Magnetic core
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The magnetic core is a key component in electrical and electromechanical devices such as electromagnets, transformers and inductors. 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 magnets. 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.
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[edit] Commonly used magnetic core structures
[edit] 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 threaded 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.
[edit] Single "I" core
Like a cylindrical rod but square, rarely used on its own.
[edit] "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.
 a U-shaped core, with sharp corners |
 the C-shaped core, with rounded banu |
[edit] "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.
 Classical E core |
 The EFD' core allows for construction of inductors or transformers with a lower profile |
 The ER core has a cylindrical central leg. |
 the EP core is halfways between a E and a pot core |
[edit] "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.
[edit] 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 fringing and reduce electromagnetic interference.
[edit] 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.
[edit] Toroidal core
This design is based on a circular toroid (topologically the 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 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 to not exert bending moment on the core. The core is only in compression or tension, and the circular shape is more stable mechanically.
[edit] 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, 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.
[edit] 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:
[edit] 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.
[edit] Eddy current loss
The induction of eddy currents within the core causes a resistive loss. The higher the resistance of the core material the lower the loss. Lamination of the core material can reduce eddy current loss.
[edit] 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.
[edit] 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 materials are used to concentrate the field. Most high-permeability material are ferromagnetic or ferrimagnetic.
[edit] 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.[1]
Iron is desirable to make magnetic cores, as it can withstand high levels of magnetic field (up to 2.16 teslas at ambient temp[2]).
It's also used because, unlike "hard" iron, it does not remain magnetised when the field is removed, which is often important.
[edit] Laminated 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 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
[edit] 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 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.
[edit] 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.
[edit] 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 spheres 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 inductors and transformers.
[edit] Iron powder
Powdered cores made of hydrogen reduced iron have higher permeability but lower Q. They are used mostly for electromagnetic interference filters and low-frequency chokes, mainly in switched-mode power supplies.
[edit] Ferrite
Ferrite ceramics are used for high-frequency applications. The ferrite materials can be engineered with a wide range of parameters.
[edit] See also
[edit] References
- ^ Soft iron core Text - Physics Forums Library
- ^ Daniel Sadarnac, Les composants magnétiques de l'électronique de puissance, cours de Supélec, mars 2001 [in french]
[edit] External links
- Toroid Winding Calculator - Online calculator for ferrite and iron-powder coil winding calculations.
- EMERF, the Electric Motor Education and Research Foundation
