Editing Transformer (section)

==Construction==

===Cores===
;[[File:Transformer winding formats.jpg|thumb|Core form = core type; shell form = shell type]]
Closed-core transformers are constructed in 'core form' or 'shell form'. When windings surround the core, the transformer is core form; when windings are surrounded by the core, the transformer is shell form.<ref name="Del Vecchio2002-10"/> Shell form design may be more prevalent than core form design for distribution transformer applications due to the relative ease in stacking the core around winding coils.<ref name="Del Vecchio2002-10">{{harvnb|Del Vecchio|Poulin|Feghali|Shah|2002|pp=10–11, Fig. 1.8}}</ref> Core form design tends to, as a general rule, be more economical, and therefore more prevalent, than shell form design for high voltage power transformer applications at the lower end of their voltage and power rating ranges (less than or equal to, nominally, 230&nbsp;kV or 75&nbsp;MVA). At higher voltage and power ratings, shell form transformers tend to be more prevalent.<ref name="Del Vecchio2002-10"/><ref name="HRTSG (2005)">{{cite web|author=Hydroelectric Research and Technical Services Group|title=Transformers: Basics, Maintenance, and Diagnostics|url=http://permanent.access.gpo.gov/lps113746/Trnsfrmr.pdf|publisher=U.S. Dept. of the Interior, Bureau of Reclamation|access-date=Mar 27, 2012|page=12}}</ref><ref name="US Army Corps of Engineers1994">{{cite conference|title=EM 1110-2-3006 Engineering and Design – Hydroelectric Power Plants Electrical Design|book-title=Chapter 4 Power Transformers|year=1994|author=US Army Corps of Engineers|page=4-1<!-- hyphen is appropriate here -->|url=https://archive.org/details/U.S._Army_Corps_of_Engineers_Engineering_and_Design_Hydroelectric_Power_Plants_E}}
</ref> Shell form design tends to be preferred for extra-high voltage and higher MVA applications because, though more labor-intensive to manufacture, shell form transformers are characterized as having inherently better kVA-to-weight ratio, better short-circuit strength characteristics and higher immunity to transit damage.<ref name="US Army Corps of Engineers1994"/>

====Laminated steel cores====
[[Image:Transformer.filament.agr.jpg|thumb|Shell type transformer with laminated core showing edges of laminations at the top of the photo]]
[[File:EI-transformer core interleaved with flux paths.png|thumb|Interleaved E-I transformer laminations showing air gap and flux paths]]
Transformers for use at power or audio frequencies typically have cores made of high permeability [[silicon steel]].<ref name="Hindmarsh1977-29">{{harvnb|Hindmarsh|1977|pp=29–31}}</ref> The steel has a permeability many times that of [[free space]] and the core thus serves to greatly reduce the magnetizing current and confine the flux to a path which closely couples the windings.<ref name="Gottlieb">{{harvnb|Gottlieb|1998|p=4}}</ref> Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy current losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires.<ref name="allan"/> Later designs constructed the core by stacking layers of thin steel laminations, a principle that has remained in use. Each lamination is insulated from its neighbors by a thin non-conducting layer of insulation.<ref name="Kulkarni2004-36">{{harvnb|Kulkarni|Khaparde|2004|pp=36–37}}</ref> The [[#Transformer universal EMF equation|transformer universal EMF equation]] can be used to calculate the core cross-sectional area for a preferred level of magnetic flux.<ref name="Say1983"/>

The effect of laminations is to confine [[eddy current]]s to highly elliptical paths that enclose little flux, and so reduce their magnitude. Thinner laminations reduce losses,<ref name="Hindmarsh1977-29"/> but are more laborious and expensive to construct.<ref name="McLyman2004-3-9">{{harvnb|McLyman|2004|pp=3-9 to 3-14}}</ref> Thin laminations are generally used on high-frequency transformers, with some of very thin steel laminations able to operate up to 10&nbsp;kHz.

[[Image:Laminering av kärna.svg|thumb|upright|Laminating the core greatly reduces eddy-current losses]]

One common design of laminated core is made from interleaved stacks of [[E-shaped]] steel sheets capped with [[I-shaped]] pieces, leading to its name of '''E-I transformer'''.<ref name="McLyman2004-3-9"/> Such a design tends to exhibit more losses, but is very economical to manufacture. The cut-core or C-core type is made by winding a steel strip around a rectangular form and then bonding the layers together. It is then cut in two, forming two C shapes, and the core assembled by binding the two C halves together with a steel strap.<ref name="McLyman2004-3-9"/> They have the advantage that the flux is always oriented parallel to the metal grains, reducing reluctance.

