DESCRIPTION
INVERTER CONTROL DEVICE
AND INVERTER CONTROL METHOD
TECHNICAL FIELD
The present invention relates to an inverter control device,
particularly to a device for controlling an inverter circuit for
controlling welding output power used in a welding machine that;
processes a processed object by arc discharge, and to a method of
controlling the circuit.
BACKGROUND ART
A device (e.g. welding machine) that discharges arc between an
electrode and a processed object (referred to as parent material,
hereinafter) to melt the parent material for processing typically
includes a power control circuit for controlling an output current;
flowing between the electrode and the parent material or output;
voltage applied between them.
In recent years, such a power control circuit has been usually
formed of an inverter circuit including a high-speed switching element
and a power conversion transformer, becoming widely used as an
inverter-controlled welding machine.
Such an inverter-controlled welding machine typically includes a
full-bridge inverter circuit. The welding machine drives a power
semiconductor element (e.g. IGBT and MOSFET) composing a bridge
circuit, at an inverter frequency (usually from several kHz to
approximately 100 kHz). Simultaneously, the welding machine
compares an output current to an output current set value (or output;
voltage to an output voltage set value) to control conduction time of the
power conversion transformer, thereby obtaining output with current;
or voltage characteristics preferable for welding output power.
As full-bridge inverter control method, some conventional
inverter-controlled welding machines use pulse-width modulation
(referred to as PWM hereinafter), which controls conduction time of a
switching element. Others use phase control method (also referred to
as phase shift method), which controls conduction timing of a switching
element (refer to patent literature 1).
Further, others can use a method in which features of PWM and
phase control method are merged; one bridge circuit out of the two is
controlled with a fixed conduction width; and the other undergoes
pulse-width modulation (referred to as one-side bridge fixed conduction
width PWM control, hereinafter).
Hereinafter, a description is made of welding machines by the
three methods: PWM, phase control, and one-side bridge fixed
conduction width PWM control.
First, PWM above is described using FIG. 11.
FIG. 11 shows an outline structure of substantial parts of an arc
welding machine including an inverter control circuit by conventional
PWM.
In FIG. 11, first rectifier 5 rectifies three- or single-phase AC input.
First switching element 1 and second switching element 2 convert;
output from first rectifier 5 to an alternating current. Second
rectifier 7 rectifies output from power conversion transformer 6.
Output current detector 8 detects an output current. Current
detecting part 9 converts a signal from output current detector 8 to a
feedback signal. Output power setting part 12 is provided to
preliminarily set average and effective values during a predetermined
period, of a welding current or welding voltage as output from the
welding machine. Error amplification part 11 determines an error
between a signal output from current detecting part 9 and a signal set
by output power setting part 12, and amplifies the error. Inverter
driving basic pulse generating part 13 generates a driving waveform
fundamental for inverter control. Pulse-width modulating part (PWM
part, hereinafter) 14 outputs a control signal for controlling conduction
widths of switching elements 1 and 2 according to an error
amplification signal from error amplification part 11. Driving circuits
21, 22, 23, and 24 convert the control signal to a drive signal for driving
switching elements 1 and 2 according to a signal output from
pulse-width modulating part 14, and outputs the drive signal. Here,
inverter control part 29 enclosed by the dashed-dotted line includes
inverter driving basic pulse generating part 13 and pulse-width
modulating part 14.
To control output power of a non-consumable electrode arc welding
machine (e.g. TIG welding machine), current control is usually-
performed in which an output current is made equal to a current set;
value. To control output power of a consumable electrode arc welding
machine (e.g. MAG welding machine), meanwhile, voltage control is
performed in which output voltage is made equal to a voltage set value.
The operation principles of an inverter used for output control of the
above-described arc welding machines are the same, and thus a
description is made of current control (controlled for a constant current
value) as an operation example of an inverter.
Three or single-phase AC input rectified by first rectifier 5 is
converted to an alternating current with a high frequency by a
full-bridge inverter circuit composed of switching elements 1, 2, 3, and
4, and then is input to the primary side of transformer 6. Here,
switching elements 1 and 2 compose first switching circuit 25, and
switching elements 3 and 4 compose second switching circuit 26. The
secondary-side output of transformer 6 is rectified by second rectifier 7
and is supplied to an electrode and parent material (both are arc loads,
not shown) through output terminals 38 and 39.
An output current from the welding machine is detected by output;
current detector 8, and a detection signal proportional to the output,
current is input to error amplification part 11 from output current,
detector 8 through current detecting part 9. Error amplification part
11 compares an output power set value from output power setting part
12 to a current signal from current detecting part 9, and outputs an
error amplification signal between both. The error amplification
signal is converted by pulse-width modulating part 14 to driving pulses
with a width corresponding to the magnitude of the error amplification
signal on a basis of a basic pulse waveform for inverter driving
generated by inverter driving basic pulse generating part 13.
The driving pulses are separated one by one alternately into two
series to become 2-series drive signals for inverter driving. One series
is input to driving circuits 21 and 24 as a signal for driving switching-
elements 1 and 4 simultaneously; the other is input to driving circuits
22 and 23 as a signal for driving switching elements 2 and 3
simultaneously.
These drive signals are converted to those suitable for driving
switching elements 1 to 4 by respective driving circuits 21 to 24, and
are input to switching elements 1 to element 4.
As a result that switching elements 1 and 4; and switching
elements 2 and 3 simultaneously conduct alternately, output from first
rectifier 5 is converted to an alternating current. The alternating
current is input to the primary winding of transformer 6; converted to
output power suitable for welding; and output from the secondary-
winding of transformer 6. Output from the secondary winding of
transformer 6 is converted to a direct current by second rectifier 7 and
is output from the welding machine as welding output power.
Here, error amplification part 11 has an amplification factor as
high as 100 times to 1,000 times for example. This allows
maintaining constant current characteristics according to an output;
current set value even for a change in output voltage due to a change in
load condition of output.
A description is made of an operation example of a welding
machine by PWM later using FIG. 14.
Next, a description is made of the above welding machine by phase
control method using FIG. 12.
FIG. 12 shows an outline structure of substantial parts of an arc
welding machine including an inverter control circuit by conventional
phase control method. In the following drawings, the same component
is given the same reference mark, and its description may be omitted.
In FIG. 12, phase control part 15 outputs a control signal for
controlling conduction of switching elements 1 to 4 according to an
error amplification signal from error amplification part 11.
Three- or single-phase AC input rectified by first rectifier 5 is
converted to an alternating current with a high frequency by a
full-bridge inverter circuit composed of switching elements 1, 2, 3, and
4, and then is input to the primary side of transformer 6 through
capacitor 10. The secondary-side output of transformer 6 is rectified
by second rectifier 7 and is supplied to an electrode and parent
material (both are arc loads, not shown) through output terminals 38
and 39.
An output current from the welding machine is detected by output;
current detector 8, and a detection signal proportional to the output,
current is input to error amplification part 11 from output current;
detector 8 through current detecting part 9. Error amplification part;
11 compares an output power set value from output power setting part
12 to a signal from current detecting part 9, and outputs an error
amplification signal between both. The error amplification signal is
converted by phase control part 15 to driving pulses with a phase
difference corresponding to the level (magnitude) of the error
amplification signal on a basis of a basic pulse waveform for inverter
driving generated by inverter driving basic pulse generating part 13.
Inverter driving basic pulse generating part 13 outputs inverter
driving basic pulse for driving first switching element 1 and second
switching element 2 composing first switching circuit 25 alternately
with a fixed conduction width. Here, first switching circuit control
part 27 has inverter driving basic pulse generating part 13 to control
first driving circuit 21 and second driving circuit 22. Second
switching circuit control part 28 has phase control part 15 to control
third driving circuit 23 and fourth driving circuit 24. The inverter
driving basic pulses are converted to a signal suitable for driving-
switching elements 1 and 2 by driving circuits 21 and 22, and is input;
to switching elements 1 and 2.
A phase control signal generated by phase control part 15 works
for outputting driving pulses for alternately driving third switching
element 3 and fourth switching element 4 composing second switching-
circuit 26 with a phase difference corresponding to an error
amplification signal in relation to operation of first switching circuit 25.
These drive pulses are converted to a signal suitable for driving
switching elements 3 and 4 by driving circuits 23 and 24 and are input
to switching elements 3 and 4.
Then, during a period when a conduction period of switching-
element 1 coincides with that of switching element 4, a primary current
flows through transformer 6 from first switching element 1 to fourth
switching element 4. Meanwhile, during a period when a conduction
period of switching element 2 coincides with that of switching element
3, a primary current flows through transformer 6 from third switching
element 3 to second switching element 2. In this way, output from
first rectifier 5 is converted to an alternating current; is converted to
output power suitable for welding! and is output from the secondary
winding of transformer 6. Output from the secondary winding of
transformer 6 is converted to a direct current by second rectifier 7 and
is output from the welding machine as welding output power.
Here, error amplification part 11 has an amplification factor as
high as 100 times to 1,000 times, which allows maintaining constant
current characteristics corresponding to an output current set value
even for a change in output voltage due to a change in load condition of
output.
An operation example of a welding machine by phase control
method is described later using FIG. 15.
Next, a description is made of the above welding machine by
one-side bridge fixed conduction width PWM control method using FIG.
13.
