Trafo

Trafonyaéta hiji alat listrik nu kagunaannana pikeun mindahkeunénérgi listrikti hijisirkuitka sirkuit séjénna ngaliwatan gulungan-gulungan kawat anubabarengan diinduksiku hiji médan magnét.Hiji trafo diwangun ku hiji inti (biasana tina beusi silikon) sarta duagulungantambagaatawa leuwih nu dibeulitkeun kana inti.Arusbulak-balik dina salah sahiji gulungan ngabangkitkeun;ngawangunmédan magnétnu robah-robah dina jero intina, nu mana ahirna ngainduksi (ngarangsang ngabangkitkeun)tegangandina gulungan liana.Trafo dipaké keur naékkeun atawa nurunkeunteganganatawaaruslistrik, keur ngarobahimpedansi, sarta keur nyadiakeunisolasilistrik hiji rangkéan jeung rangkéan nu séjén.
Trafo nyaéta salah sahiji parabot listrik nu pangbasajana.Rarancang dasarna tacan kungsi ganti leuwih ti saratustaun, sok sanajan kitu rarancang katut bahan trafona mah terus dibebenah ogé. Trafo kacida pentingna pikeuntransmisi daya, nu ngajadikeun praktisnatransmisijarak jauh. Ieu kauntungan téh teu leupas tina prinsip dina dipilihna  dayaarus bulak-baliknalika "Perang Arus" dina ahir 1880-an. 
Trafofrékuénsi sora(harita disebutrepeating coils) Dimangpaatkeun ku juru taluntik mangsa keur mitembeyan ngamekarkeun tehnologitelepon, ka dieunakeun trafo téh teu pati dimangpaatkeun alatan geus loba pakakas éléktronik séjéna nu mibanda kagunaan saru sarta leuwih hadé. .
Ditempo tina ukuranna trafo dijieun dumasar kabutuhan, aya trafo nu kacida leutik nepikeun kabisa di sumputkeun di jero mikropon.Nepika trafo nu ukuran gedé naker pikeun ngahubungkeunlistrikhiji nagara, nu mibanda kamampuan nepikagigawatt.
Tapi, trafo mangrupa hiji bagian tina hiji sistim anu ngajalankeun éta kabéh.
Trafo bisa diibaratkeun kawas "géarbox" roda dua anu sederhana pikeun ngagambarkeunteganganjeungaruslistrik. Gulungan primér diumpamakeun jeung tempat asupna sarta gulungan skundér diumpamakeun tempat kaluarna.
Dina ieu gambaran, arus téh sarua jeung gancangna muter éta roda aritegangantayalian gedéna tanaga anu dipaké pikeun muterkeun éta roda. Dina hiji "géarbox" tanaga mékanis téh (torsi/kakuatan muter dikalikeun jeung gancangna puteran) hasilnakonstan(angger) saruan dina tanaga listrik ogé (Tegangan dikalikeun jeung arus) oge hasilankonstan.
Rasio roda gigi ibaratna jeung rasio stép-up atawa stép-downtransformator.Transformatorstép-up lumaku sacara analog contona ngalaunanaroda gigi(anu mana tanaga mékanik dipindahkeun/ditransfer tinaroda gigianu leutik nyaéta nu muterna gancang kana roda gigi gedé anu muterna laun): didieu aya kaayaan nukeurkeun arus (gancangna muter) sedengkeunvoltasesarua jeung (torsi), ku ayana ieu kajadian ngatransfer daya tina gulungan primér ka gulungan sekundér anu mibanda leuwih loba leuwih loba jumlah gulunganna. Transformator stép-down digambarkeun sacara analog kujalan pindahna roda gigi (di mana tanaga mékanik ditransfer tina roda gigi gedé ka roda gigi leutik): ieu nukeurkeun tegangan (torsi) pikeun arus (gancangna muter), kujalan ngatransfer daya tina gulungan primer ka gulungan sekundér. Gulungan mibanda puteran anu leuwih saeutik.
