Coil design and fabrication: basic design and modifications

I n a sense, coil design for induc-tion heating is built upon a largestore of empirical data whosedevelopment springs from sev-

eral simple inductor geometries such asthe solenoid coil. Because of this, coildesign is generally based on experi-ence. This series of articles reviewsthe fundamental electrical consider-ations in the design of inductors anddescribes some of the most commoncoils in use.

Basic design considerationsThe inductor is similar to a transformerprimary, and the workpiece is equiva-lent to the transformer secondary (Fig.1). Therefore, several of the charac-teristics of transformers are useful inthe development of guidelines for coildesign.

One of the most important featuresof transformers is the fact that the ef-ficiency of coupling between the wind-ings is inversely proportional to thesquare of the distance between them.In addition, the current in the primaryof the transformer, multiplied by thenumber of primary turns, is equal tothe current in the secondary, multipliedby the number of secondary turns. Be-cause of these relationships, there areseveral conditions that should be keptin mind when designing any coil forinduction heating:1) The coil should be coupled to thepart as closely as feasible for maxi-mum energy transfer. It is desirablethat the largest possible number ofmagnetic flux lines intersect the work-piece at the area to be heated. Thedenser the flux at this point, the higherwill be the current generated in the part.

2) The greatest number of flux lines in asolenoid coil are toward the center ofthe coil. The flux lines are concentratedinside the coil, providing the maximumheating rate there.3) Because the flux is most concen-trated close to the coil turns them-selves and decreases farther from

them, the geometric center of the coil isa weak flux path. Thus, if a part wereto be placed off center in a coil, thearea closer to the coil turns would in-tersect a greater number of flux linesand would therefore be heated at ahigher rate, whereas the area of thepart with less coupling would be heatedat a lower rate; the resulting patternis shown schematically in Fig. 2. Thiseffect is more pronounced in high-fre-quency induction heating.4) At the point where the leads andcoil join, the magnetic field is weaker;therefore, the magnetic center of theinductor is not necessarily the geomet-ric center. This effect is most appar-ent in single-turn coils. As the numberof coil turns increases and the fluxfrom each turn is added to that fromthe previous turns, this condition be-comes less important. Due to the im-practicability of always centering thepart in the work coil, the part shouldbe offset slightly toward this area. Inaddition, the part should be rotated, ifpractical, to provide uniform exposure.5) The coil must be designed to pre-vent cancellation of the magnetic field.The coil on the left in Fig. 3 has noinductance because the opposite sidesof the inductor are too close to eachother. Putting a loop in the inductor(coil at center) will provide someinductance. The coil will then heat aconducting material inserted in theopening. The design at the right pro-vides added inductance and is morerepresentative of good coil design.

Because of the above principles,some coils can transfer power morereadily to a load because of their abil-ity to concentrate magnetic flux in thearea to be heated. For example, threecoils that provide a range of heatingbehaviors are:

• a helical solenoid, with the partor area to be heated located within thecoil and, thus, in the area of greatestmagnetic flux;

Coil design and fabrication:basic design and modifications

InductionInductionInductionInductionInduction

by STANLEY ZINN and S. L. SEMIATIN

S. Zinn is executive vice president, Ameritherm,Inc., Rochester , N.Y.; (716) 427-7840.S.L.Semiatin is a project manager in the Center forMaterials Fabrication at Battelle Columbus Divi-sion; (614) 424-7742.This article is excerpted from the book “Ele-ments of Induction Heating,” published by Elec-tric Power Research Institute (EPRI) and dis-tributed by ASM International, (516) 338-5151and used with permission of EPRI

32 HEAT TREATING/JUNE 1988

Ep = primary voltage (V); Ip = primary current (A); Np =number of primary turns; Is = secondary current (A); Ns =number of secondary turns; Es = secondary voltage (V); Rl= load resistance(Ω)

Fig. 1: Electrical circuit illustrating theanalogy between induction heating and thetransformer principle.

Fig. 2: Induction heating pattern producedin a round bar placed off center in a roundinduction coil.

Fig. 3: Effect of coil design on Inductance(from F. W. Curtis, High Frequency Induc-tion Heating, McGraw-Hill, New York, 1950)

• a pancake coil, with which the fluxfrom only one surface intersects theworkpiece; and

• an internal coil for bore heating, inwhich case only the flux on the outsideof the coil is utilized.In general, helical coils used to heat roundworkpieces have the highest values ofcoil efficiency and internal coils have thelowest values (Table I). Coil efficiencyis that part of the energy delivered tothe coil that is transferred to theworkpiece. This should not be confusedwith overall system efficiency.

Besides coil efficiency, heating pat-tern, part motion relative to the coil,and production rate are also important.Because the heating pattern reflectsthe coil geometry, inductor shape isprobably the most important of thesefactors. Quite often, the method bywhich the part is moved into or out ofthe coil can necessitate large modifi-cations of the optimum design. Thetype of power supply and the produc-tion rate must also be kept in mind. Ifone part is needed every 30 secondsbut a 50-second heating time is re-quired, it will be necessary to heatparts in multiples to meet the desiredproduction rate. Keeping these needsin mind, it is important to look at a widerange of coil techniques to find themost appropriate one.

Medium-to-high-frequencySimple solenoid coils are often reliedon in medium-to-high-frequency ap-plications such as heat treatment.These include single- and multiple-turntypes. Fig. 4 illustrates a few of themore common types based on the sole-noid design. Fig. 4a is a multiturn,single-place coil, so called because itis generally used for heating a singlepart at a time. A single-turn, single-place coil is also illustrated (Fig. 4b).Fig. 4c shows a single-turn, multiplacecoil. In this design, a single turn inter-acts with the workpiece at each part-heating location. Fig. 4(d) shows amultiturn, multiplace coil.

More often than not, medium-to-high-frequency applications require spe-cially configured or contoured coils withthe coupling adjusted for heat uniformity.In the simplest cases, coils are bent orformed to the contours of the part (Fig.5). They may be round (Fig. 5a), rect-angular (Fig. 5b), or formed to meet aspecific shape such as the cam coil (Fig.

