Magnetic Flux Controllers in InductionHeating and MeltingRobert Goldstein, Fluxtrol, Inc.

MAGNETIC FLUX CONTROLLERS arematerials other than the copper coil that are usedin induction systems to alter the flow of the mag-netic field. Magnetic flux controllers used inpower supplying components are not consideredin this article.Magnetic flux controllers have been in exis-

tence since the development of the inductiontechnique. Michael Faraday used two coils ofwire wrapped around an iron core in his experi-ments that led to Faraday’s law of electromagneticinduction, which states that the electromotiveforce (emf) induced in a circuit is directly propor-tional to the time rate of change of the magneticflux through the circuit. After the developmentof the induction principle, magnetic flux control-lers, in the formof stacks of laminated steel, foundwidespread use in the development of transfor-mers for more efficient transmission of energy(Ref 1, 2).Magnetic cores gained widespread use in the

transformer industry because they increasedthe amount of magnetic flux produced withthe same alternating current. The higher themagnetic flux, the higher the emf, which resultsin an increase in energy transfer efficiency fromthe primary winding to the secondary winding.Similar to transformers, magnetic cores were

used on early furnaces for induction melting(Ref 1, 2). The benefits of magnetic flux control-lers vary depending on the application. Forinduction heating, magnetic flux controllers canprovide favorable and unfavorable paths for mag-netic flux to flow, resulting in increased heating indesired areas and reduced the heating in undesir-able areas, respectively. Magnetic flux controllersare not used in every induction heating applica-tion, but their use has increased (Ref 3, 4).

Magnetic Circuits in InductionApplications

Induction heating applications are similar totransformers with a short circuited secondarywinding. The primary winding of the circuitis the induction coil and the secondary is the

workpiece. For both cases, there are three closedloops: flow of current in the coil, flow of mag-netic flux, and flow of current in the workpiece.In most cases, the difference between induc-

tion heating applications and transformers is thatthe magnetic circuit is open. The magnetic fieldpath includes not only the area with the control-ler, but also the workpiece surface layer and theair between the surface and controller, whichcannot be changed. Therefore, the reluctance ofthe magnetic path only partially depends on themagnetic permeability of the controller (Ref 3).Reluctance is a term used in magnetics analo-gous to resistance in electricity.Figure 1 shows a diagram of the magnetic

circuit for a single-turn induction coil with amagnetic flux concentrator heating a cylindricalworkpiece from the outside. The magnetic fluxin the system is equal to the ampere turns ofthe coil divided by the reluctance of the magneticcircuit. The reluctance of the magnetic circuitconsists of three basic components: the back pathfor magnetic flux, the coupling gap, and theworkpiece. When a magnetic flux concentrator

is applied, it strongly reduces the reluctance ofthe back path for the magnetic flux (Ref 3). Allinduction heating systems can be described inthis way.The benefits of a magnetic flux concentrator

on the electrical parameters for a given applica-tion depends on the ratio of the reluctance ofthe back path for magnetic flux to the overallreluctance in the system. It is also possible tobreak down basic system components into sub-components to determine the most economicaluse of magnetic flux controllers in a given appli-cation. Other benefits of magnetic flux control-lers, such as shielding areas from heating, canalso be understood by describing induction sys-tems in this manner.

Role of Magnetic Flux Controllersin Induction Systems

Magnetic flux controllers are powerful toolsin induction heating technology. In general,two primary reasons to use magnetic flux con-trollers in induction systems are to reduce andincrease magnetic fields in a given region.Reducing magnetic fields is done by usingeither soft-magnetic materials or electricallyconductive materials in a closed loop perpen-dicular to the flow of the magnetic flux betweenthe coil and the desired lower field area. Soft-magnetic materials improve induction coilparameters, while highly conductive materialshave a negative effect. Soft-magnetic materialsare primarily used to increase magnetic fields.The benefits of using magnetic flux control-

lers in an induction heating system include:

� Improvement of induction coil and processefficiency

� Improvement of coil power factor� Reduction in coil current� Reduction in unintended heating of machine

components� Reduction in undesired heating of areas of

the workpiece

ASM Handbook, Volume 4C, Induction Heating and Heat TreatmentV. Rudnev and G.E. Totten, editors

Copyright # 2014 ASM InternationalW

All rights reservedwww.asminternational.org

Fig. 1 Magnetic circuit in a single turn coil withmagnetic flux concentrator heating cylindricalpart from the outside: F = total. Source: Ref 3

� Precise control of the magnetic field andresulting heat pattern

� Improvement in efficiency of high-frequencypower supplying circuitry

� Reduction of external magnetic fields inclose proximity to the coil

In most applications, more than one of thesebenefits usually occurs (Ref 3–7). Recently,better understanding of the role magnetic fluxcontrollers in induction heating systems has beenobtained through comprehensive studies usingcomputer simulation and experiments, such asthose conducted at Fluxtrol, Inc. Results showthat a magnetic flux controller properly usedis typically beneficial in an induction heatingsystem (Ref 3). Magnetic flux controllers playdifferent roles in induction heating installations.Depending on the application, they are referredto as concentrators, controllers, diverters, cores,impeders, yokes, shunts, and screens. The effectsof controllers in different types of induction heat-ing applications are described here.

Materials for MagneticFlux Control

Two main categories of materials for mag-netic flux control are electrically conductivematerials and magnetic materials. Electricallyconductive materials typically are used in theform of shunts and screens to reduce externalmagnetic fields.Twomain forms ofmagnetic materials are hard

and soft. The difference is the amount of flux den-sity that remains after they are no longer exposedto a magnetic field. Hard magnetic materialsretain a significant amount of the magnetic field,while soft materials retain almost no magneticfield when the source is turned off. Soft magneticmaterials are used almost exclusively as con-centrators, controllers, diverters, cores, impeders,yokes, shunts, and screens for magnetic flux con-trol in induction systems.

Highly Conductive Materialsfor Field Reduction

Highly conductive materials in the form ofclosed loops are commonly used in inductionsystems to reduce the level of magnetic fieldsin certain areas. The effect on the distributionof the magnetic field is described by Lenz’sLaw, which states an induced electromotiveforce always produces a current whose mag-netic field opposes the original change inmagnetic flux. This current creates a field ofreaction that influences the distribution of themagnetic field, and increases both the reluc-tance of the magnetic circuit and the currentin the coil to produce the same amount of heatin the workpiece. These closed loops, com-monly referred to as Faraday, or “robber” ringscarry high frequency currents, which result inheating by Joule losses. The losses reduce the

efficiency of the system, and are not desirable.Therefore, nonmagnetic materials with highelectrical conductivity are preferred for use asFaraday rings. Copper is the most commonmaterial used, but other materials, such as alu-minum, are used due to cost and/or weightconsiderations.

