US2286024A - Coreless induction furnace for melting metals - Google Patents

Description

June 9, 1942. M. TAMA ET AL CORELESS INDUCTION FUHNACE FOR MELTING METALS Filed OC. 25, 1940 4 Sheets-Sheet l June 9, 1942. M. TAMA ET AL CORELESS INDUCTION FURNACE FOR MELTING METALS Filed OCJC. 25, 1940 4 Shee-Lzcz; Tf

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June 9, 1942. M. TAMA ET AL CORELESS INDUCTION FURNACE FOR MELTING METALS Filed Oat. 25, 1940 4 Sheena-Sheet .3

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June 9, 1942. M TAMA ET AL 2,286,024

CORELESS INDUCTION FURNACE FOR MELTING METALS Filed Oct. 25, 1940 4 Sheets-Sheet f.

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that it can 'he cleaned, in most without emptying the iurnace.

.ccording to the present Invention "oc adapted to the melting of steel..

i fore set forth, are expensive and the eiciency u Nevertheless, many such f haces have been 'lt for the .celtica as only with these furnaces was it possible to use .pie durable refractory iioing. It is esn ialiy diiiicult to the refractory lining, with basic linings consisting of magnesite. The main reason for using a coreless iuinace for i o ing steel instead of the closed magnetic cirype induction furnace, is because it has been impossible to produce a. refractory lining of such e. complicated shape as the secondary channel of the iron-core furnace. Magnesite linings are subject to considerable expansion and con'ti'action with changes of the temperature, and thus the complicated melting channels were thereby easily destroyed.

The present invention enables the employment of a lining which is almost as simple as that of the high-frequency furnace hithevto imo-wn. whilst using ordinary altel'hatina Current freouehcies. furnaces for melting steel often huilt foi` very hielt pov/ei. This can only he achieve-:l conveniently. if three-phase current can ne employed. The present invention also allows the use of polyphase current which was hitheito impossible with a coi'eless furnace. llt is iiossisle to use with one furnace designed ac coiding 'to 'the 'present invention a power of 530 to idOf@ law. employing three-phase current of alternating current frequency.

With the foregoing arid othei' objects in view. the present invention movides a coreless indue on fumace for melting metals having' a groove extension oi the crucible (secondary groove), situated in such way with respect to the nriman; coil as to embrace as many of the ux lines oi' the primary coil oossible.

While the coreless furnaces hitherto known had a cylindrical Crucible in which the secondary current could ow on the entire surface of the cylinder. provision is made by the oresent invention. for the current to flow in a suecia] secondary groove. similar in some respects to the secondary channel of an iron core furnace. but with the important difference that the channel is always onen towards the metal container on one side. 'in such a groove the current ioivs mainly in those portions in proximity to the primary coil.

By varying the height of the secondary groove it is possible to change the resistance and selfinductance of the thus formed secondary. The secondary groove should be placed as close as pos-- sible to the primacy coil, in order to embrace as many iiux lines as possible, i. e. the coupling shoi d be as close as possible. .lieu/eve?, some iestrictions are imposed hy the minimum Wall ie secondary channel is made so easily accessible .it

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"s the eiractoz-zy material, which y between the molten metal and the i y coil order to prevent the metal 'ii-om meshing tluo'ugl he secondary groove of the furnace is best dimeiisioi'ied when its inductive iesistance is about equal to its ohmic resistance. The theoiy hereinafter -isjolained will permit the size of the secondary groove which will give the best ehi `cy and power factor to he determined, u seduction of the secondary groove allows the electrical behavior of the furnace to be much more controlled than was possible with the smsie cylindiical c'rucible where, with a given diam eter and height of the Crucible, the depth of penetration determined the ohmic and inductive resistance of the secondary.

hitherto looown were usually of the Queda *one and a length at least equal to its da i f eter, electrical reasons, as hereinafter explained, call foi' a secondary channel of very small heigl which consequently necessitates a comparatively short coil, having s. length varying between. one fifth and one hfteentn of its diameter, and een orally consisting of more than one layer. This type of coil usually gives the best magnetic couplincr with the seconda-ry. For the sake of brevity, a furnace according to the present invention will hereinafter be referred to as a short coil furnace7 and a furnace of the type hitherto em ployed as a long coil furnace, although these are :not the only features in which such furnaces differ.