FIG. 13 shows an outline structure of substantial parts of an arc
welding machine including an inverter control circuit by conventional
one-side bridge fixed conduction width PWM control method.
FIG. 13 shows the configuration of FIG. 12 in which PWM part 14
is substituted for phase control part 15. Hereinafter, the operation is
described.
Inverter driving basic pulse generating part 13 outputs inverter
driving basic pulses for driving first switching element 1 and second
switching element 2 composing first switching circuit 25 alternately
with a fixed conduction width. The inverter driving basic pulses are
converted to a signal suitable for driving switching elements 1 and 2 by
driving circuits 21 and 22, and the signal is input to switching
elements 1 and 2.
The error amplification signal input from error amplification part;
11 is converted by PWM part 14 to driving pulses with a width
corresponding to the level (magnitude) of the error amplification signal
on a basis of a basic pulse waveform for inverter driving generated by-
inverter driving basic pulse generating part 13. The driving pulses
are input one by one alternately to driving circuits 23 and 24 as a
signal for driving third switching element 3 and fourth switching
element 4.
Then, during a period when a conduction period of switching
element 1 coincides with that of switching element 4, a primary current
flows through transformer 6 from first switching element 1 to fourth
switching element 4. Meanwhile, during a period when a conduction
period of switching element 2 coincides with that of switching element
3, a primary current flows through transformer 6 from first switching
element 3 to second switching element 2. In this way, output from
first rectifier 5 is converted to an alternating current; is converted to
output power suitable for welding; and is output from the secondary
winding of transformer 6. Output from the secondary winding of
transformer 6 is converted to a direct current by second rectifier 7 and
is output from the welding machine as welding output power.
An operation example of the above welding machine by one-side
bridge fixed conduction width PWM control method is described later
using FIGs. 16A through 16C
Next, a description is made of the above welding machine that
exercises control by the three types of methods using FIGs. 14A
through 14C, 15A through 15C, and 16A through 16C.
FIGs. 14A through 16C are schematic diagrams showing operation
of an inverter of an arc welding machine including a conventional
inverter control circuit. FIGs. 14A through 14C show operation by
PWM method.' FIGs. 15A through 15C, by phase control method; and
FIGs. 16A through 16C, by one-side bridge fixed conduction width
PWM control method.
FIGs. 14A, 15A, and 16A show operation states at low output (i.e.
short inverter conduction period); FIGs. 14B, 15B, and 16B, at middle
output (i.e. middle-range inverter conduction period); and FIGs. 14C,
15C, and 16C, at high output (i.e. long inverter conduction period).
FIGs. 14A through 16C schematically show conduction states of first
switching element 1 through fourth switching element 4, conduction
periods of the inverter circuit; and waveforms of a primary current;
through transformer 6.
In FIGs. 14A through 16C, a part indicated by an arrow, of an
operation waveform of first switching element 1 to fourth switching
element 4 shows how the waveform changes during output control. An
arrow appended at the falling edge of a waveform shows that the edge
moves back and forth, and the waveform expands and contracts to
change the conduction period. An arrow appended at the top of a
waveform shows that the waveform does not expand or contract, the
conduction period does not change, and the entire waveform moves
back and forth on along the time axis. This indicates that the phase of
a waveform changes to control output as shown by the inverter
conduction period. A horizontally striped part of the waveform of a
primary current through a transformer represents a regenerative
current.
First, a description is made of an operation example of a welding
machine by PWM method using FIGs. 14A through 14C. FIG. 14A
shows operation at low output, where the switching element does not
conduct (a transformer current is not flowing) due to such as delay
operation (described later) of the driving circuit during minimum
power output. FIG. 14B shows an operation example at middle
output; and FIG. 14C, at high output. Both first switching circuit 25
and second switching circuit 26 are operating with PWM method.
Here, a description is made of the following situation using FIGs.
10A and 10B. That is, a switching element does not conduct due to
such as delay operation of the driving circuit during minimum power
output; a transformer current does not flow; and a transformer current
becomes unstable near the minimum conduction width.
FIGs. 10A and 10B are schematic diagrams showing waveforms at;
some points of a switching element and a driving circuit, particularly
for a combination of switching element 3 and driving circuit 23 out of
the four switching elements and four driving circuits shown in FIG. 11.
FIG. 10A shows an outline structure of driving circuit 23 using pulse
transformer 31. FIG. 10B shows current waveforms at points A
through C shown in FIG. 10A.
Driving circuit 23 shown in FIG. 10A is one including third
switching element 3, inverter control part 29, pulse transformer
operating transistor 30, pulse transformer 31, gate resistance 32, and
capacitance 33 inside the gate of third switching element 3.
According to FIG. 10A, a drive signal output from inverter control
part 29 is delayed at transistor 30 and pulse transformer 31 composing
above-described driving circuit 23. Therewith, the signal is deformed
by gate resistance 32 and capacitance 33 inside the gate of third
switching element 3. In other words, as shown in FIG. 10B, the
waveform at point A enters a state of delayed and reduced conduction
time at point C where operation of third switching element 3 is shown.
Accordingly, conduction (i.e. a flow of a transformer current) becomes
unstable when the conduction time approaches the minimum
conduction width, which sometimes causes a transformer current not to
flow.
Next, a description is made of an operation example of a welding
machine by phase control method using FIGs. 15A through 15C. FIGs.
15A through 15C show operation examples of an arc welding machine
including an inverter control circuit by conventional phase control
method. In all the areas of FIGs. 15A, 15B, and 15C, first switching
circuit 25 shown in FIG. 12 is operating with a predetermined
conduction width, and second switching circuit 26 is operating while
undergoing phase control on first switching circuit 25. When second
switching circuit 26 becomes nonconducting in this situation, a
transformer current ceases to flow. Consequently, second switching
circuit 26 interrupts a transformer current and first switching circuit;
25 does not, thereby preventing heat generation caused by switching.
However, the large area size of the waveform indicated by the
horizontal stripes in the waveform of a transformer current brings
about a large regenerative current, thereby causing the regeneration
diode of the switching element to generate more heat.
Here, a description is made of a regenerative current in phase
control method using FIGs. 8A and 8B.
FIGs. 8A and 8B show changes in operating state of the inverter of
a welding machine according to conventional phase control method.
FIG. 8A shows the entire waveform for one cycle. FIG. 8B shows a
conduction state of the switching element and a circuit current for
periods indicated by Tl through T5 in FIG. 8A.
In FIG. 8A, LI (the part surrounded by the oval solid line)
indicates that switching loss is generated; L2 (the part surrounded by
the oval broken line), is not generated. According to FIG. 8A, first
switching element 1 indicated by Q1 does not interrupt a transformer
current, and thus a conventional turn-off power loss is not generated.
As indicated by T3 in FIG. 8B, however, first switching element 1
(indicated by Ql) and third switching element 3 (indicated by Q3) are
in a conduction state for a long time, which causes a regenerative
current to flow for a long time. Since this regenerative current is
interrupted, a regeneration turn-off power loss is generated.
Next, a description is made of an operation example of a welding
machine by one-side bridge fixed conduction width PWM control
method using FIGs. 16A through 16C.
FIGs. 16A through 16C show operation examples of an arc welding
machine including inverter control part 29 by conventional pulse-width
modulation with one-side bridge fixed conduction width. FIG. 1(5A
shows operation at low output, where the third and fourth switching
elements do not conduct (a transformer current is not flowing) due to
delay operation of the driving circuit during minimum power output;.
FIG. 16B shows operation at middle output; and FIG. 16C, at high
output. Second switching circuit 26 shown in FIG. 13 is operating
with PWM method in relation to first switching circuit 25. At this
moment, second switching circuit 26 interrupts a transformer current
and first switching circuit 25 does not interrupt, thereby preventing
heat generation caused by switching.
Here, a description is made of the path of a charging current for a
capacitor of a snubber in one-side fixed conduction width PWM method.
FIGs. 9A and 9B schematically show a charging current path of a
snubber capacitor for a switching element, near a minimum
transformer current. FIG. 9A shows operation of phase control
method; and FIG. 9B, of one-side fixed conduction width PWM method.
In FIG. 9A, first switching element 1 and third switching element
3 are in a conduction state near a minimum current by phase control
method. Accordingly, a charging current to second snubber capacitor
36 flows from first rectifier 5 to second snubber capacitor 36 through
first switching element 1. A charging current to fourth snubber
capacitor 37 flows from first rectifier 5 to fourth snubber capacitor 37
through third switching element 3. Accordingly, voltages at both ends
of transformer 6 become nearly equal, and thus a charging current does
not flow through transformer 6. Here, second snubber resistance 34
and fourth snubber resistance 35 are connected in parallel with
transformer 6 placed therebetween.
In FIG. 9B, meanwhile, only first switching element 1 becomes in a
conduction state near a minimum current by one-side fixed conduction
width PWM method. Accordingly, both charging currents to second
snubber capacitor 36 and fourth snubber capacitor 37 flow through
first switching element 1, which causes the charging currents to flow
through transformer 6. A current flowing through transformer 6 thus
causes unintended output at the secondary side of transformer 6.
The above-described pulse-width modulation is performed in an
inverter-controlled welding machine by conventional PWM method and
by one-side bridge fixed conduction width pulse-width modulation.