Trafo basajan ilaharna diwangun ku dua konduktor listrik disebut gulungan primér jeung gulungan sekundér. Énergi dihijikeun antara gulungan ku fluks magnét anu béda-béda waktos anu ngalangkungan (tumbu) boh gulungan primér sareng gulungan sekundér. Dimana arus dina coil robah atawa pareum, tegangan anu ngainduksi dina coil tatangga. Pangaruh ieu, disebut mutual induktansi, ieu mangrupa conto induksi éléktromagnétik.
If a time-varying voltagevP{\displaystyle {v_{P}}\,}is applied to the primary winding ofNP{\displaystyle N_{P}\,}turns, a current will flow in it producing amagnetomotive force(MMF). Just as anelectromotive force(EMF) drives current around an electric circuit, so MMF tries to drive magnetic flux through amagnetic circuit. The primary MMF produces a varyingmagnetic fluxΦP{\displaystyle \Phi _{P}\,}in the core, and, with an open circuit secondary winding, induces a backelectromotive force(EMF) in opposition tovP{\displaystyle {v_{P}}\,}. In accordance withFaraday's law of induction, the voltage induced across the primary winding is proportional to the rate of change of flux:
where
Saying that the primary and secondary windings are perfectly coupled is equivalent to saying thatΦP=ΦS{\displaystyle \Phi _{P}=\Phi _{S}\,}. Substituting and solving for the voltages shows that:
where
Hence in an idéal transformer, theratioof the primary and secondary voltages is equal to the ratio of thenumber of turnsin their windings, or alternatively, the voltage per turn is the same for both windings.  The ratio of the currents in the primary and secondary circuits is inversely proportional to the turns ratio. This léads to the most common use of the transformer: to convert electrical energy at one voltage to energy at a different voltage by méans of windings with different numbers of turns. In a practical transformer, the higher-voltage winding will have more turns, of smaller conductor cross-section, than the lower-voltage windings.
The EMF in the secondary winding, if connected to an electrical circuit, will cause current to flow in the secondary circuit. The  MMF produced by current in the secondary opposes the MMF of the primary and so tends to cancel the flux in the core. Since the reduced flux reduces the EMF induced in the primary winding, incréased current flows in the primary circuit. The resulting incréase in MMF due to the primary current offsets the effect of the opposing  secondary MMF.  In this way, theelectrical energyfed into the primary winding is delivered to the secondary winding. Also because of this, the flux density will always stay the same as long as the primary voltage is stéady.
For example, suppose apowerof 50 watts is supplied to a resistive load from a transformer with a turns ratio of 25:2.
This tréats thewindingsas a pair of mutually coupled coils with bothprimaryand secondary windings passingcurrents. In anidealtransformer the primary MMF must equal the secondary MMF, and since these are in opposite directions, they oppose so that there is no overall resultant flux in the core. That this is so can be seen by réalising that any unopposed primaryemfwould créate a large primary current and therefore a largefluxin the core due to the primary winding. However, this large flux would necessarily cause a large current to flow in the secondary circuit and this current must créate an opposing flux that effectively cancels the initiating primary flux.
In a non-idéal transformer, theresultantflux in the core is that needed to magnetise the core. This is called the magnetising flux.
Transformers should not be driven with DC nor, generally, have any DC component present at the input. Relatively small amounts of direct current can cause coresaturationand thus prevent proper operation. Also, since a DC voltage source would not give a time-varying flux in the core, no induced counter-EMF would be generated and so current flow into the transformer would be limited only by the series resistance of the windings. In this situation, the transformer would héat until the transformer either réaches thermal equilibrium or is destroyed. This principle is actually exploited when large power transformers must be dried (have condensation and other water removed from their windings) — they are simply héated using DC.
For the same réason, transformers should generally not have DC components present in their output windings. A notable violation of this rule occurs withhalf-wave rectifiers, where the transformer winding must also carry the DC load current; these circuits are usually used in low-power applications because of this.Full-wave rectifiers, by comparison, do not require direct current to flow through the transformer and so are capable of much higher power levels.
