US1992784A - Electric furnace system - Google Patents

Description

Feb. 26, 1935. N STANSEL 1,992,784

ELECTRIC FURNACE SYSTEM Filed Jan. 12, 1953 lll Inventor Numan RStansel,

yMW

His Attorney.

' Patented Feb. 26, E935 umrco A STATES 1,992,? ELECTRIC FURNACE SYSTEM Numan R. Stansel, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Applicationlanuary 12, 1933, Serial No. 651,340 Claims. (01. 13-13) My invention relates to electric furnace systems, more particularly to electric arc furnace regulator systems and has for its object the provision of a simple and reliable system for preventll ing the lowering of the electrodes into the bath in the event of certain abnormal conditions of operation.

My invention has special application to threephase are furnaces in which the bath or charge is the common electrode of the three arcs, such as used in the manufacture of ferrous alloys. In the manufacture of ferrous alloys in the three phase are furnace certain materials such as lime, etc., are added from time to time to the charge while the furnace is in operation. The addition of this material is usually a manual operation and it sometimes happens that a quantity of this material lodges directly under the tip of an electrode and is suilicient in quantity tobridge the gap between this electrode and the bath and thus extinguish the arc. The are is thus replaced with an ohmic resistance with the result that the current in that particular electrode is reduced and the currents in the other two electrodes are correspondingly reduced. The electrode regulator which is responsive to the currents in the electrodes thereupon lowers the electrodes in an attempt to reestablish the normal current condition, and as a result the electrodes may be lowered into the charge and the charge may thereby be contaminated with carbon from the electrodes. This replacement of the arc by the lime or other material also results in a very substantial drop in the voltage between that particular electrode and the charge, the arc voltages of the other electrodes actually increasing, and in carrying out my invention I provide control means responsive to this drop in voltage across the are which is extinguished for disabling the electrode adjusting means for all of the electrodes so as to prevent lowering of the electrodes. For a more complete understanding of my invention reference should be had to the accompanying drawing, the single figure of which shows in diagrammatic form an arc furnace regulator system embodying my invention.

Referring to the drawing, I have shown my invention in one form as applied to a furnace regulator system such as described and claimed in a copending application of Frederick E. Ackley, Serial No. 639,625, filed October 26, 1932, assigned to the same assignee as this invention. It will be understood, however, that my invention has general application to are furnace regulators.

I have shown my invention in one form as applied to an arc furnace 10 comprising a crucible 11 for a metallic charge or bath 12 and suitable electrodes 13, 14 and 15 extending into the crucible into arcing relation with the charge. The elec- 6 trodes are each adjustable by means of a suitable electric motor. This adjusting means is shown diagrammatically for the electrode 16 only and comprises a suitable reversible electric motor 18 shown as a direct current motor provided with a shunt field winding 17 which motor .is connected to rotate a fixed gear wheel 18 having its bore threaded to receive a screwrod 19 attached to the electrode 15. This motor 16 is controlled through suitable reversing switches 20 and 21 in response to the current in the circuit of the elec-.

trode 15 so as to vary the adjustment of the electrode and thereby maintain the predetermined current in the circuit. As shown the current response is obtained by means of a current transformer 22 in the circuit of the electrode 15. It will be understood that any suitable current responsive control means between the current transformer. 22 and the reversing switches 20 and 21 of the motor may be used. The particular control means shown which forms no part of my invention, it being described and claimed in the aforesaid Ackley application, will be described hereinafter.

In carrying out my invention I suitably arrange the' control circuits for the individual electrode adjusting motors in such manner that ourrent is supplied to all three control or regulator circuits through a common conductor 25 connected to one side of a suitable supply source 26, such as a direct current supply source. I also provide means responsive to a predetermined voltage drop between any one electrode and the bath for opening this common controlcircuit so as to disable the electrode adjusting means. For this purpose I provide three under- voltage relay switches 27, 28 and 29 which are biased'to their open positions and are normally held closed by operating coils 30,31 and 32 connected respectively across the electrodes 13, 14 and 15 and the charge. As shown one common connection for the three operating coils 30, 31 and 32 is made through a conductor 33, which is connected in turn to a bottom electrode 34 in the furnace, this bottom electrode being in electrically conducting relation with the charge 12. The remaining terminals of the coils are connected respectively to the electrode circuits.

With this arrangement the operating coils 30, 31 and 32 are constructed and arranged electrically so as to hold their respective switches closed when the voltage drop between the respective electrode and the charge is greater than a predetermined minimum voltage to which the voltage drops in case the arc is extinguished by the addition of lime or other material underneath that particular electrode. Under normal conditions of operation therefore, that is, with arcs formed between each electrode and the charge, the switches 27, 28 and 29 are held closed and the current regulators for the electrodes operate automatically to raise and lower the electrodes to maintain predetermined currents in the electrode circuit.

