US3493664A - Control system for electric arc furnace - Google Patents

Control system for electric arc furnace Download PDF

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US3493664A
US3493664A US711624A US3493664DA US3493664A US 3493664 A US3493664 A US 3493664A US 711624 A US711624 A US 711624A US 3493664D A US3493664D A US 3493664DA US 3493664 A US3493664 A US 3493664A
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tap changer
contact
tap
electrode
relay
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US711624A
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Sidney M Kapell
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CBS Corp
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Westinghouse Electric Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F29/00—Variable transformers or inductances not covered by group H01F21/00
    • H01F29/02—Variable transformers or inductances not covered by group H01F21/00 with tappings on coil or winding; with provision for rearrangement or interconnection of windings
    • H01F29/04—Variable transformers or inductances not covered by group H01F21/00 with tappings on coil or winding; with provision for rearrangement or interconnection of windings having provision for tap-changing without interrupting the load current
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B7/00—Heating by electric discharge
    • H05B7/02—Details
    • H05B7/144—Power supplies specially adapted for heating by electric discharge; Automatic control of power, e.g. by positioning of electrodes
    • H05B7/148—Automatic control of power
    • H05B7/152—Automatic control of power by electromechanical means for positioning of electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/25—Process efficiency

Definitions

  • a control system for an electric arc furnace having a hearth and at least one electrode connectable to a power supply includes regulator means for providing a control signal responsive to the electrical condition of the arc furnace, electrode drive means which adjusts the axial position of the electrode in response to the control signal, tap changer means for adjusting the voltage applied to the electrode from the power supply, and means for reducing the current flowing through the tap changer means, immediately prior to and during a tap change, to reduce the arcing at the arcing contacts of the tap changer.
  • the invention relates in general to electric arc furnaces, and more particularly to control systems for electric arc furnaces.
  • Electric arc furnaces are often operated above their nameplate ratings during certain portions of the arc furnace operating cycle, such as during the melt down period.
  • the furnace transformer and the main current carrying contacts of the underload tap changer are thus called upon to carry short time overloads, i.e., higher than rated current, and they are expected to do so without deleteriously affecting their performance, or resulting in a substantial impairment of their expected operating life.
  • the underload tap changer is called upon to change taps during thew. periods of higher than rated current, the arcing contacts may be seriously stressed, burned and pitted, resulting in a short useful operating life of the arcing contacts, necessitating costly down-time of the arc furnace for tap changer maintenance.
  • the present invention is a new and improved control system for electric arc furnaces of the type which utilize tap changing underload, which overcomes the disadvantages of prior art electric arc furnace apparatus and control.
  • the invention is a new control system for an electric arc furnace, having a hearth, at least one electrode, and a power supply.
  • the control system includes regulator means for providing a control signal in response to the electrical condition of the arc furnace, such as electrode current and electrode-to-hearth voltage, electrode drive means which controls the position of the electrode relative to the melt or furnace charge, in response to the control signal from the regulator, tap changer means for adjusting the voltage applied to the electrode from the power supply, and means for reducing the electrode current immediately prior to, and during the time the tap changer means is making a tap change.
  • the means for reducing the electrode current, and thus the current through the tap changer is initiated by the signal which initiates a tap change, and includes means for delaying the start of the actual mechanical tap change by the tap changer mechanism and drive means, until the electrode current and the tap changer current have been reduced.
  • the regulator means is of the type which operates upon the principle of regulating the electrode current and electrode-to-hearth voltage to a fixed or preset ratio, in response to signals responsive to the magnitudes of the electrode current and the electrode-to-hearth voltage.
  • the means for reducing the electrode current reduces the magnitude of the signal responsive to the electrode-to-hearth voltage, simulating a reduction in electrode-to-hearth voltage.
  • control signal from the regulator causes the control signal from the regulator to change the electrode position in the direction, relative to the melt, which reduces the electrode current to the value which will return the voltage-current ratio, as determined by the regulator, to its preset value.
  • the control automatically removes its temporary modification of the electrode-to-hearth voltage responsive signal, allowing the electric arc furnace to return to its normal operation.
  • FIGURE 1 is a schematic diagram of electric arc furnace apparatus and control, constructed and arranged according to the teachings of an embodiment of the invention.
  • FIG. 2 is a schematic diagram of tap changer control apparatus which may be used with the electric arc furnace apparatus and control shown in FIG. 1.
  • the exemplary form of the invention illustrated comprises an electric arc furnace 10, which includes electrodes 12, 14 and 16, a furnace transformer 18, a regulating transformer 20, and a power supply or source 22 of alternating potential.
  • the position of each of the electrodes is controlled by regulator means which is responsive to the electrical condition of its associated electrode.
  • the position of electrode 16 is controlled by regulator means 24. Since the regulator means for each of the electrodes 12, 14 and 16 would be similar, only regulator means 24 for regulating the position of electrode 16 is shown in FIG. 1.
  • Electric arc furnace 10 which is illustrated in FIG. 1 as being three-phase, but which may be single-phase if desired, includes a container or hearth 26, which is grounded at 28, and which contains a charge or melt 30.
  • the electrodes 12, 1-4 and 16 are suspended over the melt 30, with their lower ends being in close proximity thereto.
  • the furnace transformer 18, in this instance, is a three-phase transformer having a primary 32, which may be connected in Y or delta, and a secondary winding 34 which is normally connected delta, as shown in FIG. 1, or in any other desired configuration.
  • the secondary wind- 3 ing 34 is connected to electrodes 12, 14 and 16 via electrical conductors 36, 38 and 40, respectively, and the primary winding 32 is connected to regulating transformer 20 via conductors 42, 44 and 46.
  • Regulating transformer 20 may be of the type shown in FIG. 1, having a Y-connected excitation winding 48, tapped phase windings 50, 52 and 54, each having a plurality of taps T, numbered consecutively across the tapped winding, a delta connected tertiary (not shown) for the circulation of third harmonic currents, and tap changer means 56, 58 and 60, associated with tapped windings 50, 52 and 54, respectively.
  • Each of the tap changer means 56, 58 and 60 includes tap changer drive and control means, such as tap changer drive and control means 62 associated with tap changer means 60.
  • Tap changer drive and control means 62 has terminals 65, 67 and 69, which are connected to a source 64 of AC control potenital, via conductors 66, 68 and 70, respectively.
  • Each of the tap changer means 56, 58 and 60 such as tap changer means 60, includes a preventive autotransformer 72, having its outer terimnals connected to the tap changer mechanism 74, which includes two movable contact arms for sequentially changing the effective tap position of the tap changer without interrupting the circuit.
  • the center terminal of the preventive autotransformer 72 is connected to one of the phases of the source 22 of alternating potential.
  • the ends of the tapped winding 54 are connected to the stationary contacts of a reversing switch 76, and the movable contact arm of thereversing switch 76 is connected to conductor 46, which is connected to primary winding 32 of the furnace transformer 18.
  • the voltage from AC source 22 supplied to the primary winding 32 of arc furnace transformer 18 is modified by the tapped windings 50, 52 and 54, with the voltage across the tapped windings either adding to, or subtracting from, the source potential, depending upon whether the position of the reversing switch connects the tapped windings in series aiding or series opposing, with respect to the source potential 22.
  • the tap changing underload electrical system for electric arc furnace shown in FIG. 1 is one of several embodiments which may be used.
  • the regulating transformer may be connected in the secondary of furnace transformer 18, instead of in the primary.
  • the axial positions of the electrodes 12, 14 and 16 are each controlled by a regulator.
  • the regulator means 24 for controlling the position of electrode 16 relative to the melt 30, may be of any suitable type. For example, it maybe of the type shown in FIG. 1, wherein it obtains a measure of the electrode-to-he'arth voltage, and a meassure of the electrode current, and regulates the position of the electrode to provide a predetermined ratio of current to voltage; or, the regulator may be of the type which obtains a measure 'of the electrode current, and regulates the electrode current to predetermined values at various portions of the electric arc furnace operating cycle.
