US2748240A - Induction heating systems - Google Patents

Description

May 29, 1956 B. E. MGARTHUR INDUCTION HEATING SYSTEMS Filed July 29, 1952 INVENTOR INDUCTION HEATING SYSTEMS Bruce E. McArthur, Youngstown, Ohio, assigner to Magnethermic Corporation, Youngstown, Ehio, a corporation of Ohio Application July 29, 1952, Serial No. 303,342

2 Claims. (Cl. 219--10.75)

The present invention relates to improvements in electrical induction furnaces, and relates more particularly to the provision of improvements adapted to effect differing advantageous distribution of the heating effect of alternating currents directed through the inductor coil or coils thereof, along the length of an elongated metal work-piece telescoped radially within the induction coil winding convolutions.

The invention hereof, although not limited thereto, nds ready application to that type of induction furnace of which that disclosed in the prior pending application of Lackner et al., Serial No. 205,766 filed January 12, 1951, now Patent No. 2,676,234 issued February 2, i954, is an example, and which disclosed a furnace having a multi-section winding comprising a coil unit consisting of plurality of relatively axially aligned, induction heating coils, each said coil consisting of a helical conductor which is preferably disposed in a single layer of convolutions, and the coils of the unit being each energized by a different flow of single phase alternating current, and said coil unit being adapted for the reception of an elongated metal work-piece telescoped into a heating compartment provided within the conductor convolutions thereof.

ln the operation of electrical induction furnaces of the general type exemplified by that of said prior application, wherein the coil unit comprises a set of three longitudinally successive induction heating coils, l find that, when the three induction coils of the coil unit are energized, the portions of an elongated metal workpiece, which are disposed nearer to those regions of the three-coil unit at which opposite longitudinal ends of the intermediate of said coils are respectively adjacent to a longitudinal end of a respectively different one of the other coils of the unit, are heated more slowly, and

therefore do not, in the same period of time, achieve the same degree of heating as do the portions of the work-piece which are disposed radially within the middle convolutions of each coil of the unit.

The effect of a greater concentration of magnetic lines of force in the longitudinally medial regions of each energized coil of the unit, and the effects of variations in heat radiation in different longitudinal portions of the work-piece are contributing factors to the above result, but the principal contributing factor resides in the difference in phase of the three magnetic ux elds, which result from concurrent energization of the three aligned induction coils, and such factors is mainly adversely effective in the heating of portions of the length of the elongated metal work-piece which are nearer to the regions where opposite longitudinal ends of different of said coils are adjacent.

rThe improvements hereof are directed toward the amelioration and correction of such undesired factors, whereby an elongated metal work-piece may have different portions of its length heated more nearly to the same desired temperature.

Again it is often desirable to effect herein so-called i nited States Patent CII 'ice

tapered heating of an elongated metal Work-piece to achieve predetermined gradients of inductive heating effort in different portions of its length, whereby dierent relatively longitudinally displaced portions of the work-piece may be heated, either to the same, or to respectively different predetermined temperatures, under conditions wherein the said work-piece portions are of the same, or different cross-sectional areas.

Different of the present improvements are adapted to singly, or co-operatively, achieve the above recited desired results or any thereof.

An object of my invention, therefore, is to provide improved means to achieve concurrent heating of different longitudinally aligned portions of an elongated metal work-piece by directly alternating current through the winding convolutions of an induction heating coil unit in an improved manner to cause each of said portions to achieve closely corresponding elevated temperatures, in a desired period of time.

Another object of my invention is to achieve the foregoing object in an induction heating furnace and system, when the said portions of the Work-piece are of variant cross-sectional area.

Another object is to provide for the heating of different longitudinal portions of an elongated metal workpiece by induction heating thereof, in such manner that portions of the work-piece which are relatively longitudinally displaced may be concurrently heated to predetermined variant temperatures.

Another object of my invention is to provide improved means which are adapted to effect avoidance of relative concentration of magnetic flux lines of force in the regions of a metal work-piece which are nearest the longitudinal middle of any single coil, or of a plurality of coils, of a coil unit, as a result of energization of said unit by a flow of alternating current directed through the unit.