An attempt to exercise control with an inverter conduction width of a
minute (approximately 1 us) pulse width causes delay time in the drive
path between inverter control part 29 and a switching element,
particularly, delay time in the driving circuit and operation delay time
in the switching element. Consequently, the switching element
cannot be driven, or highly accurate control cannot be exercised in
practice.
At this point, as shown in FIGs. 10A and 10B, a drive waveform
signal output from inverter control part 29 activates switching element
3 through points A and B shown in FIG. 10A. On this occasion,
however, the waveform at each point is deformed as shown in FIG. 10B
due to delay operation in circuit components of driving circuit 23 and
gate input capacitance 33 of third switching element 3. As shown in
FIG. 10B, the conduction waveform of third switching element 3 at
point C is not only delayed but is shortened in conduction width
compared to the waveform at point A. Then, as shown in FIG. IOC,
switching element 3 ceases to conduct as the drive signal width from
inverter control part 29 becomes narrower.
This state is one such that a switching element is not conducting
at minimum output in FIG. 14A showing an operation example of a
welding machine by PWM control. Such a situation is of a problem
particularly when requiring stable control on an output current in a
range of several amperes, as in a TIG welding machine.
This phenomenon undesirably causes heat generation of an
element and transformer saturation due to an unstable transformer
current near a minimum drive width because the switching element is
inadequately driven due to insufficient power for driving the gate of
the switching element.
As shown in FIG. 9B, in operation by one-side bridge fixed
conduction width pulse-width modulation near a minimum current, a
charging current to a snubber capacitor causes a primary current to
flow through transformer 6, thereby generating unintended output at
the secondary side of the transformer. Accordingly, a large
capacitance of the snubber capacitor leads to difficulty in control at low
output, which prevents an output current or output voltage of the
welding machine from falling to a minimum output.
An inverter-controlled welding machine by conventional phase
control method does not need to expand and contract the driving pulse
width of a switching element, and thus is not affected by delay time in
the drive path, allowing control with a high degree of accuracy even at
low output.
In the above case, however, the switching elements composing first
switching circuit 25 and second switching circuit 26 simultaneously
conduct for a relatively long time. This brings about a large
regenerative current, thereby causing more heat generation in a
regeneration diode contained in a switching element and a higher
switching loss at the transistor.
As described above, phase control method involves a large
regenerative current and difficulty in preventing heat generation in
the device. Meanwhile, PWM control method has difficulty in
controlling a minute current well accurately.
[Prior art document]
[Patent literature]
[Patent literature l] Japanese Patent Unexamined Publication No.
2004-322189
SUMMARY OF THE INVENTION
The present invention, in order to solve the above-described
problems, provides an inverter control device that prevents a
regenerative current to suppress heat generation for whatever the
magnitude of a signal such as an error amplification signal of the
inverter control device and controls an output current well accurately.
An inverter control device of the present invention includes: a first
rectifier rectifying AC input; a first switching element and a second
switching element inserted between the outputs of the first rectifier,
composing a first switching circuit, series-connected; a thirdswitching
element and a fourth switching element inserted between the outputs
of the first rectifier, composing a second switching circuit,
series-connected; a power conversion transformer, one primary winding
of which is connected to the junction between the first and second
switching elements and the other primary winding of which is
connected to the junction between the third and fourth switching
elements! a second rectifier rectifying output from the power
conversion transformer; an output power detecting part detecting an
output current or output voltage from the second rectifier! an output
power setting part for preliminarily setting an output current or
output voltage! an error amplification part determining an error
between signals from the output power detecting part and the output
power setting part, and outputting the error,' and an inverter control
part outputting a signal for controlling operation of the first and
second switching circuits according to a signal from the error
amplification part. The inverter control part includes- a first
switching circuit control part outputting a drive signal for alternately
bringing the first and second switching elements composing the first
switching circuit into conduction; and a second switching circuit;
control part outputting a drive signal for alternately bringing the third
and fourth switching elements composing the second switching circuit
into conduction. The second switching circuit control part includes: a
pulse-width modulating part generating a conduction width that is
time during which the third and the fourth switching elements are kept
in conduction, according to a signal from the error amplification part,
and outputting the conduction width; a phase control part generating
conduction time during which the third and fourth switching elements
are kept in conduction, where the time is a phase difference relative to
the conduction time for the first and second switching elements,
according to a signal from the error amplification part, and outputting
the phase difference,' and a signal changing part accepting a signal
from the pulse-width modulating part and a signal from the phase
control part, and outputting one of the signal from the pulse-width
modulating part and the signal from the phase control part according
to a signal from the error amplification part.
This configuration allows the inverter control device to exercise
control by PWM control method and phase control method. Hence,
when an error amplification signal is larger than a predetermined
threshold, PWM control method is used to prevent a regenerative
current to suppress heat generation of a switching element. When
smaller, phase control method is used to enable an output current to be
controlled well accurately.
The inverter control device of the present invention includes: a
first rectifier rectifying AC input; a first switching element and a
second switching element inserted between the outputs of the first
rectifier, composing a first switching circuit, series-connected; a third
switching element and a fourth switching element inserted between
the outputs of the first rectifier, composing a second switching circuit,
series-connected; a power conversion transformer, one primary winding
of which is connected to the junction between the first and second
switching elements and the other primary winding of which is
connected to the junction between the third and fourth switching
elements; a second rectifier rectifying output from the power
conversion transformer; an output power detecting part detecting an
output current or output voltage from the second rectifier; an output,
power setting part for preliminarily setting an output current or
output voltage; an error amplification part determining an error
between signals from the output power detecting part and the output-
power setting part, and outputting the error; and an inverter control
part outputting a signal for controlling operation of the first and
second switching circuits according to a signal from the error
amplification part. The inverter control part includes: a first
switching circuit control part outputting a drive signal for alternately
bringing the first and second switching elements composing the first
switching circuit into conduction.* and a second switching circuit-
control part outputting a drive signal for alternately bringing the third
and fourth switching elements composing the second switching circuit;
into conduction. The second switching circuit control part includes: a
pulse-width modulating part generating a conduction width that is
time during which the third and the fourth switching elements are kept;
in conduction, according to a signal from the error amplification part;,
and outputting the conduction width; a phase control part generating a
drive signal with a phase difference relative to a drive signal from the
first switching circuit control part according to an error amplification
signal; a driving pulse width changing part changing a driving pulse
width from the phase control part according to the error amplification
signal; and a signal changing part accepting a signal from the
pulse-width modulating part and a signal from the phase control part,
and outputting either of the signals according to a signal from the error
amplification part.
This configuration allows the inverter control device to exercise
control by PWM control method and phase control method. Hence,
when an error amplification signal is larger than a predetermined
threshold, PWM control method is used to prevent a regenerative
current to suppress heat generation of a switching element. When
smaller, phase control method is used to enable an output current to be
controlled well accurately.
The inverter control device of the present invention includes: a
first rectifier rectifying AC input.* a first switching element and a
second switching element inserted between the outputs of the first
rectifier, composing a first switching circuit, series-connected; a third
switching element and a fourth switching element inserted between
the outputs of the first rectifier, composing a second switching circuit,
series-connected; a power conversion transformer one primary winding
of which is connected to the junction between the first and second
switching elements and the other primary winding of which is
connected to the junction between the third and fourth switching
elements; a second rectifier rectifying output from the power
conversion transformer; an output power detecting part detecting an
output current or output voltage from the second rectifier,' an output;
power setting part for preliminarily setting an output current or
output voltage,' an error amplification part determining an error
between signals from the output power detecting part and the output;
power setting part, and outputting the error; and an inverter control
part outputting a signal for controlling operation of the first and
second switching circuits, according to a signal from the error
amplification part. The inverter control part includes-' a first
switching circuit control part outputting a drive signal for alternately
bringing the first and second switching elements composing the first
switching circuit into conduction! and a second switching circuit
control part outputting a drive signal for alternately bringing the third
and fourth switching elements composing the second switching circuit
into conduction. The second switching circuit control part includes: a
pulse-width modulating part generating a conduction width that is
time during which the third and the fourth switching elements are kept
in conduction, according to a signal from the error amplification part,
and outputting the conduction width,' an additional driving pulse
generating part outputting a drive signal to be added to the beginning
of a drive signal output from the pulse-width modulating part! and a
combining part combining output from the pulse-width modulating
part with output from the additional driving pulse generating part.
This configuration allows the inverter control device to exercise
control by PWM control method and phase control method. Hence,
when an error amplification signal is larger than a predetermined
threshold, PWM control method is used to prevent a regenerative
current to suppress heat generation of a switching element. When
smaller, phase control method is used to enable an output current to be
controlled well accurately.
The inverter control method of the present invention is a method of
controlling an inverter control device including: a first rectifier
rectifying AC input; a first switching element and a second switching
element inserted between the outputs of the first rectifier, composing a
first switching circuit, series-connected; a third switching element and
a fourth switching element inserted between the outputs of the first
rectifier, composing a second switching circuit, series-connected; a
power conversion transformer one primary winding of which is
connected to the junction between the first and second switching
elements and the other primary winding of which is connected to the
junction between the third and fourth switching elements; a second
rectifier rectifying output from the power conversion transformer; an
output power detecting part detecting an output current or output,
voltage from the second rectifier; an output power setting part for
preliminarily setting an output current or output voltage! an error
amplification part determining an error between signals from the
output power detecting part and the output power setting part, and
outputting the error! and an inverter control part outputting a signal
for controlling operation of the first and second switching circuits
according to a signal from the error amplification part. The method
using an inverter control part includes: a pulse width change
controlling step of changing conduction time during which the third
and fourth switching elements are kept in conduction according to a
signal from the error amplification part; and a phase controlling step of
changing conduction time during which the third and fourth switching
elements are kept in conduction according to a signal from the error
amplification part so that the conduction time has a phase difference
relative to the conduction time of the first and the second switching
elements. When the magnitude of the error amplification signal is
within a predetermined first range, the pulse width controlling stop is
performed. When the magnitude is within a predetermined second
range that is smaller than the first range, at least the phase
controlling step is performed.
