US3483300A - Electric arc furnaces - Google Patents

Electric arc furnaces Download PDF

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Publication number
US3483300A
US3483300A US731890A US3483300DA US3483300A US 3483300 A US3483300 A US 3483300A US 731890 A US731890 A US 731890A US 3483300D A US3483300D A US 3483300DA US 3483300 A US3483300 A US 3483300A
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electrode
conductors
tubes
bus
inductance
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Expired - Lifetime
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US731890A
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English (en)
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Lawrence Mcgee
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United Steel Companies Ltd
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United Steel Companies Ltd
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B7/00—Heating by electric discharge
    • H05B7/02—Details
    • H05B7/11—Arrangements for conducting current to the electrode terminals

Definitions

  • a three-phase electric arc furnace having three electrodes each energized from a common supply transformer by an arrangement of flexible and rigid conduct-ors. There are at least two rigid conductors connected in parallel for each electrode, those for the two outer electrodes lying in respective vertical planes and those for the inner electrode lying in a horizontal plane, this arrangement presenting a particularly low inductance.
  • This invention relates to three-phase electric arc furnaces and in particular to the electrical connections between the supply transformer and the electrode holders.
  • the three electrodes extend vertically downwards into the furnace shell, each being held in this position by a respective electrode holder.
  • the three holders Normal practice is for the three holders to be located so that the electrode axes are at the apices of an equilateral triangle.
  • Each holder is carried adjacent one end of a respective horizontally extending electrode arm.
  • the three arms extend above the furnace shell and are generally parallel, often diverging at a small angle to one another towards the furnace shell. Accordingly when viewed from above there are two outer electrode arms carrying outer electrode holders and a centre electrode arm carrying the centre electrode holder.
  • Each arm is normally carried by a respective vertical mast by which it and thus the associated electrode can be displaced vertically to adjust the arc length.
  • the three masts are located between the furnace shell and the supply transformer which is electrically connected to the electrodes by an arrangement of flexible conductors and rigid conductors, normally bus-tubes.
  • the flexible conduct'ors are necessary to cater for the relative movement between the electrode arms and the supply transformer. These conductors are connected to the rigid conductors which extend above the arms to the electrode holders which are electrically connected to the electrodes.
  • Such a furnace may be considered as belonging to one of three categories depending respectively on whether it has a power rating of below mva., of between 10 mva. and mva., or above 20 mva.
  • a furnace of any rating it is important that the power dissipation in the three phases should be equal and hence that the impedances of the electrical connections between the supply transformer and electrode holders should be equal for each phase so as to give phase-balance.
  • phase-balancing becomes increasingly difficult.
  • the inductance of the secondary connections i.e. those between the transformer secondary terminals and the electrode holders
  • a three-phase electric arc furnace of the kind previously described that is to say including three electrodes each held by a respective electrode holder, and three generally parallel, horizontally extending, vertically displaceable electrode arms each carrying one of the holders adjacent one end
  • at least two rigid electrode supply conductors connected in parallel extend horizontally from each of the two outer electrode holders above the arms to respective flexible conductors connected to the supply transformer and are located with those from each holder lying in a respective one of a pair of substantially vertical, substantially parallel, spaced apart planes
  • at least two rigid electrode supply conductors connected in parallel extend horizontally from the centre electrode holder above the arms to respective flexible conductors connected to the supply transformer, are located substantially symmetrically between the two'vertical planes, and lie in a respective substantially horizontal plane.
  • Such a furnace which therefore includes six or more substantially parallel electrode supply conductors, normally bus-tubes, has the major advantage over conventional furnaces that with suitable separations between the conductors, it has a particularly low inductance.
  • conductor separations can be chosen in which besides presenting alow inductance, the inductance of the total secondary connections leading to the three electrode holders are equal thus giving phase-balance. Accordingly the problems previously discussed may be largely overcome in a simple cheap manner.
  • the flexible conductors themselves have a certain amount of inductance.
  • Conventional practice is to group the flexible conductors of each phase in a bundle.
  • the usual effect of this on the conductors per phase is that the bundle of flexible conductors for the centre electrode holder, being located between the other two bundles, has a lower inductance than the outer phases thus contributing to phase unbalance.
