetic fieldthrough the secondarywinding. This varying magnetic fieldinduces a varying electromotive force (EMF), or "voltage", inthe secondary winding. This effect is called inductivecoupling.

If a load is connected to the secondary, current will flow inthe secondary winding, and electrical energy will betransferred from the primary circuit through the transformerto the load. In an ideal transformer, the induced voltage in

the secondary winding (Vs) is in proportion to the primaryvoltage (Vp) and is given by the ratio of the number of turnsin the secondary (Ns) to the number of turns in the primary(Np) as follows:
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Basic principles

The transformer is based on two principles: first, that an

electric current can produce a magnetic field(electromagnetism) and second that a changingmagnetic field within a coil of wire induces a voltageacross the ends of the coil (electromagneticinduction). Changing the current in the primary coilchanges the magnetic flux that is developed. Thechanging magnetic flux induces a voltage in thesecondary coil.

Induction law

The voltage induced across the secondary coil may becalculated from Faraday's law of induction, which statesthat:
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Where Vs is the instantaneous voltage, Ns is the number ofturns in the secondary coil and is the magnetic fluxthrough one turn of the coil. If the turns of the coil areoriented perpendicularly to the magnetic field lines, the fluxis the product of the magnetic flux densityB and the areaAthrough which it cuts. The area is constant, being equal tothe cross-sectional area of the transformer core, whereas themagnetic field varies with time according to the excitation ofthe primary. Since the same magnetic flux passes throughboth the primary and secondary coils in an ideal transformer,the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for Vs and Vp gives thebasic equation for stepping up or stepping down the voltage

Np/Ns is known as the turns ratio, and is the primaryfunctional characteristic of any transformer. In the case ofstep-up transformers, this may sometimes be stated as thereciprocal, Ns/Np. Turns ratio is commonly expressed as anirreducible fraction or ratio: for example, a transformer withprimary and secondary windings of, respectively, 100 and150 turns is said to have a turns ratio of 2:3 rather than0.667 or 100:150.
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Ideal power equation

.

Energy losses

An ideal transformer would have no energy losses, andwould be 100% efficient. In practical transformers, energy isdissipated in the windings, core, and surrounding structures.Larger transformers are generally more efficient, and thoserated for electricity distribution usually perform better than98%.
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Experimental transformers using superconducting windingsachieve efficiencies of 99.85%. The increase in efficiency cansave considerable energy, and hence money, in a largeheavily loaded transformer; the trade-off is in the additional

initial and running cost of the superconducting design.

Losses in transformers (excluding associated circuitry) varywith load current, and may be expressed as "no-load" or"full-load" loss. Winding resistance dominates load losses,whereas hysteresis and eddy currents losses contribute toover 99% of the no-load loss. The no-load loss can besignificant, so that even an idle transformer constitutes adrain on the electrical supply and a running cost. Designingtransformers for lower loss requires a larger core, good-

quality silicon steel, or even amorphous steel for the coreand thicker wire, increasing initial cost so that there is atrade-offbetween initial cost and running cost (also seeenergy efficient transformer).

Transformer losses are divided into losses in the windings,termed copper loss, and those in the magnetic circuit,termed iron loss. Losses in the transformer arise from:

Winding resistance

Current flowing through the windings causes resistiveheating of the conductors. At higher frequencies, skin effectand proximity effect create additional winding resistanceand losses.

Hysteresis losses

Each time the magnetic field is reversed, a smallamount of energy is lost due to hysteresis within thecore. For a given core material, the loss is proportional
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to the frequency, and is a function of the peak fluxdensity to which it is subjected.

Eddy currents

Ferromagnetic materials are also good conductors anda core made from such a material also constitutes asingle short-circuited turn throughout its entire length.Eddy currents therefore circulate within the core in aplane normal to the flux, and are responsible forresistive heating of the core material. The eddy currentloss is a complex function of the square of supplyfrequency and inverse square of the material thickness.Eddy current losses can be reduced by making the coreof a stack of plates electrically insulated from eachother, rather than a solid block; all transformersoperating at low frequencies use laminated or similarcores.

