US1795926A - Induction furnace - Google Patents

Description

March 10, 1931. P. H. BRACE 1,795,926

INDUCTION FURNACE Filed July 27, 1926 WITNESSES: 5.47%

!NVENTOR l brier/z Brace.

. ATTORNEY Patented Mar. 10, 1931 UNITED STATES PATENT OFFICE IPOBTER H. BRAOTE, OF WILKINSBURG, PENNSYLVANIA, ASSIGNOB TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYL VANIA INDUCTION FURNACE Application filed July 27,

induction furnace that shall be operable to melt metallic materials at low frequency.

Another object of my invention is to provide an induction furnace for heating metallic materials which does not have a magnetic core interlinking with the material to be heated.

Another object of my invention is to provide a low-frequency induction furnace for the melting of metals that shall not be subjected to the well known pinch-effect common to low-frequency induction furnaces of the iron-core type.

Induction furnaces heretofore have been of twogeneral types. High-frequency furnaces, which do not have an iron core interlinking with the magnetic circuit and lowfrequency furnaces which have an iron core interlinking with the material to be heated and upon which the induction coil is wound.

In high-frequency furnaces, the induction coil usually is wound around the crucible containing material to be heated. The heating of the material is produced by the action of a high-frequency magnetic field interlinking therewith. The degree of heating produced in a material of a given specific resistivity is dependent upon the total change of magnetic-flux therein per unit of time. For a given coil having a certain number of turns and a given current flowing therein, a magnetic 'field will be established bythe coil, the-strength of which is determined by the reluctance of the magnetic circuit. The

- total change of flux linked with the charge produced by the coil per unit of time, under the above conditions will then be substantially proportional to the square root of the frequency of the current flowing therein, assuming an alternating current to be applied to the coil.

I have found, by experiment, that the power factor of a typical induction coil and the material to be heated thereby, without iron in the magnetic-circuit, is substantially between 22% and 25% at commercial frequencies of 1926. Serial No. reams.

the order of cycles per second, and that the power factor for the same coil, at frequencies of the order of 500 cycles per second, is approximately 12%. It is, therefore, evident that the kilo-volt amperes necessar in order to transmit a required number of Kilo-watts to the material to be heated will be materially less for 60 cycles per second than for' 500 cyclesper second. 7

- I have discovered also that it is practicable to produce a total change of flux per unit of time sufficient to produce commercially useful heating in current-conducting material to be heated or melted in an induction furnace without an iron core by energizing the coil from a source of alternating current of commer'cial frequency, by increasing the number of ampere turns per unit of axial length of induction coil, as compared with that necessary to produce the same heating effect at a higher frequency.

I have successfull melted one hundred and fifty pounds of stee in an induction furnace by energizing the inductor coil from a source of sixty-cycle alternating current. The coil employed in this particular furnace was approximately eight inches in diameter. Since inductive melting of metals, in a furnace of the type which I propose to use, is more efficient when the diameter of the coil. is greater than in my experimental furnace referred to above, low-frequency inductive melting of metals becomes more advantageous when coils of large diameters are used because the well known skin effect becomes more pronounced. Low-frequency heating, therefore, lends itself satisfactorily to industrial-type furnaces of large ca acities. Induction furnaces having coils 0 large diameters may be employed to successfully melt metals, such as copper and low-resistance non-magnetic materials at commercial frequencies.

I have found also that by employing a partial magnetic core, so disposed on the outside of the inductor coil as to provide a ferrous magnetic path substantially between the ends of the coil, the power factor may be Inateriallyincreased. This is an advantage which cannot be commercially available at frequencies employed heretofore in ironless mpendent upon the resistivity ofithe material and the distribution of the current induced therein. The heat generated in a magnetic material to be heated when starting from a cold condition will be a combination of joulean and hysteresis energy. For a given di-' ameter of coil and material to be heated, the current becomes concentrated more and more at the periphery thereof'as the frequency is increased. In this ,case, the heat is generated at the surface of the material and is conducted inwardly to the portions in which the current density is very low and, for practical pur oses, is nil.

