electromagnet theory

8/9/2019 Electromagnet theory

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An electromagnetis a type of magnetin which themagnetic fieldis produced by anelectric

current. The magnetic field disappears when the current is turned off. Electromagnets usually

consist of a large number of closely spaced turns of wire that create the magnetic field. The wire

turns are often wound around a magnetic coremade froma ferromagneticor ferrimagneticmaterial such as iron; the magnetic core concentrates

themagnetic fluxand makes a more powerful magnet.

The main advantage of an electromagnet over apermanent magnetis that the magnetic field can

be quickly changed by controlling the amount of electric current in the winding. However unlike a

permanent magnet that needs no power an electromagnet requires a continuous supply of

electrical energy to maintain a magnetic field.

Electromagnets are widely used as components of other electrical devices suchasmotorsgeneratorsrelays loudspeakershard disks !"# machines scientific instruments

andmagnetic separationequipment. Electomagnets are also employed in industry for picking up

and moving heavy iron ob$ects such as scrap iron and steel. %&'

History%edit'

(turgeon)s electromagnet *+&,

-ne of Henry)s electromagnets that could lift hundreds of pounds *+/s
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0anish scientistHans 1hristian 2rsteddiscovered in *+&/ that electric currents create magnetic

fields. 3ritish scientist 4illiam (turgeoninvented the electromagnet in *+&,.%'%,'His first

electromagnet was a horseshoe5shaped piece of iron that was wrapped with about *+ turns of

bare copper wire 6insulatedwire didn)t exist yet7. The iron was varnishedto insulate it from the

windings. 4hen a current was passed through the coil the iron became magneti8ed and

attracted other pieces of iron; when the current was stopped it lost magneti8ation. (turgeon

displayed its power by showing that although it only weighed seven ounces 6roughly &// grams7

it could lift nine pounds 6roughly , kilos7 when the current of a single5cell battery was applied.

However (turgeon)s magnets were weak because the uninsulated wire he used could only be

wrapped in a single spaced out layer around the core limiting the number of turns.

3eginning in *+&9 :( scientist oseph Henrysystematically improved and populari8ed the

electromagnet.%*,

Electromagnets are very widely used in electric and electromechanicaldevices includingB

!otorsandgenerators

Transformers
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"elays including reed relaysoriginally used in telephone exchanges

Electric bellsand bu88ers

oudspeakersandearphones

Actuators

!agnetic recordingand data storage equipmentB tape recordersC1"shard disks

!"#machines

(cientific equipment such as mass spectrometers

@article accelerators

!agnetic locks

!agnetic separationequipment used for separating magnetic from nonmagnetic

material for example separating ferrous metal from other material in scrap.

#ndustrial lifting magnets

Electromagnetic suspensionused for !ADEC trains

#nduction heatingfor cooking manufacturing and hyperthermia therapy

Electromagnet used in theTevatronparticle accelerator ?ermilab :(A

aboratory electromagnet. @roduces & T field with &/ A current.

!agnet in amass spectrometer
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A1 electromagnet on thestatorof anelectric motor

!agnets in an electric bell

Physics%edit'

The magnetic field lines of a current5carrying loop of wire pass through the center of the loop concentrating the

field there

1urrent 6#7 through a wire produces a magnetic field 637. The field is oriented according to the right5hand rule.
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An electric current flowing in a wire creates a magnetic field around the wire due toAmpere)s

law6see drawing below7. To concentrate the magnetic field in an electromagnet the wire is

wound into acoilwith many turns of wire lying side by side.%&'The magnetic field of all the turns of

wire passes through the center of the coil creating a strong magnetic field there. %&'A coil forming

the shape of a straight tube 6ahelix7 is called a solenoid.%*'%&'

The direction of the magnetic field through a coil of wire can be found from a form of the right5

hand rule.%='%9'%+'%>'%*/'%**'#f the fingers of the right hand are curled around the coil in the direction of

current flow 6conventional currentflow ofpositive charge7 through the windings the thumb

points in the direction of the field inside the coil. The side of the magnet that the field lines

emerge from is defined to be the north pole.

