Magnetic core
1516916
225136462
2008-07-12T02:00:00Z
Wtshymanski
139104
all toroids are circular; copyedit
The '''magnetic core''' is a key component in electrical and electromechanical devices such as [[electromagnet]]s, [[transformer]]s, and [[inductor]]s. A magnetic core is a magnetic material with a high [[magnetic permeability]], but are usually chosen to be magnetically 'soft', that is, they are made of materials that do not maintain a significant magnetic field when external field is removed, unlike 'hard' magnetic materials.
The use of a magnetic core can enormously concentrate the strength and increase the effect of [[magnetic fields]] produced by [[electric currents]] and [[permanent magnet]]s. The properties of the device will depend crucially on the following factors:
* the [[geometry]] of the magnetic core.
* the amount of air gap in the [[magnetic circuit]].
* the properties of the core material (especially [[Permeability (electromagnetism)|permeability]] and [[hysteresis]]).
* the operating [[temperature]] of the core.
* whether the core is [[laminated]] to reduce [[eddy currents]].
== Commonly used magnetic core structures ==
=== Straight cylindrical rod ===
Most commonly made of [[Ferrite (iron)|ferrite]] or a similar material, and used in [[radio]]s especially for tuning an [[inductor]]. The rod sits in the middle of the [[coil]] and small adjustments of the rod's position will fine tune the [[inductance]]. Often the rod is [[screw thread|thread]]ed to allow adjustment with a screwdriver. In radio circuits, a blob of [[wax]] or [[resin]] is used once the inductor has been tuned to prevent the core from moving.
The presence of the high permeability core increases the [[inductance]] but the field must still spread into the [[air]] at the ends of the rod. The path through the air ensures that the [[inductor]] remains [[linear]]. In this type of inductor [[radiation]] occurs at the end of the rod and [[electromagnetic interference]] may be a problem in some circumstances.
=== Single "I" core ===
Like a cylindrical rod but square, rarely used on its own.
=== "C" or "U" core ===
''U'' and ''C''-shaped cores are the simplest solution to form a closed magnetic circuit, when used alongside a ''I'' or another ''C'' or ''U' core.
<gallery>
Image:U_core.png|a U-shaped core, with sharp corners
Image:C_core.png|the C-shaped core, with rounded corners
</gallery>
=== "E" core ===
E-shaped core are more symmetric solutions to form a closed magnetic system. Most of the time, the electric circuit is wound around the center leg, whose section area is twice that of each individual outer leg.
<gallery>
Image:E_core.png|Classical ''E'' core
Image:EFD_core.png|The ''EFD''' core allows for construction of inductors or transformers with a lower profile
Image:ER_core.png|The ''ER'' core has a cylindrical central leg.
Image:EP_core.png|the ''EP'' core is halfways between a ''E'' and a ''pot'' core
</gallery>
==== "E" and "I" core ====
Sheets of suitable iron stamped out in shapes like the ([[sans-serif]]) [[letter]]s "E" and "I", are stacked with the "I" against the open end of the "E" to form a 3-legged structure. Coils can be wound around any leg, but usually the center leg is used. This type of core is much used for power transformers, autotransformers, and inductors.
<!-- diagram would be good here-->
[[Image:ER core assembly.png|thumb|left|200px|Construction of an inductor using two ''ER'' cores, a plastic bobbin and two clips. The bobbin has pins to be soldered to a [[printed circuit board]].]]
[[Image:ER core assembly exploded.png|thumb|250px|Exploded view of the previous figure showing the structure]]
==== Pair of "E" cores ====
Again used for iron cores. Similar to using an "E" and "I" together, a pair of "E" cores will accommodate a larger coil former and can produce a larger [[inductor]] or [[transformer]]. If an air gap is required, the centre leg of the "E" is shortened so that the air gap sits in the middle of the coil to minimise [[magnetic fringing|fringing]] and reduce [[electromagnetic interference]].
{{clear}}
[[Image:RM core.png|thumb|left|150px|a pot core of 'RM' type]]
=== Pot core ===
Usually ferrite or similar. This is used for [[inductor]]s and [[transformer]]s. The shape of a pot core is round with an internal hollow that almost completely encloses the coil. Usually a pot core is made in two halves which fit together around a coil former ([[bobbin]]). This design of core has a [[Electromagnetic shielding|shield]]ing effect, preventing [[radiation]] and reducing [[electromagnetic interference]].
[[Image:Toroid core.png|thumb|150px|A toroidal core]]
=== Toroidal core ===
This design is based on a [[toroid]] (the same shape as a [[doughnut]]). The coil is wound through the hole in the torus and around the outside. An ideal coil is distributed evenly all around the circumference of the torus. This [[geometry]] will turn the [[magnetic field]] around into a full loop and thus will constrain virtually all of the field to the core material. All of the core material is covered with wire, so none of the core is "wasted" on completing the magnetic circuit. This makes a highly [[energy efficiency|efficient]] and low [[radiation]] [[transformer]]. It is popular for applications where the desirable features are: high [[specific power]] per mass and [[volume]], low [[mains hum]], and minimal [[electromagnetic interference]]. One such application is the [[power supply]] for a hi-fi [[audio amplifier]]. The main drawback that limits their use for general purpose applications, is the inherent difficulty of winding wire through the center of a torus. Unlike a split core (a core made of two elements, like a pair of ''E'' cores), specialized machinery is required for automated winding of a toroidal core. Toroids have less audible noise, such as mains hum, because the magnetic forces do not exert bending moment on the core. The core is only in compression or tension, and the circular shape is more stable mechanically.
