s\ mi mi ii 111 mi ii i inn ii i ii OJII Eur°Pean Patent Office <*S Office europeen des brevets (11) EP 0 858 246 A2 (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 electri- cally conductive materials includes a plurality of individ- ual coil turns, each turn lying in a plane substantially perpendicular to a longitudinal axis of the coil and com- prising an electrical conductor formed into an annulus. The conductor has first and second terminals for con- necting the turn to an electrical circuit. The first and sec- ond terminals are adjacent each other at a preselected circumferential position on the annulus and are physi- cally 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. CM < CO CM CO LO CO o Q_ LU Printed by Xerox (UK) Business Services 2.16.3/3.4 1 EP 0 858 246 A2 2 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 fur- nace, which has flat top and bottom surfaces. naces with an improved coil and magnetic shunt design. In conventional helically wound induction coils, cur- rent 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 ter- mination 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, "heat- ing" 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.