Induction Coil and Coreless Induction Furnace Employing Same

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(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: (51) int. CI.6: H05B 6/22, H05B 6/36 12.08.1998 Bulletin 1998/33

(21) Application number: 97304802.8

(22) Date of filing: 02.07.1997

(84) Designated Contracting States: • Peysakhovich, Vitaly A. AT BE CH DE DK ES Fl FR GB GR IE IT LI LU MC Moorestown, New Jersey 08057 (US) NLPTSE • Prabhu, Satyen A. Voorhees, New Jersey 08043 (US) (30) Priority: 10.02.1997 US 797148 . Krupnick, Laurence A. Huntingdon Valley, Pennsylvania 19006 (US) (71) Applicant: INDUCTOTHERM CORP. Rancocas, New Jersey 08073-0157 (US) (74) Representative: Perkins, Sarah Stevens, Hewlett & Perkins (72) Inventors: ■! Serjeants' Inn • Fishman, Oleg S. Fleet Street Maple Glen, Pennsylvania 1 9002 (US) London EC4Y 1 LL (GB) • Mortimer, john H. Mt. Laurel, New Jersey (US)

(54) Induction coil and coreless induction furnace employing same

(57) An induction coil for inductively heating electrically conductive materials includes a plurality of individual coil turns, each turn lying in a plane substantially perpendicular to a longitudinal axis of the coil and comprising an electrical conductor formed into an annulus. The conductor has first and second terminals for connecting the turn to an electrical circuit. The first and second terminals are adjacent each other at a preselected circumferential position on the annulus and are physically and electrically isolated from each other. The first terminal of one turn is located adjacent and electrically connected to the second terminal of an adjacent turn. The first terminal of a selected one of the plurality of turns forms a first coil terminal and the second terminal of a different selected one of the plurality of turns forms a second coil terminal.