Magnetostriction

Magnetic flux in a ferromagnetic material, such as thecore, causes it to physically expand and contract

slightly with each cycle of the magnetic field, an effectknown as magnetostriction. This produces the buzzingsound commonly associated with transformers that cancause losses due to frictional heating. This buzzing isparticularly familiar from low-frequency (50 Hz or 60Hz) mains hum, and high-frequency (15,734 Hz (NTSC)or 15,625 Hz (PAL)) CRT noise.

Mechanical losses

In addition to magnetostriction, the alternatingmagnetic field causes fluctuating forces between theprimary and secondary windings. These incite
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vibrations within nearby metalwork, adding to thebuzzing noise and consuming a small amount of power.

Stray losses

Leakage inductance is by itself largely lossless, sinceenergy supplied to its magnetic fields is returned to thesupply with the next half-cycle. However, any leakageflux that intercepts nearby conductive materials suchas the transformer's support structure will give rise toeddy currents and be converted to heat. There are alsoradiative losses due to the oscillating magnetic field butthese are usually small.

Core form and shell form transformers

Core form = core type; shell form = shell type

As first mentioned in regard to earliest ZBD closed-coretransformers, transformers are generally considered to be
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either core form or shell form in design depending on thetype of magnetic circuit used in winding construction (seeimage). That is, when winding coils are wound around thecore, transformers are termed as being of core form design;

when winding coils are surrounded by the core, transformersare termed as being of shell form design. Shell form designmay be more prevalent than core form design fordistribution transformer applications due to the relative easein stacking the core around winding coils. Core form designtends to, as a general rule, be more economical, andtherefore more prevalent, than shell form design for highvoltage 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.Shell form design tends to be preferred for extra highvoltage and higher MVA applications because, though morelabor intensive to manufacture, shell form transformers arecharacterized as having inherently better kVA-to-weightratio, better short-circuit strength characteristics and higherimmunity to transit damage.

Equivalent circuit

The physical limitations of the practical transformer may bebrought together as an equivalent circuit model (shownbelow) built around an ideal lossless transformer. [58] Powerloss in the windings is current-dependent and is representedas in-series resistances Rp and Rs. Flux leakage results in afraction of the applied voltage dropped without contributingto the mutual coupling, and thus can be modeled asreactances of each leakage inductanceXp andXs in serieswith the perfectly coupled region.

Iron losses are caused mostly by hysteresis and eddy currenteffects in the core, and are proportional to the square of thecore flux for operation at a given frequency. [59] Since the coreflux is proportional to the applied voltage, the iron loss canbe represented by a resistance RC in parallel with the idealtransformer.
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A core with finite permeability requires a magnetizingcurrent Im to maintain the mutual flux in the core. Themagnetizing current is in phase with the flux. Saturationeffects cause the relationship between the two to be non-

linear, but for simplicity this effect tends to be ignored inmost circuit equivalents.[60] With a sinusoidal supply, the coreflux lags the induced EMF by 90 and this effect can bemodeled as a magnetizing reactance (reactance of aneffective inductance)Xm in parallel with the core losscomponent, Rc. Rc andXm are sometimes together termedthe magnetizing branch of the model. If the secondarywinding is made open-circuit, the current I0 taken by themagnetizing branch represents the transformer's no-loadcurrent.[58]

The secondary impedanceRs andXs is frequently moved (or"referred") to the primary side after multiplying thecomponents by the impedance scaling factor (Np/Ns)

2.

Testing of Transformers

The structure of the circuit equivalent of a practicaltransformer is developed earlier. The performance
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parameters of interest can be obtained by solving that circuitfor any load conditions. The equivalent circuit parametersare available to the designer of the transformers from thevarious expressions that he uses for designing the

transformers. But for a user these are not available most ofthe times. Also when a transformer is rewound with differentprimary and secondary windings the equivalent circuit alsochanges. In order to get the equivalent circuit parameterstest methods are heavily depended upon. From the analysisofthe equivalent circuit one can determine the electricalparameters. But if the temperature rise of the transformer isrequired, then test method is the most dependable one.

There are several tests that can be done on the transformer;

however a few common ones are discussed here.

Winding resistance test

This is nothing but the resistance measurement of thewindings by applying a small d.c voltage to the winding andmeasuring the current through the same. The ratio gives thewinding resistance, more commonly feasible with highvoltage windings. For low voltage windings a resistance-bridge method can be used. From the d.c resistance one canget the a.c. resistance by applying skin effect corrections.

