US1279928A - Three-phase electric furnace and method of operating same. - Google Patents

Description

F.-T.SNYDER. THREE PHASE ELECTRIC FURNACE AND METHOD 0F OPERATING SAME.

APPLICATION FILEDMAR. ll, 1918. 1,279,928. n Patentedsept. 24,1918.

STATES PATENT OFFICE.

FREDERICK fr. SNYDER, or OAK PARK, ILLINOIS.

THREE-PHASE ELECTRIC FUBNACE AND METHOD 0F` OPERATING SAME.

Application led March 11, 1918.

To all whom it may concern.'

Be it known that I, FREDERICK T. SNYDER, a citizen of the United States, residing at Oak Park, in the county of Cook and State of Illinois, have -invented a certain new andl rent source of power the arrangement. commonly employed has been to connect a carbon electrode to each of the three phases usually through a transformerresulting in the use of three electrodes. These electrodes project through the roof of the furnace into the melting chamber and terminate a short distance above the molten bath. Under these conditions the resistance between the ends of the three electrodes is substantially equal. Hence the currents flowing in each of the three phases are substantially equal and the phases are well balanced.

Each electrode consumed duringhthe op.

eration of an Kelectric furnace, owever, constitutes an additional source of expense and for this reason it is desirable to reduce the number of electrodes employed as far as consistent with other factors to be considered. Attempts have been made in the past to reduce the number of upper electrodes to two, using one bottom contact vwhich latter does not require frequent renewal. For this purpose, however, it has been necessary to transform the three-phase current to twophase current before itis supplied to the furnace. In one case the three-phase current has been transformed to two-phase current in such manner that the two phases are 90 apart. In another case two phases are producedwhich are 120 apart. To these and other systems, however, various objecf tions have been made based on the fact that the balance between the three phases is not as complete as it should be; or that special transformers are required; or that other inherent difficulties exist.

The objects of the present invention are to provide means and a method whereby threephase current may be supplied directly to an electric furnace having two carbon electrodes and a bottom contact; to accomplish this result without special transformers and Speccatipn of Letters Patent.

Patented Sept. 24, 1918.

Serial No. 221,609.

in general with a minimum amount of spe cial equipment; and to take advantage of the heretofore objectionable fact that a reactance is necessary in order to stabilize the arc and that the unavoidable reactance of the furnace shell has distorted the current balance in the syst'ems heretofore proposed and to so design and arrange this unavoidable shell reactance with reference to the Y`furnace and its circuit as to cause it to perform an additional and advantageous function in reducing the voltage between each upper electrode and the bottom contact as compared with the voltage between the two electrodes, thereby equalizing the iow of current.

Other advantageous features will be ap.- parent from the following description taken in conjunction with the drawings in which- Figure 1 is a diagram of the complete circuit connections in the preferred form of the invention. v'

Fig. 2 is a simplified diagram of the secondaries of the transformer and other furnace connections, and

Figs. 3 and 4 are further diagrams illus trating phase relations.

The structure of the electric furnace proper may vary in accordance with different requirements, but consists preferably of refractory walls 1 of cylindrical form reinforced by a cylindrical steel shell 2. Suitableelectrodes 3,4 consisting preferablyof carbon are adjustablysupported above the furnace and extend throughthe roof of the furnace projecting into the melting chamber with their lower ends Vat substantially equal distances above the molten bath. A suitable bottom contact 5 is also provided which is preferably a metallic water cooled structure, the details of which,'as well as other structural detailsof the furnace, form no part of the present invention.

rIhe furnace is supplied with current from any suitable source of polyphase current preferably a threephase source supplying the three-wire circuit 6, 7, 8. Three transformers 9, 10, 11 are connected to the power circuit preferably in the manner shown. The transformers may be of standard construction employin, either the well known delta or Y connections. Said transformers comprise primary coils 12. 13. 14 respectively and secondary coils 15, 16, 17 vrespectively, said secondary coils consisting preferably of two windings in series.

One end of each of the secondary coils 15 and 17 are connected at a common point 18, the other' ends being connected at the polnts 19 and 20 with the terminals of the third secondary coil 16. A suitable conductor 21 connects the junction 18 with the bottom contact 5 of the furnace, said circuit including a suitable switch 22 and a reactance-23. This reactance, which is the main reactance, is inserted at this point for a double purpose. It stabilizes the two arcs, each of which is in series with it, through the circuit to the bottom contact and through the molten bath. Also it is so designed as to cause a voltage drop in the current phase through the bottom contact equal to the voltage drop in each arc, so that with equal currents flowing through the two electrodes and the bottom contact there will be an equal voltage drop from the bath to the external current supply along each of the three circuits.