I a coil and a material to be heated are of the respective diameters mentioned above, the skin effect will be diminished very materially, and the current density will become uniform throughout, as the frequencies become lower. For a given efficiency of power transfer from the induction coil to the material to be heated, it is necessary, therefore, at low frequencies to increase the dimensions of the coil and charge and to so construct the coil that its diameter is materiall larger than the diameter of the charge 0 material in order .that the current induced in the material will be concentrated at its periphery and thereby produce efficient heating thereof. I Stated in another way, if it is intended to melt a charge of metal of a given diameter by.

4 may be filled with insulating material and thereby protect the coil from the high tem- .perature of the metal when molten. In highfrequency induction furnaces, the annular distance between the coil and charge should be made as small as possible in order that a proper power factor may be obtained.

The fact that the diameters of the induction coil and the material to be heated must,

the order of commercial frequency may be employed in my induction furnace to good advantage, the operation and first cost thereof will be considerably less than for frequencies of from approximately 500 siyclesper'sec'ond and upwards because stan ard equipment, such as transformers andstatic or synchronous condensers, may be used.

In. many industrial establishments, syn; chronous condensers are available as parts of the equipment, and an induction furnace, of the type which Ipropose to use, may be connected to the power circuit of the indus-\ trial establishment without materially affecting the power factor of the power circuit as a whole. This is an advantage over highfrequency induction heating because, at the higher frequencies, either a special motorgenerator set must be used for generating the high-frequency current, or a spark gap, con-. denser and induction-coil circuit energized from a source of commercial frequency-alternating current must be employed.

In practicing my invention, I provide a crucible, an induction coil wound therearound and a source of commercial-frequency alternating current therefor. A partial mag. netic core is provided also for decreasing the reluctance of the air path for the magnetic field set up by the coil. The core is disposed on the outside of the coil to provide a low-reluctance magnetic path substantially between the ends of the coil, thereby avoiding the undesirable feature of having acore interlinking with the material to be heated. Means are also provided for tilting thefurnace whenever it is desirable to pour molten material from the crucible.

In the single sheet of drawings, Fig. 1 is a view, in vertical section, of an induction furnace embodying my invention;

iron core employed in the device illustrated in Fig. 1'; v

Fig. 3 is a schematic diagram of an iron-- less induction furnace energized from a source of low-frequency alternating current; and a Fig. 2 is a detail view of a portion of as Fig. 4 is a plan view of a lamination of magnetic material employed in the core member illustrated in Figs. 1 and 2.

In Fig. 1 of the drawings, an induction furnace 11 comprises a tilting and supporting frame 12, an inductive heating unit 13, a furnace-supporting means 14 and acover member 15. The frame 12 comprises, in general, a horizontal base member 16' and a pair 1 of upwardly extending members 17'which are provided with bearings 18.

The furnace 13comprises a crucible 19, an induction coil 21 disposed therearound, an insulating casing 22 and a core member 23 disposed circumferentially around the casing22. The crucible 19 may be made of any suitable refractory material, such as zirconium silicate.

As shown in Fig. 1, the annular space between the coil and charge is quite large in order that current induced in a charge of metal, to be melted in the crucible, will penetrate the charge to a depth necessary to produce efiicient heating, as hereinbefore stated. The annular space between the coil and crucible may be filled with suitable insulating material in order to protect the coil from excessive heat and also to protect it from molten metal in the event the crucible 19 should crack.

The induction coil 21 comprises a plurality of turns 24 and is provided with a terminal 25 which joins the uppermost turn thereof, a terminal 26 that is connected to the lowermost turn of the coil 21 and a terminal 27 which is connected to a turn 29 of the coil 21 at some suitable intermediate point between the uppermost and lowermost turns. The terminals 25, 26 and 27 extend through a base member 31 of the casing 22 through the iron core 23, a supporting base 32 and a clamping member 33. Suitable insulating bushings 34 are provided for insulating the terminal members 25, 26 and 27 from the core 23 and the base members 32 and 33. The intervening space between the crucible, the coil 21 and. the casing 22 may be filled with suitable pulverulent insulating material, such as zirconium silicate.