Polarity TestThis is needed for identifying the primary and secondaryphasor polarities. It is must for poly phase connections. Botha.c. and d.c methods can be used for detecting the polarities


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of the induced emfs. The dot method discussed earlier isused to indicate the polarities. The transformer is connectedto a low voltage a.c. source with the connectionsmade as shown in the fig. 18(a). A supply voltage Vs is

applied to the primary and the readings of the voltmetersV1, V2 and V3 are noted. V1 : V2 gives the turns ratio. If V3readsV1V2 then assumed dot locations are correct (for theconnection shown). The beginning and end of the primaryand secondary may then be marked by A1 A2 and a1a2respectively.If the voltage rises from A1 to A2 in theprimary, at any instant it does so from a1 to a2 inthe secondary. If more secondary terminals are present dueto taps taken from the windings they can be labeled as a3,

a4, a5, a6. It is the voltage rising from smaller numbertowardslarger ones in each winding. The same thing holds good ifmore secondarys are present.

Open Circuit Test

(a)Physical Arrangement(b)Equivalent Circuit

Figure 19: No Load TestAs the name suggests, the secondary is kept open circuitedand nominal value of the input voltage is applied to theprimary winding and the input current and power aremeasured. In Fig. V,A,W are the voltmeter, ammeter andwattmeter respectively. Let these meters read V1, I0 and W0respectively. Fig. shows the equivalent circuit ofthe transformer under this test. The no load current at ratedvoltage is less than 1 percent of nominal current and hence

the loss and drop that take place in primary impedance r1+jxl1due to the no load current I0 is negligible. The activecomponent Ic of the no load current I0Sometimes thenominal value of high voltage itself may not be known, or indoubt, especially in a rewound transformer. In such cases anopen circuit characteristics is first obtained, which is a graph


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showing the applied voltage as a function of the no loadcurrent. This is a non linear curve as shown in Fig. 20. Thisgraph is obtained by noting the current drawn bytransformer at different applied voltage, keeping the

secondary open circuited. Theusual operating point selected for operation lies at somestandard voltage around the kneepoint of the characteristic. After this value is chosen as thenominal value the parametersare calculated as mentioned above.

Short Circuit TestThe purpose of this test is to determine the series branchparameters of the equiv-

alent circuit of Fig. 21(b). As the name suggests, in this testprimary applied voltage, thecurrent and power input are measured keeping thesecondary terminals short circuited. Letthese values be Vsc, Isc and Wsc respectively. The supplyvoltage required to circulate ratedcurrent through the transformer is usually very small and isof the order of a few percentof the nominal voltage. The excitation current which is only 1

percent or less even at ratedvoltage becomes negligibly small during this test and henceis neglected. The shunt branchis thus assumed to be absent. Also I1 = I

2 as I0 0. Therefore Wsc is the sum of thecopper losses in primary and secondary put together. Thereactive power consumed is thatabsorbed by the leakage reactance of the two windings.

Load TestLoad Test helps to determine the total loss that takes place,when the transformeris loaded. Unlike the tests described previously, in thepresent case nominal voltage is applied


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across the primary and rated current is drown from thesecondary. Load test is used mainly1. to determine the rated load of the machine and thetemperature rise

2. to determine the voltage regulation and efficiency of thetransformer.Rated load is determined by loading the transformer on acontinuous basis and observ-ing the steady state temperature rise. The losses that aregenerated inside the transformeron load appear as heat. This heats the transformer and thetemperature of the transformerincreases. The insulation of the transformer is the one to getaffected by this rise in the

temperature. Both paper and oil which are used forinsulation in the transformer start get-ting degenerated and get decomposed. If the flash point ofthe oil is reached the transformergoes up in flames. Hence to have a reasonable lifeexpectancy the loading of the transformermust be limited to that value which gives the maximumtemperature rise tolerated by theinsulation. This aspect of temperature rise cannot be

guessed from the electrical equivalentcircuit. Further, the losses like dielectric losses and strayload losses are not modeled in theequivalent circuit and the actual loss under load conditionwill be in error to that extent.Many external means of removal of heat from thetransformer in the form of different coolingmethods give rise to different values for temperature rise ofinsulation. Hence these permitdifferent levels of loading for the same transformer. Hencethe only sure way of ascertainingthe rating is by conducting a load test.It is rather easy to load a transformer of small ratings. As therating increases itbecomes difficult to find a load that can absorb the requisitepower and a source to feed the


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necessary current. As the transformers come in variedtransformation ratios, in many casesit becomes extremely difficult to get suitable loadimpedance.