5c). Pancake coils (Fig. 5d) are gener-ally utilized when it is necessary to heatfrom one side only or when it is not pos-sible to surround the part. Spiral coils(Fig. 5e) are generally used for heatingbevel gears or tapered punches. Inter-nal bores can be heated in some caseswith multiturn inductors (Fig. 5f). It isimportant to note that, with the excep-tion of the pancake and internal coils,the heated part is always in the centerof the flux field.

Regardless of the part contour, themost efficient coils are essentially modi-fications of the standard, round coil. Aconveyor or channel coil, for example,can be looked at as a rectangular coilwhose ends are bent to form “bridges”in order to permit parts to pass throughon a continuous basis. The parts, how-ever, always remain “inside” the chan-nels where the flux is concentrated. Fig.6 illustrates similar situations in whichthe areas to be hardened are beside thecenter of the coil turns, and thus are keptin the area of heaviest flux.

Internal coilsHeating of internal bores, whether forhardening, tempering, or shrink fitting, isone of the major problems most com-monly confronted. For all practical pur-poses, a bore with a 0.44-inch (1.1-cm)internal diameter is the smallest that canbe heated with a 450-kHz power sup-ply. At 10 kHz, the practical minimumID is 1.0 inch (2.5-cm).

Tubing for internal coils should bemade as thin as possible, and the boreshould be located as close to the sur-face of the coil as is feasible. Becausethe current in the coil travels on the in-side of the inductor, the true coupling ofthe maximum flux is from the ID of thecoil to the bore of the part. Thus, theconductor cross section should be mini-mal, and the distance from the coil ODto the part (at 450 kHz) should approach0.062-inch (0.16-cm). In Fig.7a, forexample, the coupling distance is toogreat; coil modification improves the de-sign, as shown in Fig. 7b. Here, the coiltubing has been flattened to reduce thecoupling distance, and the coil OD hasbeen increased to reduce the spacingfrom coil to work.

More turns, or a finer pitch on aninternal coil, will also increase the fluxdensity. Accordingly, the space be-tween the turns should be no more thanone-half the diameter of the tubing, andthe overall height of the coil should not

Fig. 4: Typical configurations for induc-tion coils: (a) multiturn, single place; (b)single-turn, single place; (c) single-turn,multiplace (from F. W. Curtis, High Fre-quency Induction Heating, McGraw-Hill,New York, 1950)

Fig. 5: Multiturn coils designed for heat-ing parts of various shapes: (a) round;(b) rectangular; (c) formed; (d) pancake;(e) spiral-helical; (f) internal (from F. W.Curtis, High Frequency Induction Heat-ing, McGraw-Hill, New York, 1950)

Fig. 6: Coil modifications for localizedheating (from F. W. Curtis, High Fre-quency Induction Heating, McGraw-Hill,New York, 1950)

Fig. 7: Induction coils designed for in-ternal (bore) heating (from F. W. Curtis,High Frequency Induction Heating ,McGraw-Hill, New York, 1950)

33HEAT TREATING/JUNE 1988

(a) Round (b) Rectangular (c) Formed

(e) Spherical-helical(d) Pancake (e) Internal

Area to behardened

Area to behardened

Coil (c)in position

Too deep MinimumKeep close

Flat tubing

Small hole

Coupling

exceed twice its diameter. Figs. 7c and7d show special coil designs for heatinginternal bores. The coil in Fig. 7d wouldnormally produce a pattern of four ver-tical bands, and therefore the part shouldbe rotated for uniformity of heating.

Internal coils, of necessity, utilize verysmall tubing or require restricted cool-ing paths. Further, due to their compara-tively low efficiency, they may needvery high generator power to produceshallow heating depths.

Coil characterizationBecause magnetic flux tends to con-centrate toward the center of thelength of a solenoid work coil, the heat-ing rate produced in this area is gen-erally greater than that produced to-ward the ends. Further, if the part be-ing heated is long, conduction and ra-diation remove heat from the ends ata greater rate. To achieve uniformheating along the part length, the coilmust thus be modified to provide bet-ter uniformity. The technique of ad-justing the coil turns, spacing, or cou-pling with the workpiece to achieve auniform heating pattern is sometimesknown as “characterizing” the coil.

There are several ways to modifythe flux field. The coil can be decoupledin its center, increasing the distancefrom the part and reducing the flux inthis area. Secondly, and more com-monly, the number of turns in the cen-ter (turn density) can be reduced, pro-ducing the same effect. A similar ap-proach - altering a solid single-turninductor by increasing its bore diam-eter at the center - achieves the sameresult.

In Fig. 8a, the coil turns have beenmodified to produce an even heatingpattern on a tapered shaft. The closerturn spacing toward the end compen-

sates for the decrease in couplingcaused by the taper. This techniquealso permits “through the coil” load-ing or unloading to facilitate fixturing.A similar requirement in the heat treat-ment of a bevel gear is shown in Fig.8b. Here, because of the greater parttaper, a spiral-helical coil is used. Witha pancake coil, decoupling of the cen-ter turns provides a similar approachfor uniformity.

Multiturn vs. single-turnHeating-pattern uniformity require-ments and workpiece length are thetwo main considerations with regardto the selection of a multiturn vs. asingle turn induction coil. A fine-pitch,multiturn coil closely coupled to theworkpiece develops a very uniformheating pattern. Similar uniformity can

be achieved by opening up the cou-pling between the part and the coil sothat the magnetic flux pattern in-tersecting the heated area is more uni-form. However, this also decreases en-ergy transfer. Where low heating ratesare required, as in through heating forforging, this is acceptable. When highheating rates are needed, however, it issometimes necessary to maintain closecoupling. The pitch of the coil must beopened to prevent overloading of thegenerator.

Because the heating pattern is a mir-ror image of the coil, the high flux fieldadjacent to the coil turns will produce aspiral pattern on the part. This is called“barber poling,” and can be eliminatedby rotating the workpiece during heat-ing. For most hardening operations,which are of short duration, rotationalspeeds producing not less than 10 revo-lutions during the heating cycle shouldbe used.