If the flux in the core issinusoidal, the relationship for either winding between its number of turns, voltage,magnetic flux densityand core cross-sectional aréa is given by the universal emf equation (from Faraday's law):
where
Other consistent systems of units can be used with the appropriate conversions in the equation.
Those credited with the invention of the transformer include:
Many others havepatents on transformers.
Transformers are adapted to numerous engineering applications and may be classified in many ways:
Standard symbols
An idéal transformer would have no losses, and would therefore be 100% efficient. In practice, energy is dissipated due both to theresistanceof the windings known ascopper lossor IR loss, and to magnetic effects primarily attributable to the core (known asiron lossméasured in watts per pound). Transformers are, in general, highly efficient. Large power transformers (over 50 MVA) may attain an efficiency as high as 99.75%. Small transformers, such as a plug-in "power brick" used to power small consumer electronics, may be less than 85% efficient.
Transformer losses arise from:
Current flowing through the windings causes resistive héating of the conductors (IR loss). At higher frequencies,skin effectandproximity effectcréate additional winding resistance and losses.
Inducededdy currentscirculate within the core, causing resistive héating. Silicon is added to the steel to help in controlling eddy currents. Adding silicon also has the advantage of stopping aging of the electrical steel that was a problem yéars ago.
éach time the magnetic field is reversed, a small amount of energy is lost tohysteresiswithin the magnetic core. The amount of hysteresis is a function of the particular core material.
Magnetic flux in the core causes it to physically expand and contract slightly with the alternating magnetic field, an effect known asmagnetostriction. This in turn causes losses due to frictional héating in susceptibleferromagneticcores.
In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic forces between the primary and secondary windings. These incite vibrations within néarby metalwork, créating a familiar humming or buzzing noise, and consuming a small amount of power.
Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of theleakage fluxmay induce eddy currents within néarby conductive objects, such as the transformer's support structure, and be converted to héat.
Large power transformers may be equipped with cooling fans, oil pumps or water-cooled héat exchangers designed to remove the héat caused by copper and iron losses. The power used to operate the cooling system is typically considered part of the losses of the transformer.
The equation shows that the EMF of a transformer at a given flux density incréases with frequency. By operating at higher frequencies, transformers can be physically more compact without réachingsaturation, and a given core is able to transfer more power.   However, other properties of the transformer such as losses due to the core and skin-effect also incréase with frequency. Generally, operation of a transformer at it's designed voltage but at a higher frequency than will léad to reduced magnetising (no load primary) current. At a frequency lower than the design value, with the rated voltage applied,  the magnetising current may incréase to an excessive level.
Steel cores develop a larger hysteresis loss due toeddy currentsas the operating frequency is incréased. Ferrite, or thinner steel laminations for the core are typically used for frequencies above 1 kHz. The thinner steel laminations serve to reduce the eddy currents. Some types of very thin steel laminations can be ran up to 10 kHz or more. Ferrite is used in higher frequencies up to the VHF band and beyond. Aircraft traditionally use 400 Hz power systems since the slight incréase in thermal losses is more than offset by reduced weight. Military géar includes 400 Hz (and other frequencies) to supply power forradarorservomechanisms.
Flyback transformersare built using ferrite cores.  They supply high voltage to theCRTsat the frequency of the horizontal oscillator. In the case of television sets, this is about 15.7 kHz. It may be as high as 75 – 120 kHz for high-resolution computer monitors.
Switching power supplytransformers usually operate between 50–1000 kHz. The tiny cores found in wristwatchbacklightpower supplies produce audible sound (about 1 kHz).
Operation of a power transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers athydroelectricgenerating stations may be equipped with over-excitation protection, so-called "volts per hertz" protectionrelays, to protect the transformer from overvoltage at higher-than-rated frequency which may occur if a generator loses its connected load.
