US2908736A - Electrode lead arrangement for three phase electric furnace - Google Patents

Description

H. ERNST Oct. 13, 1959 ELECTRODE LEAD ARRANGEMENT FOR THREE PHASE ELECTRIC FURNACE Filed June 6, 1958 IN VENTOR. Z2

rnst V uftcarzzey United States Patent ELECTRODE LEAD ARRANGEMENT FOR PHASE ELECTRIC FURNACE 2,908,736 Patented Oct. 13, 1959 contact elements which are not shown but are well known in the art.

Hans Ernst, Duisburg, Germany, assignor to DEMAG- Elektrometallurgie G.m.b.H., Duisburg, corporation of Germany Application June 6, 1958, Serial No. 740,443

Claims priority, application Germany June 8, 1957 4 Claims. (Cl. 13-9) Germany, a

This invention relates to electric arc furnaces and more particularly to the electrode leads of a three phase electric arc furnace.

section.

Electrode leads in a three phase electric arc furnace A i are generally disposed in planar spaced apart relation with one electrode lead midway between the other two. In the operation of such furnaces, particularly those where the current exceeds 20,000 amperes, it has been 5 i lex inductance which is a combination of self inductance found that when equal currents are flowing in the outer electrodes, the arc voltages of these electrodes are not equal. Thls phenomenon was due to difierences in the inductances of the electrode leads of prior art electric are furnaces and often resulted in unequal power output in the two outer electrodes with consequent nonuniform electrode consumption and heat radiation.

Prior art arrangements for minimizing this unbalance in the inductances of the phases, for example inserting a balancing transformer between the two outer phases, were expensive and often inefficient.

It is a primary object of the invention to minimize the inequality of inductances between the electrode leads in a three phase electric arc furnace.

Other objects and advantages of the invention will be apparent from the following detailed description of the invention taken in view of the accompanying drawing in which:

Fig. 1 is a vertical sectional view through an electric arc furnace having electrode leads according to a preferred embodiment of the invention;

Fig. 2 is a sectional view through the electrode leads taken along line 2-2 of Fig. 1;

Fig. 3 is a modification of the electrode leads shown in Fig. 2; and

Fig. 4 is a sectional view of the electrode leads taken along line 4-4 of Fig. 1. I

Referring to the drawing, Fig. 1 shows a three phase electric furnace 10 having a shell 12 with suitable refractory lining and three electrodes, only two of which 14 and 16 are shown, supported on and secured to electrode supporting arms 18 by suitable electrode clamping devices 20 which are well known in the art. Each of the electrode supporting arms 18 is mechanically connected to a suitable positioning device such as a hy draulic ram 22 which is controlled by a servomechanism indicated at 24 and which, in turn, responds to signals derived from the arc current and are voltage by means that are not shown but are well known in the art.

The three electrodes are electrically connected to the phases of a three phase transformer 26 by electrode leads 28. Each of the electrode leads 28 includes a first bus bar 30 secured to a transformer terminal 32, a second bus bar 34 mounted on its respective electrode supporting arm 18 by support members 36, and a flexible center portion 38 electrically connecting first bus bar 30 to second bus bar 34 and which allows the electrodes to be raised and lowered without stressing bus bars 30 and 34. Conducting members 40 electrically connect second bus bars 34 to their respective electrodes through Throughout the descriptionand the appended claims, the expression mean geometric distance of the cross section is intended to be generic to circular as well as noncircular cross sections such as shown in Fig. 2 and to bunched conductors as shown in Fig. 4 and to be the equivalent of the mean geometric radius which is a mathematical quantity commonly used in electrical engineering in the determination of the total flux linkages in a conductor of circular cross section. The inductance per unit length of each of the electrode leads is a comand mutual inductance, and the inductances per unit length of the three electrode leads in a three phase furnace energized by balanced three phase currents having a sequence ABC are given by the expressions:

a at ac' bc E 5% L K(ln gla r r and r are the mean geometric distances of the cross sections of the leads of the phases A, B, and C, respectively.