The secondary groove may be of a rectangular. trapezoidai or triangular cross-section, and in all such cases it is advisable to round off the edges in order to avoid cracking of the refractoiy materlal. The trapezcidal cross-section appears to be the most convenient. the short side o the tiapeaoid being situated as close as possible to the primary coil.

A further feature of the invention resides in the fact that the primary coil and the secondary groove pertaining thereto are situated at the lowest nat of the crucible. By virtue of this arrangement, the following advantages ase achieved:

i. it is possible to start the melting operation always with a charge of liquid metal and with the normal power. As the entire power of the furnace is produced in the secondary groove, one can always he as sured that the short coil furnace absorbs the entire power, even if it be started with only a small charge, which only lls the secondary groove. With a long coil furnace it is necessary to lill up the entire Crucible. if the whole power is to be used. A short coil furnace can be filled and partly emptied during the melting process, but the power consumption remains entirely constant; such furnace is always operating at its rated power input.

2. The movement of the charge caused by electrodynamic forces is produced at the lowest part of the furnace. In cases Where this movement is very pronounced, the height of the Crucible is chosen so as to counteract this movement by hydrostatic pressure, and to have the metal surface where a, m0vement generally is not desired, at sufiicient distance from the space Where the movement is originated.

T .which eme eefew bottom fall sere l ln omer 'fao ov secondary cil-eolie fo For line same we;

away from the moo lll ezapportlne; means. as for :le etal Shoal@ preferably oi e melf-Nel o high resistivity, suole..

tie

"te-el.

11eme has ehe howl@ in fore vided several o direct comm fr groove end the con-mln l umeee, as :ls clearly sho lo mg. El wmelz. .s cross-section ln the .ff-93 of lle.

orf` f e @he sake of simplicity, f. *ms arrangement there is also provided lulther cylindrical space 2'6 which is llled with metal. when smelling the furnace. This :space being compareively small, -the furnace een be Q .w' @ma E. mommy commenced oeeretlng with small quantity of @nu WCG-@lary metal. in MD. 6' the arrows show' the mot-lon 35 o the metal wil-,hln ehe tube like openings 25 and the secondary gloove 25, A sump 2l is pro vided umlemeolzh the groove like extension.

l loe is provided with a Crucible @.l, insulating layer casing N, a cov 5.' E(

e pou'llzg nose 33 shown in Fig. 7. The coil 313 is easily aocessble from outslce. The coil which will heeinafter be more ully oeef'lbed consise of wo or three layers o' copper moes having a special cross section.

Figs. 9 and l0 show detalls of the shore coil 17.1 elevation and llo plan. 013.137 one layer o lille voll is. shown in lthe drawings. 'The primary coil is made of copper tube of rectau'lgularerasesestion, the side o the rectangle being im inline direction of the coil axis.

coil le wound. in such a way that the end s le lo e plone right angles with. coll mais. This is one in order to enable the ool oe pressed Loeeher by gwo sim-ole insu loting rines and 3l, The coil thus pressed e solid piece and a base for the 1-e a phase furnace indicates me primary eoll J are sito. ed eonoentllcally, elle second# imam? coll. p

yl each l heee insu l. L 'me Crucible oy En l fraotory lining which oanbe pressed into it. Each the mooxfedlle extensol ls reetangularly layer of the coll will generally have its own elmo-ed, Fie. 2 a Llapezold and in 3 iriwater connect- ions 38 and 35 and connecting I i oo cables 5l) and lll, the latter being connected eimOCO-SS ther to the current supply or to the next coil n Poll femme; will; on the eem@ layer. The separate cooling of each loyer gives es m0.; f serloecl will 'eeeloe so Figs. a much more efcent Cooling Systenl j but a.dap."3d [fo imm wwwmain' ll shows another embodiment of the lnmll'm QWG @5 vention in which the flat primary col 42 sul "1 A j im ported by nog 5G has more or less une seme pmmoly collo f our; l are eltualecl side by side oliale bottom of the furnace, whereas in @he ,manr as the meow-mw extensmn 5s and Shown in Fig 5 two secondmy is sllfualed ooncentncally below this c'ham'lel. amdv QJ and Wm primary Cous g5 and ,lo Tlltehopepmgicof the channel 1s in the dlrectlon v svg'hg I ,4 Ol @C01 LXM,

