Patent Publication Number: US-2019184483-A1

Title: Shielded metal arc welding system and welding power supply for shielded metal arc welding

Description:
FIELD 
     The present disclosure relates to a system for shielded metal arc welding, and also to a welding power supply for shielded metal arc welding. 
     BACKGROUND 
     Shielded metal arc welding is known as one of the conventional welding methods. JP-A-06-126459, for example, discloses a shielded metal arc welding system. Shielded metal arc welding, unlike CO 2  arc welding, does not need to use shielded gas, so that welding can be performed with relatively simple apparatus. 
       FIG. 8A  is a block diagram showing an example of a conventional shielded metal arc welding system. The illustrated shielded metal arc welding system includes a coated electrode B, an electrode holder C for holding the coated electrode B, and a welding power supply A 100  for supplying electric power to the coated electrode B via the electrode holder C. The welding power supply A 100  is provided with a transformer  300  for transforming AC power. The AC power from a commercial power supply D is inputted to the primary side of the transformer  300 , and outputted from the secondary side of the transformer  300  after the transformation. The welding power supply A 100  has an output terminal a 1  connected to a workpiece W and another output terminal b 1  connected to the electrode holder C. 
       FIG. 8B  is a block diagram showing a welding power supply A 101  simulating the above-noted welding power supply A 100 . As shown in  FIG. 8B , the welding power supply A 101  includes a rectifying/smoothing circuit  1 , an inverter circuit  2 , a transformer  3 , a rectifying/smoothing circuit  5 , an inverter circuit  7  and a controlling circuit  800 . The AC power inputted from the three-phase commercial power supply D is converted into DC power by the rectifying/smoothing circuit  1  and then converted into high-frequency power by the inverter circuit  2 . The high-frequency power is transformed by the transformer  3 , and converted into DC power by the rectifying/smoothing circuit  5 . This DC power is converted into AC power by the inverter circuit  7  and outputted from the welding power supply A 101 . The controlling circuit  800  controls switching of the inverter circuit  2  to perform feedback control so that the output current from the welding power supply A 101  follows the target current. Also, the controlling circuit  800  controls switching of the inverter circuit  7  so that sinusoidal alternating current is outputted as with the welding power supply A 100 . 
     In the above welding power supply A 101 , however, there is room for improvement in terms of power factors. 
     To elaborate on that above issue, the inventors performed welding using the welding power supply A 101  with input of AC power from a commercial power supply D.  FIG. 9  is a diagram showing the waveforms of input voltage Vin, input current Iin and output current Iout. The frequency of the commercial power supply D is 60 Hz. The frequency of the electric power outputted from the welding power supply A 101  is 50 Hz. The input voltage Vin is 100 V (effective value), the input current Iin is 20 A (effective value), and the target current of the output current Iout is 100 A (effective value). The input voltage Vin and the input current Iin in the figure indicate the measurements of the input voltage and the input current for one of the three phases, and the other two phases have waveforms phase-shifted by 120° and 240°, respectively. As shown in  FIG. 9 , the waveform of the output current Iout from the inverter circuit  7  is sinusoidal (i.e., ever-changing) due to the control of the controlling circuit  800 . In such a case, if the output current Iout is large (the absolute value of the instantaneous value is large) when the input voltage Vin is low (the absolute value of the instantaneous value is small), large input current Iin flows to maintain the output. Conversely, in the case where the output current Iout is small when the input voltage Vin is high, the input current Iin cannot flow much. In such a case, most of the flowing current is charging current for the smoothing circuit, which increases the reactive power. As shown in  FIG. 9 , the input current Iin is not stable, changing rapidly in accordance with the relationship between the input voltage Vin and the output current Iout. Moreover, since the reactive power is increased, the power factor is low. Note that the waveform of the input voltage Vin differs depending on the phase. Even if the power factor is not so low for one phase, the power factor is low for other phases, so that the power factor as a whole becomes low. The power factor actually measured was about 68%. 
     SUMMARY 
     The present disclosure has been proposed under the foregoing circumstances and aims to provide a shielded metal arc welding system with improved power factor. 
     According to a first aspect of the disclosure, there is provided a welding power supply for supplying electric power to a coated electrode. The welding power supply may include a rectifying circuit that converts AC power inputted to the rectifying circuit into DC power, and an inverter circuit that converts the DC power into AC power to be outputted to the coated electrode. The inverter circuit outputs a square-wave current. 
     According to a second aspect of the disclosure, there is provided a shielded metal arc welding system that may include a welding power supply in accordance with the above-noted first aspect, and a coated electrode that receives power supply from the welding power supply. 
     According to the welding power supply of the first aspect or the shielded metal arc welding system of the second aspect, the waveform of the current outputted from the inverter circuit is square. Hence, the output current is maintained at the peak level except the short period of time in which the direction of the current changes. Thus, less reactive power is generated, which is advantageous to increasing power factor. 