An additional conductor 2-1 extends from the juncture 19 to one of the upper electrodes 3 and a similar conductor 25 extends from the'juncture 20 to the other upper electrode 4. These two circuits include also reactances 26 and 27. These reactances include the inherent and unavoidable reactance of the circuits, to which is addeelsuch further reactance as may be necessary to make the two arc circuit currents, equal the contact circuit current when the main reactance ,has been adjusted to the amount required to stabilize the arcs at they arc voltage used. The reactance 23, on the other hand, constitutes the main reactance and consists primarily of the furnace shell and such additional reactance as may be required under given conditions to accomplish the stabilizingl ofthe arcs previously referredlto and at `the same time balance the three currents.

ploying them as to render their use highly advantageous.

The diagram Fig. 3 indicates the open circuit volt-age between the three terminals representing the two upper electrodes and the bottom contact. This voltage may be assumed to be 150 volts. In other words, the.

voltage between the two upper electrodes will be 150 volts and the voltage between each up per electrode and thc bottom cont-act will be 150 volts. When the current flows it is controlled largely by the resistance of the arcs. It is apparent, therefore, that in the absence of special compensating means much more current will flow from each electrode to the bottomcontact through one arc in each case than will flow from one upper electrode to the other through two arcs in series. The three phases, therefore, will be unbalanced. In my present invention this excess current through the contact is reduced 'and made equal to the current through the two elec` trode circuit by the voltage drop of the main reactance.

In the diagram Fig. 4, the same terminals are also shown and the junction point 28 represents the molten bath. It will be seen that the angle between the three phases is 120 in each case.

The stabilizing reactance composed chiefly of the furnace shell is at an angle to the two phases including the arcs instead of being 1n the phase with each arc. The two arcs are thereby stabilized with one reactance. If the current begins to. increase in either arc, the cross section of the arc increases, thereby reducing its resistance and further increasing the flow of current through said arc. A portion of this increase current flows through the main reactance increasing the voltage drop through said reactance. This reduces the component of voltage available for the arc in question by increasing the angle between the two -arc phases as indicated in dotted lines, the uncture point moving from 28 to 29, for example. As a result, the flow of current through said arc is decreased. If, on the other hand, the current through either arc tends to decrease below the normal value the reverse operation takes place, the current through the arc increases (seelowered junction oint 30), and normal conditions are there ore automatically restored. This arrangement, therefore, provides for an inherent regulation stabilizing both arcs with a single reactance, also giving balanced loads on all of the three phases of a three-phase circuit and permitting three-phase current to be supplied directly to the furnace, said phases being 120o apart:

`Vith the operating voltage values assumed in Fig. l it will be seen that the three arms of the star represent approximately a .voltagedrop of 63 volts. Assuming that the shell reactance under given conditions corresponds Ato a volt-age drop of 41 volts, it will be necessary to provide additional reactancev equivalent to a voltage drop equal to the difference'betwecn 63 and 11, which is 22 volts, represented by reactance 23. In order to balance the phases it will be necessary therefore tov adjust thereactances 26, 27 to values, also corresponding to 22 volts. The two arcs are then adjusted to correspond -by two electrode cranes 31, 32. The circuit to said electrodes from the conductors 24, 25 is through or adjacent to said cranes.` In order to render the electrode circuits noninductive the said cranes are arranged adjacent and parallel, whereby the currents flowing through said cranes, being partially in opposite directions, the inductance of one partially offsets the inductance of the other.

During the operation of an electric furnace, it is sometimes desirable to reduce the power supplied thereto. With the present arrangement, this reduction can be effected simply bv opening ythe switch 22 in the circuit of the bottoni. contact. Such vopening cuts olf two of the three equally loaded current paths, reducing the power input to one-third of the full load. A reduction to approximately one-sixth of lthe full power may be accomplished at the same time by changing the primary connections of the transformers from delta to star. In a properly designed furnace, the metal may be kept hot with one-sixth of the full load input.

Vhere it has been attempted heretofore to obtain a balanced .three-phase load with a furnace of this type, z'. e., one having two upper electrodes, a higher voltage is used between the said electrodes than between each electrode and the bath. This higher voltage tends to cause leakage of current through the hot roof, thereby decreasing the load available for effective work in the furnace. With my delta arrangement, the voltage between the two electrodes is no higher than the voltage between each electrode and the bath, whereby for a given load and' current, the leakage through the roof can be kept at aminimum.

