Electrode lift control device

Abstract

To provide an electrode lifting control device which optimizes the efficiency of dissolution of scrap.SOLUTION: An electrode lifting control device comprises: impedance deviation calculation means which calculates an impedance deviation by subtracting a previously set reference impedance from a calculated actual measurement impedance on the basis of the arc current and arc voltage; and inverter target speed setting means which outputs a speed reference to an inverter that drives a motor for lifting / lowering an electrode on the basis of operation speed reference characteristics expressing the speed reference of the lifting / lowering speed of the electrode with respect to the impedance deviation. In the operation speed reference characteristics, the operation speed reference becomes 0 when the impedance deviation is 0, and the operation speed reference is not 0 and the inclination of the operation speed reference with respect to the impedance deviation is equal to or greater than 0 when the impedance deviation is not 0.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] An embodiment of the present invention relates to an electrode lifting control device for an AC arc furnace. [Background technology]

[0002] In an AC arc furnace, scrap is melted by an arc discharge between the electrode and the scrap. An electrode lifting controller maintains the arc discharge by adjusting the distance between the electrode and the scrap, heating and melting the scrap.

[0003] The electrode lifting / lowering control device may be, for example, a constant impedance control device that controls the impedance between the electrode and the scrap to a constant value. In the constant impedance control, the electrode is raised and lowered to control the actual impedance value to follow a target value.

[0004] When the deviation between the target and measured impedance values is small, attempting to make the measured impedance value follow the target value can cause hunting in the electrode lifting and lowering operation, resulting in unstable behavior. Therefore, when the impedance deviation ΔZ is small, a dead zone is set to stop the electrode lifting and lowering.

[0005] Although providing a dead zone in constant impedance control improves the stability of the machine control system, it does not necessarily result in efficient energy input to the scrap under optimal arc discharge. Depending on the state of the arc discharge, the arc current may decrease, resulting in insufficient energy input to the scrap and longer time required to melt the scrap. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-126417 Summary of the Invention [Problem to be solved by the invention]

[0007] Embodiments of the present invention provide an electrode lift control system that optimizes the efficiency of scrap melting. [Means for solving the problem]

[0008] An embodiment of the present invention is an electrode lifting control device that controls the elevation of the electrode of an AC arc furnace that melts scrap by arc discharge formed between the electrode and the scrap. The electrode lifting control device includes: impedance deviation calculation means that calculates an actual measured impedance based on an arc current flowing through the electrode and an arc voltage between the electrode and the scrap, and calculates an impedance deviation by subtracting a predetermined reference impedance from the actual measured impedance; and inverter target speed setting means that outputs an operating speed reference, which is a speed reference for the electrode lifting speed relative to the impedance deviation, to an inverter that drives an electric motor that lifts and lowers the electrode by mechanical coupling with the electrode, based on an operating speed reference characteristic. The operation speed reference characteristic has a high-precision control characteristic in a range from 0 to a first threshold value where the impedance deviation is smaller than a positive first threshold value and larger than a second threshold value where the impedance deviation is smaller than a negative second threshold value, and a first sensitivity characteristic having a linear characteristic that is continuous between the first threshold value and the second threshold value and has a positive coefficient in a range from equal to or larger than the first threshold value and equal to or smaller than the second threshold value. The high-precision control characteristic includes a range having a differential coefficient smaller than the coefficient of the first sensitivity characteristic, and the range includes a range from a third threshold value where the impedance deviation is larger than 0 and smaller than the first threshold value to the first threshold value, and a range from the second threshold value to a fourth threshold value where the impedance deviation is larger than the second threshold value and smaller than 0. [Effects of the Invention]

[0009] According to an embodiment, an electrode lift control device that optimizes the efficiency of scrap melting is provided. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a schematic block diagram illustrating an electrode lift control device according to an embodiment. [Figure 2] 5A and 5B are schematic diagrams for explaining the operation of the electrode lifting control device according to the embodiment. [Figure 3] 5A and 5B are schematic diagrams for explaining the operation of the electrode lifting control device according to the embodiment. [Figure 4] 5 is a graph illustrating the operation of the electrode lift control device according to the embodiment. FIG. [Figure 5] 5A and 5B are schematic diagrams for explaining the operation of the electrode lifting control device according to the embodiment. [Figure 6] 10A and 10B are schematic diagrams for explaining the operation of an electrode lifting control device according to a modified example of the embodiment. [Figure 7] FIG. 10 is a graph showing the operational characteristics of an electrode lifting control device according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0011] Each embodiment will be described below with reference to the drawings. The drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the size ratio between parts, etc. are not necessarily the same as those in reality. Furthermore, even when the same part is shown, the dimensions and ratios may be different depending on the drawing. In the present specification and the drawings, elements similar to those described above with reference to the previous drawings are designated by the same reference numerals, and detailed descriptions thereof will be omitted as appropriate.

[0012] FIG. 1 is a schematic block diagram illustrating an electrode lift control device according to an embodiment. FIG. 1 shows a portion of an AC arc furnace including an electrode lifting control device 6 and an electrode 2 whose elevation is controlled by the electrode lifting control device 6. In the AC arc furnace, a furnace transformer 1 is connected to an AC bus. The electrode 2 is connected to the secondary side of the furnace transformer 1. A furnace body 3 is disposed below the electrode 2, and scrap 4 is charged into the furnace body 3. A built-in current transformer 15 is provided on the secondary side of the furnace transformer 1, and the current transformer 15 detects the arc current IFB flowing between the electrode 2 and the scrap 4. The current transformer 15 does not have to be built-in, as long as it can detect the line current flowing through the furnace transformer 1. An auxiliary transformer 16 is connected to the secondary side of the furnace transformer 1 to detect the arc voltage VFB between the electrode 2 and the scrap 4.

[0013] The electric motor 8 is mechanically coupled to the electrode 2 via an electrode lifting mechanism, which is simply shown by a dashed line in Fig. 1. The electrode 2 descends toward the scrap 4 charged in the furnace body 3 and ascends away from the scrap 4, depending on the direction of rotation of the electric motor 8.

