US3395239A - Arc furnace electrode magnetic circuit forming structure for use therein - Google Patents

Description

July 30, 1968 BRUNING ET AL 3,395,239

ARC FURNACE ELECTRODE MAGNETIC CIRCUIT FORMING STRUCTURE FOR USE} THEREIN Filed Oct. 29, 1964 FIG. 2A.

FIG. IA.

PRIOR ART a FIG. 2B.

PRIOR ART FIG. 3.

PRIOR ART WITNESSESI INVENTORS 5? Armin M. Bruning 0nd 0% C. Geerge A. Kemeny AT ORNEY United States Patent M 3,395,239 ARC FURNACE ELECTRODE MAGNETlC CIRCUIT FORMING STRUCTURE FOR USE THEREIN Armin M. Bruning and George A. Kemeny, Franklin Township, Export County, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 29, 1964, Ser. No. 407,475 8 Claims. (Cl. 13-18) ABSTRACT OF THE DISCLOSURE An electrode for use in a furnace having a melt composed at least partially of ferromagnetic material which is normally raised to a temperature above the Curie polnt of the material and which has an arcing surface upon which the ferromagnetic material is deposited and cooled to a temperature below the Curie point, includes a generally annular electrode face member which provides the arcing surface, an annular field coil mounted with respect to the electrode face member in a manner to set up a magnetic field in the space between the arcing surface and the melt having lines of force in a direction to exert a force on the arc and cause it to move substantially continuously, and a generally annular magnetic yoke member so mounted with respect to the electrode face member and the field coil as to provide a low reluctance path for at least a substantial part of that portion of the total path of the magnetic lines of force not utilized in creating a magnetic field between the arcing surface and the melt, the yoke member having at least a sufficiently large permeability and cross section in accordance with the strength of the magnetic field, which magnetic field is made sufficiently strong to saturate deposited and cooled ferromagnetic material from the melt, that the yoke member remains unsaturated and the majority of the magnetomotive force of the magnetic field remains available to create a field of substantial magnitude between the arcing surface and the melt which continues to move the arc.

This invention relates to improvements in arc furnace electrodes of the type in which a magnetic field is produced and maintained at the arcing surface to cause substantially continual movement of the arc, and more particularly to an improved electrode having means for maintaining a strong magnetic field in the gap between the arcing surface of the electrode and the melt or another electrode, and which is substantially unaffected by the deposit of ferromagnetic material from the melt upon the arcing surface of the electrode.

In high power are heaters, it is necessary to continually move the are over the surface of the electrode to prevent the very hot are spot from remaining at one position on the electrode for a sufficient time interval that substantial melting of the electrode occurs, releasing the cooling fluid flowing in the electrode near the arcing surface, or to prevent material from being evaporated or Sublimated from the arcing surface thereby shortening the life of the electrode. This is usually accomplished by using an annular electrode and providing a magnetic field which is usually produced by a field coil or coils disposed near the arcing surface, the coil being energized by, for example, a direct current and setting up a sustained field which, interacting with the arc, produces a force which causes the arc to move substantially continuously in one direction of rotation around the annular electrode so long as the polarity of the arc remains unchanged. If an alternating current produces the arc and a direct current energizes the field coil, the direction of rotation changes 3,395,239 Patented July 30, 1968 periodically in accordance with double the frequency of the alternating current.

When an electrode is used in an arc furnace where the melt is at least partially composed of ferromagnetic material such as iron, it is almost impossible to prevent the formation of a crust or layer of the material of the melt upon the arcing surface of the electrode. Whereas the ferromagnetic material in the melt may be above the Curie point in temperature so that the permeability of the very hot ferromagnetic material in the melt is substantially unity, and the magnetic field of the coil is not substantially affected thereby, the material deposited upon the electrode is much cooler and in many cases will be below the Curie point in temperature, and can have a permeability many orders of magnitude above the permeability of the surrounding air or gases, with the result that the lines of flux of the magnetic field formed by the field coil are substantially all confined to paths passing through the crust or layer of melt deposited upon the electrode. The magnetomotive force of the magnetic field resulting from energization of the coil is substantially all utilized in the formation of a magnetic field in an air path on the side of the electrode away from the melt, since very little magnetomotive force is needed to force the magnetic flux lines through the high permeability material of the layer of cooled melt material on the arcing surface. The result of the confinement of the lines of flux of the magnetic field to the ferromagnetic layer on the side of the electrode toward the melt is that no appreciable magnetic field is available in the gap between electrode and melt to exert a force on the arc and cause the arc to move over the surface of the electrode, or more precisely over the surface of the layer of ferromagnetic material, with the result that the arc may be localized, and may burn a hole in the electrode very quickly.

