US1543908A - Method of operating electric furnaces - Google Patents

Description

June 36, 1925.

w. DYRSSEN METHOD OF OPERATING ELECTRIC FURNACES 7 Sheets-Sheet 1 I Filed March 50, 1922 v A ORNEY June 30, 1925.

VoL T465 4, PERCENT/76E Fawn? EICTOR m Farm/v65 1,543,908 w. DYR'SSEN METHOD OF OPERATING ELECTRIC FURNACES 7 Sheets-Sheet 2 Filed March 30. 1922 l dz" Vc/f "8 5 pd f K/LOWHTTSW;

RNEY

June 30, 1925.

Filed March 30, 1922 7 Sheets-Sheet 5 83 QQQ gem ATTORNEY June 30, 1925. 1,543,908

W.. DYRSSEN METHOD OF OPERATING ELECTRIC FURNACES Filed March 30, 1922 7 Sheets-Sheet 7 I IL 6f 7 o 6 INVENTOR Wa/demarflyrsxn y BY 65 Y fi. M

ATTO NEY Patented June 30, 1925.

wammun mms'snn, or NEW You, at. Y.

ICE.

METHOD OF OPERATING ELECTRIC FURNACES.

Application filed xan'm ao, 192.2.

7 '0 all whom it may cancer/2..

Be it known that I, WALDEMAR DYnssEN, a citizen of the United States, and resident of the city, county, and State of New York. have invented certain new and useful Iniprovements in Methods of OperatingElectric Furnaces, of which the following is a specification. v

This invention relates to a method of supplying electrical energy to metallurgical furnaces to control the power input to suit the operating conditions during the melting down and the tinishingperiods.

In the usual practice, where the electric power is furnished to the furnace by means of a transformer, by changing the flow of current in the electrodes by varying the length (that is, the resistance) of the are a small amount at a fixed transformer voltage ratio or ata fixed transformer connection. The varying of the current has however the effect that the l LMMF. or voltage of the cred at incrcasi-al clll'i'cllt ing causes:

First: l'tesislance voltage drop in the cirruit including transformer, busbars, cables, electrode llUlllOl'S, electrodes and in the bath. This loss is proportional to the current, but may increase at a somewhat higher rate than the current due to increased temperature of the copper conductors at higher rurrei'u's whirhincreases their resistance.

Her-0nd: Lower power factor at increased current. The power factor is dependent upon the total reactive resistance of the in stallation, that is, the reactive resistance of the complete low tension or furnace circuit, including the husbars, cables, electrodes and (he bath, the reactive resistance of the trans former proper and the reactive resistance of any additional reactance that may he added due to the follow- I m the high tension circuit of the transformer. The total reactive resistance causes a reactive voltage drop which is propor; tional to the respective currents passing through the res )ective reactance. This reactive voltagerop can be expressed as a percentage of the imposed voltage. There is a fixed rela ion between the percentage of the power input is varied arc itself is low age during the melt Serial No. 548,057.

reactance and the power factor accordin to the following formula:

. ,c P 1 100 R" wherein I is the power factor and R the reactance in percent. From this formula it can be seen that the power factor is lowered at increased reactance (that is increased current) at lower currents slowly but at higher currents niore rapidly. For instance, at

- Per cent. 20% rea'ctance the power factor is-.- 98

I reactance the power factor is 91.6 reactance the power factor is. 80'

'llnrd: Resistance and reaotance in the high ter power supply lineand the source of power to the furnace installation. This cav of lower, arc voltage at increased currents is usually relatively small butin some cases it might be considerable.

The usual practice, as described above, is

by using a differsometimes deviated from out transformer voltage ratio or at a diiferout transformer connection during the melt ing down period and the finishing period, which gives a. eonsid ably higher are voltv ing down period than during the finishing period. The higher are voltage is ohjwztionahle from several points of view, such destruction of the walls and roof of: the ,1" arnaee and damage to the n etal in physical and ciien'rieal ways.

