Welding system and polarity switching method

Abstract

This invention provides a welding system that, when multiple welding power supplies are connected in parallel to output AC power, can prevent crosscurrents caused by polarity switching of each welding power supply and reduce surge voltages generated by external inductance. [Solution] The control device 1 transmits a synchronization signal to the welding power supply devices 2a and 2b. When the control circuit 28a receives the synchronization signal, it stops the DC power supply, and when the absolute value of the output current of the inverter circuit 25a falls below the first threshold current, it turns off both switching elements and transmits a first permission signal to the welding power supply device 2b. When the control circuit 28a receives a second permission signal from the welding power supply device 2b, it turns on one of the switching elements. When the control circuit 28b receives the first permission signal, it turns on both switching elements, and when the absolute value of the output current of the inverter circuit 25b falls below the second threshold current, it turns off one of the switching elements and transmits a second permission signal to the welding power supply device 2a.

Description

Technical Field

[0001] The present invention relates to a welding system and a polarity switching method.

Background Art

[0002] Conventionally, submerged arc welding is known. Submerged arc welding involves spraying granular flux onto the workpiece, feeding a welding wire into the flux, and generating an arc between the tip of the welding wire (electrode) and the workpiece to perform welding. In submerged arc welding, for example, welding is performed while moving the welding location by running a carriage along the welding line. In submerged arc welding, thick plates can be welded at high efficiency by flowing a large current through a thick welding wire. When a large current (e.g., 2000 A) is required for welding, it is difficult to output the current with a single welding power supply device, so it is conceivable to connect multiple welding power supply devices in parallel.

[0003] When connecting multiple welding power supply devices in parallel to output AC power, it is necessary to align the polarity switching timing of each welding power supply device. If the switching timings do not match, there is a problem that a cross current occurs where the output current of the welding power supply device that switched first flows into the welding power supply device with a delayed switching. To solve this problem, Patent Document 1 discloses a method of transmitting a synchronization signal for instructing the polarity switching timing of multiple welding power supply devices by high-speed communication.

[0004] On the other hand, Patent Document 2 discloses an AC arc welding power supply device that can reduce the surge voltage generated by an external inductance and applied to a switching element during polarity switching. In the invention of Patent Document 2, during polarity switching, both of the series-connected switching elements are turned on. As a result, the current flowing through the external inductance is consumed by the arc and decreases. Then, when the output current becomes less than or equal to a predetermined value, one of the series-connected switching elements is turned off to perform polarity switching.

[0005] When using the AC arc welding power supply device described in Patent Document 2, even if the synchronization signal is transmitted via high-speed communication using the method described in Patent Document 1, the timing at which multiple parallel-connected welding power supplies actually switch polarities will not coincide precisely. This is because variations in the response time of the control circuits of each welding power supply device, variations in switching time due to the switching elements, and variations in the external inductance of each welding power supply device cause the timing at which the output current of each welding power supply device falls below a predetermined value to not coincide. Since the polarity switching timing does not coincide precisely, crosscurrent may occur. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2021-053695 [Patent Document 2] Japanese Patent Application Publication No. 2-235574 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The present invention was conceived under the circumstances described above, and aims to provide a welding system that can prevent crosscurrent due to polarity switching of each welding power supply unit when multiple welding power supply units are connected in parallel to output AC power, and can reduce surge voltage generated by external inductance. [Means for solving the problem]

[0008] A welding system provided by a first aspect of the present invention comprises a first welding power supply and a second welding power supply, the output sides of which are connected in parallel, and a control device for controlling the first welding power supply and the second welding power supply, wherein the first welding power supply comprises a first DC power supply having a first DC reactor, a first inverter circuit having two first switching elements connected in series and converting DC power input from the first DC power supply into AC power, and a first control circuit for controlling the first DC power supply and the first inverter circuit, wherein the second welding power supply comprises a second DC power supply having a second DC reactor, a second inverter circuit having two second switching elements connected in series and converting DC power input from the second DC power supply into AC power, and a second control circuit for controlling the second DC power supply and the second inverter circuit, and the first inductor of the first DC reactor The inductance is smaller than the second inductance of the second DC reactor. The control device transmits a synchronization signal to the first welding power supply and the second welding power supply to instruct them on the timing to switch polarity. When the first control circuit receives the synchronization signal, it stops the first DC power supply, turns off both of the two first switching elements when the absolute value of the output current of the first inverter circuit becomes less than or equal to the first threshold current, and transmits a first permission signal to the second welding power supply. When the first control circuit receives a second permission signal from the second welding power supply, it turns on one of the first switching elements. When the second control circuit receives the first permission signal, it turns on both of the two second switching elements, turns off one of the second switching elements when the absolute value of the output current of the second inverter circuit becomes less than or equal to the second threshold current, and transmits a second permission signal to the first welding power supply.

[0009] In a preferred embodiment of the present invention, the second control circuit further shuts down the second DC power supply when it receives the first permission signal.

[0010] In a preferred embodiment of the present invention, the second control circuit increases the output current of the second DC power supply when it receives the synchronization signal.

[0011] In a preferred embodiment of the present invention, the first DC power supply does not have the first DC reactor, and the second DC reactor is a saturable reactor.