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Description shape. Welding or brazing lengths of copper tubing together to make a large enough coil present readily Field of the Invention apparent disadvantages of their own. The pitch of the helical winding, especially when The present invention relates to induction heating 5 large diameter tubing is used in high power furnaces, and melting apparatus, such as for heating and melting causes complications in mounting the coil in the fur- metals, and relates particularly to coreless induction furnace, which has flat top and bottom surfaces. naces with an improved coil and magnetic shunt design. In conventional helically wound induction coils, current flows from the bottom turn to the top turn (or vice Background of the Invention 10 versa) and the returns vertically down (or up) via the termination tube to the bottom (or top) turn. While the mag- Induction heating apparatus such as induction fur- netic field Hc of the coil windings is concentrated inside naces or ladles for heating or melting metals operate on the furnace in the direction of the axis of the coil, the the principle of inducing eddy currents in an object magnetic field H, of the current in the termination tube is (sometimes referred to as the load) to be heated. The 15 spread in the area around the tube in the plane of the eddy currents cause the load to act as its own heat coil turns. This stray magnetic field induces eddy cur- source. Power is generated in the load by resistive heat- rents in surrounding metal objects, causing them to ing caused |>y the eddy currents, according to the well- become heated. known P=l R heating principle. As used herein, "heating" is used broadly to encompass not only raising the 20 Summary of the Invention temperature of a material without causing the material to change state, but also melting, wherein the tempera- One aspect of the invention is an induction coil for ture of a material is raised sufficiently to cause it to inductively heating electrically conductive materials. change state. The induction coil comprises a plurality of individual coil In a typical induction furnace, metal to be heated is 25 turns, each turn lying in a plane substantially perpendic- contained in a crucible, and a generally helical induction ular to a longitudinal axis of the coil and comprising an coil surrounds the crucible. The induction coil is water electrical conductor formed into an annulus. The con- cooled. The crucible is usually made of a ceramic ductor has first and second terminals for connecting the refractory material. The eddy currents are induced in turn to an electrical circuit. The first and second termi- the load by passing a high-frequency alternating current 30 nals are adjacent each other at a preselected circumfer- through the induction coil to generate a time-varying ential position on the annulus and are physically and magnetic field, or induction field. Depending upon the electrically isolated from each other. The first terminal of magnitude and frequency of the alternating current in one turn is located adjacent and electrically connected the induction coil, and on other design considerations, to the second terminal of an adjacent turn. The first ter- the induction field can be used for melting, heating, 35 minal of a selected one of the plurality of turns forms a and/or stirring a quantity of molten metal in the crucible. first coil terminal and the second terminal of a different The induction field can also be used for heat treating selected one of the plurality of turns forms a second coil workpieces, and for other procedures. terminal. In virtually all coreless induction furnaces, the Another aspect of the invention is an induction fur- induction coil is constructed of several turns of heavy 40 nace comprising a refractory vessel for holding a quan- wall copper tubing shaped into the form of a helix. Alter- tity of electrically conductive material to be heated, an nating electrical current is conducted through the coil induction coil generally surrounding the vessel for via termination tubes connected to the top and bottom inductively heating electrically conductive material in turns of the coil. Heat generated in the coil turns is the vessel, and a plurality of magnetic shunt assemblies removed by water pumped through the copper tubing. 45 arranged circumferentially around the induction coil for Often, the same termination tubes are used for to con- directing magnetic flux generated by the induction coil nect the coil to both a cooling water supply and a source to the material to be heated in the vessel. The induction of electrical current. As a practical matter, the termina- coil comprises a plurality of individual coil turns, each tion tubes usually are located near each other at one turn lying in a plane substantially perpendicular to a lon- end of the coil. 50 gitudinal axis of the coil and comprising an electrical The coil is preferably made from one continuous conductor formed into an annulus. The conductor has length of copper tubing, or sections of tubing welded or first and second terminals for connecting the turn to an brazed into one continuous length. One drawback of electrical circuit. The first and second terminals are this method of construction is that is does not have adjacent each other at a preselected circumferential great hoop strength. Another is that it is necessary to position on the annulus and are physically and electri- maintain large inventories of copper tubing for making cally isolated from each other. The first terminal of one coils, and to have machinery for winding the copper tub- turn is located adjacent and electrically connected to ing (which is often of large diameter) into a helical the second terminal of an adjacent turn. The first termi-