Soft Magnetic Materials forMagnetic Flux Control

Soft magnetic materials most commonly usedin induction systems are laminations and softmagnetic composites. Soft magnetic ferrites areused occasionally in some high-frequency appli-cations. The main requirements are that it shouldhave a relative magnetic permeability >1 andshould not have a good electrically conductivepath for strong eddy current generation.Placing the material in the path of magnetic

flux lowers the reluctance of that part of themagnetic circuit. Therefore, it requires less cur-rent to drive the magnetic flux through thatpart of the circuit, and a higher percentage offlux flows in the magnetic material than wouldflow in the same space containing only air.However, this positive effect has some limita-tions because the magnetic circuit is almostalways open.For the same coil current, workpiece power

increases with increasing concentrator per-meability very fast initially, then approachesthe threshold value asymptotically. At thesame time, losses in the induction coil oftenincrease slowly with increasing concentratorpermeability.Computer simulation was used to demon-

strate the diminishing positive effect of veryhigh permeabilities in induction applications.In a real induction heating application, the mag-netic permeability of a magnetic flux controllerdepends on magnetic flux density, frequency,and temperature of the controller itself (notthe part it is heating). In the study, permeabilityin the magnetic flux controller is considered tobe constant in the cross-section and at a fixedvalue for each calculation. The study was notconducted for a particular material, but is usedonly to show the effect of conductor permeabil-ity on induction coil parameters.Figure 2 shows the effects of magnetic per-

meability on coil current and efficiency forheating a flat plate using a single leg of aninductor at frequencies of 3 and 10 kHz. Simi-lar studies show that in most induction heatingapplications, the threshold value for workpiecepower occurs when concentrator permeabilityis <100. In high-frequency induction heatingapplications, the threshold value occurs atlower levels of permeability. Therefore, incre-asing permeability to higher values will notimprove coil parameters significantly (Ref 3).In some cases, use of materials with higherthan optimal magnetic permeability reducesinduction coil lifetimes without any benefits(Ref 3, 8, 9).

Other important properties of soft magneticmaterials, such as good saturation flux density,stable mechanical properties, low magneticlosses, chemical resistance, and resistance toelevated temperature depend on the application.Laminations are commonly made of coated

thin sheets of silicon electrical steel with 3 or4% silicon. The laminates are cut (by waterjet, laser, CNC, and electrical discharge machin-ing) or stamped to the required shape for use ininduction coils.For induction coils, multiple laminations are

stacked between mechanical supports calledkeepers (Fig. 3). The large lamination crosssection is oriented so it is in the plane of theflow of the magnetic field; intense eddy currentheating occurs if it is not in the plane.Lamination thickness varies based on fre-

quency used to limit eddy-current losses fromthe in-plane magnetic field. The lamination coat-ing prevents an electrical connection betweenindividual laminations. In line-frequency appli-cations, individual laminations are typicallybetween 0.020 to 0.040 in. (0.5 to 1 mm) thick.Laminations can be as thin as 0.002 in. in appli-cations using higher frequencies.Laminations have very high magnetic perme-

ability and saturation flux density. They alsohave high temperature resistance limited pri-marily by the coating. The primary drawbacksof laminations are intense heating in 3-D mag-netic fields and limited frequency range (up toabout 30 kHz).Soft Magnetic Composites consist of a

soft-magnetic component (typically iron andiron-base alloy powder metal) and a dielectriccomponent (usually an organic polymer binder).The soft-magnetic component provides a favor-able path in which the magnetic field can flow.The dielectric component electrically insulatesmagnetic particles from each other to limit eddycurrent losses.Two main forms of soft-magnetic composites

are machinable and formable. Machinablematerials are commonly produced via powdermetallurgy compaction techniques and heattreated to improve magnetic and mechanicalproperties. Machining soft-magnetic compo-sites is easier than machining laminations.The composites are applied on induction coilsusing an adhesive and mechanical supports(Fig. 4).Machinable-soft magnetic composites have

good magnetic permeability and saturation fluxdensity, as well as good temperature resistance,which is limited primarily by the polymerbinder. Unlike laminations, soft-magnetic com-posites can be tailored to work in the entirerange of frequencies for induction heating andmelting, and perform well in 3-D magneticfields (Ref 3–7).Formable soft-magnetic composites are shaped

around the induction-coil surfaces, held in placemechanically, and cured in an oven, which fixestheir shape. Magnetic properties of formablesoft magnetic composites are not as favorable asthose of machinable composites, and are used in

634 / Equipment

instances of irregular geometries and use of amachined material is difficult.Figure 5 shows relative magnetic permeabil-

ity versus magnetic field strength of some com-mon soft-magnetic composites (Ref 4). Theirdiverse properties offer the opportunity for finecontrol using different materials on the sameinduction coil.

Design Guidelines for UsingMagnetic Flux Controllers onInduction Coils

The decision of where to use a magnetic fluxcontroller depends strongly on the shape of theinduction heating coil. Many induction coils are

hybrids and have components made of morethan one of the basic coil types. Therefore, itis important to understand how magnetic fluxflows and how it is affected by a magnetic fluxcontroller to properly apply it on more complexinduction coils.

