Electric heating – Metal heating – By arc
Reexamination Certificate
2003-04-16
2004-09-28
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130320
Reexamination Certificate
active
06797922
ABSTRACT:
This invention relates to a power supply apparatus for a welder and, more particularly, to such power supply apparatus which is adapted to supply a pulsating current to a welder load when it demands a small current.
BACKGROUND OF THE INVENTION
For welding small electronic components, laser is frequently used. Laser, however, requires special equipment, and a power supply apparatus for laser is expensive. Recently, therefore, there has been an increasing demand for TIG-welding of such components with a small current.
FIG. 1
shows a block circuit diagram of a prior art TIG welder. An AC voltage from a three-phase commercial AC power supply connected to input terminals
2
a
,
2
b
and
2
c
is applied to a rectifier
4
and is rectified. The output of the rectifier
4
is smoothed by a smoothing capacitor
6
into a DC voltage. The DC voltage is then applied to an inverter
8
, where it is converted to a high-frequency voltage. The high-frequency voltage is applied to a transformer
10
, where it is voltage-transformed to a high-frequency voltage having a given magnitude. The resultant voltage-transformed, high-frequency voltage is rectified by rectifying diodes
12
,
12
, and smoothed by a smoothing reactor
14
before it is supplied through output terminals
16
a
and
16
b
to a workpiece
18
a
and a torch
18
b
. A current detector
20
detects the current supplied to the workpiece
18
a
and torch
18
b
, and develops a current-representative signal representing the magnitude of the detected current. The current-representative signal is applied to a control circuit
22
. The control circuit
22
receives also a reference current signal from a reference source
24
. The control circuit
22
supplies such a control signal to a drive circuit
26
as to make the difference between the current-representative signal and the reference current signal zero (0). In response to the control signal, the drive circuit
26
controls conduction periods of semiconductor switching devices of the inverter
8
, so that a load current corresponding to the reference current can be supplied to the workpiece
18
a
and torch
18
b
. A high-frequency generator
28
is coupled between the output terminal
16
b
and the smoothing reactor
14
via a coupling coil
30
. The high-frequency generator
28
is used to break the insulation provided by the gap between the workpiece
18
a
and the torch
18
b
to thereby cause arcing therebetween to start. The high-frequency generator
28
applies a high-frequency voltage of from 5 kV to 20 kV at a frequency of from 1 MHz to 3 MHz between the workpiece
18
a
and the torch
18
b
for a short time for strting the welder. A capacitor
31
is for bypassing a high-frequency signal from the high-frequency generator
28
.
Welder users sometimes desire to TIG weld small-sized electronic components with a small current of, for example, about 1 A. However, as is seen from
FIG. 2
, in order to sustain arcing, a load current of 0.5 A or greater is required. A smaller load current will cause interruption of arcing. Furthermore, a DC current transformer is sometimes used as the current detector
20
. In such case, one current transformer must detect a current ranging from a small current of, for example, 1 A, up to a large current of 300 A, for example. At smaller current, such current transformer cannot develop a current-representative signal with linearity. In order to secure the linearity, a Hall-effect current detector may be used, but a Hall-effect current detector tends to produce drifts in the current-representative signal for a small current, and, therefore, such drifts must be controlled. To eliminate the need for such drift control, a series combination of a DC power supply
32
, a current-limiting resistor
34
and a switch
36
is connected between the output terminals
16
a
and
16
b
. When a small current of, for example, 1 A, is required to flow through the workpiece
18
a
and the torch
18
b
, the switch
36
is closed. The DC power supply
32
may be provided, for example, by rectifying the commercial AC power supply, or by rectifying a voltage induced across an additional secondary winding wound on the transformer
10
.
However, in order to provide the DC power supply
32
, at least a rectifier is additionally required. When the transformer
10
is provided with an additional secondary winding, insulation must be provided between the additional secondary winding and the secondary winding to which the diodes
12
are connected. Thus, an isolation transformer must be used as the transformer
10
. Further, when a large current is supplied to the workpiece
18
a
and torch
18
b
, the DC power supply
32
need not supply current, and, therefore, the switch
36
is opened. Like this, when a welder is arranged to operate with a small current as well as a large current, a complicated circuit arrangement is required.
A TIG welder may be provided with a sequencing circuit (not shown) and a sequencer control
38
. The sequencing circuit is used for initiating operations, such as supplying of inert gas, and the sequencer control
38
is used for controlling the sequencing circuit. The sequencer control
38
detects when current flows through the workpiece
18
a
and torch
18
b
, and operates the sequencing circuit. When the load current is small, e.g. of 1 A, a current-representative signal sufficient to initiate the operation of the sequencing circuit is not available.
An object of the present invention is to provide a power supply apparatus for a welder, e.g. a TIG welder, which has a simple circuit arrangement, can perform TIG welding with a small current, and can make, without fail, a sequencing circuit operate even when the load current is small.
SUMMARY OF THE INVENTION
A power supply apparatus for a welder according to the present invention has a DC power supply, which, for example, may provide a DC signal by rectifying and smoothing an AC supply or, more specifically, a commercial AC supply. The DC signal from the DC power supply is converted to a high-frequency signal in high-frequency converting means, which may be high-frequency switching means, e.g. an inverter. A transformer transforms the high-frequency signal from the high-frequency converting means. DC converting means converts the transformed, high-frequency signal from the transformer to a DC signal and supplies a load current to a welder load. The load current can be set to a desired value within a range of current values. The DC converting means may be one including rectifying means and smoothing means. Pulse current supplying means is provided, which, when the load current to be supplied to the load from the DC converting means is set to a value smaller than a predetermined current value, supplies a pulse current having a peak value larger than the predetermined current value.
With the above-described arrangement, when the set load current value is below the predetermined current value, a pulse current having a peak value larger than the predetermined current value is supplied. The pulse current can enable, without fail, a sequencing circuit which controls supply of inert gas, for example. In addition, arcing can be sustained as long as the pulse current having a peak value larger than the predetermined current value is flowing.
The welder power supply apparatus may be provided with control means for controlling the high-frequency converting means in such a manner as to make the load current flowing through the welder load have the set current value. The control means receives a load-current representative signal representing the load current. The load-current representative signal may be provided from, for example, load current detecting means. The control means receives also a reference current signal representing the predetermined current value. The control means controls the high-frequency converting means in such a manner as to make the difference between the load-current representative signal and the reference current signal become zero (0). The pulse current supplying means contr
Danjo Kenzo
Ishii Hideo
Katooka Masao
Morimoto Takeshi
Morris LLP Duane
Sansha Electric Manufacturing Company Limited
Shaw Clifford C.
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