The initiai stage of trea trio furnaces iucludesfa down the scrap when the 10d of melting is K necessary.

to use igl power,

gnietal in elecinaximuin ofpower s used in order to shorten this period the charge he refinin period it.

slugs ar'e'to be ineltthe bath has to be-heated prevents the falling off of power input in the proportional relation (or in extreme cases m actual amount) at increased. current as is the case m the usual operation of electrio furnaces. I do not employ a long de structive are or a high are voltage as is now the case in some other methods of operation.

To enable those skilled in the art to clearly visualize the operation 0 my ,improved method, I have prepared t e accompanying drawings in which Fig. 1 shows a furnace and the circuits for controlling the power input, the electrodes being connected in star to a three-phase transformer;

Fig. 2 is asimilar view showing the electrodes connecting in .delta to athree-phase transformer;

Fig. 3 is a diagram showing graphically the theoretical relationships of resistance and reactance, transformer voltages etc., to maintain a constant arc voltage at different loads; 5 g

Fig. 4 is a diagramillustrating an extreme case in which the transformer voltage is kept constant and wherein by means of variable reactance, the arc voltage is kept constant at all loads;

Fig. 5 is a diagram similar to Fig. 3 but showing the voltage and other values varied in steps;

Fig. 6 represents diagrammatically the different electrical characteristics in a furnace installation from the time the cold scrap is melted down until the finished or refined metal is poured. The values shown in this figure are based on the characteristics shown in Fig. 5;

Fig. 7 is a diagram showing a case wherein a higher are voltage is constantl mainta'ined;

Fig. 8 shows diagrammatically a furnace and the circuits for controlling the power input as indicated graphically in Figs. 5 and 6.-

Referring first to Fig. 1, three-phase current is supplied to the primary windin 10. 12 and14 which are connected to fee wires 16, 18 and 20 leading to a suitable three-phase alternator. not shown. The primary windingsof the transformer excite the secondary windings 22, 24 and 26 which are connected in star. with the electrodes 28. 30 and 32 of the furnace 34. This is a typical circuit arrangement and a certain E. M. F. has to be used in order to strilie arcs between the tips 40, 42 and 44 of the electrodes and the charge 50 in the furnace. The voltage between the tip of an electrode and the bath is herein called the arc voltage and referred to by the symbol Va.

The arrangement shown in Fig. 2 is similar except that the electrodes are connected to the coils 22, 24' and '26 in delta and consequently the arc voltage is equal to the difference in potential between the tips o f; any two of the electrodes divided by Signifying the voltage between the tips of the electrodes in this case by We, it follows trodes and the charge in the furnace. The power produced in each arc is equal to the product of the arc Volta e. and the current strength or amperage. T e power factor in the actual arc is always practically According to my improved method, I

maintain the arc voltage constant during all periods of operation and at any input of power or current.

At increasing amperagesthe power factor drops and the voltage loss in thedeads in-.

creases and in order to maintain a constant E. M. F. at the arc-I increase the transformer voltage. This enables me to increase the k. w. input in the arcs in direct proportion to the amperage.

Fig. 3 illustrates graphically the electrical conditions in an installation operating according to my method when using a furnace of the Heroult type of about six tons capacity. I

- When initially melting down the charge I use a current of about 8600 amperes and maintain an E. M. F. at the arc of 45 volts. At this amperage I use a a transformer E. M. F. of about 63 volts when the transformer is connected in star,.or a transformer phase voltage of about 109 when connected in delta. After the initial melting down period the charge in the furnace is in a molten state and less power is required to carry on the refining operation. During this stage I reduce the current to approximately 4000 amperes. And in order to maintain the arc voltage constant at this current strength 1 it is-necessary to reduce the transformer E. M. F. to about 51 volts between the phase and neutral. or to a phase voltage of about 88.