Another object of my invention is to provide an improved induction coil means for an induction coil unit, which, when energized is adapted to effect improved distribution of different degrees of magnetornotive force to different portions of the length of a metal work-piece in which is surrounded by winding convolutions of said unit, to cause the different portions of the length of said work-piece to concurrently achieve predetermined relatively different temperatures in said work-piece portions.

Another yobject of my invention is to concurrently achieve at relatively low cost, predetermined degrees of heating temperatures, by an electrical induction method, in each of the differently longitudinally disposed portions of an elongated metal work-piece of uniform or variant cross-sectional form, in a relatively short, fixed period of tlme.

Another object of my invention is to achieve the above and other objects of my invention in an electrical and induction heating system through the use of three separate single phase currents derived from a three-phase readily available commercial source of power, without substantial unbalance of the load on each of the power leads extending from said source, in an electro-thermally efficient manner.

Other objects of the invention, and the invention itself, together with the instrumentalities employed to achieve the results thereof, will be obvious to those skilled in the induction heating art from the following exemplary descriptions of embodiments of the invention, and the accompanying drawings, whereof Fig. 1 is a side elevational view of a three-coil unit of an induction furnace system which is an embodiment of my invention, said unit being shown in elevation and having a portion indicated as broken away to show an interior furnace compartment, and an electrical system 3 for delivering energizing electrical currents to said coils, said system being shown diagrammatically.

Ai 2 is a diagrammatic View of electrical system for achieving variant heating of different portions length of an elongated metal work-piece disposed in the furnace compartment of an induction furnace.

Figs. 3 and r are conventional graphs each of which illustrate the phase relationships of conventional threephase voltage and/or of magnetomotive forces, which are employed in the electrical and electromagnetic systems which are involved in the operation of the furnace of Fig. 1.

Fig. 5 is a showing of an induction coil winding which is a modification of any of the coils of the unit of Fig. 1.

Figs. 6 and 7 are cross-sectional views of different portions of the water-cooled conductor employed in the coil winding of Fig. 5.

Referring now to the drawings, in all of which like parts are designated by like reference characters, the heating coil unit of Fig. l comprises three coils 1, 2 and 3 of separate helically wound electrical conductors liti, each conductor having tubular portion, as indicated at 11, and each tubular portion il is preferably integrally bonded to a rectangular solid copper bar 12, to increase the conductivity of the conductor. The tubular portion is adapted to receive a ow of coolant liquid to maintain the coil conductors cool during operation of the furnace, as fully explained in the said prior application of Lackner et al., previously referred to.

As in the induction coil unit shown in the said prior application, the coils It, 2 and 3 are disposed in axial alignment leing preferably telescoped over a heat and electrical insulating tube 66 which is impervious to moisture and which is preferably formed of a fiberglass and plastic composition, and preferably providing a stainless steel tube 6d telescoped within the tube 65, said tube being split longitudinally as indicated at 65, the interior of said tube (5S alfording an elongated cylindrical furnace compartment. Mounting and clamping elements for the conductors, as at 67 and 68, are provided as more fully disclosed in said prior application for patent.

At 2l, 22 and 23, three different single phase stepdown transformers are shown, the secondary windings Zia, and 23a, thereof, being connected by respective pairs of conductors 3l, 32 and 33 to different of the induction coils i, 2 and 3 of the unit which is generally indicated by the reference character 123.

The energizing current from the three-phase power line conductors Si, 52 and 53 are led through contacts of the ganged switches 6i, 62 and 63, which preferably are concurrently actuatable to opened and closed positions by the electromagnet as indicated at itl, and through the conductors 71, 72 and 73 and the different pairs of end terminals 8l, 82 and 83 of the respective primary coils f2; and 93 of the diiferent transformers 2l, and 23, in the manner shown to cause three separate currents of different single phases derived from the three phase leads 5l, 52 and 53 to be respectively communicated to different of the primary windings of the three transformers.

Electrical circuit changing means are provided by the switches at fil, i2 and 42T) of Fig. l, whereby the different longitudinal portions of the metal work-piece W, which are respectively enclosed by different of the coils 1, 2 and 3, may be heated to differing temperatures as a result of relatively increasing or decreasing the strength of alternating electrical current directed to different of the said coils.

