US3139473A

US3139473A - Arc furnace control system

Info

Publication number: US3139473A
Application number: US77913A
Authority: US
Inventors: Clifford W Morris
Original assignee: FMC Corp
Current assignee: FMC Corp
Priority date: 1960-12-23
Filing date: 1960-12-23
Publication date: 1964-06-30
Anticipated expiration: 1981-06-30

Description

June; 30;, c. w; MORRIS;

ARQ FURNACE: common SYSTEM 2 Sheets-Sheet 1 Filed Deon 2.6,, 1960:

. INVENTOR CLIFFORDVLMORRIS June 30, 1964 c. w. MORRIS 3,139,473

ARC FURNACE CONTROL SYSTEM Filed Dec. 23, 1960 2 Sheets$heet 2 INVENTOR CLIFFORD W. MORRIS ATTORN United States Patent 3,139,473 ARC FURNACE CONTROL SYSTEM 7 Clifford W. Morris, Modesto, Califl, assignor to FMC Corporation, a corporation of Delaware Filed Dec. 23, 1960, Ser. No. 77,913 9 Claims. (Cl. 13-13) This invention relates to a control system for electric arc furnaces and particularly to an arrangement wherein the furnace electrodes are moved with respect to the crucible material to maintain a substantially constant electrode tip-to-charge resistance.

One use for which the invention has been found especially useful is in conjunction with barium oxide. In the reduction of barium carbonate to barium oxide in an electric arc furnace, it is required that the electrode tip be positioned with respect to the crucible material in order to furnish substantially constant resistance between the tip and the material. With substantially constant resistance, the power delivered to the crucible material by the arc to elevate the temperature of the material Will be maintained substantially constant.

Barium carbonate at temperatures beyond the eutectic one and near the conversion temperature bubbles and boils with a rolling and splashing action in the crucible. Therefore, a sensitive and fast acting control system is required in order to maintain even a semblance of constant electrode tip-to-charge resistance and consequently constant currents to provide a uniform delivery of heat. If the control does not respond rapidly enough, the electrode movement may lag far behind the resistance and current changes with the result that an oscillating condition is set up accompanied by alternately moving of the electrode, resulting in a heavy over-current condition ertial'problems.

One of the objects of the present invention is to provide an arrangement for rapidly moving the furnace electrodes with respect to the crucible material to maintain 'uniforrn heating of the material.

A further object of the invention is to provide a means for controlling the position of the furnace electrodes with respect to the materialin a crucible or the like with provisions for readily adjusting the control system for changes in the material and the temperature to which it is to be subjected.

A still further object of the invention is to provide means for controlling a hydraulic actuator for moving the electrodes with high sensitivity and rapid response.

In one aspect of the invention the control system is employed with an A.C. electric arc furnace having movable electrode means. The furnace may be single phase or polyphase. Motor means are provided to. raise and lower the electrodes, said motor means preferably being a double acting, hydraulically operated servo motor. Means are provided for producing a first control signal in response to the current flowing to the electrode means. Other means are provided for producing a second control signal in response to the voltage across the electrodes and furnace. A saturable reactor is included which has gate windings connected to both the second control signal and torque motor or similar arrangement for operating a hy- H draulic valve controlling the servo motor. The control winding means of the saturable reactor provides a predetermined bias to the reactor and in addition a feedback or control connection from the first control signal circuit is made to thecontrol winding in an opposing relationship to the bias so that an increase in electrode current flow will tend to increase the gate winding impedance and thereby decrease the signal fed to the differential control means. The bias also may be connected so as to accentuate the second control signal.

These and other objects, advantages and features of the invention will become apparent from the following description and drawings.

7 In the drawings:

FIG. 1 is an elevational view of the electric arc furnace and the means for positioning the electrodes with respect to the crucible material; and

FIG. 2 is a schematic representation of the control circuit for the positioning of the electrodes.