In the event, however, that the voltage between any one electrode and the charge decreases to the predetermined minimum for which the operating coils 30, 31 and 32 are adjusted, the switch 27, 28 or 29 operated by that coil opens and consequently prevents adjustment of any of the electrodes. Such a condition might arise, for example, from the accidental throwing of a shovelful of lime underneath one of the electrodes in sufficient quantity to extinguish the arc. and replace it by a resistance. It will be understood that an arc is a conduction of current through gases and does not have the property of resistivity. It is furthermore a well known voltampere characteristic of an are that the voltage across the arc varies inversely with current through it. In other words, assuming a given arc length and supply voltage a decrease in the current through the are such as by inserting additional resistance in series with it produces an increase in the voltage acrossthe arc.

Now when the arc is extinguished and replaced by the lime or other material added which is of high electrical resistivity, the effect is to decrease the voltage between the particular electrode and the bath to such a low value that the corresponding under- voltage switch 27, 28 or 29 drops open, and furthermore to decrease the current through the particular electrode. This latter effect produces also a decrease in currents in the circuits of the other two electrodes and these decreases in current because of the characteristics of the arc, produce increased voltages across the remaining two arcs. Since a fixed voltage only is available from the source of supply 52, the increased voltages across the two remaining arcs result in a lower available voltage across the lime; that is, between the electrode under which the lime is thrown and the bath. The increases in the voltages across the two arcs effectively prevents the opening of undervoltage relays which might be provided individually for the arcs and consequently without some protective means these two electrodes wouldbe lowered into contact with the bath, thereby producing carbon contamination of the bath, before the voltage across the arc is reduced sufiiciently to cause operation of individual undervoltage relays. with my improved system, however, the opening of the undervoltage relay associated with the electrode under which the lime or other material is thrown effectively disables the control means for all of the electrodes.

Referring now in detail to the current responsive control system for the electrodev motor 16, the control system comprises a contact device 35 provided with two pairs of spaced contacts 36,

37 and 38, 39, shown as spring contact arms which are mounted in-insulated relation to each other on a suitable supporting member 40. This member 40 is mounted on a pivot shaft 42 and is connected to anelectric torque motor 43 for movement on its pivot. The torque motor 43 is connected to be energized from the current transformer 22 in the circuit of the electrode 15. The torque motor is biased in one direction by means of the spring 44, the torque applied by the motor being in opposition to the spring. An adjustable rheostat 45 is connected across the current transformer 22 for adjusting the current response of the torque motor. Also a suitable fluid damping device 46 is connected to the torque motor.

A rotating contact member shown as a starwheel 4'7 is mounted between the pair of contact arms 38, 39, it being secured to a shaft 48 driven by a suitable synchronous driving motor 50 which is connected through a transformer 51 to the three phase source of supply 52 for the furnace. The shaft 48 is parallel with the pivot shaft 42 although this relation has been modifled somewhat in the drawing for purposes of clarity. During the operation of the furnace the star-wheel 47 is rotated between the pair of contacts 36, 37 at a substantially constant speed. The distance between the contacts is somewhat greater than the diameter of the star-wheel so that when the contacts are in a central position, corresponding to the desired current in the circuit, the star-wheel is free to rotate without touching either contact.

In the operation of the system, a decrease in current in the circuit of the electrode 15 below a predetermined value to be maintained, for which the torque motor 43 is adjusted by means of the resistance 45, results in a decrease in the torque of the motor 43 whereby the contacts are turned about the pivot 42 in a clockwise direction, as viewed in the drawing, by means of the spring 44. This brings the contact 36 into engagement with the star-wheel whereby a circuit is closed from one side of the supply source 26 through the conductor 25, the switches 27, 28 and 29 which are normally closed, the conductor 53; a'collector ring 54 and thence through the shaft 48 or by means of a suitable conductor to the star-wheel 47. From the star-wheel the circuit continues through the contact 36, the conductor 55, an operating coil 56, and conductor 57 back to the opposite side of the supply source 26. The operating coil 56 picks up a switch 20 in the circuit of the electrode motor 16 thus closing a circuit to start the motor in the direction to lower the electrode. The closing of the switch 20 at the same time opens a switch 57 which is mechanically interlocked with the switch 20. The circuit of the motor 16 may be traced from oneside of the supply source 26 through the conductor 58, the resistance 60, v

the resistance 61, the switch 20, conductor 62, the series field 63 of the motor, the armature of the motor, conductor 64, a normally closed switch 65, the resistance 66, and conductor 57 back to the opposite side of the supply source.

the control circuit is broken by disengagement of the contact. The dynamic braking circuit for the motor 16 may be traced from one side of the armature through the conductor 62, switch 57, the resistance 66, switch 65, and the conductor 64 back to the other side of the armature. The, particular dynamic braking switch 57 or 65 which is open when the motor is in operation is immediately closed, when the control circuit is opened, by its respective operating coil 6'7 or 68. These coils are connected in parallel across the motor armature.