The furnace-supporting means 1% comprises the clamping member 33, the base member 32. which may be made of any suitable electric-insulating material, or which may be Y made of a ferrous material and insulated from the core 23, annular link 35 of insulating material disposed between the upper portion of the core member and an annular clamping-ring member 36. The annular ring member 36 and the base members and 33 are maintained in rigid relation by a plurality of tie-rod members 37 that provided with suitable threaded portions 38 and which interfit with nut members ll and lock washers By providing th e required tension in the tiered members 31,,

the core member and the furnace struc-- turn 18 may be maintained in rugged and compact relation. The particular structure of the furnace illustrated in. i may he modified and changed from the sh o wing illus mas, and .l do not wish, thererore, to be ited to the particular design, shown.

. 1 pair of upright members are suitably secured to the members and 33 by screw-bolt members s4. lock washers as. T h e upr' cl members as are provided with a p1 of bus that are adapted to be received by a aearings 1%}, thereby prcvrdin" means whereby the furnace 11 may be tilted to cause material contained within the crucible 12' to ilow'from a spout 47 thereof.

The cover member 15 is so arranged as to stcr within the annular ring 36 and. to

cover the opening in the crucible; The cover member 15 comprises an annular electric in sulating member 48, a refractory arch 49, an annular shell member 51, a base member 52 and a topclosure member 53. The space between the closure member 53 and the arch 49 may be filled with suitable heat-insulating material 54. A member 55 of cone shape is disposed Within an opening 56 of the cover member 51, to thus provide a means for observing operating conditions within the crucible 19 and also for charging the furnace 11. The cover member 53 is suitably secured to the casing member 51 by screw-bolt members 57 and the lock washers 58. The casing 51 is also suitablysecured to the insulating member 52 by a plurality of screw-bolt members 59 and lock washers 61.

In Fig. 2 of the drawings, I have illustrated a particular form in which it may be desirable to arrange the core member 23. The core member comprises a plurality of laminations 62, a plan View of which-is illus trated in Fig. 4. The laminations 62 are arranged substantially as shown in Fig. 2 and may be arch b und by a plurality of wedging members 63 on y one of which is shown and a clamping ring 64. The clamping ringfi l may be provided with aturn-buckle of any known type, such that the wedging member 63 may beforced inwardly to tightly clamp the core member 23 in a circumferential direction.

In Fig. 3 of the drawings, a schematic diagram or the induction furnace 11 is illustrated and comprises a crucible 19, an induction coil 65, energized from a transformer 66, which in turn is energized from a source of low-frequency alternating current 67. A condenser 68 is connected across the terminals of the induction coil 65 in order to correct for the inherently low power factor caused thereby. It is to be understood that the condenser 68 may be of the static or synchronous type and it is immaterial to the satisfactory operation of the furnace which type is used at frequencies between 25 to substantially 300 cycles per second.

The induction furnace ll, illustrated in Fig. 3, is not provided with an iron core, is the induction furnace illustrated in Fig. l. The operation of the furnace in Fig. is substantially the same the operation of device shown in Fig. It, except "that is necessary to supply a larger condenser capacity to correct for the lower power-factor thereof.

By my invention l have rovided an induction furnace which may or may not u.- an iron core in its magnetic circuit and one that is operable to produce melting of cue rent-conducting materials at low frequencies of the order of from 25 cycles to 350 cycles core that does not interlink with the meterial to be heated to thereby eliminate the pinch-effect in the material to be heated which is common to low-frequency induction furnaces in which molten material forms a short-circuiied turn around the magnetic core.

It is to be understood that the induction coil. employed in the furnace embodying my invention may be made hollow and of oval 10 section, so as to beedapted to lit into a limited amountoi space and may beertificially cooled by passing water or any other suit able cooling medium therethroiigh in order to keep the-materiel of which ihe coil is made 5 at a safe operating temperature.

Various modifications may be made in the device embodying my invention without (leparting from the spiritimd scope thereofu I desire therefore, that only such limitations shall be placed thereon as are imposed by the prior art and. the appended claim.

I claim as my invention:

In aninduction furnace comprising an induction coil and e orucible disposed Within said coil so-es i0 loe in'the direct path of its magnetic axis, in combination a cylindrical core member located on the outside of said coil comprising a plurality of metallic megnetio radially spaced laminations, and means for arch-binding said leminetioiis,

In testimony whereof, I have hereunto subscribed my name one 241th day of Jul 1926.

PQRTMR H. ER GE,

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