Further, the temperature rise of the transformer is due to thelosses that take placeinside the transformer. The efficiency of the transformer isabove 99% even in modest sizeswhich means 1 percent of power handled by the transformeractually goes to heat up themachine. The remaining 99% of the power has to bedissipated in a load impedance externalto the machine. This is very wasteful in terms of energy also.( If the load is of unity power

factor) Thus the actual loading of the transformer is seldomresorted to. Equivalent lossmethods of loading and Phantom loading are commonlyused in the case of transformers.

The load is applied and held constant till the temperaturerise of transformer reaches asteady value. If the final steady temperature rise is lowerthan the maximum permissiblevalue, then load can be increased else it is decreased. That

load current which gives themaximum permissible temperature rise is declared as thenominal or rated load current andthe volt amperes are computed using the same.In the equivalent loss method a short circuit test is done onthe transformer. Theshort circuit current is so chosen that the resulting losstaking place inside the transformeris equivalent to the sum of the iron losses, full load copperlosses and assumed stray loadlosses. By this method even though one can pump inequivalent loss inside the transformer,the actual distribution of this loss vastly differs from thattaking place in reality. Thereforethis test comes close to a load test but does not replace one.


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Types of transformer

Audio transformers

Audio transformers are those specifically designed for use in audio circuits.

They can be used to block radio frequency interference or the DC

component of an audio signal, to split or combine audio signals, or to

provide impedance matching between high and low impedance circuits, such

as between a high impedance tube (valve) amplifieroutput and a low

impedance loudspeaker, or between a high impedance instrument output and

the low impedance input of a mixing console.

Such transformers were originally designed to connect different telephone

systems to one another while keeping their respective power suppliesisolated, and are still commonly used to interconnectprofessional audio

systems or system components.

Being magnetic devices, audio transformers are susceptible to external

magnetic fields such as those generated by AC current-carrying conductors.

"Hum" is a term commonly used to describe unwanted signals originating

from the "mains" power supply (typically 50 or 60 Hz). Audio transformers

used for low-level signals, such as those from microphones, often include

magnetic shielding to protect against extraneousmagnetically-coupledsignals.
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Output transformer

Early audio amplifiers used transformers forcoupling between

stages, i.e., for transferring signal without connecting different

operating voltages together. It was realized that transformersintroduced distortion; furthermore they produced significant

frequency-dependentphase shifts, particularly at higher frequencies.

The phase shift was not problematical in itself, but made it difficult

to introduce distortion-cancelling negative feedback, either over a

transformer-coupled stage or the whole amplifier. Where they were

used as a convenient way to isolate stages while coupling signals,

transformers could be eliminated by using capacitor coupling. The

transformer coupling the output of the amplifier to the loudspeaker,however, had the important requirement to couple the high

impedance of the output valves with the low impedance of the

loudspeakers. With the 1940s Williamson amplifieras a much-

quoted early example, audio amplifiers with hitherto unprecedented

low distortion were produced, using designs with only one

transformer, the output transformer, and large overall negative

feedback. Some attempts to design transformer less amplifiers were

made, for example using very-low-impedance powertriodes (such

as the 6080, originally designed for power regulation), but were notwidely used. The design of output transformers became a critical

requirement for achieving low distortion, and carefully-designed,

expensive components were produced with minimal inherent

distortion and phase shift. Bulletins Ultra-Lineartransformer design

was used in conjunction with Williamson's principles, allowing

pentode orbeam tetrode output devices to produce the higher power

of a pentode than a triode, and lower distortion than either type.

Some earlyjunction transistoramplifiers used transformers in the

signal path, both interstage and output, but solid-state designs were

rapidly produced with suitably low impedance to drive loudspeakers

without using transformers, allowing very large amounts of

feedback to be applied without instability.
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Since the replacement of thermionic by solid-state electronics,

signal transformers, including output transformers, are rarely or

never used in modern audio designs. A few very expensive valve

audio amplifiers are produced for vacuum-tube audio enthusiasts,

and they require well-designed output transformers.