This method allows inverter control by PWM control method and
phase control method. Hence, when an error amplification signal is
larger than a predetermined threshold, PWM control method is used to
prevent a regenerative current to suppress heat generation of a
switching element. When smaller, phase control method is used to
enable an output current to be controlled well accurately.
As described thereinbefore, the present invention allows using two
control methods: PWM control method and phase control method.
Hence, when an error amplification signal is larger than a
predetermined threshold, PWM control method is used to prevent a
regenerative current to suppress heat generation of a switching
element. When smaller, phase control method is used to enable an
output current to be controlled well accurately.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows an outline structure of substantial parts of the
inverter control device of an arc welding machine according to the first
exemplary embodiment of the present invention.
FIG. 2A is a schematic diagram of operation of the components of
the inverter control device of an arc welding machine according to the
first exemplary embodiment of the present invention.
FIG. 2B is a schematic diagram of operation of the components of
the inverter control device of an arc welding machine according to the
first exemplary embodiment of the present invention.
FIG. 2C is a schematic diagram of operation of the components of
the inverter control device of an arc welding machine according to the
first exemplary embodiment of the present invention.
FIG. 3 shows an outline structure of substantial parts of the
inverter control device of an arc welding machine according to the
second exemplary embodiment of the present invention.
FIG. 4A is a schematic diagram of operation of the components of
the inverter control device of an arc welding machine according to the
second exemplary embodiment of the present invention.
FIG. 4B is a schematic diagram of operation of the components of
the inverter control device of an arc welding machine according to the
second exemplary embodiment of the present invention.
FIG. 4C is a schematic diagram of operation of the components of
the inverter control device of an arc welding machine according to the
second exemplary embodiment of the present invention.
FIG. 5 shows an outline structure of substantial parts of the
inverter control device of an arc welding machine according to the third
exemplary embodiment of the present invention.
FIG. 6A is a schematic diagram of operation of the components of
the inverter control device of an arc welding machine according to the
third exemplary embodiment of the present invention.
FIG. 6B is a schematic diagram of operation of the components of
the inverter control device of an arc welding machine according to the
third exemplary embodiment of the present invention.
FIG. 6C is a schematic diagram of operation of the components of
the inverter control device of an arc welding machine according to the
third exemplary embodiment of the present invention.
FIG. 7A illustrates inverter operation of an inverter control device
according to the third exemplary embodiment of the present invention.
FIG. 7B illustrates inverter operation of the inverter control
device according to the third exemplary embodiment of the present
invention.
FIG. 8A illustrates inverter operation by phase control method.
FIG. 8B illustrates inverter operation by phase control method.,
FIG. 9A shows a snubber charge path.
FIG. 9B shows a snubber charge path.
FIG. 10A shows an outline structure of a driving circuit.
FIG. 10B shows waveforms at some parts of the driving circuit.
FIG. 11 shows an outline structure of substantial parts of a
welding machine by conventional pulse-width modulation.
FIG. 12 shows an outline structure of substantial parts of a
welding machine by conventional phase control method.
FIG. 13 shows an outline structure of substantial parts of a
welding machine by conventional one-side bridge fixed conduction
width PWM control method.
FIG. 14A is a schematic diagram of operation of the inverter of a
welding machine by conventional pulse-width modulation.
FIG. 14B is a schematic diagram of operation of the inverter of the
welding machine.
FIG. 14C is a schematic diagram of operation of the inverter of the
welding machine.
FIG. 15A is a schematic diagram of operation of the inverter of a
welding machine by conventional phase control method.
FIG. 15B is a schematic diagram of operation of the inverter of the
welding machine by conventional phase control method.
FIG. 15C is a schematic diagram of operation of the inverter of the
welding machine by conventional phase control method.
FIG. 16A is a schematic diagram of operation of the inverter of a
welding machine by one-side bridge fixed conduction width PWM
control method.
FIG. 16B is a schematic diagram of operation of the inverter of the
welding machine by one-side bridge fixed conduction width PWM
control method.
FIG. 16B is a schematic diagram of operation of the inverter of the
welding machine by one-side bridge fixed conduction width PWM
control method.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Hereinafter, a description is made of some exemplary embodiments
of the present invention with reference to the related drawings. In
the following drawings, the same component is given the same
reference mark, and its description may be omitted. The scope of the
present invention is not limited by these exemplary embodiments.
FIRST EXEMPLARY EMBODIMENT
A description is made of an arc welding machine using an inverter
control device according to the first exemplary embodiment using FIG.
1 and FIGs. 2A through 2C. FIG. 1 shows an outline structure of
substantial parts of the arc welding machine. FIGs. 2A through 2C
are schematic diagrams showing operation of the components of the arc
welding machine. FIGs. 2A through 2C illustrate operation of the arc
welding machine, specifically operation of a switching element, an
inverter conduction period, and a waveform of a transformer primary
current at low output (FIG. 2A), middle output (FIG. 2B), and high
output (FIG. 2C) for welding.
Here, low, middle, and high output for welding are classified on the
basis of the magnitude of an error amplification signal from error
amplification part 11 (described later), for example. Specifically, if
the magnitude is lower than a predetermined first threshold, the
output is low! if between the predetermined first threshold and a
predetermined second threshold, middle; and if higher than the
predetermined second threshold, high.
First and second thresholds can be determined by such as results
of actual welding.
As shown in FIG. 1, the inverter control device of an arc welding
machine includes first rectifier 5, first switching element 1 and second
switching element 2, third switching element 3 and fourth switching-
element 4, power conversion transformer 6, second rectifier 7, an
output power detecting part (e.g. voltage detecting part 20), output
power setting part 12, error amplification part 11, and inverter control
part 29. Here, first rectifier 5 rectifies AC input. First switching
element 1 and second switching element 2 are inserted between the
outputs of first rectifier 5>' compose first switching circuit 25; and are
series-connected. Third switching element 3 and fourth switching
element 4 are inserted between the outputs of first rectifier 5; compose
second switching circuit 26! and are series-connected. One primary
winding of power conversion transformer 6 is connected to the junction
between first switching element 1 and second switching element 2, and
the other primary winding is connected to the junction between third
switching element 3 and fourth switching element 4. The primary-
winding of transformer 6 has capacitor 10 serially inserted therein.
Second rectifier 7 rectifies output from transformer 6. The output
power detecting part includes voltage detecting part 20 and current
detecting part 9. Voltage detecting part 20 detects voltage output
from second rectifier 7. Current detector 8 detects a current output
from second rectifier 7. Current detecting part 9 converts a signal
from current detector 8 to a feedback signal. Error amplification part,
11 determines an error between an output current detection signal
from current detecting part 9 and an output setting signal from output
current setting part 12, and amplifies the error. Inverter control part
29 controls operation of first switching circuit 25 and second switching
circuit 26 according to an error amplification signal from amplification
part 11.
Inverter control part 29 includes first switching circuit control
part 27 and second switching circuit control part 28. Here, first
switching circuit control part 27 generates a drive signal for
alternately bringing first switching element 1 and second switching
element 2 into conduction; second switching circuit control part 28
generates a drive signal for alternately bringing third switching
element 3 and fourth switching element 4 into conduction.
First switching circuit control part 27 includes inverter driving
basic pulse generating part 13 driving first switching element 1 and
second switching element 2 with a constant conduction width.
Generating part 13 provides a predetermined conduction time (e.g. the
entire half-cycle time minus dead time of a switching element).
Second switching circuit control part 28 includes pulse-width
modulating part 14, phase control part 15, and signal changing part 19.
Here, modulating part 14 generates a conduction width corresponding
to a drive signal input from inverter driving basic pulse generating
part 13 of first switching circuit control part 27 and an error
amplification signal input from error amplification part 11, and
outputs the conduction width. Phase control part 15 generates a drive
signal with a phase difference corresponding to a drive signal input
from inverter driving basic pulse generating part 13 of first switching
circuit control part 27 and an error amplification signal input from
error amplification part 11. Signal changing part 19 outputs an
output signal selectively from pulse-width modulating part 14 and
from phase control part 15, to third driving circuit 23 and fourth
driving circuit 24.
First driving circuit 21 controls driving of first switching element
1; second driving circuit 22, second switching element 2; third driving
circuit 23, third switching element 3; and fourth driving circuit 24,
fourth switching element 4.
As described later, this configuration allows the inverter control
device to exercises control by two types of methods^ PWM control and
phase control. Hence, when an error amplification signal is larger
than a predetermined threshold, PWM control method is used to
prevent a regenerative current to suppress heat generation of a
switching element. When smaller, phase control method is used to
enable an output current to be controlled well accurately.