If part rotation is not feasible, heat-ing uniformity can be increased by us-ing flattened tubing, by putting a step inthe coil, or by attaching a liner to thecoil. Flattened tubing should be placedso that its larger dimension is adjacentto the workpiece. The stepping of coilturns (Fig. 9) provides an even, hori-zontal heating pattern. Stepping is eas-ily accomplished by annealing the coilafter winding and pressing it betweentwo boards in a vise. A coil liner is asheet of copper soldered or brazed tothe inside face of the coil. This liner ex-pands the area over which the currenttravels. Thus, a wide field per turn canbe created. The height of this field canbe modified to suit the application by con-trolling the dimensions of the liner. Whena liner is used, the current path from thepower supply passes through the con-necting tubing (Fig.10). Between thetwo connections, the tubing is used solelyfor conduction cooling of the liner.

In fabricating coils with liners, it isnecessary only to tack-braze the tubingto the liner at the first and last connectionpoints, with further tacks being usedsolely for mechanical strength. The re-mainder of the common surfaces be-tween tubing and liner can then be filledwith a low temperature solder for maxi-mum heat conduction, because the coil-water temperature will never exceed theboiling point of water, which is well be-low the flow point of the solder. Thismay be necessary because the coppermay be unable to conduct heat fast



Coil design

Table 1: Typical coupling efficiencies for induction coils

Fig. 8: Adjustment (“characterization”) ofinduction heating patterns for severalparts by varying the coupling distance orturn spacing (from F. W. Curtis, High Fre-quency Induction Heating, McGraw-Hill,New York, 1950)

Fig. 9: Induction coil with an offset (step)used to provide heating uniformity

Path of windings narrowerat small end Parallel

Variation in couplingfor even heating

More intense heatat small end

Parallel coil; heating patternuneven

Coil is parallelto axis

Coil slightly conical; heating pattern even

enough from the inside of the coil.In multiturn coils, as the heated length

increases, the number of turns gener-ally should increase in proportion. In Fig.11a, the face width of the coil is in pro-portion to the coil diameter. In Fig. 11b,the ratio of the coil diameter to face widthis not suitable; the multiturn coil shownin Fig. 11c provides a more acceptableheat pattern. Multiturn coils of this typeare generally utilized for large-diameter,single-shot heating, in which the quenchmedium can be sprayed between the coilturns (Fig. 11d).

When the length of the coil exceedsfour to eight times its diameter, uniformheating at high power densities becomesdifficult. In these instances, single-turnor multiturn coils that scan the length ofthe workpiece are often preferable.Multiturn coils generally improve the ef-ficiency, and therefore the scanning rate,when a power source of a given ratingis used. Single-turn coils are also effec-tive for heating bands that are narrowwith respect to the part diameter.

The relationship between diameterand optimum height of a single-turn coilvaries somewhat with size. A small coilcan be made with a height equal to itsdiameter because the current is concen-trated in a comparatively small area.With a larger coil, the height should notexceed one-half the diameter. As thecoil opening increases, the ratio is re-duced — i.e., a 2-inch (5.1-cm) ID coilshould have a 0.75-inch (1.91-cm) maxi-mum height, and a 4-inch (10.2-cm) IDcoil should have a 1.0-inch (2.5-cm)height. Fig. 12 shows some typical ra-tios.

Coupling distancePreferred coupling distance depends onthe type of heating (single-shot or scan-ning) and the type of material (ferrousor nonferrous). In static surface heat-ing, in which the part can be rotated butis not moved through the coil, a couplingdistance of 0.060 inch (0.15 cm) frompart to coil is recommended. For pro-gressive heating or scanning, a couplingdistance of 0.075 inch (0.19 cm) is usu-ally necessary to allow for variations inworkpiece straightness. For throughheating of magnetic materials, multiturninductors and slow power transfer areutilized. Coupling distances can be looserin these cases — on the order of 0.25 to0.38 inch (0.64 to 0.95 cm). It is impor-

tant to remember, however, that processconditions and handling dictate coupling.If parts are not straight, coupling mustdecrease. At high frequencies, coil cur-rents are lower and coupling must be

increased. With low and medium fre-quencies, coil currents are considerablyhigher and decreased coupling can pro-vide mechanical handling advantages. Ingeneral, where automated systems areused, coil coupling should be looser.

The coupling distances given aboveare primarily for heat treating applica-tions in which close coupling is required.In most cases, the distance increaseswith the diameter of the part, typical val-ues being 0.75, 1.25, and 1.75 inches (19,32 and 44 mm) or billet-stock diametersof approximately 1.5, 4 and 6 inches (38,102, and 152 mm), respectively.

Effects of part irregularitiesWith all coils, flux patterns are affectedby changes in the cross-section or massof the part. As shown in Fig. 13 (p. 36),when the coil extends over the end of ashaft-like part, a deeper pattern is pro-duced on the end. To reduce this effect,the coil must be brought to a point evenwith or slightly lower than the end ofthe shaft. The same condition exists inheating of a disk or a wheel. The depthof heating will be greater at the endsthan in the middle if the coil overlaps thepart. The coil can be shortened, or thediameter at the ends of the coil can bemade greater than at the middle, therebyreducing the coupling at the former lo-cation.

Just as flux tends to couple heat to agreater depth at the end of a shaft, itwill do the same at holes, long slots, orprojections (Fig. 14, p. 36). If the partcontains a circular hole, an additionaleddy-current path is produced that willcause heating at a rate considerablyhigher than that in the rest of the part.The addition of a copper slug to the holecan effectively correct or eliminate thisproblem. The position of the slug (Fig.15, p. 36) can control the resultant heat-ing pattern. In addition, the slug will mini-mize hole distortion if the part must bequenched following heating.