    
    
     
       DRAWINGS 
         FIG. 1  is a block diagram showing the entire configuration of a shielded metal arc welding system according to a first embodiment; 
         FIG. 2A  is a diagram showing an example of a charging circuit according to the first embodiment; 
         FIG. 2B  is a diagram showing an example of a discharging circuit according to the first embodiment; 
         FIG. 3  is a time chart for explaining control of a restriking circuit and shows waveforms of various signals of a welding power supply; 
         FIG. 4  is a waveform diagram for explaining the waveform of input current of the welding power supply according to the first embodiment; 
         FIG. 5  shows the relationship between the output frequency and the power factor; 
         FIG. 6  is a block diagram showing the entire configuration of a shielded metal arc welding system according to a second embodiment; 
         FIG. 7  is a block diagram showing the entire configuration of a shielded metal arc welding system according to a third embodiment; 
         FIG. 8A  is a block diagram showing an example of conventional shielded metal arc welding system; 
         FIG. 8B  is a block diagram showing a shielded metal arc welding system including a welding power supply with an inverter circuit, which simulates the welding power supply shown in  FIG. 8A ; and 
         FIG. 9  is a waveform diagram for explaining the waveform of input current of the welding power supply shown in  FIG. 8B . 
     
    
    
     EMBODIMENTS 
     Embodiments of the present disclosure are described below with reference to the accompanying drawings. 
       FIGS. 1-4  are views for explaining a shielded metal arc welding system according to a first embodiment.  FIG. 1  is a block diagram showing the internal configuration of a welding power supply A 1  according to the first embodiment and shows the entire configuration of the shielded metal arc welding system.  FIG. 2A  is a circuit diagram showing an example of a charging circuit  63  of the welding power supply A 1 .  FIG. 2B  is a circuit diagram showing an example of a discharging circuit  64  of the welding power supply A 1 .  FIG. 3  is a time chart for explaining control of a restriking circuit  6  and shows waveforms of various signals of the welding power supply A 1 .  FIG. 4  is a waveform diagram for explaining the waveform of input current of the welding power supply according to the first embodiment. 
     As shown in  FIG. 1 , the shielded metal arc welding system includes a welding power supply A 1 , a coated electrode B and an electrode holder C. The electrode holder C is a part to be held by an operator in performing welding. The electrode holder C holds the coated electrode B and is configured to cause AC current inputted from the welding power supply A 1  to flow through the coated electrode B. The welding power supply A 1  converts the AC power inputted from a commercial power supply D and outputs the converted power via the output terminals a 1  and b 1 . The output terminal a 1  is connected to a workpiece W by a cable, and the output terminal b 1  is connected to the electrode holder C by a cable. In the welding power supply A 1 , the tip of the coated electrode B is brought into contact with a workpiece W and then pulled away to form an arc. 
     The welding power supply A 1  includes a rectifying/smoothing circuit  1 , an inverter circuit  2 , a transformer  3 , a rectifying/smoothing circuit  5 , a restriking circuit  6 , an inverter circuit  7 , a controlling circuit  8 , a current sensor  91 , voltage sensors  92  and  93 , and an auxiliary power supply circuit  10 . 
     The rectifying/smoothing circuit  1  converts the AC power inputted from the commercial power supply D into DC power and outputs the DC power. The rectifying/smoothing circuit  1  includes a rectifying circuit that rectifies an AC current and a smoothing capacitor for smoothing the rectified current. The configuration of the rectifying/smoothing circuit  1  is not particularly limited. 
     For example, the inverter circuit  2  is a single-phase full-bridge type PWM control inverter and has four switching elements. The inverter circuit  2  converts the DC power inputted from the rectifying/smoothing circuit  1  into high-frequency power by switching the switching elements based on output control driving signals inputted from the controlling circuit  8 , and outputs the high-frequency power. The inverter circuit  2  may be a half-bridge inverter or another type of inverter circuit as long as it can convert DC power to high-frequency power. 
     The transformer  3  transforms the high-frequency voltage outputted from the inverter circuit  2  and outputs it to the rectifying/smoothing circuit  5 . The transformer  3  includes a primary winding  3   a,  a secondary winding  3   b  and an auxiliary winding  3   c.  The input terminals of the primary winding  3   a  are connected to respective output terminals of the inverter circuit  2 . The output terminals of the secondary winding  3   b  are connected to respective input terminals of the rectifying/smoothing circuit  5 . The secondary winding  3   b  is provided with a center tap separately from the two output terminals. The center tap of the secondary winding  3   b  is connected to an output terminal b 1  via a connection line  4 . The output voltage from the inverter circuit  2  is transformed in accordance with the winding turns ratio of the primary winding  3   a  and the secondary winding  3   b  and inputted into the rectifying/smoothing circuit  5 . The output terminals of the auxiliary winding  3   c  are connected to respective input terminals of the charging circuit  63 . The output voltage from the inverter circuit  2  is transformed in accordance with the winding turns ratio of the primary winding  3   a  and the auxiliary winding  3   c  and inputted into the charging circuit  63 . Since the secondary winding  3   b  and the auxiliary winding  3   c  are insulated from the primary winding  3   a,  the current inputted from the commercial power supply D is prevented from flowing to the circuits on the secondary side or the charging circuit  63 . 