[0014] The electrode lifting control device 6 is connected to a current transformer 15 and an auxiliary transformer 16. The electrode lifting control device 6 receives the detected values of the arc current IFB detected by the current transformer 15 and the arc voltage VFB detected by the auxiliary transformer 16.

[0015] The electrode lift control device 6 is connected to an inverter 7 that drives an electric motor 8. The inverter 7 outputs a pulse signal PC, which is generated in accordance with the rotation speed and rotation direction of the electric motor 8, to the electrode lift control device 6.

[0016] The electrode lifting control device 6 calculates the measured impedance ZFB based on the arc current IFB and the arc voltage VFB. The electrode lifting control device 6 estimates the position of the electrode 2 based on the pulse signal PC. When the electrode 2 is in the high-speed lifting region, the electrode lifting control device 6 outputs a high-speed speed reference V1* to the inverter 7 regardless of the measured impedance ZFB. When the electrode 2 is in the operating region, the electrode lifting control device 6 outputs an operating speed reference V2* to the inverter 7 so that the measured impedance ZFB follows the preset reference impedance ZREF. The high-speed lifting region and the operating region are regions in which the electrode 2 lifts and lowers, and the operating region is a region closer to the scrap 4 than the high-speed lifting region. The upper limit value of the high-speed speed reference V1* is set sufficiently larger than the upper limit value of the operating speed reference V2*.

[0017] The inverter 7 acquires a speed feedback signal VFBK from a speed sensor provided in the electric motor 8. The inverter 7 drives the electric motor 8 to raise and lower the electrode 2 so that the speed feedback signal VFBK follows the high-speed speed reference V1* or the operating speed reference V2* output from the electrode lifting control device 6.

[0018] In an AC arc furnace, at least one electrode 2 is provided for each phase of the three-phase AC. The electrodes 2 provided for each phase are subjected to the above-described electrode lift control by an electrode lift control device 6. The following describes the electrode lift control for one of the three phases, but unless otherwise specified, it is assumed that the electrode lift control device 6 described below is provided for each phase of the three-phase AC and controls the AC arc furnace.

[0019] The configuration of the electrode lifting control device 6 will be described in detail. The electrode lift control device 6 includes an impedance deviation calculation unit 9 and an inverter target speed setting unit 12. The electrode lift control device 6 further includes an electrode position measurement unit 13, an inverter operation control unit 11, and an inverter acceleration / deceleration setting switching unit 14. The electrode lift control device 6 also includes an electrode operation stop command unit 10.

[0020] In this example, the electrode lifting control device 6 is provided in the electrode lifting main panel 5. For example, the electrode lifting main panel 5 is installed in an operation room in a plant where an AC arc furnace is installed. For example, the electrode lifting main panel 5 is connected to a control panel on which buttons, levers, gauges, etc. are arranged to be operated by an operator who operates the AC arc furnace, and the electrode lifting main panel 5 also includes a connection interface with the control panel. The electrode lifting control device 6 is, for example, a programmable logic controller (PLC), and the functions of each unit described below are realized by one or more steps constituting a PLC program.

[0021] The impedance deviation calculation unit (impedance deviation calculation means) 9 receives as input the detection values of the arc current IFB detected by the current transformer 15 and the arc voltage VFB detected by the auxiliary transformer 16. The impedance deviation calculation unit 9 calculates the measured impedance ZFB using the respective detection values of the arc current IFB and the arc voltage VFB. A reference impedance ZREF is set in advance in the impedance deviation calculation unit 9. The impedance deviation calculation unit 9 outputs the impedance deviation ΔZ obtained by calculating the difference between the reference impedance ZREF and the measured impedance ZFB to the inverter operation control unit 11.

[0022] The electrode operation stop command unit 10 is connected to operation buttons and the like provided on the operation panel. When an operation is performed by an operator operating the operation buttons and the like, the electrode operation stop command unit 10 outputs an operation command to the inverter operation control unit 11. When a stop operation is performed by the operator, the electrode operation stop command unit 10 outputs a stop command to the inverter operation control unit 11.

[0023] Based on the operation command, inverter operation control section (inverter operation control means) 11 executes processing to start the lifting and lowering operation of electrode 2. Based on the stop command, inverter operation control section 11 executes processing to stop the lifting and lowering operation of electrode 2.

[0024] When inverter operation control unit 11 receives an operation command, it determines the current position of electrode 2 based on the position information of electrode 2 output by electrode position measurement unit 13. The position of electrode 2 at the start of operation is, for example, an initial value of 0, which is the upper limit position to which electrode 2 can be raised or lowered. Based on the information on the current position of electrode 2, inverter operation control unit 11 selects and outputs either high-speed reference V1* or operating speed reference V2* as the speed reference for raising or lowering electrode 2. The upper limit value of high-speed reference V1* is greater than the upper limit value of operating speed reference V2*.

[0025] As will be described in detail later with reference to FIGS. 2 and 3 , in the electrode lift control device 6 of this embodiment, the region in which the electrode 2 can be lifted and lowered is divided into at least two regions. The region in which the electrode 2 is located farther from the scrap 4 is defined as a high-speed lift region R1, and the region in which the electrode 2 is located closer to the scrap 4 than the high-speed lift region R1 is defined as an operating region R2. When the inverter operation control unit 11 determines that the position of the electrode 2 is in the high-speed lift region R1, it selects a high-speed speed reference V1* and outputs it to the inverter target speed setting unit 12. When the inverter operation control unit 11 determines that the position of the electrode 2 is in the operating region R2, it outputs an operating speed reference V2* to the inverter target speed setting unit 12. The inverter operation control unit 11 outputs the impedance deviation ΔZ to the inverter target speed setting unit 12 together with the high-speed speed reference V1* or the operating speed reference V2*.