When the electrode leads 28 are disposed in a common plane and the distances d and d are equal as shown in Fig. 2, the expressions for the inductance per unit length of the electrode leads becomes:

It can be seen from these expressions that the inductances of the electrode leads of phases B and C can be made equal either by varying the distance between leads or the mean geometric distances of the cross sections of the leads. However, because the inductance increases as the distance between electrode leads increases and because it is desirable to keep this inductance as small as possible, the electrode leads are preferably disposed as close together as electrical clearance will permit. In accordance with the invention, the inductances of the electrode leads of phases B and C are equalized by metric distances of their cross sections. This desired relationship in the sizes of the leads is determined by equating the inductances L and L of these leads:

From this expression the relative dimensions of the electrode leads of phases-B and C to achieve equality of inductance can be readily determined. This relationship is maintained for all three pontions of the electrode leads, i.e., the first bus bars 30, the second bus bars 34, and the flexible centerportions 38.

Each flexible conducting portion 38 generally comprises a group of bunched individual conducting members X, Y and Z as shown in Fig. 4. The term mean geometric distance of the cross section is not conventionally applied to such bunched conductors, but in an analogous manner the inductance per unit length of such a bunched conductor is dependent upon both the mean geometric distance of the cross section of the individual conductors and the distances of the individual conductors from each other. In accordance with the invention the individual conductors X, Y and Z of the electrode lead of phase C are either spaced from each other a greater distance than that between the individual conductors X, Y and Z' of the electrode lead of phase B or have a different size than the individual conductors of phase B, or a combination of both such factors may be utilized as illustrated in Fig. 4. This variation in size and spacing between individual conductors to effect equal inductances of the two outer flexible portions most.

effective when equal numbers of individual conductors are utilized in the two leads and the conductors have different spacing in a vertical plane as shown in Fig. 4.

While only a preferred embodiment of the invention has been illustrated and described, many modifications and variations thereof will be obvious to those skilled in the art, and consequently it is intended to cover in the appended claims all such variations and modifications which fall within the true spirit and scope of. the invent-ion.

I claim: l a

1. In combination with a three phase electric arc furnace having three electrodes, an electrode lead connecting each of said electrodes with a source of three phase electrical energy, said electrode leads being spaced relative to each other, one of said electrode leads being disposed substantially midway between the other two, said 2. In a combination with a three phase electric arc furnace having three electrodes energized by a three phase transformer, an electrode lead connecting each of said electrodes with one of the phases of said transformer, said electrode leads being spaced relative to each other, one of said electrode leads being disposed substantially midway between the other two, said electrode leads being substantially coplanar through a substantial portion of their lengths, the logarithm of the mean geometric distance of the cross section of one of said other electrode leads being equal to the square root of the sum of the squares of the logarithm of the mean geometric distance of the cross section of the second of said other electrode leads plus a constant.

3. In combination with a three phase electric arc furnace having three electrodes, a three phase transformer, three electrode leads, one of said electrode leads connecting each of said electrodes with one of the phases of said transformer, said electrode leads being spaced relative to each other with one of said electrode leads disposed substantially midway between the other two, each of said electrode leads including a first bus bar a portion electrically connected to its corresponding electrode, a second bus bar portion connected to one of the phases of said transformer, and a flexible conducting portion connecting one of said first bus bars with the corresponding one of said second bus bars, correspond-- r ing bus bar portions being substantially coplanar, said flexible portions of said electrode leads being of substantially equal length, .the logarithm of the mean geometric distance of the cross section of each of the portions of one of the outer electrode leads'being equal to the square root of the sum of the squares of the logarithm of the mean geometric distance of the cross section of the corresponding portions of the other of said outer electrode leads plus a constant.

4. In a three phase electric arc furnace having three electrodes, an electrode lead connecting eachof said electrodes with one of the phases of a three phase source of electrical energy, said electrode leads' being spaced from each other with one of said electrode leads substantially midway between the other two, each of said leads including a first bus bar portion connected to one of said electrodes, a second bus bar portion connected to said source of electrical energy, and a flexible portion connecting said first and second bus bar portions, the

corresponding bus bar portions being substantially coplanar, the mean geometric distance of the cross section of each of said portions of one of the outer electrode leads being greater than the mean geometric distance of the cross section of the corresponding portion of the other of said outer leads, the flexible portion of each electrode lead including a group of individual conductors, the individual conductors of said one electrode lead having a different effective distance from each other than the individual conductors of the other outer electrode lead.

No references cited.

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