[0012] A polarity switching method provided by a second aspect of the present invention is a polarity switching method in a welding system comprising a first welding power supply and a second welding power supply, the output sides of which are connected in parallel, wherein the first DC power supply of the first welding power supply comprises a first DC reactor, the second DC power supply of the second welding power supply comprises a second DC reactor, the first inductance of the first DC reactor is smaller than the second inductance of the second DC reactor, and when switching polarity, first the first DC power supply is stopped, and when the absolute value of the output current of the first inverter circuit of the first welding power supply becomes less than or equal to a first threshold current, both of the two first switching elements connected in series of the first inverter circuit are turned on, then both of the two second switching elements connected in series of the second inverter circuit of the second welding power supply are turned on, and when the absolute value of the output current of the second inverter circuit becomes less than or equal to a second threshold current, one of the second switching elements is turned off and one of the first switching elements is turned on. [Effects of the Invention]

[0013] According to the present invention, when switching polarity, the first control circuit stops the first DC power supply and turns off both of the two first switching elements when the absolute value of the output current of the first inverter circuit falls below the first threshold current. Subsequently, the second control circuit turns on both of the two second switching elements and turns off one of the second switching elements when the absolute value of the output current of the second inverter circuit falls below the second threshold current. This switches the polarity of the second welding power supply, but since both of the first switching elements are off, no lateral current flows to the first welding power supply. Subsequently, the first control circuit switches the polarity of the first welding power supply by turning on one of the first switching elements. At this time, since the polarity of the second welding power supply has been switched, no lateral current flows. In this way, the welding system according to the present invention can prevent lateral current due to the switching of polarity of each welding power supply. Furthermore, when both of the first switching elements are turned off, the output current of the first inverter circuit decreases, and when switching the polarity of the second welding power supply, the output current of the second inverter circuit decreases, so the surge voltage generated by the external inductance can be reduced. [Brief explanation of the drawing]

[0014] [Figure 1] This is a diagram illustrating the welding system according to the first embodiment, and is a block diagram showing the overall configuration of the welding system. [Figure 2] This is a block diagram showing the internal configuration of the control device and multiple welding power supply units. [Figure 3] This is an example of a flowchart used to explain polarity switching processes. [Figure 4] This is a timing chart to explain the polarity switching process. [Figure 5] This is an example of a current waveform when the control circuit's waveform command signal is a sine wave signal. [Figure 6] This is a diagram illustrating the welding system according to the second embodiment, and a timing chart illustrating the polarity switching process. [Modes for carrying out the invention]

[0015] Hereinafter, preferred embodiments of the present invention will be specifically described with reference to the accompanying drawings.

[0016] 〔First Embodiment〕 FIG. 1 and FIG. 2 are diagrams for explaining a welding system A1 according to the first embodiment. FIG. 1 is a block diagram showing the overall configuration of the welding system A1. FIG. 2 is a block diagram showing the internal configurations of the control device and a plurality of welding power supply devices.

[0017] The welding system A1 is a welding system for performing submerged arc welding. As shown in FIG. 1, the welding system A1 includes a control device 1, welding power supply devices 2a and 2b, a welding torch 3, a carriage 4, a wire feeding device 5, a wire reel 6, a spraying device 7, and an electrode 8. The welding system A1 travels the carriage 4 along the weld line of the workpiece W, while spraying granular flux 79 from the spraying device 7 mounted on the carriage 4, and feeding the welding wire into the flux 79 by the wire feeding device 5. The welding wire is supplied from the wire reel 6. The welding power supply devices 2a and 2b convert the AC power supplied from the commercial power supply P into power suitable for welding and output it, and generate an arc between the electrode 8, which is the tip portion of the welding wire, and the workpiece W inside the flux 79. Welding is performed by the heat of the arc. Thereby, welding is performed along the weld line of the workpiece W. Instead of using the carriage 4, the workpiece W may be moved or rotated.

[0018] The welding torch 3 guides the welding wire fed by the wire feeder 5 to the welding location. The tip of the welding wire becomes an electrode 8 protruding from the tip of the welding torch 3. The welding torch 3 is equipped with a contact tip (not shown) located at its tip and conductive to the welding power supply unit 2. The welding power supply unit 2 supplies a welding current to the welding wire in contact with the contact tip. The welding torch 3 is mounted on a trolley 4 and moves with the movement of the trolley 4. Therefore, the electrode 8 is also mounted on the trolley 4 and moves with the movement of the trolley 4. The welding torch 3 may be directly fixed to the trolley 4, or it may be indirectly fixed via an arm or the like.

[0019] The control device 1 performs various controls on the welding system A1. The control device 1 includes a control board equipped with a microcontroller and communication module, an operation panel for operation, and a display panel for display. The control device 1 may be mounted on the trolley 4, or it may be located separately from the trolley 4 (for example, adjacent to the welding power supply unit 2). Alternatively, the control device 1 may be a general-purpose computer with a program installed to perform various controls on the welding system A1. The control device 1 makes the trolley 4 travel at a predetermined travel speed. The travel speed is set according to the material and thickness of the workpiece W. The control device 1 instructs the spraying device 7 to start and stop spraying flux 79. Note that the start and stop of flux spraying by the spraying device 7 may be performed manually. The control device 1 instructs each wire feeder 5 to start and stop feeding the welding wire, and to set the feeding speed of the welding wire. The feeding speed is set according to the set welding current, etc.

