coil design & fabrication

7/30/2019 Coil Design & Fabrication

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In 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 common

coils 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 the

square 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 desirable

that 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. In

addition, 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 more

representative 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; I

s= secondary current (A); N

s=

number of secondary turns; Es

= secondary voltage (V); Rl

= load resistance()

Fig. 1: Electrical circuit illustrating the

analogy between induction heating and the

transformer principle.

Fig. 2: Induction heating pattern produced

in a round bar placed off center in a round

induction coil.

Fig. 3: Effect of coil design on Inductance

(from F. W. Curtis, High Frequency Induc-

tion Heating, McGraw-Hill, New York, 1950)

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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 of

coil 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 bridgesin 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 minimum

ID is 1.0 inch (2.5-cm).Tubing for internal coils should be

made 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, for

example, 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 localized

heating (from F. W. Curtis, High Fre-


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 be

hardened

Area to be

hardenedCoil (c)

in position

Too deep Minimum

Keep closeFlat tubing

Small hole

Coupling


3/12

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 very

small 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 modify

the 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 closer

turn 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 uniform

heating 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 calledbarber 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 is

necessary 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) of

induction heating patterns for several

parts by varying the coupling distance or

turn spacing (from F. W. Curtis, High Fre-


New York, 1950)

Fig. 9: Induction coil with an offset (step)

used to provide heating uniformity

Path of windings narrower

at small end Parallel

Variation in coupling

for even heating

More intense heat

at small end

Parallel coil; heating pattern

uneven

Coil is parallel

to axis

Coil slightly conical;

heating pattern even


4/12

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 shown

in 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. multiturn

coils depending on the length-to-diameter

ration of the workpiece (from F. W. Curtis,

High Frequency Induction Heating, McGraw-

Hill, New York, 1950)

Fig. 12: Typical proportions of various

single-turn coils (from F. W. Curtis, High

Frequency Induction Heating, McGraw-Hill,

New York, 1950)

Coil leads

Coil liner

Connection from generator

to coil (braze points)

Top view

Tubing soft-soldered to

coil liner for maximum

surface-to-surface

cooling

Side view showing actual shape of coil

89-mm (3 1/2) PD gear 12.7 mm (1/2)

Coil

Gear 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.)


5/12



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 in

which 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 while

tests 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 the

heating 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 in

high frequency induction heating (from F.

W. Curtis, High Frequency Induction Heat-


Fig. 15: Control of the heating pattern at a

hole 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 in

certain parts that results from the tendency

for 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 two

different regions of a workpiece by wind-

ing the turns in opposite directions (from

F. W. Curtis, High Frequency Induction Heat-


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

Multiturn

coil

Heat


6/12

29HEAT TREATING/AUGUST 1988

Coil design and fabrication:

part 2, specialty coilsby STANLEY ZINN and S. L. SEMIATIN


Coil 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 can

be 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 the

depth 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 a

deeper 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 the

design of a master coil with changeable

inserts (from M.G. Lozinski, Industrial Ap-

plications of Induction Heating, Pergamon

Press, London, 1969)

Fig. 2: Inductor with a relief designed for

the hardening of the lateral surface of a tem-

plate (from M.G. Lozinski, Industrial Appli-

cations of Induction Heating, Pergamon

Press, London, 1969)


7/12


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 the

upper, 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 on

the 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 which

it 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 is

generally 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 preclude

either 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 and

suffer mechanical abuse as well.Therefore, they should be designedfor simple replacement as required.Fig. 5 depicts an arrangement forthe use of either ceramic or metalpi ns that comp ensa tes fo r thes eproblems. 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 a

screw 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 and

quench (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 heating

crankshaft journals (from M.G. Lozinski, In-

dustrial Applications of Induction Heating,

Pergamon Press, London, 1969)


8/12

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), so

named 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.Water

cooling, 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 for

fixing the position of a split inductor on a

crankshaft journal (from M.G. Lozinski, In-

dustrial Applications of Induction Heating,

Pergamon Press, London, 1969)

Fig. 6: Schematic illustration of a butterfly

coil: (a) coil construction (arrows indicate

reinforcing type of curent flow in coil); and

(b) coupling between the turns of the coil

and the end of a bar to produce a uniform

heating pattern

Fig. 7: Two types of split-return coils (from

C. 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; sketch

in (b) indicates the current path in the work-

piece (from F. W. Curtis, High Frequency

Induction Heating, McGraw-Hill, New York,

1950)

Fig. 9: Typical channel coil used to heat the

edges of discrete lengths of rectangular bar

stock; end of coil is decoupled by bendingto prevent overheating of ends (from F. W.