A steel core's [[remanence]] means that it retains a static magnetic field when power is removed. When power is then reapplied, 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.<ref name="Harlow2004-2">{{harvnb|Harlow|2004|loc=§2.1.7 & §2.1.6.2.1 in Section §2.1 Power Transformers by H. Jin Sim and Scott H. Digby in Chapter 2 Equipment Types}}</ref> Overcurrent protection devices such as [[fuse (electrical)|fuses]] must be selected to allow this harmless inrush to pass.

On transformers connected to long, overhead power transmission lines, induced currents due to [[Geomagnetically induced current|geomagnetic disturbances]] during [[Geomagnetic storm|solar storms]] can cause [[Saturation (magnetic)|saturation of the core]] and operation of transformer protection devices.<ref>{{Cite journal
| last1 = Boteler| first1 = D. H.| last2 = Pirjola | first2= R. J.| last3 = Nevanlinna | first3 = H.| title = The Effects of Geomagnetic Disturbances On Electrical Systems at the Earth's Surface| journal = Advances in Space Research| doi = 10.1016/S0273-1177(97)01096-X| volume = 22| issue = 1| pages = 17–27| year = 1998| bibcode = 1998AdSpR..22...17B}}</ref>

Distribution transformers can achieve low no-load losses by using cores made with low-loss high-permeability silicon steel or [[Amorphous#Metallic glass|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.<ref>{{cite journal| last =Hasegawa| first = Ryusuke| title = Present Status of Amorphous Soft Magnetic Alloys| journal =Journal of Magnetism and Magnetic Materials| volume =215-216| issue = 1| pages = 240–245| date = June 2, 2000| doi = 10.1016/S0304-8853(00)00126-8 | bibcode =2000JMMM..215..240H}}</ref>

====Solid cores====
Powdered iron cores 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 magnetic permeability with high bulk electrical [[resistivity]]. For frequencies extending beyond the [[Very high frequency|VHF band]], cores made from non-conductive magnetic [[ceramic]] materials called [[ferrite (magnet)|ferrites]] are common.<ref name="McLyman2004-3-9"/> Some radio-frequency transformers also have movable cores (sometimes called 'slugs') which allow adjustment of the [[coupling coefficient (inductors)|coupling coefficient]] (and [[bandwidth (signal processing)|bandwidth]]) of tuned radio-frequency circuits.

====Toroidal cores====
[[Image:Small toroidal transformer.jpg|thumb|right|Small toroidal core transformer]]
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 (magnet)|ferrite]].<ref name="McLyman2004-3-1">{{harvnb|McLyman|2004|p=3-1}}</ref> A strip construction ensures that the [[grain boundary|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.<ref name="Say1983"/> {{rp|485}} 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 minimizes the length of wire needed and provides screening to minimize the core's magnetic field from generating [[electromagnetic interference]].

Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages 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 main disadvantages are higher cost and limited power capacity (see [[#Classification parameters|Classification parameters]] below). Because of the lack 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 of megahertz, to reduce losses, physical size, and weight of inductive components. A drawback of toroidal transformer construction is the higher labor cost of winding. This is because it is necessary to pass the entire length of a coil winding through the core aperture each time a single turn is added to the coil. As a consequence, toroidal transformers rated more than a few kVA are uncommon. Relatively few toroids are offered with power ratings above 10&nbsp;kVA, and practically none above 25&nbsp;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.<ref>{{Cite web|url=http://www.magneticsmagazine.com/main/articles/toroidal-line-power-transformers-power-ratings-tripled/|title=Toroidal Line Power Transformers. Power Ratings Tripled. {{!}} Magnetics Magazine|website=www.magneticsmagazine.com|access-date=2016-09-23|archive-url=https://web.archive.org/web/20160924114636/http://www.magneticsmagazine.com/main/articles/toroidal-line-power-transformers-power-ratings-tripled/|archive-date=2016-09-24|url-status=dead}}</ref>

====Air cores====
A transformer can be produced by placing the windings near each other, an arrangement termed an "air-core" transformer. An air-core transformer eliminates loss due to hysteresis in the core material.<ref name="calvert2001"/> The magnetizing inductance is drastically reduced by the lack of a magnetic core, resulting in large magnetizing currents and losses if used at low frequencies. Air-core transformers are unsuitable for use in power distribution,<ref name="calvert2001"/> but are frequently employed in radio-frequency applications.<ref>{{cite web| first = Reuben| last = Lee| title = Air-Core Transformers| work = Electronic Transformers and Circuits|url=http://www.vias.org/eltransformers/lee_electronic_transformers_07b_22.html | access-date=May 22, 2007}}</ref> Air cores are also used for [[Transformer types#Resonant transformer|resonant transformers]] such as [[tesla coil]]s, where they can achieve reasonably low loss despite the low magnetizing inductance.