lo; um u The 'primary coil is supported by a frame [33 ore eraely of nommegnetlo material of high Speresistance. this figure the metal is indi by the Crucible by 5, the heat; usw

the same di While the construction of uch coil more complicanIl than that of ordinary ndrical one, fields are ren tbly by its use.

ure of this fn nace is a second This :furnace Airesents comhi nation. of the short cou furnace and the ltnown long coil furnace. The coil can. be operated either at the commercial frequency or at higher l'reouency.

The height of the metal bath in the crucible must 'ce so chosen as to allow only so much sti fine movement originated by the electroffnetio movement at the bottom of the secondary Q'cove to be ransferred to the surface of the bath as is necessary for carrying-out the metallurgical process in the furnace. When melting light metals the pressure head of the molten metal must be greater than when melting heavy metals. The height should be chosen so as to prevent any undesired oxidation oi' the liquid metal on the surface of the metal bath due to the stirring movement in said bath.

It is known that a mutual repulsion takes place between primary coil and secondary groove. 'The metal heated in the groove-like extension (see Fic, 6) is thus repelled towards the center and liorvs into the cylindrical space 2l'. rhe space from. which the metal is moved now re ceivcs cold metal coming from the upper parts of the furnace through the ducts iil and flowing in the direction indicated by the arrows in order to prevent overheating of the metal. An abrupt reversal of the direction of flow of the metal docs not take place, nor are there "dead spots, where the metal might remain for some time or reverse its direction. This embodiment of the invention. is extremely advantageous when melting light metal alloys, because the oxides and other chemical compounds are continuously washed away by the moving metal and are carried towards the conter, there to sink to the sump of the furnace which is provided for the accumulation of such oxides.

The continuous supply of cold metal through the ducts 2t (Fig. t) to the 'nost heated haft of the secondary groove renders it possible to apply much more power than With the 'previously described types, so that a furnace in accordance with the present invention can Work with much more power than a furnace having a secondary groove of the same dimensions but Without thc ducts 2S.

An important characteristic of the invention is the phenomenon of displacement of the current which will cause the current to flow almost entirely in the `groove-like extension if the height of the channel is sufficiently large. Therefore, in the furnace an electrodynamic force is set up which causes the metal to be repelled from the primary coil, and an upward movement of metal will occur as for instance indicated by arrows in the embodiment of the invention shown in Fig. il..

lin the modihcation illustrated in Fig.. l2 the second coil. m serves the purpose of increasing outer of the Iii it operated i uency the furnace can be charged f y with small pieces of metal, which ly loc melted by currents induced from coil @ne piece after the other Will sinh inte.

coil and the remainder of donc by low requency.

The lone coil however, be used to nro'a duce additional movement oi' the metal. r this it is or lbly supplied with alt .ine current of a low i' uency and the ment of the metal can be controlied by the power applied to said second coll.

The depth of the secondary groove shown in Fie. l2 being half the difference between the diameter of the secondary groove 53 and J Crucible should not be too small. this depth will be chosen greater than double the depth of penetration of the current, but more than three to four times such depth.

Since one of the main advantages of the pres#- ent invention is an improvement of the electrical efficiency and power factor, it is necessary to explain the theory of the coreless induction furn nace precisely, A practical theory on the rather complicated matter has been developed by Professor Esrnarch (Wissenschaftliche `veroeffentlichungen aus dem Siemens Konzern, vol. X) and. his rather simple calculation has proved to be in accordance with the results of many experiments. Although this theory was based on the long coil furnace, the same principle can be applied to a short coil furnace with a special secondary channel, according to the present invention,

The basic idea of Professor Esmarchs theory is this: The current is supposed to flow in a layer or the thickness on the surface of the charge,

.ed by the short melting will be sump of metal which is h l/lAC and no current is supposed to fiow in the inner layers. is called the depth of penetration,"