In one form of furnace with which the invention is to be used, an electric furnace 10 (FIG. 1) includes shell 11 in which is disposed a charge 12, such as barium carbonate, which is to be reduced at high temperature. Other materials, of course, can be treated. During operation, a portion of the partially converted charge remains solid and acts as a crucible. For convenience in handling, shell 11 may be mounted upon car 13 which is adapted to travel along rails 14. Furnace electrodes 15 are mounted in electrode stem 16 Which is supported by cable 17. Cable 17 is operated by cylinder assembly or servo motor 18 which is located adjacent to electric furnace 10.

The power source of servo motor 18 is hydraulic cylinder 19 which positions cylinder rod 20 in a linear manner. Hydraulic cylinder 19 is supported with respect to base plate 21 by means of uprights 22. At the lower end of cylinder rod 20, cross head 23 is attached by means of pin 24 and coupling 25. Cross head 23 is adapted to be guided in its verticalmotion by engagement with uprights 22. At the bottom portion of cross head 23 shaft 26 is mounted by means of bearing 27. Shaft 26 serves as a pivotal support for moving sheave 28 over which cable 17 extends.

One end of cable 17 is attached to eye bolt 29 which is mounted upon support 30 of cylinder assembly 18 by means of spring 31 and plate 32. Cable 17 after passing about moving sheave 28 extends across sheave 33 and sheave 34 onto the connection with stem 16. It is evident that with one end of cable 17 attached to servo motor 18, motion of the cylinder rod and the moving sheave driven by it will cause the cable to move and thereby raise or lower the electrode stem and the'electrode or electrodes extending from it. Limit switch 35 mounted upon one of the uprights engages cross head 23 at the lower extent of its stroke and terminates its. downward movement, thereby establishing an up-limit for the elec trodes. Switch 36 mounted upon support 30 is actuated by plate 32 whenever cable 17 becomes slack and permits spring 31 to expand in an upward direction. This switch serves to safeguard the equipment whenever the electrodes are. lowered to a point where they bottom upon the charge or some other obstruction.

Control system 37 (FIG. 2) is adapted to operate servo motor means 18 to maintain a substantially constant resistance condition between electrodes 15 and charge 12 disposed within shell 11. Power is delivered to electrodes '15 by means of line 38 which is connected to an electrical power source (not shown). The electric circuit from electrode 15 continues through the are between the elec- 'trode and the charge and then through the grounded shell. Control system 37 operates hydraulic servo valve 39 which in turn controls the flow of hydraulic fluid under pressure to either end of hydraulic cylinder 19 in order to raise or lower the electrodes. In FIG. 2, the ports of the valve cylinder are symbolically represented at 40 which are controlled by valve spool 41 in the usual manner. The servo valve includes a torque motor comprising pole piece 42, as well as Raise coil 43 and Lower coil 44. The coils are connected to one another at terminal 45. The electromagnetic fields induced in pole piece 42 by the differential currents across the coils rotate torque armature 46 about its central portion 47 soas to move spool 41 which is pivotally connected to the armature. Valve spool 41 is directly actuated by the torque motor.

Current transformer 48 translates the varying current in line 38 into a usable voltage signal or first control signal which substantially duplicates the current signal in amplitude and frequency. The voltage signal from current transformer 48 is applied across loading resistor 49, one end of which is grounded.

The operators control panel 50 for control system 37 has three positions designated A, B and C corresponding to Lower, Automatic and Raise modes. Control panel 50 includes

switches

51, 52 and 53 which may besimultaneously set at three positions, a, b and c. In the Automatic mode, the voltage of loading resistor 49 is applied to the primary winding 54 of isolation transformer 55. The connection is made through line 56, switch 51, terminal 51b, normally closed relay contacts 57, and line 58 to primary winding 54. The voltage signal induced in secondary winding 59 of isolation transformer 55 is then connected to furnace current adjusting potentiometer 60, one end of which is grounded. Slider 61 of potentiometer 60 connects the signal to the bridge rectifier 62, the output of which is connected to

lines

63 and 64. Line 63 is conected to one end of Raise coil 43 while line 64 is connected through line 65 to terminal 45 of the Raise coil. In this way, a DC. voltage, being a function of the electrode current, is applied to the Raise coil with the negative polarity at terminal 45. Potentiometer 60 is adjusted to establish a predetermined DC. voltage across the Raise coil 43 for a predetermined current to electrodes 15, but it does not limit the peaks of current signals impressed upon the coil.