===Windings===
[[Image:Transformer-hightolow smaller.jpg|thumb|Windings are usually arranged concentrically to minimize flux leakage.]]
[[Image:transformer min stray field geometry.svg|thumb|left|Cut view through transformer windings.
Legend: <br />
'''White''': Air, liquid or other insulating medium <br />
'''Green spiral''': [[Electrical steel|Grain oriented silicon steel]]<br />
'''Black''': Primary winding <br />
'''Red''': Secondary winding
]]
The electrical conductor used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. For small transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from [[enameled wire|enameled magnet wire]]. Larger power transformers may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of [[Transformerboard|pressboard]].<ref name="CEGB1982">{{harvnb|CEGB|1982|}}</ref>

High-frequency transformers operating in the tens to hundreds of kilohertz often have windings made of braided [[Litz wire]] to minimize the [[Skin effect|skin-effect]] and proximity effect losses.<ref name="dixon">{{Cite conference | first = Lloyd| last = Dixon| book-title = Magnetics Design Handbook| title = Power Transformer Design|url=http://focus.ti.com/lit/ml/slup126/slup126.pdf| year=2001| publisher = Texas Instruments}}</ref> 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.<ref name="CEGB1982"/> Each strand is individually insulated, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions 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.<ref name="CEGB1982"/>

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

{{anchor|Tap_(transformer)}}

Power-frequency transformers may have ''taps'' at intermediate points on the winding, usually on the higher voltage winding side, for voltage adjustment. Taps may be manually reconnected, or a manual or automatic switch may be provided for changing taps. Automatic on-load [[tap changer]]s are used in electric power transmission or distribution, on equipment such as [[arc furnace]] transformers, or for automatic voltage 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. A [[Center tap|center-tapped transformer]] is often used in the output stage of an audio power [[amplifier]] in a [[Push-pull converter|push-pull circuit]]. Modulation transformers in [[Amplitude modulation|AM]] transmitters are very similar.

===Cooling===
[[Image:Drehstromtransformater im Schnitt Hochspannung.jpg|thumb|upright|right|Cutaway view of liquid-immersed 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.]]

It is a rule of thumb that the life expectancy of electrical insulation is halved for about every 7&nbsp;°C to 10&nbsp;°C increase in [[operating temperature]] (an instance of the application of the [[Arrhenius equation]]).<ref name="Harlow2004-3">{{harvnb|Harlow|2004|loc=§3.4.8 in Section 3.4 Load and Thermal Performance by Robert F. Tillman in Chapter 3 Ancillary Topics}}</ref>

Small dry-type and liquid-immersed transformers are often self-cooled by natural convection and [[radiation]] heat dissipation. As power ratings increase, transformers are often cooled by forced-air cooling, forced-oil cooling, water-cooling, or combinations of these.<ref name="Pansini1999-32">{{harvnb|Pansini|1999|p=32}}</ref> Large transformers are filled with [[transformer oil]] that both cools and insulates the windings.<ref name="willis2004">H. Lee  Willis, ''Power Distribution Planning Reference Book'', 2004 CRC Press.  {{ISBN|978-0-8247-4875-3}}, pg. 403</ref> Transformer oil is often a highly refined [[mineral oil]] that cools the windings and insulation by circulating within the transformer tank. The mineral oil and [[electrical insulation paper|paper]] insulation system has been extensively studied and used for more than 100 years. It is estimated that 50% of power transformers will survive 50 years of use, that the average 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.<ref name="Hartley (2003)">{{cite conference|last=Hartley|first=William H. (2003)|title=Analysis of Transformer Failures|url=http://www.bplglobal.net/eng/knowledge-center/download.aspx?id=191|conference=36th Annual Conference of the International Association of Engineering Insurers|access-date=30 January 2013|page=7 (fig. 6)|url-status=dead|archive-url=https://web.archive.org/web/20131020185815/http://www.bplglobal.net/eng/knowledge-center/download.aspx?id=191|archive-date=20 October 2013}}</ref><ref name="Hartley (~2011)">{{cite web|last=Hartley|first=William H. (~2011)|title=An Analysis of Transformer Failures, Part 1 – 1988 through 1997|url=http://www.hsb.com/TheLocomotive/AnAnalysisOfTransformerFailuresPart1.aspx|publisher=The Locomotive|access-date=30 January 2013|archive-date=18 June 2018|archive-url=https://web.archive.org/web/20180618175745/http://www.hsb.com/TheLocomotive/AnAnalysisOfTransformerFailuresPart1.aspx|url-status=dead}}</ref> Prolonged operation at elevated temperature degrades insulating properties of winding insulation and dielectric coolant, which not only shortens transformer life but can ultimately lead to catastrophic transformer failure.<ref name="Harlow2004-3"/> With a great body of empirical study as a guide, [[transformer oil testing]] including [[dissolved gas analysis]] provides valuable maintenance information.