For slotted parts heated with sole-noid coils (Fig. 16, p. 36), the continuouscurrent path is interrupted by the slot,and the current must then travel on theinside of the part to provide a closed cir-cuit. This is the basis for concentratorcoils. It is of interest to note, however,that with the slot closed, the applied volt-age of the work coil causes a higher cur-rent to flow. This is due to the fact that

35HEAT TREATING/JUNE 1988

Fig. 10: Method of inserting a liner in acoil to widen the flux path

Fig. 11: Selection of single-turn vs. multiturncoils depending on the length-to-diameterration of the workpiece (from F. W. Curtis,High Frequency Induction Heating, McGraw-Hill, New York, 1950)

Fig. 12: Typical proportions of varioussingle-turn coils (from F. W. Curtis, HighFrequency Induction Heating, McGraw-Hill,New York, 1950)

Coil leads

Coil liner

Connection from generatorto coil (braze points)

Top view

Tubing soft-soldered tocoil liner for maximumsurface-to-surface

cooling

Side view showing actual shape of coil

89-mm (3 1/2”) PD gear 12.7 mm (1/2”)

CoilGear Coil

Multiturn,good

Single turn,bad

Coil

Locating stud Spray-quench ring

Gear

19.0 mm (3/4 in.)

25.4 mm(1 in.)

51 mm (2 in.)

Water cooling

25.4 mm(1 in.)

12.7 mm(1/2 in.)

12.7 mm(1/2 in.)

102 mm (4 in.)



the resistive path, now around the pe-riphery of the part, is considerablyshorter. The increase in current then pro-duces a considerably higher heating rate

Coil designwith the same coil.

Flux divertersWhen two separate regions of aworkpiece are to be heated, but are closetogether (Fig. 17), it is possible that themagnetic fields of adjacent coil turns willoverlap, causing the entire bar to beheated. To avoid this problem, suc-cessive turns can be wound in oppositedirections. By this means, the inter-mediate fields will cancel, and the fieldsthat remain will be restricted. It shouldbe noted that, as shown in Fig. 17, leadplacement is critical. Having the returninductor spaced far from the coil leadswould add unneeded losses to the sys-tem. Another example of a counterwoundcoil is shown in Fig. 18; the coil in Fig.18b is the counterwound version of theone in Fig. 18a. This type of coil can beused effectively in an application inwhich the rim of a container is to beheated while the center remains rela-tively cool.

Another technique that can be uti-lized in the above circumstances involvesthe construction of a shorted turn or “rob-ber” placed between the active coilturns. In this case, the shorted loop actsas an easy alternative path for concen-tration of the excess flux, absorbing thestray field. It is therefore sometimescalled a flux diverter. As for the activecoil turns, the robber must be watercooled to dissipate its own heat. A typi-cal construction is shown in Fig. 19.

Shorted coil turns are also used ef-fectively to prevent stray-field heatingon very large coils where the end fluxfield might heat structural frames.

Flux robbers or flux diverters can alsobe used in fabricating test coils when itis desired to determine the optimum num-ber of turns empirically. In these situa-tions, a few additional turns are providedthat can be added or removed as re-quired. These can be shorted with acopper strap or temporarily brazed whiletests are made and removed pending theoutcome of’ the heating trials.

This is the first installment of a three-part article on coil design and fabri-cation. Part two, on specialty coils,will appear in August. Part three, onfabrication, will appear in October.

Fig. 13: Effect of coil placement on theheating pattern at the end of a workpiece(from F. W. Curtis, High Frequency Induc-tion Heating, McGraw-Hill, New York, 1950)

Fig. 14: Localized overheating of sharp cor-ners, keyways, and holes most prevalent inhigh frequency induction heating (from F.W. Curtis, High Frequency Induction Heat-ing, McGraw-Hill, New York, 1950)

Fig. 15: Control of the heating pattern at ahole through use of copper slugs (from M.G. Lozinski, Industrial Applications of In-duction Heating, Pergamon Press, London,1969)

Fig. 16: Localized overheating of slots incertain parts that results from the tendencyfor induced currents to follow the part con-tour (from F. W. Curtis, High Frequency In-duction Heating, McGraw-Hill, New York, 1950)

Fig. 17: Control of heating patterns in twodifferent regions of a workpiece by wind-ing the turns in opposite directions (fromF. W. Curtis, High Frequency Induction Heat-ing, McGraw-Hill, New York, 1950)

Fig. 18: Design of pancake coils to provide(a) uniform, or overall, heating or (b) pe-ripheral heating only (from F. W. Curtis,High Frequency Induction Heating, McGraw-Hill, New York, 1950)

Fig. 19: Typical construction of a water-cooled flux robber.

Work

Coil

Keyway

Braze

Water Path

Multiturncoil

Heat

29HEAT TREATING/AUGUST 1988

Coil design and fabrication:part 2, specialty coilsby STANLEY ZINN and S. L. SEMIATIN


C oil designs are based on theheating-pattern require-ments of the application,the frequency, and the

power-density requirements. In addi-tion, the material-handling techniquesto be used for production determine,to a large extent, the coil to be used.If a part is to be inserted in a coil,moved on a conveyor, or pushed endto end, or if the coil/heat station com-bination is to move onto the part, thecoil design must take the appropriatehandling requirements into con-sideration. Accordingly, a variety ofspecialty coil designs have evolved forspecific applications.

Master work coils and coil insertWhen production requirements neces-sitate small batches (as in job-shop ap-plications) and a single-turn coil canbe used, master work coils provide asimple, rapid means of changing coildiameters or shapes to match a vari-ety of parts. In its basic form, a mas-ter work coil consists of copper tub-ing that provides both an electricalconnection to the power supply and awater-cooled contact surface for con-nection to a coil insert (N. B. Stevensand P.R. Capalongo, “Inductor forHigh-Frequency Induction Heating,”U.S. Patent 2,456,091, December 14,1948). A typical design, shown in Fig.1, consists of a copper tube that is bentinto the form of a single-turn coil andsoldered to a copper band that con-forms to the slope of the coil insert

S. Zinn is executive vice president, Ameritherm,Inc., Rochester, N.Y.; (716) 427-7840. S.L.Semiatin is a project manager in the Center forMaterials Fabrication at Battelle Columbus Divi-sion; (614) 424-7742.This article is excerpted from the book “Ele-ments of Induction Heating,” published by Elec-tric Power Research Institute (EPRI) and dis-tributed by ASM International, (516) 338-5151and used with permission of EPRI.

and is recessed. Holes in the insertsthat match tapped holes in the mastercoil securely clamp the inserts to themaster coil, providing good transfer ofelectrical energy and heat removal.Inserts are machined from copperwith a thickness that matches the re-quired heating pattern, and should besomewhat greater in thickness than thedepth of the recess for easy removal.Special coil shapes are easily config-ured. It is important to note that, be-cause of the less-than-optimal cool-ing technique, coil inserts are particu-larly well adapted to processes requir-ing short heating times or those inwhich they are also cooled by thequenching medium.