. 2 l o @30x/Mjah.

rThis formula is approximately exact for the shapes of coreless induction furnaces hitherto built. Conditions in a short coil furnace seem to be somewhat different. Experiments seem to prove that a coefficient slightly larger than 5030 is correct. However. the basic idea of imagining the current uniformly distributed in a layer of the thickness can be used with good results for the calculation of 'the short coil furnace. While the task of determining the exact depth of penetration is such a secondary groove remains for mathematicians, a calculation based on the Steinmetz formula will not be far wrong. The Steinmetz formula was first suggested by Dr. E. F. Northrup, who is responsible for the development of the high frequency furnace, for the calculation of such furnaces.

In Professor Esmarchs theory the real charge of the furnace is replaced by an electrically equal body, having the shape of a tube of the same outer diameter and same height as the charge,

s. twomfoii system is 'toen applied to eA smelly ego body. For e short coil. electric-silly edo Ai body has the elimine of s. 1

f in oeiitioi seeoiidery body fout-er diamete minus depth oi penetration) in centimeters oi primary oeil, centimeters Aiht of secondary body, izfi centimeters renee of primary ooii oross-sectioo eter a cirele having the seme ered es the orosseeetion of the secondary body in centimeters meer oi turns oi' primary coil is por centimeter my ir: cycles Fifi ooo/er dissipated in the secondary body, i. e. in the Charge, calculated by:

`t'-rzji2322R- i (3) and the inductive leed. is:

NnzjigwlLw-pzfie) Uil The iizipoi'terit difference between o short coil furnace according to the present invention and long eoii furnace is the following: In the letter, the resistance of the secondary body is es compared with. its reaotance wie. Professor Esmaroh, for the purpose of calculet- -.og the long coil furnace, neglected the term Ry, iii the expression for gi given above (2), arid wrote:

small in the furnace according to the present inven tio however, the resistance of the secondary will be oi value comparable to its reeetaiice wie, and, therefore, the exact Formulee 2, 3 and i 'will to 'tie used im* its calculation.

g. if. represents Korndoi'iers formeel coils with d ieotorigulsr eross-seotioriii iood for deteriiiiiiihg the selfwiriduotenee L; of the ri" coil according to:

:si: be determined by the equetio LzzkzizJL-g dou the eross-seotiozi of seid body ee Wleeed, for ou :ose: by a circuler suol eircom erenee.

. o shows s relation found by l-iaveleiez. between the mutuel iriduoianoe of two cireuier rings of wire situated in the same plane, and the quotient of the diameters of both rings. Th 1 relation can be used approximately for the termination of the mutuel inductdnoe in the two ooil system discussed, `which is determined by:

The use of this formula will only lead to goed results., if the height hi of the primary coil is larger than the difference of the diameters D1 and D2. Should the coil be made longer than that, the coefficient kx will have e. lower velue than showin. in the diagram. It is desirable, how ever, to obtain as high e. mutual induotence es possible, end e Coil oi e greater height than the limit given. above will therefore rarely be used.

The main fact learned from these diagrams when trying them on examples is: A variation of the height of the secondary body H2 between zero and the value of h1 will not affect the mutuei inductarice and only slightly aieot the self-im ductanoe of the secondary. The resistance of the seeondery, however, varies inversely proportion ally to the height h2.

in the equation:

deity body H2 Within the foregoing limits. Presuming this and Writing:

whereby the power dissipated in the secondary body becomes:

there is obtained the maximum oi power dissipatted in the secondary at a given current in the primary by choosing h2 so as to give the maximum o the function:

f=i+m2 which represents only the variables taken out ootioo .tio eifeiitiol celeulus shows Q :csetiimuoi to 'be ct:

msi. (lill *ht im oi the secondary moet ohteio. c. res tence oi the sive the moet sonde 'y for .t Woede the oe lo .flirt/tui` iu -'comico la es the velue oi .o2-L2 will only etn it or" the velue of the of the secoudeiy within. the limits given above will, theiefore, only meen a. slight voirie.u tion. of the inductive lood No et e. given primati? cuiieut.' This moons thot et the height ha for which the power N2 etteios e, for e given otite-eey current, the velue oi {lith-ich sporesimetely equal im practical fue cee to the power lector cos. 42) will reach its os well.