[0020] As shown in FIG. 2, the control device 1 includes a communication unit 16. The control device 1 communicates with the welding power supply devices 2a and 2b via the communication unit 16 and controls the welding power supply devices 2a and 2b. For example, when the start of welding is instructed by an operator, the control device 1 transmits a command signal for starting output to the welding power supply devices 2a and 2b via the communication unit 16. The welding power supply devices 2a and 2b that receive the command signal start power output and generate an arc between the electrode 8 and the workpiece W. Note that the method of generating the arc is not limited. Also, for example, when the stop of welding is instructed by an operator, the control device 1 transmits a command signal for stopping output to the welding power supply devices 2a and 2b via the communication unit 16. The welding power supply devices 2a and​​​​​​​​​​​Communication line 92 is a communication line connecting the control device 1 and the welding power supply units 2a and 2b, and is wired in a bus-type wiring configuration. The control device 1 and the welding power supply units 2a and 2b communicate via communication line 92, for example, according to the HCI (Host Control Interface) communication standard. The HCI communication standard is a communication standard developed to enable high-speed communication between the control device 1 and the welding power supply units 2a and 2b. In the HCI communication standard, the amount of data transmitted is reduced by reducing the amount of data head of the communication data to less than half the amount of data head of fieldbus communication. The communication speed on communication line 92 according to the HCI communication standard is faster than the communication speed on communication line 91, and is about 25 Mbps to 100 Mbps. In this embodiment, it is about 50 Mbps. The control device 1 transmits a synchronization signal to the welding power supply units 2a and 2b via communication line 92. Since communication line 92 only transmits the synchronization signal and does not transmit or receive other signals, the communication speed does not become very slow. Furthermore, the communication standard used between the control device 1 and the welding power supply units 2a and 2b via the communication line 92 is not limited to the HCI communication standard; any standard that can transmit the synchronization signal with almost no delay is acceptable. The wiring configuration of the communication line 92 is also not limited. The communication line 92 may also be a dedicated line that transmits a pulse signal switching between high and low levels as a synchronization signal from the control device 1 to the welding power supply units 2a and 2b. In this case as well, the communication line 92 can communicate at a higher speed than the communication line 91, and can transmit the synchronization signal with almost no delay.

[0024] In other words, the control device 1 and the welding power supply devices 2a and 2b are connected by two communication lines: a communication line 92 for high-speed communication of only synchronization signals, and a communication line 91 for transmitting other signals.

[0025] Communication line 93 is a communication line connecting welding power supply unit 2a and welding power supply unit 2b, and is a dedicated line that transmits the permission signal, described later, as a high-level pulse signal. Since the permission signal needs to be transmitted between welding power supply unit 2a and welding power supply unit 2b at a high transmission speed, communication line 93 may use, for example, wired communication or optical communication using optical fiber, but the communication method is not limited.

[0026] The welding power supply units 2a and 2b each convert AC power supplied from the commercial power supply P into AC power of a desired frequency and output it. The output sides of the welding power supply units 2a and 2b are connected in parallel. Specifically, one output terminal a of the welding power supply units 2a and 2b is connected to each other and connected to the workpiece W. The other output terminal b of the welding power supply units 2a and 2b is connected to each other and connected to the welding wire. As shown in Figure 2, the welding power supply unit 2a includes a rectifier and smoothing circuit 21a, an inverter circuit 22a, a transformer 23a, a rectifier and smoothing circuit 24a, an inverter circuit 25a, a current sensor 26a, a communication unit 29a, and a control circuit 28a. The welding power supply unit 2b includes a rectifier and smoothing circuit 21b, an inverter circuit 22b, a transformer 23b, a rectifier and smoothing circuit 24b, an inverter circuit 25b, a current sensor 26b, a communication unit 29b, and a control circuit 28b. Hereafter, when the welding power supply units 2a and 2b are not distinguished, they will be referred to as welding power supply unit 2. In this case, the common components are described as the rectifier / smoothing circuit 21, inverter circuit 22, transformer 23, rectifier / smoothing circuit 24, inverter circuit 25, current sensor 26, communication unit 29, and control circuit 28.

[0027] The rectifier-smoothing circuit 21 converts the AC power input from the commercial power supply P into DC power and outputs it. The rectifier-smoothing circuit 21 comprises a rectifier circuit that rectifies the AC current and a smoothing capacitor that smooths it. The configuration of the rectifier-smoothing circuit 21 is not limited. The inverter circuit 22 converts the DC power input from the rectifier-smoothing circuit 21 into high-frequency power and outputs it by switching a switching element with an output control drive signal input from the control circuit 28. The transformer 23 transforms the high-frequency voltage output by the inverter circuit 22 and outputs it to the rectifier-smoothing circuit 24.

[0028] The rectifier-smoothing circuit 24 converts the high-frequency power input from the transformer 23 into DC power and outputs it. The rectifier-smoothing circuit 24 comprises a rectifier circuit DR that rectifies the high-frequency current and a DC reactor DCL that smooths it. In this embodiment, the DC reactor DCL of the rectifier-smoothing circuit 24a of the welding power supply unit 2a and the DC reactor DCL of the rectifier-smoothing circuit 24b of the welding power supply unit 2b are both saturable reactors with the same characteristics. Saturable reactors have a large inductance value in the low current range and a small inductance value in the high current range. In this embodiment, both DC reactors DCL are designed so that their inductance values ​​change similarly from several tens of μH to several μH. The rectifier-smoothing circuit 21, inverter circuit 22, transformer 23, and rectifier-smoothing circuit 24 constitute a DC power supply that generates DC power to be output to the inverter circuit 25.

[0029] The inverter circuit 25 converts the DC power input from the rectifier and smoothing circuit 24 into AC power and outputs it by switching a switching element in response to a switching drive signal input from the control circuit 28. The inverter circuit 25 alternately switches between positive polarity, where the potential of output terminal a (connected to the workpiece W) is higher than the potential of output terminal b (connected to the contact tip of the welding torch 3), and reverse polarity, where the potential of output terminal a is lower than the potential of output terminal b.