2 3 EP 0 858 246 A2 4 nal of a selected one of the plurality of turns forms a first assembly of Figure 8, taken along the lines 9-9 in Figure coil terminal and the second terminal of a different 8. selected one of the plurality of turns forms a second coil terminal. Description of the Invention Yet another aspect of the invention resides in the 5 magnetic yokes used with the induction furnace accord- Referring now to the drawings, wherein like numer- ing to the invention. The furnace includes a plurality of als indicate like elements, there is shown in Figure 1 an magnetic yokes arranged at axially opposite ends of the induction furnace 10 with a conventional helically induction coil and a plurality of magnetic shunts wound induction coil 12 as is known in the art. Induction arranged circumferentially around the induction coil. 10 coil 12 as illustrated in Figure 1 is a conventional two Each magnetic shunt comprises a plurality of lamina- section coil, comprising oppositely wound top section tions arranged in a stack. Each lamination has lateral 12a and bottom section 12b, but induction coil 12 could edges facing the induction coil and lying along a portion equally consist of a single section. As discussed above, of the circumference of a circle having a diameter sub- induction coil 12 is constructed of heavy wall copper stantially equal to the outer diameter of the induction 15 tubing wound helically into a plurality of turns 14. AC coil turns, and each lamination has ends adjacent corre- electric current is connected to each section of the sponding axially opposite ends of the induction coil. At induction coil by means of termination tubes 16 con- least one clamp is provided for holding the laminations nected to the top and bottom turns of the coil. The sec- in said stack. A cast aluminum heat sink surrounds the tions of the coil which are supplied with AC current are stack except for the lateral edges and ends of the lami- 20 often referred to as the "active windings." The coil turns nations. 1 4 are electrically isolated from each other, and are iso- These and other aspects of the invention will be lated from the termination tubes 16 as well. Heat gener- apparent from the following description and the ated by the flow of AC current in the active windings is appended claims. removed by water pumped through the copper tubing. 25 Often, termination tubes 16 are used to connect the Description of the Drawings active windings to both the AC current and a source of cooling water. For the purpose of illustrating the invention, there is Although it is omitted from Figure 1 for clarity, those shown in the drawings a form which is presently pre- skilled in the art will understand that a refractory vessel ferred; it being understood, however, that this invention 30 or crucible is placed inside the helically wound induction is not limited to the precise arrangements and instru- coil 12. The crucible holds metal to be inductively mentalities shown. heated by induction coil 1 2. Also omitted for clarity is the Figure 1 illustrates a conventional helically wound furnace shell, which surrounds and supports the coil 12, induction coil for a coreless induction furnace according the crucible, and the other furnace elements. Typically, to the prior art. 35 the furnace shell is metal, and the furnace may also Figure 2 illustrates an induction coil according to include other metal components. the present invention. Induction coil 12 generates an electromagnetic field Figure 3 is a partial sectional view of a coreless Hc, illustrated by the broken lines in Figure 1 . Usually, a induction furnace incorporating an induction coil magnetic assembly comprising circular yokes and verti- according to the present invention. go cal shunts (omitted from Figure 1 for clarity) are placed Figure 4 is a side elevational view, partially in sec- around the outer circumference of induction coil 12. The tion, of a portion of a coreless induction furnace incor- vertical shunts are made of thin laminations of electro- porating an induction coil according to the present magnetic steel, similar to a transformer coil. The lamina- invention, showing how individual coil turns are inter- tions are clamped into a sold stack by one or more "U"- connected. 45 shaped or "C"-shaped brackets made of a non-mag- Figure 5 is a top plan view, partially broken away, of netic metal, such as stainless steel, aluminum, or cop- the coreless induction furnace illustrated in Figure 3. per alloys. The surface of the lamination stacks facing Figure 6 is a transverse sectional view taken along the induction coil 12 are typically curved to follow the the lines 6-6 in Figure 3, and illustrating a magnetic contour of the outer circumference of the coil 12. This is shunt arrangement for directing magnetic flux gener- so done by supporting the laminations in a stack such that ated by the induction coil. the surfaces of the laminations which face the induction Figure 7 is a side view of a portion of an induction coil 12 are parallel to and have the same curvature as coil according to the present invention, partially in sec- the outer circumference of coil 12. The sides of the tion, illustrating how coolant flows from one coil turn to shunts not facing the coil are protected from stray mag- another. 55 netic flux by non-magnetic copper or aluminum side Figure 8 is an isometric view of one of the magnetic plates. The side plates may be water-cooled to remove shunt assemblies illustrated in Figure 7. excess heat. Figure 9 is a sectional view of the magnetic shunt Often, the magnetic assembly extends axially