Our invention overcomes this disadvantage of the prior art and solves the problem of maintaining a strong magnetic field in front of the electrode, that is, between the electrode and the surface of the melt notwithstanding a deposit or layer of cooled ferromagnetic material upon the electrode itself.

In summary, we provide a yoke composed of magnetic material of high permeability, of sufficient size not to become saturated by the magnetic field of the coil, the yoke being, if desired, annular in shape and semicircular in cross-section and substantially completing the enclosure of the field coil by providing with the electrode face member a substantially complete enclosure (in crosssection). Preferably the ends of the yoke terminate at the edges of the electrode face member or arcing surface, which can be semicircular in cross-section, or if desired, the ends of the yoke overlap portions of the electrode face member. Since the yoke is of high permeability material and provides a low reluctance path for the magnetic lines of force passing through the yoke, very little of the magnetoznotive force of the field coil is utilized in setting up a magnetic field in the yoke, and the magnetomotive force of the field coil is utilized in setting up a strong magnetic field in front of the arcing surface of the electrode and extending for some distance beyond, that is, in the gap between the electrode and the melt.

The magnetic field set up by the coil is preferably substantially greater in magnitude than that required to saturate the layer of melt, while not being great enough to saturate the material of the yoke. If now a layer of cooled ferromagnetic material forms on the face of the electrode, very little magnetic field force is used in maintaining a magnetic field in the yoke area which leaves most of the magnetomotive force available for saturating the layer of ferromagnetic material and to produce a strong field in the gap between the electrode or electrode face member and the melt, and the magnetic field in this location is thus not substantially decreased in amplitude. As a result, a strong field is available to cause the arc to move continuously over the face of the electrode, or over the deposit or layer of material on the electrode, protecting the electrode from hot spots and melting or evaporation.

In an other embodiment of our invention, the yoke member is annular in shape and arcuate in cross-section, and comprises, for example, an annular ring with an arcuate cross-section of substantial-1y greater than 180, for example about 315, with an annular gap of about 45 therein, and with an annular electrode face member arcuate in cross-section disposed adjacent the gap between the sides of the yoke, the electrode face member being disposed on the side of the gap toward the coil, with the field coil located substantially at or near the center of the arcuate electrode face member and near the center of the yoke. An arc taking place between the melt and the electrode face member passes through the 45 gap in the yoke, and the yoke provides a very strong magnetic field between the electrode face member and the melt, and one which is substantially unaffected by the deposit of a layer of ferromagnetic material on the electrode even while the deposit is cooled below the Curie point and has a high permeability. The surface of the yoke at least in the vicinity of the gap and the electrode is electrically insulated to prevent the are striking to the yoke or forming a conductive path through the yoke between the electrode and the melt.

Our invention is suitable for use with the non-consumable electrode described and claimed in the copending application of A. M. Bruning for Electric Arc Furnace and Non-Consumable Electrode for Use Therein, Ser. No. 407,327, filed Oct. 29, 1964, and assigned to the assignee of the instant invention, and is also suitable for use with the non-consumable electrode of S. M. De Corso and C. B. Wolf, described and claimed in their copending application entitled Non-Consumable Arc Electrode," Ser. No. 407,327, filed Oct. 29, 1964, and assigned to the assignee of the instant invention.

An object is to provide a new and improved electrode offering advantages over any now existing in the art.

