DESCRIPTION
ALTERNATING-CURRENT WELDING METHOD AND
ALTERNATING-CURRENT WELDING DEVICE
TECHNICAL FIELD
The present invention relates to a method and device for
performing AC (Alternating-Current) arc welding in which a
negative-polarity period and a positive-polarity period are alternated
BACKGROUND ART
In terms of environmental issues, aluminum and magnesium
materials have been used in recent years for buildings, vehicles, etc.
because of their lightweight and highly recyclable natures. These
materials are generally joined by AC arc welding. However, in the
case of applying AC arc welding to, for example, an aluminum material,
arc interruption may occur when the polarity is switched from positive
(the electrode is negative) to negative (the electrode is positive) or vice
versa.
Arc interruption reduces workability, and also cools the melt
pool, possibly causing weld defects. The conventional way to solve
these problems is to regenerate the arc by applying a high-frequency
high voltage between the electrode and the base material.
The operation of a conventional AC arc welding device to
address arc interruption will be described as follows with reference to
Figs. 9 and 10. Fig. 9 is a schematic configuration view of the
conventional AC arc welding device. Fig. 10 shows the change in

welding control signals with time when arc interruption occurs during
conventional AC arc welding control.
The operation of the AC arc welding device having the structure
shown in Fig. 9 will be described with reference to Fig. 10. This is a
non-consumable electrode AC arc welding device in which a
positive-polarity period and a negative-polarity period are alternated.
In Fig. 9, AC arc welding device 1 includes welding output unit
2, AC frequency controller 3, current detection unit 4, arc interruption
detecting unit 6, and high-voltage generator 16. Welding output unit
2 outputs a welding output. AC frequency controller 3 controls an AC
frequency. Current detection unit 4 detects a welding current. Arc
interruption detecting unit 6 detects arc interruption from the
detection result of current detection unit 4. High-voltage generator
16 applies a high voltage between electrode 9 and base material 12.
Electrode 9 is provided in welding torch 10. The welding output from
welding output unit 2 is applied between electrode 9 and base material
12 so as to create arc 11 used for welding.
In Fig. 10, a time E1 is when the arc is extinguished, and a time
E2 is when the arc is reignited.
In Fig. 9, welding output unit 2 includes primary and secondary
inverters for alternating the positive-polarity period and the
negative-polarity period based on the output of AC frequency controller
3. Welding output unit 2 receives commercial power (for example,
three-phase 200V) from outside of AC arc welding device 1 and outputs
welding voltage and current suitable for welding.
A negative polarity means that arc plasma electrons move in the
direction from base material 12 to electrode 9, and that electrode 9 is

positive, and base material 12 is negative. A positive polarity, on the
other hand, means that arc plasma electrons move in the direction
from electrode 9 to base material 12, and that electrode 9 is negative,
and base material 12 is positive.
Current detection unit 4, which can be composed of a current
transformer (CT), detects the welding current, and sets an arc
interruption signal high when arc interruption occurs, and low when
the arc is present.
Arc interruption detecting unit 6, which can be composed of a
CPU, determines the occurrence of arc interruption from a current
detection signal received from current detection unit 4.
AC frequency controller 3, which can be composed of a CPU,
controls the welding output at a predetermined AC frequency, and
outputs positive and negative control signals determined based on the
AC frequency to welding output unit 2.
Welding output unit 2 includes IGBTs or other similar devices
which switch the output polarity based on the positive and negative
control signals. When the positive control signal is high, welding
output unit 2 changes the output polarity such that electrons move
from electrode 9 to base material 12, thereby providing the
positive-polarity period. When the negative control signal is high, on
the other hand, welding output unit 2 changes the output polarity such
that electrons move from base material 12 to electrode 9, thereby
providing the negative-polarity period.
The welding current and voltage from welding output unit 2 are
supplied to welding torch 10 to create arc 11 between the tip of
electrode 9 and base material 12 so as to perform AC arc welding.