Curtis, High Frequency Induction Heating,


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, High


New York, 1950)

Entire coil is constructed ofrectangular copper tubing

with water flowingthrough for cooling

Full current flows

in center leg

Section A-A

Induced eddycurrents followparallel to coil

currents and aremost intense

under center leg

Current splits to return

along 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 s urfaces of largesprocket teeth one tooth at a time (welding fixture not shown)

Sprocket wheel

Area to behardened

Return

leg-1

/2ofto

talcur

rent,I

To generator

Liner

Current path

Strip to be

heated

Coil

Work

Bend

Coil

Liner


9/12


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 afill 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: Lindberg

Cycle-Dyne Inc.)

Fig. 12: Multiturn channel coil with a liner

added to control the heating pattern (from

F.W. Curtis, High Frequency Induction Heat-

ing, McGraw-Hill, New York, 1950

Fig. 13: Development of the heating pattern

in parts moved through a channel coil.

Channel-type coil

Condenser can

Leads

Solder ring

Direction of travel


10/12

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 tubing

for a solenoid induction coil (from M. G.

Lozinsky, Industrial Applications of Induction

Heating, Pergamon Press, London, 1969)

Because of its low resistivity,

fully annealed, high-conductiv-

ity copper is most commonly

used in the fabrication of in-

duction heating coils. The copper is

typically in a tubular form, with a mini-

mum outer diameter of 0.125 inch (0.32

cm) to allow for water cooling. Mate-

rial of this kind is available in a wide

range of cross sec t ions ( round,

square, and rectangular) and sizes.

Selection of tubing

In addition to the 12R loss due to its

own resistivity, the coil surrounds the

load and absorbs addit ional heat

through radiation and convection

from the heated surface. Therefore, it

is essential that the tubing selected

for the work coil have a sufficient

cooling path to remove this heat. Oth-

erwise, the resistivity of the copper

will increase due to the temperature

increase, thus creating greater coillosses. In some instances, such as

large coils, it may be necessary to

break up the individual water paths

in a coil to prevent overheating and

possible coil failure.

Another factor in the selection of

tubing for induction coils relates to

the fact that the current in the work

coils is traveling at a specific refer-

ence depth tha t depends on the

power-supply frequency and the re-

sistivity of the copper. Accordingly,

the wall thickness of the coil tubing

should be selected to reference-depth

limits similar to those used for induc-

tion heating of copper. Suggested wall

thicknesses for various frequencies are

shown in Table I (p.40). However, cop-

per availability must be considered, and

often wall thicknesses less than twice

the reference depth are used with only

a nominal loss in overall coil efficiency.

Square copper tubing is also com-

mercially available and is frequently

used in coil fabrication. It offers a con-

siderable advantage in that it couples

more flux to the part per turn than round

tubing (Fig. 1). Moreover, it is more eas-

ily fabricated in that it will not collapse

as readily on bending. It is also easily

mitered to create sharp, close bends as

required. If only round tubing is avail-able, it can be flattened in a vise or other

simple device to adjust the resultant

thickness dimension. This flattening

can be done with minimal decrease in

dimension of the water-flow path.

Coil forming

In fabrication of copper coils, it must be

noted that the copper work hardens with

increasing 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, then

cooling it rapidly in water. These in-

termediate anneals prevent fracture of

the tubing during fabrication.

In some forming operations, it may

be desirable to fill the coil with sand or

salt to preclude collapse of the tubing.

In addition, there are several low-tem-

perature alloys-with melting points be-

low 212F (100C)-that are normally usedto perform this same function. When the

coil is completed, it is immersed in boil-

ing water. The alloy then flows out

freely and can be reused at another time.

With any of these techniques, once

filled, the tubing acts as a solid rod dur-

ing forming and can be simply cleared

on completion.