Another object of the invention is to provide a new and improved arc furnace electrode having means for maintaining a magnetic field between the arcing surface and the melt despite the formation of a, deposit of relatively cool ferromagnetic material on the arcing surface.

A further object is to provide a new and improved arc furnace electrode having a yoke of ferromagnetic material disposed in predetermined position with respect to the arcing surface, and a field coil adjacent thereto for maintaining a strong magnetic field in the space between the arcing surface and the melt.

These and other objects will become more clearly apparent after a study of the following specification, when taken in connection with the accompanying drawings, in which:

FIGURES 1A, 1B and 1C illustrate an electrode of the prior art and the magnetic field around the electrode under certain conditions of operation normally encountered in service;

FIGS. 2A and 2B illustrate our electrode according to one embodiment of our invention, and show the magnetic field under similar conditions of operation; and

FIG. 3 illustrates an electrode with a yoke according to a second embodiment of our invention.

Referring now to the drawings for a more detailed understanding of the invention, in which like reference numerals are used throughout to designate like parts, and in particular to FIG. 1A thereof, there is shown an electrode face member according to the prior art generally designated 10, having a field coil 11 disposed at the approximate center thereof as taught by the aforementioned copending application of De Corso and Wolf and an are 12 between the electrode and the melt 13. Particular reference is made now to FIG. 1B, where the magnetic flux lines in the magnetic field around the electrode face member 10 of the prior art under idealized conditions are shown.

In FIG. 1B, let it be assumed that the central coil 11 carries a current giving a magnetizing force of NI ampere turns. It is further assumed that all materials have a permeability of unity (the melt 13 of scrap metal is assumed non-magnetic or above the Curie point temperature), and that the return path of the current in the field coil 11 is far enough removed so that the flux lines approximate the field of a circular current carrying wire. Under these somewhat idealized conditions, there is a magnetic field between the electrode surface and the melt which will provide the necessary forces to produce the required are movement. Furthermore, one 'half of the magnetizing force NI will be effective in establishing the flux in the portion of the flux path ABC of FIG. 1B, and one half NI will establish the flux in the portion CDA of the flux path of FIG. 1B. Thus, under these ideal conditions of FIG. 1B, the required magnetic field is produced and are rotation can be maintained.

It is likely however that ferromagnetic material will attach itself to the relatively cool electrode surface, and that at least a portion of this layer of ferromagnetic material on the electrode will be below the Curie point temperature, and thus will have a relative magnetic permeability up to 3 or 4 orders of magnitude above that of the surrounding air or gas. FIG. 1C shows the magnetic field around an electrode according to the prior art, with a layer of ferromagnetic material 15 attached to or deposited on the electrode. Under these circumstances, depending on the magnitude of NI and the thickness of the layer, it is unlikely that the layer of ferromagnetic material 15 on the electrode will be saturated, and the flux field will resemble that shown in FIG. 1C. Practically all of the magnetizing force is used up in the air gap portion ABC of the path, and the small fraction of NI required to force the flux through the layer of iron will not be sufficient to establish an adequate magnetic field in the gap between the electrode and the melt. In FIG. 1C, it is possible that more than 99% of the magnetizing force NI will be utilized in the path portion ABC. The total flux lines in FIG. 1C will be in the order of twice those in FIG. 1B, but almost all of the flux lines will pass through the ferromagnetic layer 15 and not the arc gap in front of the electrode and between the electrode and the melt. It should be noted that if the arc burns a hole through the iron layer 15 to the electrode surface, there will be little or no magnetic field at the arc spot on the electrode to tend to move the are from its contact point at the electrode surface, and a burn through may quickly occur.

The ferromagnetic layer may consist of iron splashed or otherwise built up on the electrode, or of scrap magnetically drawn to the electrode, or of iron so located in the vicinity of the electrode as to virtually short out the magnetic field in the arc gap.