High-voltage generator 16 applies a high frequency high voltage
(generally, 12 kV) between electrode 9 and base material 12 when the
arc interruption signal received from arc interruption detecting unit 6
is high. When the arc interruption signal is low, high-voltage
generator 16 stops applying the high frequency high voltage.
As shown in Fig. 10, the arc interruption signal goes high at the '
time E1 when the arc is extinguished during normal welding.
High-voltage generator 16 applies a high frequency high voltage (for
example, 12 kV) to regenerate the arc between electrode 9 and base
material 12.
Applying the high frequency high voltage between electrode 9
and base material 12 breaks the isolation between them, thereby
regenerating the arc. At the time E2 when the arc is regenerated,
high-voltage generator 16 stops applying the high frequency high
voltage.
AC frequency controller 3 operates at an AC frequency
predetermined for normal welding both while the arc is being
interrupted and when the arc is present.
As described hereinbefore, according to the conventional
method for performing AC arc welding using AC arc welding device 1,
the arc is regenerated by applying a high frequency high voltage at the
occurrence of arc interruption (see, for example, Patent Literature l).
In this conventional method, however, the high frequency high
voltage applied to reignite the arc at the occurrence of arc interruption
may damage the surface of the weld bead or cause communication
failure due to the high frequency high voltage.

Citation List
Patent Literature
PTL l: Japanese Patent Examined Publication No.
62-32027
SUMMARY OF THE INVENTION
The present invention provides a method and device for
performing AC arc welding in which the arc is reignited without
applying a high frequency high voltage at the occurrence of arc
interruption.
The method for performing AC arc welding of the present
invention alternates a negative-polarity period and a positive-polarity
period at a first AC frequency. This method includes a detection step
for detecting arc interruption during welding, and a control step for,
upon detection of the arc interruption in the detection step, reigniting
the arc by outputting a high level of a positive control signal or a
negative control signal for a period shorter than a first period
corresponding to the first AC frequency.
This method allows the arc to be reignited without applying a
high frequency high voltage at the occurrence of arc interruption.
With this method, AC arc welding can be performed without damaging
the surface of the weld bead or causing communication failure due to
the high frequency high voltage.
The AC arc welding device of the present invention in which a
negative-polarity period and a positive-polarity period are alternated
includes a first AC-frequency setting unit, an output detection unit, an
arc interruption detecting unit, and an AC frequency controller. The

first AC-frequency setting unit sets a first AC frequency
predetermined for normal welding. The output detection unit detects
a welding current or a welding voltage. The arc interruption
detecting unit detects arc interruption based on the detection result of
the output detection unit. The AC frequency controller controls the
AC frequency based on the detection result of the arc interruption
detecting unit. Upon detection of arc interruption, the arc is reignited
by outputting a high level of a positive or negative control signal for a
shorter period than a first period corresponding to the first AC
frequency.
With this structure, the arc can be reignited without applying a
high frequency high voltage at the occurrence of arc interruption. As
a result, AC arc welding is performed without damaging the surface of
the weld bead or causing communication failure due to the high
frequency high voltage.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a schematic configuration view of an AC arc welding
device according to a first exemplary embodiment of the present
invention.
Fig. 2 shows the change in the AC frequencies of control signals
with time in AC arc welding according to the first exemplary
embodiment of the present invention.
Fig. 3 is a schematic configuration view of an AC arc welding
device according to a second exemplary embodiment of the present
invention.
Fig. 4 shows the change in a positive-polarity period and a

negative-polarity period of control signals with time in AC arc welding
according to the second exemplary embodiment of the present
invention.
Fig. 5 is a schematic configuration view of an AC arc welding
device according to a third exemplary embodiment of the present
invention.
Fig. 6 shows the change in a positive-polarity period and a
negative-polarity period of control signals with time in AC arc welding
according to the third exemplary embodiment of the present invention.
Fig. 7 is a schematic configuration view of an AC arc welding
device according to a fourth exemplary embodiment of the present
invention.
Fig. 8 shows the change in a positive-polarity period and a
negative-polarity period of control signals with time in AC arc welding
according to the fourth exemplary embodiment of the present
invention.
Fig. 9 is a schematic configuration view of a conventional AC
arc welding device.
Fig. 10 shows the change in welding control with time when arc
interruption occurs during conventional AC arc welding control.
DESCRIPTION OF EMBODIMENTS
Embodiments of the present invention will be described as
follows with reference to the accompanied drawings. In these
drawings, the same reference numerals are used for the same
components, and hence the description thereof may be omitted.