     The rectifying/smoothing circuit  5  converts the high-frequency power inputted from the transformer  3  into DC power and outputs the DC power. The rectifying/smoothing circuit  5  includes a full-wave rectifying circuit  51  that rectifies high-frequency current, and DC reactors  52  for smoothing the rectified current. The configuration of the rectifying/smoothing circuit  5  may be varied. In this disclosure, a combination of the rectifying/smoothing circuit  1 , the inverter circuit  2 , the transformer  3  and the rectifying/smoothing circuit  5  may be considered as a single rectifying circuit. 
     The DC reactors  52  are arranged on the positive-electrode-side connection line and on the negative-electrode-side connection line, respectively, that connect the full-wave rectifying circuit  51  and the inverter circuit  7 . These two DC reactors  52  are coupled to each other. The DC reactors  52  release the stored energy at the time of polarity switching, thereby serving to prevent arc extinction. In the present embodiment, the self-inductance of each DC reactor  52  is set relatively low by improving the coupling of the two DC reactors  52 . In the present embodiment, since the restriking circuit  6  applies restriking voltage in switching the polarity, the self-inductance of each DC reactor  52  can be low. In the present embodiment, the fluctuation of the input current is reduced, and the fluctuation of the current inputted into the DC reactor  52  is also reduced as will be described later, which also allows the self-inductance of each DC reactor  52  to be reduced. It is sufficient that the self-inductance of the DC reactors  52  is from about 20 to about 70 μH. In the present embodiment, the self-inductance is about 50 μH. 
     The inverter circuit  7  may be a single-phase full bridge inverter of PWM control and has two switching elements. The output terminal of the inverter circuit  7  is connected to the output terminal a 1 . In the inverter circuit  7 , the switching elements are switched based on switching driving signals inputted from the controlling circuit  8  so as to alternately change the potential of the output terminal of the inverter circuit  7  (the potential of the output terminal a 1 ) between the potential of the output terminal on the positive electrode side and the potential of the output terminal on the negative electrode side of the rectifying/smoothing circuit  5 . By this operation, the inverter circuit  7  performs alternate switching between the forward polarity (where the potential of the output terminal a 1  connected to the workpiece W is higher than the potential of the output terminal b 1  connected to the coated electrode B via the electrode holder c) and the reversed polarity (where the potential of the output terminal a 1  is lower than that of the output terminal b 1 ). In this manner, the inverter circuit  7  converts the DC power inputted from the rectifying/smoothing circuit  5  into AC power and outputs the AC power. The current outputted from the inverter circuit  7  has a square waveform, meaning that the direction of the current changes at the time when the polarity changes and otherwise the amplitude is maintained at the maximum or the minimum value in a certain period of time. The inverter circuit  7  may have a configuration different from that described above as long as it outputs square wave alternating current. 
     The restriking circuit  6  is arranged between the rectifying/smoothing circuit  5  and the inverter circuit  7 . The restriking circuit  6  applies restriking voltage across the output terminals a 1  and b 1  of the welding power supply A 1  at the time of switching the output polarity of the welding power supply A 1 . The restriking voltage is a high voltage applied to achieve reliable restriking at the time of switching the polarity. Arc extinction is likely to occur when the output polarity switches from the forward polarity to the reversed polarity. In the present embodiment, therefore, the restriking circuit  6  applies the restriking voltage only when the polarity switches from the forward polarity to the negative polarity and does not apply the restriking voltage when the polarity switches from the reversed polarity to the forward polarity. The restriking circuit  6  includes a diode  61 , a restriking capacitor  62 , a charging circuit  63  and a discharging circuit  64 . 
     The diode  61  and the restriking capacitor  62  are connected in series to each other and in parallel to the input side of the inverter circuit  7 . The diode  61  has an anode terminal connected to the input terminal on the positive electrode side of the inverter circuit  7  and a cathode terminal connected to one of the terminals of the restriking capacitor  62 . One of the terminals of the restriking capacitor  62  is connected to the cathode terminal of the diode  61 , and the other terminal of the restriking capacitor  62  is connected to the input terminal on the negative electrode side of the inverter circuit  7 . The restriking capacitor  62  has a predetermined capacitance and is charged with a restriking voltage that will be added to the output from the welding power supply A 1 . The restriking capacitor  62  is charged by the charging circuit  63  and discharged by the discharging circuit  64 . Cooperating with the diode  61 , the restriking capacitor  62  absorbs the surge voltage at the time of switching the inverter circuit  7 . That is, the restriking capacitor  62  also functions as a snubber circuit for absorbing surge voltage. 