Effects of Magnetic Flux Controllerson Common Coil Styles

Outer Diameter (OD) Coils. The basicmagnetic circuit for an OD coil (Fig. 1) wasdescribed previously. Computer simulation is

used to visualize and quantify the effects ofa magnetic flux controller on an induction heat-ing system (Ref 3–7). Figure 6 shows magneticfield lines and current density in a single-turnOD coil for heating a cylindrical copper work-piece with and without the use of a magneticflux controller. The same voltage (similar valueof magnetic flux) is applied to each coil. Theprocess was simulated using Cedrat Technolo-gies Flux 2D software. A copper workpiecewas used so current density values in the induc-tion coil and workpiece were similar to makevisualization of the effects easier.The magnetic field is distributed in a smaller

area for the coil with the magnetic flux control-ler, and the resulting current density in the partis concentrated under the coil heating face.Nearly all of the power induced into the partis useful, and leads to an increase in tempera-ture in the desired heating area. Nearly all ofthe current in the coil with the magnetic fluxcontroller flows on the heating face.In the coil without amagnetic flux controller, a

significant portion of the current flows in areasoutside of the heating face, which does not helpheating the part, but instead, draws additionalpower from the power supply. A significant por-tion of current in the coil flows up the sides of theturn and around the back; this current is not use-ful and only leads to additional losses in the coil,busswork, and matching components.The reduction in current in the coil and con-

centration of power under the heating face iscalled the concentrator effect (Ref 3, 10, 11),and can be explained by considering the systemfrom a magnetic circuit point of view (seeFig. 1). The concentrator effect is a result ofthe magnetic flux controller lowering the

Fig. 2 Effect of magnetic permeability on coil current (a) and efficiency (b); curves generated from computer simulation of heating a flat plate using a single leg of an inductor;50 kW in the part under the coil face. Source: Ref 3.

Fig. 3 Induction coil with laminations stacked bet-ween mechanical supports. Courtesy of TuckerInduction.

Fig. 4 Induction coil with soft magnetic composites.Source: Ref 3

Magnetic Flux Controllers in Induction Heating and Melting / 635

reluctance of this portion of the magnetic cir-cuit and reducing the need for current in thisarea of the induction coil to drive the magneticflux around the back path.The primary benefits of magnetic flux con-

trollers on OD induction coils are:

� Higher efficiency� Improved heat pattern control (ability to heat

fillets, not overheat shoulders, obtain sharpertransition zones, etc.)

� Better use of power in the workpiece (energysavings)

� Reduction in the induction coil current(reduced losses in power supplying circuitry)

� Reduction in heating of unintended areas ofthe part

� Reduction in heating of machine/structuralcomponents

� Reduction in external magnetic fields

The aspect ratio of the induction coil must beconsidered to determine the impact of a mag-netic flux controller on an OD coil. Two keyvariables are the ratio of coil length to diameter,and the ratio of the coupling gap to the lengthof the coil. Magnetic flux controllers are mostbenefitial when the length-to-diameter ratio andcoupling gap-to-length ratio are small. The effectsof the magnetic flux controller are reduced asthese ratios increase. For very large coupling gapsrelative to coil length, a magnetic flux controllercan result in lower electrical efficiency.Inner Diameter (ID) Coils. The basic mag-

netic circuit for an ID coil is shown in Fig. 7.The return path for magnetic flux is on theinside of the coil. Power density in the workpieceis proportional to the magnetic flux densitysquared, and flux density is magnetic flux dividedby the cross-sectional area through which it isflowing. The magnetic impedance of an area isdirectly proportional to the length of the regionand inversely proportional to the cross-sectionalarea. Therefore, the magnetic resistance (Rm)component is a higher percentage of the totalreluctance on an ID coil than it is on an OD coil.Due to the larger influence of Rm, the effect

of a magnetic flux controller on ID coils is muchlarger than on OD coils (Ref 3, 12). Benefits ofmagnetic flux controllers on ID induction coilsinclude:

� Improved coil efficiency (energy savings)� Better use of power in the workpiece (energy

savings)� Reduction in induction-coil current (reduced

losses in power supplying circuitry, coil leads)� Reduction in heating of unintended areas of

the part� Improved heat pattern control

Magnetic flux controllers significantly improvenearly all ID coils. To determine the impact

Fig. 5 Magnetic permeability of some Fluxtrol soft-magnetic composite materials as a function of magnetic fieldstrength.

Color Shade ResultsQuantity : |Current density| A/(square mm)

Phase (Deg): 0Scale / Color0 / 14.812514.8125 / 29.62529.625 / 44.437544.4375 / 59.2559.25 / 74.062574.0625 / 88.87588.875 / 103.6875103.6875 / 118.5118.5 / 133.3125133.3125 / 148.125148.125 / 162.9375162.9375 / 177.75177.75 / 192.5625192.5625 / 207.375207.375 / 222.1875222.1875 / 237

Color Shade ResultsQuantity : |Current density| A/(square mm)

Phase (Deg): 0Scale / Color0 / 14.812514.8125 / 29.62529.625 / 44.437544.4375 / 59.2559.25 / 74.062574.0625 / 88.87588.875 / 103.6875103.6875 / 118.5118.5 / 133.3125133.3125 / 148.125148.125 / 162.9375162.9375 / 177.75177.75 / 192.5625192.5625 / 207.375207.375 / 222.1875222.1875 / 237

Fig. 6 Magnetic field lines and current density for a single-turn OD coil with (left) and without (right) a magnetic fluxcontroller. Courtesy of Fluxtrol, Inc. Fig. 7 Magnetic circuit for ID coil. Source: Ref 3

636 / Equipment

on the coil, it is necessary to consider the coilaspect ratio. Similar to OD coils, two key vari-ables are the coil length-to-diameter ratio andthe coupling gap-coil length ratio. Unlike ODcoils, aspect ratios that benefit most from amagnetic controller are inverted. Magnetic fluxcontrollers provide the greatest benefit when thetwo ratios are larger.Linear Coils. Three basic classes of linear

induction coils are:

� Coils with return current on the oppositeside of the part (e.g., single-shot coils, ovalcoils, channel coils)

� Coils with one primary heating leg anddistributed return on the same side of thepart (vertical loop, split-n-return coils)

� Coils with two active heating directionsand return on the same side of the part(hairpin)

The difference between linear induction coilsand cylindrical coils is there is no natural fluxconcentrating/de-concentrating effect from thechange in radius in linear coils. Therefore, theonly other electromagnetic effects present arethe proximity effect between coil and work-piece and the proximity effect between the coilturns themselves in the case where the return ison the same side of the part.A hairpin inductor (magnetic circuit shown

in Fig. 8) is used to illustrate the concept of amagnetic circuit for linear coils. If the coil legsare the same size and part properties under eachleg are the same, each leg produces one half ofthe magnetic flux in the system. Magnetic fluxflows around the legs of each turn. In the region

between the two turns, magnetic flux from eachturn are additive, and the all magnetic flux fromthe system flows through this area. At the sametime, the cross-section through which the mag-netic flux must flow is smaller than the areasof the back path for magnetic flux. Thus, theRm value is high on hairpin coils, and the bulkof Rm resides in the region between the twoturns (Ref 3).This phenomenon also occurs in other types

of linear inductors (e.g., vertical loop, split-n-return) where the return is on the same sideof the inductor; the bulk of Rm resides in thearea between the turns that are in the oppositedirection. Use of a magnetic flux controllerin this area is very beneficial. The benefit isgreatest for aspect ratios where the distancebetween turns relative to the coupling gap issmall. The benefit is less when the heat faceis long and the return is on the opposite sideof the part.