FIRST EXEMPLARY EMBODIMENT
Fig. 1 is a schematic configuration view of an AC arc welding
device according to a first exemplary embodiment of the present
invention. Fig. 2 shows the change in the AC frequencies of control
signals with time in AC arc welding according to the first exemplary
embodiment. More specifically, Fig. 2 shows the change in the
amplitude of each of a welding current, a positive control signal, a
negative control signal, and an arc interruption signal with time.
The operation of the AC arc welding device having the structure
shown in Fig. 1 will be described with reference to Fig. 2. This is a
non-consumable electrode AC arc welding device in which
negative-polarity period and a positive-polarity period are alternated.
In Fig. 1, AC arc welding device 1 includes welding output unit
2, AC frequency controller 3, current detection unit 4, voltage detection
unit 5, arc interruption detecting unit 6, first AC-frequency setting
unit 7, and second AC-frequency setting unit 8. Welding output unit 2
outputs a welding output. AC frequency controller 3 controls an AC
frequency. Current detection unit 4 detects a welding current.
Voltage detection unit 5 detects a welding voltage. Arc interruption
detecting unit 6 detects arc interruption from the output of either
current detection unit 4 or voltage detection unit 5. First
AC-frequency setting unit 7 sets a first AC frequency. Second
AC-frequency setting unit 8 sets a second AC frequency. The welding
output from welding output unit 2 is applied between electrode 9
provided in welding torch 10 and base material 12 so as to create arc
11.
In Fig. 2, F1 in period P1 represents a first AC frequency of the

positive and negative control signals, and F2 represents a second AC
frequency of the positive and negative control signals. A time E1 is
when the arc is extinguished, and a time E2 is when the arc is
reignited.
In Fig. 1, welding output unit 2 includes unillustrated primary
and secondary inverters for alternating the positive-polarity period'
and the negative-polarity period based on the output of AC frequency
controller 3. Welding output unit 2 receives commercial power (for
example, three-phase 200V) from outside of AC arc welding device 1,
and outputs welding voltage and current suitable for welding.
The primary inverter is generally driven by a pulse width
modulation (PWM) operation or a phase shift operation. The primary
inverter is composed of unillustrated insulated gate bipolar transistors
(IGBTs), a primary rectifier diode, a smoothing electrolytic capacitor, a
transformer for power conversion, and other components.
The secondary inverter is generally composed of unillustrated
insulated gate bipolar transistors (IGBTs), and switches the polarity of
the welding voltage or current.
A negative polarity means that arc plasma electrons move in the
direction from base material 12 to electrode 9, and that electrode 9 is
positive, and base material 12 is negative. A positive polarity, on the
other hand, means that arc plasma electrons move in the direction
from electrode 9 to base material 12, and that electrode 9 is negative,
and base material 12 is positive.
Current detection unit 4, which can be composed of a current
transformer (CT), detects the welding current. Voltage detection unit
5 detects the welding voltage.

Arc interruption detecting unit 6, which can be composed of a
CPU, receives a current detection signal from current detection unit 4.
If the current detection signal reaches a predetermined current
detection level (for example, 1A) or below indicating that the arc is
being interrupted, unit 6 determines the occurrence of arc interruption,
and sets the are interruption signal high (arc interruption'
determination level). If the current detection signal reaches a
predetermined current detection level (for example, 3A) or above
indicating that the arc is present, unit 6 sets the arc interruption
signal low (arc determination level).
Arc interruption detecting unit 6 receives a voltage detection
signal from voltage detection unit 5. If the voltage detection signal
reaches arc interruption voltage (for example, 60V) or above indicating
that the arc is being interrupted, unit 6 may determine the occurrence
of arc interruption. Arc interruption detecting unit 6 then may set
the arc interruption signal high (arc interruption determination level).
If the voltage detection signal reaches another predetermined voltage
detection level (for example, 50V) or below, unit 6 may set the arc
interruption signal low (arc determination level), indicating that the
arc is present.
First AC-frequency setting unit 7, which can be composed of a
CPU, sets a first AC frequency F1 as an AC arc frequency
predetermined for normal welding. Second AC-frequency setting unit
8, which can be composed of a CPU, sets a second AC frequency F2
higher than the first AC frequency. The first and second AC
frequencies Fl and F2 can be, for example, 70 Hz and 400 Hz,
respectively.

AC frequency controller 3, which can be composed of a CPU,
controls the welding output based on the arc interruption signal from
arc interruption detecting unit 6, and the output of each of first and
second AC-frequency setting units 7 and 8. AC frequency controller 3
outputs positive and negative control signals determined based on the
AC frequencies to welding output unit 2.
The ratio of the positive-polarity period to the negative-polarity
period is predetermined (for example, the proportion of the
negative-polarity period in the AC cycle is 30%). This ratio is
generally called a "cleaning width".
Welding output unit 2 makes the IGBTs switch the output
polarity based on the positive and negative control signals from AC
frequency controller 3. When the positive control signal is high, the
output polarity is switched to make the positive-polarity period and to
move electrons from electrode 9 to base material 12. When the
negative control signal is high, on the other hand, the output polarity
is switched to make the negative-polarity period and to move electrons
from base material 12 to electrode 9.
The welding current and voltage from welding output unit 2 are
supplied to welding torch 10 to create arc 11 between tip 9a of electrode
9 and base material 12 so as to perform AC arc welding. Electrode 9
can be made of tungsten, and base material 12 can be made, for
example, of aluminum or magnesium as an object to be welded.
In AC arc welding, arc interruption tends to occur when the
polarity is switched, especially from the positive-polarity period to the
negative-polarity period. When arc interruption is detected, as shown
in Fig. 2, a high level of the negative control signal, and then a high