  • regulator means 24 obtains a measure of the electrode-to-hearth voltage of electrode 16 by a potential transformer 78, which has a primary winding 80 connected between the electrode 16 and the grounded hearth 26, and a secondary winding 82 connected to the input terminals 81 and 83 of a full-wave, singlephase bridge rectifier 84, through an adjustable resistor 86 and an adjustable voltage calibrating resistor 88.
  • resistor 88 is shorted by two serially connected normally closed contacts 98 and 100, which are responsive to the tap changer drive and control means 62, as indicated generally by dotted line 102.
  • a resistor 90 is connected across the positive and negative output terminals 85 and 87, respectively, of bridge rectifier 84, and the unidirectional voltage across resistor is smoothed in a wave filter network, comprising inductor 92 and capacitor 94.
  • One end of capacitor 94 is connected to the negative terminal 87 of bridge rectifier 84, one end of inductor 92 is connected to the positive terminal 85 of bridge rectifier 84, and the other ends of capacitor 94 and inductor 92 are connected together at junction 96.
  • the magnitude of the unidirectional potential across capacitor 94 is responsive to the electrodeto-hearth voltage of electrode 16.
  • Regulator means 24 obtains a measure of the electrode current flowing through electrode 16 by a current transformer 104 disposed in inductive relation with conductor 40, which is connected across a resistor 106.
  • the voltage developed across resistor 106, due to the current flow therethrough from current transformer 104, may be transformed to a usable magnitude in a transformer 108.
  • Transformer 108 has a primary winding 110 connected across the resistor 106, and a secondary winding connected through adjustable impedance means 118 to the input terminals 112 and 114 of a full-wave, single-phase bridge rectifier circuit 116.
  • the positive and negative output terminals and 122, respectively, of bridge rectifier 116 are connected across a resistor 124, and the unidirectional potential across resistor 124 is smoothed or filtered in a capacitor 128.
  • One end of inductor 126 is connected to the positive terminal 120 of bridge rectifier 116, one end of capacitor 128 is connected to the negative terminal 122 of bridge rectifier 116, and their remaining ends are connected together at junction 130.
  • the magnitude of the unidirectional potential across capacitor 124 is responsive to the electrode current of electrode 16.
  • the unidirectional voltages responsive to the electrode current and voltage are compared, and a signal developed proportional to the deviation of the electrode current and voltage from a predetermined ratio, by connecting the negative terminals 87 and 122 of bridge rectifiers 84 and 116 in common, and by connecting the stationary portion of an adjustable resistor 132, having a movable contact arm 134, across the junctions 96 and 130.
  • the movable arm of resistor 106 adjusts the magnitude of the voltage developed across it for a given primary electrode current.
  • the movable arm of resistor 106 is adjusted to set the desired ratio of electrode current to electrode-to-hearth voltage.
  • the movable arm 134 of resistor 132 is adjusted to set the magnitude of the output error signal, if and when the unidirectional voltages responsive to electrode current and voltage changes and become unbalanced.
  • Electrode drive means 140 controls the position of electrode 16. Electrode drive means 140 may include a direct current motor 146 having an armature 148 connected to conductors 142 and 144, and a field winding 150 connected to a source 152 of unidirectional potential through an adjustable resistor 154.
  • the armature 148 of direct current motor 146 may be coupled to a rotatable take up drum 158, which unwinds, or winds, a cable 162 which is guided by pulley to lower or lift the electrode 16, according to the direction of rotation of the armature 148, and thus according to the polarity of the error signal.
  • the control signal will drop to zero, and the drive means 140 will be deactivated, leaving the electrode 16 in the new position until receiving another control signal to move the electrode to a different operating position.
  • the furnace transformer 18 and regulating transformer 20 may be operated above their nameplate ratings.
  • This type of apparatus is designed to accommodate short time overloads without damage. However, if the tap changer apparatus is called upon to change taps during an overload, the arcing contacts of the tap changer mechanism may be damaged to the extent of impairing their useful operating life.
  • This invention solves this problem by using the signal tfor changing taps to initiate a modification of the regulator circuit to reduce the electrode current, and thus the tap changer current, by a predetermined fixed amount.
  • the tap change signal is also utilized to initiate a delay in the actual mechanical tap change for a short time period, which time period is selected to allow the electrode current to be reduced to its new lower operating value. This reduced current condition is maintained throughout the tap change cycle, and when the tap change has been completed, the regulator is returned to its unmodified condition.
  • the regulator 24 may be modified to reduce the magnitude of the current flow by simulating a reduction in the electrode-to-hearth voltage. This will change the apparent ratio of current to voltage, and the regulator will act to raise the electrode and reduce the electrode current until the current and the simulated electrode-tohearth voltage are in the predetermined selected ratio, which changes the actual ratio.
  • the simulated reduction in electrode-to-hearth voltage is accomplished by connecting an adjustable resistor 86 in series with the electrode-to-hearth voltage responsive signal, and by shorting the resistor with two serially connected normally closed contacts 98 and 100.
  • the opening of either contact will connect the resistor 86 to reduce the magnitude of the signal applied to the bridge rectifier 84.
  • the unidirectional voltage across capacitor 94 will be reduced, and a control signal will be provided between the arm 134 of resistor 132 and junction 96, which has a polarity which will cause the electrode 16 to be raised until the current responsive signal across capacitor 128 is reduced to the magnitude where it will again provide the predetermined ratio with the voltage across capacitor 94.
  • the selected setting of adjustable resistor 86 will determine the magnitude of the reduction in electrode current.
  • contact 98 is responsive to the initial tap changing signal, opening to initiate the reduction in the electrode current.
  • Delay means is utilized in the tap changer control 62 to provide a predetermined short delay interval between the start of the tap change signal and the start of the actual mechanical tap change.
  • Contact 98 remains open until the tap changer cycle is initiated, at which time contact 100' will open and remain open until the tap change cycle has been completed.
  • Contact 98 closes after contact 100 opens, and contact 100 closes after the completion of the tap change cycle, thus shorting resistor 86 after the completed tap change, returning the regulator 24 to its unmodified condition.
  • FIG. 2 is a schematic diagram of tap changer drive and control means which may be used to provide these functions.
  • terminals 65, 67 and 69 of the tap changer drive and control means 62 shown in FIG. 2 may be connected to a conventional three wire, singlephase source 64 of alternating potential, with terminal 67 being grounded at 162, and with the voltage between terminals 65 and 69 being substantially twice the voltage between terminal 65 and ground, or between terminal 69 and ground.
  • the tap changer drive and control means 62 will first be described as it is conventionally constructed, and then the changes required in the conventional control circuit to practice the teachings of the invention will be described.
  • the tap changer drive and control means 62 includes a tap changer drive motor 164, which drives the tap changer means 60 shown in FIG. 1, as indicated by dotted line 166, a raise relay R having an electromagnetic coil 168 and contacts R1, R2, R3 and R4, a lower relay L having an electromagnetic coil 170 and contacts L1, L2, L3 and L4, a brake relay B having an electromagnetic coil 172 and a contact B1, an operators tap changer control switch 174 having normally open contacts 175 and 177, which is usually located remotely, such as at the operators control panel, a local tap changer control switch 176 having normally open contacts 179 and 181, which is usually located at the tap changer, a remote-local selector switch 178 having a normally closed contact 183 and a normally open contact 185, for selecting which of the tap changer control switches is to operate the tap changer, and contacts TC1, TC2 and T03 which are cam operated contacts responsive to the tap changer means 60 shown in FIG. 1.
  • the tap changer drive motor may have two windings, a lower winding 180 connected between terminals 182 and -184, which, when energized, drives the motor shaft in a direction to connect the tap changer to a lower numbered tap, and a raise winding 186 connected between terminals 188 and 184, which when energized, drives the motor shaft in a direction to connect the tap changer to a higher numbered tap.
  • the lower winding 180 of tap changer drive motor 164 is connected across terminals 65 and 69 via contact L1 of relay L, which is a normally open contact, i.e., open when the electromagnetic coil 170 of relay L is deenergized.