!uch stronger magnetic fields can be produced if a magnetic core of

a softferromagnetic6orferrimagnetic7 material such as iron is placed inside the coil.%*'%*&'%&'%*'A

core can increase the magnetic field to thousands of times the strength of the field of the coilalone due to the high magnetic permeabilityF of the material.%*'%&'This is called a ferromagnetic5

core or iron5core electromagnet. However not all electromagnets use cores and the very

strongest electromagnets such as superconducting and the very high current electromagnets

which have important uses cannot use them due to saturation.
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Ampere's law%edit'

For definitions of the variables below, see box at end of article.

The magnetic field of electromagnets in the general case is given byAmpere)s awB

which says that the integral of the magneti8ing field H around any closed loop of the field is

equal to the sum of the current flowing through the loop. Another equation used that gives

the magnetic field due to each small segment of current is the3iotG(avart law. 1omputing

the magnetic field and force exerted by ferromagnetic materials is difficult for two reasons.

?irst because the strength of the field varies from point to point in a complicated wayparticularly outside the core and in air gaps where fringing fieldsand leakage fluxmust be

considered. (econd because the magnetic field 3 and force arenonlinearfunctions of the

current depending on the nonlinear relation between 3 and H for the particular core material

used. ?or precise calculations computer programs that can produce a model of the magnetic

field using the finite element methodare employed.

Magnetic core%edit'

The material of a magnetic core6often made ofironor steel7 is composed of small regions

calledmagnetic domainsthat act like tiny magnets 6seeferromagnetism7. 3efore the current

in the electromagnet is turned on the domains in the iron core point in random directions so

their tiny magnetic fields cancel each other out and the iron has no large scale magnetic

field. 4hen a current is passed through the wire wrapped around the iron its magnetic field

penetrates the iron and causes the domains to turn aligning parallel to the magnetic field

so their tiny magnetic fields add to the wire)s field creating a large magnetic field that

extends into the space around the magnet. The effect of the core is to concentrate the field

and the magnetic field passes through the core more easily than it would pass through air.

The larger the current passed through the wire coil the more the domains align and the

stronger the magnetic field is. ?inally all the domains are lined up and further increases in
http://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit&section=4http://en.wikipedia.org/wiki/Ampere's_Lawhttp://en.wikipedia.org/wiki/Ampere's_Lawhttp://en.wikipedia.org/wiki/Biot%E2%80%93Savart_lawhttp://en.wikipedia.org/wiki/Leakage_fluxhttp://en.wikipedia.org/wiki/Nonlinearhttp://en.wikipedia.org/wiki/Finite_element_methodhttp://en.wikipedia.org/wiki/Finite_element_methodhttp://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit&section=5http://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Magnetic_domainshttp://en.wikipedia.org/wiki/Magnetic_domainshttp://en.wikipedia.org/wiki/Ferromagnetismhttp://en.wikipedia.org/wiki/Ferromagnetismhttp://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit&section=4http://en.wikipedia.org/wiki/Ampere's_Lawhttp://en.wikipedia.org/wiki/Biot%E2%80%93Savart_lawhttp://en.wikipedia.org/wiki/Leakage_fluxhttp://en.wikipedia.org/wiki/Nonlinearhttp://en.wikipedia.org/wiki/Finite_element_methodhttp://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit&section=5http://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Magnetic_domainshttp://en.wikipedia.org/wiki/Ferromagnetism


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current only cause slight increases in the magnetic fieldB this phenomenon is

calledsaturation.

4hen the current in the coil is turned off in the magnetically soft materials that are nearly

always used as cores most of the domains lose alignment and return to a random state and

the field disappears. However some of the alignment persists because the domains have

difficulty turning their direction of magneti8ation leaving the core a weak permanent magnet.

This phenomenon is called hysteresisand the remaining magnetic field is called remanent

magnetism. The residual magneti8ation of the core can be removed by degaussing. #n

alternating current electromagnets such as are used in motors the core)s magnetisation is

constantly reversed and the remanence contributes to the motor)s losses.

Magnetic circuit the constant Bfield approximation%edit'

!agnetic field (green)of a typical electromagnet with the iron core Cforming a closed loop with two air

gaps Gin it.

BG magnetic field in the core

BFG fringing fields. #n the gaps Gthe magnetic field lines bulge out so the field strength is less than

in the coreB BF B

BLGleakage flux;magnetic field lines which don)t follow complete magnetic circuit

LG average length of the magnetic circuit used in eq. * below. #t is the sum of the length Lcorein the iron

core pieces and the lengthLgapin the air gaps G.