level of the positive control signal are outputted for a shorter period
than the first period corresponding to the first AC frequency Fl. The
alternate output of the high level of the positive and negative control
signals reignites the arc. After the arc is reignited, the AC frequency
of the positive and negative control signals is returned to the original
first AC frequency Fl, thereby continuing AC arc welding.
As described hereinbefore, AC arc welding device 1 of the
present invention performs welding in which the negative-polarity
period and the positive-polarity period are alternated. AC arc
welding device 1 includes first AC'frequency setting unit 7, an output
detection unit including current detection unit 4 and/or voltage
detection unit 5, arc interruption detecting unit 6, and AC frequency
controller 3. First AC-frequency setting unit 7 sets the first AC
frequency Fl predetermined for normal welding. The output
detection unit detects a welding current or voltage. Arc interruption
detecting unit 6 detects arc interruption based on the detection result
of the output detection unit. AC frequency controller 3 controls the
AC frequency based on the detection result of arc interruption
detecting unit 6. Upon detection of arc interruption, the arc is
reignited by outputting a high level of the positive or negative control
signal for a period shorter than the first period corresponding to the
first AC frequency Fl.
With this structure, the arc can be reignited without applying a
high frequency high voltage at the occurrence of arc interruption. As
a result, AC arc welding is performed without damaging the surface of
the weld bead or causing communication failure due to the high
frequency high voltage.

The following is a detailed description, with reference to Fig. 2,
of the operation of AC arc welding device 1 to reignite the arc at the
occurrence of arc interruption.
As shown in Fig. 2, AC welding is performed at the first AC
frequency Fl (for example, 70 Hz) during normal welding. If the arc
is extinguished at the time El when the positive-polarity period is'
switched to the negative-polarity period, the arc interruption signal
goes high at the time El. In this case, AC frequency controller 3
outputs to welding output unit 2 an instruction to switch from the first
AC frequency Fl (for example, 70 Hz), which is predetermined for
normal welding, to the second AC frequency F2 (for example, 400 Hz).
As a result, the welding is performed at the second AC frequency F2.
Thus, while arc interruption is going on, the AC frequency is at
the second AC frequency F2. The positive and negative control
signals go high and low alternately at the second AC frequency F2. If
the arc is reignited at the time E2, the arc interruption signal goes low
at the time E2. In this case, AC frequency controller 3 instructs
welding output unit 2 to switch from the second AC frequency F2 to the
first AC frequency Fl predetermined for normal welding. As a result,
welding is continued at the first AC frequency Fl.
AC arc welding device 1 further includes second AC-frequency
setting unit 8 for setting the second AC frequency F2 higher than the
first AC frequency Fl. When arc interruption detecting unit 6 detects
arc interruption, AC frequency controller 3 switches from the first AC
frequency Fl to the second AC frequency F2. With this structure, the
arc can be reignited without applying a high frequency high voltage at
the occurrence of arc interruption.

As described above, if arc interruption is detected during
welding, the first AC frequency (for example, 70 Hz) predetermined for
normal welding is switched to the second AC frequency (for example,
400 Hz). This makes the arc much more likely to be reignited to
continue welding.
The reason for this is as follows. Immediately after arc
interruption occurs, the space between electrode 9 and base material
12 is in the arc atmosphere. Therefore, the arc can be easily
regenerated by repeating switching the polarity at intervals shorter
than in normal welding.
The arc interruption is terminated in a short time (several
hundreds of microseconds), preventing the arc from being extinguished
completely. It becomes unnecessary to apply a high frequency high
voltage (for example, 12 kV) from the high-voltage generator to
regenerate the arc as in the conventional example. The arc can be
regenerated at a voltage as low as used in normal welding. This
eliminates the problems of communication failure, the adverse effect
on surrounding electronic devices, and the damage of the surface of the
weld bead because no high voltage is applied to base material 12.
AC arc welding described in the present first exemplary
embodiment uses a non-consumable electrode, but may alternatively
use a consumable electrode to provide a similar effect.
The method of the present invention for performing AC arc
welding in which a negative-polarity period and a positive-polarity
period are alternated at the first AC frequency Fl includes a detection
step and a control step. The detection step detects arc interruption
during welding. The control step, upon detection of the arc