[0030] In the illustrated example, the inverter circuit 25 is a single-phase full-bridge circuit including four switching elements SW1 to SW4. Each of the switching elements SW1 to SW4 is an IGBT (Insulated-Gate Bipolar Transistor), but is not limited to this; other transistors such as MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) and bipolar transistors may also be used. Switching elements SW1 and SW2 are connected in series, with the emitter terminal of SW1 connected to the collector terminal of SW2. The collector terminal of SW1 is connected to the positive input terminal of the inverter circuit 25, and the emitter terminal of SW2 is connected to the negative input terminal of the inverter circuit 25. The connection point between switching elements SW1 and SW2 is connected to output terminal b. Similarly, switching elements SW3 and SW4 are connected in series with each other and then connected in parallel between the input terminals. The connection point between switching elements SW3 and SW4 is connected to output terminal a. Each switching element SW1 to SW4 is individually connected in antiparallel to a diode. Each switching element SW1 to SW4 of the inverter circuit 25 receives a switching drive signal from the control circuit 28 and switches between on and off according to the switching drive signal. Note that the configuration of the inverter circuit 25 is not limited to a full-bridge circuit; for example, a half-bridge circuit may also be used.

[0031] The current sensor 26 detects the output current of the welding power supply unit 2, and in this embodiment, it is located on the connecting wire that connects one output terminal of the inverter circuit 25 to output terminal a. The output current of the welding power supply unit 2 detected by the current sensor 26 is approximately equal to the current flowing through the electrode 8. The location where the current sensor 26 is located is not limited. The current sensor 26 detects a current detection signal (analog signal) corresponding to the output current of the welding power supply unit 2 and outputs it to the control circuit 28.

[0032] The communication unit 29 communicates with the communication unit 16 of the control device 1 (described later) via communication lines 91 and 92. The communication unit 29 receives command signals from the control device 1 (communication unit 16) via communication line 91, instructing the start and stop of power output, as well as current command values, and inputs them to the control circuit 28. The information that the communication unit 29 transmits and receives with the control device 1 (communication unit 16) via communication line 91 is not limited. The communication unit 29 also receives a synchronization signal from the control device 1 (communication unit 16) via communication line 92 and inputs it to the control circuit 28.

[0033] Furthermore, the communication unit 29 communicates with the communication unit 29 of other welding power supply units 2 via the communication line 93. The communication unit 29 transmits and receives permission signals to and from the communication unit 29 of other welding power supply units 2 via the communication line 93.

[0034] The control circuit 28 is a circuit for controlling the welding power supply unit 2 and is implemented by, for example, a microcomputer. The control circuit 28 controls the driving and stopping of the inverter circuits 22 and 25 based on the output start command and output stop command received from the control device 1 (communication unit 16) via the communication unit 29. Specifically, when the control circuit 28 receives an output start command, it starts outputting a drive signal to the inverter circuits 22 and 25, and when it receives an output stop command, it stops outputting the drive signal.

[0035] The control circuit 28 detects the output current value, which is a digital signal, based on the current detection signal input from the current sensor 26. The output current value is the instantaneous value of the output current, obtained by converting the current detection signal (analog signal) into a digital signal. The control circuit 28 also calculates the effective current value from the output current value.

[0036] The control circuit 28 generates an output control drive signal to control the switching elements of the inverter circuit 22 based on the effective current value and the current command value input from the control device 1, and outputs it to the inverter circuit 22. In other words, the control circuit 28 performs feedback control so that the effective current value matches the current command value.

[0037] Furthermore, the control circuit 28 generates a switching drive signal to control the switching elements of the inverter circuit 25 based on the output current value and the waveform command signal generated internally, and outputs it to the inverter circuit 25. In other words, the control circuit 28 performs feedback control so that the waveform of the output current matches the waveform commanded by the waveform command signal. Note that the control circuit 28 may also generate the switching drive signal based only on the waveform command signal without using the output current value. In this embodiment, the waveform command signal is a square wave signal.

[0038] Furthermore, the control circuit 28 switches polarity based on the synchronization signal received from the control device 1 (communication unit 16). The polarity switching process performed in welding system A1 will be described below.

[0039] Polarity switching is performed when a synchronization signal is received from the control device 1. When the control circuit 28a of the welding power supply device 2a receives the synchronization signal, it stops the supply of DC current to the inverter circuit 25a by stopping the inverter circuit 22a. Then, when the output current of the welding power supply device 2a becomes approximately "0", the control circuit 28a turns off all of the switching elements SW1 to SW4 of the inverter circuit 25a. Note that the control circuit 28a may turn off only the series-connected switching elements SW1 and SW2, or only the series-connected switching elements SW3 and SW4. Then, the control circuit 28a sends a permission signal to the welding power supply device 2b via the communication line 93.

[0040] When the control circuit 28b of the welding power supply unit 2b receives a synchronization signal, it increases the output current of the inverter circuit 22b, thereby increasing the supply of DC current to the inverter circuit 25b. By compensating for the decrease in the output current of the welding power supply unit 2a with the increase in the output current of the welding power supply unit 2b, the welding system A1 can suppress the decrease in output current and thus suppress arc interruption. However, if arc interruption does not occur even when the output current of the welding power supply unit 2a decreases, the control circuit 28b does not need to increase the output current of the inverter circuit 22b.

[0041] When control circuit 28b receives a permission signal from welding power supply device 2a via communication line 93, it stops the inverter circuit 22b, thereby stopping the supply of DC power to inverter circuit 25b. Control circuit 28b also turns on all of the switching elements SW1 to SW4 of inverter circuit 25b. Control circuit 28b may turn on only the series-connected switching elements SW1 and SW2, or only the series-connected switching elements SW3 and SW4. As a result, the energy stored in the external inductance is consumed by the arc, and the output current of welding power supply device 2b gradually decreases. Control circuit 28b does not necessarily have to stop inverter circuit 22b. However, in order to suppress the current flowing through the switching elements SW1 to SW4 of inverter circuit 25b, it is desirable to suppress or stop the output of inverter circuit 22b.