In machining of coil inserts, caremust be taken to relieve sharp cor-ners, unless it is desired to have adeeper heating pattern in these loca-tions. Fig. 2 shows the effect of sharpcorners on a closely coupled part. Fluxfrom both inductor sides couples to thecomer, which, due to a lack of mass,tends to overheat relative to the restof the pattern. Decoupling of the coilfrom these locations provides the de-sired pattern but tends to reduce over-all efficiency, thus slowing the heat-ing rate and resulting in a deeper case.Relieving or decoupling of only thecorners is a better alternative, par-ticularly when a solid, inductor isused, and the relief can be machinedas required.

Coils for induction scannersCoils for progressive hardening (scan-ning) are built using two techniques.The simpler of the two employs asimple single-turn or multiturn coil witha separate quench ring that can bemounted on the scanner (Fig. 3a,p.30). For larger production runs, adouble chamber coil that incorporates

Fig. 1: Schematic illustration showing thedesign of a master coil with changeableinserts (from M.G. Lozinski, Industrial Ap-plications of Induction Heating, PergamonPress, London, 1969)

Fig. 2: Inductor with a relief designed forthe hardening of the lateral surface of a tem-plate (from M.G. Lozinski, Industrial Appli-cations of Induction Heating, PergamonPress, London, 1969)


30 HEAT TREATING/AUGUST 1988

both coil cooling and quenching capa-bilities is often the preferred choice.The scanning inductor shown in Fig.3b is typical of the latter type of de-sign. Cooling water flows through theupper, or inductor, chamber to keepthe copper resistivity low. Thequenchant is sprayed from perfora-tions in the beveled face onto theworkpiece as it exits from the induc-tor. The beveled face normally is atan angle of 300 to the vertical, so thatthere is some soaking time betweenthe end of induction heating and thequenching operation. This delay timehelps to increase uniformity. Properchoice of the spray direction also re-duces the amount of fluid runback onthe shaft, which could cause variationin bar temperature and result in un-even hardness. Well-directed quench

Specialty coilsspray holes are required inasmuchas “barber poling” can occur due toerratic or misdirected quenchantthat precools the part ahead of themain quench stream.

Split coilsSplit coils are generally utilized as alast resort for applications in whichit is difficult to provide a high enoughpower density to the area to beheated without very close coupling,and where part insertion or removalwould then become impossible. Onesuch situation is the hardening ofjournals and shoulders in crank-shafts. In this case, the split-coildesign would also include the abilityto quench through the face of theinductor. Typical methods of hing-ing split inductors are shown inFig. 4.

It should be noted that with a splitinductor, good surface-to-surfacecontact must be made between the

faces of the hinged and fixed por-tions of the coil. Generally, these sur-faces are faced with silver or spe-cial alloy contacts that are matchedto provide good surface contact.Clamps are used to ensure closureduring heating. High currents at highfrequency pass through this inter-face, and the life of the contact isgenerally limited due to both wearand arcing.

Coolant for the coil chamber of asplit inductor is carried by flexiblehoses that bypass the hinge so that ex-cessive heating does not occur in themovable section during the cycle. Thequench chamber is fed by a separatehose arrangement. The face of thequench chamber is closest to the workduring heating, and therefore carriesmost of the current. Accordingly, itmust be sufficiently thick to precludeeither melting or distortion during theheating cycle.

With split coils it is also frequent-ly necessary to provide some meansof locating the part in the coil tomaintain the proper coupling dis-tance. Ceramic pins or buttons arefrequently secured to the face of theinductor. These pins contact the partduring the heating cycle and estab-lish rigid relative positioning betweenpart and coil. However, they aresubject to thermal shock during theheating and quenching cycles andsuffer mechanical abuse as well.Therefore, they should be designedfor simple replacement as required.Fig. 5 depicts an arrangement forthe use of either ceramic or metalpins that compensates for theseproblems. Here, the ceramic pin isapproximately 0.25 inch (0.64 cm)in diameter and 0.5 inch (1.3 cm)long with a 0.27-inch (0.69) head di-ameter. The rubber packing absorbsthe clamping stress. A threaded tubepasses through the chamber, and ascrew presses the pins against theshaft. In Fig. 5b, a 0.125 inch (0.32cm) nichrome pin is used with a ce-ramic tube as an insulator. Being incompression, the tube undergoescomparatively high loads withoutbreaking. The metal pin provideslonger life in these conditions thanthe ceramic pin.

Butterfly coilsOne of the most difficult heating chal-

Fig. 3: Inductor/quench designs for induc-tion scanning: (a) separate coil and quench;and (b) two-chamber, integral coil andquench (from F.H. Reinke and W. H. Gowan,Heat Treatment of Metals, Vol. 5, No. 2,1978, p. 39)

Fig. 4: Diagram (a) and schematic illustra-tion (b) of a split inductor used for heatingcrankshaft journals (from M.G. Lozinski, In-dustrial Applications of Induction Heating,Pergamon Press, London, 1969)

31HEAT TREATING/AUGUST 1988

lenges is the creation of an even heat-ing pattern at the end of a bar or shaft.Patterns developed with a pancakeinductor produce a dead spot at thecenter, due to field cancellation in thisarea.

The butterfly coil (Fig. 6), sonamed because of its appearance, uti-lizes two specially formed pancakecoils. The current paths of the adja-cent sides are aligned so that they areadditive. The “wings” of the butterflymay be bent up to decouple their fieldsfrom the shaft, or, if heat is requiredin this location, they may be coupledwith the shaft itself. In winding thiscoil, it is important that all center turnsbe wound in the same direction so thatthey are additive. Further, only theseturns should couple directly with thepart to produce the desired pattern.