[0026] The inverter operation control unit 11 acquires the detection values output from the current transformer 15 and the auxiliary transformer 16. For example, by acquiring the detection value of the arc current IFB from the current transformer 15 at the start of operation, the inverter operation control unit 11 stores the position of the electrode 2 where arc discharge starts between the electrode 2 and the scrap 4 as the scrap touch position Pc. The scrap touch position Pc may be detected by acquiring the arc voltage VFB instead of or together with the arc current IFB. Hereinafter, unless otherwise specified, the start of arc discharge will be determined by detecting the arc current IFB.

[0027] 5, the inverter operation control unit 11 detects the arc current IFB and limits the arc current IFB to a predetermined current limit value. The predetermined current limit value is set according to a preset rate of change of a speed reference, and the current limit value in the operating region is set to a value greater than the current limit value in the high-speed lift region.

[0028] An electrode position measuring unit (electrode position measuring means) 13 acquires a pulse signal PC output from the inverter 7 in accordance with the rotation speed and rotation direction of the electric motor 8, and calculates the position of the electrode 2 by integrating the pulse signals. The electrode position measuring unit 13 outputs information on the calculated position of the electrode 2 to the inverter operation control unit 11. In this example, the inverter 7 has a pulse counter function, and the electrode position measuring unit 13 uses the pulse signal PC output by the pulse counter function, but the pulse counter function need not necessarily be built into the inverter 7, and may be provided on the electric motor side.

[0029] The inverter target speed setting unit (inverter target speed setting means) 12 outputs an appropriate speed reference to the inverter acceleration / deceleration setting switching unit 14 based on the high-speed speed reference V1*, the operating speed reference V2* and the impedance deviation ΔZ output from the inverter operation control unit 11.

[0030] More specifically, when inverter target speed setting unit 12 acquires high speed reference V1*, it outputs a preset high speed reference upper limit value V1 as the high speed reference V1*, regardless of the impedance deviation ΔZ, to inverter acceleration / deceleration setting switch unit 14. When inverter target speed setting unit 12 acquires operating speed reference V2*, it extracts operating speed reference V2* set in accordance with the impedance deviation ΔZ and outputs it to inverter acceleration / deceleration setting switch unit 14.

[0031] As will be described in detail later with reference to FIG. 4, the inverter target speed setting unit 12 extracts an operation speed reference V2* corresponding to the impedance deviation ΔZ from the operation speed reference characteristic 12a. When ΔZ=0, V2*=0 of the operation speed reference characteristic 12a, and when ΔZ≠0, V2*≠0. The differential coefficient, i.e., the slope, of the operation speed reference characteristic 12a is positive, and depending on the range of ΔZ, the slope may be partially zero. In other words, the operation speed reference characteristic 12a has a characteristic such that d(V2*) / d(ΔZ)≧0, and V2* is an increasing function of ΔZ.

[0032] More specifically, the operating speed reference characteristic 12a has a sensitivity characteristic with a linear characteristic when ΔZ≧+ΔZ1 (>0, first threshold value) and when ΔZ≦-ΔZ2 (<0, second threshold value). When -ΔZ2<ΔZ<+ΔZ1, the characteristic includes a derivative coefficient smaller than the coefficient of the sensitivity characteristic. For example, the derivative coefficient when -ΔZ2<ΔZ<+ΔZ1 is set smaller than the coefficient of the sensitivity characteristic in most of the range of -ΔZ2<ΔZ<+ΔZ1. Most of the range of -ΔZ2<ΔZ<+ΔZ1 refers to the range excluding the vicinity of 0 for ΔZ. For example, when ΔZ>0, the derivative coefficient is set smaller than the coefficient of the sensitivity characteristic in the range of 10% of +ΔZ1 (third threshold value) or more, and when ΔZ<0, the derivative coefficient is set smaller than the coefficient of the sensitivity characteristic in the range of 10% of -ΔZ2 (fourth threshold value) or less. The threshold value defining the vicinity of ΔZ is set to an appropriate value.

[0033] The electrode lifting control device 6 according to the embodiment controls the distance between the electrode 2 and the scrap 4 within the entire range of the impedance deviation ΔZ based on the operating speed reference characteristic 12a, thereby enabling highly accurate and stable electrode lifting control.

[0034] The data of the operation speed reference characteristic 12a is stored in advance, for example, in a memory unit of the electrode lifting control device 6. The data format of the operation speed reference characteristic 12a is not important. The operation speed reference characteristic 12a may be stored in the memory unit, for example, in table format, or may be stored in the memory unit, for example, in the form of a mathematical formula including an approximation formula.

[0035] The inverter acceleration / deceleration setting switching unit (inverter acceleration / deceleration setting switching means) 14 applies a speed reference change rate during acceleration and a speed reference change rate during deceleration for each speed reference based on the high-speed speed reference V1* or the operating speed reference V2* output from the inverter target speed setting unit 12. The inverter acceleration / deceleration setting switching unit 14 sequentially outputs the speed reference to which the speed reference change rates during acceleration and deceleration have been applied to the inverter 7.

[0036] More specifically, for the high-speed speed reference V1*, a reference speed change rate during high-speed acceleration (first reference speed change rate during acceleration) and a reference speed change rate during high-speed deceleration (first reference speed change rate during deceleration) are set in advance. Also, for the operating speed reference V2*, a reference speed change rate during operating acceleration (second reference speed change rate during acceleration) and a reference speed change rate during operating deceleration (second reference speed change rate during deceleration) are set in advance. The inverter acceleration / deceleration setting switch 14 determines the magnitude of the acquired speed reference and applies one of the reference speed change rates during acceleration and deceleration.

