Magnetic Core

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The magnetic core is a key component in electrical devices such as electromagnets, transformers and inductors. Its role is to increase the strength and effect of magnetic fields produced by electric currents. 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 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 the ferrite or similar material, and used in radios especially for tuning an inductor. The rod sits in the middle of the coil and small adjustments of the rod position will fine tune the inductance. Often the rod is screw threaded to allow adjustment with a screwdriver. In radio circuits, a dob 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.

Image:U_core.png|a U-shaped core, with sharp cornersImage:C_core.png|the C-shaped core, with rounded corners

"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.

Image:E_core.png|Classical E coreImage:EFD_core.png|The EFD' core allows for construction of inductors or transformers with a lower profileImage: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

"E" and "I" core Sheets of suitable iron stamped out in shapes like the (sans-serif) letters "E" and "I", are stacked with the "I" against the open end of the "E" to form 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.

.

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 and reduce electromagnetic interference.

Pot core Usually ferrite or similar. This is used for inductors and transformers. 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 shielding effect, preventing radiation and reducing electromagnetic interference.

Toroidal core This design is based on a circular toroid, similar in shape to a doughnut. The coil is wound through the hole in the doughnut and around the outside, an ideal coil is distributed evenly all around the circumference of the doughnut. This geometry will turn the magnetic field around into a full loop and thus will naturally keep the majority of the field constrained within the core material. It makes a highly energy efficiency and low radiation transformer, popular in hi-fi audio amplifiers where desirable features are: high power (physics), small volume and minimal electromagnetic interference. It is, however, more difficult to wind an electrical circuit around it than with a splitable core (a core made of two elements, like two E). Automatic winding of a toroidal core requires a specific machinery.

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), 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.

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 of eddy currents within the core causes a resistive loss. The higher the electrical resistance of the core material the lower the loss. Lamination of the core material can reduce eddy current loss.

Movement of magnetic domains As the magnetic field changes, some magnetic domains grow while others shrink,thus the walls of the domains can be said to move. This movement absorbs energy.

Common magnetic core materials Laminated silicon steel Iron is desirable to make magnetic cores, as it can withstand high levels of magnetic field (up to 2.16 Tesla (unit)s at ambient temp Daniel Sadarnac, Les composants magnétiques de l'électronique de puissance, cours de Supélec, mars 2001 french). However, as it is a relatively good conductor, it cannot be used in bulk form: Intense eddy currents 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 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 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 aloying 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 Powdered cores made of carbonyl iron, a highly pure iron, have high stability of parameters across a wide range of temperatures and magnetic flux levels, with excellent Q factors between 50 kHz and 200 MHz. Carbonyl iron powders are basically constituted of micrometer-size balls of iron wrapped in an isolating layer. This is equivalent to a microscopic laminated magnetic circuit (see silicon steel, above), hence reducing the eddy currents.

A popular application of carbonyl iron-based magnetic cores is in broadband inductors.

Iron powder Powdered cores made of hydrogen reduced iron have higher permeability but lower Q. They are used mostly for electromagnetic interference electronic filter and low-frequency chokes, mainly in switched-mode power supply.

Ferrite Ferrite (magnet) are used for high-frequency applications. The ferrite materials can be engineered with a wide range of parameters.

See also

inductor
transformer
Ferrite (magnet)

References

External links

Toroid Winding Calculator - Online calculator for ferrite and iron-powder coil winding calculations.

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