Cooling of Magnetic FluxControllers

The service life of induction heating coilswith magnetic flux controllers varies from daysto years. Cooling the magnetic flux controllerminimizes mechanical damage and increasesservice life.Heat Sources in Magnetic Flux Control-

lers. All magnetic flux controllers generateand absorb heat when exposed to an alternatingmagnetic field. The amount of heat dependson induction system parameters, induction coiland part geometry, magnetic flux controller

material, part temperature, and the atmospheresurrounding the coil.Highly conductive nonmagnetic materials are

heated by eddy currents, radiation and conduc-tion from the hot part, and convection fromthe hot environment. The main heat source isfrom eddy currents. Because Faraday rings usethe eddy-current reaction field, the primaryways to reduce losses is to use low-resistivitymaterials and increase the length of the shield-ing face.Soft-magnetic materials are heated by eddy

currents, heat from thermal sources, hysteresislosses, and sometimes by conductive heat trans-fer from the copper coil. Unlike highly conduc-tive nonmagnetic materials, shielding in soft-magnetic materials is by concentration of fieldsof reaction rather than by reduction of the fields.Therefore, soft-magnetic materials are segmentedperpendicular to the plane of the magnetic fieldat a small fraction of reference depth to minimizeeddy currents.The electrical resistivities of laminations

are similar to those of steels, so individual lam-ination thickness should be between 0.002 to0.040 in. depending on frequency. With properlamination thickness, eddy current losses in2-D magnetic fields make up a small portionof the heat that needs to be removed. In 3-Dmagnetic fields, eddy current losses grow rapidlyin laminations and are larger than the other heatsources (Ref 3).Soft-magnetic composites typically have

electrical resistivities several orders of mag-nitude higher than those for laminations. Ingeneral, composites that will be used in higherfrequency applications should have higherelectrical resistivities. Therefore, the allowablethickness of soft-magnetic composites is muchlarger, enabling fabrication of the concentratorfrom one or only a few components. Globaleddy current losses in soft-magnetic compositesgenerally make up a small fraction of the totalheat (Ref 3).For soft-magnetic composites, besides global

eddy current losses, there are also eddy currentsin the individual magnetic particles. Similar toeddy currents in larger bodies, losses in individ-ual particles are controlled by particle size andcomposition. Smaller particles are typicallyused for higher frequencies. In most inductionapplications, eddy current losses in individualparticles make up only a small portion of thetotal heat (Ref 3).When the correct magnetic flux controller is

used, hysteresis losses make up the largest com-ponent of heat that must be removed. Hysteresislosses are due to internal friction of magneticdomains as they reorient in the presence of analternating magnetic field.A hysteresis loop is used to characterize mag-

netic hysteresis for a given magnetic material(Fig. 9). Key points on the curve are saturationflux density, remanence, and coercive force. Sat-uration flux density is the threshold value forthe magnetization of the material. Remanenceis the magnetism that remains after removingFig. 8 Magnetic circuit for a hairpin inductor coil. Source: Ref 3.

Magnetic Flux Controllers in Induction Heating and Melting / 637

the source of the magnetic field. Coercive forceis a measure of how much magnetic field mustbe applied in the opposite direction to removethe remanence.Losses in a magnetic material due to hystere-

sis are proportional to the product of the area ofthe hysteresis loop and the frequency. For goodsoft-magnetic materials, the hysteresis loopshould be as narrow as possible. Minimizinghysteresis losses is accomplished through alloy-ing elements and by stress relieving the mate-rial (Ref 3).Heat Removal in Magnetic Flux Control-

lers. The cooling method depends on thetype of magnetic flux controller used and theprocess environment. Power density losses inFaraday rings are higher than in soft-magneticmaterials. However, removing this heat is eas-ier due to the nature of the materials. Depend-ing on the magnitude of eddy current losses,Faraday rings can be cooled using internalwater cooling, free convection, and conductionto a large structure.Losses in soft-magnetic materials (approxi-

mately directly proportional to frequency) aresmall compared with those in the copper coil,which are usually proportional to the square rootof frequency. For low frequencies, the ratiobetween copper losses and soft magnetic lossesis large. It is smaller at higher frequencies, butcopper losses are still the dominant factor intotal coil losses below several MHz.For soft-magnetic materials operating at low

frequencies or low power densities, cooling byfree convection is sufficient to maintain a satis-factory temperature provided the atmosphere is ata temperature low enough to remove heat and themagnetic core is not continuously exposed todirect radiation from a hot part.For medium frequencies and power densities,

the most common method to remove heat is con-duction to the water-cooled copper (Fig. 10).A thermally conductive adhesive is placedbetween the soft-magnetic material and coppertubing to facilitate heat removal. Desirablethermal conductivities of laminations and soft-magnetic composites are 15 to 100 timeslower than that for copper, and the thermal con-ductivity of adhesives is even lower. Adhesivelayer thickness, consistency of gap filling,and adhesive thermal conductivity each play acritical role in heat removal from the magneticflux controller. The use of a 0.010-in. (0.25-mm)layer of an epoxy adhesive (1 W/mK thermalconductivity) with Fluxtrol materials is suffi-cient for heat removal in most cases (Ref 4).In instances of heavy loading, a supplementalwater-cooling plate applied to the backsideof the magnetic flux controller increases heatextraction.Direct water cooling could be required in appli-

cations involving very heavy loading. Coolingmethods include water channels in the concentra-tor (for soft-magnetic composite only, Fig. 11),supplemental water drizzle and quench head forquench-in-place applications. A coating on watercooling channels is recommended when using

direct water cooling of magnetic flux controllersto prevent water cooling-system contamination.Computer simulation shows that Fluxtrol 100material can withstand continuous operation at

a magnetic flux density of 1 T, a frequency of1 kHz, and direct exposure to radiation from a1200 �C (2190 �F) part using direct water cooling(Fig. 12).