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.<ref name="De Keulenaer2001"/> Air-cooled dry transformers can be more economical 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 natural convection or fins. Some large transformers employ electric fans for forced-air cooling, pumps for forced-liquid cooling, or have [[heat exchangers]] for water-cooling.<ref name="willis2004"/> An oil-immersed transformer may be equipped with a [[Buchholz relay]], which, depending on severity of gas accumulation due to internal arcing, is used to either trigger an alarm or de-energize the transformer.<ref name="Harlow2004-2"/> Oil-immersed transformer installations usually include fire protection measures such as walls, oil containment, and fire-suppression sprinkler systems.

[[Polychlorinated biphenyl]]s (PCBs) have properties that once favored their use as a [[coolant|dielectric coolant]], though concerns over their [[Persistent organic pollutant|environmental persistence]] led to a widespread ban on their use.<ref>{{Cite web| title = ASTDR ToxFAQs for Polychlorinated Biphenyls| year = 2001|url=https://wwwn.cdc.gov/TSP/ToxFAQs/ToxFAQsLanding.aspx?id=140&tid=26
| access-date = June 10, 2007 }}</ref>
Today, non-toxic, stable [[silicone]]-based oils, or [[fluorocarbon|fluorinated hydrocarbons]] may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault.<ref name="De Keulenaer2001"/><ref name="Kulkarni2004-2">{{harvnb|Kulkarni|Khaparde|2004|pp=2–3}}</ref> However, the long life span of transformers can mean that the potential for exposure can be high long after banning.<ref name=":0">{{Cite web|title=What silicone wristbands say about chemical exposure in Uruguayan children|url=http://www.buffalo.edu/news/releases/2020/07/015.html|access-date=2022-01-28|website=www.buffalo.edu|language=en}}</ref>

Some transformers are gas-insulated.  Their windings are enclosed in sealed, pressurized tanks and often cooled by [[nitrogen]] or [[sulfur hexafluoride]] gas.<ref name="Kulkarni2004-2"/>

Experimental power transformers in the 500–1,000&nbsp;kVA range have been built with [[liquid nitrogen]] or [[liquid helium|helium]] cooled [[superconductivity|superconducting]] windings, which eliminates winding losses without affecting core losses.<ref name="Mehta (1997)">{{cite journal|last=Mehta|first=S.P.|author2=Aversa, N.|author3=Walker, M.S.|title=Transforming Transformers [Superconducting windings]|journal=IEEE Spectrum|date=Jul 1997|volume=34|issue=7|pages=43–49|doi=10.1109/6.609815|url=http://www.superpower-inc.com/files/T141+IEEE+Spectrum+XFR.pdf|access-date=14 November 2012}}</ref><ref name="Pansini1999-66">{{harvnb|Pansini|1999|pp=66–67}}</ref>

===Insulation===
[[File:Substation transfomer.jpg|thumb|Substation transformer undergoing testing.]]
Insulation must be provided between the individual turns of the windings, between the windings, between windings and core, and at the terminals of the winding.

Inter-turn insulation of small transformers may be a layer of insulating varnish on the wire. Layer of paper or polymer films may be inserted between layers of windings, and between primary and secondary windings. A transformer may be coated or dipped in a polymer resin to improve the strength of windings and protect them from moisture or corrosion. The resin may be impregnated into the winding insulation using combinations of vacuum and pressure during the coating process, eliminating all air voids in the winding. In the limit, the entire coil may be placed in a mold, and resin cast around it as a solid block, encapsulating the windings.<ref name="Lane (2007)">{{cite web|last=Lane|first=Keith (2007)|title=The Basics of Large Dry-Type Transformers|url=http://ecmweb.com/content/basics-large-dry-type-transformers|publisher=EC&M|access-date=29 January 2013|date=June 2007}}</ref>

Large oil-filled power transformers use windings wrapped with insulating paper, which is impregnated with oil during assembly of the transformer. Oil-filled transformers use highly refined mineral oil to insulate and cool the windings and core. 
Construction of oil-filled transformers requires that the insulation covering the windings be thoroughly dried of residual moisture before the oil is introduced. Drying may be done by circulating hot air around the core, by circulating externally heated transformer oil, or by vapor-phase drying (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.

===Bushings===
Larger transformers are provided with high-voltage insulated [[Bushing (electrical)|bushings]] made of polymers or porcelain. A large bushing can be a complex structure since it must provide careful control of the [[electric field gradient]] without letting the transformer leak oil.<ref name="Ryan2004-416">{{harvnb|Ryan|2004|pp=416–417}}</ref>