These theoretical transformer voltages for obtaining constant arc voltage are graphiindicated by the curve Vt in Fig. 3. are voltage is represented by the ight line Va which shows a constant F. at, all amperages The ciirves d Vi represent respectively ,thereactive,

or lead and the resistance loss in i The curve Vdt'represents the transformer voltages. The -.line A. represents thek. w. input in three the curve P. F .-refers ;tO- -th8 power and the curve R12. shows the per- 1 gaoi reactanceat difierentloadsi rc-m the diagram it willbe seen that the ance loss per lead and the reactivelead is assumed to increase at'a rate as the amperage is increased. I nee reduces the power factor at an'iperages and in: order to main-i .e are voltage constant while the power decreases, the transformer 'voltage ,vary as shown by the curve Vt if the transformers are connected :instar. -01? if the electrodes are connectedfinqdelta I use tl? transformer voltages shown bythe 'curve which are "1.73 'timesthe star trans :lanl fl'lfi' voltages. V y g in the-curves as drawn in Fig. 3, If'have lll'lfltl that all the reactanceis' located ie low tension circuit, in which'case'the' ht line. If, however,part of the reis located on the high tension side or as additional reactance. coil, V? the shape of a curve, as indicatedby 9. ed line Vr', because the-reactive voltage i in the high tension reactance is ro-. me}. to the high tension current,"w ich *ren at a faster rate than the low tenircuit. It, therefore, thehighftensiori e voltage is by calculation,

1d, added to the low tension reactive 4 he total'reactive 'volta e will in somewhat faster than the ow: tension ode current. 7 alt thereof, is that the curves Rp, 1d V lt will be "modified as shown tied lines R72, PFQVi andydt. above it hasbeen assumed tliat the. rta'i'ice is constant or unchaijiged at The reactance espe'ciallyjthe addi-. ctance, in the lngh tension line, ally he changed or cutin and out at "ll. in which case the curve Vv' ean be made to take anysha e whatever, at the time influencing the other curves. .An

"so is shown in Fig. 4, where the ..ca of variable reactance, the are voltage constant at all loads. IILPI'ECtlC8, lied of operation would be practical within certain limits because it neces- .tes a very high amount of reactance coils a iow amperage, which, when the am-- ,ive voltage V represented" transformer, either in the transformer [approximately SlX tons capacity,

transj about 52 r voltage is kept constant and by magnetic.

reduced towards zero, must be inthod of. operation is not limited three-electrode, or threearc furnace, but would apply as well to any electric arc furnace. The arcs can be formed independent of the bath, or between the elec-' trodes and the bath, and themethod of operation can be applied to any type of electric arc furnace, regardless of the num- I her -of electrodes ployed." v

praeticefit is usually not convenient toxa'rrange the. electrical apparatus so that and kind of current emthe transformer voltage, as shownin Fig. 3.

'c'urve Vt and Vdt, or the reactance curves. asshown in Fig. 4, V1" and R1), are smooth or vary continuously with the current. It is usually necessary to make the necessary variations in steps, which does not give an absolutely constant arc volta e, but which however does not materially c range the are voltage.

' In Fig. is illustrated this method of It represents my method asapphase .Heroult furnace of two volt ;a'ges are used and one extra additional reactance. At currents between about 6400 and 8600 amp'eres, a transformer voltage of 119 delta and 69 star is used and the extra readtanceis not connected in the circuit. The power factor varies between, 88 and 94; and the arc voltage between 52 and 58. A.-t currents betweenabout 4500 and 6400 amperes, the extra reactance is connected into the circuit at the same tl'iLhSfOl'll'lGl? voltage. In this casethe arc voltage varies between and 5 8. For currents lower than about e500 amperes a lower transformer voltageis used, that is, about 97 delta and 56 star and the arc voltage varies" between 50 and 56.- s

At the start of melting a heat of cold steel or iron scrap there is however an additional reactance inqthe circuit due to the steel or scrap. This reactance gradually v diminishes as the temperature of the charge .'ncr'eases and-entirely disappears when the whole charge becomes red hot and non- I have estimated that this additional reactance causes a reactive voltage drop of to volts at about 8600 amperes per electrode. 7 age to be- 23, the total reactive voltage is indicated at point V? in Fig. 5. The value of the corresponding transformer voltages, 'power'factor, per cent reactance, arc voltage operation. plied to a three Assuming the reactive volt and power in arcs are indicated at Vt, vdt f R29 Va and KW In this case it is,