Fig. 11 Soft magnetic composite material with channels for direct water cooling. Source: Ref 4

Fig. 10 The most common method to remove heat from a magnetic flux controller is by conduction to the water-cooled copper coil. Source: Ref 3

Fig. 9 Hysteresis loop for magnetic materials. Source: Ref 3

638 / Equipment

Determining the Thickness of SoftMagnetic Material

The amount of soft-magnetic materialrequired is determined by considering magneticflux density, mechanical strength, and physicalspace available for the application. Magneticflux density is important because both magneticpermeability and magnetic losses are dependenton it. Magnetic flux density should not exceedthe saturation flux density of the magnetic con-troller unless there is limited available spaceand only partial shielding is required.To determine magnetic loading, it is neces-

sary to determine the magnetic flux density Bin the magnetic material. For a long solenoid,it can be determined from Eq 1 if the magnitudeof voltage on the coil head is known, or fromEq 2 if the magnitude of the coil current isknown. In an electrical circuit, the magneticpermeability of the material is a function ofmagnetic flux density:

B ¼ U

oSN(Eq 1)

B ¼ �IN

l(Eq 2)

where B is magnetic flux density, U is coil volt-age, o is angular frequency, S is cross-sectionalarea, N is number of turns, m is magnetic per-meability, and l is length.For most other coil styles, Eq 1 and 2 can

provide a rough estimate of flux density. Eq 1is better for ID coils. Computer simulation pro-vides the best information.

Magnetic Flux Control in InductionMelting Systems

Three types of induction melting systems arechannel-type, crucible-type, and cold crucible.Only the role of magnetic flux controllers inan induction melting system is discussed here.Details of each type of induction melting sys-tem are discussed elsewhere in this Volume.

Channel Type Furnaces

Channel melting furnaces are similar to eithera transformer or an ID coil (Fig. 13). They shouldalways have a magnetic flux controller or bettersystem efficiency. Channel furnaces operate atlow frequencies and are used primarily to pro-duce large volumes of metal. For this reason,laminations are the material of choice due tolower cost and availability in large sizes.

Crucible Type Furnaces

Crucible type furnaces consist of a solenoidinduction coil and a ceramic refractory or con-ductive (graphite or metal) crucible. Magneticflux controllers are used on these furnaces pri-marily to reduce external magnetic fields, whichcan cause heating of the equipment structure, orcan necessitate a very large footprint due to occu-pational safety limits for human exposure toelectromagnetic fields.Large Low-Frequency Melting Furnaces.

Faraday rings and shunts are used in many largefurnaces; shunts are used on the OD of the

induction coil and a combination of Faradayrings and counterwound turns are used on thetop and bottom of the coil. A crucible meltingfurnace is shown in Fig. 14. The design is typicalof low-frequency (tens to hundreds of Hertz) fur-naces used to melt large volumes of metal.Shunts used in these furnaces generally are

silicon steel laminations contained in a metalsupport structure. Silicon-steel shunts workwell on the back of the induction coil becausethe field is regular, in one direction, and of asimple geometry. Shunt assemblies with lami-nation shunts are less expensive than alternativematerials such as soft-magnetic composites forlarge low-frequency furnaces.Incorporating laminations on the top and bot-

tom of the induction coil is more difficult,which is why a combination of counterwoundturns and/or Faraday rings is used to reducethe level of magnetic fields in this area. Thesemethods effectively control external fields, butreduce coil efficiency.

Fig. 12 Computer simulation of heat removal from soft-magnetic material exposed to a magnetic flux density of 1 T,a frequency of 1 kHz, and a 1200 �C part by direct water cooling. Source: Ref 4

Fig. 13 Schematic of channel-type induction meltingfurnace. Source: Ref 13

Fig. 14 Sketch of induction crucible-type (coreless)melting furnace. Source: Ref 2

Magnetic Flux Controllers in Induction Heating and Melting / 639

An alternative method is to use a soft-magneticcomposite ring on the top of the induction coilbacked by a Faraday ring. The composite stronglyreduces the level of magnetic field to which theFaraday ring is exposed with lower losses thanusing counterwound turns. Losses of this type ofassembly are much lower while providing thesame level of shielding. A drawback of thisapproach is higher initial cost. Capital versusoperating costs should be considered when choos-ing between top and bottom shielding.Medium-Size Medium-Frequency Melting

Furnaces. Magnetic flux controllers are usedfor the same purposes on 1 to 10-kHz frequencymedium-size furnaces as for larger ones, butdifferent materials are used. Many of these fur-naces operate in a controlled atmosphere, wheremagnetic flux controllers are more importantthan in open air furnaces. The controllers couldbe used to shield the chamber from heating.Furnace size must be larger without the use ofmagnetic flux controllers, increasing the capitalcost of the installation.Lamination thickness decreases with increas-

ing frequency making them more difficult towork with. Also, shount pack structural materialsheat up more at higher frequencies, resulting isan increased cost of using laminate shunt packs.Due to these factors, the use of soft-magnetic

composites is becoming more popular formedium-size melting furnaces. Figure 15 showsa medium-size melting coil with a Fluxtrol 100soft-magnetic composite. This makes it possibleto incorporate magnetic poles on the top andbottom of the induction coil, so it is not neces-sary to counterwind the upper and lower turnsor use a Faraday ring. Therefore, the entirelength of the induction coil works together tocreate the internal magnetic field.Another benefit of magnetic flux controllers

in these furnaces is when multiple melting potsare placed in close proximity to each other. Thefurnaces can be controlled using either onepower supply or multiple power supplies. How-ever, the magnetic fields of different pots inter-act if they are placed too close to one another.

This phenomenon is called cross-talking, and itcan cause nonuniform melting pot performance.Magnetic flux controllers greatly reduces theamount of cross-talking.