interruption in the detection step, reignites the arc by outputting a
high level of a positive or negative control signal for a period shorter
than a first period corresponding to the first AC frequency Fl.
This method allows the arc to be reignited without applying a
high frequency high voltage at the occurrence of arc interruption.
With this method, AC arc welding can be performed without damaging
the surface of the weld bead or causing communication failure due to
the high frequency high voltage.
In the control step, welding may be performed at the second AC
frequency F2 higher than the first AC frequency Fl.
With this method, the arc can be reignited with a simple circuit
and a simple system design without applying a high frequency high
voltage at the occurrence of arc interruption.
Upon detection of the arc while welding is performed at the
second AC frequency F2, the AC frequency may be returned to the first
AC frequency Fl.
With this method, arc welding can be smoothly continued under
the original welding conditions even if arc interruption occurs.
SECOND EXEMPLARY EMBODIMENT
Fig. 3 is a schematic configuration view of AC arc welding
device 21 according to a second exemplary embodiment of the present
invention. Fig. 4 shows the change in a positive-polarity period and a
negative-polarity period of control signals with time in AC arc welding
according to the second exemplary embodiment. AC arc welding
device 21 using a non-consumable electrode will be described as follows
with reference to Figs. 3 and 4.

The present second exemplary embodiment mainly differs from
the first exemplary embodiment in counting the time of arc
interruption and outputting a control signal to switch the polarity if
the arc interruption continues for a predetermined time.
In Fig. 3, AC arc welding device 21 includes first
predetermined'period setting unit 13 and time keeper 15 for counting
time, in addition to most of the components included in the first
exemplary embodiment. In Fig. 4, a time E3 is when a first
predetermined period Tl has passed since the occurrence of arc
interruption at the time El.
In Fig. 3, first predetermined-period setting unit 13, which can
be composed of a CPU, sets a first predetermined period (for example,
100 microseconds) shorter than the negative-polarity period in normal
welding. Time keeper 15, which can be composed of a CPU, counts the
time since arc interruption occurred in the negative-polarity period
during which the negative control signal is being outputted. AC
frequency controller 3, which can be composed of a CPU, transmits a
control signal to welding output unit 2 based on the arc interruption
signal from arc interruption detecting unit 6, and the output of each of
first AC-frequency setting unit 7, first predetermined-period setting
unit 13, and time keeper 15. This controls the welding output.
In Fig. 4, if the arc is extinguished at the time El when the
positive-polarity period is switched to the negative-polarity period
during welding at the first AC frequency Fl, the arc interruption
signal goes high at the time El. When the arc interruption signal
goes high, time keeper 15 starts to count the time of arc interruption in
the negative-polarity period. At the time E3 when the time counted

by time keeper 15 has passed the first predetermined period Tl shorter
than the negative-polarity period in normal welding, AC frequency
controller 3 outputs a positive control signal to welding output unit 2.
As a result, the negative-polarity period is switched to the
positive-polarity period.
In the case where the first AC frequency is 70 Hz, and the'
proportion of the negative-polarity period in the AC cycle is 30%, the
negative-polarity period can be, for example, 4.28 milliseconds. The
first predetermined period Tl can be, for example, 100 microseconds.
In Fig. 4, the arc is reignited at the time E3 when the
negative-polarity period is switched to the positive-polarity period.
When the arc is reignited, welding is continued at the first AC
frequency Fl predetermined for normal welding.
As described above, AC arc welding device 21 further includes
first predetermined-period setting unit 13 for setting a first
predetermined period shorter than the negative-polarity period, and
time keeper 15 for counting the period of arc interruption. In AC arc
welding device 21, if arc interruption continues for the first
predetermined period in the negative-polarity period during which AC
frequency controller 3 is outputting the negative control signal,
controller 3 outputs a positive control signal to switch to the
positive-polarity period.
This structure allows the arc to be reignited without applying a
high frequency high voltage at the occurrence of arc interruption.
According to the method of the present invention for performing
AC arc welding, in the negative-polarity period during which the
negative control signal is being outputted, if arc interruption continues