Split-return inductorsIf a narrow band of heat is requiredand heating must be accomplishedfrom one surface only, the split-re-turn inductor offers distinct ad-vantages (Fig. 7). With this design,the center runner of the work coilcarries twice the current of each ofthe return legs. The pattern on theworkpiece, being a mirror image ofthe coil, produces four times as much

heat under the center leg as in eachof the return loops. With proper bal-ancing, the high-heat path can thenbe extremely narrow, while the heatproduced in each of the return legsis insufficient to affect the remain-der of the part.

Tapped coilsInduction coils can be provided withtaps to allow for differences inheated length. A typical applicationis a forging coil for heating “off theend” of a bar, in which provisionmust be made to adjust the lengthbeing heated. Taps are brazed to thework coil at locations where a wa-ter-cooled strap can be moved fromtap to tap. The active portion of thecoil is then between the power-sup-ply connection and the tap.Watercooling, however, should be main-tained through all portions of the coil,both active and inactive.

Transverse-flux coilsIn heating of parts that have a longlongitudinal axis and a thin cross-sec-tion, a circular coil wrapped aroundthe workpiece produces a heatingpattern (Fig. 8) that, due to couplingdistances, is effective only at theedges. In transverse-flux heating,however, the coil is designed to set

Fig. 5: Design of metal and ceramic pins forfixing the position of a split inductor on acrankshaft journal (from M.G. Lozinski, In-dustrial Applications of Induction Heating,Pergamon Press, London, 1969)

Fig. 6: Schematic illustration of a butterflycoil: (a) coil construction (arrows indicatereinforcing type of curent flow in coil); and(b) coupling between the turns of the coiland the end of a bar to produce a uniformheating pattern

Fig. 7: Two types of split-return coils (fromC. A. Tudbury, Basics of Induction Heating,Vol. 1, John F. Rider, Inc., New York, 1960)

Fig. 8: Illustration (a) of one type of trans-verse coil for heating a thin section; sketchin (b) indicates the current path in the work-piece (from F. W. Curtis, High FrequencyInduction Heating, McGraw-Hill, New York,1950)

Fig. 9: Typical channel coil used to heat theedges of discrete lengths of rectangular barstock; end of coil is decoupled by bendingto prevent overheating of ends (from F. W.Curtis, High Frequency Induction Heating,McGraw-Hill, New York, 1950)

Fig. 10: Use of a liner on a single-turn chan-nel coil to provide a wider heating patternon the workpiece (from F. W. Curtis, HighFrequency Induction Heating, McGraw-Hill,New York, 1950)

Entire coil is constructed ofrectangular copper tubing

with water flowingthrough for cooling

Full current flowsin center leg

Section A-A

Induced eddycurrents followparallel to coil

currents and aremost intense

under center leg

Current splits to returnalong side conductors

Return circuit foreddy currents spread

over two relativelylarge areas

Induced currents notdense enough herefor material to reach

hardening temperature

(a) Split-return coil for annealing of seam welds in pipe or tube.(b) Split return inductor for hardening of surfaces of largesprocket teeth one tooth at a time (welding fixture not shown)

Sprocket wheel

Area to behardened

Return leg - 1/2 of total current, I

To generator

Liner

Current path

Strip to beheated

Coil

Work

Bend

Coil

Liner


32 HEAT TREATING/AUGUST35HEAT TREATING/JUNE 198836HEAT TREATING/JUNE 1988 1988

up a flux field that is perpendicularto the sheet or similar part. In thisway, the path of the eddy currentsis changed so that it is parallel to themajor axis of the work. For example,in the manufacture of items such ashacksaw blades, the steel moves be-tween the turns of the coil and theeddy-current path is a circular oneacross the flat of the blade. For heat-ing of wide sheet materials, speciallydesigned transverse-flux inductorshave, in recent years, also becomeavailable.

Conveyor/channel coilsOften when power densities are lowand heating cycles not extremelyshort, parts can be processed by useof a turntable or conveyor in a con-tinuous or indexing mode. The coilmust then be designed to permit easyentry and exit of the part. The sim-plest conveyor or channel coil usedin these situations is a modificationof the hairpin inductor (Fig. 9,p.31). With the indexing technique,in which the part is at rest in the coilduring the heating cycle, the ends ofthe hairpin can be decoupled to pre-vent overheating of the ends. Theseraised portions or bridges also facili-

tate passage of the part through thecoil. When a wide heating zone is tobe produced on the part, couplingover a greater area can be accom-plished through the addition of a linerto the coil turn (Fig. 10, p. 31), ormore ampere turns can also be pro-duced with a multiturn channel in-ductor (Fig. 11). Channel-coil lin-ers may also be configured to pro-duce specialized heating patternswhere greater heat densities are re-quired in specific areas (Fig. 12). During design of heating opera-tions using channel coils, there is a“fill factor” that must be consideredfrom an efficiency standpoint. Theunused portions of the coil appearas lead losses. Therefore, partsmust be as close as possible to eachother, without touching, to utilize thefull capabilities of the inductor. An-other important consideration in theuse of a channel coil is the fact thatthose areas of the workpiece clos-est to the coil receive the greatestportion of the flux and therefore heatthe fastest (Fig. 13). If conductionthrough the part is slow, the partshould be rotated while passingthrough the coil. Sufficient time (inan indexing conveyor or turntable)or speed variation (in a continuous-motion device) must be provided toallow heat uniformity to occur in partareas farthest from the coil turns.

Specialty coils

Fig. 11: Multiturn channel coil used to in-crease the ampere turns coupled to an in-duction heated workpiece (source: LindbergCycle-Dyne Inc.)

Fig. 12: Multiturn channel coil with a lineradded to control the heating pattern (fromF.W. Curtis, High Frequency Induction Heat-ing, McGraw-Hill, New York, 1950

Fig. 13: Development of the heating patternin parts moved through a channel coil.

Channel-type coil

Condenser can

Leads

Solder ring

Direction of travel

39HEAT TREATING/OCTOBER 1988

Coil design and fabrication:part 3, fabrication principlesby STANLEY ZINN and S. L. SEMIATIN

S. Zinn is executive vice president, Ameritherm,Inc., Rochester, N.Y.; (716) 427-7840. S.L.Semiatin is a project manager in the Center forMaterials Fabrication at Battelle Columbus Divi-sion; (614) 424-7742. This article is excerpted from the book “Ele-ments of Induction Heating,” published by Elec-tric Power Research Institute (EPRI) and ASMInternational and distributed by ASM Interna-tional, (516) 338-5151 Used with permissionof ASM International and EPRI.