[0037] The operation of the electrode lifting control device 6 according to the embodiment will be described in detail. First, the areas in which the electrode 2 moves up and down and the setting of the speed of movement for each area will be described. 2 and 3 are schematic diagrams for explaining the operation of the electrode lift control device according to the embodiment. FIG. 2 shows the area in which the electrode 2 can be raised and lowered. As shown in FIG. 2 , the electrode 2 can be raised and lowered between an upper electrode limit position Pa and a lower electrode limit position Pd. For example, limit switches are provided at the upper electrode limit position Pa and the lower electrode limit position Pd, respectively. The electrode lifting control device 6 recognizes whether the electrode 2 is at the upper or lower limit position based on signals from the limit switches and prohibits the electrode 2 from being raised or lowered beyond those limits. In the following specific examples, unless otherwise specified, the upper electrode limit position Pa is the initial position of the electrode 2 at the start or restart of operation. Restarting operation refers to the resumption of operation after an abnormal condition is detected during operation and the abnormal condition is eliminated after the electrode 2 is retracted. An example of an abnormal condition during operation is when the arc current between the electrode 2 and the scrap 4 becomes excessive.

[0038] A deceleration completion position Pb is set between the electrode upper limit position Pa and the electrode lower limit position Pd. Furthermore, a scrap touch position Pc is set between the deceleration completion position Pb and the electrode lower limit position Pd. A high-speed lifting region R1 is defined as the region from the electrode upper limit position Pa to the deceleration completion position Pb. An operational region R2 is defined as the region from the deceleration completion position Pb to the scrap touch position Pc. An operable region R3 is defined as the region from the deceleration completion position Pb to the electrode lower limit position Pd.

[0039] The scrap touch position Pc is set by the inverter operation control unit 11 detecting that the electrode 2 has descended and that an arc current IFB has flowed between the electrode 2 and the scrap 4. The inverter operation control unit 11 stores the position where the arc current IFB begins to flow and sets it as the scrap touch position Pc. The scrap touch position Pc is set in this manner every time operation is started and restarted. The scrap touch position Pc is updated and stored every time a scrap touch is performed.

[0040] The deceleration completion position Pb is set at a position spaced upward from the scrap touch position Pc by a margin distance ΔL. The margin distance ΔL is a fixed value that is set in advance to absorb calculation errors in the movement distance of the electrode 2. An initial value for the deceleration completion position Pb is set in advance at the start of operation. When the scrap touch position Pc is updated, the deceleration completion position Pb is also updated. The area above the deceleration completion position Pb is designated as a high-speed lifting region R1, and the area below is designated as an operational region R2 and an operable region R3. The operable region R3 is the range in which the scrap touch position Pc can be updated and includes the range in which the electrode can be lowered to its maximum.

[0041] In the high-speed lifting region R1, the electrode 2 moves up and down at a high-speed standard upper limit V1. In the operating region R2, the electrode 2 moves up and down at an operating speed standard V2* determined according to the impedance deviation ΔZ, with an upper limit of the operating speed standard upper limit V2. The value of the high-speed standard upper limit V1 is sufficiently greater than the value of the operating speed standard upper limit V2.

[0042] FIG. 3 shows the changes over time of the high speed reference V1* and the operating speed reference V2*. The dashed line in Figure 3 shows the time variation of the high speed reference V1*, and the solid line in Figure 3 shows the time variation of the operating speed reference V2*.

[0043] 3, in the case of high-speed speed reference V1*, the electrode 2 is accelerated at a reference speed change rate during high-speed acceleration (first reference speed change rate) α1, and when it reaches the high-speed speed reference upper limit value V1, it travels at a constant speed at the high-speed speed reference upper limit value V1 and is decelerated at a reference speed change rate during high-speed deceleration (first reference speed change rate during deceleration) β1. In the case of operating speed reference V2*, it is accelerated at a reference speed change rate during operating acceleration (second reference speed change rate) α2, and in this example, it travels at a constant speed at the operating speed reference upper limit value V2 and is decelerated at a reference speed change rate during operating deceleration (second reference speed change rate during deceleration) β2.

[0044] The reference speed rate of change during high-speed acceleration α1, the reference speed rate of change during high-speed deceleration β1, the reference speed rate of change during operational acceleration α2, and the reference speed rate of change during operational deceleration β2 are preset fixed values, and are applied by the inverter acceleration / deceleration setting switch unit 14. Furthermore, the high-speed reference speed upper limit value V1 is a preset fixed value, the operating speed reference V2* is set according to the impedance deviation ΔZ, and the operating speed reference upper limit value V2 is the upper limit of the operating speed reference V2*.

[0045] In the electrode lifting control device 6 of this embodiment, the relationship between the reference speed change rate during high-speed acceleration α1 and the reference speed change rate during operational acceleration α2 is set to be α1<α2. Also, the relationship between the reference speed change rate during high-speed deceleration β1 and the reference speed change rate during operational deceleration β2 is set to be β1<β2.

[0046] In the high-speed lifting region R1, the high-speed acceleration speed reference rate of change α1 and the high-speed deceleration speed reference rate of change β1 are set to sufficiently small values. Therefore, even if the high-speed speed reference upper limit value V1 is set to a sufficiently large value, overshooting and undershooting due to time changes in the high-speed speed reference V1* can be suppressed. Therefore, the electrode lifting control device 6 can quickly lift and lower the electrode 2 to a desired position with high accuracy in the high-speed lifting region R1.

[0047] In the case of the operation region R2, the reference speed change rate during operation acceleration α2 and the reference speed change rate during operation deceleration β2 are set to sufficiently large values. On the other hand, the magnitude of the reference operating speed upper limit V2 is set to a value smaller than the magnitude of the reference high speed upper limit V1. As a result, overshooting and undershooting due to changes in the operating speed standard V2* over time can be suppressed, and the electrode lifting control device 6 can quickly lift and move the electrode 2 to a desired position with high precision.

[0048] Setting of the lifting speed of the electrode 2 in the operation region R2 will be described using the operation speed reference characteristic 12a. FIG. 4 is a graph showing the operation of the electrode lift control device according to the embodiment. 4 is a graph showing the operating speed reference characteristic 12a. In this graph, the horizontal axis represents the impedance deviation ΔZ, the vertical axis represents the operating speed reference V2*, and the operating speed reference V2* is plotted against the impedance deviation ΔZ. In the electrode lifting control device 6 according to the embodiment, when the electrode 2 is in the operation region R2, the operation speed reference V2* corresponding to the impedance deviation ΔZ is applied in accordance with the operation speed reference characteristic 12a.