  • the lower winding 180 is also connected across terminals 65 and 69 via contact R4 of relay R, which is a normally closed contact, and through contact B1 of the brake relay B, which is a normally open contact.
  • the raise winding 186 of tap changer drive motor 164 is connected across terminals 65 and 69 via contact R1 of relay R, which is a normally open contact.
  • the raise winding 186 is also connected across terminals 65 and 69 via contact L4 of relay L, which is a normally closed contact, and through contact B1 of the brake relay B.
  • the electromagnetic coil 170 of relay L is connected from terminal 67 to terminal 65 through the circuit which includes normally closed contact R3 from relay R, normally closed contact TC2 from the tap changer apparatus 60, and then either through contacts 179 and of switches 176 and 178, respectively, or through contacts 175 and 183 of switches 174 and 178, respectively.
  • the contact TD1 will be assumed to be a solid conductor for the present discussion.
  • a sealing circuit for relay L is provided between terminals 65 and 67 via normally closed contact R3 of relay R, normally open contact L2 of relay L, and normally open contact TC1 from the tap changer apparatus 60.
  • the electromagnetic coil 168- of relay R is connected from terminal 67 to terminal 65 through the circuit which includes normally closed contact L3 from relay L, normally closed contact TC3 from the tap changer apparatus 60, and then either through contacts 181 and 185 of switches 176 and 178, respectively, or through contacts 177 and 183 of switches 174 and 178, respectively.
  • Contact TD1 will again be assumed to be a solid conductor for the present discussion.
  • a sealing circuit for relay R is provided between terminals 65 and 67 via normally closed contact L3 of relay L, normally open contact R2 of relay R, and normally open contact TC1 from the tap changer means 60.
  • Electromagnetic coil 172 of brake relay B is connected across terminals 65 and 67 through normally open contact TC1 of the tap changer apparatus 60, an adjustable resistor 190, and a diode 192.
  • a capacitor 194 is connected across the electrical coil 172 of brake relay B.
  • Contact TC1 is a normally open contact responsive to the tap changer apparatus 60. It closes once the tap changer starts to move, and remains closed until the tap changer completes its tap change cycle. Normally closed contacts TC2 and TC3 are responsive to cams on the tap changer apparatus, with contact TC2 opening when the tap changer is on its lowest numbered tap position, and with contact TC3 opening when the tap changer is on its highest numbered tap position. Contacts TC2 and TC3 are thus interlocks which prevent the tap changer from being driven past its intended operating range.
  • the operator actuates the remote-local selector switch 178 to select the tap changer switch which will initiate a tap change.
  • the local tap changer switch is usually used only for adjustment, test and set-up purposes, with the tap changer switch located at the remote operators panel being utilized during the actual operation of the arc furnace 10. If the remote tap changer switch -174 is selected, contact 183 will close and contact 185 will open, as shown in FIG. 2. If the local tap changer switch 176 is selected, contact 183 will open and contact 185 will close. For purposes of example, it will be assumed that the remote tap changer switch 174 is selected. Thus, contact 183 will be closed.
  • the tap changer switch 174 will switch the tap changer switch 174 to the lower position and will keep the switch in this position for a short period of time, which is suflicient to allow the lower relay L to pickup and seal-in.
  • Indicating means such as a light 196 may be connected to be energized once the relay L seals in.
  • the operator moves the switch 174 to the lower position until the indicating light 196 is energized, at which time the switch is re- .leased and it returns to its neutral position. If the tap changer is not already on its lowest numbered tap position, contact TC2 will be closed, and normally closed contact R3 of relay R will be closed.
  • the operator will switch tap changer switch 174 to the raise position, and will keep the switch in this position for a short period of time sufficient to allow the raise relay R to pickup and seal-in.
  • Indicating means 96 may also be connected to be energized once relay R seals in. Thus, the operator moves switch 174 to the raise position until indicating light 1% comes on, at which time the switch may be released and returned to its neutral position. If the tap changer is not already on its highest numbered tap position, contact TC3 will be closed, and
  • the operation of the tap changer would be similar, except now contact 185 will be closed, and the operator would initiate the raise cycle of the tap changer by momentarily closing contact 181 of the local tap changer switch 176.
  • the electromagnetic coil 172 of the brake relay B will also be disconnected from the source 64 of alternating potential, but capacitor 194 will discharge through the electromagnetic coil 172, keeping the electrical coil 172 energized for a short period of time after the energized relay R, or the energized relay L, is deenergized. This time will depend upon the voltage magnitude to which capacitor 194 had been charged, and is thus determined by the setting of resistor 190.
  • contact B1 will be closed for a short period of time after contact TC1 opens, connecting both the raise and lower coils 186 and 180 of the tap changer drive motor across terminals 65 and 69, which applies a braking force to the motor, stopping the tap changer at its newly selected tap position.
  • relay B After capacitor 194 has discharged to a certain voltage magnitude, relay B will drop out, opening its contact B1, and removing the simultaneous energization of the raise and lower windings of the tap changer drive motor 164.
  • the tap changer drive and control means 62 may be modified, according to the teachings of the invention, by adding contacts 200 and 202 to the operators tap changer control switch 174, with contact 200 being arranged to operate with contact 175, and contact 202 being arranged to operate with contact 177. Or, a single contact may be used if it is arranged to close and open when either contact 175 or contact 177 is closed and opened.
  • Contacts 200 and 202 are connected in parallel, with one side of the parallel arrangement being connected to contact 183 of the remote-local selector switch 178, and the other side being connected to terminal 67 through the electromagnetic coil 204 of a first auxiliary relay AI, which has a normally closed contact 98, and also through the electromagnetic coil 206 of a time delay TD, which has a normally open contact TD1.
  • Contacts 175 and 177 of tap changer control switch 174 instead of being connected directly to contact 183 of selector switch 178, are connected to contact 183 through contact TD1 of time delay relay TD.
  • the electrical coil 210 of a second auxiliary relay All having a normally closed contact 100, is connected to be energized when cam operated contact TC1 closes.
  • normally .closed contacts 98 and 100 associated with relays AI and AII are serially connected across resistor 86 in the regulator 24 shown in FIG. 1.
  • the operation of the tap changer drive and control means changed in accordance with the teachings of the invention, will now be described.
  • the operator sets the selector switch 178 to operate the tap changer from the operators tap changer switch 174. If the operator wishes to change the tap changer to a lower numbered tap position, tap changer switch 174 would be moved to the lower position, and held there, which closes contacts 175 and 200.
  • the closing of contact 200 energizes the electromagnetic coil-s 204 and 206 of relays AI and TD.
  • relay AI When relay AI is energized, contact 98 will open, placing resistor 86 in the regulator circuit, and the regulator will start to reduce the electrode current and the tap changer current.
  • relay TD When relay TD is energized, it will start its timed period, with its contact TD1 closing at the end of this period.
  • tap changer switch 174 will be moved to the raise position and held there, which closes contacts 177 and 202.
  • the closing of contact 202 energizes the electromagnetic coils 204 and 206 of relays AI and TD.
  • relay AI When relay AI is energized, contact 98 will open, placing resistor 86 in the regulator circuit and the regulator will start to reduce the electrode current.
  • relay TD When relay TD is energized, it will start its timed period, with its contact TD1 closing at the end of this period. When contact TD1 closes, the electrode current and the tap changer current have already been reduced by the regulator to a lower magnitude.
  • relay R When contact TD1 closes, the electromagnetic coil 168 of relay R will be energized, starting the tap changer drive motor, sealing in relay R through contacts TC1 and R2, and relay All will be energized, opening its contact 100.
  • the indicating light 196 will also be energized when contact TC1 closes. The operator may then release the tap changer switch 174 after the indicating light is energized, as the raise relay R will then be sealed in, and contact 100 will now be open to maintain the regulator in its modified setting.
  • relays AI and TD When the tap changer switch 174 is allowed to return to its neutral position, relays AI and TD will be deenergized, allowing contact 98 to close, and relay TD to reset, opening its contact TD1.