The furnace which is herein shown and described, is thus adapted to be energized by low voltage alternating currents of the different three phases, each of the same low frequency such as 60 cycles, derived from a threephase source, the latter being here represented in the drawings by the conductors Si, 52 and 53 of a threephase high voltage power line. The set of single phase step-down transformers 2l, 22 and 23, serve to reduce the voltage of currents of each of the three phases which are respectively directed through the three induction coil windings l., 2 and 3 of the furnace, and serve also to afford a ready means of separately delivering, as from the secondary windings of said transformers three different low voltage currents, each being of relatively different of the three phases of a three phase system, to diiierent of the said induction coils, in such a manner as to facilitate reversal of the direction of current flow through an intermediate one of said coils, for the purpose of relatively reversing the phase of the magnetic linx induced by such current, and thereby improving the over-all efficiency of the electrical induction operation, as later herein set forth.

'lhe first or left-hand conductor of the secondary coil Ela of the transformer 21, is shown as connected to the terminal 9i at the left end of the induction coil 1 and the second or right-hand conductor 3l is connected to the opposite terminal 92 at the rightcnd of the said coil il.

The arrangement for the coil 3 is the same as that said coil i, the rst and second of the conductors 33 being respectively connected to the left-hand and righthand end terminals of said coil 3; however, in the case of the conductors 32 which lead from the secondary transformer winding 22a, the first or left-hand terminal of the secondary transformer winding 22a is connected to the right-hand terminal of the coil 2, and the right-hand terminal there of said winding 22a is connected to the left-hand terminal of said induction coil 2. While, therefore, separate flows of single phase current are thus directed respectively through the respective coils 1, 2 and 3; in the case of the coil 2, current is directed in the relatively reversed direction through the convolutions of said coil 2.

It will be understood that in the showing of Fig. l each of the coil windings is of like form and viewed from the left end of the coil unit i123, the conductors of each of the coils l, 2 and 3 are wound in the same clockwise circuitous direction, but because of the transposition of the pair of conductors 32 leading from the secondary winding 22a to the coil 2, as at Ztl, current will traverse the winding 2 in an opposite circuitous direction to that prevailing in the case of coils i and 3, and opposite to the direction of flow which would prevail for coil 2 had the pair of conductors 32 not been transposed It will be understood that such reversal of the direction of circuitous flow of current through the convolutions of the coil 2, could alternatively be achieved in other ways, such as by winding the convolutions of said coil 2 in the relatively opposite or counter-clockwise direction, or by relatively transposing the conductors of the pair 32, which lead to opposite terminals of the primary winding 92 of the transformer Z2, but the present invention is not restricted to any of the above or other obvious expcdients, and any thereof may be employed to achieve the same purpose of reversing the direction of current iiow through the intermediate of three successively disposed induction coils.

Each of the coils l, 2 and 3 is essentially an electromagnetic winding and, with respect to the work-piece W, operates as would the primary winding of a transformer having a short-circuited secondary winding, the latter' being represented by the metallic work-piece W, and the alternating current passing through each of said coils l, 2 and 3 is ellective to induce a separate field of magnetic flux of alternating polarity, which permeates the metallic material of that portion of said work-piece which is enclosed by the coil and induces therein, a heavy alternating current, of very low Voltage, which causes electro-thermal heating of the work-piece.

armate Each single alternation, or impulse of single phase current through any of said coils l, 2 and 3 creates magnetic poles of opposite polarity at the ends of the coil, said polarities being relatively reversed at each coil end during the next impulse.

Respectively north and south magnetic poles will be momentarily produced in the magnetic iiux at a rst and second of the opposite ends of the coil 2 during the period of any single unidirectional impulse of alternating current flowing through said coil, whereas except for the transposition of the conductors 32 at 20, said first and second coil ends would, at such time, be of opposite or respectively south and north polarities.