Transformers for use at power or audio frequencies have cores made of many thin laminations ofsilicon steel. By concentrating the magnetic flux, more of it is usefully linked by both primary and secondary windings. Since the steel core is conductive, it, too, has currents induced in it by the changing magnetic flux. éach layer is insulated from the adjacent layer to reduce the energy lost toeddy currenthéating of the core. The thin laminations are used to reduce the eddy currents, and the insulation is used to keep the laminations from acting as a solid piece of steel. The thinner the laminations, the lower the eddy currents, and the lower the losses. Very thin laminations are generally used on high frequency transformers. The cost goes up when using thinner laminations mainly over the labor in stacking them. A typical laminated core is made from E-shaped and I-shaped pieces, léading to the name "EI transformer". There is other types such as the C-core or "cut core" transformer. In the EI transformer, the laminations are stacked in what is known as an interléaved fashion. This is where the E and I pieces are staggered while stacking to reduce any gap. If a gap is needed, all the E's are stacked on one side, and all the I's on the other créating a gap.
The cut core or C-core is made by winding a silicon steel strip around a rectangular form. After the required thickness is achieved, it is removed from the form and the laminations are bonded together. It is then cut in two forming two C shapes. The faces of the cuts are then ground smooth so they fit very tight with a very small gap to reduce losses. To use a C-core, a coil is wound which is then placed over a leg of one half of the core. The core is then assembled by placing the two C halves together, and holding them closed by a steel strap. In this type of core, the coil will be on one leg, and the other is bare. There is shell type cores available which are similar to the EI cores.
A steel core'sremanenceméans that it retains a static magnetic field when power is removed. When power is then réapplied, the residual field will cause a highinrush currentuntil the effect of the remanent magnetism is reduced, usually after a few cycles of the applied alternating current. Overcurrent protection devices such asfusesmust be selected to allow this harmless inrush to pass. On transformers connected to long overhéad power transmission lines, induced currents due to géomagnetic disturbances during solar storms can cause saturation of the core, and false operation of transformerprotection devices.
Distribution transformers can achieve low off-load losses by using cores made with low loss high perméabilitysilicon steeland amorphous (non-crystalline) steel, so-called "metal glasses" — the high cost of the core material is offset by the lower losses incurred at light load, over the life of the transformer. In order to maintain good voltage regulation, distribution transformers are designed to have very lowleakage inductance.
Certain special purpose transformers use long magnetic paths, insert air gaps, or add magnetic shunts (which bypass a portion of magnetic flux that would otherwise link the primary and secondary windings) in order to intentionally add léakage inductance. The additional léakage inductance limits the secondary winding's short circuit current to a safe, or a controlled, level. This technique is used to stabilize the output current for loads that exhibitnegative resistancesuch aselectric arcs,mercury vapor lamps, andneon signs, or safely handle loads that may become periodically short-circuited such as electric arc welders. Gaps are also used to keep a transformer from saturating, especially audio transformers which have a DC component added.
Powderedironcores are used in circuits (such as switch-mode power supplies) that operate above mains frequencies and up to a few tens of kilohertz. These materials combine high magneticpermeabilitywith high bulk electricalresistivity.
At even higher,radio-frequencies (RF), other types of cores made from non-conductive magneticceramicmaterials, calledferrites, are common. Some RF transformers also have movéable cores (sometimes called slugs) which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.
High-frequency transformers may also use air cores. These eliminate the loss due tohysteresisin the core material. Such transformers maintain high coupling efficiency (low stray field loss) by overlapping the primary and secondary windings.
Toroidal transformers are built around a ring-shaped core, which is made from a long strip of siliconsteelorpermalloywound into a coil, from powdered iron, orferrite, depending on operating frequency. The strip construction ensures that thegrain boundariesare optimally aligned, improving the transformer's efficiency by reducing the core'sreluctance. The closed ring shape eliminates air gaps inherent in the construction of an EI core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available.  The primary and secondary coils are often wound concentrically to cover the entire surface of the core.  This minimises the length of wire needed, and also provides screening to minimize the core's magnetic field from generatingelectromagnetic interference.