3oth the leakage flux and the fringing fields get larger as the gaps are increased reducing the force

exerted by the magnet.
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#n many practical applications of electromagnets such as motors generators transformers

lifting magnets and loudspeakers the iron core is in the form of a loop ormagnetic circuit

possibly broken by a few narrow air gaps.%&'This is because the magnetic field lines are in the

form of closed loops. #ron presents much less resistance 6reluctance7 to the magnetic field

than air so a stronger field can be obtained if most of the magnetic field)s path is within the

core.%&'

(ince most of the magnetic field is confined within the outlines of the core loop this allows a

simplification of the mathematical analysis.%&'(ee the drawing at right. A common simplifying

assumption satisfied by many electromagnets which will be used in this section is that the

magnetic field strength Bis constant around the magnetic circuitand 8ero outside it. !ost of

the magnetic field will be concentrated in the core material (C). 4ithin the core the magnetic

field (B)will be approximately uniform across any cross section so if in addition the core has

roughly constant area throughout its length the field in the core will be constant. %&'This $ust

leaves the air gaps (G) if any between core sections. #n the gaps the magnetic field lines

are no longer confined by the core so they )bulge) out beyond the outlines of the core before

curving back to enter the next piece of core material reducing the field strength in the gap.

%&'The bulges (BF)are called fringing fields.%&'However as long as the length of the gap is

smaller than the cross section dimensions of the core the field in the gap will be

approximately the same as in the core. #n addition some of the magnetic field lines (BL)will

take )short cuts) and not pass through the entire core circuit and thus will not contribute to

the force exerted by the magnet. This also includes field lines that encircle the wire windings

but do not enter the core. This is called leakage flux. Therefore the equations in this sectionare valid for electromagnets for whichB

*. the magnetic circuit is a single loop of core material possibly broken by a few air

gaps

&. the core has roughly the same cross sectional area throughout its length.

. any air gaps between sections of core material are not large compared with the

cross sectional dimensions of the core.

,. there is negligible leakage flux

The main nonlinear feature of ferromagneticmaterials is that the 3 fieldsaturatesat a certain

value%&'which is around *.= to &teslas6T7 for most high permeability core steels.%*,'%*


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The magnetic field created by an electromagnet is proportional to both the number of turns in

the winding N and the current in the wire I hence this product NI in ampere5turns is given

the name magnetomotive force. ?or an electromagnet with a single magnetic circuitof which

length Lcoreof the magnetic field path is in the core material and length Lgapis in air gaps

Ampere)s aw reduces toB%*9'%&'%*+'

where

is themagnetic permeabilityof the core material at the particular Bfieldused.

is the permeability of free space 6or air7; note that in

this definition is amperes.

This is anonlinear equation because the permeabilityof the core varies with the

magnetic field B. ?or an exact solution the value ofat the Bvalue used must be

obtained from the core material hysteresis curve.%&'#f Bis unknown the equation

must be solved by numerical methods. However if the magnetomotive force is well

above saturation so the core material is in saturation the magnetic field will be

approximately the saturation value Bsatfor the material and won)t vary much with

changes in NI. ?or a closed magnetic circuit 6no air gap7 most core materials

saturate at a magnetomotive force of roughly +// ampere5turns per meter of flux

path.

?or most core materials .%*+'(o in equation 6*7

above the second term dominates. Therefore in magnetic circuits with an air gap

the strength of the magnetic field Bdepends strongly on the length of the air gap

and the length of the flux path in the core doesn)t matter much.

Force exerted by magnetic field%edit'

The force exerted by an electromagnet on a section of core material isB

The *.= T limit on the field %*,'%*='mentioned above sets a limit on themaximum

force per unit core area or pressurean iron5core electromagnet can exert;