therefore, necessary to employ a transformer theti me the cold charge is initially melted until the finished product is poured. The highest transformer" voltage 142 (Vdt) without the reactance, is used at the start. After the first three minutes, which are required in establishing steady arcs, the current is kept at the maximum strength or at 8600 amperes per electrode as closely as possible as indicated by the curve A. Due to the reactance in the charge the power factor is low, but it gradually increases as more and more of the scrap around the electrode becomes red hot and non-magnetic. The are voltage and power increase also. After about twenty minutes, when the greater part of the charge reactance has disappeared, the medium voltage or 119 is used without the reactance. At about thirty-five minutes the effect of the charge reactance has entirely disappeared and the furnace operates at a steady load. At eighty minutes much of the scrap is melted in the center and around the electrode, the furnace becomes hotter and it is necessary to reduce the power gradually by reducing the current. At one'hundred and ten monutes the charge is nearly melted and the power is reduced considerably by reducing the current and connecting the reactance in the circuit which maintains thearc voltage nearly constant. After a few more reductions of current the charge and the slag are completely melted and at one hundred andthirty-five minutes the power is shut off and the first refining slag is removed. At one hundred and forty-five minutes the slag materials for the second slag are added and a great amount of power must be used in order to quickly melt the slag and heat the steel which is still comparatively c'old., Medium voltage'without the reactance is used. At one hundred and sixty minutes the slag is melted up and the power reduced by reducing the current and using the low voltage connection, which maintains the arc voltage nearly constant. Between one hundred and sixty and two hundred minutes the metal is refined at low power. Before tapping or pouring the charge it is in this particular case desired to heat the bath to with the reactance in the circuit is used during the first twenty minutes, making the conditions about the same as when operating without the reactance in melting ordinary steel scrap In theacid electric melting process it is desirable to use a higher are voltage than in the basic process. The electrical characteristics of the same furnace shown in Fig. 5 are represented in Fig. 7 except that the furnace is operated acid. An are voltage of "ab0ut75 is desired. The extra reactance is in this case connected in the circuit during the whole heat. At start or at a current of about 8600 amperes per electrode a transformer'voltage of about 97 star or 168 delta is used, giving an arc voltage of about 73. At lower than 6000 amperes a transformer voltage of about 82 star or 142 delta is used giving an arc voltage of about 69 to 82 depending upon the amount of current used.

F ig. 8 is a diagram showing circuit connections and a furnace arranged to be operated according to Figs. 5 and 6 hereinbefore discussed.

In Fig. 8, 51 is the furnace containing themetal 50 and provided with three electrodes 52 each forming arcs 03 between their lower ends and the bath. The electrodes are connected in delta to the low tension coils 54 of a three-phase transformer. The high tension coils are 55, each of which have four connections 56, 57 58 and 59 leading therefrom. Leads 56 are connected to an oil switch 60, leads 57 to oil switch 61 and leads 58 to oil switch 62. Leads59 are connected to the three-phases of the powensupply which also are connected to the other poles of the.oil switches 60, 61 and 62 in such a way that when switch 60 is closed all of the coils55 are in the circuit and low voltage is obtained between the low tension transformer terminals. When switch 61 is closed a medium voltage is obtained and when-switch 62 is closed a high voltage is obtained. The power-lines 63 are each connected at one end to a reactance coil 64 anthalso to an oil switch 65 as shown. The other poles of the oil switch 65 are connected to main power supply lines 66 which also are connected to the other terminals of the rcact ance coils 64. \Vhen switch 65 is closed the reactance coils are by-passed by the current and when it is o ened the reactance coils are connected in t e circuit. The reactance can then be connected into the circuit independent of the oil switches 60, 61 incl 62.

and highymedium and low voltages with or without the reactance can he obtained at the low tension transformer terminals.

It will be seen that by utilizing the method described 1 can maintain a. constant or nearly constant are voltage at different loads for any kind or size of electric arc furnace and that. by said method it is possible to vary the power input in the arcs without varying or materially varying saidarc voltage. This enables me to obtain a great power input during the melting down period or whenevcrhigh power is desired at the same are voltage or at the arc voltage that most suitable to use during the refining or finishing periods or when a lower power input is desired.