Cold-Crucible Melting Furnaces

In cold-crucible furnaces, a water-cooled seg-mented copper ring is placed between the

induction coil and the melt. Magnetic flux flowsthrough the slots of the cold crucible and heatsthe melt. Water-cooled Faraday rings are oftenused on the bottom of the furnace. A metal skulltypically forms between the melt and the coppercomponents, which creates a barrier that preventscontamination of the melt by impurities.These furnaces are less efficient than coreless

melting furnaces due to electrical losses inthe segmented copper ring, the lower Faraday

Fig. 15 Medium-sized melting coil with soft-magnetic shunts. Courtesy of Fluxtrol, Inc

Traditional design

PDmax = 216 MW/m3

Color Shade ResultsQuantity : Power density W/(cubic m)Phase (Deg): 0Scale / Color0 / 13.50879E63.50879E6 / 27.01758E627.01758E6 / 40.52636E640.52636E6 / 54.03515E654.03515E6 / 67.54394E667.54394E6 / 81.05274E681.05274E6 / 94.56152E694.56152E6 / 108.0703E6108.0703E6 / 121.57909E6121.57909E6 / 135.08789E6135.08789E6 / 148.59667E6148.59667E6 / 162.10546E6162.10546E6 / 175.61426E6175.61426E6 / 189.12304E6189.12304E6 / 202.63182E6202.63182E6 / 216.14061E6

Modified design

PDmax = 279 MW/m3

Color Shade ResultsQuantity : Power density W/(cubic m)Phase (Deg): 0Scale / Color0 / 17.47247E617.47247E6 / 34.94494E634.94494E6 / 52.41741E652.41741E6 / 69.88988E669.88988E6 / 87.36234E687.36234E6 / 104.83482E6104.83482E6 / 122.30729E6122.30729E6 / 139.77976E6139.77976E6 / 157.25224E6157.25224E6 / 174.7247E6174.7247E6 / 192.19717E6192.19717E6 / 209.66965E6209.66965E6 / 227.14211E6227.14211E6 / 244.61459E6244.61459E6 / 262.08706E6262.08706E6 / 279.55952E6

Fig. 16 Comparison of power density in the melt charge derived from a 2-D simulation using Flux 2D software: (a)traditional induction coil design with laminate shunts, and (b) modified design with a soft-magneticcomposite controller with top and bottom shunts. Source: Ref 14

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ring, and thermal losses of the melt to the water-cooled copper components. Therefore, cold-crucible melting furnaces are typically used onlyin the production ofmetals that are highly reactivewhen molten or that require very low impurities.Cold-crucible processes almost always are carriedout in a controlled atmosphere.Magnetic flux controllers improve the effi-

ciency of cold-crucible furnaces. Similar to othercontrolled-atmospheremelting applications,mag-netic flux controllers are used on the OD of theinduction coil to shield the chamber from heating.Additional improvements are achieved by

using magnetic poles on the top and bottom ofthe induction coil to reduce the losses associatedwith the Faraday rings on the top and bottomof the cold crucible. Studies show up to 50%improvement in induction coil efficiency usingan induction coil with soft-magnetic compositetop and bottom poles compared with a standardcoil design (Fig. 16, 17) (Ref 14).

Magnetic Flux Control inMass-Heating Applications

The role of magnetic flux controllers in aninduction mass-heating system depends on theapplication. Of the many types of inductionmass-heating installations, the largest applica-tion is heating component part prior to a shap-ing operation, such as forging, forming, androlling. The role of magnetic flux controllerswhole-body and local area mass-heating appli-cations is discussed here. Detailed discussionof these types of installations are presentedelsewhere in this Handbook.

Whole-Body Mass Heating

Workpieces are large in most whole-bodymass-heating applications, and the part passes

through the induction coil. Part shape and induc-tion coil length are the primary determinants ofthe type of magnetic flux controller used in thesesystems.Long solenoids are used for long cylindrical

or tubular body workpieces. Soft-magnetic con-trollers do not provide significant benefits forlong solenoids (length = several times the diam-eter). Therefore, several induction coils areused to bring the billet up to the desired temper-ature distribution. In these installations, Fara-day rings at the end of the coils are the onlymagnetic flux controllers used (Fig. 18). Theyperform two functions: shielding mechanicalhandling rolls from heating and preventingcross-talking among multiple coils used in thebillet-heating line.For rectangular workpieces, the benefits of a

magnetic flux controller depend upon the aspectratio of the cross-section to be heated. Forsquare and nearly square parts, guidelines formagnetic flux controller and coil selection aresimilar to those for cylindrical and tubularparts. Magnetic flux controllers play a greaterrole for rectangular parts with one dimensionsignificantly larger than the other (e.g., slabs,sheet, strip). In this case, both longitudinal andtransverse flux heating methods are used.Longitudinal coils generate a uniform tem-

perature in the cross-section. Soft-magneticmaterials are beneficial when the length of theinduction coil is small relative to the perimeterof the coil inside cross-section.Soft-magnetic materials should be used on

transverse flux heating coils (Ref 3, 10, 15).Material selection depends on the frequency.Silicon steel laminations are used for low fre-quencies, and soft magnetic composites areused for middle and higher frequencies. Soft-magnetic composites are also used near theends of low frequency transverse flux coilswhere 3-D magnetic fields are present.

Local Area Mass Heating

In many cases, it is desirable to heat only aportion of the component for forming. Localarea mass heating can be achieved using bothstatic and continuous heating arrangements.For static heating, both single-turn and multi-

turn induction coils are used, where the coil l iscomparable to the coil diameter. Soft-magneticcontrollers applied to the induction coil improvestemperature distribution and coil efficiency due tobetter use of power in the workpiece together withreduced coil current demand.Continuous heating applications use a chan-

nel coil, which can be one turn to heat fasten-ers, for example (Fig. 19) or several turns asin a bar end heater (Fig. 20). Soft-magneticcontrollers help significantly in both cases.For single-turn channel coils, use of soft-

magnetic material reduces the power requiredto heat the desired area of the part by 20 to50% (Ref 3, 4). Soft-magnetic composites arethe material of choice because these applica-tions usually operate at 10 kHz or higher.

Fluxtrol DesignPtotal(kW)

Ptotal(kW)

Pleads Pmelt Pcoil Pcrucible I(A)

122.5 6% 47% 15.5% 31% 1600

Traditional Coil Design

Pleads Pmelt Pcoil Pcrucible I(A)

135 6.5% 31% 21% 41.5% 1900

Fig. 17 Comparison of integral power values for traditional induction coil design with laminate shunts and a Fluxtrol-designed coil with soft-magnetic composite controller withtop and bottom shunts. Source: Ref 14

Fig. 18 Billet-heating coil with Faraday rings on bothends. Courtesy of Pillar Induction, Inc.