Instrument transformers

Instrument transformers are used for measuringvoltage and current in electrical power systems, andfor power system protection and control. Where avoltage or current is too large to be conveniently usedby an instrument, it can be scaled down to astandardized low value. Instrument transformersisolate measurement, protection and control circuitryfrom the high currents or voltages present on thecircuits being measured or controlled.

Cores

Laminated steel cores
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Laminated core transformer showing edge of laminations attop of photo

Transformers for use at power or audio frequencies typicallyhave cores made of high permeabilitysilicon steel. The steelhas a permeability many times that offree space and thecore thus serves to greatly reduce the magnetizing currentand confine the flux to a path which closely couples thewindings. Early transformer developers soon realized thatcores constructed from solid iron resulted in prohibitiveeddy-current losses, and their designs mitigated this effectwith cores consisting of bundles of insulated iron wires. Laterdesigns constructed the core by stacking layers of thin steellaminations, a principle that has remained in use. Eachlamination is insulated from its neighbors by a thin non-

conducting layer of insulation. The universal transformerequation indicates a minimum cross-sectional area for thecore to avoid saturation.

The effect of laminations is to confine eddy currents tohighly elliptical paths that enclose little flux, and so reducetheir magnitude. Thinner laminations reduce losses, but are
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more laborious and expensive to construct. Thin laminationsare generally used on high frequency transformers, withsome types of very thin steel laminations able to operate upto 10 kHz.

Laminating the core greatly reduces eddy-current losses

One common design of laminated core is made from

interleaved stacks ofE-shaped steel sheets capped with I-shaped pieces, leading to its name of "E-I transformer. Sucha design tends to exhibit more losses, but is very economicalto manufacture. The cut-core or C-core type is made bywinding a steel strip around a rectangular form and thenbonding the layers together. It is then cut in two, formingtwo C shapes, and the core assembled by binding the two Chalves together with a steel strap. They have the advantagethat the flux is always oriented parallel to the metal grains,

reducing reluctance.

A steel core's remanence means that it retains a staticmagnetic field when power is removed. When power is thenreapplied, the residual field will cause a high inrush currentuntil the effect of the remaining magnetism is reduced,usually after a few cycles of the applied alternating current.
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[Overcurrent protection devices such as fuses must beselected to allow this harmless inrush to pass. Ontransformers connected to long, overhead powertransmission lines, induced currents due to geomagnetic

disturbances during solar storms can cause saturation of thecore and operation of transformer protection devices.

Distribution transformers can achieve low no-load losses byusing cores made with low-loss high-permeability siliconsteel or amorphous (non-crystalline) metal alloy. The higherinitial cost of the core material is offset over the life of thetransformer by its lower losses at light load.

Solid cores

Powdered iron cores are used in circuits such as switch-mode power supplies that operate above mains frequenciesand up to a few tens of kilohertz. These materials combinehigh magnetic permeability with high bulk electricalreceptivity. For frequencies extending beyond the VHF band,cores made from non-conductive magnetic ceramicmaterials called ferrites are common. Some radio-frequencytransformers also have movable cores (sometimes called'slugs') which allow adjustment of the coupling coefficient(and bandwidth) of tuned radio-frequency circuits.

Toroidal cores

Small toroidal core transformer
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Toroidal transformers are built around a ring-shaped core,which, depending on operating frequency, is made from along strip ofsilicon steel or perm alloy wound into a coil,powdered iron, or ferrite. A strip construction ensures that

the grain boundaries are optimally aligned, improving thetransformer's efficiency by reducing the core's reluctance.The closed ring shape eliminates air gaps inherent in theconstruction of an E-I core.The cross-section of the ring isusually square or rectangular, but more expensive cores withcircular cross-sections are also available. The primary andsecondary coils are often wound concentrically to cover theentire surface of the core. This minimizes the length of wireneeded, and also provides screening to minimize the core'smagnetic field from generating electromagnetic

interference.

Toroidal transformers are more efficient than the cheaperlaminated 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 exteriormagnetic field (about one tenth), low off-load losses (makingthem more efficient in standby circuits), single-bolt

mounting, and greater choice of shapes. The maindisadvantages are higher cost and limited power capacity(see "Classification" above). Because of the lack of a residualgap in the magnetic path, toroidal transformers also tend toexhibit higher inrush current, compared to laminated E-Itypes.