[0049] In the operating speed reference characteristic 12a, the range between ΔZ≦-ΔZ2 and +ΔZ1≦ΔZ is called the sensitivity setting control range SR. +ΔZ1 is the positive threshold value (first threshold value) of ΔZ, and -ΔZ2 is the negative threshold value (second threshold value). In the sensitivity setting control range SR, three straight lines -S1 to -S3 with different coefficients, i.e., slopes, are drawn in the range of ΔZ≦-Z2, and three straight lines +S1 to +S3 with different slopes are drawn in the range of +ΔZ1≦ΔZ. These straight lines are called sensitivity setting lines. These sensitivity setting lines indicate that the operating speed reference V2* has different sensitivities to the impedance deviation ΔZ. The operator selects one of the sensitivity setting lines depending on, for example, the state of scrap 4.

[0050] In the operating speed reference characteristic 12a, the range of -ΔZ2<ΔZ<+ΔZ1 is called the high-precision control range HR. Also, the operating speed reference characteristic in the high-precision control range HR is sometimes called the high-precision control curve. In the operating speed reference characteristic 12a, when ΔZ = -ΔZ2, V2* = -V22, and when ΔZ = +ΔZ1, V2* = +V21. In other words, in the operating speed reference characteristic 12a, the high-precision control curve is continuous with each sensitivity setting line, and when any of the sensitivity setting lines is selected by the operator, the high-precision control curve is common.

[0051] As shown in Fig. 4, in the high-precision control curve, when ΔZ = 0, V2* = 0. In the high-precision control curve, V2* is set to > 0 in the range of ΔZ > 0, and V2* is set to < 0 in the range of ΔZ < 0. Over the entire range of the operation speed reference characteristic 12a, d(V2*) / d(ΔZ) ≥ 0, and preferably d(V2*) / d(ΔZ) > 0. In other words, the operation speed reference V2* is preferably a monotonically increasing function of the impedance deviation ΔZ.

[0052] In the high-precision control curve, +V21=d(La) / dt, where La is the distance between the electrode 2 and the scrap 4.

[0053] Alternatively, in the range of ΔZ > 0, d(V2*) / d(ΔZ) is set to a slope smaller than any of the sensitivity setting lines, except near ΔZ = 0. Similarly, in the range of ΔZ < 0, d(V2*) / d(ΔZ) is set to a slope smaller than any of the sensitivity setting lines, except near ΔZ = 0. In other words, the high-precision control curve is set to suppress the sensitivity of the electrode 2 lift speed over most of the ΔZ range compared to the sensitivity setting line. Most of the ΔZ range is, for example, the range between +ΔZ1 × 10% and -ΔZ2 × 10%. Here, +ΔZ1 × 10% (the third threshold) and -ΔZ2 × 10% (the fourth threshold) are set to appropriate values based on the detection accuracy of the arc current IFB and arc voltage VFB, the calculation accuracy of the measured impedance ZFB, the setting accuracy of the high-precision control curve, etc.

[0054] A more specific explanation will be given. The impedance deviation ΔZ is calculated by the following equation (1). ΔZ=ZFB-ZREF =(VFB / IFB)-ZREF (1)

[0055] From Equation (1), when ZFB > ZREF, ΔZ > 0. Assuming VFB is constant, the detected IFB is smaller than the current value estimated from ZREF. Therefore, the electrode lifting and lowering control device 6 lowers the electrode 2 closer to the scrap 4 to increase IFB and make ZFB follow ZREF.

[0056] From Equation (1), when ZFB < ZREF, ΔZ < 0. Assuming VFB is constant, the detected IFB is larger than the current value estimated from ZREF. Therefore, the electrode lifting and lowering control device 6 raises the electrode 2 away from the scrap 4 to decrease IFB and make ZFB follow ZREF.

[0057] In the operating speed reference characteristic 12a, the positive direction of V2* is the direction in which the electrode 2 descends, and the greater ΔZ is, the greater the descending speed of the electrode 2. Also, in the operating speed reference characteristic 12a, the negative direction of V2* is the direction in which the electrode 2 ascends, and the greater |ΔZ| is, the greater the ascending speed of the electrode 2.

[0058] In the high-precision control range HR, in the range where ΔZ > 0, V2* > 0. Therefore, assuming VFB is constant, when there is a small variation in which IFB decreases from the current value estimated from ZREF, the electrode lifting and lowering control device 6 lowers the electrode 2 at a small V2* set according to the small ΔZ.

[0059] In the range where ΔZ < 0 in the high-precision control range HR, since V2* < 0, when there is an increase in a small IFB, the electrode lifting and lowering control device 6 raises it at a small operating speed reference V2* set according to the small ΔZ.

[0060] In the range where ΔZ > 0 in the sensitivity setting control range SR, the electrode lifting and lowering control device 6 lowers the electrode 2 at an operating speed reference V2* of sufficient magnitude according to the sensitivity straight line set by the operator.

[0061] In the range of ΔZ<0 in the sensitivity setting control range SR, the electrode lifting control device 6 lifts the electrode 2 at a sufficiently large operating speed reference V2* in accordance with the sensitivity line set by the operator.

[0062] In this way, the electrode lift control device 6 according to the embodiment lowers and raises the electrode 2 to eliminate minute fluctuations in the arc current IFB that occur in the operation region R2. Even for large changes in the arc current IFB that occur in the operation region R2, the electrode lift control device 6 according to the embodiment raises and lowers the electrode 2 according to the sensitivity set by the operator to quickly eliminate the fluctuations in the arc current IFB and perform constant impedance control.

