Magnetic core
Encyclopedia
A magnetic core is a piece of magnetic material
with a high permeability used to confine and guide magnetic field
s in electrical, electromechanical and magnetic devices such as electromagnet
s, transformer
s, electric motor
s, inductor
s and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds such as ferrite
s. The high permeability, relative to the surrounding air, causes the magnetic field lines to be concentrated in the core material. The magnetic field is often created by a coil of wire around the core that carries a current. The presence of the core can increase the magnetic field of a coil by a factor of several thousand over what it would be without the core.
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 a device will depend crucially on the following factors:
In many applications it is undesirable for the core to retain magnetization when the applied field is removed. This property, called hysteresis can cause energy losses in applications such as transformers. Therefore 'soft' magnetic materials with low hysteresis, such as silicon steel, rather than the 'hard' magnetic materials used for permanent magnets, are usually used in cores.
circuits to prevent energy losses called core losses that occur in magnetic cores. The absence of normal core losses permits a higher Q factor
, so air core coils are used in high frequency resonant circuits, such as up to a few megahertz. However, losses such as proximity effect
and dielectric losses are still present.
Most commonly made of 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 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.
This type of core is most likely to be found in car ignition coils.
) letters
"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.
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
.
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 shield
ing effect, preventing radiation
and reducing electromagnetic interference
.
(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. The symmetry
of this geometry creates a magnetic field
of circular loops inside the core, and the lack of sharp bends will constrain virtually all of the field to the core material. This not only makes a highly efficient
transformer
, but also reduces the electromagnetic interference
radiated by the coil.
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.
The ring is essentially identical in shape and performance to the toroid, except that inductors commonly pass only through the center of the core, without wrapping around the core multiple times.
The ring core may also be composed of two separate C-shaped hemispheres secured together within a plastic shell, permitting it to be placed on finished cables with large connectors already installed, that would prevent threading the cable through the small inner diameter of a solid ring.
. 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.
or inductor
, some of the power that would ideally be transferred through the device is lost in the core, resulting in heat
and sometimes noise
. There are various reasons for such losses, the primary ones being:
of the core material changes by expansion and contraction of the tiny magnetic domains it is composed of, due to movement of the domain wall
s. This is a lossy process, because the domain walls get "snagged" on defects in the crystal structure and then "snap" past them, dissipating energy as heat. This is called hysteresis loss. It can be seen in the graph of the B field versus the H field for the material, which has the form of a closed loop. The amount of energy lost in the material in one cycle of the applied field is proportional to the area inside the hysteresis loop. Hysteresis loss increases with higher frequencies as more cycles are undergone per unit time.
of eddy current
s 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, as can making the core of a magnetic material nonconductive, like ferrite.
materials are used to concentrate the field.
Most high-permeability material are ferromagnetic or ferrimagnetic.
is used in magnetic assemblies, 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.
Iron is desirable to make magnetic cores, as it can withstand high levels of magnetic field
without saturating
(up to 2.16 tesla
s at ambient temperature.)
It is also used because, unlike "hard" iron, it does not remain magnetised when the field is removed, which is often important in applications where the magnetic field is required to be repeatedly switched.
Unfortunately, due to the electrical conductivity of the metal, at AC frequencies a bulk block or rod of soft iron can often suffer from large eddy currents circulating within it that waste energy and cause undesirable heating of the iron.
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.
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 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. 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.
, 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.
filters
and low-frequency chokes, mainly in switched-mode power supplies
.
are used for high-frequency applications. The ferrite materials can be engineered with a wide range of parameters. As ceramics, they are essentially insulators, which prevents eddy currents, although losses such as hysteresis losses can still occur.
is a variety of alloys that are non-crystalline or glassy. These are being used to create high efficiency transformers. The materials can be highly responsive to magnetic fields for low hysteresis losses and they can also have lower conductivity to reduce eddy current losses. China is currently making wide spread industrial and power grid usage of these transformers for new installations.