[0042] When the output current of the welding power supply unit 2b drops sufficiently, the control circuit 28b activates the inverter circuit 22b and turns off the switching elements SW1 to SW4 that were on before the polarity switching process. This switches the polarity of the welding power supply unit 2b. At this time, since all the switching elements SW1 to SW4 of the inverter circuit 25a of the welding power supply unit 2a are off, no current flows from the welding power supply unit 2b to the welding power supply unit 2a. The control circuit 28b also sends a permission signal to the welding power supply unit 2a via the communication line 93.

[0043] When the control circuit 28a receives a permission signal from the welding power supply unit 2b via the communication line 93, it activates the inverter circuit 22a and turns on the switching elements SW1 to SW4, which were off before the polarity switching process. This switches the polarity of the welding power supply unit 2a. Note that the timing of the control circuit 28a activating the inverter circuit 22a is not limited to when it receives a permission signal from the welding power supply unit 2b, but can be anytime after all of the switching elements SW1 to SW4 of the inverter circuit 25a have been turned off. After all of the switching elements SW1 to SW4 of the inverter circuit 25a have been turned off, even if the inverter circuit 22a outputs power, it will not be supplied to the arc.

[0044] Figure 3 is an example of a flowchart illustrating the polarity switching process. Figure 3(a) shows an example of the polarity switching process performed by the control circuit 28a of the welding power supply unit 2a, and Figure 3(b) shows an example of the polarity switching process performed by the control circuit 28b of the welding power supply unit 2b. Each polarity switching process is initiated when a synchronization signal is received from the control device 1.

[0045] In the polarity switching process performed by the control circuit 28a shown in Figure 3(a), first, the supply of DC current is stopped (S1). Specifically, the control circuit 28a stops the output control drive signal output to the inverter circuit 22a, thereby stopping the inverter circuit 22a and the supply of DC current to the inverter circuit 25a. Next, it is determined whether or not the output current of the welding power supply device 2a has become approximately "0" (S2). Specifically, the control circuit 28a compares the absolute value of the detected output current Ia of the welding power supply device 2a with a first threshold current I1. The first threshold current I1 is a threshold used to determine that the output current of the welding power supply device 2a has stopped flowing, and is set to a value close to "0". Note that the first threshold current I1 is not limited.

[0046] If the absolute value of the output current Ia is greater than the first threshold current I1, it is determined that the output current is not yet "0" (S2: NO), and the process returns to step S2, where the determination in step S2 is repeated. When the absolute value of the output current Ia becomes less than or equal to the first threshold current I1, it is determined that the output current is almost "0" (S2: YES), and all of the switching elements SW1 to SW4 of the inverter circuit 25a are turned off (S3). Specifically, the control circuit 28a stops the switching drive signal that it outputs to the inverter circuit 25a. Next, a permission signal is sent to the welding power supply device 2b (S4).

[0047] Next, it is determined whether or not a permission signal has been received from the welding power supply unit 2b (S5). If the permission signal has not been received (S5: NO), the process returns to step S5 and the determination in step S5 is repeated. If the permission signal has been received (S5: YES), the polarity of the welding power supply unit 2a is switched (S6), and the polarity switching process is completed. Specifically, the control circuit 28a activates the inverter circuit 22a by outputting an output control drive signal, and outputs a switching drive signal that turns on the switching elements SW1~SW4, which were off before the polarity switching process. This switches the polarity of the welding power supply unit 2a.

[0048] In the polarity switching process performed by the control circuit 28b shown in Figure 3(b), first, the supply of DC current is increased (S11). Specifically, the control circuit 28b increases the output current of the inverter circuit 22a by outputting an output control drive signal generated by increasing the current command value to the inverter circuit 22b. Next, it is determined whether or not a permission signal has been received from the welding power supply device 2a (S12). If no permission signal has been received (S12: NO), the process returns to step S12 and the determination in step S12 is repeated.

[0049] If a permission signal is received (S12: YES), the supply of DC current is stopped (S13). Specifically, the control circuit 28b stops the output control drive signal that it outputs to the inverter circuit 22b, thereby stopping the inverter circuit 22b and the supply of DC current to the inverter circuit 25b. Next, all of the switching elements SW1 to SW4 of the inverter circuit 25b are turned on (S14). Specifically, the control circuit 28b outputs a high-level switching drive signal to the switching elements SW1 to SW4 of the inverter circuit 25b.

[0050] Next, it is determined whether the output current of the welding power supply unit 2b has decreased sufficiently (S15). Specifically, the control circuit 28b compares the absolute value of the detected output current Ib of the welding power supply unit 2b with the second threshold current I2. The second threshold current I2 is a threshold value used to determine whether the output current of the welding power supply unit 2b has decreased sufficiently, and is set to a current value (for example, around 100A) that suppresses the induced voltage of the external inductance to an allowable voltage. Note that the second threshold current I2 is not limited.

[0051] If the absolute value of the output current Ib is greater than the second threshold current I2, it is determined that the output current has not yet decreased sufficiently (S15: NO), and the process returns to step S15, where the determination in step S15 is repeated. When the absolute value of the output current Ib becomes less than or equal to the second threshold current I2, it is determined that the output current has decreased sufficiently (S15: YES), and the polarity of the welding power supply unit 2b is switched (S16). Specifically, the control circuit 28b activates the inverter circuit 22b by outputting an output control drive signal, and outputs a switching drive signal that turns off the switching elements SW1~SW4 that were on before the polarity switching process. This switches the polarity of the welding power supply unit 2b. Next, a permission signal is sent to the welding power supply unit 2a (S17), and the polarity switching process ends.