FIGs. 2A through 2C show operating states of an inverter
according to the embodiment. FIG. 2A shows operation states at low
output (i.e. short inverter conduction period); FIG. 2B, at middle
output (i.e. middle-range inverter conduction period); and FIG. 2C, at
high output (i.e. long inverter conduction period). FIGs. 2A through
2C schematically show conduction states, conduction periods of an
inverter circuit." and waveforms of a primary current through
transformer 6, for first switching element 1 through fourth switching
element 4.
In FIGs. 2A through 2C, a part indicated by an arrow, of an
operation waveform of first switching element 1 to fourth switching
element 4 shows how the waveform changes during output control. An
arrow appended at the falling edge (dot-marked part) of a waveform
shows that the edge moves back and forth, and the waveform expands
and contracts to control output as indicated by the inverter conduction
period. An arrow appended at the top of a waveform shows that the
waveform does not expand or contract, and the entire waveform moves
back and forth for operation. This indicates that the phase of the
waveform changes to control output as shown by the inverter
conduction period. A horizontally striped part of the waveform of a
primary current through a transformer represents a regenerative
current as described under Background Art.
A description is made of operation of an arc welding machine
configured as above. In FIG. 1, three- or single-phase AC input;
rectified by first rectifier 5 is converted to an alternating current with
a high frequency by a full-bridge inverter circuit composed of switching
elements 1, 2, 3, and 4, and then input to the primary side of
transformer 6. The secondary-side output of transformer 6 is rectified
by second rectifier 7 and is supplied to an electrode and parent
material (i.e. both are arc loads, not shown) through output terminals
38 and 39 of the arc welding machine.
An output current from an arc welding machine is detected by
current detector 8, and a feedback signal proportional to the output
current is input from current detector 8 to error amplification part 11
through current detecting part 9. Error amplification part 11
compares an output current set value from output power setting part
12 to a feedback signal from current detecting part 9, and outputs an
error amplification signal between both. The error amplification
signal is input to pulse-width modulating part 14, phase control part
15, and signal changing part 19.
Inverter driving basic pulse generating part 13 outputs inverter
driving basic pulses for driving first switching element 1 and second
switching element 2 composing first switching circuit 25 alternately
with a fixed conduction width.
The inverter driving basic pulses are converted to a signal suitable
for driving first switching element 1 and second switching element 2 by
first driving circuit 21 and second driving circuit 22, and is input to
first switching element 1 and second switching element 2.
Pulse-width modulating part 14 accepts a basic pulse waveform for
inverter driving generated by inverter driving basic pulse generating
part 13. Driving pulses are generated with a width corresponding to
the level of an error amplification signal from error amplification part
11 on a basis of the basic pulse waveform. The driving pulses are
separated one by one alternately into two series for third driving
circuit 23 and for fourth driving circuit 24, and are input to signal
changing part 19 as 2-series drive signals for inverter driving.
Phase control part 15 accepts a basic pulse waveform for inverter
driving generated by inverter driving basic pulse generating part 13.
Driving pulses are generated with a phase difference corresponding to
the level of an error amplification signal in relation to the basic pulse
waveform. The driving pulses are separated one by one alternately
into two series: for third driving circuit 23 and for fourth driving
circuit 24, and are input to signal changing part 19 as 2-series drive
signals for inverter driving.
Signal changing part 19 outputs a drive signal input selectively
from pulse-width modulating part 14 and from phase control part 15 to
third driving circuit 23 and fourth driving circuit 24, according to the
level of an error amplification signal from error amplification part 11.
The drive signal output from signal changing part 19 is converted to a
signal suitable for driving third switching element 3 and fourth
switching element 4 by third driving circuit 23 and fourth driving
circuit 24, and is input to third switching element 3 and fourth
switching element 4.
During a period when a conduction period of first switching
element 1 coincides with that of fourth switching element 4, a primary
current flows through transformer 6 from first switching element 1 to
fourth switching element 4. Meanwhile, during a period when a
conduction period of second switching element 2 coincides with that of
third switching element 3, a primary current flows through
transformer 6 from third switching element 3 to second switching
element 2. In this way, output from first rectifier 5 is converted to an
alternating current; is converted to output power suitable for welding;
and is output from the secondary winding of transformer 6. The
output from the secondary winding of transformer 6 is converted to a
direct current by second rectifier 7 and is output from the welding
machine as welding output power.
When the magnitude of an error amplification signal is larger than
a predetermined threshold (i.e. a large transformer current conduction
width), signal changing part 19 outputs a drive signal from pulse-width
modulating part 14; when smaller (i.e. a small transformer current;
conduction width), outputs a drive signal from phase control part 15.
In this way, third switching element 3 and fourth switching
element 4 composing second switching circuit 26 are driven with
pulse-width modulation at high output; with phase control method at
low output. Accordingly, a regenerative current can be better
suppressed at high output as compared to phase control method; a
small current can be better accurately controlled as compared to PWM
method.
The pulse width for phase control method is determined as a pulse
width for pulse-width modulation when switching is made from
pulse-width modulation to phase control method.
Here, first switching element 1 and second switching element 2
composing first switching circuit 25 are alternately operated according
to a signal from inverter driving basic pulse generating part 13.
independently of the output level, with a fixed conduction width by
first driving circuit 21 and second driving circuit 22.
Next, a description is made of operation of the inverter control
device of an arc welding machine according to the first exemplary
embodiment using FIGs. 2A through 2C.
FIGs. 2A through 2C show operation examples of the components
of an arc welding machine, namely circuit operation examples of an
inverter control device. FIG. 2A for control operation at low output
including a part near a minimum conduction width and FIG. 2B for
control operation at middle output show examples where second
switching circuit 26 is operating while being controlled with phase
control method in relation to first switching circuit 25. FIG. 2C shows
an example where the switching circuit is operating while being
controlled with pulse-width modulation.
Here, the driving pulse width of a drive signal undergoing phase
control in FIGs. 2A and 2B is set to the drive signal width at a time
point when Operation by pulse-width modulation in FIG. 2C changes to
operation by phase control in FIG. 2B. This setting makes control
shift smoothly from operation by pulse-width modulation to operation
by phase control.
The drive signal width at a time point when operation by
pulse-width modulation changes to operation by phase control is
assumed to be a drive signal width smaller than 50% if the maximum
conduction width is 100%, for example.
As shown in FIG. 1, the primary winding of transformer 6 is
provided with capacitor 10 serially. Capacitor 10 enables reducing a
regenerative current as shown by horizontal stripes in a transformer
primary current waveform in FIGs. 2A to 2C. This prevents a
switching element from generating heat due to a regenerative current,
unlike by conventional phase control method, even if control is
exercised by phase control method. Here, setting the capacitance of
capacitor 10 to several uF for an arc welding machine with an output
class of 350 A is most effective for suppressing a regenerative current,
which has been experimentally proven.
Setting operation of first switching circuit 25 to near a maximum
conduction width results in a transformer primary current being
interrupted by third switching element 3 and fourth switching element
4. Herewith, first switching element 1 and second switching element
2 do not interrupt a current, thereby significantly reducing switching
loss in first switching element 1 and second switching element 2 to
suppress heat generation.
As described above, with the arc welding machine using the
inverter control device of the first exemplary embodiment, first
switching circuit 25 (i.e. one of the two switching circuits) is driven
with a fixed conduction width; second switching circuit 26 (i.e. the
other switching circuit) is driven with pulse-width modulation at high
output and by phase control method at low output. Further, capacitor
10 serially connected to the primary side of transformer 6 suppresses a
regenerative current. These facts allow implementing an inverter
control device in which advantages of pulse-width modulation and
phase control method are merged, and allow implementing an arc
welding machine using an inverter control device enabling highly
accurate control at low output while significantly suppressing heat
generation in a switching element.
Switching between pulse-width modulation method and phase
control method is made as follows. That is, if a signal from error
amplification part 11 is larger than a predetermined threshold.
pulse-width modulation is used; if smaller, phase control method is
used.
In the first exemplary embodiment, the description is made of
current control using output current detector 8 and output current
detecting part 9. Besides, it is obvious that voltage control where the
output current detector is replaced with output voltage detecting part
20 follows the same operation. Voltage control is suitable for
consumable electrode welding! and current control, for
non-consumable.
In this embodiment, a programmable integrated logic element may
be used such as a CPU, DSP, and FPGA as inverter control part 29.
SECOND EXEMPLARY EMBODIMENT
A description is made of an arc welding machine including an
inverter control device of the second exemplary embodiment using FIG.
3 and FIGs. 4A through 4C. FIG. 3 shows an outline structure of
substantial parts of the inverter control device. FIGs. 4A through 4C
illustrate operation of the inverter control device, specifically
operation of a switching element, an inverter conduction period, and a
waveform of a transformer primary current at low output (FIG. 4A),
middle output (FIG. 4B), and high output (FIG. 4C) for welding.
In this embodiment, a component or portion same as that in the
first embodiment is given the same reference mark to omit its detailed
description.
The principal point different from the first embodiment is the
configuration of inverter control part 29. Specifically, the second
embodiment includes driving pulse width changing part 17 as
described later. Further, signal changing part 19 outputs an output
signal selectively from pulse-width modulating part 14, phase control
part 15, and driving pulse width changing part 17, to third driving
circuit 23 and fourth driving circuit 24.