  • this effect is no longer necessarily undesirable as by a suitable choice of the separation between the rigid conductors, e.g. bus-tubes, which lie in the substantially horizontal plane or planes, the inductance of these conductors can be made less than, equal to or greater than that of the rigid conductors extending to the outer electrode holders.
  • the overall effect is that the total inductances of the phases are equal (it is of course understood that the inductances of the electrodes and the transformer are symmetrical).
  • This is particularly desirable in that it is achieved using the conventional arrangement of flexible conductors.
  • the separation between the rigid conductors can be so chosen that these conductors have equal inductance. This entails the use of an arrangement of flexible conductors also having equal inductance if phase-balance is to be obtained.
  • each pair can provide a go and return flow path for cooling water.
  • a reduction in the inductance of the secondary connections can also be achieved by completing the delta connection of the supply transformer at the junction of the flexible and rigid conductors.
  • This entails six bundles of flexible conductors connecting the six transformer terminals to the rigid conductors.
  • the flexible conductors secured to the outer transformer terminals are connected between the outer and inner phase rigid conductors while the two inner bundles of flexible conductors are connected between the inner transformer terminals and the outer phase rigid conductors.
  • FIGURE 1 is a side elevation of the furnace
  • FIGURE 2 is a schematic illustration showing the electrical connections
  • FIGURE 2a is a schematic illustration showing an alternative arrangement of the electrical connections.
  • FIGURE 3 is a more detailed side elevation partly cut away showing the rigid electrode supply bus-tubes
  • FIGURE 4 is a plan view corresponding to FIGURE 3;
  • FIGURE 5 is an outline section taken along the line V-V of FIGURE 3;
  • FIGURE 6 is an outline section taken along the line VI-VI of FIGURE 3.
  • FIGURE 7 is a graph showing the effect on the inductance of varying the separation of the centre-phase bustubes.
  • the furnace includes a supply transformer 1 having three primary or input terminals 2 and six secondary or output terminal 3, the latter of which are labelled individually a a b b c 0 for reasons which will become apparent later.
  • Each of the three transformer secondary windings is connected between a respective pair of the secondary terminals 3.
  • Six sets of flexible conductors each set comprising a bundle of eight conductors, electrically connect each secondary terminal 3 to a respective pair of electrode supply rigid bus-tubes as shown in FIGURE 2.
  • This arrangement of flexible conductors has low inductance because the two bundles of each phase hang close together and the mutual inductances between them are 180 apart and therefore fully subtractive.
  • FIGURE 2a The alternative arrangement of FIGURE 2a on the other hand has a higher inductance because the subtractive mutual inductances between the phases are 120 apart and therefore only half-subtractive.
  • the furnace includes six bus-tubes 6-11 in all of which only three indicated at 6, 7 and 8 are visible in FIGURE 1.
  • the :bus-tubes 6 and 7 are supported on and electrically insulated from a horizontal electrode arm 12 which itself is carried by a vertically displaceable mast 15.
  • the two bus-tubes 6 and 7 are connected electrically so as to be in parallel by a cross-member 27.
  • An electrode holder 18 is carried adjacent the end of the arm 12 and holds a cylindrical electrode 21.
  • the bus-tubes 8 and 9 are supported on and electrically insulated from an identical horizontal electrode arm 13 which itself is supported by an identical vertically displaceable mast 16.
  • These two :bus-tubes are connected to an electrode holder 19 which holds an electrode 22 and are connected together electrically by a cross-member 28.
  • the two remaining bus-tubes 10 and 11 are supported on and electrically insulated from an identical horizontal electrode arm 14 carried by a third vertically displaceable mast 17 and are connected by means of an identical electrode holder 20 to the third electrode 23.
  • a cross-member 29 connects the bus-tubes 10 and 11 together.
  • the three electrode arms 12-14 are generally parallel, converging at a small angle towards the electrodes 21-23.
  • the latter are located with their axes at the apices of an equilateral triangle and extend downwardly into a furnace shell 24 (see FIGURE 1).
  • the charge to :be melted is placed in this and the heat of the three arcs which occur between the electrodes 21-23 and the metal surface melts the charge in the usual way.
  • the three masts 15, 16 and 17 are adjusted vertically so as to adjust the corresponding arc lengths and thus maintain optimum operating conditions.
  • the shell 24 is tipped on racks 25 and the metal thus poured out.
  • All of the bus-tubes have an external diameter of 12.7 cm. (5 inches) and an internal diameter of 9.5 cm. (3.75 inches).
  • the length of the'four longest bus-tubes, that is to say those connected to the outer electrode holders 18 and 20, is 8.5 metres.
  • the three electrode arms have an external diameter of 40.7 cm. (16 inches) and an internal diameter of 35.6 cm. (14 inches).
  • brackets 26 made of stainless steel. These brackets are electrically insulated from the bus-tubes and are designed to withstand the maximum electromagnetic separation force that can occur between adjacent bus-tubes.
  • FIGURE 5 illustrates the disposition of the bus-tubes 6-11 over the greater part of their length and also how they are disposed in relation to the three electrode arms 12-14, which are shown at the same height. It can be seen from this figure that the two bus-tubes 6 and 7 lie in a vertical plane while the two :bus-tubes 10 and 11 also lie in a vertical plane spaced apart from the first plane. The two bus-tubes 8 and 9 lie in a horizontal plane and are symmetrically placed with respect to the other four bus-tubes. All of the bus-tubes are parallel to one another and extend horizontally. The spacings indicated in FIGURE 5 are set forth in the table given below.