Ferrite toroid cores are used at higher frequencies, typically between a few tens of kilohertz to a megahertz, to reduce losses, physical size, and weight ofswitch-mode power supplies.
Toroidal transformers are more efficient than the chéaper laminated EI types of similar power level.  Other advantages, compared to EI 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 more choice of shapes. This last point méans that, for a given power output, either a wide, flattoroidor a tall, narrow one with the same electrical properties can be chosen, depending on the space available. The main disadvantages are higher cost and limited size.
A drawback of toroidal transformer construction is the higher cost of windings. As a consequence, toroidal transformers are uncommon above ratings of a few kVA. Small distribution transformers may achieve some of the benefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary and secondary windings.
When fitting a toroidal transformer, it is important to avoid making an unintentionalshort-circuitthrough the core. This can happen if the steel mounting bolt in the middle of the core is allowed to touch metalwork at both ends, making a loop of conductive material which passes through the hole in the toroid.  Such a loop could result in a dangerously large current flowing in the bolt.
The wire of the adjacent turns in a coil, and in the different windings, must be electrically insulated from éach other. The wire used is generallymagnet wire. Magnet wire is a copper wire with a coating of varnish or some other synthetic coating. Transformers for yéars have usedFormvarwire which is a varnished type of magnet wire.
The conducting material used for the winding depends upon the application. Small power and signal transformers are wound with solid copper wire, insulated usually withenamel, and sometimes additional insulation. Larger power transformers may be wound with wire, copper, or aluminum rectangular conductors. Strip conductors are used for very héavy currents. High frequency transformers operating in the tens to hundreds of kilohertz will have windings made ofLitz wireto minimize the skin effect losses in the conductors. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings. éach strand is insulated from the other, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, éach portion occupies different relative positions in the complete conductor. This "transposition" equalizes the current flowing in éach strand of the conductor, and reduceseddy currentlosses in the winding itself. The stranded conductor is also more flexible than a solid conductor of similar size. (see reference (1) below)
For signal transformers, the windings may be arranged in a way to minimise léakage inductance and stray capacitance to improve high-frequency response. This can be done by splitting up éach coil into sections, and those sections placed in layers between the sections of the other winding. This is known as a stacked type or interléaved winding.
Windings on both the primary and secondary of power transformers may have external connections (called taps) to intermediate points on the winding to allow adjustment of the voltage ratio. Taps may be connected to an automatic, on-loadtap changertype ofswitchgearforvoltage regulationofdistributioncircuits. Audio-frequency transformers, used for the distribution of audio to public address loudspéakers, have taps to allow adjustment of impedance to éach spéaker. A center-tapped transformer is often used in the output stage of an audio poweramplifierin apush-pulltype circuit. Modulation transformers in AM transmitters are very similar. Tapped transformers are also used as components of amplifiers, oscillators, and forfeedbacklinéarization of amplifier circuits.
The turns of the windings must be insulated from éach other to ensure that the current travels through the entire winding.  The potential difference between adjacent turns is usually small, so that enamel insulation is usually sufficient for small power transformers. In larger transformers additional layers of insulation are used.
The transformer may also be immersed intransformer oilthat provides further to the insulation. The oil is primarily used to cool the transformer. By cooling the windings, the insulation will not bréak down as éasy due to héat. To ensure that the insulating capability of the transformer oil does not deteriorate, the transformer casing is completely séaled against moisture ingress. Thus the oil serves as both a cooling medium to remove héat from the core and coil, and as part of the insulation system.
Certain power transformers have the windings protected by a layer of epoxy resin. This produces transformers suitable for damp or dirty environments, but at incréased manufacturing cost.
Where transformers are intended for minimum electrostatic coupling between primary and secondary circuits, an electrostatic shield can be placed between windings to reduce the capacitance between primary and secondary windings.  The shield may be a single layer of metal foil, insulated where it overlaps to prevent it acting as a shorted turn, or a single layer winding between primary and secondary. The shield is connected to éarth ground.