Particular reference is made now to FIG. 2A which shows an electrode having a yoke according to our invention. The electrode face member 20 preferably composed of non-magnetic material, has a field coil 21, a yoke 24 and an are 22 to the melt 23. FIG. 2A shows an idealized and somewhat simplified version of the magnetic circuit of our invention, which as aforementioned includes the heavy ferromagnetic yoke 24. In the apparatus of FIG. 2A, a very small fraction of the magnetizing force NI is utilized in the path portion A'B'C' while the air gap path portion C'D'A' will utilize practically all the available magnetizing force. For a given magnetizing force NI, the number of lines of flux will be about the same as in FIG. 1C except that in the apparatus of our invention the magnetic field is where it is needed, that is, at the arcing surface of the electrode face member and between the electrode face member and the melt.

Particular reference is made now to FIG. 2B, which shows electrode apparatus according to our invention after a layer of ferromagnetic material 25 has been deposited on the electrode face member 20. It is seen from the study of FIG. 2B that the presence of the layer of ferromagnetic material 25 does not substantially alter the location of the lines of flux. The yoke 24 in the path portion A'B'C' is sufiiciently heavy to prevent magnetic saturation of the yoke iron, and therefore only a small fraction of the total magnetomotive force or magnetizing force NI is used in this portion of the flux path, and the remainder of the magnetizing force is utilized in setting up the magnetic field in the path portion C'D'A. With most of the NI available in this path portion, the magnetizing f-orce available saturates the ferromagnetic layer 25 on the electrode face member 20, and there will then be established an adequate magnetic field in the arc gap adjacent the saturated iron layer and between the electrode and the melt. The total number of magnetic field lines in the electrode-to-melt gap in FIG. 2B may be orders of magnitude larger than the number of lines in the gap in FIG. 1C, even though the magnetizing force NI remains substantially the same. In short, in our invention the number of lines of force and the flux density distribution in the air gap between the electrode face member and the melt need not be significantly less after the formation of the layer of ferromagnetic material on the electrode.

Particular reference is made now to FIG. 3, in which another embodiment of our invention is shown. In FIG. 3 the yoke 34, which is annular, and arcuate-shaped in cross-section, extends considerably further than 180 as is the case in FIG. 2B; in FIG. 3 the arcuate-shaped yoke 34 may extend for example 315 in cross-section and have an annular gap of 45 therein. The electrode face member 30 is disposed within the yoke, at or near the opening between the sides of the yoke, the field coil 31 being shown substantially centrally disposed with respect to the yoke, and also at the virtual center of the arcuateshaped electrode face member. The arc 32 passes through the gap between the sides of the yoke member 34, and between the electrode face member 30 and the melt 33. It will be apparent that a very strong magnetic field 37 is maintained between the sides of the yoke member in the space between the electrode face member and the melt. Yoke 34 is preferably sufficiently heavy not to be saturated by the magnetic field of coil 31.

For proper operation of the configuration shown in FIG. 3, are attachment to the yoke is prevented by ceramic insulation 35, and the yoke in the vicinity of the arc is suitably cooled.

Preferably the electrode 20 is composed of non-magnetic material. However, if the magnetic field set up by the coil 21 is sufiiciently strong to saturate both the layer of deposited ferromagnetic material and an electrode composed of magnetic material, magnetic material could be employed.

Other suitable insulation, in addition to a heat resistant ceramic, may be used at 35 in FIG. 3.

Any convenient means or structure, not shown for convenience of illustration, may be used for supporting or securing the yoke, field coil, and electrode face member in desired positions with respect to each other.

Whereas our invention has been described with reference to an are occurring between the electrode and the melt, the arc could occur between two electrodes. The term surface of opposite polarity includes the melt or another electrode.

Our invention is suitable for use with water cooled electrodes.

Whereas the ferromagnetic yoke of the electrode has been illustrated in an idealized simplified version, it should be understood that this yoke can also be part of the structural components of the electrode configuration.

The yoke may also form a part of the coolant manifolding means.

Whereas the invention has been illustrated with respect to a yoke generally circular or arcuate in cross section, and with respect to an electrode face member generally arcuate in cross section, other shapes could be employed. The yoke-electrode assembly could be square, with the yoke providing three sides of the square cross-section.