3 5 EP 0 858 246 A2 6 above and below the top and bottom of the induction coil tion coil 24 to comprise two or more sections. As with 12 to capture magnetic flux which curves around the conventional induction coil 12, induction coil 24 has a ends of the coil 1 2 and direct that flux into the interior of plurality of individual turns 28, and electrical and cooling the coil to improve the coupling efficiency of the coil 12 water connections are made to coil 12 via termination to an object being inductively heated by the coil 12. In 5 tubes 30. Termination tubes 30 are essentially of the such cases, additional turns 1 8 may be wound above same construction as termination tubes 1 6 known in the and below the active windings. The additional turns 1 8 art and, as in the prior art, connect opposite ends of are not connected to the AC current, but are used to induction coil 24 to an electrical current source and a support the refractory crucible in the area where the source of cooling water. magnetic yokes extend above and below the active w Induction coil 24 differs from conventional induction windings. These additional turns are often referred to as coil 1 2 most prominently with respect to how the individ- "cooling turns." ual turns 28 are formed. Instead of being wound from a As noted above, the pitch of the helically-wound continuous length of copper tubing, induction coil 24 is induction coil 12 causes complications in mounting the made up of a plurality of discrete, individual turns 28 of coil 1 2 in the furnace, which has flat top and bottom sur- is copper tubing formed into a flat annulus, or ring, which faces. To compensate for the tilt of the coil turns 14 due are connected together to form a complete coil 24. An to the pitch, support fins 20, typically metallic, are individual turn 28 is illustrated in a top plan view in Fig- welded to the top and bottom turns of each coil section ure 6. Turn 28 is circular in shape, and has any desired 1 2a and 1 2b. When the induction coil 1 2 consists of two inner diameter, outer diameter, tubing diameter, and oppositely wound sections 1 2a and 1 2b, as illustrated in 20 tubing wall thickness. Those dimensions can be deter- Figure 1 , fins 20 are required in the center of the fur- mined by the coil designer depending on the particular nace also, between the two coil sections. Since the fins application of the coil 24, and do not differ materially 20 are metallic and are welded to the coil turns, they will from the design considerations for conventional induc- conduct current and therefore generate heat. To remove tion coils. that heat, the fins 20 need to be water cooled. 25 As seen in Figure 2, each turn 28 is flat, or planar, Also as noted above, current in the helically wound so that the individual turns 28, when connected together induction coil 12 flows in the vertical portions of the ter- to make up a complete coil 24, form a coil in the shape mination tubes 16, and that current generates a mag- or a right circular cylinder instead of a helix. Each turn netic field H, around the termination tubes. While the has two ends 32 and 34 which overlap and are joined magnetic field Hc generated by the active windings of 30 together at an overlap joint at a selected circumferential induction coil 12 is concentrated inside the furnace 10 position on the annulus, as best seen in Figures 6 and along the axial direction of the coil 1 2, the magnetic field 7. Each end 32 and 34 is sealed by a plate 36 so that, H, around the termination tubes 16 spreads out in the when the ends 32 and 34 overlap, plates 36 prevent area around the tubes in the plane of the turns 1 4. Mag- communication between the interior of the tubing at netic field H, induces eddy currents in surrounding 35 ends 32 and 34 across the overlap joint. Plates 36 also metal objects, such as the furnace shell, causing them serve to mechanically join the ends 32 and 34 together to become heated. with appropriate fasteners such as a nut and bolt Still further, the magnetic field H, which crosses the arrangement 38 or other fasteners. A thin electrically cooling turns 18 at each end of the induction coil 12 insulating layer 40 is located between ends 32 and 34, induces eddy currents in the cooling turns, causing 40 so that ends 32 and 34 are electrically, as well as phys- additional losses which contribute to reduced furnace ically, isolated from each other. Thus, when ends 32 and efficiency. 34 are connected to an electrical current source, current These problems can be eliminated by the induction will flow from one end of the turn to the other. It can be coil of the present invention. A furnace 22 incorporating seem that this construction provides a turn which is flat, an induction coil 24 according to the present invention is 45 and which is a mechanically closed but electrically open illustrated in Figure 2. As with the furnace 10 illustrated circle. in Figure 1 , some of the non-essential details of the fur- Each turn 28 has two flow openings 42 and 44 by nace 22 are omitted from Figure 2 for the sake of clarity. means of which a coolant, such as coiling water, can be However, those skilled in the art will have no trouble supplied. As illustrated by the arrows in Figure 7, the understanding the invention in spite of those omissions, so coolant entering the tubing making up the turn through Furnace 22 comprises induction coil 24 and a opening 42 encounters the plate 36 closing end 32, and refractory vessel or crucible 26 located inside induction is directed to the right as viewed in the figure. The cool- coil 24. The crucible holds metal to be inductively ant circulates through the turn until it reaches the plate heated by induction coil 24. Omitted for clarity is the fur- closing end 34. When the coolant encounters plate 36 nace shell, which surrounds and supports the coil 24, ss closing end 34, it is directed out of the tubing making up the crucible 26, and the other furnace elements. Induc- turn 28 through opening 44. Thus, the coolant makes tion coil 24 is illustrated as a single section coil, one "round trip" through the turn before exiting. although it is within the scope of the invention for induc- Two or more turns 28 may be connected together at