The voltage of electrodes 15 with respect to ground of the arc voltage is connected by means of line 66 to primary winding 67 of isolation transformer 68. In the Automatic mode, the arc voltage is impressed across the upper portion of primary winding 67 which leads to line 69, terminal 52b, switch 52, line 70, normally closed relay contact 71 and line 72 connected to tap connector 73. The voltage may be adjusted by selectively placing tap connector 73 in circuit with terminals 74 or 75.

The output of isolation transformer 68 is connected to the common terminal of

gate windings

76 and 77 of magnetic amplifier or saturable reactor system 78. When the alternating signal connected to the gate windings is positive, a circuit is completed through gate winding 77, rectifier 79a, line 80 and line 65 to terminal 45 at the servo valve operator. The return path of the circuit is through Lower coil44, line 81, rectifier 79b and line 82 to ground. For a negative going signal, the cricuit extends from ground, line 83, rectifier 790, line 80, line 65 to Lower coil 44. The return path is through line 81, rectifier 79d, to gate winding 76. Bias for the saturable reactor or magnetic amplifier 78 may be provided by means of potentiometer 84 connected across

terminals

85 and 86 adjacent to the rectifiers which energize b-ias winding 87. Potentiometer 84 is adjusted so as to energize bias winding 87 and thereby bring to a minimum the high inductive impedance of magnetic amplifier 78 so that a practically unimpeded and uninfluenced signal may reach lower coil 44. Magnetic amplifier 78 is provided with a signal, which is in effect a feedback signal, to control winding 88 which is energized by a circuit extending from line 63 through potentiometer 89 and line .90. The magnitude of the feedback or third control signal is determined by the setting of potentiometer 89.

With increasing current through line 38 to electrodes 15, the current signal appearing across

lines

63 and 64 of bridge rectifier 62 increases. This results in an increased signal being connected from line 63 to control winding 88 of saturable reactor 78 which opposes the signal in bias winding 87. As a result of these opposing signals, the overall impedance of saturable reactor 78 increases so that the output voltage across terminals and 86 is reduced, thereby reducing the voltage applied to Lower coil 44. At the same time, the increased current signal from bridge rectifier 62 has caused an increased signal across Raise coil 43 with the result that the combined coil signals double the torque response applied to armature 46. The combined effect of the coil signals causes a large response in servo valve 39 with the result that the control rapidly positions hydraulic cylinder 19 to restore the desired operating conditions.

When the arc current decreases and the voltage across the arc increases, the voltage signal applied to primary winding 67 of isolation transformer 68 increases. At the same time, current transformer 48 delivers a reduced current signal to bridge rectifier 62. Consequently, the signal delivered from line 63 to control winding 88 of magnetic amplifier 78 is also reduced. The reduced energy of control winding 88 in opposition to that of bias winding 87 results in an increase in the saturation of magnetic amplifier 78. This causes a decrease in the impedance of the magnetic amplifier so that a greater proportion of the increased arc voltage signal coupled from isolation transformer 68 is delivered at

terminals

85 and 86 of the magnetic amplifier. In turn, the greater proportion of the increased arc voltage is impressed across Lower coil 44 of servo valve 39. Thus, with a decrease in current at the arc, a decreased voltage is applied to Raise coil 43 and an increased voltage is applied to Lower coil 44 so that the torque response of armature 46 is effectively doubled to drive the servo valve to lower the electrodes.

Thus, it may be seen that the control system is capable of increasing the difference between the voltage and current produced signals reaching the servo valve in aproportional manner and also having the difference signal returned to a zero value at a rate proportional to that at which original signal differences at the electrode return to zero. In this way, the control system is capable of taking advantage of the high frequency response characteristics and the load signal differential threshold of the torque motor in servo valve 39.