The phase relations of the three fields separately in duced by different of the coils l, 2 and 3 would, except for the previously described reversal of the direction of current iow through the coil 2, correspond to that of the three phases of a three-phase system, which is portrayed by the conventional vector diagram of Fig. 3, wherein the divergent solid lines A, B and C indicate a three-phase, 120 displacement between each pair of the separate single phases of the respectively different three fields of liux, under the assumed condition that there has been no reversal of the direction of current, which iiows through the coil 2, and induces the magnetic iiux of the phase C.

Under the condition of reversed current, as described, flowing through said coil 2, the phase relations between different of the three tiux fields is so altered, that the resultant phase relationship between said three tiux fields, may also be shown by the same vector diagram of Fig. 3 when the dotted line Cx is substituted for the line C, to indicate that, as a result of reversal of polarity of the magnetic iiux which is induced by reversed direction of iiow of alternating current passing through the coil 2, the phase C has been replaced by the relatively opposite phase Cx and is displaced by an angle of only 60, respectively from the phases A and B, i. e. the phase relations between the three flux fields are now like those between three successive of the phases of a six-phase periodic system.

In considering the showings of the graphs X and Y of Fig. 4, as applied to the phase relations between three fields of magnetic tiux, the horizontal line t is a so-called time line, the sinusoidal lines a, b and c, each of which alternatively extend above and below the line t, respectively relates to the respective fields of magnetic force induced by the relatively separate tiows of alternating current passing through the respective coils l, 2 and 3, to respectively show the periods, measured horizontally on the time line t, when the successive impulses of magnetic force of the respective fields of iiux, are of positive polarity when above the line t, or of negative polarity when below the line t, In the graphs X and Y, the equally spaced vertical lines divide the time line t into 60, i, e. 1%; cycle, time periods.

The graphs X and Y are alike with respect to the sinusoidal lines, or waves, a and b which respectively indicate the time and polarity relations of the fields of magnetic force respectively induced by the separate liows of alternating current through the induction coils l and 3, but differ with respect to the showing of the dotted line wave c, where graph X at c, shows relative time and polarity relations between the field of magentic force induced by the flow of alternating current through the coil 2 to the fields induced by the iiows through coils l. and 3, under the assumption that the iti-feed conductors 32 are not transposed at 20 as shown in Fig. l, and where graph Y shows the effect of reversal of the direction of current tiow through coil w.

Therefore, as also shown in Fig. 3, the graph Y by the dotted line c thereof, is shown to take a phase position which is displaced 180 along the time line t, from its position shown in graph X as a result of such reversal of the direction of current liow through the coil 2.

As shown in Y, the positive and negative impulses of said wave c are each interspaced in time, measured along the line t, from impulses of like polarity, of said waves a and b, by a period equal to 1%; of a complete cycle, and expressed otherwise as best indicated by Fig. 3, is relatively displaced from the like impulses of each of waves a and b by a phase angle of 60.

Thus the successive waves, a, c and b, of alternating magnetic ux impulses in the three fields of iiux are shown to be relatively in the same relative phase relations to each other, as are three successive phases of a sixphase periodic system.

In this manner the displacement in phase of the magnetic tiux induced by current iiow through coil 2, relative to the phases of iiux induced by the current ows through coils l and 3 is so reduced, that the three fields of liuX induced by currents through the coils l. and 3, that the aggregate magnetization more closely approaches that which would take place were a single phase current passed through the three coils in the same circuitous direction of iiow, and resultantly the electro-thermal efficiency of the furnace is Very greatly improved, particularly with respect to those portions of the pair of fields of iiux which are induced by respective of the pair of Coils l and 3, or the pair of coils Z and 3, in those regions where opposite ends of each coil of said pairs, are in adjacent relation; this result is achieved since heating of the portion of the work-piece in said regions, is much less adverseiy influenced by the eii'ect of the reduction of relative phase displacement, as between adjacent portions of each of said pairs of flux fields.