\Yhilc I have described my method hy quotii'ig specific current strengths and E.

M. Fail is to he understood that these examplcs are to he construed in an illustrative rather than in a limiting sense.

ll'hat I claim is: I

l. The method of operating an electric arc furnace which consists in maintaining a sul'istantially or approximately constant E. M. F. at the are regardless of variations in the transformer voltage and varying the amperage to control the magnitude of the power input. I

2 The method of melting and refining metal in an electric arc furnace which consists in supplying current of high amperage during the melting down period and supplying a current of comparatively lower amperage during the refining period and maintaining the arc voltage substantially or approximately constant during both periods regardless of variations in the transformer voltage.

The method of operating an electric arc furnace which consists in maintaining a substantially or approximately constant E. M. F. at the artist varying amperages and loads by hhanging the transformer E. M. F. to compensate for losses due to resistance in the conductors and the drop in E; M. F. due to the lower power factor at increasing amperagcs.

4. The method of operating an electric arc furnace which consists in increasing the transformer voltage in steps at increasing amperages to compensate for drop in E. M. F. due to resistance and rcactance and maintaining the arc voltage substantially or approximately constant at all loads.

5. The method of operating an electric arc furnace which consists in maintaining a substantially or approximately constant M. F. at the are at varying amperages and loads by decreasing reactance at increasing loads and varying the power factor to compensate for losses in E. M. F. due to resistance and reactence.

6. The method of operating an electric arc furnace which consists in decreasing the rcartance in s eps at increasing ampcrages to compensate for drop in E. M. F. due to resistance and reactance and maintaining the arc wiltage substantially or approximatcly constant at all loads.

7. The method of rn'ieri-iting an electric arc furnace which consists in maintaining a sul'istantially or approximately constant M. F. at the are at varying ainporages and loads. uy changin the transformer E. M. F. and by changing the reactance of the in 'stallation to compensate for losses due to resistance and reaetance.

S. The method of operating an electric arc furnace which consists in increasing the transformer voltage in steps and decreasing the reactance in steps at increasing amperages to con'ipensate for drop in E. M. F. due

to resistance and reactance and maintaining the arc voltage substantially or approximatelv constant at all loads.

5). The method of melting and refining metal in an electric arc furnace which consists in supplying a currentof maximum amperages and at a higher F. M. F. at the transformer during the melting down pe riod and a current of approximately onehalf of said maximum amperageand of a lower voltage at the transformer during the t-ially or approximately the same l. M. F. at the are during these periods.

10. The method of melting and refining metal in anelectric arc furnace which consists in supplying'a current of maximum amperages and at a lower reactive resistance during the melting down period and a current of approximately one-half of said maximum amperage and at a higher reactive resistance during the refining period and maintaining substantially or approximately the same M. F. at the are during these periods.

11. The method of melting and refining metal in an electric arc furnace which consists in supplying a current of maximum amperage at a higher transformer voltage and at a lower reactive resistance during the melting down period and supplying a current of considerably lower amperage at a lower transformer voltage and a higher reactive resistance during the refining period and maintaining substantially or approximately the same E. M. F.- at the are during all periods.

12. In the melting or treating of steel in an electric arc furnace, the method which consists in maintaining a substantially or approximately constant E. M. F. at the are regardless of variations in the transformer voltage and varying the amperage to controlthe magnitude of the power input.

13. In the melting or treating of steel in an electric arc furnace, in which an arc is refining period and maintaining suhstan struck between an electrode and the charge, consists in maintaining an E. M. F. at the the method which consists in maintaining are which varies approximately 15 per cent 10 a substantially or approximately constant above or below a mean constant and vary- E. M. F. at the arc andvarying the transing the amperage to control the magnitude former voltage and the amperage to control of o power input.

the magnitude of the power input. In witness -whereof, have hereunto 14 In the melting or treatingof steel in signed my name. an electric arc furnace, the method which- 1 WALDEMAR DYRSSEN.

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