for the first predetermined period shorter than the negative-polarity
period, the high level of the positive control signal is outputted in the
control step so as to switch to the positive-polarity period.
This method allows the arc to be reignited without applying a
high frequency high voltage at the occurrence of arc interruption.
As described hereinbefore, if arc interruption occurs in the
negative-polarity period and continues for the first predetermined
period Tl (for example, 100 microseconds), the arc becomes more likely
to be reignited by switching to the positive-polarity period. The
reason for this is considered as follows. The shorter the first
predetermined period Tl, the space between electrode 9 and base
material 12 remains in the arc atmosphere, allowing the arc to be
easily regenerated. Experimental results indicate that the first
predetermined period Tl is preferably within 1 millisecond.
According to the present second exemplary embodiment, arc
interruption is terminated in a short time (for example, several
hundreds of microseconds), preventing the arc from being extinguished
completely. It becomes unnecessary to apply a high frequency high
voltage (for example, 12 kV) to regenerate the arc. This eliminates
the problems of communication failure, the adverse effect on
surrounding electronic devices, and the damage of the surface of the
weld bead.
AC arc welding described in the present second exemplary
embodiment uses a non-consumable electrode, but may alternatively
use a consumable electrode to provide a similar effect.
THIRD EXEMPLARY EMBODIMENT

Fig. 5 is a schematic configuration view of AC arc welding
device 31 according to a third exemplary embodiment of the present
invention. Fig. 6 shows the change in a positive-polarity period and a
negative-polarity period of control signals with time in AC arc welding
according to the third exemplary embodiment. The AC arc welding
device of the present third exemplary embodiment using a
non-consumable electrode will be described as follows with reference to
Figs. 5 and 6.
The present third exemplary embodiment mainly differs from
the second exemplary embodiment in counting the time of arc
interruption occurred in the positive-polarity period during which the
positive control signal is being outputted, and outputting a control
signal to switch the polarity if the arc interruption continues for a
predetermined time.
In Fig. 5, AC arc welding device 31 includes second
predetermined-period setting unit 14 in addition to most of the
components included in the second exemplary embodiment. In Fig. 6,
a time E4 is when a second predetermined period T2 has passed since
the occurrence of arc interruption at the time El.
In Fig. 5, second predetermined-period setting unit 14, which
can be composed of a CPU, sets a second predetermined period T2 (for
example, 100 microseconds) shorter than the positive-polarity period
in normal welding. Time keeper 15, which can be composed of a CPU,
counts the time of arc interruption occurred in the positive-polarity
period during which the positive control signal is being outputted. AC
frequency controller 3, which can be composed of a CPU, transmits a
control signal to welding output unit 2 based on the arc interruption

signal from arc interruption detecting unit 6, and the output of each of
first AC-frequency setting unit 7, second predetermined-period setting
unit 14, and time keeper 15. This controls the welding output.
In Fig. 6, if the arc is extinguished at the time El when the
negative-polarity period is switched to the positive-polarity period
during normal welding at first AC frequency Fl, the arc interruption
signal goes high at the time El. When the arc interruption signal
goes high, time keeper 15 starts to count the time of arc interruption in
the positive-polarity period. At the time E4, AC frequency controller
3 outputs a negative control signal to welding output unit 2. As a
result, the positive-polarity period is switched to the negative-polarity
period. The time E4 is when the time counted by time keeper 15 has
passed the second predetermined period T2 shorter than the
positive-polarity period in normal welding.
In the case where the first AC frequency Fl is 70 Hz, and the
proportion of the negative-polarity period in the AC cycle is 30%, the
positive-polarity period can be, for example, 10 milliseconds. . The
second predetermined period T2 can be, for example, 100 microseconds.
In Fig. 6, the arc is reignited at the time E4 when the
positive-polarity period is switched to the negative-polarity period.
When the arc is reignited, welding is continued at the first AC
frequency Fl predetermined for normal welding.
As described above, AC arc welding device 31 further includes
second predetermined-period setting unit 14 for setting a second
predetermined period shorter than the positive-polarity period, and
time keeper 15 for counting the period of arc interruption. In AC arc
welding device 31, if arc interruption continues for the second