Whereas we have shown and described our invention with respect to some embodiments thereof which give satisfactory results it should be understood that changes may be made and equivalents substituted without departing from the spirit and scope of the invention.

We claim as our invention:

1. In an electrode for use in an arc furnace of the type in which the melt consists at least partially of ferromagnetic material, in combination, an annular electrode face member composed of non-magnetic conductive material with an arcing surface, the electrode face member being adapted to be connected to a source of potential to produce an arc therefrom to a surface of opposite polarity, magnetic field producing coil means mounted near the electrode face member, at least a substantial portion of the lines of force of the magnetic field normally taking a path part of which partially encircles the electrode face member and steps up a magnetic field in the space between the electrode face member and the surface of opposite polarity, said magnetic field causing the arc to substantially continuously move around the electrode face member, ferromagnetic material from the melt being occasionally deposited on the arcing surface of the electrode face member and forming a layer thereon, at least a portion of the layer of ferromagnetic material being normally cooled below the Curie point of the ferromagnetic material and having a high permeability relative to the permeability of the gases adjacent the electrode face member, and an annular yoke composed of ferromagnetic material mounted adjacent the electrode face member on the side thereof away from the arcing surface and in a position whereat the yoke forms a low reluctance path for at least a substantial part of that portion of the total path of the magnetic lines of force not utilized in creating a magnetic field between the arcing surface and the melt, the yoke being composed of a material having a permeability and a sufficient cross-section whereby the material of the yoke is not saturated by the magnetic field set up by the coil means, the low reluctance of the yoke providing that little of the magnetomotive force of the magnetic field is utilized in providing a flux path through the yoke, the strength of the magnetic field of the coil means being substantially greater than that required to saturate that ferromagnetic material of the layer on the electrode face member which is cooled below the Curie point, whereby the major portion of the magnetomotive force of the magnetic field is utilized in providing a magnetic field in the space between the layer of deposited ferromagnetic material and the surface of opposite polarity.

2. A non-consumable electrode for use in an arc furnace where the melt consists at least partially of ferromagnetic material comprising, in combination, an annular electrode face member arcuate-shaped in cross-section and composed of a non-magnetic material, the electrode face member being adapted to be connected to means for bringing a current thereto to form an are between the arcing surface of the electrode face member and a surface of opposite polarity, annular coil means partially enclosed by and mounted centrally with respect to the electrode face member for setting up a magnetic field between the arcing surface of the electrode face member and the surface of opposite polarity, said magnetic field causing the arc to move substantially continuously over the arcing surface of the annular electrode face member, and generally annular yoke means composed of magnetic material having the electrode face member mounted therein and partially enclosing the coil means, the yoke means he ing substantially arcualte in cross-section with the annular gap between the sides of the yoke means adjacent the arcing surface of the electrode face member, an are from the arcing surface of the electrode face member passing through said gap between the sides of the yoke means, the electrode face member having a layer of ferromagnetic material from the melt deposited thereon, at least a portion of the layer of material being cooled to a temperature below the Curie point of the ferromagnetic material, the yoke means providing a low reluctance path and maintaining a strong magnetic field in the space adjacent the arcing surface of the electrode face member and in the space between the sides of the yoke means notwithstanding the deposit of cooled ferromagnetic material on the arcing surface of the electrode face member.

3. An electrode for use in an arc furnace having ferromagnetic material as a portion of the melt and in which ferromagnetic material from the melt is normally deposited as a layer on the electrode, the material of the layer being cooled below the Curie temperature of the material, comprising, in combination, an annular yoke member composed of magnetic material having a pre determined arcuate cross-section, with an annular gap therein facing toward the melt, an annular electrode face member composed of non-magnetic material having a predetermined arcuate cross-section and mounted within the yoke member, the electrode face member and the yoke member being mounted in predetermined positions with respect to each other with the arcing surface of the electrode face member adjacent the gap in the yoke member, coil means for setting up a magnetic field mounted inside the yoke member and near the electrode face memher, the magnetic field being sufficiently strong to saturate the ferromagnetic material of the deposited and cooled layer but not strong enough to saturate the yoke member whereby a magnetic field set up near the electrode face member to cause the arc to move over the surface of the electrode face member is maintained at at least a predetermined strength by the loke member notwithstanding the deposit of cooled ferromagnetic material on the arcing surface of the electrode face member.