Thus both the efficiency oud the power factor ofi-Ii for desieoM in the secondary channel that all give resistance Re equal to wie.

Sie, considering e furnace in which this condh tion is ezeotly fulfilled, the following simplified expressions con oe used for the determination of the power N2 and the inductive lood No:

eporommeteiy eouol to the potrei octor, in e. @tactical uruece becomes:

NE ZLgw The lest equation contains neither the number of turns nor the diameter itself but only the quotient of the diameters D1 oud De. It has hereihbefore been explained that it is impossible to vary k2 very much by varying the height of the secondary channel within the known limits. Neithei can En oe varied vei'y much by changing the dimensions of the primary coil. ka, however, the coeilclent of mutuel inductonces, is very much dependent on. the quotient of the diameters lili Dz. This leads to the conclusion that the maximum power factor attains-.ble in e short coil motion?, by choosing the height ha of the eccoodoi'y eccoidlog; to Condition ifi will he chieiiy correct for o ence of 'iutezieh i Condition is met, iuroeee o i e. given Quotient will have approximately the power factor shown izo. this dimmen.

it will be seen from the seme that a short coil furnace ourlet practical conditions will have a. for better power factor than the careless iur-- neces hitheito iuiowu.

ih practical cases will have o. velue lying tween it?, ond 0.92. A power factor of 0.16 to i' con thus ice obtained. The best long coil iuf- :ue-ces have e. power ietctoiq of about 0.1.

Generally speaking, in e. short coil furnace the power No dislspated in the charge will be iai: superior to the secondary power of e long coil furnace ci? the seme diameter oud the seme sito pero toms. With a iurnace according to the present invention it will therefore be possible to obtain electrical elcieneies of 8G to 90%.

The results of the foregoing theory have been` confirmed by exoeiiments. The maximum eti ciency und roower .iector was obtained at heights oi the secoudarychannei very close to the calculated velues.

Condition li also leads to the fact, that the height h2 giving the maximum eolciency and power factor is dependent on the frequency and the specic resistance of the material only. Colcuiatious essumihg the Steinmetz. formula to be short coil furnace leed to a niioo g h2 lei/f k2 for practical cases will'have o. value between 22 and 27 lsee Fig. 14). is c. rough formule for determining the height of the secondary chaooel we moy, therefore, Write:

This formula does not apply oi course, to the iumece shown in Fis. 11. in this furnace the width of the secondary channel must be determined to fulli Condition 14 by a. complete calculetlori.

In all the embodiments shown only the main parts o the furnaces have been shown, which are necessary to characterize the present invention. Other details of such furnaces, as are known to any expert, will be mentioned only for the sake of completeness; for example all such formule.

furnaces should be provided with e. tilting device, in order to pour out the metal. The fur-N 'oece con be tilted either about the axis of the pouring nose or about the center of gravity of the furnace. However, the metal could also be removed by means of a syphon. The cooling and the supporting of the coil can be performed by any o the known methods. Water cooling and oir cooling devices have been shown in the fore L' come embodiments.

noted use or o union and or is to be performed, We We claim is:

induction furnace for melting con 'ig a Crucible, an groove f .ettore section ol the oi'ucible Wall t vvitn the inner space of 'the crucible, uated at the extremity of said a nositlon corresponding 5v ii/. 'lg

e said primary coil, the latter e extremity of said grooveu melting "'ficiole, at least one ann ie bxl'ttoni section of the cruci- "og tl'r inner space ol itayer p ry coil eitueaoli, annular groove, lresponding generally to an en gtn oi groove fre l l,o the i er oi the r-ccteo, away l x e location coil, the latter being shaped c it substantially only the metal in the Verity of said groove.