4 7 EP 0 858 246 A2 8 their flow openings 42 and 44 by connectors 46. Each prises a plurality of rectangular packs 62 of transformer connector 46 comprises a short length of pipe 48 which iron laminations 64. The number of packs is equal to the has a center portion 50 in the shape of a hex nut, so that number of vertical shunts 60. The yokes 58 are fabri- the connector can be turned by a wrench. (Although the cated by placing the lamination packs 62 into a circular connector 46 as described and illustrated is assumed to 5 mold. If desired, copper cooling tubes (not shown) can be a one-piece connector, it is not so limited, and may also be placed in the mold. The copper cooling tubes, if be made up of more than one piece.) The outer diame- used, would have appropriate terminations for connect- ter portions of pipe 48 which extend axially from center ing the tubes to a source of coolant. After the lamination portion 50 are threaded to engage corresponding packs 62 and cooling tubes, if desired, are placed in the threads in flow openings 42 and 44 in turn 28. The 10 mold, the mold is filled with molten aluminum. Once the threads on the outer diameter portions of pipe 48 have aluminum solidifies, the circular yoke is removed from opposite senses, however, i.e., one is right-handed and the mold. the other is left-handed, so that when two turns are to The vertical shunts 60, best seen in Figures 8 and be connected by connector 46, rotation of the connector 9, like the yokes 58, comprise packs 66 of iron lamina- in the clockwise sense threads connector 46 into both 15 tions 68. The laminations 68 have a length equal to the turns simultaneously and rotation in the counterclock- axial length of the induction coil 24. The shunts 60 are wise sense threads connector 46 out of both turns fabricated by clamping the laminations 68 to form packs simultaneously (or vice versa). 66. As the laminations are clamped, they are arranged Pipe 48, being hollow, enables coolant to flow from so that the edges 70 of the laminations which will face the outlet flow opening 44 of one turn 28 to the inlet flow 20 the induction coil 24 follow a curvature substantially opening 42 of the adjacent turn 28. Thus, connector 46 equal to the outer diameter of the coil. To facilitate this provides a mechanical and fluid flow connection arrangement, the laminations are placed in a specially between adjacent turns. Connector 46 is conductive, so designed mold 72 which has side walls 74 and a curved that it also provides an electrical connection between guide plate 76 which is used to support the laminations adjacent turns. 25 70 and provide the desired curvature. The widths of When the desired number of turns are connected to laminations 68 at the ends of the pack 66 may be form a coil 24, electrical and coolant connections are trimmed, if necessary, to have the inner edges 70 con- made at the turns 28 located at opposite ends of the form to the curvature of the guide plate 76. Once the coil. For example, as shown in Figures 2 and 4, termina- laminations 68 are in place, they are held in place by tion tubes 52 may be connected to the top and bottom 30 clamps 78. If desired, copper cooling tubes 80 are turns of the coil using connectors 46. Termination tubes located along the length of shunts 60, and are inserted would otherwise be the same as the termination tubes through holes in clamps 78. The ends of cooling tubes 16 known in the art. Since each turn 28 of the coil 24 is 80 are provided with terminations by which they may be flat, the resulting coil is in the shape of a right circular connected to a source of coolant. Once all of the lami- cylinder. This makes it very simple to mechanically sup- 35 nations, clamps, and cooling tubes are in place, the port the coil, as shown in Figure 3. mold 70 is filled with molten aluminum. Once the alumi- As shown in Figure 3, the turns 28 are supported by num 82 solidifies, shunt 60 is complete. insulating spacers 54 between adjacent turns. Alternat- Although not shown in the figures, the ends of the ing pairs of turns are held together by straps 56. Prefer- mold 70 are arranged so that the molten aluminum cov- ably, straps 56 comprise several wraps of KEVLAR tape 40 ers only the lateral sides of the lamination pack 66 and or other insulating tape which has a high tensile the side 84 that faces away from the induction coil 24. strength. However, as those skilled in the art will appre- The ends 86 and 88 and the side that faces the induc- ciate, other ways of supporting and mounting the turns tion coil remain exposed. may be used without departing from the scope of the As best seen in Figure 3, the yokes 58 are placed at invention. 45 both ends of the induction coil 24 and are then con- As illustrated in Figure 2, the magnetic field Hc of nected together by tie rods 86. The tie rods 86 are even coil 24 extends outside the coil for some distance. located at regular intervals around the circumference of The field outside the coil can be problematic and inter- the yokes 58, as can be seen in Figure 6. The tie rods fere with external equipment, and at the very lease 86 connect the two yokes 58 and compress the coil leads to furnace inefficiencies. To solve that problem, a so turns 28 between the yokes 58 to minimize turn move- magnetic system comprising magnetic yokes and ment and coil vibration when in use. Once the coil turns shunts is used. The magnetic system provides a low 28 are properly compressed, the shunts 60 are placed reactance return path for the magnetic field outside the circumferentially around the coil 24, with the guide plate coil. The magnetic system comprises composite yokes 76 facing the coil. The shunts 60 are placed so that the 58 placed at the top and bottom of the coil 24, and a plu- 55 lamination packs 66 of the shunts 60 are in alignment rality of vertical shunts 60 magnetically connecting the with the lamination packs 62 of circular yokes 58. The yokes 58. shunts 60 are held in place by a system of horizontal As best seen in Figure 3, circular yokes 58 com- bars 88, through which compression rods 90 are