In the inverter control device of the arc welding machine in FIG. 3,
second switching circuit control part 28 composing inverter control
part 29 includes driving pulse width changing part 17 changing a
driving pulse width from phase control part 15. Changing part 17
thus changes a driving pulse width from phase control part 15,
eventually resulting in output from driving pulse width changing part
17 being a signal with both its phase and pulse width changed.
FIGs. 4A through 4C show operating states of the inverter control
device according to the second embodiment. FIG. 4A shows operation
states at low output (i.e. short inverter conduction period); FIG. 413, at
middle output (i.e. middle-range inverter conduction period); and FIG.
4C, at high output (i.e. long inverter conduction period). The figures
schematically show conduction states, conduction periods of the
inverter circuit; and waveforms of a primary current through
transformer 6, for first switching element 1 through fourth switching
element 4.
In FIGs. 4A through 4C, a part indicated by an arrow, of an
operation waveform of first switching element 1 to fourth switching
element 4 shows how the waveform changes during output control. An
arrow appended at the edge (falling edge of the waveform) shows that
the edge moves back and forth, and the waveform expands and
contracts. An arrow appended at the top of a waveform shows that the
entire waveform moves back and forth for operation; the phase of the
waveform changes to control output by a conduction period as shown by
the inverter conduction period. A horizontally striped part of the
waveform of a primary current through a transformer represents a
regenerative current.
A description is made of operation of the inverter control device cf
an arc welding machine structured as above.
In FIG. 3, a portion indicated by the same reference mark as that,
in FIG. 1 operates in the same way as in the first embodiment, and
thus its detailed description is omitted.
An error amplification signal having been input from error
amplification part 11 to second switching circuit control part 28 is
input to pulse-width modulating part 14 and phase control part 15.
Pulse-width modulating part 14 generates driving pulses with a
width based on the level (magnitude) of an error amplification signal
on a basis of a basic pulse waveform for inverter driving generated by-
inverter driving basic pulse generating part 13. The driving pulses
are separated one by one alternately into two series and are output as
2-series drive signals for inverter driving.
Phase control part 15 generates driving pulses having a phase
difference based on the level of an error amplification signal, in
relation to a basic pulse waveform for inverter driving generated by
inverter driving basic pulse generating part 13. The driving pulses
are separated one by one alternately into two series and are input to
signal changing part 19 as well as to driving pulse width changing part
17 as 2-series drive signals for inverter driving.
Driving pulse width changing part 17 changes the drive signal
width input from phase control part 15 according to the level of an
error amplification signal and inputs the drive signal to signal
changing part 19. Here, setting is made so that driving pulse width
changing part 17 changes the drive signal width so as to be inversely
proportional to the level of the error amplification signal. Herewith, a
low level of the error amplification signal causes inverter output to
decrease and to expand the drive signal width, resulting in being
similar to regular phase control operation. For example, the width
expands at a part temporally before a rising edge of a switching
element composing first switching circuit 25 as shown in FIG. 4A.
Signal changing part 19 outputs a drive signal selectively from
pulse-width modulating part 14, phase control part 15, and driving
pulse width changing part 17, according to the level of an error
amplification signal.
In this way, third switching element 3 and fourth switching
element 4 composing second switching circuit 26 can be driven with
pulse-width modulation at high output; with phase control method (a
relatively short driving pulse width) at middle output; the driving
pulse width expands as output decreases while operating with phase
control method at low output; and can be driven in the same state as
with regular phase control method at minimum output.
Here, if an error amplification signal is lower than a
predetermined first threshold, a drive signal from driving pulse width
changing part 17 is output; if higher than the first threshold and lower
than a predetermined second threshold, from phase control part 15;
and if higher than the second threshold, from pulse-width modulating
part 14. First and second thresholds can be determined to
appropriate values for the welding by such as results of actual welding.
That is, the inverter control method according to the first,
embodiment of the present invention is one for an inverter control
device of the first and second embodiments, particularly including a
pulse width change controlling step and phase controlling step as a
method using inverter control part 29. Here, the pulse width change
controlling step changes time during which third switching element 3
and fourth switching element 4 are kept in conduction, according to a
signal from error amplification part 11. The phase controlling step
changes conduction time for third switching element 3 and fourth
switching element 4 so as to contain a phase difference in relation to
conduction time for first switching element 1 and second switching
element 2, according to a signal from error amplification part 11.
In the inverter control method of the first and second embodiments,
when the magnitude of the error amplification signal is within a
predetermined first range, the pulse width controlling step is
performed; and when the magnitude is within a predetermined second
range that is smaller than the first range, at least the phase
controlling step is performed.
This method allows inverter control method to exercise control by
PWM control method and phase control method. Hence, when an
error amplification signal is larger than a predetermined threshold,
PWM control method is used to prevent a regenerative current to
suppress heat generation of a switching element. When smaller,
phase control method is used to enable an output current to be
controlled well accurately.
When an error amplification signal is within a predetermined
third range that is smaller than the second range, the inverter control
part may perform both the pulse width controlling step and phase
controlling step.
With this method, when an error amplification signal is larger
than a predetermined threshold, PWM control method is used to
prevent a regenerative current to suppress heat generation of a
switching element.' when smaller, phase control method is used to
enable an output current to be controlled well accurately.
The following arrangement may be made. That is, the first and
second ranges are continuous with each other. The phase controlling
step performed when the magnitude of an error amplification signal is
within the second range is performed with a pulse width fixed to that
when the error amplification signal is at the minimum within the first
range.
With this method, when an error amplification signal is larger
than a predetermined threshold, PWM control method is used to
prevent a regenerative current to suppress heat generation of a
switching element; when smaller, phase control method is used to
enable an output current to be controlled well accurately.
A drive signal output from signal changing part 19 is converted to
a signal suitable for driving third switching element 3 and fourth
switching element 4 by third driving circuit 23 and fourth driving
circuit 24, and is input to third switching element 3 and fourth
switching element 4. Here, a signal output from signal changing part
19 is changed according to a signal output from error amplification
part 11.
During a period when a conduction period of first switching
element 1 coincides with that of fourth switching element 4, a primary
current flows through transformer 6 from first switching element 1 to
fourth switching element 4. Meanwhile, during a period when a
conduction period of second switching element 2 coincides with that of
third switching element 3, a primary current flows through
transformer 6 from third switching element 3 to second switching
element 2. In this way, output from first rectifier 5 is converted to an
alternating current; is converted to output power suitable for welding;.
and is output from the secondary winding of transformer 6. The
output from the secondary winding of transformer 6 is converted to a
direct current by second rectifier 7 and is output from the welding
machine as welding output power.
FIGs. 4A through 4C show operation examples of the inverter
control device of an arc welding machine according to the second
exemplary embodiment. FIG. 4A shows control operation at low
output. As shown in FIG. 4A, as the state changes from low output to
minimum output, second switching circuit 26 expands its driving pulse
width with its phase shifting in relation to that of first switching
circuit 25. Eventually, second switching circuit 26 is in a state same
as that by conventional phase control method at zero output.
FIG. 4B shows control operation at middle output. Second
switching circuit 26 is operating with phase control method in relation
to first switching circuit 25 while maintaining a certain conduction
width.
FIG. 4C shows control operation at high output. Second
switching circuit 26 is operating with pulse-width modulation in
relation to first switching circuit 25.
As shown in FIG. 3, with the inverter control device of the second
embodiment, the primary winding of transformer 6 is provided with
capacitor 10 serially, which enables reducing a regenerative current
compared to a case where capacitor 10 is not provided, as shown by the
horizontal stripes in a transformer primary current waveform in FIGs.
4A through 4C. In whichever state of FIGs. 4A to 4C, a regenerative
current can be reduced. This shows that heat generated by a
regenerative current in phase control method can be significantly
suppressed.
Setting is made so that first switching circuit 25 operates with
near a maximum conduction width. This leads to a transformer
primary current being interrupted by third switching element 3 and
fourth switching element 4. Herewith, first switching element 1 and
second switching element 2 do not interrupt a current, thereby
significantly reducing switching loss in first switching element 1 and
second switching element 2 to suppress heat generation.
Operating in the same way as in conventional phase control
operation at minimum output prevents a transformer current from
conducting due to a charging current to second snubber capacitor 36.
As described above, with an arc welding machine of the second
exemplary embodiment, first switching circuit 25 (i.e. one of the two
switching circuits) is driven with a fixed conduction width; second
switching circuit 26 (i.e. the other switching circuit) is driven with
pulse-width modulation at high output; and with phase control method
at middle output, to suppress a regenerative current by capacitance 10
serially connected to the primary side of the transformer. These facts
allow implementing an inverter welding machine in which advantages
of pulse-width modulation and phase control method are merged;
enable highly accurate control at low output while significantly
suppressing heat generation in a switching element.
At low output, both phase control method and pulse-width
modulation are performed, resulting in a drive signal same as that
with a drive signal added to a part temporally before a drive signal
output by pulse-width modulation. Herewith, conduction of a
switching element can be stabilized even at a minute conduction width.
With the inverter control device of the second embodiment, the
description is made of current control using output current detector 8
and output current detecting part 9. Besides, it is obvious that
voltage control where the output current detector is replaced with
output voltage detecting part 20 follows the same operation.