[[Image:Planar.png|thumb|150px|left|A planar 'E' core]]
=== Planar core ===
A planar core consists of two flat pieces of magnetic material, one above and one below the coil. It is typically used with a flat coil that is part of a [[printed circuit board]]. This design is excellent for [[mass production]] and allows a high [[power (physics)|power]], small [[volume]] [[transformer]] to be constructed for low cost. It is not as ideal as either a '''pot core''' or '''toroidal core''' but costs less to produce.
[[Image:Planar core assembly.png|thumb|250px|left|A planar inductor]]
[[Image:Planar core assembly exploded.png|thumb|250px|left|Exploded view that shows the spiral track made directly on the printed circuit board]]
{{clear}}
== Core loss ==
In a [[transformer]] or [[inductor]], some of the power that would ideally be transferred through the device is lost in the core, resulting in [[heat]]. There are various reasons for such losses, the primary ones being:
=== [[Hysteresis]] loss ===
The larger the area of the [[hysteresis]] loop, the more loss per cycle. Hysteresis loss increases with higher [[frequencies]] as more cycles are undergone per unit time.
=== [[Eddy current]] loss ===
The [[Electromagnetic induction|induction]] of [[eddy current]]s within the core causes a resistive loss. The higher the [[electrical resistance|resistance]] of the core material the lower the loss. [[Lamination]] of the core material can reduce eddy current loss.
=== Movement of [[magnetic domain]]s ===
As the magnetic field changes, some [[magnetic domain]]s grow while others shrink,
thus the walls of the domains can be said to move. This movement absorbs energy.
== Common magnetic core materials ==
Having no magnetically active core material (an "air core") gives incredibly poor performance in most situations, so a wide range of high-[[permeability (electromagnetism)|permeability]] materials are used to concentrate the field.
Most high-permeability material are [[ferromagnetic]] or [[ferrimagnetic]].
===Soft iron===
"Soft" [[iron]] is used in electromagnets and in some electric motors; and it can create a field as much as 50,000 times more intense than with an air core.<ref>[http://www.physicsforums.com/archive/index.php/t-164613.html Soft iron core Text - Physics Forums Library<!-- Bot generated title -->]</ref>
Iron is desirable to make magnetic cores, as it can withstand high levels of [[magnetic field]] (up to 2.16 [[Tesla (unit)|tesla]]s at ambient temp<ref>Daniel Sadarnac, ''Les composants magnétiques de l'électronique de puissance'', cours de Supélec, mars 2001 [in french]</ref>).
It's also used because, unlike "hard" iron, it does not remain magnetised when the field is removed, which is often important.
===Laminated silicon steel===
{{main|Silicon steel}}
Although iron is a relatively good conductor, it cannot be used in bulk form with a rapidly changing field, such as in a transformer, as intense [[eddy current]]s would appear due to the magnetic field, resulting in huge losses (this is used in [[induction heating]]).
Two techniques are commonly used together to increase the resistivity of iron: lamination and alloying of the iron with silicon
====Lamination====
[[Lamination|Laminated]] magnetic cores are made of thin, insulated iron sheets. Using this technique, the magnetic core is equivalent to many individual magnetic circuits, each one receiving only a small fraction of the magnetic flux (because their section is a fraction of the whole core section). Furthermore, these circuits have a [[Electrical resistance|resistance]] that is higher than that of a non-laminated core, also because of their reduced section. From this, it can be seen that the thinner the laminations, the lower the eddy currents.
====Silicon alloying====
A small addition of [[silicon]] to Iron (around 3%) results in a dramatic increase of the resistivity, up to four times higher. Further increase in Silicon concentration impairs the steel's mechanical properties, causing difficulties for rolling.
Among the two types of [[silicon steel]], grain-oriented (GO) and grain non-oriented (GNO), GO is most desirable for magnetic cores. It is [[anisotropic]], offering better magnetic properties than GNO in one direction. As the magnetic field in inductor and transformer cores is static (compared to that in electric motors), it is possible to use GO steel in the preferred orientation.
===carbonyl iron===
{{main|carbonyl iron}}
Powdered cores made of [[carbonyl iron]], a highly pure iron, have high stability of parameters across a wide range of [[temperature]]s and [[magnetic flux]] levels, with excellent [[Q factor]]s between 50 kHz and 200 MHz. Carbonyl iron powders are basically constituted of micrometer-size [[sphere]]s of iron coated in a thin layer of [[electrical insulation]]. This is equivalent to a microscopic laminated magnetic circuit (see silicon steel, above), hence reducing the [[eddy currents]], particularly at very high frequencies.
A popular application of carbonyl iron-based magnetic cores is in high-frequency and broadband [[inductor]]s and [[transformer]]s.
===Iron powder===
Powdered cores made of [[hydrogen reduced iron]] have higher permeability but lower Q. They are used mostly for [[electromagnetic interference]] [[electronic filter|filters]] and low-frequency chokes, mainly in [[switched-mode power supply|switched-mode power supplies]].
===Ferrite===
{{main|Ferrite (magnet)}}
[[Ferrite (magnet)|Ferrite ceramics]] are used for high-frequency applications. The ferrite materials can be engineered with a wide range of parameters.
==See also==
* [[inductor]]
* [[transformer]]
* [[Ferrite (magnet)]]
* [[Magnetic core memory]]
===References===
{{reflist}}
==External links==
* [http://www.66pacific.com/calculators/toroid_calc.aspx Toroid Winding Calculator] - Online calculator for ferrite and iron-powder coil winding calculations.
* [http://www.smma.org/emerf.html EMERF, the Electric Motor Education and Research Foundation]
[[Category:Electromagnetic components]]
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[[fr:Circuit magnétique]]
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