     The charging circuit  63  is a circuit for charging the restriking capacitor  62  for generating the restriking voltage and connected in parallel to the restriking capacitor  62 .  FIG. 2A  shows an example of the charging circuit  63 . As shown in the figure, the charging circuit  63  includes a rectifying/smoothing circuit  63   c  and an isolated forward converter  63   d.  The rectifying/smoothing circuit  63   c  includes a rectifying circuit that performs full-wave rectification of AC voltage and a smoothing capacitor for smoothing the rectified voltage. The rectifying/smoothing circuit  63   c  converts the high-frequency voltage inputted from the auxiliary winding  3   c  of the transformer  3  into DC voltage. The isolated forward converter  63   d  raises the DC voltage inputted from the rectifying/smoothing circuit  63   c  and outputs it to the restriking capacitor  62 . The isolated forward converter  63   d  is provided with a drive circuit  63   a  for driving the switching element  63   b.  The drive circuit  63   a  outputs a pulse signal for driving the switching element  63   b  based on a charging circuit driving signal inputted from a charge controller  86 . While the charging circuit driving signal is on (e.g. high-level signal), the drive circuit  63   a  outputs a predetermined pulse signal to the switching element  63   b.  This causes the restriking capacitor  62  to be charged. While the charging circuit driving signal is off (e.g. low-level signal), the drive circuit  63   a  does not output a pulse signal. Thus, charging of the restriking capacitor  62  is interrupted. In this way, based on the charging circuit driving signal, the charging circuit  63  performs switching between the state for charging the restriking capacitor  62  and the state for not charging the restriking capacitor  62 . Note that the drive circuit  63   a  maybe dispensed with, and the charge controller  86  may directly input a pulse signal as the charging circuit driving signal into the switching element  63   b.  The configuration of the charging circuit  63  may be varied. For example, the charging circuit  63  maybe provided with a step-up chopper circuit, a step-down chopper circuit, instead of the isolated forward converter  63   d.  Further, the power to be supplied to the charging circuit  63  is not limited to the power from the auxiliary winding  3   c  of the transformer  3 . For example, the transformer  3  may not include the auxiliary winding  3   c,  and the power may be supplied from the secondary winding  3   b  or other power supplies. 
     The discharging circuit  64  discharges the restriking voltage charged in the restriking capacitor  62 . The discharging circuit  64  is connected between the connection point of the diode  61  and the restriking capacitor  62  and the connection line  4  that connects the center tap of the secondary winding  3   b  and the output terminal b 1 .  FIG. 2B  shows an example of the charging circuit  64 . As shown in the figure, the discharging circuit  64  includes a switching element  64   a  and a current limiting resistor  64   b.  In the present embodiment, the switching element  64   a  is an IGBT (Insulated Gate Bipolar Transistor). The switching element may be a bipolar transistor, a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or the like. The switching element  64   a  and the current limiting resistor  64   b  are connected in series to each other and connected in series to the restriking capacitor  62 . The collector terminal of the switching element  64   a  is connected to one of the terminals of the current limiting resistor  64   b,  and the emitter terminal of the switching element  64   a  is connected to the connection line  4  via the connection line  64   c.  Note that the current limiting resistor  64   b  may be connected to the emitter side of the switching element  64   a . The discharge controller  85 , which will be described later, inputs a discharging circuit driving signal to the gate terminal of the switching element  64   a . While the discharging circuit driving signal is on (e.g. high-level signal), the switching element  64   a  is in the ON state. In this state, the restriking voltage charged in the restriking capacitor  62  is discharged via the current limiting resistor  64   b.  While the discharging circuit driving signal is off (e.g. low-level signal), the switching element  64   a  is in the OFF state. In this state, discharge of the restriking voltage is interrupted. In this way, based on the discharging circuit driving signal, the discharging circuit  64  is switched between the state for discharging the restriking capacitor  62  and the state for not discharging the restriking capacitor  62 . 
     The current sensor  91  detects the output current from the welding power supply A 1 . In the present embodiment, the current sensor  91  is arranged on the connection line  71  that connects the output terminal of the inverter circuit  7  and the output terminal a 1 . In the present embodiment, current may flow from the inverter circuit  7  toward the output terminal a 1  (which is referred to as “positive” state), or may flow from the output terminal a 1  toward the inverter circuit  7  (which is referred to as “negative” state). The current sensor  91  detects the instantaneous value of the output current and inputs it to the controlling circuit  8 . The current sensor  91  may have any configuration as long as it detects the output current from the connection line  71 . Further, the position of the current sensor  91  is not limited to the illustrated one. For example, the current sensor  91  may be placed on the connection line  4 . 