The operators control 50 enables the operator to select the Lower, Automatic and Raised modes of operation. Control 50 is energized from A.C. source 91 which is connected to primary winding 92 of transformer 93. In the Automatic mode, switches 51, 52, 53am set in the B position. In the Lower mode, switches 51, 52, 53 are set in the A position. In the A position, switch 51 by means of line 56 grounds the current signal from current transformer 48 to terminal 51a. At the same time, switch 53 connects secondary winding 94 of transformer 93 through switch 95, line 96, relay contacts 71 and line 72 to tap connection 73. This results in an increased signal being applied to magnetic amplifier or saturable reactor means 78 with the result that an increased signal is impressed across Lower coil 44 of the servo valve. In this manner, the electrode 15 can be manually directed to descend toward charge 12 in shell 11.

To establish the Raise mode, switches 51, 53 are set in the C position. Switch 51 again grounds the output of current transformer 48. Switch 53 connects secondary winding 94 through switch 95, relay contacts 97 and line 58 to primary winding 54 of isolation transformer 55. In this way, an increased signal is applied to the isolation transformer and ultimately through the bridge rectifier to Raise coil 43 of the servo valve. As a result of this, servo motor 19 elevates the electrodes away from the crucible.

motor may" have a resistance of 70 ohms.

Merely by way of example, the servo valve torque The current circuits can be set to provide nine volts D.C. across one torque motor winding when 3,000 amperes flow in the electrode circuit. The Voltage sensing control circuit can be adjusted to furnish nine volts DC. to the other torque motor coil at 150 volts A.C.

If less sensitivity is desired, a fixed bias can be supplied to the saturable reactor in place of the bias furnished by winding 87 which is dependent upon the voltage appearing across the gate windings.

It should be apparent that variations can be made in the construction as needed without departing from the spirit of the invention except as defined in the appended claims.

What is claimed is:

1. In a control system for electric arc furnaces having electrode means connected to a source of electrical energy, the combination including motor means connected to said electrode means for raising and lowering the same, means producing a first control signal in response to the voltage across said electrode-means and furnace, saturable reactor means having gate winding means connected to said second control signal and to differential control means for said motor means, means connecting said first signal to said differential control means, control winding means for said saturable reactor means providing a predetermined bias thereto, and a feedback connection from said first control signal to said control winding means connected in opposing relationship to said predetermined bias, so that increase in current flow will tend to increase impedance in said gate winding means and thereby decrease the signal fed to said differential control means, so that changes in current and voltage relations in said electrode means will be accentuated and thereby rapidly adjust said electrode means.

2. In a control system for electric arc furnace means having electrode means connected to a source of electrical energy, said electrical energy being adapted to form an are at said electrode means, the combination including means for raising or lowering the electrode means with respect to the furnace, means for producing a first control signal in response to the current'flowing in said electrode means, means for producing a second control signal in response to the voltage across thearc, saturable reactor -means having gate winding means connected to said means for producing a second control signal, control winding means connected to said means for producing a first control signal, winding means for providing bias to said saturable reactor, said control winding means and said bias winding means being in opposingrelationship in said saturable reactor, and differential control means for said means for raising and lowering the electrode means, said differential control means being connected to said means for producing a first control signal and to said gate Winding means, so that an increase in said first control signal with increased electrode current will increase theimpedance of said gate winding presented to the reduced second control signal accompanying said increased electrode current and thereby decrease the signal fed by said gate winding means to said differential control means, so that changes in current and voltage relations in said electrode means will be accenuated and thereby rapidly adjust said electrode means.