Instead of a single set of coils l, 2 and 3, a plurality of such sets may be employed for a single furnace, whereby there may be one, two, three etc. sets of three coils each, successively arranged in relative axial alignment with the coils of the set 123, corresponding induction coils of the different sets being preferably connected in parallel, or in series, with each other, so as to be energized preferably, by current of the same phase from a secondary winding of the same transformer, and in all cases, by any of the expedients above mentioned or by any equivalent expedient, current of the phase corresponding to that flowing through the coil Z, should be similarly reversely directed through each intermediate coil of each set. Alternatively, a duplicate set of transformers like those at 2l, 22 and 23 or separate additional secondary Windings of the same, may be provided for each of the sets of coils.

The above description relates more particularly to an electrical system, of which an embodiment is shown in Fig. l, and which involves a reversal of the circuitous direction of single phase current flow traversing a medial one, only, of a set of three inductor coil sections, and to which system, the graphs of Figs. 3 and i are applied, the said electrical system being herein claimed.

i now proceed to describe structural features shown in Figs. 2, 5, 6 and 7 which relate to refinements in flux distribution which may be employed in connection with the above described electrical. system or other electrical systems for low frequency induction furnaces.

To effect substantially like, or a desired variation of distribution of heating effect in dierent of the longitudinal portions of a work-piece which is telescoped within longitudinally successive of a plurality of induction coils which are respectively energized by alternating currents of differing phases of a multi-phase power system, or to improve the distribution of heating effect, by an induction furnace having one or more induction coil windings, over the length of a work-piece telescoped within induction coil convolutions, the instrumentalities now to be described adapted for the distribution of heating effect along the length of work-piece, may be singly, or conjointly employed to singly or co-operatively achieve advantageous results.

rPhe phase of my invention relating, more particularly,

amaca@ to the provision of so-called distributed heating of an elongated metal Work-L ieee, may involve the use of the tap switches itl, and lil f 1Eig. l, the switching Jiurnpers of 2, aud/or the coil winding of 5, or alternately, ci obvious modifiu ms and equivalents thereof, some of which later des s.

ln each of the Witches at dl, d2. and d3, the adjusted position of each of the switch blades 4in, and in @l in wans of operation of an] of said switch by moving its blade f' srown at Sii to the Contact as shown to a contact further to the that end of the Worllis disrosed within the encircling convoll be heated to a lesser degree by ed current flowing through the coil than will that posits end of the worn-piece which is shown exposed ,n l at the left side of the drawing.

lt will also be cle.; that, movement of any of the switch blades or to their extreme position tov/ard the left, to right, the portion of the billet W enclosed y ti socie-,ted induction coil l, 2 or 3,

tively lower or higher temperato pcc ,t t' e, response to the different voltages applied to an,v such coil, by any such adjustment of its contre a The three switch blades f f' 2a and (i3d, when adjusted to the position show l, will progressi ely cause the portion of the billet W enclosed by the coil l to be l` ter than that enclosed by the coil Z and to cause the porton of the billet enclosed by the coil 2 to be also than that enclowl by the coil 3, during a. nor pe such relative adjustments of the th switch blades are adapted to a condition where the billet, immediately Vtiter hea Lng to a given temperature at that end which is enclosed by tbe coil l, is to be then forced through ertrusion with said end which is hottest being first directed through the extrusion dies; relatively foli A a through the dies have reased by the elfect g' the extrusion process, ^cssively operated-upon portions of the est. "sion, progressively achieve approxies as they are passing through the ztially lov/cr t by friction d heat generated whezeby the sur billet. dur mately like r extrusion process.

Under the conditions where the Work-piece is of relatively difle uniforrl is high; surrounds portion the work-piece which is of lesser mass at a rein. vely lower voltage, as predetermined by the adjusted position of its associated controlling switch.

in Fig. 2, an induction heating coil, which may alone provide the sole induction heating coil of a furnace, or alternatively7 liig. 2 may be considered as a disclosure of heat distribu ve instrumentalitics for one or more of a plurality of coils employed in coil unit such as that of Fig. l, is shown at 25, said coil comprising a single circuitous coil winding which extends between end terminals and and surrounds a work-piece in the manner of the coil unit of Fig. l, and is provided with intermediate tap connectors at l'7l, .lV/2;, 1173, T274, W5 and 176.