     The voltage sensor  92  detects the voltage between the terminals of the restriking capacitor  62 . The voltage sensor  92  detects the instantaneous value of the voltage between the terminals and inputs it to the controlling circuit  8 . The voltage sensor  93  detects the voltage between the output terminals a 1  and b 1 . The voltage sensor  93  detects the instantaneous value of the voltage between the terminals and inputs it to the controlling circuit  8 . 
     The auxiliary power supply circuit  10  is a power supply that applies an auxiliary voltage across the output terminals a 1  and b 1 . The welding power supply A 1  supplies power for welding in response to the timing when the tip of the coated electrode B (held in contact with the workpiece W) is separated from the workpiece W. To detect the timing, the auxiliary power supply circuit  10  applies an auxiliary voltage lower than the no-load voltage. For example, the auxiliary voltage is a DC voltage of 20 V. Application of a high no-load voltage during suspension of the welding operation can be dangerous for operators if they touch the coated electrode B by mistake. Thus, the auxiliary voltage to be applied may preferably be lower than the no-load voltage so as not to harm the operator. The auxiliary power supply circuit  10  for applying an auxiliary voltage may be provided separately from the power supply for supplying the power for welding. Instead of providing the auxiliary power supply circuit  10 , the output from the inverter circuit  2  may be decreased to provide low no-load voltage. Part of the output power from the inverter circuit  2  is supplied to the auxiliary power supply circuit  10  via an auxiliary winding (not shown) of the transformer  3 . The auxiliary power supply circuit  10  converts the inputted AC voltage into DC voltage and outputs it. As a variation, the auxiliary power supply circuit  10  may be configured to receive power from other power supply sources. The auxiliary power supply circuit  10  outputs the auxiliary voltage in response to a signal from a switching unit  87 . 
     The controlling circuit  8  controls the welding power supply A 1  and its function maybe implemented by a microcomputer, for example. To the controlling circuit  8 , the instantaneous value of the output current is inputted from the current sensor  91 , the instantaneous value of the voltage between the terminals of the restriking capacitor  62  is inputted from the voltage sensor  92 , and the instantaneous value of the voltage between the output terminals a 1  and b 1  is inputted from the voltage sensor  93 . The controlling circuit  8  outputs a driving signal to each of the inverter circuit  2 , the inverter circuit  7 , the charging circuit  63  and the discharging circuit  64 . The controlling circuit  8  includes a current controller  81 , a target current setter  82 , a polarity switching controller  83 , a discharge controller  85 , a charge controller  86  and a switching unit  87 . 
     The current controller  81  controls the inverter circuit  2  for achieving feedback control with respect to the output current from the welding power supply A 1 . The current controller  81  converts the instantaneous value signal of the output current inputted from the current sensor  91  into an absolute value signal by using an absolute value circuit. Based on the deviation between the absolute value signal and the target current value inputted from the target current setter  82 , the current controller  81  generates an output control driving signal for controlling the switching elements of the inverter circuit  2  by PWM control. The current controller  81  forwards generated output control driving signals to the inverter circuit  2  upon receiving a start signal from the switching unit  87 . 
     The polarity switching controller  83  controls the inverter circuit  7  to switch the output polarity of the welding power supply A 1 . The polarity switching controller  83  generates a switching driving signal that is a pulse signal for controlling the switching elements to switch the output polarity of the inverter circuit  7 . The polarity switching controller  83  outputs switching driving signals to the inverter circuit  7  upon receiving a start signal from the switching unit  87 . The switching driving signal is outputted also to the discharge controller  85 . 
     As shown in  FIG. 3 , the output current (see (b)) from the welding power supply A 1  changes in accordance with the switching driving signal (see (a)). When the switching driving signal is on, the potential of the output terminal a 1  (workpiece W) is higher than the potential of the output terminal b 1  (coated electrode B) (i.e., forward polarity), while the switching driving signal is off, the potential of the output terminal a 1  (workpiece W) is lower than the potential of the output terminal b 1  (coated electrode B) (i.e., reversed polarity). The output current from the welding power supply A 1  continues to decrease after the switching driving signal is changed from ON to OFF (time t 1  in  FIG. 3 ), changing its polarity at time t 2  and reaching the minimum current value. On the other hand, the output current from the welding power supply A 1  continues to increase after the switching driving signal is changed from OFF to ON (time t 4  in  FIG. 3 ), changing its polarity at time t 5  and reaching the maximum current value. The time taken for the output current of the welding power supply A 1  to change from the maximum current value to the minimum current value and the time taken for the output current of the welding power supply A 1  to change from the minimum current value to the maximum current value are sufficiently short as compared with the period of the switching driving signal (hence the period of the output current). Thus, the output current can be considered as having a form of square waves. 