3. In a control system for electric arc furnace means having electrode means connected to a source of electrical energy, said electrical energy forming an arc at said electrode means, the combination including means for raising or lowering the electrode means with respect to the furnace, means for producing a first control signal in response to the current flowing in said electrode means, means for producing a second control signal in response to the voltage across the arc, saturable reactor means having gate winding means connected to said means for producing a second control signal, control Winding means connected to said means for producing a first control signal, winding means connected to said gate winding means for providing bias to said saturable reactor, said control winding means and said bias winding means being in opposing relationship in said saturable reactor, and differential control means for said means for raising and lowering the electrode means, said differential control means being connected to said means for producing a first control signal and to said gate winidng means, so that an increase in said first control signal with increased electrode current will increase the impedance of said gate winding presented to the reduced second control signal accompanying said increased electrode current and thereby de crease the signal fed by said gate winding means to said differential control means, so that changes in current and voltage relations in said electrode means will be accentuated and thereby rapidly adjust said electrode means.

4. In a control system for electric arc furnace means having electrode means connected to a source of electrical energy, said electrical energy forming an are at said electrode means, the combination including means for raising or lowering the electrode means with respect to the furnace, means for producing a first control signal in response to the current flowing in said electrode means,

first transformer means connected to said first signal producing means, means for producing a second control signal in response to the voltage across the arc, second transformer means connected to said second signal producing means, saturable reactor means having gate windingmeans connected to said second transformer means, control winding means connected to said first transformer, winding means for providing bias to said saturable reactor, said control winding means and said bias winding means being in opposing relationship in said saturable reactor, and differential control means for said means for raising and lowering the electrode means, said differential control means being connected to said first transformer means and to said gate winding means, so that an increase in said first control signal with increased electrode current will-increase the impedance of said gate winding presented to the reduced second control signal accompanying said increased electrode current and thereby decrease the signal fed by said gate winding means to said differential control means, so that changes in current and voltage relations in said electrode means will be accentuated and thereby rapidly adjust said electrode means.

5. In a control system for electric arc furnaces having electrode means connected to a source of A.C., the combination including hydraulically operated motor means connected to said electrode means for raising and. lowering the same, first circuit means producing a first control signal in response to the current flowing in said electrode means, second circuit means producing a second control signal in response to the voltage across said electrode means, saturable reactor means having gate winding means connected to said second circuit means and second control signal, differential'motor means having one portion connected to said gate winding means, means connecting said first signal to another portion of said differential motor means, control Winding means for said saturable reactor means having an electric bias source connected thereto, a connection from said first circuit means for feeding a signal to said control winding means in opposing relationa ship to said bias, and means operated by said differential D.C. control signal in response to the voltage across said electrode means, saturable reactor means having gate winding means connected to said second circuit means and second control signal, differential torque motor means having one portion connected to said gate winding means, means connecting said first signal to another portion of said differential motor means, control winding means for said saturable reactor means having an electric bias source connected thereto, a connection from said first circuit means for feeding a signal to said control winding means in opposing relationship to said bias, and hydraulic valve means directly operated by said differential torque motor means connected to said hydraulically operated motor means for operating the same, so that changes in current and voltage relations in said electrode means will be accentuated and thereby rapidly adjust said electrode means.

7. In a control system for electric arc furnaces having electrode means connected to a source of A.C., the combination including hydraulically operated motor means connected to said electrode means for raising and lowering the same, first circuit means having rectifier means therein for producing a first D.C. control signal in response to the current flowing in said electrode means, second circuit means having rectifier means therein for producing a second D.C. control signal in responseto the voltage across said electrode means, saturable reactor means having gate winding means connected to said second circuit means and second control signal, differential torque motor means having one portion connected to said gate winding means, means connecting said first signal to another portion of said differential motor means, control winding means for said saturable reactor means having an electric bias source connected thereto, a connection from said first circuit means for feeding a D.C. signal proportional to said first signal to said control winding means in opposing relationship to said bias, and hydraulic valve means directly operated by said differential torque motor means connected .to said hydraulically operated motor means for operating the same, so that changes in current and voltage relations in said electrode means will be accentuated and thereby rapidly adjust said electrode means.

