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.
http://en.wikipedia.org/wiki/Ampere's_circuital_lawhttp://en.wikipedia.org/wiki/Ampere's_circuital_lawhttp://en.wikipedia.org/wiki/Electromagnetic_coilhttp://en.wikipedia.org/wiki/Electromagnetic_coilhttp://en.wikipedia.org/wiki/Electromagnetic_coilhttp://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Helixhttp://en.wikipedia.org/wiki/Helixhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Electromagnet#cite_note-Hyperphysics-1http://en.wikipedia.org/wiki/Electromagnet#cite_note-Hyperphysics-1http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Right_hand_grip_rulehttp://en.wikipedia.org/wiki/Right_hand_grip_rulehttp://en.wikipedia.org/wiki/Electromagnet#cite_note-6http://en.wikipedia.org/wiki/Electromagnet#cite_note-7http://en.wikipedia.org/wiki/Electromagnet#cite_note-7http://en.wikipedia.org/wiki/Electromagnet#cite_note-8http://en.wikipedia.org/wiki/Electromagnet#cite_note-8http://en.wikipedia.org/wiki/Electromagnet#cite_note-9http://en.wikipedia.org/wiki/Electromagnet#cite_note-10http://en.wikipedia.org/wiki/Electromagnet#cite_note-10http://en.wikipedia.org/wiki/Electromagnet#cite_note-11http://en.wikipedia.org/wiki/Conventional_currenthttp://en.wikipedia.org/wiki/Conventional_currenthttp://en.wikipedia.org/wiki/Positive_chargehttp://en.wikipedia.org/wiki/Positive_chargehttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Soft_magnetic_materialhttp://en.wikipedia.org/wiki/Soft_magnetic_materialhttp://en.wikipedia.org/wiki/Ferromagnetichttp://en.wikipedia.org/wiki/Ferrimagnetichttp://en.wikipedia.org/wiki/Ferrimagnetichttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Electromagnet#cite_note-Hyperphysics-1http://en.wikipedia.org/wiki/Electromagnet#cite_note-Gates-12http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Electromagnet#cite_note-Shipman-13http://en.wikipedia.org/wiki/Permeability_(electromagnetism)http://en.wikipedia.org/wiki/Permeability_(electromagnetism)http://en.wikipedia.org/wiki/Electromagnet#cite_note-Hyperphysics-1http://en.wikipedia.org/wiki/Electromagnet#cite_note-Hyperphysics-1http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Ampere's_circuital_lawhttp://en.wikipedia.org/wiki/Ampere's_circuital_lawhttp://en.wikipedia.org/wiki/Electromagnetic_coilhttp://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Helixhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Electromagnet#cite_note-Hyperphysics-1http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Right_hand_grip_rulehttp://en.wikipedia.org/wiki/Right_hand_grip_rulehttp://en.wikipedia.org/wiki/Electromagnet#cite_note-6http://en.wikipedia.org/wiki/Electromagnet#cite_note-7http://en.wikipedia.org/wiki/Electromagnet#cite_note-8http://en.wikipedia.org/wiki/Electromagnet#cite_note-9http://en.wikipedia.org/wiki/Electromagnet#cite_note-10http://en.wikipedia.org/wiki/Electromagnet#cite_note-11http://en.wikipedia.org/wiki/Conventional_currenthttp://en.wikipedia.org/wiki/Positive_chargehttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Soft_magnetic_materialhttp://en.wikipedia.org/wiki/Ferromagnetichttp://en.wikipedia.org/wiki/Ferrimagnetichttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Electromagnet#cite_note-Hyperphysics-1http://en.wikipedia.org/wiki/Electromagnet#cite_note-Gates-12http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-2http://en.wikipedia.org/wiki/Electromagnet#cite_note-Shipman-13http://en.wikipedia.org/wiki/Permeability_(electromagnetism)http://en.wikipedia.org/wiki/Electromagnet#cite_note-Hyperphysics-1http://en.wikipedia.org/wiki/Electromagnet#cite_note-Merzouki-28/9/2019 Electromagnet theory
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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
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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&.
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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§ion=9http://en.wikipedia.org/wiki/Loudspeakerhttp://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit§ion=10http://en.wikipedia.org/wiki/Magnetic_circuithttp://en.wikipedia.org/wiki/Ampere_(unit)http://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit§ion=9http://en.wikipedia.org/wiki/Loudspeakerhttp://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit§ion=10http://en.wikipedia.org/wiki/Magnetic_circuithttp://en.wikipedia.org/wiki/Ampere_(unit)8/9/2019 Electromagnet theory
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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#.
http://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit§ion=11http://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit§ion=12http://en.wikipedia.org/wiki/LNCMIhttp://en.wikipedia.org/wiki/LNCMIhttp://en.wikipedia.org/wiki/Direct_currenthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Magnetic_fluxhttp://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit§ion=11http://en.wikipedia.org/w/index.php?title=Electromagnet&action=edit§ion=12http://en.wikipedia.org/wiki/LNCMIhttp://en.wikipedia.org/wiki/Direct_currenthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Magnetic_flux8/9/2019 Electromagnet theory
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"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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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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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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