Magnetic core

The high permeability, relative to the surrounding air, causes the magnetic field lines to be concentrated in the core material.

"Soft" magnetic materials with low coercivity and hysteresis, such as silicon steel, or ferrite, are usually used in cores.

A former may still be used; a piece of material, such as plastic or a composite, that may not have any significant magnetic permeability but which simply holds the coils of wires in place.

"Soft" (annealed) iron is used in magnetic assemblies, direct current (DC) electromagnets and in some electric motors; and it can create a concentrated field that is as much as 50,000 times more intense than an air core.

In order to reduce the eddy current losses mentioned above, most low frequency power transformers and inductors use laminated cores, made of stacks of thin sheets of silicon steel: Laminated magnetic cores are made of stacks of thin iron sheets coated with an insulating layer, lying as much as possible parallel with the lines of flux.

A small addition of silicon to iron (around 3%) results in a dramatic increase of the resistivity of the metal, up to four times higher.

[citation needed] The higher resistivity reduces the eddy currents, so silicon steel is used in transformer cores.

Further increase in silicon concentration impairs the steel's mechanical properties, causing difficulties for rolling due to brittleness.

Among the two types of silicon steel, grain-oriented (GO) and grain non-oriented (GNO), GO is most desirable for magnetic cores.

Many materials require careful heat treatment to reach their magnetic properties, and lose them when subjected to mechanical or thermal abuse.

[4] High mechanical strength and corrosion resistance are also common properties of metallic glasses which are positive for this application.

[5] Powder cores consist of metal grains mixed with a suitable organic or inorganic binder, and pressed to desired density.

Due to large difference of densities, even a small amount of binder, weight-wise, can significantly increase the volume and therefore intergrain spacing.

Lower permeability materials are better suited for higher frequencies, due to balancing of core and winding losses.

Relatively high hysteresis and eddy current loss, operation limited to lower frequencies (approx.

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, especially higher power ones.

The "C-type" particles can be prepared by heating the E-type ones in hydrogen atmosphere at 400 °C for prolonged time, resulting in carbon-free powders.

Used in applications with high DC current bias (line noise filters, or inductors in switching regulators) or where low residual flux density is needed (e.g. pulse and flyback transformers, the high saturation is suitable for unipolar drive), especially where space is constrained.

This includes coils wound on a plastic or ceramic form in addition to those made of stiff wire that are self-supporting and have air inside them.

The presence of the high permeability core increases the inductance, but the magnetic field lines must still pass through the air from one end of the rod to the other.

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 a 3-legged structure.

This design is excellent for mass production and allows a high power, small volume transformer to be constructed for low cost.

It is popular for applications where the desirable features are: high specific power per mass and volume, low mains hum, and minimal electromagnetic interference.

The main drawback that limits their use for general purpose applications is the inherent difficulty of winding wire through the center of a torus.

Toroids have less audible noise, such as mains hum, because the magnetic forces do not exert bending moment on the core.

This process causes losses, because the domain walls get "snagged" on defects in the crystal structure and then "snap" past them, dissipating energy as heat.

The net energy that flows into the inductor expressed in relationship to the B-H characteristic of the core is shown by the equation[12] This equation shows that the amount of energy lost in the material in one cycle of the applied field is proportional to the area inside the hysteresis loop.

Physical mechanisms for anomalous loss include localized eddy-current effects near moving domain walls.

Material manufacturers provide data on core losses in tabular and graphical form for practical conditions of use.

Magnetic field ( green ) created by a current-carrying winding (red) in a typical magnetic core transformer or inductor, with the iron core C forming a closed loop, possibly with air gaps G in it. The drawing shows a section through the core. The purpose of the core is to provide a closed high permeability path for the magnetic field lines.
B – magnetic field in the core will be approximately constant across any cross section
B F – "fringing fields". In the gaps G the magnetic field lines "bulge" out, so the field strength is less than in the core: B F < B
B L – leakage flux ; magnetic field lines which don't follow complete magnetic circuit
(left) Eddy currents ( I, red ) within a solid iron transformer core. (right) Making the core out of thin laminations parallel to the field ( B, green ) with insulation between them ( C ) limits the eddy currents to circulate within each individual lamination, reducing the total current. In this diagram the field and currents are shown in one direction, but they actually reverse direction with the alternating current in the transformer winding.
Typical EI Lamination.
Ferrite rods are simple cylinders of ferrite that can be wound around.
A toroidal core
On the left, a non-adjustable ferrite rod with connection wires glued to the ends. On the right, a molded ferrite rod with holes, with a single wire threaded through the holes.
A ferrite ring on a computer data cable.