[0063] Next, the current limiting characteristics of the inverter 7 when it is overloaded in the high-speed lifting region R1 and the operating region R2 will be described. FIG. 5 is a schematic diagram for explaining the operation of the electrode lift control device according to the embodiment. Figure 5 shows the time characteristics of the current limit value when the inverter 7 is overloaded. Current limit characteristic J1 represents the time characteristics of the current limit value of the inverter 7 in the high-speed lift-down region R1. Current limit characteristic J2 represents the time characteristics of the current limit value of the inverter 7 in the operating region R2. Time t1 is the time required to accelerate the electrode 2 from 0 to the high-speed speed reference upper limit value V1 at the high-speed acceleration speed reference rate of change α1. Time t2 is the time required to accelerate the electrode 2 from 0 to the operating speed reference upper limit value V2 at the operating acceleration speed reference rate of change α2. As described above, V1 > V2 and α1 < α2, so t1 > t2.

[0064] Generally, in an inverter, the overload capacity is expressed as the time integral of the output torque, and since the output torque is proportional to the inverter's output current, it can be calculated as the time integral of the output current. When the inverter's output current is limited, the overload capacity can be calculated by multiplying the current limit value by time, as in the example of Figure 5. If the overload capacity is constant, in the electrode lifting control device 6 according to the embodiment, t1 > t2, and therefore the current limit value J1max in the high-speed lifting region R1 is set to a value smaller than the current limit value J2max in the operating region R2.

[0065] More specifically, the overload capacity is constant regardless of time, and therefore the relationship of the following equation (2) holds. J1max×t1=J2max×t2 (2)

[0066] That is, in the electrode lifting control device 6 according to the embodiment, a current limit value according to the lifting region is applied so that the electrode 2 can achieve appropriate acceleration.

[0067] Although the above description has been given in the case of acceleration, the same applies when decelerating the electrode 2, because V1>V2 and β1<β2, the current limit time in the high-speed lift-up region R1 is set longer than the current limit time in the operating region R2. The current limit value is set so that the product of the current limit value and the current limit time is constant.

[0068] Next, a series of lifting and lowering operations of the electrode 2 will be described. At the start of operation, the electrode 2 is positioned, for example, at the electrode upper limit position Pa. A limit switch provided at the electrode upper limit position Pa sets the output of the electrode position measuring unit 13 to an initial value, for example, 0.

[0069] An operation command is generated by an operator's operation. Upon receiving the operation command, the inverter operation control unit 11 determines whether to output the high-speed speed reference V1* or the operating speed reference V2* to the inverter target speed setting unit 12 based on the position information of the electrode 2 output from the electrode position measuring unit 13. At the start of operation, the output of the electrode position measuring unit 13 is 0, so the inverter operation control unit 11 determines that the electrode 2 is in the high-speed lifting / lowering region R1 and outputs the high-speed speed reference V1* to the inverter target speed setting unit 12.

[0070] The inverter target speed setting unit 12 sets the high speed reference V1* to the high speed reference upper limit value V1, and outputs it to the inverter acceleration / deceleration setting switching unit .

[0071] The inverter acceleration / deceleration setting switch 14 applies the high-speed acceleration reference speed change rate α1 and the high-speed deceleration reference speed change rate β1 to the high-speed speed reference V1*, and sequentially outputs the high-speed speed reference V1* to the inverter 7.

[0072] The electrode 2 accelerates from the electrode upper limit position Pa at a high-speed acceleration reference speed change rate α1, and then descends at a high-speed reference upper limit value V1.

[0073] The electrode position measuring unit 13 receives the pulse signal PC from the inverter 7, continuously calculates the position of the electrode 2, and outputs the calculation result to the inverter operation control unit 11.

[0074] The inverter operation control unit 11 outputs the operating speed reference V2* to the inverter target speed setting unit 12 so that the electrode 2 completes deceleration to the operating speed reference upper limit value V2 at the deceleration completion position Pb. An initial value is set for the deceleration completion position Pb. The inverter operation control unit 11 calculates the position at which the electrode 2 decelerates to the operating speed reference V2* based on the high-speed acceleration speed reference rate of change α1, the high-speed speed reference upper limit value V1, and the high-speed deceleration speed reference rate of change β1, and outputs the operating speed reference V2* together with the impedance deviation ΔZ to the inverter target speed setting unit 12 when the electrode 2 reaches the calculated position.

[0075] At the start of operation, no arc current IFB flows, so ΔZ=∞, and the inverter target speed setting unit 12 outputs the operating speed reference upper limit value V2 as the operating speed reference V2* to the inverter acceleration / deceleration setting switching unit 14. Note that, in order to prevent the electrode 2 from colliding with the scrap 4, the operating speed reference V2* may be set to a value sufficiently smaller than the operating speed reference upper limit value V2 at the start of operation. Alternatively, the electrode 2 may be stopped once at the deceleration completion position Pb, and the operator may manually lower the electrode 2 to touch the scrap.

[0076] When the inverter operation control unit 11 detects that the electrode 2 has descended and that the arc current IFB has flowed, the inverter operation control unit 11 stores this position as the scrap touch position Pc. The inverter operation control unit 11 adds a preset margin distance ΔL to the stored scrap touch position Pc to calculate the deceleration completion position Pb, and stores the calculated deceleration completion position Pb as a new deceleration completion position Pb.

[0077] After the scrap touch, the electrode 2 is controlled to move up and down within the operation region R2. Within the operation region R2, the electrode 2 operates in accordance with the operation speed reference characteristic 12a.

[0078] For example, when the inverter operation control unit 11 detects an overcurrent in the arc current IFB, it moves the electrode 2 out of the operation region R2 and retreats it to the high-speed lifting region R1 in order to protect the electrode 2, etc. In the retreat operation of the electrode 2 in the high-speed lifting region R1, the inverter operation control unit 11 moves the electrode 2 to the electrode upper limit position Pa.

[0079] When melting of the scrap 4 by heating with the arc current is completed, the inverter operation control unit 11 moves the electrode 2 to the electrode upper limit position Pa. This completes one operation unit.