Fig. 1: Comparative heating patterns pro-duced by using round vs. square tubingfor a solenoid induction coil (from M. G.Lozinsky, Industrial Applications of InductionHeating, Pergamon Press, London, 1969)

Because of its low resistivity,fully annealed, high-conductiv-ity copper is most commonlyused in the fabrication of in-

duction heating coils. The copper istypically in a tubular form, with a mini-mum outer diameter of 0.125 inch (0.32cm) to allow for water cooling. Mate-rial of this kind is available in a widerange of c ross sec t ions ( round ,square, and rectangular) and sizes.

Selection of tubingIn addition to the 12R loss due to itsown resistivity, the coil surrounds theload and absorbs addit ional heatthrough radiation and convectionfrom the heated surface. Therefore, itis essential that the tubing selectedfor the work coil have a sufficientcooling path to remove this heat. Oth-erwise, the resistivity of the copperwill increase due to the temperatureincrease, thus creating greater coillosses. In some instances, such aslarge coils, it may be necessary tobreak up the individual water pathsin a coil to prevent overheating andpossible coil failure. Another factor in the selection oftubing for induction coils relates tothe fact that the current in the workcoils is traveling at a specific refer-ence dep th tha t depends on thepower-supply frequency and the re-sistivity of the copper. Accordingly,the wall thickness of the coil tubingshould be selected to reference-depth

limits similar to those used for induc-tion heating of copper. Suggested wallthicknesses for various frequencies areshown in Table I (p.40). However, cop-per availability must be considered, andoften wall thicknesses less than twicethe reference depth are used with onlya nominal loss in overall coil efficiency. Square copper tubing is also com-mercially available and is frequentlyused in coil fabrication. It offers a con-siderable advantage in that it couplesmore flux to the part per turn than roundtubing (Fig. 1). Moreover, it is more eas-ily fabricated in that it will not collapseas readily on bending. It is also easilymitered to create sharp, close bends asrequired. If only round tubing is avail-able, it can be flattened in a vise or othersimple device to adjust the resultantthickness dimension. This flatteningcan be done with minimal decrease indimension of the water-flow path.

Coil formingIn fabrication of copper coils, it must benoted that the copper work hardens withincreasing deformation. Thus, most fab-ricators anneal the tubing every few

bends to relieve this condition by heat-ing the tubing until it is bright red, thencooling it rapidly in water. These in-termediate anneals prevent fracture ofthe tubing during fabrication. In some forming operations, it maybe desirable to fill the coil with sand orsalt to preclude collapse of the tubing.In addition, there are several low-tem-perature alloys-with melting points be-low 212°F (100°C)-that are normally usedto perform this same function. When thecoil is completed, it is immersed in boil-ing water. The alloy then flows outfreely and can be reused at another time.With any of these techniques, oncefilled, the tubing acts as a solid rod dur-ing forming and can be simply clearedon completion.

Bracing of coilsBecause electric currents flow in boththe workpiece and the coil, magnetomo-tive forces between the two are devel-oped. The magnitudes of the forces de-pend on the magnitudes of the currents.If sufficiently large, the forces maycause the part to move in the coil. If thepart has a large mass, however, the coilwill tend to move relative to theworkpiece. The turns may also tend tomove relative to each other. It is impor-tant, therefore, that the coil turns besuitably braced to prevent movementand possible turn-to-turn shorting. Fur-thermore, coil motion relative to the partmust be prevented to avoid undesirablechanges in the heating pattern.

Much of the acoustic noise gener-ated during low-frequency operationsalso occurs due to coil vibration, muchas a speaker coil and magnet structurework in an audio system. Bracing andphysical loading of the coil to restrictits movement will aid in reducing thiscondition. On very large, high-currentcoils, the magnetomotive force exerted


40 HEAT TREATING/OCTOBER 1988

can be extremely large, and if properbracing is not provided; the coil maygradually work harden and finally fail. Typical bracing techniques are il-lustrated in Fig. 2. In Fig. 2a, brassstuds are brazed to every other turn.These studs are then secured to in-sulator posts to hold them in a fixedrelation to each other. Nuts on eachside of the stud at the insulator allowadjustment for characterization of theheating pattern. In Fig. 2b, the insu-lation has been contoured to hold theturns relative to each other after theend turns are secured with studs. The insulation used for bracing ap-plications must meet the criteria forthe coil design. In addition to the in-stallation being capable of withstand-ing the hea t rad ia ted f rom theworkpiece, its electrical capabilitiesmust permit it to with-stand the volt-age between the mounting studs orthe turn-to-turn voltages of the coil.This is of particular concern whenusing high-voltage RF coils where upto 12,000v may be impressed acrossthe total coil. It may be necessary inthese instances to provide slots be-tween the stud locations in the insu-lator boards to increase the electricalcreepage path between the studs. Itmay also be necessary to increase theheat-resistant characteristics of theinsulation by facing the area exposedto the heated surface with a sheet ofhigh-temperature insulation.

For purposes of rigidity, clean-

ness, and protection, it is sometimesdesirable to encapsulate work coils ina plastic or refractory material. Thesame kind of care with respect to volt-age and temperature characteristicsmust be taken with these materials aswith insulating boards. For low-tem-perature induction heating applica-tions, epoxy encapsulation of the coilis quite common. For heating of steelbillets, coils are usually cast in a re-fractory cement to prevent scale fromthe part from falling between the

turns. In coating of coils with refrac-tory materials, care must be taken tomatch the pH of the refractory to that ofthe material being heated; for example,an acidic refractory is required for theferrous scale that drops off during high-temperature heating of steels.

Design considerationsAll coils represent an inductance tothe tank circuit. However, in practice,the working portion of the coil may infact be only a small portion of the in-ductance presented to the tank. Be-tween the output terminals of thegenerator or heat station and the heat-ing portion of the work coil, there maybe a considerable distance of outputlead. In any case, some finite distanceexists between the heat-station termi-nations and the actual coil. Designand construction of these work-coill eads can be a major fac tor indetermining job feasibility.