  • the electrode current may be reduced prior to and during a tap change by connecting a resistor in the circuit of the regulator which measures the current, and setting the regulator to provide the desired regulation with this resistor connected therein.
  • Relays AI and All would each have a normally open contact connected across this resistor, with the contacts being connected in parallel with each other. The operation of either relay would thus short this resistor, simulating an increase in electrode current, causing the regulator to reduce the electrode current and maintain the current at this reduced magnitude until the tap change is completed.
  • a new and improved control system for an electric arc furnace which automatically lowers the electrode current, and thus the tap changer current, when a tap change is ordered by the operator.
  • the tap change signal immediately initiates the reduction in electrode current, while the application of the tap change signal to the tap changer drive motor control is delayed for a short period of time sufficient to allow the regulator to reduce the electrode current to its new magnitude. After this delay period, the tap change signal is applied to the tap change drive control, initiating the mechanical operation of the tap changer.
  • the arcing contacts of the tap changer are called upon to interrupt a lower magnitude of current than it would normally have to interrupt at a similar point in the arc furnace cycle, which substantially increases the life of the arcing contacts.
  • the time between maintenance periods may be substantially extended, resulting in less furnace down time.
  • a control system for an electric arc furnace having a hearth, and at least one electrode connectable to a power supply comprising:
  • electrode drive means connected in circuit relation with said regulator means, for moving the electrode in response to the control signal therefrom,
  • tap changer means for adjusting the voltage applied to the electrode from the power supply
  • said regulator means is of the current-voltage balance type
  • said means for reducing the magnitude of the current flowing through said tap changer means when said tap changer means is adjusting the voltage applied to the electrode includes means for simulating in the regulator means a reduced voltage between the electrode and the furnace hearth.
  • said tap changer means includes a tap changer mechanism, drive means for said tap changer mechanism, and control means for actuating said drive means in response to a tap change signal, and including delay means for delaying the start of said tap changer drive means after said tap changer control means receives the tap change signal, until the current flowing through said tap changer means has been reduced.
  • said regulator means is of the current-voltage balance type, having a first circuit for providing a signal responsive to a measure of the electrode current, a second circuit for providing a signal responsive to a measure of the electrodeto-furnace hearth voltage, and means comparing the signals of said first and second circuits to provide said control signal, and wherein the means for reducing the magnitude of the current flowing through said tap changer means when said tap changer means is adjusting the voltage applied to the electrode, includes means for reducing the magnitude of the signal provided by said second circuit.
  • said tap changer means includes a tap changer mechanism, drive means for said tap changer mechanism, and control means for actuating said drive means in response to a tap change signal, and including delay means for delaying the start of said tap changer drive means after said tap changer control means receives the tap change signal, until the current flowing through said tap changer means has been reduced.
  • control system for an electric arc furnace having a hearth and at least one electrode connectable to a power supply comprising:
  • regulator means of the current-voltage balance type providing a control signal responsive to the electrode current and electrode-to-furnace hearth voltage
  • electrode drive means connected in circuit relation with said regulator means for moving the electrode in response to the control signal therefrom
  • tap changer means for adjusting the voltage applied to the electrode from the power supply, said tap changer means including a tap changer mechanism, drive means driving the tap changer mechanism, tap changer control means for actuating said drive means in response to a tap change signal, means for reducing the magnitude of the current flowing through said tap changer mechanism when the control means receives a tap change signal, and delay means for delaying the start of said tap change drive means after said tap changer control means receives the tap change signal, until the current flowing through said tap changer mechanism has been reduced.
  • said regulator means has a first circuit for providing a signal responsive to a measure of the electrode current, a second circuit for providing a signal responsive to a measure of the electrode voltage, and means comparing the signals of said first and second circuits to provide said control signal, and wherein the means for reducing the magnitude of the current flowing through said tap changer mechanism includes means for reducing the magnitude of the signal provided by said second circuit.
  • the means for reducing the magnitude of the signal provided by the second circuit of said regulator means includes impedance means which is connected to reduce the magnitude of the signal provided by said second circuit from the time the tap change signal is applied to the tap changer control means, until said tap changer mechanism completes the change.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Plasma & Fusion (AREA)
  • Vertical, Hearth, Or Arc Furnaces (AREA)
  • Furnace Details (AREA)
  • Discharge Heating (AREA)

Description

Feb. 3, 1970 s. MLKAPELL 3,493,654
CONTROL SYSTEM FOR ELECTRIC ARC FURNACE Filed March 8, 1968 2 Sheets-Sheet 1 w w W .3 M m 1 A m a 5 YM B mw n $20". g 92 w mm.
w 3T my J g mm 6528 92 mm ma 1| 5921013 m E p Nm On S 5528 88 6 mm B 1 mm I: N J 0N T W 8 o m SE28 5% Ll J g NN Feb. 3, 1970 s. M. KAPELL CONTROL SYSTEM FOR ELECTRIC ARC FURNACE Filed March 8, 1968 r 2 Sheets-Sheet. 2
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United States Patent O Int. Cl. H05b 7/18 U.S. CI. 13-12 8 Claims ABSTRACT OF THE DISCLOSURE A control system for an electric arc furnace having a hearth and at least one electrode connectable to a power supply. The control system includes regulator means for providing a control signal responsive to the electrical condition of the arc furnace, electrode drive means which adjusts the axial position of the electrode in response to the control signal, tap changer means for adjusting the voltage applied to the electrode from the power supply, and means for reducing the current flowing through the tap changer means, immediately prior to and during a tap change, to reduce the arcing at the arcing contacts of the tap changer.
BACKGROUND OF THE INVENTION Field of the invention The invention relates in general to electric arc furnaces, and more particularly to control systems for electric arc furnaces.
Description of the prior art Electric arc furnaces must have the flexibility of being able to be operated at any one of a plurality of voltage levels, in order to perform such functions as holding, refining, bore down, clean bottom and melt. Therefore, tap changing under load voltage regulating transformers are being used to regulate the primary voltage applied to the arc furnace transformer. The tap changer mechanism may be operated in response to an operators selector switch, which the operator manually controls; or, it may be automatically operated in response to computer control, or power sensitive relays.
Electric arc furnaces are often operated above their nameplate ratings during certain portions of the arc furnace operating cycle, such as during the melt down period. The furnace transformer and the main current carrying contacts of the underload tap changer are thus called upon to carry short time overloads, i.e., higher than rated current, and they are expected to do so without deleteriously affecting their performance, or resulting in a substantial impairment of their expected operating life. However, if the underload tap changer is called upon to change taps during thew. periods of higher than rated current, the arcing contacts may be seriously stressed, burned and pitted, resulting in a short useful operating life of the arcing contacts, necessitating costly down-time of the arc furnace for tap changer maintenance.
SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved control system for electric arc furnaces of the type which utilize tap changing underload, which overcomes the disadvantages of prior art electric arc furnace apparatus and control.
More specifically, the invention is a new control system for an electric arc furnace, having a hearth, at least one electrode, and a power supply. The control system includes regulator means for providing a control signal in response to the electrical condition of the arc furnace, such as electrode current and electrode-to-hearth voltage, electrode drive means which controls the position of the electrode relative to the melt or furnace charge, in response to the control signal from the regulator, tap changer means for adjusting the voltage applied to the electrode from the power supply, and means for reducing the electrode current immediately prior to, and during the time the tap changer means is making a tap change. The means for reducing the electrode current, and thus the current through the tap changer, is initiated by the signal which initiates a tap change, and includes means for delaying the start of the actual mechanical tap change by the tap changer mechanism and drive means, until the electrode current and the tap changer current have been reduced. In one embodiment of the invention, the regulator means is of the type which operates upon the principle of regulating the electrode current and electrode-to-hearth voltage to a fixed or preset ratio, in response to signals responsive to the magnitudes of the electrode current and the electrode-to-hearth voltage. The means for reducing the electrode current reduces the magnitude of the signal responsive to the electrode-to-hearth voltage, simulating a reduction in electrode-to-hearth voltage. This causes the control signal from the regulator to change the electrode position in the direction, relative to the melt, which reduces the electrode current to the value which will return the voltage-current ratio, as determined by the regulator, to its preset value. When the tap change has been completed, the control automatically removes its temporary modification of the electrode-to-hearth voltage responsive signal, allowing the electric arc furnace to return to its normal operation.
