Editing Transformer (section)

===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.