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 a

switched-mode power supply. A drawback of toroidaltransformer construction is the higher labor cost of winding.

This is because it is necessary to pass the entire length of acoil winding through the core aperture each time a singleturn is added to the coil. As a consequence, toroidaltransformers are uncommon above ratings of a few kVA.Small distribution transformers may achieve some of the
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benefits of a toroidal core by splitting it and forcing it open,then inserting a bobbin containing primary and secondarywindings.

Air cores

A physical core is not an absolute requisite and a functioningtransformer can be produced simply by placing the windingsnear each other, an arrangement termed an "air-core"transformer. The air which comprises the magnetic circuit isessentially lossless, and so an air-core transformereliminates loss due to hysteresis in the core material. Theleakage inductance is inevitably high, resulting in very poorregulation, and so such designs are unsuitable for use in

power distribution. They have however very high bandwidth,and are frequently employed in radio-frequency applications,for which a satisfactory coupling coefficient is maintained bycarefully overlapping the primary and secondary windings.

They're also used for resonant transformers such asTeslacoils where they can achieve reasonably low loss in spite ofthe high leakage inductance.

Windings

Windings are usually arranged concentrically to minimizeflux leakage.

The conducting material used for the windings depends uponthe application, but in all cases the individual turns must beelectrically insulated from each other to ensure that the
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current travels throughout every turn. For small power andsignal transformers, in which currents are low and thepotential difference between adjacent turns is small, thecoils are often wound from enameled magnet wire, such as

Formvar wire. Larger power transformers operating at highvoltages may be wound with copper rectangular stripconductors insulated by oil-impregnated paper and blocks ofpressboard.

Cut view through transformer windings. White: insulator.Green spiral: Grain oriented silicon steel. Black: Primarywinding made ofoxygen-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 bothwindings. Since most cores are at least moderatelyconductive they also need insulation. Bottom: Lowestcapacitance for one end of the secondary winding needed forlow-power high-voltage transformers. Bottom left: Reductionofleakage inductance would lead to increase of capacitance.

High-frequency transformers operating in the tens to

hundreds of kilohertz often have windings made of braidedLitz wire to minimize the skin-effect and proximity effectlosses. Large power transformers use multiple-strandedconductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings. Each strand is individually insulated, andthe strands are arranged so that at certain points in the
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winding, or throughout the whole winding, each portionoccupies different relative positions in the completeconductor. The transposition equalizes the current flowing ineach strand of the conductor, and reduces eddy current

losses in the winding itself. The stranded conductor is alsomore flexible than a solid conductor of similar size, aidingmanufacture.

For signal transformers, the windings may be arranged in away to minimize leakage inductance and stray capacitanceto improve high-frequency response. This can be done bysplitting up each coil into sections, and those sections placedin layers between the sections of the other winding. This isknown as a stacked type or interleaved winding.

Power transformers often have internal connections or tapsat intermediate points on the winding, usually on the highervoltage winding side, for voltage regulation control purposes.Such taps are normally manually operated, automatic on-load tap changers being reserved, for cost and reliabilityconsiderations, to higher power rated or specializedtransformers supplying transmission or distribution circuitsor certain utilization loads such as furnace transformers.Audio-frequency transformers, used for the distribution ofaudio to public address loudspeakers, have taps to allowadjustment of impedance to each speaker. A center-tappedtransformer is often used in the output stage of an audiopower amplifier in a push-pull circuit. Modulationtransformers in AM transmitters are very similar.

Certain transformers have the windings protected by epoxyresin. By impregnating the transformer with epoxy under avacuum, one can replace air spaces within the windings with

epoxy, thus sealing the windings and helping to prevent thepossible formation of corona and absorption of dirt or water.

This produces transformers more suited to damp or dirtyenvironments, but at increased manufacturing cost.
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Cooling

Cutaway view of oil-filled power transformer. Theconservator (reservoir) at top provides oil-to-atmosphereisolation. Tank walls' cooling fins provide required heatdissipation balance.