     The output frequency, i.e., the frequency of the output power (output voltage, output current) from the inverter circuit  7 , becomes the same as the frequency of the switching driving signal. The frequency of the switching driving signal (output frequency) can be set appropriately and can be changed depending on the welding operation. 
     The discharge controller  85  controls the discharging circuit  64 . Based on the switching driving signal inputted from the polarity switching controller  83 , the discharge controller  85  generates a discharging circuit driving signal for controlling the discharging circuit  64  and outputs it to the discharging circuit  64 . The discharging circuit driving signal is inputted also to the charge controller  86 . 
     The discharge controller  85  generates the discharging circuit driving signal in such a manner that the discharging circuit driving signal is on when the output current from the welding power supply A 1  changes from positive to negative. Specifically, the discharge controller  85  generates a pulse signal that switches to ON when the switching driving signal is switched from ON to OFF (time t 1  in  FIG. 3 ) and that switches to OFF (at time t 3  in  FIG. 3 ) after the lapse of a predetermined time period T 1  since the pulse signal was switched to ON. The discharge controller  85  outputs this pulse signal as the discharging circuit driving signal (see (c) in  FIG. 3 ). 
     Time period T 1  is the period during which the discharge state is maintained. The time period T 1  is set to cover the timing (time t 2  in  FIG. 3 ) at which the output current of the welding power supply A 1  changes from positive to negative. 
     The manner in which the discharge controller  85  generates the discharging circuit driving signal is not limited to the above. It is only required that the restriking voltage is applied when the output current from the welding power supply A 1  changes from positive to negative, so that the discharging circuit driving signal is only required to become ON before the output current changes from positive to negative and become OFF after the output current is changed from positive to negative. For example, the discharge controller  85  may generate the discharging circuit driving signal based on the output current from the welding power supply A 1 . Specifically, the discharge controller  85  may switch the discharging circuit driving signal to OFF when the instantaneous value of the output current drops below a predetermined current I. The current I 1  is a current value between the minimum current value and zero for determining the completion of the arc restriking. The current I 1  is set to a value that enables reliable determination of the changing of the output current direction even if the value of the detected output current contains a certain error. 
     The charge controller  86  controls the charging circuit  63 . The charge controller  86  generates a charging circuit driving signal for driving the charging circuit  63  based on the discharging circuit driving signal inputted from the discharge controller  85  and the instantaneous value of the voltage between the terminals of the restriking capacitor  62  inputted from the voltage sensor  92 , and outputs the charging circuit driving signal to the charging circuit  63 . 
     As shown in  FIG. 3 , when the direction of the output current changes (time t 2  in  FIG. 3 ) after the discharging circuit driving signal ((c) in  FIG. 3 ) is switched to ON (time t 1  in  FIG. 3 ), the voltage between terminals of the restriking capacitor  62  (see (e) in  FIG. 3 ) drops sharply because of the flow of the restriking current. The restriking capacitor  62  needs to be charged with the restriking voltage before the timing of the next discharge. When the restriking capacitor  62  is charged to the target voltage V 0 , further charging is not necessary. Thus, the charge controller  86  generates the charging circuit driving signal so that the charging circuit driving signal is on from when the restriking capacitor  62  is discharged till when the restriking capacitor  62  is charged to the target voltage V 0 . Specifically, the charge controller  86  generates a pulse signal that switches to ON when the discharging circuit driving signal inputted from the discharge controller  85  is switched from ON to OFF (time t 3  in  FIG. 3 ) and that switches to OFF when the voltage between the terminals of the restriking capacitor  62  has reached the target voltage V 0  (time t 6  in  FIG. 3 ). The charge controller  86  outputs this pulse signal as the charging circuit driving signal (see (d) in  FIG. 3 ). 
     The manner in which the charge controller  86  generates the charging circuit driving signal is not limited to the above. The timing of the start and end of charging is not limited, and it is only required that the restriking capacitor  62  is charged with the restriking voltage before the timing of next discharge. 
     The switching unit  87  switches the voltage applied to the output terminals a 1  and b 1  between the auxiliary voltage outputted from the auxiliary power supply circuit  10  and the voltage outputted from the inverter circuit  7 . When the welding power supply A 1  is activated, the switching unit  87  outputs a start signal to the auxiliary power supply circuit  10  to cause the auxiliary power supply circuit  10  to output the auxiliary voltage. When the voltage between the output terminals a 1  and b 1  inputted from the voltage sensor  9  rises to a threshold value after once dropped to approximately zero, the switching unit  87  determines that the tip of the coated electrode B is separated from the workpiece W after brought into contact with the workpiece W and hence performs the voltage switching. Specifically, the switching unit  87  outputs a stop signal to the auxiliary power supply circuit  10  to make the auxiliary power supply circuit  10  stop outputting the auxiliary voltage, and outputs a start signal to the current controller  81  and the polarity switching controller  83  to start the output from the inverter circuit  7 . Note that the determination by the switching unit  87  that the tip of the coated electrode B is separated from the workpiece W after brought into contact with the workpiece W may be performed based on the output current detected by the current sensor  91 . 