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
with a high permeability used to confine and guide magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s in electrical, electromechanical and magnetic devices such as electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
s, transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s, electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s, inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds such as ferrite
Ferrite (magnet)
Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
s. The high permeability, relative to the surrounding air, causes the magnetic field lines to be concentrated in the core material. The magnetic field is often created by a coil of wire around the core that carries a current. The presence of the core can increase the magnetic field of a coil by a factor of several thousand over what it would be without the core.
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 a device will depend crucially on the following factors:
- the geometryGeometryGeometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....
of the magnetic core. - the amount of air gap in the magnetic circuitMagnetic circuitA magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other...
. - the properties of the core material (especially permeabilityPermeability (electromagnetism)In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
and hysteresisHysteresisHysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
). - the operating temperatureOperating temperatureAn operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
of the core. - whether the core is laminated to reduce eddy currents.
In many applications it is undesirable for the core to retain magnetization when the applied field is removed. This property, called hysteresis can cause energy losses in applications such as transformers. Therefore 'soft' magnetic materials with low hysteresis, such as silicon steel, rather than the 'hard' magnetic materials used for permanent magnets, are usually used in cores.
Air core
A coil not containing a magnetic core is called an air core coil. 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. Air core coils generally have a much lower inductance than similarly sized ferromagnetic core coils, but are used in radio frequencyRadio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
circuits to prevent energy losses called core losses that occur in magnetic cores. The absence of normal core losses permits a higher Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
, so air core coils are used in high frequency resonant circuits, such as up to a few megahertz. However, losses such as proximity effect
Proximity effect
Proximity effect may refer to:*Proximity effect *Proximity effect , an increase in bass or low frequency response when a sound source is close to a microphone...
and dielectric losses are still present.
Straight cylindrical rod
Ferrite (iron)
Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material...
or a similar material, and used in radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
s especially for tuning an inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
. The rod sits in the middle of the coil
Coil
A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
and small adjustments of the rod's position will fine tune the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
. Often the rod is thread
Screw thread
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread...
ed to allow adjustment with a screwdriver. In radio circuits, a blob of wax
Wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
or resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
is used once the inductor has been tuned to prevent the core from moving.
The presence of the high permeability core increases the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
but the field must still spread into the air at the ends of the rod. The path through the air ensures that the inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
remains linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
. In this type of inductor radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
occurs at the end of the rod and electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
may be a problem in some circumstances.
Single "I" core
Like a cylindrical rod but square, rarely used on its own.This type of core is most likely to be found in car ignition coils.
"C" or "U" core
U and C-shaped cores are used with I or another C or U core to make a square closed core, the simplest closed core shape. Windings may be put on one or both legs of the core."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."E" and "I" core
Sheets of suitable iron stamped out in shapes like the (sans-serifSans-serif
In typography, a sans-serif, sans serif or san serif typeface is one that does not have the small projecting features called "serifs" at the end of strokes. The term comes from the French word sans, meaning "without"....
) letters
Letter (alphabet)
A letter is a grapheme in an alphabetic system of writing, such as the Greek alphabet and its descendants. Letters compose phonemes and each phoneme represents a phone in the spoken form of the language....
"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.
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 inductorInductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
or transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
. 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
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
.
Pot core
Usually ferrite or similar. This is used for inductorInductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s and transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
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
Bobbin
A bobbin is a spindle or cylinder, with or without flanges, on which wire, yarn, thread or film is wound. Bobbins are typically found in sewing machines, cameras, and within electronic equipment....
). This design of core has a shield
Electromagnetic shielding
Electromagnetic shielding is the process of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive and/or magnetic materials. Shielding is typically applied to enclosures to isolate electrical devices from the 'outside world' and to cables to isolate...
ing effect, preventing radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
and reducing electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
.
Toroidal core
This design is based on a toroidToroid
Toroid may refer to*Toroid , a doughnut-like solid whose surface is a torus.*Toroidal inductors and transformers which have wire windings on circular ring shaped magnetic cores.*Vortex ring, a toroidal flow in fluid mechanics....