roughlyB

#n more intuitive units it)s useful to remember that at *T the magnetic

pressure is approximately , atmospheres or kgIcm&.
http://en.wikipedia.org/wiki/Ampere_(unit)http://en.wikipedia.org/wiki/Magnetomotive_forcehttp://en.wikipedia.org/wiki/Magnetic_circuithttp://en.wikipedia.org/wiki/Magnetic_circuithttp://en.wikipedia.org/wiki/Electromagnet#cite_note-17http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Electromagnet#cite_note-Fitzgerald-18http://en.wikipedia.org/wiki/Magnetic_permeabilityhttp://en.wikipedia.org/wiki/Magnetic_permeabilityhttp://en.wikipedia.org/wiki/Ampereshttp://en.wikipedia.org/wiki/Nonlinear_equationhttp://en.wikipedia.org/wiki/Nonlinear_equationhttp://en.wikipedia.org/wiki/Magnetic_permeabilityhttp://en.wikipedia.org/wiki/Hysteresis_loophttp://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Numerical_analysishttp://en.wikipedia.org/wiki/Electromagnet#cite_note-Fitzgerald-18http://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit&section=8http://en.wikipedia.org/wiki/Electromagnet#cite_note-Pauley-14http://en.wikipedia.org/wiki/Electromagnet#cite_note-Pauley-14http://en.wikipedia.org/wiki/Electromagnet#cite_note-Short-16http://en.wikipedia.org/wiki/Magnetic_pressurehttp://en.wikipedia.org/wiki/Magnetic_pressurehttp://en.wikipedia.org/wiki/Magnetic_pressurehttp://en.wikipedia.org/wiki/Magnetic_pressurehttp://en.wikipedia.org/wiki/Ampere_(unit)http://en.wikipedia.org/wiki/Magnetomotive_forcehttp://en.wikipedia.org/wiki/Magnetic_circuithttp://en.wikipedia.org/wiki/Electromagnet#cite_note-17http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Electromagnet#cite_note-Fitzgerald-18http://en.wikipedia.org/wiki/Magnetic_permeabilityhttp://en.wikipedia.org/wiki/Ampereshttp://en.wikipedia.org/wiki/Nonlinear_equationhttp://en.wikipedia.org/wiki/Magnetic_permeabilityhttp://en.wikipedia.org/wiki/Hysteresis_loophttp://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Numerical_analysishttp://en.wikipedia.org/wiki/Electromagnet#cite_note-Fitzgerald-18http://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit&section=8http://en.wikipedia.org/wiki/Electromagnet#cite_note-Pauley-14http://en.wikipedia.org/wiki/Electromagnet#cite_note-Short-16http://en.wikipedia.org/wiki/Magnetic_pressurehttp://en.wikipedia.org/wiki/Magnetic_pressure


10/14

Diven a core geometry the 3 field needed for a given force can be

calculated from 6&7; if it comes out to much more than *.= T a larger core

must be used.

Closed magnetic circuit%edit'

1ross section of lifting electromagnet like that in above photo showing cylindrical construction. The

windings (C)are flat copper strips to withstand the orent8 force of the magnetic field. The core is formed by the

thick iron housing (D)that wraps around the windings.

?or a closed magnetic circuit 6no air gap7 such as would be found in an electromagnet lifting apiece of iron bridged across its poles equation 6*7 becomesB

(ubstituting into 6&7 the force isB

J#t can be seen that to maximi8e the force a core with a short flux path Land a wide cross

sectional areaAis preferred 6this also applies to magnets with an air gap7. To achieve this in

applications like lifting magnets 6see photo above7 and loudspeakersa flat cylindrical design is

often used. The winding is wrapped around a short wide cylindrical core that forms one pole and

a thick metal housing that wraps around the outside of the windings forms the other part of the

magnetic circuit bringing the magnetic field to the front to form the other pole.

Force between electromagnets%edit'

JThe above methods are applicable to electromagnets with a magnetic circuit and do not apply

when a large part of the magnetic field path is outside the core. An example would be a magnet

with a straight cylindrical core like the one shown at the top of this article. ?or electromagnets 6or

permanent magnets7 with well defined )poles) where the field lines emerge from the core the

force between two electromagnets can be found using the )Dilbert model) which assumes the

magnetic field is produced by fictitious )magnetic charges) on the surface of the poles with pole

strength and units ofAmpere5turn meter. !agnetic pole strength of electromagnets can be

found fromB

The force between two poles isB
http://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit&section=9http://en.wikipedia.org/wiki/Loudspeakerhttp://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit&section=10http://en.wikipedia.org/wiki/Magnetic_circuithttp://en.wikipedia.org/wiki/Ampere_(unit)http://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit&section=9http://en.wikipedia.org/wiki/Loudspeakerhttp://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit&section=10http://en.wikipedia.org/wiki/Magnetic_circuithttp://en.wikipedia.org/wiki/Ampere_(unit)


11/14

J

This model doesn)t give the correct magnetic field inside the core and thus gives incorrect

results if the pole of one magnet gets too close to another magnet.

ide effects%edit'

There are several effects other than the production of a magnetic field which are often importantB

!hmic heating%edit'

arge aluminum busbars carrying current into the electromagnets at theK1!#6aboratoire Kational des1hamps !agnLtiques #ntenses7 high field laboratory.