[0052] Note that the processes shown in the flowcharts in Figure 3 are examples, and the polarity switching processes are not limited to those described above.

[0053] Figure 4 is a timing chart illustrating the polarity switching process. Figure (a) shows the time variation of the synchronization signal output by the control device 1. Figure (b) shows the time variation of the output current Ia of the welding power supply device 2a. Figure (c) shows the time variation of the output current Ib of the welding power supply device 2b. Figure (d) shows the time variation of the output current Iout of the welding system A1. Note that the vertical and horizontal axes of the timing chart shown in Figure 4 have been enlarged or reduced as appropriate for ease of understanding, and the waveforms shown have also been simplified, exaggerated, or emphasized for ease of understanding. The same applies to Figure 6.

[0054] At time t0, a synchronization signal is transmitted from control device 1. In response, the control circuit 28a of welding power supply device 2a stops the inverter circuit 22a, causing the output current Ia to decrease as shown in Figure (b). On the other hand, the control circuit 28b of welding power supply device 2b increases the output current of the inverter circuit 22b, causing the output current Ib to increase as shown in Figure (c). By increasing the output current Ib to compensate for the decrease in output current Ia, the decrease in the output current Iout of welding system A1 is suppressed, as shown in Figure (d).

[0055] At time t1, the output current Ia becomes "0", causing the control circuit 28b of the welding power supply unit 2b to stop the inverter circuit 22b. As a result, the output current Ib decreases, as shown in Figure (c). Since the output current Ia is "0", the output current Iout of the welding system A1 decreases in accordance with the decrease in output current Ib, as shown in Figure (d).

[0056] At time t2, the polarity of welding power supply unit 2b is switched because the output current Ib falls below the second threshold current I2. At this time, all switching elements SW1 to SW4 of the inverter circuit 25a of welding power supply unit 2a are turned off, so no current flows from welding power supply unit 2b to welding power supply unit 2a. Subsequently, the polarity of welding power supply unit 2a is also switched. After the polarity is switched, the absolute values ​​of the output currents Ia and Ib increase, and the absolute value of the output current Iout of welding system A1 also increases to reach the set current. Note that when switching from reverse polarity to positive polarity, the waveforms in Figures 4(b), (c), and (d) will have their positive and negative signs reversed.

[0057] Figure 5 shows an example of current waveforms when the waveform command signal from the control circuit 28 is a sinusoidal signal. The dashed line shows the waveform of the output current Ia of the welding power supply unit 2a, the thin solid line shows the waveform of the output current Ib of the welding power supply unit 2b, and the thick solid line shows the waveform of the output current Iout of the welding system A1. As shown in the figure, at time t0, a synchronization signal is transmitted from the control device 1, and in response, the output current Ia decreases, and the output current Ib increases to compensate for the decrease in output current Ia. At time t1, the output current Ia becomes "0", causing the inverter circuit 22b of the welding power supply unit 2b to stop, and the output current Ib decreases. Subsequently, at time t2, the output current Ib becomes less than or equal to the second threshold current I2, causing the polarity of the welding power supply unit 2b to switch, and the polarity of the welding power supply unit 2a to also switch. In this way, the output current Iout has a sinusoidal waveform.

[0058] Next, the operation and effects of the welding system A1 according to this embodiment will be described.

[0059] According to this embodiment, when switching polarity, the control circuit 28a stops the inverter circuit 22a, thereby stopping the supply of DC current to the inverter circuit 25a, and when the output current of the welding power supply device 2a becomes approximately "0", it turns off all of the switching elements SW1 to SW4 of the inverter circuit 25a. Subsequently, the control circuit 28b turns on all of the switching elements SW1 to SW4 of the inverter circuit 25b. Then, when the output current of the welding power supply device 2b has decreased sufficiently, the control circuit 28b turns off the switching elements SW1 to SW4 that were on before the polarity switching process, thereby switching the polarity of the welding power supply device 2b. When the polarity of the welding power supply device 2b is switched, all of the switching elements SW1 to SW4 of the inverter circuit 25a are off, so no lateral current flows from the welding power supply device 2b to the welding power supply device 2a. Subsequently, the control circuit 28a turns on the switching elements SW1 to SW4 that were off before the polarity switching process, thereby switching the polarity of the welding power supply device 2a. At this time, since the polarity of the welding power supply device 2b has been switched, no lateral current flows. Thus, welding system A1 can prevent crosscurrent caused by switching the polarity of welding power supplies 2a and 2b. Furthermore, when all switching elements SW1 to SW4 of inverter circuit 25a are turned off, the output current of welding power supply 2a decreases, and when the polarity of welding power supply 2b is switched, the output current of welding power supply 2b decreases. Therefore, welding system A1 can reduce surge voltage generated by external inductance.

[0060] Furthermore, according to this embodiment, the control circuit 28b can reduce the output current of the welding power supply device 2b by turning on all of the switching elements SW1 to SW4 of the inverter circuit 25b, thereby dissipating the energy stored in the external inductance through the arc. Also according to this embodiment, the control circuit 28b compares the absolute value of the output current Ib with a second threshold current I2. The second threshold current I2 is set to a current value that suppresses the induced voltage of the external inductance to an allowable voltage. As a result, the control circuit 28b can determine that the output current Ib of the welding power supply device 2b has decreased sufficiently.