(the same shape as a doughnut
Doughnut
A doughnut or donut is a fried dough food and is popular in many countries and prepared in various forms as a sweet snack that can be homemade or purchased in bakeries, supermarkets, food stalls, and franchised specialty outlets...
). 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. The symmetry
Symmetry
Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...
of this geometry creates a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
of circular loops inside the core, and the lack of sharp bends will constrain virtually all of the field to the core material. This not only makes a highly efficient
Energy conversion efficiency
Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...
transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
, but also reduces the electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
radiated by the coil.
It is popular for applications where the desirable features are: high specific power
Specific power
In physics and engineering, surface power density or sometimes simply specific power is power per unit area.-Applications:* The intensity of electromagnetic radiation can be expressed in W/m2...
per mass and volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
, low mains hum
Mains hum
Mains hum, electric hum, or power line hum is an audible oscillation of alternating current at the frequency of the mains electricity, which is usually 50 Hz or 60 Hz, depending on the local power line frequency...
, and minimal electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
. One such application is the power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
for a hi-fi audio amplifier
Audio amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...
. 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.
Ring or bead
The ring core may also be composed of two separate C-shaped hemispheres secured together within a plastic shell, permitting it to be placed on finished cables with large connectors already installed, that would prevent threading the cable through the small inner diameter of a solid ring.
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 boardPrinted circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
. This design is excellent for mass production
Mass production
Mass production is the production of large amounts of standardized products, including and especially on assembly lines...
and allows a high power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
, small volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
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 transformerTransformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
or inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
, some of the power that would ideally be transferred through the device is lost in the core, resulting in heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
and sometimes noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
. There are various reasons for such losses, the primary ones being:
Hysteresis loss
When the magnetic field through the core changes, the magnetizationMagnetization
In classical electromagnetism, magnetization or magnetic polarization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material...
of the core material changes by expansion and contraction of the tiny magnetic domains it is composed of, due to movement of the domain wall
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
s. This is a lossy process, because the domain walls get "snagged" on defects in the crystal structure and then "snap" past them, dissipating energy as heat. This is called hysteresis loss. It can be seen in the graph of the B field versus the H field for the material, which has the form of a closed loop. The amount of energy lost in the material in one cycle of the applied field is proportional to the area inside the hysteresis loop. Hysteresis loss increases with higher frequencies as more cycles are undergone per unit time.
Eddy current loss
The inductionElectromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
of eddy current
Eddy current
Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...
s within the core causes a resistive loss. The higher the resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of the core material the lower the loss. Lamination of the core material can reduce eddy current loss, as can making the core of a magnetic material nonconductive, like ferrite.
Magnetic core materials
Having no magnetically active core material (an "air core") provides very low inductance in most situations, so a wide range of high-permeabilityPermeability (electromagnetism)
In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
materials are used to concentrate the field.
Most high-permeability material are ferromagnetic or ferrimagnetic.
Soft iron
"Soft" ironIron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
is used in magnetic assemblies, 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.
Iron is desirable to make magnetic cores, as it can withstand high levels of magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
without saturating
Saturation (magnetic)
Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetizing field H cannot increase the magnetization of the material further, so the total magnetic field B levels off...
(up to 2.16 tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
s at ambient temperature.)
It is also used because, unlike "hard" iron, it does not remain magnetised when the field is removed, which is often important in applications where the magnetic field is required to be repeatedly switched.
Unfortunately, due to the electrical conductivity of the metal, at AC frequencies a bulk block or rod of soft iron can often suffer from large eddy currents circulating within it that waste energy and cause undesirable heating of the iron.
Laminated silicon steel
Because 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 currentEddy current
Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...
s would appear due to the magnetic field, resulting in huge losses (this is used in induction heating
Induction heating
Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...
).