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inserted. One end of the compression rods 90 bears minal of a selected one of the plurality of turns form- against the cast aluminum portion 82 of the shunts 60. ing a first coil terminal and the second terminal of a Tightening the compression rods 90 holds the shunts 60 different selected one of the plurality of turns form- in place and compresses the induction coil 24 in the ing a second coil terminal. radial direction. 5 The entire induction coil 24 and magnetic yoke and 2. An induction furnace comprising shunt assembly may be mounted in either a steel shell or a steel frame furnace body. a refractory vessel for holding a quantity of It will be appreciated from the foregoing description electrically conductive material to be heated, that the induction coil of the present invention and the 10 an induction coil generally surrounding the ves- induction furnace constructed using the coil provide sel for inductively heating electrically conduc- several advantages. The coil is almost exactly in the tive material in the vessel, the coil comprising a shape of a right circular cylinder. This simplifies mount- plurality of individual coil turns, each turn lying ing the coil in the furnace body. Each coil turn can be in a plane substantially perpendicular to a lon- manufactured separately, eliminating the need to inven- 15 gitudinal axis of the coil and comprising an tory tube stock and to handle a heavy, one piece coil electrical conductor formed into an annulus, until final assembly. The electrical current ascends the the conductor having first and second terminals coil from turn to turn via the connectors 46, all of which for connecting the turn to an electrical circuit, are in a line along the coil circumference, instead of the first and second terminals being adjacent being distributed along the winding. The electrical cur- 20 each other at a preselected circumferential rent in the termination tubes flows in the direction oppo- position on the annulus and being physically site to the electrical current in the connectors, which and electrically isolated from each other, the minimizes stray magnetic fields due to current flow in first terminal of one turn being adjacent and the termination tubes. Since the individual turns are fab- electrically connected to the second terminal of ricated separately, they can be made in different sizes 25 an adjacent turn, the first terminal of a selected and connected together as desired when constructing a one of the plurality of turns forming a first coil coil. Thus, the top and bottom turns may have different terminal and the second terminal of a different dimensions (such as cross section) to minimize losses. selected one of the plurality of turns forming a Moreover, no cooling turns are needed. Since the mag- second coil terminal, and netic system is molded from aluminum, heat generated 30 a magnetic assembly arranged around the in the laminations of the yokes and shunts is very effi- induction coil for directing magnetic flux gener- ciently conducted to the cooling tubes for heat removal. ated by the induction coil to the material to be The coil and magnetic assembly is self-contained and is heated in the vessel. independent of the design of the furnace body. Other advantages and benefits of the invention will 35 3. An induction furnace according to claim 2, wherein suggest themselves to those skilled in this art. the magnetic assembly comprises a plurality of The present invention may be embodied in other magnetic yokes arranged at axially opposite ends specific forms without departing from the spirit or essen- of the induction coil and a plurality of magnetic tial attributes thereof and, accordingly, reference should shunts arranged circumferentially around the induc- be made to the appended claims, rather than to the fore- 40 tion coil. going specification, as indicating the scope of the invention. 4. An induction furnace according to claim 3, wherein each magnetic shunt comprises Claims 45 a plurality of laminations arranged in a stack, 1. An induction coil for inductively heating electrically each lamination having lateral edges facing the conductive materials, comprising a plurality of indi- induction coil and lying along a portion of the vidual coil turns, each turn lying in a plane substan- circumference of a circle having a diameter tially perpendicular to a longitudinal axis of the coil substantially equal to the outer diameter of the and comprising an electrical conductor formed into so induction coil turns, each lamination having an annulus, the conductor having first and second ends adjacent corresponding axially opposite terminals for connecting the turn to an electrical cir- ends of the induction coil, cuit, the first and second terminals being adjacent at least one clamp for holding the laminations each other at a preselected circumferential position in said stack, and on the annulus and being physically and electrically 55 a heat sink surrounding the stack except for the isolated from each other, the first terminal of one lateral edges and the ends of the laminations. turn being adjacent and electrically connected to the second terminal of an adjacent turn, the first ter- 5. An induction furnace according to claim 4, wherein