In this embodiment, a programmable integrated logic element may
be used such as a CPU, DSP, and FPGA as inverter control part 29.
THIRD EXEMPLARY EMBODIMENT
A description is made of the inverter control device of an arc
welding machine according to the third exemplary embodiment using
FIGs. 5 through 7. FIG. 5 shows an outline structure of substantial
parts of the inverter control device. FIGs. 6A through 6C
schematically illustrate operation of the inverter control device,
specifically operation of a switching element, an inverter conduction
period, and a waveform of a transformer primary current at low output
(FIG. 6A), middle output (FIG. 6B), and high output (FIG. 6C) for
welding. FIGs. 7A and 7B illustrate inverter operation, showing
changes in operating state. FIG. 7A shows the entire waveform for
one cycle. FIG. 7B shows conduction states of the switching elements
and operating states of a circuit current in the time areas indicated by
Tl through T5 in FIG. 7A.
In FIG. 7A, part LI surrounded by the solid-line oval indicates
that switching loss is generated; and L2 surrounded by the broken-line
oval, not generated.
In the third embodiment, a component or portion same as that in
the first and second embodiments is given the same reference mark to
omit its detailed description.
As shown in FIG. 5, with the inverter control device of the arc
welding machine according to the third embodiment, inverter control
part 29 includes pulse-width modulating part 14, additional driving
pulse generating part 16, and first combining part 40 and second
combining part 41 combining output from pulse-width modulating part
14 with output from additional driving pulse generating part 16.
FIGs. 6A through 6C show operating states of the inverter control
device according to the third embodiment. FIG. 6A shows an
operation state at low output (i.e. a short inverter conduction period);
FIG. 6B, at middle output (i.e. middle-range); and FIG. 6C, at high
output (i.e. a long inverter conduction period). The figures
schematically show conduction states, conduction periods of the
inverter circuits; and waveforms of a primary current through the
transformer, for first switching element 1 through fourth switching
element 4.
In FIGs. 6A through 6C, a part indicated by an arrow, of an
operation waveform of first switching element 1 to fourth switching
element 4 shows how the waveform changes during output control. An
arrow appended at the falling edge (indicated by a black dot) shows
that the edge moves back and forth, and the waveform expands and
contracts. A horizontally striped part of the waveform of a primary
current through the transformer represents a regenerative current.
A description is made of operation of the inverter control device of
the arc welding machine structured as the above.
An error amplification signal input from error amplification part
11 to second switching circuit control part 28 is input to pulse-width
modulating part 14. Pulse-width modulating part 14 generates
driving pulses with a width corresponding to the level of the error
amplification signal on a basis of a basic pulse waveform for inverter
driving generated by inverter driving basic pulse generating part 13.
These driving pulses are separated one by one alternately into two
series and are output as 2-series drive signals for inverter driving.
Additional driving pulse generating part 16 outputs a signal for
appending driving pulses for a certain period of time immediately
before driving pulses output from pulse-width modulating part 14.
Output from additional driving pulse generating part 16 and that
from pulse-width modulating part 14 are combined by first combining
part 40 and second combining part 41, and are output as a drive signal
produced by expanding driving pulses from pulse-width modulating
part 14.
This drive signal is converted to a signal suitable for driving third
switching element 3 and fourth switching element 4 by third driving
circuit 23 and fourth driving circuit 24, and is input to third switching
element 3 and fourth switching element 4.
The current converted to an alternating current by the circuit
composed of first switching element 1 through fourth switching
element 4 is input to the primary winding of transformer 6; is
converted to output power suitable for welding,' and is output from the
secondary winding of transformer 6. Output from the secondary
winding is converted to a direct current by second rectifier 7 and is
output from the welding machine as welding output power.
FIG. 6A shows control operation at low output. Additional
driving pulse generating part 16 adds driving pulses for a certain
period of time immediately before driving pulses output from
pulse-width modulating part 14. Herewith, driving pulses to second
switching circuit 26 are secured for a certain period of time (not zero)
even at minimum output and at zero output. As a result, a conduction
width determined by driving first switching circuit 25 and second
switching circuit 26 changes to zero continuously. This enables
controlling a minute current that is difficult to control by pulse-width
modulation alone.
FIGs. 6B and 6C show control operation at middle output and high
output, and show that second switching circuit 26 is operating with
pulse-width modulation in relation to first switching circuit 25.
With the inverter control device according to the third embodiment,
a regenerative current reduces at capacitor 10 as shown by a
horizontally striped part of a transformer primary current waveform in
FIGs. 6A, 6B, and 6C. Accordingly, in whichever state of FIGs. 6A to
6C, a regenerative current reduces steeply. This shows that heat of a
switching element generated by a regenerative current can be
significantly suppressed.
Setting operation of first switching circuit 25 to near a maximum
conduction width results in a transformer primary current being
interrupted by third switching element 3 and fourth switching element
4. Herewith, first switching element 1 and second switching element
2 do not interrupt a current, thereby significantly reducing switching
loss in first switching element 1 and second switching element 2 to
suppress heat generation.
FIGs. 7A and 7B illustrate changes in operating state of the
inverter control device of the arc welding machine according to the
third embodiment. FIG. 7A shows the entire waveform for one cycle.
FIG. 7B shows conduction states of the switching elements and a
circuit current in the time areas indicated by Tl through T5 in FIG.
7A.
In FIG. 7A, part LI surrounded by the solid line indicates that
switching loss is generated; and L2 surrounded by the broken line, not
generated. In FIG. 7A, first switching element 1 indicated by Ql does
not interrupt a transformer current, and a regenerative current
reduces steeply, resulting in no turn-off power loss.
As shown by T3 and T4 in FIG. 7B, a regenerative current is not
generated. Then, as shown by the waveform of fourth switching
element 4 indicated by Q4 in FIG. 7A, a new drive signal added
immediately before point A of the old drive signal causes fourth
switching element 4 (indicated by Q4) to start to conduct earlier than
first switching element 1 (indicated by Ql). Accordingly, loss while
fourth switching element 4 is on can be reduced as well.
As described above, with the inverter control device of the third
exemplary embodiment, first switching circuit 25 (i.e. one of the two
switching circuits) is driven with a fixed conduction width; second
switching circuit 26 (i.e. the other switching circuit) is driven with
pulse-width modulation. Besides the above, adding a new drive signal
of a short period immediately before the old drive signal enables
stabilizing conduction of a switching element at minute conduction.
In other words, an inverter control device according to the third
embodiment exercises control by PWM method while adding a drive
signal output from additional driving pulse generating part 16 to a
drive signal output from pulse-width modulating part 14. Here, if
welding output power is near minimum output, pulse-width
modulating part 14 outputs an extremely short drive signal. With
this arrangement alone, output is unstable as described under
Background Art and becomes zero due to such as delay caused by the
characteristics of the driving circuit, making difficult to control a
minute current well accurately.
Meanwhile, the inverter control device of the third embodiment
additionally combines a drive signal output from additional driving
pulse generating part 16 with a drive signal output from pulse-width
modulating part 14. Herewith, the resulting drive signal is to have a
certain length even if pulse-width modulating part 14 outputs a short
drive signal near minimum welding output power. Herewith, an
output current does not become zero even near minimum output, which
enables controlling a minute current.
With the inverter control device of the third embodiment, the
description is made of current control using output current detector 8
and output current detecting part 9. Besides, it is obvious that
voltage control where the output current detector is replaced with
output voltage detecting part 20 follows the same operation.
In the third embodiment, a programmable integrated logic element
may be used such as a CPU, DSP, and FPGA as inverter control part 29.
In the first through third embodiments described above, a reactor
is serially connected to the primary winding of transformer 6 and
capacitor 10, and other capacitors are parallelly connected to switching
elements 1 through 4. Herewith, the first through third embodiments
can be used in combination with a software switching circuit using a
resonance phenomenon.
In the first through third embodiments described above, a drive
signal output from inverter driving basic pulse generating part 13 in
order to drive first switching circuit 25 has a fixed conduction width.
However, if the drive signal turns off with a delay relative to off timing
of a drive signal for second switching circuit 26, advantages same as
those in the first through third embodiments are obtained.
Accordingly, the conduction width of a drive signal output from
inverter driving basic pulse generating part 13 may change between off
timing of a drive signal for second switching circuit 26 and a maximum
conduction width of first switching circuit 25.
Adding a polarity inversion function to second rectifier 7 enables
the first through third embodiments described above to be applied to
an arc welding machine with AC output.
INDUSTRIAL APPLICABILITY
An arc welding machine of the present invention drives one of the
two switching circuits with a fixed conduction width and changes the
control method of the other switching circuit between pulse-width
modulation, phase control method, and drive signal width control
method by phase control method. This implements highly accurate
control while suppressing heat generation of a switching element.
namely an inverter controlled welding machine with low heat:
generation and high power efficiency, which also means
environmentally friendly and industrially useful.