predetermined period in the positive-polarity period during which AC
frequency controller 3 is outputting the positive control signal,
controller 3 outputs the negative control signal to switch to the
negative-polarity period.
This structure allows the arc to be reignited without applying a
high frequency high voltage at the occurrence of arc interruption.
According to the method of the present invention for performing
AC arc welding, in the positive-polarity period during which the
positive control signal is being outputted, if arc interruption continues
for the second predetermined periods shorter than the positive-polarity
period, the high level of the negative control signal is outputted in the
control step so as to switch to the negative-polarity period.
This method allows the arc to be reignited without applying a
high frequency high voltage at the occurrence of arc interruption.
As described hereinbefore, if arc interruption occurs in the
positive-polarity period and continues for the second predetermined
period T2 (for example, 100 microseconds), the arc becomes more likely
to be reignited by switching to the negative-polarity period. The
reason for this is considered as follows. The shorter the second
predetermined period T2, the space between electrode 9 and base
material 12 remains in the arc atmosphere, allowing the arc to be
easily regenerated. Experimental results indicate that the second
predetermined period T2 is preferably within 1 millisecond.
According to the present third exemplary embodiment, arc
interruption is terminated in a short time (for example, several
hundreds of microseconds), preventing the arc from being extinguished
completely. It becomes unnecessary to apply a high frequency high

voltage (for example, 12 kV) to regenerate the arc. This eliminates
the problems of communication failure, the adverse effect on
surrounding electronic devices, and the damage of the surface of the
weld bead.
AC arc welding described in the present third exemplary
embodiment uses a non-consumable electrode, but may alternatively
use a consumable electrode to provide a similar effect.
FOURTH EXEMPLARY EMBODIMENT
Fig. 7 is a schematic configuration view of AC arc welding
device 41 according to a fourth exemplary embodiment of the present
invention. Fig. 8 shows the change in a positive-polarity period and a
negative-polarity period of control signals with time in AC arc welding
according to the fourth exemplary embodiment. The AC arc welding
device of the present fourth exemplary embodiment using a
non-consumable electrode will be described as follows with reference to
Figs. 7 and 8.
The present fourth exemplary embodiment mainly differs from
the first to third exemplary embodiments in counting the time of arc
interruption occurred in the negative-polarity period during which the
negative control signal is being outputted, and outputting a control
signal to switch the polarity if the arc interruption continues for a
predetermined time. If the arc interruption continues for another
predetermined time, a control signal is outputted to switch the polarity
again.
In Fig. 8, a time E5 is when the second predetermined period T2
has passed since the time E3, and a time E6 is when the first

predetermined period Tl has passed since the time E5.
In Fig. 7, AC frequency controller 3, which can be composed of a
CPU, controls the welding output based on the arc interruption signal
from arc interruption detecting unit 6, and the output of each of first
AC-frequency setting unit 7, first predetermined-period setting unit 13,
second predetermined-period setting unit 14, and time keeper 15.
In Fig. 8, the arc is extinguished and the arc interruption signal
goes high at the time El when the positive-polarity period during
normal welding at the first AC frequency (for example, 70 Hz) is
switched to the negative-polarity period.
Time keeper 15 counts the time of arc interruption in the
negative-polarity period. AC frequency controller 3 outputs a positive
control signal to terminate the negative-polarity period at the time E3.
The time E3 is when the time counted by time keeper 15 has passed the
first predetermined period Tl (for example, 100 microseconds).
In Fig. 8, the arc is not reignited at the time E3 in spite that the
polarity is switched. In this case, the arc interruption is going on, and
time keeper 15 counts the time of arc interruption since the time E3
during which the positive control signal is being outputted. At the
time E5 when the time counted by time keeper 15 has passed the
second predetermined period T2 (fox example, 100 microseconds), AC
frequency controller 3 outputs the negative control signal to terminate
the positive-polarity period.
In Fig. 8, the arc is not reignited at the time E5 in spite that the
polarity control signal is changed again. In this case, the arc
interruption is going on, and time keeper 15 counts the time of arc
interruption since the time E5 during which the negative control signal

is being outputted. At the time E6 when the time counted by time
keeper 15 has passed the first predetermined period T1 (for example,
100 microseconds), AC frequency controller 3 outputs the positive
control signal to terminate the negative-polarity period.
In Fig. 8, the arc is reignited at the time E6 when the polarity is
switched from negative to positive. When the arc is reignited,
welding is continued at the first AC frequency F1 predetermined for
normal welding.
As described hereinbefore, if arc interruption occurs, the arc
becomes more likely to be reignited by repeating switching between the
two polarity control signals at short intervals.
According to the present fourth exemplary embodiment, arc
interruption is terminated in a short time (for example, several
hundreds of microseconds), preventing the arc from being extinguished
completely. It becomes unnecessary to apply a high frequency high
voltage (for example, 12 kV) to regenerate the arc. This eliminates
the problems of communication failure, the adverse effect on
surrounding electronic devices, and the damage of the surface of the
weld bead.
AC arc welding described in the present fourth exemplary
embodiment uses a non-consumable electrode, but may alternatively
use a consumable electrode to provide a similar effect.
INDUSTRIAL APPLICABILITY
As described hereinbefore, according to the present invention,
arc interruption, when it occurs, can be terminated in a short time to
regenerate the arc without applying a high frequency high voltage.