BRIEF DESCRIPTION OF THE DRAWINGS Further advantages, uses and embodiments of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:
FIGURE 1 is a schematic diagram of electric arc furnace apparatus and control, constructed and arranged according to the teachings of an embodiment of the invention; and
FIG. 2 is a schematic diagram of tap changer control apparatus which may be used with the electric arc furnace apparatus and control shown in FIG. 1.
DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. 1 in particular, the exemplary form of the invention illustrated comprises an electric arc furnace 10, which includes electrodes 12, 14 and 16, a furnace transformer 18, a regulating transformer 20, and a power supply or source 22 of alternating potential. The position of each of the electrodes is controlled by regulator means which is responsive to the electrical condition of its associated electrode. For example, the position of electrode 16 is controlled by regulator means 24. Since the regulator means for each of the electrodes 12, 14 and 16 would be similar, only regulator means 24 for regulating the position of electrode 16 is shown in FIG. 1.
Electric arc furnace 10, which is illustrated in FIG. 1 as being three-phase, but which may be single-phase if desired, includes a container or hearth 26, which is grounded at 28, and which contains a charge or melt 30. The electrodes 12, 1-4 and 16 are suspended over the melt 30, with their lower ends being in close proximity thereto.
The furnace transformer 18, in this instance, is a three-phase transformer having a primary 32, which may be connected in Y or delta, and a secondary winding 34 which is normally connected delta, as shown in FIG. 1, or in any other desired configuration. The secondary wind- 3 ing 34 is connected to electrodes 12, 14 and 16 via electrical conductors 36, 38 and 40, respectively, and the primary winding 32 is connected to regulating transformer 20 via conductors 42, 44 and 46.
Regulating transformer 20 may be of the type shown in FIG. 1, having a Y-connected excitation winding 48, tapped phase windings 50, 52 and 54, each having a plurality of taps T, numbered consecutively across the tapped winding, a delta connected tertiary (not shown) for the circulation of third harmonic currents, and tap changer means 56, 58 and 60, associated with tapped windings 50, 52 and 54, respectively. Each of the tap changer means 56, 58 and 60 includes tap changer drive and control means, such as tap changer drive and control means 62 associated with tap changer means 60. Tap changer drive and control means 62 has terminals 65, 67 and 69, which are connected to a source 64 of AC control potenital, via conductors 66, 68 and 70, respectively.
Each of the tap changer means 56, 58 and 60, such as tap changer means 60, includes a preventive autotransformer 72, having its outer terimnals connected to the tap changer mechanism 74, which includes two movable contact arms for sequentially changing the effective tap position of the tap changer without interrupting the circuit. The center terminal of the preventive autotransformer 72 is connected to one of the phases of the source 22 of alternating potential. The ends of the tapped winding 54 are connected to the stationary contacts of a reversing switch 76, and the movable contact arm of thereversing switch 76 is connected to conductor 46, which is connected to primary winding 32 of the furnace transformer 18. Thus, the voltage from AC source 22 supplied to the primary winding 32 of arc furnace transformer 18 is modified by the tapped windings 50, 52 and 54, with the voltage across the tapped windings either adding to, or subtracting from, the source potential, depending upon whether the position of the reversing switch connects the tapped windings in series aiding or series opposing, with respect to the source potential 22.
It is to be understood that the tap changing underload electrical system for electric arc furnace shown in FIG. 1 is one of several embodiments which may be used. For example, the regulating transformer may be connected in the secondary of furnace transformer 18, instead of in the primary.
The axial positions of the electrodes 12, 14 and 16 are each controlled by a regulator. The regulator means 24 for controlling the position of electrode 16 relative to the melt 30, may be of any suitable type. For example, it maybe of the type shown in FIG. 1, wherein it obtains a measure of the electrode-to-he'arth voltage, and a meassure of the electrode current, and regulates the position of the electrode to provide a predetermined ratio of current to voltage; or, the regulator may be of the type which obtains a measure 'of the electrode current, and regulates the electrode current to predetermined values at various portions of the electric arc furnace operating cycle.
More specifically, regulator means 24 obtains a measure of the electrode-to-hearth voltage of electrode 16 by a potential transformer 78, which has a primary winding 80 connected between the electrode 16 and the grounded hearth 26, and a secondary winding 82 connected to the input terminals 81 and 83 of a full-wave, singlephase bridge rectifier 84, through an adjustable resistor 86 and an adjustable voltage calibrating resistor 88. For purposes which will be hereinafter described, resistor 88 is shorted by two serially connected normally closed contacts 98 and 100, which are responsive to the tap changer drive and control means 62, as indicated generally by dotted line 102.
A resistor 90 is connected across the positive and negative output terminals 85 and 87, respectively, of bridge rectifier 84, and the unidirectional voltage across resistor is smoothed in a wave filter network, comprising inductor 92 and capacitor 94. One end of capacitor 94 is connected to the negative terminal 87 of bridge rectifier 84, one end of inductor 92 is connected to the positive terminal 85 of bridge rectifier 84, and the other ends of capacitor 94 and inductor 92 are connected together at junction 96. Thus, the magnitude of the unidirectional potential across capacitor 94 is responsive to the electrodeto-hearth voltage of electrode 16.
Regulator means 24 obtains a measure of the electrode current flowing through electrode 16 by a current transformer 104 disposed in inductive relation with conductor 40, which is connected across a resistor 106. The voltage developed across resistor 106, due to the current flow therethrough from current transformer 104, may be transformed to a usable magnitude in a transformer 108. Transformer 108 has a primary winding 110 connected across the resistor 106, and a secondary winding connected through adjustable impedance means 118 to the input terminals 112 and 114 of a full-wave, single-phase bridge rectifier circuit 116. The positive and negative output terminals and 122, respectively, of bridge rectifier 116 are connected across a resistor 124, and the unidirectional potential across resistor 124 is smoothed or filtered in a capacitor 128. One end of inductor 126 is connected to the positive terminal 120 of bridge rectifier 116, one end of capacitor 128 is connected to the negative terminal 122 of bridge rectifier 116, and their remaining ends are connected together at junction 130. Thus, the magnitude of the unidirectional potential across capacitor 124 is responsive to the electrode current of electrode 16.
The unidirectional voltages responsive to the electrode current and voltage are compared, and a signal developed proportional to the deviation of the electrode current and voltage from a predetermined ratio, by connecting the negative terminals 87 and 122 of bridge rectifiers 84 and 116 in common, and by connecting the stationary portion of an adjustable resistor 132, having a movable contact arm 134, across the junctions 96 and 130.
The movable arm of resistor 106 adjusts the magnitude of the voltage developed across it for a given primary electrode current. The movable arm of resistor 106 is adjusted to set the desired ratio of electrode current to electrode-to-hearth voltage.
The movable arm 134 of resistor 132 is adjusted to set the magnitude of the output error signal, if and when the unidirectional voltages responsive to electrode current and voltage changes and become unbalanced.
Any deviation of the electrode current and voltage from the predetermined ratio will provide a polarized error or control signal across adjustable arm 134 and junction 96, which may be amplified in power amplifier means 136. Power amplifier means 136 may be of any suitable type, such as a power amplifier of the solid state, rotating or magnetic amplifier type. The amplified control signal is applied to electrode drive means 140 via conductors 142 and 144. Electrode drive means 140 controls the position of electrode 16. Electrode drive means 140 may include a direct current motor 146 having an armature 148 connected to conductors 142 and 144, and a field winding 150 connected to a source 152 of unidirectional potential through an adjustable resistor 154. The armature 148 of direct current motor 146 may be coupled to a rotatable take up drum 158, which unwinds, or winds, a cable 162 which is guided by pulley to lower or lift the electrode 16, according to the direction of rotation of the armature 148, and thus according to the polarity of the error signal. When the ratio of the electrode current and voltage returns to the preselected ratio, the control signal will drop to zero, and the drive means 140 will be deactivated, leaving the electrode 16 in the new position until receiving another control signal to move the electrode to a different operating position.