Having thus vdescribed my invention,`

what I claim as new and desire to secure by Letters Patent of the Unitedy States is:v

1. The method of operating electric fu`rnaces having current paths of different resistances, from a source of polyphase current, which consists in providing reactance.

in the path of lowest resistance of such amount as to automatically equalize the curw rents'flowing in the different phases of said polyphase system. l I

2. The method of operating electric furnaces having two upper 'electrodes and a bottom contact which consists in supplying three-phase current directly to the furnace terminals and reducing the flow of current between each upper electrode and said bottom contact by means of a reactance in the phase of said bottom contact whereby the Y three phases are balanced.

3. The method of operating electric furnaces having two upper carbon electrodes and a bottom metallic contact from a source ofthree-phase current, which consists in reducing the current flow between each' upper electrode and said bottom contact to balance the flow through the two arcs in series.

4. The method of operating electric furnaces having upper electrodes and a bottom contact which consists in providing a reactalice in series with said bottom contact sufiicient to cause a voltage drop equal to the voltage drop through the arcs between each.

of said electrodes and a bath in contact with said contact.

5. The method of operating multi-phase electric are furnaces which consists -in stabilizing the arc by` means of a voltage drop which is out of phase with the current causing said arc. r

6. The combination with a source of threephase current, of an electric furnace having two upper electrodes and a third terminal in conducting relation with the molten bath, each of said three terminals being directly connected with one of said phases.

7 The combination with a source of threephase current, of an electric furnace having two upper carbon *electrodesv and a bottom metallic contact, three transformers connected to said three-phase circuit and coi` nected also tosaid three terminals, the three phases supplied to said furnace being 120O apart.

8. The combination with a source of constant potential polyphase electric current of an electric furnace having current paths of different resistance/and with reactance in the paths of lower resistance of such amount as to substantially equalize the currents flownected between said transformer `coils andv said bottom contact`v to stabilize two arcs in thefurnace and to balance the current in the three phases.

11, In combination, a three-phase circuit,

an electric furnace having two upper electrodes and a bottom contact, transformers connected to said three-phase furnace and to the three terminals of said furnace, and a reactance connected in the phase which includes said bottom contact.

1'2. An electric furnace having two upper electrodes and a bottom contact, three secondary transformer coils connected in delta, an arc stabilizing reactance connected in series between one juncture of said transformer coils and said bottom contact and further reactance in series between the other two junctures of said secondary coils and said upper electrodes.

13. An electric furnace having a plurality of electrodes and a bottom contact, said furnace having a shell of magnetic material adapted to form a reactance in the circuits passing through said contact.

14. An electric furnace having a plurality of electric circuits in which the sum of the reactance ohms and the resistance ohms for the different circuits are substantially equal.

15. An electric furnace having a plurality of electric circuits, the reactance of one of the circuits being substantially lower than the reactauce of another of the circuits.

16. In combination, a three-phase circuit, an electric furnace having two upper electrodes and a bottom contact connected with said circuit, providing two arcs between said electrodes and the metal bath, a reactance in one phase of the furnace circuit and the said arcs being arranged in the other two phases of said furnace circuits.

17. The combination with an electric furnace having a plurality of electrodes and a bottom contact, of circuits supplying said electrodes and said contact, said electrode circuits being close together and separated from the circuit supplying said contact.

18. An electric furnace with a plurality of electrodes and a bottom contact in combination with electric circuits supplying said electrodes and contact,l the circuits through said electrodes which do not pass through said contact being non-inductive as compared with the circuits through said contact.

19. The combination of an electric furnace having a plurality of electrodes and a bottom contact of a switch in the circuit to said contact, the opening of said switch serving to reduce the power supply to said furnace for constant position of the electrodes.

20. The combination with an electric furnace having three terminals providing for two arcs, of a source of polyphasc current therefor, and a single reactance stabilizing said two arcs, the current through said arcs differing in phase.

21. The combination with an electric arc furnace comprising an upper electrode and a bottom contact, of a source of current therefor, the drop in voltage in the circuit of said contact being equal to the voltage drop across said arc stream.

22. The combination with an electric furnace having two upper electrodes and a bottom contact, of a source of polyphase -current for the two arcs between said electrodes and the bath, the drop in voltage due to the reactance of the circuit of said contact being equal to the voltage drop across each of said arc streams.

23. The combination with an electric furnace having two upper electrodes and a bottom contact, of a three-phase supply circuit and three transformers connected to said supply circuit and to the furnace terminals corresponding to said electrodes and said contact, ,the connection being such as to maintain a balanced load on said supply circuit.

In witness whereof I hereunto subscribe my name this ninth day of March A. D.,

FREDERICK T. SNYDER.

Copies of this patent may be obtained for ve cents each, by addressing the Commissioner of Patents,

Washington, D. C.

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