The effect of lead construction onsystem performance can be best un-derstood with respect to the tank cir-cuit of which it is a part (Fig. 3). Thecoil/load inductance is representedby L2. Each lead connecting the tankcapacitor to the coil has its own in-ductance (L1, L3). If the voltage in thetank, ET, is impressed across the to-tal of these inductances, then somevoltage drop appears across each.The full voltage will thus never ap-pear across the work coil. If the induc-tance of the coil (L2) is approximately10 times the total inductance of theleads (L1 plus L3) or greater, a maxi-mum of 10% of the total voltage willbe lost in the leads. Any loss less thanthis can be considered nominal.

Some coils have many turns, alarge cross-sectional area, and thusfairly high inductance. Hence, thecomparative lead inductance is small.As the frequency increases, coils of-ten become smaller in size, and theirinductance and inductive reactancedecrease. As the distance betweenthe heat station and coil increases,therefore, these lead inductances canbecome critical.

Several coil designs that illustratethe effect of lead design are shown inFigs. 4 and 5. In Fig. 4a, a coil withleads far apart is depicted. The spacebetween the leads presents an induc-

Coil design

Fig. 2: Typical techniques for bracing ofinduction-coil turns (from F.W. Curtis, HighFrequency Induction Heating, McGraw-Hill,New York, 1950)

Table I: Selection of copper tubing for induction coils.

41HEAT TREATING/OCTOBER 1988

tance almost equal to that of the coil.Thus, a major portion of the voltagewill not appear in the working area. Abetter design (Fig. 4b) minimizes this gapand thus improves heating efficiency. Fig.5b also shows single-turn, multiplace coilswith an extremely poor and an improvedlead design.

Another factor to consider is the in-teraction of the leads with nearby metalstructures. Because all leads have someinductance, they can act as work coils.Thus, a conductor placed within their fieldwill be heated. Leads placed adjacent tometal structures will tend to heat them. Inaddition to unwanted heat, this loss re-duces the power available to the load. Itis important that lead-to-lead separationbe minimized and proximity to metallicstructural members be considered. When-ever possible, duct housings, trays, orconduits must be of low-resistivity or in-sulating materials, such as aluminum orplastic.

Typical lead designInduction heating lead designs typi-cally make use of water-cooled cop-per plates or tubes. When coi l vol tages are low(X800v), a low-inductance structureknown as a fishtail is often utilized. Afishtail is a pair of parallel copperplates that are water cooled to main-tain low resistivity. They are placedwith their wide bus faces parallel, andare either separated physically withair as an insulator or held together bynylon bolts and nuts with teflon or asimilar material acting as a spacer.Extending from the heat station to apoint as close as possible to the op-erating area of the work coil, theypresent minimum inductance and pro-vide maximum power at the coil. De-pending on condi t ions and con-struction, efficient runs of up to 15feet are practical. The thickness of thecopper plates should be consistentwith the frequency, as noted in TableI, and cooling-water paths and sizesmust be consistent with the powerbeing transmitted as well. The copperplates should increase in width withgenerator power and the distance ofthe run. Moreover, they should bespaced as close together as possible

with only enough space for proper insu-lation to prevent arcing.

As the coil inductance increases (e.g.,as the number of turns or the coil diam-eter increases), lead length becomes lesscritical, and plain copper tubing leads thenbecome more practical. However, largercoils also require higher terminal voltages.These leads must also be kept as close aspossible to each other while maintainingsufficient spacing to prevent arcing. How-ever, good practice still dictates that coilleads be kept to a minimum length and

that copper tubing sizes be used thatare consistent with frequency, current,and cooling requirements.

Rigid leads, whether tubing or bus,built to the above guidelines are inher-ently more effective than flexible, wa-ter-cooled cable. In some cases, how-ever, it is absolutely necessary to useflexible connections. There are severalvariations in flexible leads, but it mustbe kept in mind that the inductive leadlosses in flexible cables are usuallymuch greater than those for rigid con-nections. The most common flexible leadis generally used in applications similarto tilt-type induction melting furnacesand consists of a water-cooled, spiral-wound inner conductor (similar to BXcable, but made of copper) with an outerinsulating covering. These leads areused in pairs with one for each lead con-nection. Not only must they be sizedfor current and frequency, but the insu-lation must be capable of handling thevoltage rating of the system. Flexibleleads should be tied together with in-sulating straps.

Coaxial leads are also available andmay be rigid or flexible. They consist ofan inner conductor and an outer sheathor housing that is also used as the re-turn conductor. This outer sheath isgenerally at ground potential. In addi-tion to providing an extremely low-inductance lead, the outer ground actsto eliminate possible strong radiation orinductive coupling to adjacent struc-tures.

Rigid coaxial lead is generally quiteexpensive and is usually limited to thoseapplications where it is imperative totransmit high power at high frequencyover some distance.

Another type of coaxial cable is thewater-cooled type generally used at ra-dio frequencies. It consists of a low-in-ductance, braided inner conductor thatruns through a water-cooled tube, andan outer return braid that is also watercooled. This construction is generallyutilized with medium-to-high-induc-tance coils because its constructiondoes not greatly minimize lead induc-tance but does provide flexibility. Thislast type of lead is most common whenthe operator must physically move the coilfrom part to part as in bottle sealing.

Fig. 3: Schematic circuit diagram indicat-ing the inductance of the coil leads andinduction coil itself: L1, L3-lead inductances;L2-induction-coil inductance; C1-tank capaci-tance; E1-tank voltage

Fig. 4: Effect of coil-lead spacing on lead in-ductance; closer spacing, as in (b), reduceslead inductance and thus power losses (fromF.W. Curtis, High Frequency Induction Heat-ing, McGraw-Hill, New York, 1950)

Fig. 5: Lead construction for multiplace in-ductors; lead design in (b) is preferablebecause of lower heat inductance (from F.W.Curtis, High Frequency Induction Heating,McGraw-Hill, New York, 1950)

Excessive inductance

Coil design and fabrication: basic design and modifications

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