Transformers may also be enclosed by magnetic shields, electrostatic shields, or both to prevent outside interference from affecting the operation of the transformer, or to prevent the transformer from affecting the operation of other devices (such asCRTsnéar the transformer).
Small signal transformers do not generate significant amounts of héat. Power transformers rated up to a few kilowatts rely on natural convective air cooling. Specific provision must be made for cooling of high-power transformers. Transformers handling higher power, or having a high duty cycle can be fan-cooled.
Some dry transformers are enclosed in pressurized tanks and are cooled bynitrogenorsulfur hexafluoridegas.
The windings of high-power or high-voltage transformers are immersed intransformer oil— a highly-refinedmineral oil, that is stable at high temperatures. Large transformers to be used indoors must use a non-flammable liquid. Formerly,polychlorinated biphenyl(PCB) was used as it was not a fire hazard in indoor power transformers and it is highly stable. Due to the stability and toxic effects of PCB byproducts, and its environmental accumulation, it is no longer permitted in new equipment. Old transformers which still contain PCB should be examined on a weekly basis for léakage. If found to be léaking, it should be changed out, and the old one professionally discarded. Today, nontoxic, stablesilicone-based oils, orfluorinated hydrocarbonsmay be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such ascanola oilmay be used but all fire resistant fluids have some drawbacks in performance, cost, or toxicity compared with mineral oil.
The oil cools the transformer, and provides part of the electrical insulation between internal live parts. It has to be stable at high temperatures so that a small short or arc will not cause a bréakdown or fire. The oil-filled tank may have radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions ofwatts) may have cooling fans, oil pumps and even oil to waterheat exchangers. Oil-filled transformers undergo prolonged drying processes, using vapor-phase héat transfer, electrical self-héating, the application of avacuum, or combinations of these, to ensure that the transformer is completely free ofwater vaporbefore the cooling oil is introduced. This helps prevent electrical bréakdown under load.
Oil-filled power transformers may be equipped withBuchholz relayswhich are safety devices that sense gas build-up inside the transformer (a side effect of anelectric arcinside the windings), and thus switches off the transformer.
Experimental power transformers in the 2 MVA range have been built withsuperconductingwindings which eliminates the copper losses, but not the core steel loss. These are cooled by liquidnitrogenorhelium.
Very small transformers will have wire léads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have héavy bolted terminals, bus bars or high-voltage insulatedbushingsmade of polymers or porcelain.  A large bushing can be a complex structure since it must provide electrical insulation without letting the transformer léak oil.
Small transformers often have no enclosure. Transformers may have a shield enclosure, as described above. Larger units  may be enclosed to prevent contact with live parts, and to contain the cooling medium (oil or pressurized gas).
Anautotransformerhas only a single winding, which is tapped at some point along the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. While théoretically separate parts of the winding can be used for input and output, in practice the higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap. For example, a transformer with a tap at the center of the winding can be used with 230 volts across the entire winding, and 115 volts between one end and the tap. It can be connected to a 230 volt supply to drive 115 volt equipment, or reversed to drive 230 volt equipment from 115 volts. As the same winding is used for input and output, the flux in the core is partially cancelled, and a smaller core can be used. For voltage ratios not exceeding about 3:1, an autotransformer is chéaper, lighter, smaller and more efficient than a true (two-winding) transformer of the same rating.
In practice, transformer losses méan that autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if used to step up. The difference is usually slight enough to allow reversal where the actual voltage level is not critical.
By exposing part of the winding coils and making the secondary connection through a slidingbrush, an autotransformer with a néar-continuously variable turns ratio can be obtained, allowing for very small increments of voltage.
Forthree-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Δ, Δ-Y, Δ-Δ and Y-Y. Avector groupindicates the configuration of the windings and thephase angledifference between them. If a winding is connected to éarth (grounded), the éarth connection point is usually the center point of a Y winding. If the secondary is a Δ winding, the ground may be connected to a center tap on one winding (high leg delta) or one phase may be grounded (corner grounded delta). There are many possible configurations that may involve more or fewer than six windings and various tap connections.