This eliminates the problems of communication failure and the damage
of the surface of the weld bead due to the application of a high
frequency high voltage, thereby obtaining excellent welding results.
The method and device of the present invention for performing AC arc
welding are particularly applicable to industries using aluminum and
magnesium materials, such as automotive and construction industries.
REFERENCE MARKS IN THE DRAWINGS
1, 21, 31, 41 AC arc welding device
2 welding output unit
3 AC frequency controller
4 current detection unit
5 voltage detection unit
6 arc interruption detecting unit
7 first AC-frequency setting unit
8 second AC-frequency setting unit
9 electrode

10 welding torch
11 arc
12 base material
13 first predetermined-period setting unit
14 second predetermined-period setting unit
15 time keeper
16 high-voltage generator

We Claim:
1. A method for performing AC arc welding in which a
negative-polarity period and a positive-polarity period are alternated
at a first AC frequency, the method comprising:
a detection step for detecting arc interruption during welding;
and
a control step for, upon detection of the arc interruption in the
detection step, reigniting the arc by outputting a high level of a
positive control signal or a negative control signal for a period shorter
than a first period corresponding to the first AC frequency.
2. The method of claim 1, wherein
in the control step, welding is performed at a second AC
frequency higher than the first AC frequency.
3. The method of claim 2, wherein
upon detection of the arc while welding is performed at the
second AC frequency, the AC frequency is returned to the first AC
frequency.
4. The method of claim 1, wherein
in the negative-polarity period during which the negative
control signal is being outputted, if arc interruption continues for a
first predetermined period shorter than the negative-polarity period,
the high level of the positive control signal is outputted in the control
step so as to switch to the positive-polarity period.

5. The method of claim 1, wherein
in the positive-polarity period during which the positive control
signal is being outputted, if arc interruption continues for a second
predetermined period shorter than the positive-polarity period, the
high level of the negative control signal is outputted in the control step
so as to switch to the negative-polarity period.
6. The AC arc welding device in which a negative-polarity period
and a positive-polarity period are alternated, the device comprising:
a first AC-frequency setting unit for setting a first AC frequency
predetermined for normal welding,'
an output detection unit for detecting a welding current or a
welding voltage;
an arc interruption detecting unit for detecting arc interruption
based on a detection result of the output detection unit; and
an AC frequency controller for controlling an AC frequency
based on a detection result of the arc interruption detecting unit,
wherein
upon detection of the arc interruption, the arc is reignited by
outputting a high level of a positive control signal or a negative control
signal for a period shorter than a first period corresponding to the first
AC frequency.
7. The AC arc welding device of claim 6, further comprising:
a second AC-frequency setting unit for setting a second AC
frequency higher than the first AC frequency, wherein

upon detection of the arc interruption by the arc interruption
detecting unit, the AC frequency controller switches the first AC
frequency to the second AC frequency.
8. The AC arc welding device of claim 6, further comprising:
a first predetermined-period setting unit for setting a first
predetermined period shorter than the negative-polarity period; and
a time keeper for counting a period of arc interruption, wherein
if the arc interruption continues for the first predetermined
period in the negative-polarity period during which the AC frequency
controller is outputting the negative control signal, the AC frequency
controller outputs the positive control signal to switch to the
positive-polarity period.
9. The AC arc welding device of claim 6, further comprising:
a second predetermined-period setting unit for setting a second
predetermined period shorter than the positive-polarity period,' and
a time keeper for counting a period of arc interruption, wherein
if the arc interruption continues for the second predetermined
period in the positive-polarity period during which the AC frequency
controller is outputting the positive control signal, the AC frequency
controller outputs the negative control signal to switch to the
negative-polarity period.

ABSTRACT

If arc interruption occurs during AC arc welding, a first AC
frequency predetermined for normal welding is switched to a second
AC frequency higher than the first AC frequency. This allows the arc
to be reignited without applying a high frequency high voltage, thereby
preventing the damage of the surface of the weld bead or
communication failure due to the high frequency high voltage.