8. In a control system for electric arc furnaces having electrode means connected to a source of A.C., the combination including hydraulically operated motor means connected to said electrode means for raising and lowering the same, first circuit means having rectifier means therein for producing a first D.C. control signal in response to the current flowing in said electrode means, second circuit means having rectifier means therein and an isolation tranformer for producing a second D.C. control signal in response to the voltage across said electrode means, satu rable reactor means having gate winding means connected to said second circuit means and second control signal, differential torque motor means having one portion connected to said gate winding means, means connecting said firstsignal to another portion oftsaid differential motor means, control winding means for said saturable reactor means having an electric bias source connected thereto,

aconnection from said first circuit means for feeding a D.C. signal proportional to said first signal to said control winding means in opposing relationship to said bias, and hydraulic valve means directly operated by said differential torque motor means connected to said hydraulically operated motor means for operating the same, so that changes in current and voltage relations in'said electrode means will be accentuated and thereby rapidly adjust said electrode means.

9. In a control system for electric arc furnace means having electrode means connected to a source of A.C., said A.C. being adapted to form an are at said electrode means, the combination including hydraulically operated servo motor means for raising or lowering the electrode means with respect to the furnace and a charge therein, transformer means associated with the AC. supply to said electrode means for producing a first control signal in response to the current flowing in said electrode means,

I means connected across said furnace for producing a second control signal in response to the voltage across the same, saturable reactor means having gate winding means connected to said means for producing a second control signal, control winding means for said saturable reactor means, means connecting D.C. to said control winding means for providing bias to said saturable reactor, rectifier means connected to said transformer means, means connecting the D.C. signal derived from said first control signal through said rectifier means to said control winding means in opposing relationship to said bias, and differential means responsive to said first and second signals connected to valve means for controlling the raising and lowering of the electrode means, so that an increase in said first control signal with increased electrode current will increase the impedance of said gate winding presented to the reduced second control signal accompanying said increased electrode current and thereby decrease the signal fed by said gate winding means to said differential control means and thereby rapidly adjust said electrode means.

References Cited in the file of this patent UNITED STATES PATENTS 2,717,326 Gunton Sept. 6, 1955 2,884,580 Sauter et al Apr. 28, 1959 2,921,107 Toothman et a1 Jan. 12, 1960

Claims

1. IN A CONTROL SYSTEM FOR ELECTRIC ARC FURNACES HAVING ELECTRODE MEANS CONNECTED TO A SOURCE OF ELECTRICAL ENERGY, THE COMBINATION INCLUDING MOTOR MEANS CONNECTED TO SAID ELECTRODE MEANS FOR RAISING AND LOWERING THE SAME, MEANS PRODUCING A FIRST CONTROL SIGNAL IN RESPONSE TO THE VOLTAGE ACROSS SAID ELECTRODE MEANS AND FURNACE, SATURABLE REACTOR MEANS HAVING GATE WINDING MEANS CONNECTED TO SAID SECOND CONTROL SIGNAL AND TO DIFFERENTIAL CONTROL MEANS FOR SAID MOTOR MEANS, MEANS CONNECTING SAID FIRST SIGNAL TO SAID DIFFERENTIAL CONTROL MEANS, CONTROL WINDING MEANS FOR SAID SATURABLE REACTOR MEANS PROVIDING A PREDETERMINED BIAS THERETO, AND A FEEDBACK CONNECTION FROM SAID FIRST CONTROL SIGNAL TO SAID CONTROL WINDING MEANS CONNECTED IN OPPOSING RELATIONSHIP TO SAID PREDETERMINED BIAS, SO THAT INCREASE IN CURRENT FLOW WILL TEND TO INCREASE IMPEDANCE IN SAID GATE WINDING MEANS AND THEREBY DECREASE THE SIGNAL FED TO SAID DIFFERENTIAL CONTROL MEANS, SO THAT CHANGES IN CURRENT AND VOLTAGE RELATIONS IN SAID ELECTRODE MEANS WILL BE ACCENTUATED AND THEREBY RAPIDLY ADJUST SAID ELECTRODE MEANS.