The transformer T comprises a primary winding l and a secondary winding S, and may preferably, but not ne essarily in most cases, be provided With a switch llli i those at di, 42 or 43 and having 'i955 of contacts including the termicts and lid? at its two sides, with conducextending between the successive taps u so forth, proceeding inwardly from to taps on different convolutions of the P, of the transformer" T.

current from the transformer may be c by swinging the clade toward the the right to respectively reduce or increase the voltage induced in the Secondary Winding S, in the mane above describec switches di, 4i?, and 43, and

described results.

. t as the six successive taps lol. to le@ orovided at intervals from end-to-end S and similarly successive taps and may be provided for which is generally indicated at a series of detachably conting j pers such as 255i, 1&2, .i553 ed for relatively varying the coil inplishcd, for enc as to interconnect the transformer with the induction coil tap 26, Whered by t ie portions of the transformer C, ch is located between the taps lati i, fered to that portion of the induction ich between the coil taps 'ri' and the terln sach a case the heating effect of such por- ,ted in said coil were the upper #ed to the tap led and the moved to the tap l@ of sullicient number of rmer secondary Winding it portions of the inen; resulting from diferent unit lengths of i coil may be er A, Zed by the application or n e or variant voltages from selected portions of the transformer secondary, S, and, therefore, uniform, tapered, or variant heating of different portions of the length a billet can be readily accomplished.

The tap switches and/ or switching jumpers shown have various advantageous applications, for example, they can be adjusted to vary the input and consequently change the production rate, and when billets shorter than full coil. lengths are used, the tap switch or switching jumpers controlling the empty or partially empty coil section can e adjusted to effect zero or any higher proper input of energizatiou for that section or portion thereof to maintain the desired billet heat pattern. In such a case, the alternating currents directed to each of the above three successive groups of convolutions, proceeding from left to right, will be of relative respective strengths in the respective ratios of 71/2, 5 and 6.

Lance Erst, second and tiird groups have convolutions in the respective relative numbers of 4, 5 and 6, and respectively occupy lengths of the coil 25 which are in direct proportion to the number of convolutions, the first group will be of 4x l/z or 30x ampere turn excitation and the magnetic elccts thereof, `will be distributed over only 4,135 of the total length of the coil, whereas the second or middle group will be of 6x 6 or 36x ampere turn excitation distributed over W15 of the coil length, and the excitation by the third group will be by 5x 5 or 25x ampere turns, distributed over 5/15 of the coil length.

n l coil can be nesten by cu e, or the same, or

By such an arrangement, the respective excitations per unit length of the coil by currents through the three groups of convolutions, causes the work-piece enclosed by these portions of the coil, to be heated at commensurably different rates, whereby in a short predetermined heating period the left end of the work-piece will be of still lower temperature.

Thus, such an arrangement of taps of the transformer winding S, and coil 2, will afford tapered degrees of temperature proceeding from left to right, to an elongated work-piece disposed within the convolutions of the coil 2S.

rl`he above example shows how, with a sutlicient number of variably adjustable jumpers and taps for the transformer secondary winding and for the induction coil, different portions of the induction coil can be heated by current resulting from different voltage, or the same, or different unit lengths of the induction coil may be energized by the application of like or variant voltages from selected portions of the transformer secondary, S, and therefore by suitable adjustment of the jumpers, uniform, tapered, or variant heating of different portions of the length of a billet can be readily accomplished.

The tap switches and/ or switching jumpers shown have various advantageous application, for example, they can be adjusted to vary the input and consequently change the production rate, and when billets shorter than full coil lengths are used, the switching jumpers controlling the empty or partially empty coil section disposed at an end of the coil, can be adjusted to effect zero or any higher proper input of energization for that section or portion thereof, to economically maintain the desired degree of heating in the different longitudinal portions of the billet.

rThe positions of taps on each of the windings of each transformer secondary and on each of the induction heating coils, may be varied within the scope of this embodiment, and the relatively interconnected jumpers may be changed in number to meet different conditions, for instance in applying the system of Fig. 2 to an intermediate of the coils of a three-coil unit, such as the coil 2 of Fig. 1, jumper ISI may extend from the tap transformer 152 to the left hand terminal of the coil 2, jumper 184 may extend from the transformer tap 169 to the right hand terminal of coil 2, while jumper 182 may extend from the transformer tap 167 to a tap 5 on a convolution of the coil 2 which is near the left hand end of the coil, which the jumper 183 may extend from the tap 160 of the transformer to a tap 6 which is affixed to a convolution of the coil 2.