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the heat sink comprises cast aluminum.

6. An induction furnace according to claim 5, wherein the cast aluminum heat sink includes at least one passage therein through which a coolant medium s may flow.

7. An induction furnace according to claim 6, wherein the passage comprises a copper tube. 10 8. An induction furnace comprising

a refractory vessel for holding a quantity of electrically conductive material to be heated, an induction coil generally surrounding the ves- is sel for inductively heating electrically conductive material in the vessel, the coil comprising a plurality of individual coil turns, each turn lying in a plane substantially perpendicular to a longitudinal axis of the coil and comprising an 20 electrical conductor formed into an annulus, the conductor having first and second terminals for connecting the turn to an electrical circuit, the first and second terminals being adjacent each other at a preselected circumferential 25 position on the annulus and being physically and electrically isolated from each other, the first terminal of one turn being adjacent and electrically connected to the second terminal of an adjacent turn, the first terminal of a selected 30 one of the plurality of turns forming a first coil terminal and the second terminal of a different selected one of the plurality of turns forming a second coil terminal, and a plurality of magnetic yokes arranged at axially 35 opposite ends of the induction coil and a plurality of magnetic shunts arranged circumferentially around the induction coil, wherein each magnetic shunt comprises 40 a plurality of laminations arranged in a stack, each lamination having lateral edges facing the induction coil and lying along a portion of the circumference of a circle having a diameter substantially 45 equal to the outer diameter of the induction coil turns, each lamination having ends adjacent corresponding axially opposite ends of the induction coil, at least one clamp for holding the lamina- so tions in said stack, and a cast aluminum heat sink surrounding the stack except for the lateral edges and ends of the laminations.

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