il. careless induction .urnace for melting -tals comprising a crucible, an annular groove the bottom section of the Crucible Wall comn rou-nicating with the inner space of the said crucible, a primary coil situated at the extremity said groove, in a position corresponding gerrto an. extension of the length of said groove tbe free opening of saine into the inner space of the crucible being directed away from the location. the said primary coil, said coil being con struct-e as snort coil having a'height approximately varying between 1/5 and 765 of its diameter tous being shaped to induce current substantially only in the metal in the extremity of said A l' l induction furnace for melting ,s a crucible, an annular groove i section of tbe crucble Wall, a mi.

mary coil situated at the extremity ci said groove, a position corresponding generally to an tension of 'the length of groove the free of sanie into tbe inner space ci the cruc'- nary coil, the latter being shaped to il@ cuco current substantially only in the metal Y :Lne extremity of the said. grcmve7 a plurality o1" enteral ducts and a central channel Conner'Y tno said groove with the inner space oi cruclble.

A induction furnace for n" tais comp loing crucible, an annular grot, the bottom section of the cruclble wall, primary coil situa :o at the extremity o' groove, in position corresponding generally an extension of the length of said groove tb free opening of same into the inner space of t Crucible boing directed away from the loca e primary coil, til latter iduce current substantially only tire in. the extremity of the said groove, a plu A coreless induction furnace i'or naetals comprising a Crucible, an annular groove 3 the bottom section of the crucible Wal nc.'

with tile space of tnesaid ble, a pi mary coil situated at the extremity said groove, in a position corresponding generally to extension of the length of said groove tree opening of same into the inner space of the Crucible being directed away from the location the primary coil, said primary coil having U-shaped cross section and embracing the end of the groove with its flanges for the purpose of concentrating the induced current in the metal the extremity oi the groove.

'7. A coreless induction furnace lor melting metals comprising e, crucible, an annular groove in the bottom section of the Crucible Wall coml municating with the inner space of the said Crucible, a primary coil situated at the extremity of said groove, in a position corresponding generally to an extension of the length of said groove the free opening of same into the inner space of tire crucible being directed away from the location of the said primary coil, the latter be" shaped to :induce current substantially only the metal in the extremity of said groove and an additional long coil surrounding the upper part of the Crucible above the said groove.

il. A coreless induction furnace for melting metals comprising a cruoible, an annular groove in the bottom section of the crucible wall communicating with the inner space of the said Crucible, a primary coil situated at the extremity of said groove, in a position corresponding generally to an extension of the length of said groove the free opening of same into the inner space of the crucible being directed away from the 1ocation of the said primary coil, the latter being shaped to induce current substantially only in the metal in the extremity of said groove, said groove and said Iprimary coil having substantially the saine diameter.

9. A coreless induction furnace for melting metals comprising a Crucible, an annular groove in the bottom section of the crucible wall communicating with the inner space of the said crucible'a primary coil situated at the extremity of said groove, in a position corresponding genorally to an extension of the length of said groove in the botto.-v

tlie free opening o1" same into the inne? space of the Crucible being directed away from the loco tion of the seid primary coil, the latter being shaped to induce current substantially only in the metal in the extremity of seid groove and teilig constructed as an annular @at coil located below the seid groove.

l0, A corclcss induction furnace for melting motels compising a crucible, en annuler groove section of the Crucible Well commuriceting 1.-' 1 the inne? space of the seid ofsucible, e coil situated et the extremity of said groove, iii s, position coresponding generally to au exteusou of the length of said groove the opening of same into the inner space of the crucifole directed owoy from the loca-tion of the said primary coil, the letter being shaped izicluce current substantially only in the metal in the extemity of seid groove, the height of the metal column in the Crucible being controlled fieseoee suoli a manner es to counteract by its pressure heeel the metal movement created in the vicinity o the groove to such an extent as to prevent surface (nudisti-on.l

ll il coielcss induction furnace for melting metals comprising a Crucible, an annular groove in bottom section of the ciuoible wall communicating with the inner space ol? the saisi Crucible, e primary coil situated et the extremity of said groove, in e position corresponding gel.,- erally to en extension ci the length o said groove the free opening of same into the inner space the ciucilole being directed away from the locs tion of the soie. primary coil, the vlatter being shaped. to induce current substantiolly only in the motel in the extremity of said groove and e sump below the said groove for the accumule.- tlon ci solid impurities.

MANUEL TAMA. MARIO TAMA,

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