     The operation and advantages of the shielded metal arc welding system according to the present embodiment are described below. 
     According to the present embodiment, arc welding is performed by forming an arc between the tip of the coated electrode B and the workpiece W using the AC power outputted from the welding power supply A 1 . The controlling circuit  8  controls the inverter circuit  7  so that the waveform of the output current Iout becomes square. Accordingly, the output current Iout is always maintained at the peak level except the short period of time in which the direction of the current changes. Thus, reactive power is unlikely to be generated, which leads to increased power factor. 
       FIG. 4  is a waveform diagram for explaining the waveform of the input current of the welding power supply A 1 . Welding was performed using the welding power supply A 1  by actually inputting AC power from a commercial power supply D.  FIG. 4  shows the waveforms of the input voltage Vin, the input current Iin and the output current Iout which were actually measured in welding. Since the frequency of the commercial power supply D is 60 Hz, the input frequency is 60 Hz. The output frequency is set to 50 Hz. The effective value of the input voltage Vin is 100 V, the effective value of the input current Iin is 20 A, and the target current of the output current Iout is 100 A. The input voltage yin and the input current Iin in the figure indicate the measurements of the input voltage and the input current of one of the three phases inputted from the commercial power supply D. The input voltage and the input current of other two phases have waveforms phase-shifted by 120° and 240°, respectively, from the waveform of the input voltage Vin and the input current Iin shown in  FIG. 4 . As shown in  FIG. 4 , due to the control of the inverter circuit  7  with the controlling circuit  8 , the waveform of the output current Iout is square. Accordingly, the output current Iout is always kept at the peak level except the short period of time in which the direction of the current changes. Thus, the input current Iin does not change greatly and is stable. The power factor actually measured in the welding power supply A 1  was about 77%, which is considerable improvement as compared with the power factor (68%) in the case of the welding power supply A 100  shown in  FIG. 8B . 
     Moreover, according to the present embodiment, the self-inductance of the DC reactors  52  (e.g. about 50 μH) is considerably lower than the self-inductance (e.g. about 165 μH) in a conventional configuration. This contributes to power factor enhancement. Further, since the responsiveness is improved, the system can deal with high output frequencies. 
     According to the present embodiment, the discharge controller  85  of the welding power supply A 1  generates the discharging circuit driving signal in such a manner that the discharging circuit driving signal is on when the output current from the we power supply A 1  changes from positive to negative. The discharge controller  85  inputs the discharging circuit driving signal to the discharging circuit  64 . Thus, the discharging circuit  64  discharges and applies the restriking voltage charged in the restriking capacitor  62  when the output current from the welding power supply A 1  changes from positive to negative. Thus, arc extinction at the time when the output polarity of the welding power supply A 1  switches from forward polarity to reversed polarity is prevented. 
     According to the present embodiment, the discharging circuit  64  controls the discharge based on the discharging circuit driving signal inputted from the discharge controller  85 . The discharging circuit driving signal (see (c) in FIG.  3 ) switches to ON when the switching driving signal (see (a) in  FIG. 3 ) is switched, and then switches to OFF after the lapse of the time period T 1 . Thus, the discharging circuit driving signal is always on when the output current from the welding power supply A 1  changes from positive to negative. This allows the discharging circuit  64  to properly apply the restriking voltage. 
     According to the present embodiment, the charging circuit  63  controls the charge based on the charging circuit driving signal inputted from the charge controller  86 . The charging circuit driving signal (see (d) in  FIG. 3 ) switches to ON when the discharging circuit driving signal (see (c) in  FIG. 3 ) is switched to OFF. Thus, the charging circuit  63  starts charging immediately after the discharge. Thus, sufficient charging time is secured before the next discharge. The charging circuit driving signal switches to OFF when the voltage between the terminals of the restriking capacitor  62  (see (e) in  FIG. 3 ) reaches the target voltage V 0 . Thus, the charging circuit  63  does not excessively charge the restriking capacitor  62 . 
     The present embodiment has described the welding power supply A 1  in which the AC power having a frequency of 60 Hz (input frequency) is inputted from the commercial power supply D and AC power having a frequency of 50 Hz (output frequency) is outputted. However, the present disclosure is not limited to this. The input frequency is not limited, and the output frequency is not limited either. The output frequency may be set to a desired frequency, and some frequencies may contribute to further improvement of the power factor. 