  • FIGURE 6 illustrates the disposition of the six bustubes adjacent the connections to the flexible conductors 4.
  • Th p c g g is 28.0 cm. (11 i ch
  • the inductance per foot of the bus-tubes may be calculated using the normal mathematical formulae. With the dispositions indicated, the average inductance per metre of all the bus-tubes is 0.335 ,uH (0.102 ,uH/fOOt). The inductance of the centre bus-tubes 8 and 9 is higher than this being 0.374 tH/metre (0.114 H/foot).
  • furnaces designed with flexible conductor bundles such as in FIGURE 2a, having low inductance in the centre phase bundle that the separations should be such that the inductance of the centre bus-tubes 8 and 9 is higher than that of the outer bus-tubes 6, 7, 10 and 11 so as to counteract the eifect of the flexible conductors 4 and thus give balance.
  • FIGURE 7 The effect of increasing the separation of the two centre bus-tubes 8 and 9 while maintaining them spaced symmetrically between the outer bus-tubes is shown in FIGURE 7 in which I represents the inductance in ,aH/metre and d represents the separation of the bustubes 8 and 9 in centimeters.
  • the outer bus-tubes and the electrode arms are located as shown in FIGURES 5 and 6.
  • the curve m gives the relationship for the centre phase conductors, the curve p that for the outer phase conductors and the curve n the average. It will be understood that the separation of the two centre bustubes 8 and 9 is chosen to make the inductance unbalance equal and opposite to that of the flexible conductors 4.
  • the effect of increasing the separation of the outer phase bus-tubes, 6 from 7 and 10 from 11, is to decrease their additive mutual inductance but without at the same time greatly decreasing their subtractive mutual inductance with the bus-tubes of the centre phase.
  • the balanced position where all phases are equal is substantially as illustrated in FIGURE 7, at the point of intersection of the three curves.
  • the bus-tubes 6 and 10 and also the bus-tubes 7 and 11 should be close together but a limitation on their minimum separation is caused by the heating effect of the current flowing in them on the centre electrode arm 13, on any occasion when any of these bus-tubes are at the same elevation.
  • the average inductance per metre of the illustrated arrangement is 0.335 ,uI-I.
  • an equivalent conventional arrangement consisting of six bus-tubes all located in the same horizontal plane has an average inductance of 0.485 aI-I/metre and moreover the centre bus-tubes are lower in inductance than the outer bus-tubes and hence with the similar effect presented by the flexible conductors, there is a considerable degree of unbalance between the phases.
  • Another equivalent conventional arrangement consists of three bus-tubes having their axes at the apices of an equilateral triangle. The size of the triangle is the smallest that the given electrode arm spacing permits. Although the inductances of these bus-tubes are substantially equal, the inductance per metre is high being of the order of 0.508 pH.
  • a three-phase electric arc furnace energized from a supply transformer and including three electrodes each held by a respective electrode holder, three generally parallel, horizontally extending, vertically displaceable electrode arms each carrying one of said holders adjacent one end, rigid electrode supply conductors extending horizontally from each of said holders above said arms, and flexible conductors connecting said rigid conductors to said supply transformer, each outer holder of said electrode holders having at least two of said rigid conductors connected in parallel and lying in a respective one of a pair of substantially vertical, substantially parallel, spaced apart planes, and from centre holder of said electrode holders at least two of said rigid conductors connected in parallel, located substantially symmetrically between said vertical planes, and lying in a respective substantially horizontal plane.
  • a furnace according to claim 1 in which said supply transformer has three electrically separate secondary windings each terminating in a respective pair of six secondary terminals, and said flexible conductors connect each terminal of each of said pairs to those of said rigid conductors connected to different electrodes.
  • a three-phase electric arc furnace comprising a supply transformer, a vertically extending inner electrode, two vertically extending outer electrodes located on either side of said inner electrode, three electrode holders each holding one of said electrodes, three generally parallel, horizontally extending, vertically displaceable electrode arms each carrying one of said holders adjacent one end, three pairs of rigid electrode supply conductors, each pair of which extends horizontally from one of said holders above said arms, and flexible electrode supply conductors connecting said rigid electrode supply conductors to said supply transformer, said rigid conductors being disposed with those pairs extending from holders of said outer electrodes being located in respective substantially vertical, substantially parallel, spaced apart planes and with that pair extending from holder of said centre electrode being located substantially symmetrically between said vertical planes in a substantially horizontal plane.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Furnace Details (AREA)
  • Discharge Heating (AREA)
US731890A 1967-06-06 1968-05-24 Electric arc furnaces Expired - Lifetime US3483300A (en)