Bracing of coils

Because electric currents flow in both

the 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 may

cause the part to move in the coil. If the

part has a large mass, however, the coil

will tend to move relative to the

workpiece. The turns may also tend to

move relative to each other. It is impor-

tant, therefore, that the coil turns be

suitably braced to prevent movement

and possible turn-to-turn shorting. Fur-

thermore, coil motion relative to the partmust be prevented to avoid undesirable

changes in the heating pattern.

Much of the acoustic noise gener-

ated during low-frequency operations

also occurs due to coil vibration, much

as a speaker coil and magnet structure

work in an audio system. Bracing and

physical loading of the coil to restrict

its movement will aid in reducing this

condition. On very large, high-current

coils, the magnetomotive force exerted


11/12


40 HEAT TREATING/OCTOBER 1988

can be extremely large, and if proper

bracing i s not provided; the coil may

gradually work harden and finally fail.

Typical bracing techniques are il-

lustrated in Fig. 2. In Fig. 2a, brass

studs are brazed to every other turn.

These studs are then secured to in-

sulator posts to hold them in a fixed

relation to each other. Nuts on each

side of the stud at the insulator allow

adjustment for characterization of the

heating pattern. In Fig. 2b, the insu-

lation has been contoured to hold the

turns relative to each other after the

end turns are secured with studs.

The insulation used for bracing ap-

pl icat ions must meet the cr iter ia for

the coil design. In addition to the in-

stallation being capable of withstand-

ing the heat radia ted f rom the

workpiece, its electrical capabilities

must permit it to with-stand the volt-

age between the mounting studs or

the turn-to-turn voltages of the coil.

This is of particular concern when

using high-voltage RF coils where up

to 12,000v may be impressed across

the total coil. It may be necessary in

these instances to provide slots be-

tween the stud locations in the insu-lator boards to increase the electrical

creepage path between the studs. It

may also be necessary to increase the

heat-resistant characteristics of the

insulation by facing the area exposed

to the heated surface with a sheet of

high-temperature insulation.

For purposes of rigidity, clean-

ness, and protection, it is sometimes

desirable to encapsulate work coils in

a plastic or refractory material. The

same kind of care with respect to volt-

age and temperature characteristicsmust be taken with these materials as

with insulating boards. For low-tem-

pera ture induct ion heat ing appl ica-

tions, epoxy encapsulation of the coil

is quite common. For heating of steel

bi llets, coils are usually cast in a re-

fractory cement to prevent scale from

the part from falling between the

turns. In coating of coils with refrac-

tory materials, care must be taken to

match the pH of the refractory to that of

the material being heated; for example,

an acidic refractory is required for the

ferrous scale that drops off during high-

temperature heating of steels.

Design considerations

All coils represent an inductance tothe tank circuit. However, in practice,

the working portion of the coil may in

fact be only a small portion of the in-

ductance presented to the tank. Be-

tween the output terminals of the

generator or heat station and the heat-

ing portion of the work coil, there may

be a considerable distance of output

lead. In any case, some finite distance

exists between the heat-station termi-

nations and the actual coil. Design

and construction of these work-coil

leads can be a major fac tor in

determining job feasibility.

The effect of lead construction on

system performance can be best un-

derstood with respect to the tank cir-

cuit of which it is a part (Fig. 3). The

coil/load inductance is represented

by L2. Each lead connecting the tank

capacitor to the coil has its own in-

ductance (L1, L

3). If the voltage in the

tank, ET, is impressed across the to-

tal of these inductances, then some

voltage 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 approximately

10 times the total inductance of the

leads (L1plus L

3) or greater, a maxi-

mum of 10% of the total voltage will

be lost in the leads. Any loss less than

this can be considered nominal.

Some coils have many turns, a

large cross-sectional area, and thus

fairly high inductance. Hence, the

comparative lead inductance is small.

As the frequency increases, coils of-ten become smaller in size, and their

inductance and inductive reactance

decrease. As the distance between

the heat station and coil increases,

therefore, these lead inductances can

become cri tical.

Several coil designs that illustrate

the effect of lead design are shown in

Figs. 4 and 5. In Fig. 4a, a coil with

leads far apart is depicted. The space

between the leads p resents an induc-

Coil design

Fig. 2: Typical techniques for bracing of

induction-coil turns (from F.W. Curtis, High


New York, 1950)

Table I: Selection of copper tubing for induction coils.