       FIG. 5  shows the relationship between the output frequency and the power factor. Specifically, with the input frequency fixed at 60 Hz, the output frequency was changed to measure the power factor.  FIG. 5  shows the measurement results. The horizontal axis indicates the output frequency, whereas the vertical axis indicates the power factor. As shown in  FIG. 5 , the power factor is highest when the output frequency is 300 Hz. Further, although there are some up-and-down variations, the power factor generally increases with increasing output frequency in the output frequency range of 300 Hz or less, whereas the power factor generally decreases with increasing output frequency in the output frequency range of 300 Hz or more. Moreover, the power factor tends to be higher than the power factors at its closest frequencies when the output frequency is a natural number multiple of the input frequency, i.e., an N multiple of the input frequency, where N is a natural number 1, 2, 3, . . . . For example, the power factor P 1  when the output frequency is 60 Hz is higher than that when the output frequency is 50 Hz or 70 Hz, and the power factor P 2  when the output frequency is 180 Hz is higher than that when the output frequency is 160 Hz or 200 Hz. Therefore, if the increase of the power factor is the only concern, it is favorable to set the output frequency to 300 Hz when the input frequency is 60 Hz. As known in the art, a higher output frequency results in greater concentration, and hence a deeper penetration and a thinner weld bead. Generally, shielded metal arc welding tends to provide shallow penetration and a thick weld bead. To make the penetration deeper and the weld bead thinner, the output frequency may be set high. To keep the penetration shallow and the weld bead thick, like the conventional shielded metal arc welding, the output frequency may be set low. The output frequency may be made switchable between the frequencies that are natural number multiples of the input frequency. This allows switching of the output frequency so as to form a weld bead of a desired shape while enhancing the power factor. 
       FIGS. 6 and 7  show other embodiments of the present disclosure. In these figures, the elements that are identical or similar to those of the foregoing embodiment are designated by the same reference signs as those used for the foregoing embodiment. 
       FIG. 6  is a block diagram of the internal configuration of a welding power supply A 2  according to the second embodiment and shows the entire configuration of the shielded metal arc welding system. Note that, in  FIG. 6 , illustration of the internal configuration of the controlling circuit  8  is omitted. The welding power supply A 2  shown in  FIG. 6  differs from the welding power supply A 1  according to the first embodiment (see  FIG. 1 ) in that the restriking circuit  6  is arranged on the output side of the inverter circuit  7 . In the present embodiment, since the restriking capacitor  62  does not function as a snubber circuit for the inverter circuit  7 , the wiring on the negative side of the diode  61  and the restriking capacitor  62  (the wiring connected to the connection line  4 ) may be dispensed with. 
     In the second embodiment again, the effects similar to those of the first embodiment are achieved. 
       FIG. 7  is a block diagram showing the internal configuration of a welding power supply A 3  according to the third embodiment and shows the entire configuration of the shielded metal arc welding system. Note that, in  FIG. 7 , illustration of the internal configuration of the controlling circuit  8  is omitted. The welding power supply A 3  shown in  FIG. 7  differs from the welding power supply A 1  according to the first embodiment (see  FIG. 1 ) in that the inverter circuit  7  is a full-bridge inverter. 
     For example, the inverter circuit  7  according to the third embodiment is a single-phase full-bridge inverter with PWM control and has four switching elements. The inverter circuit  7  has an output terminal connected to the output terminal a 1  and another output terminal connected to the output terminal b 1 . By switching the switching elements based on the switching driving signal inputted from the controlling circuit  8 , the inverter circuit  7  performs switching between the two states: the state in which the potential of the one of the output terminals of the inverter circuit  7  (the potential at the output terminal a 1 ) is the potential of the positive side output terminal of the rectifying/smoothing circuit  5 , whereas the potential of the other output terminal (potential of the output terminal b 1 ) is the potential of the negative side output terminal of the rectifying/smoothing circuit  5 ; and the state in which the potential of the one of the output terminals of the inverter circuit  7  (potential at the output terminal a 1 ) is the potential of the negative side output terminal of the rectifying/smoothing circuit  5 , whereas the potential of the other output terminal (potential of the output terminal b 1 ) is the potential of the positive side output terminal of the rectifying/smoothing circuit  5 . In this way, the inverter circuit  7  performs alternate switching between the forward polarity where the potential of the output terminal a 1  is higher than that of the output terminal b 1  and the reversed polarity where the potential of the output terminal a 1  is lower than that of the output terminal b 1 . That is, the inverter circuit  7  converts the DC power inputted from the rectifying/smoothing circuit  5  into AC power and outputs the AC power. 
     In the third embodiment again, the effects similar to those of the first embodiment are achieved. In this embodiment, the restriking circuit  6  may apply the restriking voltage also when the output polarity switches from the reversed polarity to the forward polarity. 
     The shielded metal arc welding system and the welding power supply for shielded metal arc welding according to the present disclosure are not limited to the foregoing embodiments. The specific configuration of each part of the shielded metal arc welding system and the welding power supply for the shielded metal arc welding may be varied in many ways.