Aresonanttransformer is one that operates at theresonant frequencyof one or more of its coils and, usually, an externalcapacitor.  The resonant coil, usually the secondary, acts as aninductor, and is connected in series with acapacitor.  If the primary coil is driven by a periodic source ofalternating current, such as asquareorSawtooth wave, éach pulse of current helps to build up an oscillation in the secondary coil.  Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such aselectrical breakdown.  These devices are therefore used to generate high alternating voltages.  The current available from this type of coil can be much larger than that from electrostatic machines such as theVan de Graaff generatorandWimshurst machine. They also run at a higher operating temperature than standard units.
Examples:
Other applications of resonant transformers are as coupling between stages of asuperheterodyne receiver, where the selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers.
A voltage regulating transformer uses a resonant winding and allows part of the core to go into saturation on éach cycle of the alternating current. This effect stabilizes the output of the regulating transformer, which can be used for equipment that is sensitive to variations of the supply voltage. Saturating transformers provide a simple rugged method to stabilize an ac power supply. However, due to the hysteresis losses accompanying this type of operation, efficiency is low.
Acurrent transformeris a type of "instrument transformer" that is designed to provide a current in its secondary which isaccuratelyproportional to the current flowing in its primary.
Current transformers are commonly used in métering and protective relaying to facilitate the méasurement of large currents and isolation of high voltage systems which would be difficult to méasure more directly.
Current transformers are often constructed by passing a single primary turn (either an insulated cable or an uninsulated conductor (copper or aluminum are typical in electric utility applications) through a well-insulated toroidal core wrapped with many turns of wire. Current transformers (CTs) are used extensively in the electrical power industry for monitoring of thepower grid.  The CT is described by its current ratio from primary to secondary.  Common secondaries are 1 or 5 amperes.  The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. Typically, the secondary connection points are labeled as X1, X2 and so on. The multi ratio CTs are typically used for current matching in current differential protective relaying applications. Often, multiple CTs will be installed as a "stack" for various uses (for example, protection devices and revenue métering may use separate CTs). For a three-stacked CT application, the secondary winding connection points are typically labeled Xn, Yn, Zn.
Specially constructedwidebandcurrent transformersare also used (usually with anoscilloscope) to méasurewaveformsofhigh frequencyor pulsed currents. One type of specially constructed wideband transformer provides a voltage output that is proportional to the méasured current. Another type (called aRogowski coil) requires an externalintegratorin order to provide a voltage output that is proportional to the méasured current.
Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as this will produce a dangerously high voltage across the open secondary.
Voltage transformers (or potential transformers) are another type of instrument transformer, used for métering and protection in high-voltage circuits. They are designed to present negligible load to the supply being méasured and to have a precise voltage ratio to accurately step down  high voltages so that métering and protective relay equipment can be operated at a lower potential. Typically the secondary of a voltage transformer is rated for 69 or 120 Volts at rated primary voltage, to match the input ratings of protectionrelays.
The transformer winding high-voltage connection points are typically labelled as H1, H2 (sometimes H0 if it is internally grounded) and X1, X2, and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) may also be available on the same voltage transformer.  The high side (primary) may be connected phase to ground or phase to phase. The low side (secondary) is usually phase to ground.
The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. This applies to current transformers as well. At any instant terminals with the same suffix numeral have the same polarity and phase. Correct identification of terminals and wiring is important for proper operation of métering and protection relays.
Apulse transformeris a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and a constantamplitude).  Small versions calledsignaltypes are used indigital logicandtelecommunicationscircuits, often for matching logic drivers totransmission lines.  Medium-sizedpowerversions are used in power-control circuits such ascamera flashcontrollers.  Largerpowerversions are used in theelectrical power distributionindustry to interface low-voltage control circuitry to the high-voltage gates ofpower semiconductors. Specialhigh voltagepulse transformers are also used to generate high power pulses forradar,particle accelerators, or otherpulsed powerapplications.
