Editing Transformer (section)

====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>