During certain portions of the arc furnace operating cycle, the furnace transformer 18 and regulating transformer 20 may be operated above their nameplate ratings. This type of apparatus is designed to accommodate short time overloads without damage. However, if the tap changer apparatus is called upon to change taps during an overload, the arcing contacts of the tap changer mechanism may be damaged to the extent of impairing their useful operating life.
This invention solves this problem by using the signal tfor changing taps to initiate a modification of the regulator circuit to reduce the electrode current, and thus the tap changer current, by a predetermined fixed amount. The tap change signal is also utilized to initiate a delay in the actual mechanical tap change for a short time period, which time period is selected to allow the electrode current to be reduced to its new lower operating value. This reduced current condition is maintained throughout the tap change cycle, and when the tap change has been completed, the regulator is returned to its unmodified condition.
In the embodiment of the invention shown in FIG. 1, wherein an impedance or balanced-beam type regulator is used, the regulator 24 may be modified to reduce the magnitude of the current flow by simulating a reduction in the electrode-to-hearth voltage. This will change the apparent ratio of current to voltage, and the regulator will act to raise the electrode and reduce the electrode current until the current and the simulated electrode-tohearth voltage are in the predetermined selected ratio, which changes the actual ratio.
As shown in FIG. 1, the simulated reduction in electrode-to-hearth voltage is accomplished by connecting an adjustable resistor 86 in series with the electrode-to-hearth voltage responsive signal, and by shorting the resistor with two serially connected normally closed contacts 98 and 100. The opening of either contact will connect the resistor 86 to reduce the magnitude of the signal applied to the bridge rectifier 84. Thus, the unidirectional voltage across capacitor 94 will be reduced, and a control signal will be provided between the arm 134 of resistor 132 and junction 96, which has a polarity which will cause the electrode 16 to be raised until the current responsive signal across capacitor 128 is reduced to the magnitude where it will again provide the predetermined ratio with the voltage across capacitor 94. The selected setting of adjustable resistor 86 will determine the magnitude of the reduction in electrode current.
As will be hereinafter explained, contact 98 is responsive to the initial tap changing signal, opening to initiate the reduction in the electrode current. Delay means is utilized in the tap changer control 62 to provide a predetermined short delay interval between the start of the tap change signal and the start of the actual mechanical tap change. Contact 98 remains open until the tap changer cycle is initiated, at which time contact 100' will open and remain open until the tap change cycle has been completed. Contact 98 closes after contact 100 opens, and contact 100 closes after the completion of the tap change cycle, thus shorting resistor 86 after the completed tap change, returning the regulator 24 to its unmodified condition.
The tap changer drive and control means 62 for operating contacts 98 and 100, and for instituting the time delay between the tap change signal and the actual mechanical tap change, is shown in block form in FIG. 1. FIG. 2 is a schematic diagram of tap changer drive and control means which may be used to provide these functions.
More specifically, terminals 65, 67 and 69 of the tap changer drive and control means 62 shown in FIG. 2, may be connected to a conventional three wire, singlephase source 64 of alternating potential, with terminal 67 being grounded at 162, and with the voltage between terminals 65 and 69 being substantially twice the voltage between terminal 65 and ground, or between terminal 69 and ground. The tap changer drive and control means 62 will first be described as it is conventionally constructed, and then the changes required in the conventional control circuit to practice the teachings of the invention will be described.
Basically, as illustrated in FIG. 2, the tap changer drive and control means 62 includes a tap changer drive motor 164, which drives the tap changer means 60 shown in FIG. 1, as indicated by dotted line 166, a raise relay R having an electromagnetic coil 168 and contacts R1, R2, R3 and R4, a lower relay L having an electromagnetic coil 170 and contacts L1, L2, L3 and L4, a brake relay B having an electromagnetic coil 172 and a contact B1, an operators tap changer control switch 174 having normally open contacts 175 and 177, which is usually located remotely, such as at the operators control panel, a local tap changer control switch 176 having normally open contacts 179 and 181, which is usually located at the tap changer, a remote-local selector switch 178 having a normally closed contact 183 and a normally open contact 185, for selecting which of the tap changer control switches is to operate the tap changer, and contacts TC1, TC2 and T03 which are cam operated contacts responsive to the tap changer means 60 shown in FIG. 1.
The tap changer drive motor may have two windings, a lower winding 180 connected between terminals 182 and -184, which, when energized, drives the motor shaft in a direction to connect the tap changer to a lower numbered tap, and a raise winding 186 connected between terminals 188 and 184, which when energized, drives the motor shaft in a direction to connect the tap changer to a higher numbered tap. The lower winding 180 of tap changer drive motor 164 is connected across terminals 65 and 69 via contact L1 of relay L, which is a normally open contact, i.e., open when the electromagnetic coil 170 of relay L is deenergized. The lower winding 180 is also connected across terminals 65 and 69 via contact R4 of relay R, which is a normally closed contact, and through contact B1 of the brake relay B, which is a normally open contact.
The raise winding 186 of tap changer drive motor 164 is connected across terminals 65 and 69 via contact R1 of relay R, which is a normally open contact. The raise winding 186 is also connected across terminals 65 and 69 via contact L4 of relay L, which is a normally closed contact, and through contact B1 of the brake relay B.
The electromagnetic coil 170 of relay L is connected from terminal 67 to terminal 65 through the circuit which includes normally closed contact R3 from relay R, normally closed contact TC2 from the tap changer apparatus 60, and then either through contacts 179 and of switches 176 and 178, respectively, or through contacts 175 and 183 of switches 174 and 178, respectively. The contact TD1 will be assumed to be a solid conductor for the present discussion. A sealing circuit for relay L is provided between terminals 65 and 67 via normally closed contact R3 of relay R, normally open contact L2 of relay L, and normally open contact TC1 from the tap changer apparatus 60.
The electromagnetic coil 168- of relay R is connected from terminal 67 to terminal 65 through the circuit which includes normally closed contact L3 from relay L, normally closed contact TC3 from the tap changer apparatus 60, and then either through contacts 181 and 185 of switches 176 and 178, respectively, or through contacts 177 and 183 of switches 174 and 178, respectively. Contact TD1 will again be assumed to be a solid conductor for the present discussion. A sealing circuit for relay R is provided between terminals 65 and 67 via normally closed contact L3 of relay L, normally open contact R2 of relay R, and normally open contact TC1 from the tap changer means 60.
Electromagnetic coil 172 of brake relay B is connected across terminals 65 and 67 through normally open contact TC1 of the tap changer apparatus 60, an adjustable resistor 190, and a diode 192. A capacitor 194 is connected across the electrical coil 172 of brake relay B.
Contact TC1 is a normally open contact responsive to the tap changer apparatus 60. It closes once the tap changer starts to move, and remains closed until the tap changer completes its tap change cycle. Normally closed contacts TC2 and TC3 are responsive to cams on the tap changer apparatus, with contact TC2 opening when the tap changer is on its lowest numbered tap position, and with contact TC3 opening when the tap changer is on its highest numbered tap position. Contacts TC2 and TC3 are thus interlocks which prevent the tap changer from being driven past its intended operating range.