Reference marks in the drawings
1 First switching element
2 Second switching element
3 Third switching element
4 Fourth switching element
5 First rectifier
6 Transformer
7 Second rectifier
8 Current detector
9 Current detecting part
10 Capacitor
11 Error amplification part
12 Output power setting part
13 Inverter driving basic pulse generating part
14 Pulse-width modulating part
15 Phase control part
16 Additional driving pulse generating part
17 Driving pulse width changing part
19 Signal changing part
20 Voltage detecting part
21 First driving circuit
22 Second driving circuit
23 Third driving circuit
24 Fourth driving circuit
25 First switching circuit
26 Second switching circuit
27 First switching circuit control part
28 Second switching circuit control part
29 Inverter control part
30 Transistor
31 Pulse transformer
32 Gate resistance
33 In-gate capacitance
34 Second snubber resistance
35 Fourth snubber resistance
36 First snubber capacitor
37 Second snubber capacitor
38 Output terminal
39 Output terminal
40 First combining part
41 Second combining part
We- claims; CLAIMS
1. An inverter control device comprising;
a first rectifier rectifying AC input;
a first switching element and a second switching element
disposed between outputs of the first rectifier, constituting a first
switching circuit, series-connected;
a third switching element and a fourth switching element
disposed between outputs of the first rectifier, constituting a second
switching circuit, series-connected;
a power conversion transformer, one primary winding of which
is connected to a junction between the first switching element and the
second switching element and the other primary winding of which is
connected to a junction between the third switching element and the
fourth switching element;
a second rectifier rectifying output from the power conversion
transformer;
an output power detecting part detecting one of an output
current and output voltage from the second rectifier!
an output power setting part for preliminarily setting one of an
output current and output voltage,"
an error amplification part determining an error between a
signal from the output power detecting part and a signal from the
output power setting part, and outputting the error; and
an inverter control part outputting a signal controlling
operation of the first switching circuit and the second switching circuit
according to a signal from the error amplification part,
wherein the inverter control part includes:
a first switching circuit control part outputting a drive signal
for alternately bringing the first switching element and the second
switching element constituting the first switching circuit into
conduction! and
a second switching circuit control part outputting a drive signal
for alternately bringing the third switching element and the fourth
switching element constituting the second switching circuit into
conduction,
wherein the second switching circuit control part includes:
a pulse-width modulating part generating a conduction width
that is time during which the third switching element and the fourth
switching element are kept in conduction, according to a signal from
the error amplification part, and outputting the conduction width;
a phase control part generating conduction time during which
the third switching element and the fourth switching element are kept
in conduction, the conduction time being a phase difference in relation
to conduction time of the first switching element and the second
switching element, according to a signal from the error amplification
part, and outputting the phase difference! and
a signal changing part accepting a signal from the pulse-width
modulating part and a signal from the phase control part, and
outputting one of a signal from the pulse-width modulating part and a
signal from the phase control part according to a signal from the error
amplification part.
2. The inverter control device of claim 1, wherein the signal changing
part outputs a signal:
from the pulse-width modulating part if a magnitude of an error
amplification signal is within a predetermined first range,' and
from the phase control part if the magnitude of the error
amplification signal is within a predetermined second range smaller
than the first range.
3. An inverter control device comprising:
a first rectifier rectifying AC input;
a first switching element and a second switching element
disposed between outputs of the first rectifier, constituting a first
switching circuit, series-connected;
a third switching element and a fourth switching element
disposed between outputs of the first rectifier, constituting a second
switching circuit, series-connected;
a power conversion transformer, one primary winding of which
is connected to a junction between the first switching element and the
second switching element and the other primary winding of which is
connected to a junction between the third switching element and the
fourth switching element;
a second rectifier rectifying output from the power conversion
transformer;
an output power detecting part detecting one of an output
current and output voltage from the second rectifier;
an output power setting part for preliminarily setting one of an
output current and output voltage,"
an error amplification part determining an error between a
signal from the output power detecting part and a signal from the
output power setting part, and outputting the error; and
an inverter control part outputting a signal for controlling
operation of the first switching circuit and the second switching circuit;
according to a signal from the error amplification part,
wherein the inverter control part includes.
a first switching circuit control part outputting a drive signal
for alternately bringing the first switching element and the second
switching element constituting the first switching circuit into
conduction,' and
a second switching circuit control part outputting a drive signal
for alternately bringing the third switching element and the fourth
switching element constituting the second switching circuit into
conduction,
wherein the second switching circuit control part includes^
a pulse-width modulating part generating a conduction width
that is time during which the third switching element and the fourth
switching element are kept in conduction, according to a signal from
the error amplification part, and outputting the conduction width;
a phase control part generating a drive signal of a phase
difference in relation to a drive signal from the first switching circuit
control part according to the error amplification signal;
a driving pulse width changing part changing a driving pulse
width from the phase control part according to the error amplification
signal; and
a signal changing part accepting signals from the pulse-width
modulating part, from the phase control part, and from the driving
pulse width changing part, and outputting one of the signals from the
pulse-width modulating part, from the phase control part, and from the
driving pulse width changing part, according to a signal from the error
amplification part.
4. The inverter control device of claim 3, wherein the signal changing
part outputs a signal:
from the pulse-width modulating part if a magnitude of an error
amplification signal is within a predetermined first range;
from the phase control part if the magnitude of the error
amplification signal is within a predetermined second range smaller
than the first range.' and
from the driving pulse width changing part if the magnitude of
the error amplification signal is within a predetermined third range
smaller than the second range.
5. An inverter control device comprising:
a first rectifier rectifying AC input;
a first switching element and a second switching element
disposed between outputs of the first rectifier, constituting a first
switching circuit, series-connected;
a third switching element and a fourth switching element;
disposed between outputs of the first rectifier, constituting a second
switching circuit, series-connected;
a power conversion transformer, one primary winding of which
is connected to a junction between the first switching element and the
second switching element and the other primary winding of which is
connected to a junction between the third switching element and the
fourth switching element;
a second rectifier rectifying output from the power conversion
transformer;
an output power detecting part detecting one of an output
current and output voltage from the second rectifier;
an output power setting part for preliminarily setting one of an
output current and output voltage;
an error amplification part determining an error between a
signal from the output power detecting part and a signal from the
output power setting part, and outputting the error! and
an inverter control part outputting a signal controlling
operation of the first switching circuit and the second switching circuit
according to a signal from the error amplification part,
wherein the inverter control part includes:
a first switching circuit control part outputting a drive signal
for alternately bringing the first switching element and the second
switching element constituting the first switching circuit into
conduction,' and
a second switching circuit control part outputting a drive signal
for alternately bringing the third switching element and the fourth
switching element constituting the second switching circuit into
conduction,
wherein the second switching circuit control part includes:
a pulse-width modulating part generating a conduction width
that is time during which the third switching element and the fourth
switching element are kept in conduction, according to a signal from
the error amplification part, and outputting the conduction width;
an additional driving pulse generating part for outputting a
drive signal added to a beginning of a drive signal output from the
pulse-width modulating part; and
a combining part combining output from the pulse-width
modulating part with output from the additional driving pulse
generating part.
6. The inverter control device of claim 1, further comprising a capacitor
serially connected to a primary winding of the power conversion
transformer.
7. The inverter control device of claim 1, wherein the inverter control
part is formed of a programmable integrated logic element.
8. An inverter control method for a inverter control device including:
a first rectifier rectifying AC input;
a first switching element and a second switching element
disposed between outputs of the first rectifier, constituting a first
switching circuit, series-connected;
a third switching element and a fourth switching element;
disposed between outputs of the first rectifier, constituting a second
switching circuit, series-connected;
a power conversion transformer, one primary winding of which
is connected to a junction between the first switching element and the
second switching element and the other primary winding of which is
connected to a junction between the third switching element and the
fourth switching element;
a second rectifier rectifying output from the power conversion
transformer!
an output power detecting part detecting one of an output
current and output voltage from the second rectifier;
an output power setting part for preliminarily setting one of an
output current and output voltage;
an error amplification part determining an error between a
signal from the output power detecting part and a signal from the
output power setting part, and outputting the error; and
an inverter control part outputting a signal controlling
operation of the first switching circuit and the second switching circuit
according to a signal from the error amplification part,
the inverter control method comprising:
a pulse width change controlling step of changing time during
which the third switching element and the fourth switching element
are kept in conduction, according to a signal from the error
amplification part; and
a phase controlling step of changing conduction time of the third
switching element and the fourth switching element so that the
conduction time has a phase difference in relation to conduction time of
the first switching element and the second switching element,
according to a signal from the error amplification part,
wherein the inverter control part performs:
the pulse width controlling step if a magnitude of an error
amplification signal is within a predetermined first range; and
at least the phase controlling step if the magnitude of the error
amplification signal is within a predetermined second range smaller
than the first range.
9. The inverter control method of claim 8, wherein the inverter control
part performs both the pulse width controlling step and the phase
controlling step if the magnitude of the error amplification signal is
within a predetermined third range smaller than the second range.
10. The inverter control method of one of claims 8 and 9,
wherein the first range and the second range are continuous with each
other, and
wherein the phase controlling step performed when the magnitude of
the error amplification signal is within the second range is performed
with a pulse width fixed to a pulse width when the error amplification
signal is a minimum within the first range.
11. The inverter control method of claim 8, wherein the inverter control
part performs both the pulse width controlling step and the phase
controlling step if the magnitude of the error amplification signal is
within a predetermined second range smaller than the first range.

An inverter control device of the present invention drives one of
the two switching circuits with a fixed conduction width and changes
the control method of the other switching circuit between pulse-width
modulation, phase control method, and drive signal width control
method by phase control method according to an output state, to
implement highly accurate control at low output while suppressing
heat generation of a switching element.