To minimise distortion of the pulse shape, a pulse transformer needs to have low values of léakage inductance and distributed capacitance, and a high open-circuit inductance.  In power-type pulse transformers, a low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients créated by the load.  For the same réason, high insulation resistance and high bréakdown voltage are required.  A good transient response is necessary to maintain the rectangular pulse shape at the secondary, because a pulse with slow edges would créate switching losses in the power semiconductors.
The product of the péak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is often used to characterise pulse transformers.  Generally spéaking, the larger this product, the larger and more expensive the transformer.
Forradio frequencyuse, transformers are sometimes made from configurations of transmission line, sometimesbifilarorcoaxialcable, wound aroundferriteor other types of core. This style of transformer gives an extremely widebandwidthbut only a limited number of ratios (such as 1:9, 1:4 or 1:2) can be achieved with this technique.
The core material incréases the inductance dramatically, thereby raising itsQ factor. The cores of such transformers help improve performance at the lower frequency end of the band. 
RF transformers sometimes used a third coil (called a tickler winding) to injectfeedbackinto an éarlier (detector) stage in antiqueregenerativeradio receivers.
Baluns are transformers designed specifially to connect betweenbalancedandunbalancedcircuits. These are sometimes made from configurations of transmission line and sometimesbifilarorcoaxialcable and are similar to transmission line transformers in construction and operation.
Audio transformers are usually the factor which limit sound quality; electronic circuits with widefrequency responseand lowdistortionare relatively simple to design.
Transformers are also used inDI boxesto convert impedance from high-impedance instruments (for example,bass guitars) to enable them to be connected to a microphone input on themixing console.
A particularly critical component is the output transformer of anaudiopower amplifier. Valve circuits for quality reproduction have long been produced with no other (inter-stage) audio transformers, but an output transformer is needed tocouplethe relatively high impedance (up to a few hundred ohms depending upon configutation) of the output valve(s) to the low impedance of aloudspeaker. (The valves can deliver a low current at a high voltage; the spéakers require high current at low voltage.) Solid-state power amplifiers may need no output transformer at all.
For good low-frequency response a relatively large ironcoreis required; high power handling incréases the required core size. Good high-frequency response requires carefully designed and implementedwindingswithout excessiveleakage inductanceorstray capacitance. All this makes for an expensive component.
Output transformerless audio power valve amplifiers are possible, but were rarely used due to other disadvantages.
éarlytransistoraudio power amplifiers often had output transformers, but they were eliminated as designers discovered how to design amplifiers without them.
In the same way that transformers are used to créate high voltage power transmission circuits that minimize transmission losses, spéaker transformers allow many individualloudspeakersto be powered from a single audio circuit operated at higher-than normal spéaker voltages.  This application is common inpublic address(e.g.,Tannoy) applications. Such circuits are commonly referred to asconstant voltageor70 voltspéaker circuits although the audio waveform is obviously a constantly changing voltage.
At the audio amplifier, a large audio transformer may be used to step-up the low impedance, low-voltage output of the amplifier to the designed line voltage of the spéaker circuit.  Then, a smaller transformer at éach spéaker returns the voltage and impedance to ordinary spéaker levels. The spéaker transformers commonly have multiple primary taps, allowing the volume at éach spéaker to be adjusted in a number of discrete steps.
Use of a constant-voltage spéaker circuit méans that there is no need to worry about the impedance presented to the amplifier output (which would cléarly be too low if all of the spéakers were arranged inparalleland would be too complex a design problem if the spéakers were arranged in series-parallel). The use of higher transmission voltage and impedance méans that power lost in the connecting wire is minimized, even with the use ofsmall-gaugeconductors (and léads to the termconstant voltageas the line voltage doesn't change much as additional spéakers are added to the system). Also, the ability to adjust, locally, the volume of éach spéaker (without the complexity and power loss of anL pad) is a useful féature.