Magnetic Flux Controllers in Induction Heating and Melting / 641

The effect of soft-magnetic materials onchannel coils with multiple turns for heatinglonger sections is not as strong as for shortersingle-turn coils; energy savings is typicallybetween 5 and 20% (Ref 3, 4).

Magnetic Flux Control in InductionTube-Welding Applications

Induction tube welding is a major applicationof induction heating, used in high volume pro-duction of carbon-steel tubes. It is also used toproduce stainless steel, copper, and aluminumtubing. Magnetic flux controllers play a crucialrole in tube-welding lines, both in the weldingprocess and post weld heat treatment.

Magnetic Flux Controllers forContinuous Induction Tube Welding

In an induction tube welding process, steelstrip is rolled into the form of a tube and passedthrough a cylindrical induction coil where thestrip edges are heated. The strip passes througha final set of rollers that close the profile of thetube and welding occurs. Induction weldingsystems for steel tubing operate at high fre-quencies (100�500 kHz) and high power (inthe range of 100 kW to a few MWs).Figure 21 shows a schematic of current flow

in the tube in a typical induction welding appli-cation. The induction coil induces a current thatflows around the tube OD under the face of theinduction coil. When the current reaches the openedge of the tube, it turns to close the current loop.Three options for the current to flow are:

1. Along the edges of the steel strip toward therolls, connecting at the weld apex and return-ing back on the opposite edge of the tube

2. Along the edges away from the V-shapegroove, returning on the tube OD of the tubefarther up the line

3. Along the tube ID

The first current path is the most desirableone, the second path is somewhat useful, andthe third path is undesirable. In inductionwelding applications, a soft-magnetic materialis placed inside the tube to reduce heatingalong the ID of the tube and to enhance currentflow along the tube edges toward the Vgroove. The soft-magnetic material on the tubeID of the tube is referred to as an impeder,because it impedes current flow along thetube ID.Impeders are made of soft magnetic ferrites

encased in a fiberglass tube with water cooling.The tube protects the ferrite from metal spumeand provides mechanical support for the brittleferrites. The tube also keeps the ferrites in placeas they often fracture in production due to ther-mal shock. Figure 22 shows some typical impe-ders used in the welding industry. For large pipe

Fig. 20 Continuous bar-end heater with magnetic flux concentrator. Courtesy of Fluxtrol, Inc.

Fig. 19 Channel coil for fastener heating. Courtesy ofFluxtrol, Inc.

Fig. 21 Schematic of a continuous high-frequency induction longitudinal seam tube-welding system. Source: Ref 13

Fig. 22 Impeders for small-tube welding; used forcurrent flow along the tube ID. Source: Ref 16

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welding systems, the impeder consists of multi-ple impeders assembled together, referred to asa cluster impeder (Fig. 23).Ferrites are the material of choice in most

tube-welding applications due to their low cost,high magnetic permeability at lower flux densi-ties, and low losses at high frequencies. Thedrawbacks of ferrites are low-saturation mag-netic flux density, sensitivity to thermal shock,and sensitivity of magnetic properties to tem-perature change.The life of ferrites in the case of small tubes

ranges from several hours to a week. The mag-netic flux density in the impeder core is higherdue to the smaller cross-section inside the tube,and ferrites become saturated. In some instancesof short impeder life, ferrites have been replacedwith Fluxtrol 75 soft-magnetic composite, result-ing in 10 to 30% energy savings and increasedlife (Ref 17).The efficiency of tube welding systems is

improved using an external magnetic bridge(Ref 18). A soft-magnetic composite materialplaced above the tube opening helps increasecurrent flow along the weld “V” and helpsbalance heating of the top and bottom edgesof the tube wall (Fig. 24). In some applications,the benefits of using an external magneticbridge are 20 to 30% energy savings andimproved weld quality. System improvementsfrom using a magnetic bridge depend on theconfiguration of the weld rolls. To achieve sig-nificant benefits, the magnetic bridge must beclose to the weld apex without moving the coilfarther away from it.

Magnetic Flux Controllers forSeam-Annealing Inductors

In some cases, the weld seam requires anneal-ing to relieve stresses from the welding processor to improve the microstructure. To accomplishthis, induction heating is commonly used,referred to as seam annealing. In instances ofhigh production rates and for heavy-walled pipe,several seam annealers are placed in series.Seam-annealing inductors are linear induc-

tors, either vertical loop or split-n-return,operating at 1 or 3 kHz frequency and powerlevels of hundreds of kWs. Figure 25 shows atypical seam-annealing induction coil in thefield. C-shaped soft-magnetic materials (lamina-tions or soft-magnetic composites) are placedover the central turn to concentrate heat on theweld seam. Magnetic flux controllers are notused on the return leg (vertical loop) or legs(split-n-return), because it is desirable to widely

distribute the return current to minimize heatgenerated by those turns.

Magnetic Flux Control inLocal Heat Treating Applications

Surface hardening of machinery componentsusing induction is a common application. Themain advantage of induction heat treating com-pared with other heat treatment methods isthe ability to quickly and selectively heat onlythe areas of the workpiece where propertymodification is desired. This results in lowerdistortion, more favorable residual stress distri-bution, better microstructure, significant energysavings, and the ability to place the induct-ion system in-line with other manufacturingoperations. Application of magnetic flux con-trollers, primarily in the form of soft-magnetic

Fig. 23 Cluster impeder (multiple impeders assembledtogether) for large diameter pipe welding.Courtesy of Electronic Heating Equipment, Inc.

Fig. 24 Influence of a magnetic bridge on weld temperature distribution for a 40 mm OD by 2.5 mm thick tubetraveling at a line velocity of 75 m/min; the bridge helps balance heating of the top and bottom edges ofthe tube wall. Source: Ref 4

Fig. 25 Seam annealer for tube-welding line. Courtesy of EFD Induction

Magnetic Flux Controllers in Induction Heating and Melting / 643

materials, are particularly well suited for use inheat treating applications, because they stronglyinfluence the power density value and its distri-bution along the surface of the workpiece,enabling faster heating and better heat patterncontrol (Ref 3–6).All induction heat treating processes can be

classified in two categories: single shot andscanning, based on whether the induction coilis moving (excluding rotation) relative to thepart during the heating process. A brief discus-sion of the benefits of magnetic flux controllersin these cases follows.