Thus such taps may be so located and said jumpers may so connect certain taps of the secondary transformer winding with taps of the coil 2 that not only will the groups of convolutions nearest the ends of the coil 2 be supplied with a flow of increased energizing current as compared to the flow of current through the larger number of convolutions disposed medially between said end groups of convolutions, but also the direction of current flow through all portions of the coil 2 will be reversed, as such term has been previously explained herein, and to achieve the previously described objective thereof.

Also by application of the jumper and taps system of Fig. 2, to the three-coil system of Fig. 1, the winding convolutions which are nearest the adjacent ends of all longitudinally adjoining of said coils may be energized at a greater voltage per convolution than other portions of said coils and therefore will afford a greater magnetizing force, due to increased ampere-turn effectiveness, than the same, or greater number of convolutions located in other portions of each coil length, which in the case of coil 2, would be the middle portion thereof.

The system of Fig. 2, when applied to that of Fig. l, may either supplement the effect of the reversed current through the coil 2 of Fig. 1, or may be substituted therefor.

Another embodiment of distributive heat instrumen# tality shown in Fig. 5 is effective another way, to increase the induction heating effect on portions of a work-piece which are in regions of either or both ends, of any induction coil of an induction coil unit.

Fig. 5 shows a coil 4 whose continuous winding consists of convolutions of a conductor, which in the coil-end portions 76 and 76a of the winding, is narrower than in the middle coil portion 75. lf preferred the conductor may be made from three sections of tubing, those for said end sections being shown, relatively enlarged in cross-section in Fig. 7, and in Fig. 6 the middle convolutions of the coil are, for comparison shown to be substantially wider, to cause the coil to have fewer convolutions per unit of coil length in its mid-portion than near its ends, where the convolutions are narrower, in proportion to the inverse value of flux density per convolution at the different coil portions. The middle section having its opposite ends integrally jointed by soldering, welding or the like at 77 and 77a to an end of each of the two end sections, 76 and 76a, to make the entire coil continu ously liquid and electrical conducting throughout its length, each of the said end sections, in the exemplary embodiment shown, being designed to have three-fourths as many turns as is provided for the middle section '75.

In this example the width of the narrower portions of the conductor at 76 and '76a may be one-fourth inch whereas the width of the wider portion of the conductor at 75, is three-eighths inch, and when the longitudinal extent of the middle section is twelve inches, and the length of each of the two end sections is six inches, each end section will have fifty percent (50%) more turns per unit length than will the longer middle section. Consequently the ampere-turns per unit length of the coil, and consequently the relatively densities of magnetic linx induced by equal current flowing through each of the three coil sections will be greater at the end sections. It is contemplated that the ratios of ampereturns in the different sections may be widely varied from the above described embodiment, and from that of the coil i of Fig. 5 as may be advantageous in satisfying different requirements of induction heating.

A useful modification of the disclosed coil of Fig. 5, involves a narrow convolution section as at '76 only, and with the wider convolution coil section of 75 being extended to the opposite end of the coil, with the end section 76a, of narrow convolutions, being thus eliminated. The obvious opposite construction may be had by merely reversing the positions of the two ends of the coil of said modification.

A useful application of the disclosures of Fig. 5, together with the modification thereof is to apply the coil construction of Fig. 5, as shown, to the coil 2 of Fig. l, then to employ a coil embodying the first above described modiication of the coil of Fig. 5, as coil 3 of Fig. 1, and to apply the aforesaid opposite coil construction, wherein the set of narrow convolutions is at the right end portion of the coil which is substituted for the coil 1 of Fig. l. Such improvements may be applied to Fig. l preferably in connection with the previously described reversal of current flow through the coil 2 to achieve maximum efficiency. However, when employed in connection with a three-coil unit without said reversal of current flow, marked improvement in the heating of portions of the work-piece located in the regions of adjoining coil ends may be effected, by application of the above improvements, to increase the degree of electromagnetic energization in the said regions of adjoining coil ends.