[0061] Furthermore, according to this embodiment, when switching polarity, the control circuit 28b stops the inverter circuit 22b when it receives a permission signal from the welding power supply device 2a, thereby stopping the supply of DC power to the inverter circuit 25b. This suppresses the current flowing through the switching elements SW1 to SW4 of the inverter circuit 25b.

[0062] Furthermore, according to this embodiment, when the control circuit 28b receives a synchronization signal, it increases the output current of the inverter circuit 22b, thereby increasing the supply of DC current to the inverter circuit 25b. By compensating for the decrease in the output current of the welding power supply device 2a with the increase in the output current of the welding power supply device 2b, the welding system A1 can suppress the decrease in output current and thus suppress arc interruption.

[0063] In this embodiment, the case in which the welding power supply device 2 controls output using an inverter circuit 22 has been described, but it is not limited to this. The welding power supply device 2 may also be equipped with a configuration other than an inverter circuit that controls output, such as a thyristor-controlled power supply or a movable core power supply.

[0064] Furthermore, although this embodiment describes a case where the control circuits 28 of the welding power supply unit 2a and the welding power supply unit 2b each perform polarity switching processing, it is not limited to this. For example, the control device 1 may perform polarity switching processing for the welding power supply unit 2a and the welding power supply unit 2b.

[0065] Furthermore, although this embodiment describes the case where there are two welding power supply units 2, it is not limited to this. The welding system A1 may be equipped with three or more welding power supply units 2. For example, if the welding system A1 is further equipped with a welding power supply unit 2c, the control circuit 28c of the welding power supply unit 2c performs the same control as the control circuit 28a of the welding power supply unit 2a. In other words, when switching polarity, the control circuit 28c performs the same polarity switching process as the control circuit 28a.

[0066] [Second Embodiment] Figure 6 is a diagram illustrating welding system A2 according to the second embodiment. Figure 6 is a timing chart illustrating the polarity switching process. The overall configuration of welding system A2 according to the second embodiment, as well as the internal configuration of the control device and the multiple welding power supply devices, are the same as those in Figures 1 and 2, and therefore are not shown or described. Welding system A2 according to this embodiment differs from welding system A1 according to the first embodiment in that the inductance values ​​of the DC reactor DCL are different in the rectifier-smoothing circuit 24a and the rectifier-smoothing circuit 24b.

[0067] In this embodiment, the inductance value La of the DC reactor DCL in the rectifier and smoothing circuit 24a of the welding power supply unit 2a is smaller than the inductance value Lb of the DC reactor DCL in the rectifier and smoothing circuit 24b of the welding power supply unit 2b. The smaller the inductance value La, the shorter the time (the time from time t0 to time t1 shown in Figure 6) from the time the supply of DC current to the inverter circuit 25a is stopped during polarity switching until the output current of the welding power supply unit 2a becomes almost "0". Therefore, in order to shorten the time required for polarity switching, it is desirable for the inductance value La to be smaller. On the other hand, if the inductance value Lb is too small, re-ignition may not be possible during polarity switching. Therefore, in order to perform re-ignition smoothly during polarity switching, it is desirable for the inductance value Lb to be larger.

[0068] In the first embodiment, both the welding power supply unit 2a and the welding power supply unit 2b used saturable reactors with the same characteristics as the DC reactor DCL. However, in the second embodiment, reactors with different characteristics are used as the DC reactor DCL. The DC reactor DCL of the welding power supply unit 2a has a constant inductance value of several μH from the low current range to the high current range. On the other hand, the DC reactor DCL of the welding power supply unit 2b uses a saturable reactor similar to the first embodiment, inductance value changing from several tens of μH to several μH from the low current range to the high current range. The inductance value La of the DC reactor DCL of the welding power supply unit 2a is smaller than the minimum value of the inductance value Lb of the DC reactor DCL of the welding power supply unit 2b. Note that the inductance value La is not limited, and the range of the inductance value Lb is also not limited. Furthermore, the rectifier and smoothing circuit 24a of the welding power supply unit 2a may not have a DC reactor DCL and may only be connected by a power line. In this case, the inductance value La is the inductance value of the power line and can be made even smaller. Also, the DC reactor DCL of the welding power supply unit 2b may have an inductance value of several hundred μH in the low current range in order to enable smoother re-ignition.

[0069] Figure 6 is a timing chart illustrating the polarity switching process in welding system A2 according to the second embodiment. Similar to Figure 4, Figure 6(a) shows the time variation of the synchronization signal output by control device 1. Figure 6(b) shows the time variation of the output current Ia of welding power supply device 2a. Figure 6(c) shows the time variation of the output current Ib of welding power supply device 2b. Figure 6(d) shows the time variation of the output current Iout of welding system A1.

[0070] In the welding system A2 according to the second embodiment, the inductance value La is set smaller compared to the welding system A1, so the time from when the supply of DC current to the inverter circuit 25a is stopped at time t0 until the output current Ia of the welding power supply device 2a becomes almost "0" at time t1 is shortened. Consequently, the time from when the synchronization signal is transmitted from the control device 1 until the polarity is switched (the time from time t0 to time t2) is shortened.