In case a relatively great change in electrode current is to be corrected for, then the torque motor moves contact 36 a correspondingly great distance so that the contact rides up and over the end of each projection of the star wheel to the other side by reason of the resiliency of its spring arm support thus maintaining the control circuit closed for a longer period of time with consequent increased perio of operation of the electrode motor. As the current increases due to the corrective action, the torque motor moves the contact 36 away from the star-wheel and consequently the periods of operation of the motor decrease with consequent slowing down in the rate at which the electrode is lowered. Finally when the predetermined current has been reestablished in the electrode-circuit, the contact 36 will have been moved to a position where it is nolonger engaged by the star-wheel and, the electrode motor consequently remains at rest. It will be observed that the control system causes the motor to operate for a greater part of the time upon relatively great current changes thereby giving a rapid adjustment, and for a smaller part of the time upon small current changes thereby giving a slow adjustment.

In case the current in the electrode circuit increases to a value greater than the predetermined value, the torque motor 43 moves the contact 3'7 into engagement with the star-wheel whereby a circuit is closed for the operating coil '70, this circuit leading from the star-wheel through the contact 37, conductor 71, the coil and conductor 57 to the other side of the supply source 26. This closes the switch 21 in the armature circuit of the electrode motor 16 whereby the armature is connected to the supply source 26 for opposite rotation, the braking switch 65 being at the same time opened since it is interlocked with the switch- 21. The circuit of .the motor now leads through the conductor 58, resistance 60, switch 21, conductor 64, the armature of the motor, the series field 63, conductor 62, switch 5'7 and conductor 5'7 back to the supply source 26. This produces rotation of the electrode 'motor to raise the electrode, the

motor being operated for short intervals of gradually decreasing duration, and stopped bydynamic braking, until the electrode current has been re-established, as will be understood from the previous description.

It will be observed that when the electrode is being raised the resistance 60 is included inthe armature circuit of the motor 16, but not the resistance 61 whereas both resistances are included in the circuit when the electrode is lowered. This provides for the increased torque required to raise the electrode and also tends to cause the motor to operate at a lower speed when lowering the electrode.

The contacts 38 and 39 cooperate with a conducting disc 72 mounted on the shaft 48. .This

' disc is connected to the same circuit as the starwheel 4'7. Upon a relatively great or maximum change in electrode current one or the other of these contacts is moved into engagement with the disc '72 whereby a circuit is closed for an operating coil 73 which opens a switch '74 and there-, by opens a shunt circuit connecting'the resistance '75 across the armature of the electrode motor. This-provides for increased speed of operation of the electrode motor to give a still more rapid initial adjustment than under the control of the star-wheel until contact with the disc '72 is broken, after which the star-wheel completes the adjustment as previously described.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modifications may be made, and I, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure byv Letters Patent of the United States is:

1. An electric arc furnace regulator system comprising in combination with a plurality of electrodes arranged to produce arcs to a charge, means responsive to the current in each electrode for adjusting each electrode, and means responsive to a predetermined voltage between any one of said electrodes and the charge for disabling the adjusting means for all of said electrodes.

2. An electric arc furnace regulator system comprising in combination with a plurality of electrodes arranged to produce arcs to a charge of metal in said furnace, an automatic regulating device for adjusting each of said electrodes-to maintain a predetermined current therein, and means responsive to a predetermined minimum voltage between any one of said electrodes and the charge for disabling said regulating devices.

3. An electric arc furnace regulator system comprising in combination with a plurality of electrodes, adjusting means for each of said electrodes, a common control circuit for said adjusting means, a plurality of switches one for each electrode connected in series with each other in said control circuit, and means responsive to a predetermined minimum voltage across the arc circuit of any one of said electrodes for opening the corresponding one of said switches.

4. An electric arc furnace regulator system comprising in combination with a plurality of electrodes, adjusting means for each of said electrodes, a common control circuit for said adjusting means, a plurality of switches one for each electrode connected in series with each other in said control circuit, said switches being biased to their open circuit positions, operating windings for said switches, and connections connecting said windings across the respective arc circuits of said electrodes, each of said windings being arranged to hold its respective switch in its closed circuit position under normal voltage conditions but to allow said switch to open under predeter-- mined low voltage conditions. v

5. The combination with a metal melting furnace of a plurality of electrodes arranged to produce arcs to a charge of metal in said furnace, an automatic regulating device for adjusting each of said electrodes to maintain a predetermined.

current in each of said electrodes, a common control circuit for said regulating devices, a plurality of switches one for each electrode connected in series with each other-in said control circuit, said switches being biased to their open positions, operating windings for said switches,

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