Single-Shot Heat Treating

In single-shot induction heat treating applica-tions, the position of the induction coil relativeto the heated length of the part does not move.In many single-shot heating applications, thepart rotates while heating and quenching occurto ensure uniformity of pattern. Within the fam-ily of single-shot heat treating applications, thereare several varieties of induction coil configura-tions. Common induction coil styles used forheat treating include linear coils, ID coils, encir-cling coils, and encircling/non encircling coils.Themost important role of soft-magneticmate-

rials in these applications is to control the distribu-tion of temperature and ensure good part quality.This is especially important near the ends of theinduction coil to control run-out or in central areaswhere there are geometry changes.Secondary benefits of soft-magnetic materials

in these applications are energy savings and lowercoil currents. This is especially important in app-lications such as simultaneous dual-frequencyhardening of gears, where power levels at high

frequency tend to be large. Lower currents reducethe coil kVAs, and allow for smaller transformersand matching components, which reduce overallsystem cost.Soft-magnetic materials most commonly used

for induction heat treating applications are lami-nations and soft-magnetic composites. In someapplications, a combination of laminations andsoft-magnetic composites are used on the samecoil. Laminations are more common on longerlinear coil sections at lower heat treating fre-quencies (1–10 kHz). Soft-magnetic compositesare more common on medium to high frequencylinear coils (10 kHz and up) and coils with shortlinear sections. For coils with circular sections,soft-magnetic composites are more popular inmost cases due to ease of application.

Induction Scan Heat Treatment

In induction scan heat treatment applications,the position of the induction coil relative to theheated length of the part does move. In manyscanning applications, the part rotates duringheating and quenching to ensure uniformity ofpattern. Within the family of scanning applica-tions, induction coil configurations are not asdiverse as for single shot. Common inductioncoil styles used for scanning include encirclingcoils, short linear coils, and channel coils.Encircling coils likely are the most common

for induction scanning (Fig. 26). In inductionscanning applications with encircling coils, themain benefit of a magnetic flux controller is inheat pattern control at the beginning and endof the part. Energy savings from the applicationof magnetic flux controllers is less than in thecase of single-shot coils, because much of thestray heating before and after the coil is stillused in the scanning process to contribute tothe case depth. In some instances, such as axlescan hardening, where the heat treatment inthe fillet area drives the design, magnetic fluxcontrollers improve the pattern and achieveenergy savings between 15 and 50% (Ref 4,17, 19). Figure 27 shows some induction scanhardening coils with magnetic flux controllersused for axle scan hardening.

Linear coils commonly are used for scan-ning large components, such as hardeningbearing races on slewing rings, and tooth-by-tooth hardening of large gears. Other applicationsare scan hardening of components where there isa large geometrical change. Nearly all short lin-ear scanning coils use soft magnetic materials,because they are necessary for heat pattern con-trol and provide large energy savings.Channel coils are commonly used for in-line

heat treatment and brazing of smaller components,such as hardening or tempering of nuts andbolts, brazing of cutting bits, and hardeningof saw blade teeth. Magnetic flux controllersare used for heat pattern control and efficiencyimprovement. Most of these installations oper-ate at higher frequencies, so soft-magnetic compo-sites are the material of choice when a magneticflux controller is used.

REFERENCES

1. A. Muhlbauer, History of Induction Heat-ing, Vulcan Verlag GmbH, Huyssenallee,Essen, Germany, 2008

2. V. Rudnev, et al., Handbook of InductionHeating, Marcel Dekker, New York, 2003

3. V.S. Nemkov, “Resource Guide for Induc-tion Heating, CD-R,” Fluxtrol Inc., 2006

4. www.fluxtrol.com5. R.S. Ruffini, R.T. Ruffini, and V.S. Nemkov,

Advanced Design of Induction Heat TreatingCoils, Part I: Design Principles, IndustrialHeating, June 1998.

6. R.S. Ruffini, R.T. Ruffini, and V.S. Nemkov,Advanced Design of Induction Heat TreatingCoils, Part II: Magnetic Flux Concentrationand Control, Industrial Heating, Nov 1998

7. R.T.Ruffini, V.S.Nemkov, andR.C.Goldstein,“Prospective for Improved Magnetic FluxControl in the Induction Heating Tech-nique,” presented at 20th Heat Treating Soci-ety Conference and Exposition, St. Louis,2001

8. R.C. Goldstein, et al., “Virtual Prototypingof Induction Heat Treating,” 25th HeatTreating Society Conference and Exposi-tion, Indianapolis, Sept, 2009

Fig. 26 Induction scan-hardening coils made of formedtubing. Source: Ref 4 Fig. 27 Two-turn scan hardening coil with magnetic flux controllers used for axle scan hardening. Source: Ref 4

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9. Myers, et al., Optimizing Performance ofCrankshaft Hardening Inductors, IndustrialHeating, Dec 2006

10. S. Lupi, et al., Induction Heating IndustrialApplications, France 1992

11. V.S. Nemkov, and R.C. Goldstein, DesignPrinciples for Induction Heating and Hard-ening, Handbook of Metallurgical ProcessDesign, G. Totten, K. Funatani, and L. Xie,Eds., Marcel Dekker, New York 2004

12. V.S. Nemkov, et al., “Optimal Design ofInternal Induction Coils,” Intl. Symposium

on Heating by Electromagnetic Sources,Padua, Italy, June 2004

13. Bialod, et al., Induction Heating: IndustrialApplications, UIE, 1991

14. Heidlhoff, et al., “Advancements in TiAlloy Powder Production by Close-Coupled Gas Atomization,” Advances inPowder Metallurgy & Particulate Materi-als, 2011

15. Spagnolo, et al., “Space Control Optimi-zation of Multi-Coil Transverse FluxInduction Heating of Metal Strips,” Intl.

Symposium on Heating by ElectromagneticSources, Padua, Italy, May 2010

16. www.startechnologica.com.br17. Fluxtrol internal documents18. V.S. Nemkov, Magnetic Flux Guide for

Continuous High Frequency Welding ofClosed Profiles, U.S. Patent Application20080308550, Dec, 2008

19. R.C. Goldstein, et al., “Optimizing AxleScan Hardening Inductors,” 24th HeatTreating Society Conference and Exposi-tion, Detroit, Sept 2007

Magnetic Flux Controllers in Induction Heating and Melting / 645