In the operation of the tap changer drive and control means 62, the operator actuates the remote-local selector switch 178 to select the tap changer switch which will initiate a tap change. The local tap changer switch is usually used only for adjustment, test and set-up purposes, with the tap changer switch located at the remote operators panel being utilized during the actual operation of the arc furnace 10. If the remote tap changer switch -174 is selected, contact 183 will close and contact 185 will open, as shown in FIG. 2. If the local tap changer switch 176 is selected, contact 183 will open and contact 185 will close. For purposes of example, it will be assumed that the remote tap changer switch 174 is selected. Thus, contact 183 will be closed. If the operator wishes to move the tap changer to a lower numbered tap, he will switch the tap changer switch 174 to the lower position and will keep the switch in this position for a short period of time, which is suflicient to allow the lower relay L to pickup and seal-in. Indicating means, such as a light 196 may be connected to be energized once the relay L seals in. Thus, the operator moves the switch 174 to the lower position until the indicating light 196 is energized, at which time the switch is re- .leased and it returns to its neutral position. If the tap changer is not already on its lowest numbered tap position, contact TC2 will be closed, and normally closed contact R3 of relay R will be closed. Thus, when the operator moves swtch 174 to the lower position, closing contact 175, electromagnetic coil 170 of relay L will be energized. When relay L is energized, contact L1 will close to energize the lower winding 180 of the tap changer drive motor 164, contact L4 will open to remove the brake, contact L3 will open to lock out the raise portion of the circuit, and contact L2 will close to seal in relay L as soon as the tap changer has moved sufficiently to close contact TC1 by cam action. When contact TC1 closes, indicating light 196 will be energized, the operator may then release switch 174, and the circuit will be maintained through contacts R3, L2 and TC1. If the operator had selected the local position of selector switch 178', the operation of the tap changer would be similar, except now contact 185 will be closed, and the operator would initiate the lower cycle of the tap changer by momentarily closing contact 179 of the local tap changer switch 176.
If the operator wishes to change the tap changer to a higher numbered tap position, and the remote tap changer switch 174 is selected by the selector switch 178, the operator will switch tap changer switch 174 to the raise position, and will keep the switch in this position for a short period of time sufficient to allow the raise relay R to pickup and seal-in. Indicating means 96 may also be connected to be energized once relay R seals in. Thus, the operator moves switch 174 to the raise position until indicating light 1% comes on, at which time the switch may be released and returned to its neutral position. If the tap changer is not already on its highest numbered tap position, contact TC3 will be closed, and
normally closed contact L3 of relay L will be closed. Thus, when the operator moves switch 174 to the raise position, closing contact 177, electromagnetic coil 168 of relay R will be energized. When relay R is energized, contact R1 will close to energize the raise winding 186 of the tap changer drive motor 164, contact R2 will open to remove the brake, contact R3 will open to lock out the lower portion of the circuit, and contact R2 will close to seal in relay R as soon as the tap changer has moved sufficiently to close contact TC1 by cam action. When contact TC-1 closes, indicating light 196 will be energized, the operator may release switch 174, and the circuit will be maintained through contacts L3, R2 and TC1. If the operator had selected the local position of the selector switch 178, the operation of the tap changer would be similar, except now contact 185 will be closed, and the operator would initiate the raise cycle of the tap changer by momentarily closing contact 181 of the local tap changer switch 176.
When contact TC1 closes, the electromagnetic coil 172 of the brake relay B will be energized, closing its normally open contact B1. If the lower relay L is energized, contact L4 will be open, and the closing of contact B1 has no effect on the drive motor. If the raise relay R is energized, contact R4 will be open, and the closing of contact B1 again has no effect on the drive motor. When brake relay B is energized through the diode 192, capacitor 194 will be charged to the voltage across the electrical coil 172. The voltage across coil 172 will be determined by the setting of adjustable resistor 190. When the tap changer reaches its new operating position, contact TC1 will open and both the relays R and L will be deenergized, and their contacts R4 and L4 will be closed. The electromagnetic coil 172 of the brake relay B will also be disconnected from the source 64 of alternating potential, but capacitor 194 will discharge through the electromagnetic coil 172, keeping the electrical coil 172 energized for a short period of time after the energized relay R, or the energized relay L, is deenergized. This time will depend upon the voltage magnitude to which capacitor 194 had been charged, and is thus determined by the setting of resistor 190. Thus, contact B1 will be closed for a short period of time after contact TC1 opens, connecting both the raise and lower coils 186 and 180 of the tap changer drive motor across terminals 65 and 69, which applies a braking force to the motor, stopping the tap changer at its newly selected tap position. After capacitor 194 has discharged to a certain voltage magnitude, relay B will drop out, opening its contact B1, and removing the simultaneous energization of the raise and lower windings of the tap changer drive motor 164.
The tap changer drive and control means 62 may be modified, according to the teachings of the invention, by adding contacts 200 and 202 to the operators tap changer control switch 174, with contact 200 being arranged to operate with contact 175, and contact 202 being arranged to operate with contact 177. Or, a single contact may be used if it is arranged to close and open when either contact 175 or contact 177 is closed and opened. Contacts 200 and 202 are connected in parallel, with one side of the parallel arrangement being connected to contact 183 of the remote-local selector switch 178, and the other side being connected to terminal 67 through the electromagnetic coil 204 of a first auxiliary relay AI, which has a normally closed contact 98, and also through the electromagnetic coil 206 of a time delay TD, which has a normally open contact TD1. Contacts 175 and 177 of tap changer control switch 174, instead of being connected directly to contact 183 of selector switch 178, are connected to contact 183 through contact TD1 of time delay relay TD. The electrical coil 210 of a second auxiliary relay All, having a normally closed contact 100, is connected to be energized when cam operated contact TC1 closes. As hereinbefore described, normally .closed contacts 98 and 100 associated with relays AI and AII, are serially connected across resistor 86 in the regulator 24 shown in FIG. 1.
The operation of the tap changer drive and control means, changed in accordance with the teachings of the invention, will now be described. The operator sets the selector switch 178 to operate the tap changer from the operators tap changer switch 174. If the operator wishes to change the tap changer to a lower numbered tap position, tap changer switch 174 would be moved to the lower position, and held there, which closes contacts 175 and 200. The closing of contact 200 energizes the electromagnetic coil- s 204 and 206 of relays AI and TD. When relay AI is energized, contact 98 will open, placing resistor 86 in the regulator circuit, and the regulator will start to reduce the electrode current and the tap changer current. When relay TD is energized, it will start its timed period, with its contact TD1 closing at the end of this period. When contact TD1 closes, the electrode current and tap changer current have already been reduced by the regulator to a lower magnitude. When contact TD1 closes, the electromagnetic coil 170 of relay L will be energized, starting the tap changer drive motor, scaling in relay L through contacts TC1 and L2, and relay AII will be energized opening its contact 100. The indicating light 196 will also be energized when contact TC1 closes. The operator may then release the tap changer switch 174 after the indicating light is energized, as the relay L will then be sealed in, and contact 100 will now be open to maintain the regulator in its modified setting. When tap changer switch 174 is allowed to return to its neutral position, relays AI and TD will be deenergized, allowing contact 98 to close, and relay TD to reset, opening its contact T D1.
If the operator wishes to change the tap changer to a higher numbered tap position, tap changer switch 174 will be moved to the raise position and held there, which closes contacts 177 and 202. The closing of contact 202 energizes the electromagnetic coils 204 and 206 of relays AI and TD. When relay AI is energized, contact 98 will open, placing resistor 86 in the regulator circuit and the regulator will start to reduce the electrode current. When relay TD is energized, it will start its timed period, with its contact TD1 closing at the end of this period. When contact TD1 closes, the electrode current and the tap changer current have already been reduced by the regulator to a lower magnitude. When contact TD1 closes, the electromagnetic coil 168 of relay R will be energized, starting the tap changer drive motor, sealing in relay R through contacts TC1 and R2, and relay All will be energized, opening its contact 100. The indicating light 196 will also be energized when contact TC1 closes. The operator may then release the tap changer switch 174 after the indicating light is energized, as the raise relay R will then be sealed in, and contact 100 will now be open to maintain the regulator in its modified setting. When the tap changer switch 174 is allowed to return to its neutral position, relays AI and TD will be deenergized, allowing contact 98 to close, and relay TD to reset, opening its contact TD1.
If the regulator 24 is of the type which regulates to predetermined current magnitudes at dilferent portions of the arc furnace cycle, instead of being of the impedance type shown, the electrode current may be reduced prior to and during a tap change by connecting a resistor in the circuit of the regulator which measures the current, and setting the regulator to provide the desired regulation with this resistor connected therein. Relays AI and All would each have a normally open contact connected across this resistor, with the contacts being connected in parallel with each other. The operation of either relay would thus short this resistor, simulating an increase in electrode current, causing the regulator to reduce the electrode current and maintain the current at this reduced magnitude until the tap change is completed.