The only power consumed in a 01electromagnet is due to theresistanceof the windings and is

dissipated as heat. (ome large electromagnets require cooling water circulating through pipes in

the windings to carry off the waste heat.

(ince the magnetic field is proportional to the product NI the number of turns in the

windings Nand the current Ican be chosen to minimi8e heat losses as long as their product is

constant. (ince the power dissipation ! " I#$ increases with the square of the current but only

increases approximately linearly with the number of windings the power lost in the windings canbe minimi8ed by reducing Iand increasing the number of turns Nproportionally or using thicker

wire to reduce the resistance. ?or example halving # and doubling K halves the power loss as

does doubling the area of the wire. #n either case increasing the amount of wire reduces the

ohmic losses. ?or this reason electromagnets often have a significant thickness of windings.

However the limit to increasing Nor lowering the resistance is that the windings take up more

room between the magnet)s core pieces. #f the area available for the windings is filled up more

turns require going to a smaller diameter of wire which has higher resistance which cancels the

advantage of using more turns. (o in large magnets there is a minimum amount of heat loss that

can)t be reduced. This increases with the square of the magnetic fluxB#.
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12/14

"nductive voltage spi#es%edit'

An electromagnet is a large inductor and resists changes in the current through its windings. Any

sudden changes in the winding current cause large voltage spikes across the windings. This is

because when the current through the magnet is increased such as when it is turned on energy

from the circuit must be stored in the magnetic field. 4hen it is turned off the energy in the field is

returned to the circuit.

#f an ordinary switchis used to control the winding current this can cause sparks at the terminals

of the switch. This doesn)t occur when the magnet is switched on because the voltage is limited

to the power supply voltage. 3ut when it is switched off the energy in the magnetic field is

suddenly returned to the circuit causing a large voltage spike and an arcacross the switch

contacts which can damage them. 4ith small electromagnets acapacitoris often used across

the contacts which reduces arcing by temporarily storing the current. !ore often a diodeis used

to prevent voltage spikes by providing a path for the current to recirculate through the windinguntil the energy is dissipated as heat. The diode is connected across the winding oriented so it is

reverse5biased during steady state operation and doesn)t conduct. 4hen the supply voltage is

removed the voltage spike forward5biases the diode and the reactive current continues to flow

through the winding through the diode and back into the winding. A diode used in this way is

called a flyback diode.

arge electromagnets are usually powered by variable current electronic power supplies

controlled by a microprocessor which prevent voltage spikes by accomplishing current changes

slowly in gentle ramps. #t may take several minutes to energi8e or deenergi8e a large magnet.

$orent% forces%edit'

#n powerful electromagnets the magnetic field exerts a force on each turn of the windings due to

theorent8 force acting on the moving charges within the wire. The orent8 force is

perpendicular to both the axis of the wire and the magnetic field. #t can be visuali8ed as a

pressure between the magnetic field linespushing them apart. #t has two effects on an

electromagnet)s windingsB

The field lines within the axis of the coil exert a radial force on each turn of the windings tending

to push them outward in all directions. This causes a tensile stressin the wire.

The leakage field lines between each turn of the coil exert a repulsive force between ad$acent

turns tending to push them apart.

The orent8 forces increase with B#. #n large electromagnets the windings must be firmly

clamped in place to prevent motion on power5up and power5down from causing metal fatiguein

the windings. #n the3itterdesign below used in very high field research magnets the windings

are constructed as flat disks to resist the radial forces and clamped in an axial direction to resist

the axial ones.