4. A non-consumable electrode for use in an arc furnace where the melt consists at least partially of ferromagnetic material comprising, in combination, an annular electrode face member arcuate-shaped in cross-section and composed of non-magnetic material, the electrode face member being adapted to have means connected thereto for bringing a current thereto to form an are between the arcing surface of the electrode face member and a surface of opposite polarity, coil means mounted at substantially the radial center of the electrode face member for setting up a magnetic field between the arcing surface of the electrode face member and the surface of opposite polarity, said magnetic field causing the arc to move substantially continuously over the arcing surface of the annular electrode face member, and yoke means composed of magnetic material mounted around the coil means, the yoke means being arcuate in cross-section having a cross-sectional circumference of the order of 315 with the annular gap of the order of 45 between the sides of the yoke means, said annular gap being adjacent the arcing surface of the electrode face member, an are from the arcing surface of the electrode face member passing through said gap between the sides of the yoke means, the electrode face member having a layer of material from the melt deposited thereon, at least a portion of the layer of material being cooled to a temperature below the Curie point of the ferromagnetic material, the yoke means maintaining a strong magnetic field in the space adjacent the arcing surface of the electrode face member and in the space between the sides of the yoke means notwithstanding the deposit of cooled ferromagnetic material on the arcing surface of the electrode face member.

5. An electrode according to claim 3 including in addition electrical insulation covering at least that portion of the surface of the yoke member which is near thevgap and near the electrode face member. I

6. An electrode according to claim 3 in which the arcuate electrode face member extends beyond the edges of the annular gap in the yoke member on both sides of the yoke member.

7. In an electrode, an annular shaped electrode face member of arcuate cross-section mounted transversely on the end of said electrode and adapted to be connected to a source of potential for producing an arc therefrom to a surface of opposite polarity, coil means at least partially enclosed by and mounted centrally on said electrode face member for setting up a magnetic field at least a portion of the lines of force of which encircle the ananular electrode face member and exert a force on the are which causes the arc to move in an annular path around the electrode face member, an annular shaped yoke member of arcuate cross-section mounted on said electrode face member to complete the enclosure of said coil, the yoke member being composed of ferromagnetic material and providing a low reluctance path for the magnetic lines of force, the sides of the yoke member extending at least as far as the edges of the arcuate electrode face member, the electrode face member normally having a deposit of melt thereon composed of ferromagnetic material at least a portion of which is cooled below the Curie point of the material and has a permeability substantially larger than the permeability of the surrounding gases, the strength of the magnetic field being substantially larger than that required to saturate that portion of the deposited melt material cooled below the Curie point, the yoke member being composed of material having a sufficiently high permeability and a sufficiently large crosssection whereby the yoke member is not saturated by the magnetic field and continues to form a low reluctance path whereby the major portion of the magnetomotive force of the magnetic field is utilized in setting up a magnetic field between the deposited layer on the electrode and the melt which continues to move the are.

8. A method for sustaining arc movement on an electrode employed in a furnace having a melt composed at least in part of ferromagnetic material, the electrode having electrode face means forming an arcing surface and magnetic field producing means for generating a magnetic field between the arcing surface and the melt for normally moving the are substantially continuously in a path around the electrode face means, the electrode face means having sometimes deposited thereon ferromagnetic material from the melt at a temperature below the Curie point thereof, which comprises the steps of forming a low reluctance path for magnetic flux around at least a substantial portion of the electrode face means on the side thereof away from the melt, and generating a magnetic field of predetermined strength sufficient to saturate the material of the deposit but not of sufficient strength to saturate the material forming the low reluctance path, only a small portion of the magnetomotive force of the magnetic field being used to produce flux in said low reluctance path whereby the major portion of the magnetomotive force of the magnetic field is utilized in setting up a magnetic field between the deposit and the melt which continues to move the arc.

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