In summary, there has been disclosed a new and improved control system for an electric arc furnace, which automatically lowers the electrode current, and thus the tap changer current, when a tap change is ordered by the operator. The tap change signal immediately initiates the reduction in electrode current, while the application of the tap change signal to the tap changer drive motor control is delayed for a short period of time sufficient to allow the regulator to reduce the electrode current to its new magnitude. After this delay period, the tap change signal is applied to the tap change drive control, initiating the mechanical operation of the tap changer. Thus, the arcing contacts of the tap changer are called upon to interrupt a lower magnitude of current than it would normally have to interrupt at a similar point in the arc furnace cycle, which substantially increases the life of the arcing contacts. The time between maintenance periods may be substantially extended, resulting in less furnace down time.
While the operation of the arc furnace 10 and its associated apparatus have been described relative to operator control, it is to be understood that the teachings of the invention may also be applied to an electric arc furnace which is under computer control, or relay control using power sensitive relays to initiate tap changer lower or raise tap change operating cycles.
Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit there-of, it is intended that all matter contained in the foregoing description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limiting sense.
I claim as my invention: 1. A control system for an electric arc furnace having a hearth, and at least one electrode connectable to a power supply, comprising:
regulator means for proving a control signal responsive to the electrical condition of the electrode circuit,
electrode drive means connected in circuit relation with said regulator means, for moving the electrode in response to the control signal therefrom,
tap changer means for adjusting the voltage applied to the electrode from the power supply,
and means for reducing the magnitude of the current flowing through said tap changer means when said tap changer means is adjusting the voltage applied to the electrode.
2. The control system of claim 1 wherein said regulator means is of the current-voltage balance type, and said means for reducing the magnitude of the current flowing through said tap changer means when said tap changer means is adjusting the voltage applied to the electrode includes means for simulating in the regulator means a reduced voltage between the electrode and the furnace hearth.
3. The control system of claim 1 wherein said tap changer means includes a tap changer mechanism, drive means for said tap changer mechanism, and control means for actuating said drive means in response to a tap change signal, and including delay means for delaying the start of said tap changer drive means after said tap changer control means receives the tap change signal, until the current flowing through said tap changer means has been reduced.
4. The control system of claim 1 wherein said regulator means is of the current-voltage balance type, having a first circuit for providing a signal responsive to a measure of the electrode current, a second circuit for providing a signal responsive to a measure of the electrodeto-furnace hearth voltage, and means comparing the signals of said first and second circuits to provide said control signal, and wherein the means for reducing the magnitude of the current flowing through said tap changer means when said tap changer means is adjusting the voltage applied to the electrode, includes means for reducing the magnitude of the signal provided by said second circuit.
5. The control system of claim 4 wherein said tap changer means includes a tap changer mechanism, drive means for said tap changer mechanism, and control means for actuating said drive means in response to a tap change signal, and including delay means for delaying the start of said tap changer drive means after said tap changer control means receives the tap change signal, until the current flowing through said tap changer means has been reduced.
6. The control system for an electric arc furnace having a hearth and at least one electrode connectable to a power supply comprising:
regulator means of the current-voltage balance type providing a control signal responsive to the electrode current and electrode-to-furnace hearth voltage,
electrode drive means connected in circuit relation with said regulator means for moving the electrode in response to the control signal therefrom,
tap changer means for adjusting the voltage applied to the electrode from the power supply, said tap changer means including a tap changer mechanism, drive means driving the tap changer mechanism, tap changer control means for actuating said drive means in response to a tap change signal, means for reducing the magnitude of the current flowing through said tap changer mechanism when the control means receives a tap change signal, and delay means for delaying the start of said tap change drive means after said tap changer control means receives the tap change signal, until the current flowing through said tap changer mechanism has been reduced.
7. The control system of claim 6 wherein said regulator means has a first circuit for providing a signal responsive to a measure of the electrode current, a second circuit for providing a signal responsive to a measure of the electrode voltage, and means comparing the signals of said first and second circuits to provide said control signal, and wherein the means for reducing the magnitude of the current flowing through said tap changer mechanism includes means for reducing the magnitude of the signal provided by said second circuit.
8. The control system of claim 7 wherein the means for reducing the magnitude of the signal provided by the second circuit of said regulator means includes impedance means which is connected to reduce the magnitude of the signal provided by said second circuit from the time the tap change signal is applied to the tap changer control means, until said tap changer mechanism completes the change.
References Cited UNITED STATES PATENTS 7/1962 Ravenscroft 13-13 X 8/1965 Wilson 323-435 U.S. Cl. X.R. 13-13; 32343.5
US711624A 1968-03-08 1968-03-08 Control system for electric arc furnace Expired - Lifetime US3493664A (en)

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3657455A (en) * 1970-04-30 1972-04-18 Continental Oil Co Temperature regulator
US4131754A (en) * 1977-05-02 1978-12-26 Consarc Corporation Automatic melt rate control system for consumable electrode remelting
EP0197329A2 (en) * 1985-04-03 1986-10-15 MANNESMANN Aktiengesellschaft Method for balancing electrical valves in three-phase arc furnaces, and device to carry out the method
US4677643A (en) * 1984-03-09 1987-06-30 Licentia Patent-Verwaltungs-Gmbh Device for feeding one or a plurality of electrodes in an electrothermal furnace
EP3591821A1 (en) * 2018-07-04 2020-01-08 Vestas Wind Systems A/S Controlled switching current of an on-load tap changer of a wind turbine

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2732873A1 (en) * 1977-07-21 1979-02-15 Leybold Heraeus Gmbh & Co Kg ARRANGEMENT FOR REGULATING THE DEPTH OF SUBMERGING OF MELTING ELECTRODES IN ELECTRO-SLASK MELTING FURNACES
DE102006050624A1 (en) * 2006-10-26 2008-04-30 Siemens Ag Reactance connecting device for transformer in electric arc furnace, has induction coil and standalone load switch for adjusting reactance of induction coil under load, where induction coil is provided with multiple tapping points

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3043894A (en) * 1958-05-16 1962-07-10 British Iron Steel Research Electric arc furnaces
US3204176A (en) * 1961-04-26 1965-08-31 Mc Graw Edison Co Tap changing furnace transformer

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3043894A (en) * 1958-05-16 1962-07-10 British Iron Steel Research Electric arc furnaces
US3204176A (en) * 1961-04-26 1965-08-31 Mc Graw Edison Co Tap changing furnace transformer

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3657455A (en) * 1970-04-30 1972-04-18 Continental Oil Co Temperature regulator
US4131754A (en) * 1977-05-02 1978-12-26 Consarc Corporation Automatic melt rate control system for consumable electrode remelting
US4677643A (en) * 1984-03-09 1987-06-30 Licentia Patent-Verwaltungs-Gmbh Device for feeding one or a plurality of electrodes in an electrothermal furnace
EP0197329A2 (en) * 1985-04-03 1986-10-15 MANNESMANN Aktiengesellschaft Method for balancing electrical valves in three-phase arc furnaces, and device to carry out the method
US4663764A (en) * 1985-04-03 1987-05-05 Fried. Krupp Gesellschaft mit beschrankter/Haftung Method and apparatus for electrically balancing three-phase arc discharge furnaces
EP0197329B1 (en) * 1985-04-03 1992-04-29 MANNESMANN Aktiengesellschaft Method for balancing electrical valves in three-phase arc furnaces, and device to carry out the method
EP3591821A1 (en) * 2018-07-04 2020-01-08 Vestas Wind Systems A/S Controlled switching current of an on-load tap changer of a wind turbine
US10910975B2 (en) 2018-07-04 2021-02-02 Vestas Wind Systems A/S Controlled switching current of an on load tap changer of a wind turbine
US11424704B2 (en) 2018-07-04 2022-08-23 Vestas Wind Systems A/S Controlled switching current of an on-load tap changer of a wind turbine

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