[0071] In this embodiment as well, when switching polarity, the control circuit 28a stops the inverter circuit 22a, thereby stopping the supply of DC current to the inverter circuit 25a, and when the output current of the welding power supply device 2a becomes approximately "0", it turns off all of the switching elements SW1 to SW4 of the inverter circuit 25a. Subsequently, the control circuit 28b turns on all of the switching elements SW1 to SW4 of the inverter circuit 25b. Then, when the output current of the welding power supply device 2b has decreased sufficiently, the control circuit 28b turns off the switching elements SW1 to SW4 that were on before the polarity switching process, thereby switching the polarity of the welding power supply device 2b. When the polarity of the welding power supply device 2b is switched, all of the switching elements SW1 to SW4 of the inverter circuit 25a are off, so no lateral current flows from the welding power supply device 2b to the welding power supply device 2a. Subsequently, the control circuit 28a turns on the switching elements SW1 to SW4 that were off before the polarity switching process, thereby switching the polarity of the welding power supply device 2a. At this time, since the polarity of the welding power supply device 2b has been switched, no lateral current flows. Thus, welding system A2 can prevent crosscurrent caused by switching the polarity of welding power supplies 2a and 2b. Furthermore, when all switching elements SW1 to SW4 of inverter circuit 25a are turned off, the output current of welding power supply 2a decreases, and when the polarity of welding power supply 2b is switched, the output current of welding power supply 2b decreases. Therefore, welding system A2 can reduce surge voltage generated by external inductance. In addition, welding system A2 achieves the same effects as welding system A1 due to its common configuration.

[0072] Furthermore, according to this embodiment, the inductance value La of the DC reactor DCL in the rectifier and smoothing circuit 24a of the welding power supply unit 2a is smaller than the inductance value Lb of the DC reactor DCL in the rectifier and smoothing circuit 24b of the welding power supply unit 2b. Therefore, the time required for polarity switching can be shortened compared to the case where the inductance value La is the same as the inductance value Lb. Also, since the inductance value Lb is set to be larger than the inductance value La, re-ignition can be performed smoothly when switching polarity.

[0073] In this embodiment as well, the welding system A2 may be equipped with three or more welding power supply units 2. For example, if the welding system A2 is further equipped with a welding power supply unit 2c, the control circuit 28c of the welding power supply unit 2c performs the same control as the control circuit 28a of the welding power supply unit 2a. Therefore, the inductance value Lc of the DC reactor DCL in the rectifier and smoothing circuit 24c of the welding power supply unit 2c should be set to approximately the same value as the inductance value La.

[0074] In the first and second embodiments, the welding systems A1 and A2 were described in which the wire feeder 5 feeds the welding wire and the tip portion of the welding wire protruding from the tip of the welding torch 3 is used as the electrode 8, but the system is not limited to this. The welding systems A1 and A2 may use a non-melting electrode such as tungsten as the electrode 8.

[0075] Furthermore, although the first and second embodiments were described using a submerged arc welding system as an example, the present invention is not limited thereto. The present invention can also be applied to welding systems other than submerged arc welding systems.

[0076] The welding system and polarity switching method according to the present invention are not limited to the embodiments described above. The specific configuration of each part of the welding system and polarity switching method according to the present invention can be modified in various ways. [Explanation of Symbols]

[0077] A1-A2: Welding system, 1: Control device, 2,2a,2b: Welding power supply, 25,25a,25b: Inverter circuit, 28,28a,28b: Control circuit, SW1-SW4: Switching element, DCL: DC reactor

Claims

1. The system comprises a first welding power supply unit and a second welding power supply unit whose output sides are connected in parallel, and a control device that controls the first welding power supply unit and the second welding power supply unit. The first welding power supply device is A first DC power supply having a first DC reactor, A first inverter circuit having two first switching elements connected in series, and converting DC power input from the first DC power supply into AC power, A first control circuit that controls the first DC power supply and the first inverter circuit, Equipped with, The second welding power supply device is A second DC power supply having a second DC reactor, A second inverter circuit having two second switching elements connected in series, and converting DC power input from the second DC power supply into AC power, A second control circuit for controlling the second DC power supply and the second inverter circuit, Equipped with, The first inductance of the first DC reactor is smaller than the second inductance of the second DC reactor. The control device transmits a synchronization signal to the first welding power supply unit and the second welding power supply unit to instruct them on the timing to switch polarity. When the first control circuit receives the synchronization signal, it stops the first DC power supply, and when the absolute value of the output current of the first inverter circuit becomes less than or equal to the first threshold current, it turns off both of the two first switching elements and sends a first permission signal to the second welding power supply device, and when it receives a second permission signal from the second welding power supply device, it turns on one of the first switching elements. When the second control circuit receives the first permission signal, it turns on both of the two second switching elements, and when the absolute value of the output current of the second inverter circuit becomes less than or equal to the second threshold current, it turns off one of the second switching elements and transmits the second permission signal to the first welding power supply device. Welding system.

2. When the second control circuit receives the first permission signal, it further shuts off the second DC power supply. The welding system according to claim 1.

3. When the second control circuit receives the synchronization signal, it increases the output current of the second DC power supply. The welding system according to claim 1 or 2.

4. The first DC power supply does not have the first DC reactor. The second DC reactor is a saturable reactor. The welding system according to claim 1.

5. A polarity switching method in a welding system comprising a first welding power supply and a second welding power supply with their output sides connected in parallel, The first DC power supply of the first welding power supply device is equipped with a first DC reactor. The second DC power supply of the second welding power supply device is equipped with a second DC reactor. The first inductance of the first DC reactor is smaller than the second inductance of the second DC reactor. When switching polarity, First, the first DC power supply is shut off, When the absolute value of the output current of the first inverter circuit of the first welding power supply device falls below the first threshold current, both of the two first switching elements connected in series in the first inverter circuit are turned off. Next, both of the two second switching elements connected in series in the second inverter circuit of the second welding power supply device are turned on. When the absolute value of the output current of the second inverter circuit falls below the second threshold current, one of the second switching elements is turned off and one of the first switching elements is turned on. Method for switching polarity.

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