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A magnetic core is the core of an electromagnet or inductor. The properties of an electromagnet or inductor will be influenced by the core with the most important factors being:
- the geometry of the magnetic core.
- the amount of air gap in the magnetic circuit.
- the magnetic core material (especially permeability and hysteresis).
- the temperature of the core.
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 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."E" core
E-shaped core are more symetric solutions to form a closed magnetic system. Most of the time, the electric circuit is winded around the center leg, whose section area is twice that of each individual outer leg."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 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 efficient and low radiation transformer, popular in hi-fi audio amplifiers where desirable features are: high power, 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, small volume transformer to be constructed for low cost. It is not as ideal as either a pot core or torroidal 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 gets worse at lower frequencies.
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.
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 teslas 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
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). Furhermore, 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.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
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 poweder 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 filters and low-frequency chokes, mainly in switched-mode power supplies.Ferrite
Ferrite ceramics are used for high-frequency applications. The ferrite materials can be engineered with a wide range of parameters.
See also
References
"Magnetic core"