Though it is not uncommon for oil-filled transformers to havetoday been in operation for over fifty years high temperaturedamages winding insulation, the accepted rule of thumbbeing that transformer life expectancy is halved for every 8degree C increase in operating temperature. At the lowerend of the power rating range, dry and liquid-immersedtransformers are often self-cooled by natural convection andradiation heat dissipation. As power ratings increase,

transformers are often cooled by such other means asforced-air cooling, force-oil cooling, water-cooling, or acombinations of these. The dielectric coolant used in manyoutdoor utility and industrial service transformers istransformer oil that both cools and insulates the windings.

Transformer oil is a highly refined mineral oil that inherentlyhelps thermally stabilize winding conductor insulation,
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typically paper, within acceptable insulation temperaturerating limitations. However, the heat removal problem iscentral to all electrical apparatus such that in the case ofhigh value transformer assets, this often translates in a need

to monitor, model, forecast and manage oil and windingconductor insulation temperature conditions under varying,possibly difficult, power loading conditions. Indoor liquid-filled transformers are required by building regulations inmany jurisdictions to either use a non-flammable liquid or tobe located in fire-resistant rooms. Air-cooled drytransformers are preferred for indoor applications even atcapacity ratings where oil-cooled construction would bemore economical, because their cost is offset by the reducedbuilding construction cost.

The oil-filled tank often has radiators through which the oilcirculates by natural convection. Some large transformersemploy electric-operated fans or pumps for forced-air orforced-oil cooling or heat exchanger-based water-cooling.Oil-filled transformers undergo prolonged drying processesto ensure that the transformer is completely free ofwatervapor before the cooling oil is introduced. This helps preventelectrical breakdown under load. Oil-filled transformers may

be equipped with Buchholz relays, which detect gas evolvedduring internal arcing and rapidly de-energize thetransformer to avert catastrophic failure. Oil-filledtransformers may fail, rupture, and burn, causing poweroutages and losses. Installations of oil-filled transformersusually includes fire protection measures such as walls, oilcontainment, and fire-suppression sprinkler systems.

Polychlorinated biphenyls have properties that once favoredtheir use as a dielectric coolant, though concerns over their

environmental persistence led to a widespread ban on theiruse.[89] Today, non-toxic, stable silicone-based oils, orfluorinated hydrocarbons may be used where the expense ofa fire-resistant liquid offsets additional building cost for atransformer vault. Before 1977, even transformers that werenominally filled only with mineral oils may also have beencontaminated with polychlorinated biphenyls at 10-20 ppm.
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Since mineral oil and PCB fluid mix, maintenance equipmentused for both PCB and oil-filled transformers could carry oversmall amounts of PCB, contaminating oil-filled transformers.

Some "dry" transformers (containing no liquid) are enclosedin sealed, pressurized tanks and cooled by nitrogen or sulfurhexafluoride gas.

Experimental power transformers in the 2 MVA range havebeen built with superconducting windings which eliminatesthe copper losses, but not the core steel loss. These arecooled by liquid nitrogen or helium.

Insulation drying

Construction of oil-filled transformers requires that theinsulation covering the windings be thoroughly dried beforethe oil is introduced. There are several different methods ofdrying. Common for all is that they are carried out in vacuumenvironment. The vacuum makes it difficult to transferenergy (heat) to the insulation. For this there are severaldifferent methods. The traditional drying is done bycirculating hot air over the active part and cycle this withperiods of hot-air vacuum (HAV) drying. More common forlarger transformers is to use evaporated solvent whichcondenses on the colder active part. The benefit is that theentire process can be carried out at lower pressure andwithout influence of added oxygen. This process iscommonly called vapour-phase drying (VPD).

For distribution transformers, which are smaller and have asmaller insulation weight, resistance heating can be used.

This is a method where current is injected in the windings to

heat the insulation. The benefit is that the heating can becontrolled very well and it is energy efficient. The method iscalled low-frequency heating (LFH) since the current isinjected at a much lower frequency than the nominal of thegrid, which is normally 50 or 60 Hz. A lower frequencyreduces the effect of the inductance in the transformer, sothe voltage needed to induce the current can be reduced.
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The LFH drying method is also used for service of oldertransformers.

Terminals

Very small transformers will have wire leads connecteddirectly to the ends of the coils, and brought out to the baseof the unit for circuit connections. Larger transformers mayhave heavy bolted terminals, bus bars or high-voltageinsulated bushings made of polymers or porcelain. A largebushing can be a complex structure since it must providecareful control of the electric field gradient without lettingthe transformer leak oil.
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