[0080] (Variation) FIG. 6 is a schematic diagram for explaining the operation of the electrode lift control device according to the modified example of the embodiment. In FIG. 6, the operation speed reference V2* in the embodiment and the operation speed reference V2a* in the present modified example are shown on the same time axis. As shown in Fig. 4, the electrode lifting control device 6 according to the embodiment controls the lifting of the electrode 2 even in response to minute fluctuations in the arc current IFB in the high-precision control range HR. Therefore, depending on the state of the scrap 4, the lifting and lowering operation of the electrode 2 may become violent. Therefore, in this modification, the rate of change of the speed reference when accelerating or decelerating the electrode 2 is varied depending on the time t.

[0081] As shown in FIG. 6 , in the electrode lift control device 6 according to the embodiment, the electrode 2 is accelerated at a constant reference speed change rate α2 during acceleration and decelerated at a constant reference speed change rate β2 during deceleration. In contrast, in the electrode lift control device according to this modification, the reference speed change rate α2a during acceleration is set to a smaller value at the beginning of the speed-based acceleration than at the middle of the acceleration, and is also set to a smaller value at the end of the speed-based acceleration than at the middle of the acceleration. Similarly, the reference speed change rate β2a during deceleration is set to a smaller value at the beginning of the speed-based deceleration than at the middle of the deceleration, and is also set to a smaller value at the end of the speed-based deceleration than at the middle of the deceleration. In other words, in the case of the modification, the acceleration and deceleration are made gentler at the beginning and end of the acceleration and deceleration, thereby suppressing sudden fluctuations in the lifting and lowering operation of the electrode 2. The middle of the acceleration is the period between the beginning of the acceleration and the end of the acceleration, and the middle of the deceleration is the period between the beginning of the deceleration and the end of the deceleration.

[0082] The initial stage of acceleration is, for example, a period during which the operation speed reference V2a* is 10% of the operation speed reference upper limit V2. The final stage of acceleration is, for example, a period during which the operation speed reference V2a* is 90% of the operation speed reference upper limit V2. The initial stage of deceleration is, for example, a period during which the operation speed reference V2a* is 90% of the operation speed reference upper limit V2. The final stage of deceleration is, for example, a period during which the operation speed reference V2a* is 10% of the operation speed reference upper limit V2.

[0083] The effects of the electrode lift control device 6 according to this embodiment and the modified example will be described while comparing with an electrode lift control device according to a comparative example. First, the operation of the electrode lift control device according to the comparative example will be described. FIG. 7 is a graph showing the operational characteristics of the electrode lifting control device according to the comparative example. 7 is a graph of a speed reference characteristic 112a of an electrode lift control device according to a comparative example. The speed reference characteristic 112a corresponds to the operating speed reference characteristic 12a in the embodiment. The electrode lift control device according to the comparative example controls the lifting and lowering of the electrode 2 in accordance with the speed reference characteristic 112a.

[0084] 7, in the speed reference characteristic 112a in the comparative example, the speed reference V* is 0 in the range of 0<ΔZ<+ΔZ101. In the range of ΔZ≦0, three straight lines -S1 to -S3 having different slopes are set as sensitivity setting lines, and in the range of +ΔZ101≦Z, +S1 to +S3 having different slopes are set as sensitivity setting lines. Any of the sensitivity setting lines is selected by the operator, and for any of the sensitivity setting lines, V*=0 in the range of 0<ΔZ<+ΔZ101.

[0085] The range of 0<ΔZ<+ΔZ101 is called dead zone D. The threshold value of dead zone D, +Z101, is set to, for example, about 10% to 30% of the maximum value of impedance deviation ΔZ. The upper limit value of speed standard V* is set to, for example, a value equal to the operating speed standard upper limit value V2 in the case of the electrode lifting control device 6 according to the embodiment.

[0086] In the electrode lift control device according to the comparative example, by providing a dead zone D in the range of ΔZ>0, hunting that occurs due to excessive control of the lifting operation of the electrode 2 is suppressed, and unstable operation of the electrode lift control device is prevented. On the other hand, the electrode lift control device according to the comparative example performs fixed position control that does not control the lifting of the electrode 2 in the event of minute fluctuations in the arc current IFB. As a result, the distance between the electrode 2 and the scrap 4 may not be properly maintained. In other words, in the dead zone D, by placing emphasis on control stability, it may not be possible to input appropriate energy based on the arc current IFB to the scrap 4.

[0087] Furthermore, the electrode lifting control device controls the lifting according to the sensitivity setting line in the range of ΔZ<0. Therefore, even in the case of a small fluctuation in the arc current IFB, the electrode 2 is frequently controlled to be lifted and lowered so as to be far away from the scrap 4 according to the sensitivity setting. This may result in a decrease in the actual energy that can be input to the scrap 4.

[0088] The electrode lift control device 6 according to the embodiment outputs an operation speed reference V2* corresponding to the impedance deviation ΔZ to the inverter 7 in accordance with an operation speed reference characteristic 12a having a high-precision control range HR. In the high-precision control range HR, V2* is set to 0 when ΔZ=0, V2* is set to >0 when ΔZ>0, and V2* is set to <0 when ΔZ<0. Furthermore, the operation speed reference characteristic 12a satisfies d(V2*) / d(ΔZ)≧0. Therefore, the electrode lift control device 6 can lift and lower the electrode 2 in an appropriate direction even when a slight change in the arc current IFB occurs.

[0089] In most of the high-precision control range HR, the sensitivity of V2* to ΔZ is set lower than in the sensitivity setting control range SR. Therefore, the electrode lift control device 6 converts small fluctuations in IFB into small responses to V2*, enabling stable execution of high-precision position control.

[0090] In addition, in the electrode lift control according to the comparative example, when starting operations or restarting operations after an abnormality occurs, the electrode 2 is lowered at a sufficiently slow speed from a position sufficiently far from the scrap 4. Therefore, there is a problem that it takes a long time to start arc discharge, actually start melting the scrap 4, and restart it in one operation unit.