Core losses%edit'
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13/14

J#nalternating current6A17 electromagnets used in transformers inductorsandA1

motorsandgenerators the magnetic field is constantly changing. This causes energy losses in

theirmagnetic coresthat are dissipated as heat in the core. The losses stem from two

processesB

%dd& currentsB ?rom ?araday)s law of inductionthe changing magnetic field induces

circulating electric currentsinside nearby conductors called eddy currents. The energy in these

currents is dissipated as heat in theelectrical resistanceof the conductor so they are a cause of

energy loss. (ince the magnet)s iron core is conductive and most of the magnetic field is

concentrated thereeddy currentsin the core are the ma$or problem. Eddy currents are closed

loops of current that flow in planes perpendicular to the magnetic field. The energy dissipated is

proportional to the area enclosed by the loop. To prevent them the cores of A1 electromagnets

are made of stacks of thin steel sheets orlaminations oriented parallel to the magnetic field with

an insulating coating on the surface. The insulation layers prevent eddy current from flowing

between the sheets. Any remaining eddy currents must flow within the cross section of each

individual lamination which reduces losses greatly. Another alternative is to use a ferrite core

which is a nonconductor.

'&steresis lossesB "eversing the direction of magneti8ation of the magnetic domainsin the core

material each cycle causes energy loss because of thecoercivityof the material. These losses

are calledhysteresis. The energy lost per cycle is proportional to the area of the hysteresis

loopin the B'graph. To minimi8e this loss magnetic cores used in transformers and other A1

electromagnets are made of soft low coercivity materials such as silicon steelorsoft ferrite.

The energy loss per cycle of the A1 current is constant for each of these processes so the

power loss increases linearly with frequency.

High field electromagnetsuperconducting electromagnets'
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14/14

The most powerful electromagnet in the world the ,< T hybrid 3itter5superconducting magnet at the :( Kational

High !agnetic ?ield aboratory Tallahassee ?lorida :(A

ain article*uperconducting agnet

4hen a magnetic field higher than the ferromagnetic limit of *.= T is neededsuperconducting

electromagnetscan be used. #nstead of using ferromagnetic materials these

use superconductingwindings cooled with liquid helium which conduct current withoutelectrical

resistance. These allow enormous currents to flow which generate intense magnetic fields.

(uperconducting magnets are limited by the field strength at which the winding material ceases

to be superconducting. 1urrent designs are limited to */G&/ T with the current 6&//>7 record of

.+ T.%*>'The necessary refrigeration equipment and cryostatmake them much more expensive

than ordinary electromagnets. However in high power applications this can be offset by lower

operating costs since after startup no power is required for the windings since no energy is lost

to ohmic heating. They are used in particle acceleratorsand !"#machines.

&itter electromagnets

3oth iron5core and superconducting electromagnets have limits to the field they can produce.

Therefore the most powerful man5made magnetic fields have been generated by air+

corenonsuperconducting electromagnets of a design invented by?rancis 3itterin *>

called3itter electromagnets.%&/'#nstead of wire windings a 3itter magnet consists of

a solenoidmade of a stack of conducting disks arranged so that the current moves in a helical

path through them with a hole through the center where the maximum field is created. This

design has the mechanical strength to withstand the extremeorent8 forcesof the field which

increase with B&. The disks are pierced with holes through which cooling water passes to carry

away the heat caused by the high current. The strongest continuous field achieved with a

resistive magnet is 9.< T as of * !arch &/*, produced by a 3itter electromagnet at

the"adboud :niversityHigh ?ield !agnet aboratory inKi$megen Holland.%&*'The previous

record was < T.%*>'The strongest continuous magnetic field ,< T%&/'was achieved in une &///

with a hybrid device consisting of a 3itter magnet inside a superconducting magnet.

xploding electromagnets

The factor limiting the strength of electromagnets is the inability to dissipate the enormous wasteheat so more powerful fields up to *// T%*>'have been obtained from resistive magnets by

sending brief pulses of current through them. The most powerful manmade magnetic fields have

been created by using explosives to compress the magnetic field inside an electromagnet as it is

pulsed. Theimplosioncompresses the magnetic field to values of around */// T %&/'for a few

microseconds. 4hile this method may seem very destructive there are methods to control the

blast so that neither the experiment nor the magnetic structure are harmed by redirecting the

brunt of the force radially outwards. These devices are known as destructive pulsed

electromagnets.%&&'They are used in physicsandmaterials scienceresearch to study the

properties of materials at high magnetic fields.
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