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GB26087/67A GB1166148A (en) 1967-06-06 1967-06-06 Electric Arc Furnaces

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US (1) US3483300A (enrdf_load_stackoverflow)
DE (1) DE1765526A1 (enrdf_load_stackoverflow)
FR (1) FR1569521A (enrdf_load_stackoverflow)
GB (1) GB1166148A (enrdf_load_stackoverflow)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3767831A (en) * 1972-08-23 1973-10-23 Boehler & Co Ag Geb Process and apparatus for electro-slag remelting metals and in particular steel
US3835227A (en) * 1966-12-24 1974-09-10 Inst Elektroswarki Patona An electroslag remelting system with equalized plural electrode remelting
US4414672A (en) * 1981-09-15 1983-11-08 Institut Elektrosvarki Imeni E. O. Patona Akademii Nauk Ukrainskoi Ssr Plasma-arc furnace
US4425658A (en) 1981-02-24 1984-01-10 Fried. Krupp Gesellschaft Mit Beschrankter Haftung Conductor arrangement for a three-phase electric arc furnace
US5715273A (en) * 1995-04-14 1998-02-03 Danieli & C. Officine Meccaniche Spa Secondary circuit with variable impedance for electric arc furnaces

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2517502A1 (fr) * 1981-12-01 1983-06-03 Clesid Sa Dispositif d'alimentation et de support d'electrodes pour four a arcs
DE3808683C1 (enrdf_load_stackoverflow) * 1988-03-11 1989-09-07 Mannesmann Ag, 4000 Duesseldorf, De

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2332944A (en) * 1943-10-26 Electrode holder
US2368998A (en) * 1942-05-02 1945-02-06 Nissim Raoul Electric arc furnace
US2494775A (en) * 1946-01-31 1950-01-17 Delaware Engineering Corp Arc electrode support
US2752409A (en) * 1955-04-01 1956-06-26 Shawinigan Chem Ltd Low voltage connections for electrode furnace
US3431344A (en) * 1965-11-15 1969-03-04 Westinghouse Electric Corp Control system providing supply circuit impedance balance control for electric arc furnaces
US3433878A (en) * 1965-06-10 1969-03-18 Asea Ab Line transmission in arc furnaces

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2332944A (en) * 1943-10-26 Electrode holder
US2368998A (en) * 1942-05-02 1945-02-06 Nissim Raoul Electric arc furnace
US2494775A (en) * 1946-01-31 1950-01-17 Delaware Engineering Corp Arc electrode support
US2752409A (en) * 1955-04-01 1956-06-26 Shawinigan Chem Ltd Low voltage connections for electrode furnace
US3433878A (en) * 1965-06-10 1969-03-18 Asea Ab Line transmission in arc furnaces
US3431344A (en) * 1965-11-15 1969-03-04 Westinghouse Electric Corp Control system providing supply circuit impedance balance control for electric arc furnaces

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3835227A (en) * 1966-12-24 1974-09-10 Inst Elektroswarki Patona An electroslag remelting system with equalized plural electrode remelting
US3767831A (en) * 1972-08-23 1973-10-23 Boehler & Co Ag Geb Process and apparatus for electro-slag remelting metals and in particular steel
US4425658A (en) 1981-02-24 1984-01-10 Fried. Krupp Gesellschaft Mit Beschrankter Haftung Conductor arrangement for a three-phase electric arc furnace
US4414672A (en) * 1981-09-15 1983-11-08 Institut Elektrosvarki Imeni E. O. Patona Akademii Nauk Ukrainskoi Ssr Plasma-arc furnace
US5715273A (en) * 1995-04-14 1998-02-03 Danieli & C. Officine Meccaniche Spa Secondary circuit with variable impedance for electric arc furnaces

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GB1166148A (en) 1969-10-08
FR1569521A (enrdf_load_stackoverflow) 1969-05-30
DE1765526A1 (de) 1972-01-13

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