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, lying, as much as possible, parallel with the lines of flux. 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). Because eddy currents flow around lines of flux, the laminations prevent most of the eddy currents from flowing at all, restricting any flow to much smaller, thinner and thus higher resistance regions. From this, it can be seen that the thinner the laminations, the lower the eddy currents.Silicon alloying
A small addition of siliconSilicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
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 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. 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 ironCarbonyl iron
Carbonyl iron is a highly pure iron, prepared by chemical decomposition of purified iron pentacarbonyl. It usually has the appearance of grey powder, composed of spherical microparticles. Most of the impurities are carbon, oxygen, and nitrogen.BASF invented carbonyl iron powder in 1925, and claims...
, a highly pure iron, have high stability of parameters across a wide range of temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
s and magnetic flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
levels, with excellent Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
s between 50 kHz and 200 MHz. Carbonyl iron powders are basically constituted of micrometer-size sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
s of iron coated in a thin layer of electrical insulation
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal 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
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s and transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s.
Iron powder
Powdered cores made of hydrogen reduced iron have higher permeability but lower Q. They are used mostly for electromagnetic interferenceElectromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
filters
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
and low-frequency chokes, mainly in switched-mode power supplies
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
.
Ferrite
Ferrite ceramicsFerrite (magnet)
Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
are used for high-frequency applications. The ferrite materials can be engineered with a wide range of parameters. As ceramics, they are essentially insulators, which prevents eddy currents, although losses such as hysteresis losses can still occur.
Vitreous Metal
Amorphous metalAmorphous metal
An amorphous metal is a metallic material with a disordered atomic-scale structure. In contrast to most metals, which are crystalline and therefore have a highly ordered arrangement of atoms, amorphous alloys are non-crystalline...
is a variety of alloys that are non-crystalline or glassy. These are being used to create high efficiency transformers. The materials can be highly responsive to magnetic fields for low hysteresis losses and they can also have lower conductivity to reduce eddy current losses. China is currently making wide spread industrial and power grid usage of these transformers for new installations.
See also
- Pole piecePole pieceA pole piece is a structure composed of material of high magnetic permeability that serves to direct the magnetic field produced by a magnet. A pole piece attaches to and in a sense extends a pole of the magnet, hence the name....
- BalunBalunA balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...
- Choke (electronics)Choke (electronics)A choke is a coil of insulated wire, often wound on a magnetic core, used as a passive inductor which blocks higher-frequency alternating current in an electrical circuit while passing signals of much lower frequency and direct current by having an impedance largely determined by reactance, which...
- Electromagnetic interferenceElectromagnetic interferenceElectromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
- Ferrite (iron)Ferrite (iron)Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material...
- InductorInductorAn inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
- TransformerTransformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
- Ferrite (magnet)Ferrite (magnet)Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
- Magnetic core memoryMagnetic core memoryMagnetic-core memory was the predominant form of random-access computer memory for 20 years . It uses tiny magnetic toroids , the cores, through which wires are threaded to write and read information. Each core represents one bit of information...
- Toroidal inductors and transformersToroidal inductors and transformersToroidal inductors and transformers are electronic components, typically consisting of a circular ring-shaped magnetic core of iron powder, ferrite, or other material around which wire is coiled to make an inductor. Toroidal coils are used in a broad range of applications, such as high-frequency...
- Unintentional radiatorUnintentional radiatorAn unintentional radiator or Incidental radiator is any device which creates radio frequency energy within itself, which is then unintentionally radiated from the device...
External links
- http://hyperphysics.phy-astr.gsu.edu/Hbase/magnetic/indtor.html - Online calculator for ferrite coil winding calculations.
- EMERF, the Electric Motor Education and Research Foundation
- What are the bumps at the end of computer cables?
- Understanding Ferrite Inductors, by Murata ManufacturingMurata Manufacturingis a Japanese manufacturer of electronic components, based in Nagaokakyo, Kyoto.Honorary Chairman Akira Murata started Murata Manufacturing as a personal venture in October, 1944. On December, 1950 reorganized the company into Murata Manufacturing Co., Ltd...
- How to use ferrites for EMI suppression by Fair-Rite
- Tekzilla Daily Episode 13, Explanation on Short Podcast
- Transformer Cores - All Possible Maufactured Variants