[0091] The electrode lift control device 6 according to the embodiment includes an electrode position measuring unit 13, and is therefore capable of estimating the position of the electrode 2 in the region where the electrode 2 can be raised and lowered. Based on the estimated position of the electrode 2, the electrode lift control device 6 can determine in which of the regions divided into the high-speed lifting and lowering region R1 and the operation region R2 the electrode 2 is located. In the high-speed lifting and lowering region R1, the electrode 2 can be raised and lowered at a speed higher than the standard speed in the operation region R2, thereby shortening the time required for the electrode 2 to descend when starting or restarting operation.

[0092] In the electrode lift control device 6 according to the embodiment, the speed reference change rate during acceleration in the operation region R2 is set to be larger than the speed reference change rate during acceleration in the high-speed lift region R1. The speed reference change rate during deceleration in the operation region R2 is also set to be larger than the speed reference change rate during deceleration in the high-speed lift region R1. Therefore, it is possible to quickly accelerate and decelerate the speed reference in the operation region R2 while suppressing overshoot and undershoot, thereby achieving highly accurate position control.

[0093] In the electrode lift control device 6 according to the embodiment, the speed reference change rate during acceleration / deceleration in the operation region R2 is set to be larger than the speed reference change rate during acceleration / deceleration in the high-speed lift region R1. In addition, by setting the current limit value in the operation region R2 to be larger than the current limit value in the high-speed lift region R1, it becomes possible to make maximum use of the output torque of the electric motor 8.

[0094] In the electrode lifting / lowering control device 6 according to the embodiment, the speed reference change rate during acceleration / deceleration is set to be sufficiently large so as to quickly follow fluctuations in ΔZ in the operation region R2. This may result in violent lifting / lowering of the electrode 2. As shown in the modified example of FIG. 6, by decreasing the speed reference change rate during acceleration / deceleration over time, the lifting / lowering movement of the electrode 2 can be made gentler, enabling more stable lifting / lowering movement to be achieved.

[0095] In this way, an electrode lifting control device that optimizes the efficiency of scrap melting can be realized.

[0096] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0097] 1...furnace transformer, 2...electrode, 3...furnace body, 4...scrap, 5...electrode lifting main panel, 6...electrode lifting control device, 7...inverter, 8...electric motor, 9...impedance deviation calculation unit, 10...electrode operation stop command unit, 11...inverter operation control unit, 12...inverter target speed setting unit, 12a...operation speed reference characteristic, 13...electrode position measuring unit, 14a...inverter acceleration / deceleration setting switching unit, 15...current transformer, 16...auxiliary transformer

Claims

1. 1. An electrode lifting control device that controls the lifting of electrodes of an AC arc furnace that melts scrap by arc discharge formed between the electrodes and the scrap, an impedance deviation calculation means for calculating an actual measured impedance based on the arc current flowing through the electrode and the arc voltage between the electrode and the scrap, and calculating an impedance deviation by subtracting a preset reference impedance from the actual measured impedance; an inverter target speed setting means for outputting an operation speed reference to an inverter that drives an electric motor that moves the electrode up and down by mechanically coupling with the electrode, based on an operation speed reference characteristic that indicates an operation speed reference, which is a speed reference of the lifting speed of the electrode relative to the impedance deviation; an electrode position measuring means for calculating and outputting the position of the electrode based on the rotation speed and rotation direction of the electric motor; an inverter operation control means for selecting either the operating speed reference or a high speed reference having a value greater than the magnitude of the operating speed reference based on the position of the electrode, and outputting the selected high speed reference to the inverter target setting means; Equipped with The operating speed reference characteristic is the operating speed reference is 0 when the impedance deviation is 0, and the impedance deviation has a high precision control characteristic in a range where the impedance deviation is greater than 0 and smaller than a positive first threshold value, and where the impedance deviation is smaller than 0 and greater than a negative second threshold value; a first sensitivity characteristic having a linear characteristic that is continuous with the first threshold value and the second threshold value and has a positive coefficient in a range equal to or greater than the first threshold value and equal to or less than the second threshold value; the high-precision control characteristic includes a range having a differential coefficient smaller than the coefficient of the first sensitivity characteristic, the range includes a range in which the impedance deviation is from a third threshold value greater than 0 and less than the first threshold value to the first threshold value, and a range in which the impedance deviation is from the second threshold value to a fourth threshold value greater than the second threshold value and less than 0; The inverter operation control means selecting and outputting the operating speed standard when the position of the electrode is closer to the scrap than a preset deceleration completion position; an electrode lifting control device that selects and outputs the high speed reference when the position of the electrode is farther from the scrap than the deceleration completion position;

2. When the high speed reference is selected, a first acceleration reference speed change rate is applied during acceleration and a first deceleration reference speed change rate is applied during deceleration, and the applied first acceleration reference speed change rate is output as the high speed reference; further comprising an inverter acceleration / deceleration setting switching means for, when the operating speed reference is selected, applying a second acceleration speed reference rate of change during acceleration and a second deceleration speed reference rate of change during deceleration, and sequentially outputting the applied second acceleration speed reference rate of change as the operating speed reference; the second reference speed change rate during deceleration is greater than the first reference speed change rate during acceleration, 2. The electrode lift control device according to claim 1, wherein the second reference speed change rate during deceleration is greater than the first reference speed change rate during deceleration.

3. The inverter operation control means When the high speed reference is selected, a first current limit value set in accordance with the first acceleration speed reference rate of change is applied; When the operating speed standard is selected, a second current limit value set according to the second acceleration speed standard change rate is applied; 3. The electrode lifting control device according to claim 2, wherein the product of the first current limit value and the first reference speed change rate during acceleration is equal to the product of the second current limit value and the second reference speed change rate during acceleration.

4. In the second acceleration reference speed change rate, the change rates at the initial and final stages of acceleration are smaller than the change rate at the intermediate stage of acceleration between the initial stage of acceleration and the final stage of acceleration, 4. The electrode lifting control device according to claim 2, wherein the second reference rate of change of speed during deceleration is smaller in rate of change at the beginning and end of deceleration than in rate of change at the middle of deceleration between the beginning of the deceleration and the deceleration period.

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