12/12

41HEAT TREATING/OCTOBER 1988

tance almost equal to that of the co il.

Thus, a major portion of the voltage

will not appear in the working area. A

better design (Fig. 4b) minimizes this gap

and thus improves heating efficiency. Fig.

5b also shows single-turn, multiplace coils

with an extremely poor and an improved

lead design.

Another factor to consider is the in-

teraction of the leads with nearby metal

structures. Because all leads have some

inductance, they can act as work coils.

Thus, a conductor placed within their field

will be heated. Leads placed adjacent to

metal structures will tend to heat them. In

addition to unwanted heat, this loss re-

duces the power available to the load. It

is important that lead-to-lead separation

be minimized and proximity to metallic

structural members be considered. When-

ever possible, duct housings, trays, or

conduits must be of low-resistivity or in-

sulating materials, such as aluminum or

plastic.

Typical lead design

Induction 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 structure

known as a fishtail is often utilized. A

fishtail is a pair of parallel copper

plates that are water cooled to main-tain low resistivity. They are placed

with their wide bus faces parallel, and

are either separated physically with

air as an insulator or held together by

nylon bolts and nuts with teflon or a

similar material acting as a spacer.

Extending from the heat station to a

point as close as possible to the op-

erating area of the work coil, they

present minimum inductance and pro-

vide maximum power at the coil. De-

pending on condi t ions and con-struction, efficient runs of up to 15

feet are practical. The thickness of the

copper plates should be consistent

with the frequency, as noted in Table

I, and cooling-water paths and sizes

must be consistent with the power

being transmitted as well. The copper

plates should increase in width with

generator power and the distance of

the run. Moreover, they should be

spaced 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 less

critical, and plain copper tubing leads then

become more practical. However, larger

coils also require higher terminal voltages.

These leads must also be kept as close as

possible to each other while maintaining

sufficient spacing to prevent arcing. How-

ever, good practice still dictates that coil

leads be kept to a minimum length and

that copper tubing sizes be used that

are consistent with frequency, current,

and cooling requirements.

Rigid leads, whether tubing or bus,

buil t 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 use

flexible connections. There are several

variations in flexible leads, but it must

be kept in mind that the inductive lead

losses in flexible cables are usually

much greater than those for rigid con-

nections. The most common flexible lead

is generally used in applications similar

to tilt-type induction melting furnaces

and consists of a water-cooled, spiral-

wound inner conductor (similar to BX

cable, but made of copper) with an outer

insulating covering. These leads are

used in pairs with one for each lead con-

nection. Not only must they be sized

for current and frequency, but the insu-

lation must be capable of handling the

voltage rating of the system. Flexible

leads should be tied together with in-

sulating straps.

Coaxial leads are also available and

may be rigid or flexible. They consist of

an inner conductor and an outer sheath

or housing that is also used as the re-

turn conductor. This outer sheath is

generally at ground potential. In addi-

tion to providing an extremely low-inductance lead, the outer ground acts

to eliminate possible strong radiation or

inductive coupling to adjacent struc-

tures.

Rigid coaxial lead is generally quite

expensive and is usually limited to those

applications where it is imperative to

transmit high power at high frequency

over some distance.

Another type of coaxial cable is the

water-cooled type generally used at ra-

dio frequencies. It consists of a low-in-ductance, braided inner conductor that

runs through a water-cooled tube, and

an outer return braid that is also water

cooled. This construction is generally

utilized with medium-to-high-induc-

tance coils because its construction

does not greatly minimize lead induc-

tance but does provide flexibility. This

last type of lead is most common when

the operator must physically move the coil

from part to part as in bottle sealing.

Fig. 3: Schematic circuit diagram indicat-

ing the inductance of the coil leads and

induction coil itself: L1, L3-lead inductances;L

2-induction-coil inductance; C

1-tank capaci-

tance; E1-tank voltage

Fig. 4: Effect of coil-lead spacing on lead in-

ductance; closer spacing, as in (b), reduces

lead inductance and thus power losses (from

F.W. Curtis, High Frequency Induction Heat-


Fig. 5: Lead construction for multiplace in-

ductors; lead design in (b) is preferable

because of lower heat inductance (from F.W.

Curtis, High Frequency Induction Heating,


Excessive inductance
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