Use is contemplated of coils such as that shown in Fig. 5, and of the alternate modifications thereof, singly or conjointly employed, wherein such a system of interconnecting jumpers and coil winding taps are provided to variably energize different longitudinal sections of such coil by the application of different alternating current voltages to different of said described sections, and/ or dilerent portions of any, or all, of such sections.

Having thus described my invention relating to the art of predetermined distribution of heat to different longitudinal portions of an elongated Work-piece by electrical induction equipment and wherein variant instrumentalities adapted therefor, may be employed separately or conjointly to effect various advantageous results, I am aware that the disclosures hereof will suggest to those workers in the art possessing mechanical skill obvious other embodiments and substantially equivalent instrumentaiitics which, however, are included within the invention hereof as claimed in a divisional application for patent in which i reserve the right to disclose and claim the portion of this specification which relates particularly to the subject matter of Figs. 2, 5, 6, 7, and the switches 41 42, 43, when employed as described as means to adjustably vary the heating ctiiccts in diferent portions of the length of a metai work-piece disposed within the convolutions of the coil sections 1, 2, and 3.

I hereby claim:

l. in an electrical system for an induction heating furnace of the type described for the heating of metal workpieces, which comprises an inductor which employs at least one set of three longitudinally aligned, successively arranged coil sections, each said coil section comprising a plurality of coil convolutions, said sections being disposed in end-to-end relationship to each other to jointly provide, interiorly of their convolutions, an elongated heating compartment within which metal work-pieces may be inductively heated in response to concurrent energization ot all ot" said sections, each section being energized by the eiiect of a single-phase low-frequently current passed therethrough, electrical communicating means for deriving from a source of three-phase 10W-frequency alternating current, three separate flows of single phase low frequency current which are in three-phase timed relation to each other, and electrical means for so distributing said ows as to cause a first iiow thereof to traverse the convolutions of a first endmost one of said set of coil sections, a second ilow thereof to traverse the convolutions of the intermediate of said set of coil sections, and a third flow thereof to traverse the convolutions of the relatively third section of said set of inductor coil sections, the said first, second, and third single phase ioWs being in the same phase relation to each other as are three successive phases of a three-phase cyclic system, and said electrical communicating means comprising means for causing the said second tiow to traverse the convolutions of said intermediate coil section in that circuitous direction which is relatively reversed with respect to the relatively opposite circuitous direction in which said 'first iiow traverses the couvolutions of said first end coil section and also in which said third flow traverses the convolutions of said third section., of said set of three coil sections, to cause the phase of magnetomotive force of: magnetic iiux produced by energization of said second coil section to be relatively reversed, and thereby to cause the phase relations between the magnetomotive forces which are produced by energize.- tion of the three coil sections, and which are effective to cause inductive heating of, and to create magnetic force effects upon, any said work-piece which is disposed in said compartment, to be in that relatively timed relation to each other which characterizes three successive phases of a G-phase cyclic system.

2. The electrical system for an induction heating system, substantially as set forth in claim 1, characterized by the recited electrical communicating means comprising a set of transformers having at least three primary windings energizable from said source of three-phase low-frequency alternating current, and at least one set of three electrically separated secondary windings, and by the recited electrica] means comprising conductors leading from said secondary windings to said inductor coil sections, to effect the recited distribution of current flows through the recited said coil sections.

References Cited in the tile of this patent UNITED STATES PATENTS 1,842,967 Hegel Ian. 26, 1932 1,870,619 Flanzer Aug. 9, 1932 1,943,802 Northrup Jan. 16, 1934 1,981,630 Northrup Nov. 20, 1934 1,981,631 Northrup Nov. 20, 1934 2,229,680 Somes Jan. 28, 1941 2,572,073 Strickland, Ir. et al Oct. 23, 1951 2,604,577 Strickland, Jr. et al July 22, 1952 2,618,734 Anderson Nov. 18, 1952 2,623,081 Schorg Dec. 23, 1952 FORETGN PATENTS 71,733 Switzerland Feb. 16, 1916

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