CV background for welding

Details CV background for welding CV background for welding

1999-09-23

2002-02-05

Shaw, Clifford C. (Department: 1725)

Electric heating

Metal heating

By arc

C219S130100

Reexamination Certificate

active

06344628

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to the art of welding power supplies. More specifically, it relates to welding power supplies having a CV background.
BACKGROUND OF THE INVENTION
Many known welding power supplies provide either a constant current (CC) or a constant voltage (CV) output. Generally, regulated welding power supplies operating in a CC mode monitor the output current, and control the power supply in response to the detected current and a user selected current setpoint. Likewise, regulated welding power supplies operating in a CV mode monitor the output voltage, and control the power supply in response to the detected voltage and a user selected voltage setpoint, to provide a generally constant average output voltage. Many power supplies are multi-purpose, and can be operated in either CV or CC mode. This invention relates particularly to welding power supplies having a CV mode.
Some power supplies that operate in a CV mode are phase controlled power supplies, such as the power supply used in the MM 250X®. The MM 250X® power supply allows the user to select an output voltage when in the CV mode. Power at the selected voltage is provided to a wire feeder, which feeds wire to an arc. The user selects a wire feed speed, which determines the current provided to the arc, or selects a welding current, which determines the wire feed speed.
A schematic of the MM 250® power circuitry is shown in FIG.
1
. Generally, power circuit
100
includes a transformer core
102
, having center tapped secondaries
104
and
106
. Center tapped secondary, as used herein, includes a single secondary with a center tap, or two secondaries sharing a common core with a common node. A pair of SCR's
108
and
110
form a phase controlled rectifier. The rectifier is disposed between the secondaries and the output of power source
100
. The output of the SCR's is provided through an inductor
112
(405 &mgr;H) on an output line to a positive output stud
117
. (Output line, as used herein, is the line connecting to the output studs, output connection, etc, through which power is provided to the arc.) The center tap of the secondary is connected to a negative output stud
118
. Thus, the output of power circuit
100
is applied across the output studs. Capacitors
115
and
116
filter noise. A capacitor
114
(120,000 &mgr;F capacitor bank) is provided, with inductor
112
, to maintain the voltage and current across the output studs when the SCR's are off.
When an alternating voltage is provided to the primary (not shown) of transformer
102
a voltage is induced across secondaries
104
and
106
. The voltage is rectified by SCR's
108
and
110
. Phase control of the SCR's regulates the portion of the rectified wave that is provided to the output, thus regulating the output voltage. When one of the SCR's is conducting, the current passes through the secondary, through inductor
112
, through the welding arc and back to the secondary. When both SCR's are off, the current path is from inductor
112
, through the welding arc, and back through capacitor
114
. Capacitor
114
and inductor
112
thus help to smooth the output and average the voltage provided by power source
100
, so that a generally constant voltage output is provided to the output lines (i.e., applied across the output studs).
Power supply
100
is regulated by monitoring the voltage across capacitor
114
. The monitored voltage is compared to a user selected voltage, and if the magnitude of the monitored voltage is not great enough, then SCR's
108
and
110
are turned on for greater portions of each cycle. Conversely, if the magnitude of the monitored voltage is too great, then SCR's
108
and
110
are turned on for a lesser portion of the cycle.
The design of
FIG. 1
is relatively inexpensive, reliable, and easily controlled. However, under some circumstance it is difficult to start the welding arc with such a design. Specifically, when the welding arc is initiated, the output voltage is not provided until the next time an SCR is turned on. Also, the inductance of inductor
112
, which preferably is large so that it effectively stabilizes the current output, delays the start of welding current.
Additionally, at lower voltage outputs the portion of each cycle that the SCR is on is so small that there can be discontinuities and/or undesirable ripple (when the voltage dips so low the arc is unstable) in the output voltage and/or current. Such an output has an adverse impact on the weld quality.
One prior art design that attempts to provide adequate welding starts includes a high frequency starter circuit across the welding output. Another design includes a boost circuit in parallel with the welding output. Examples of the latter include U.S. Pat. No. 3,530,359 issued to Grist, and U.S. Pat. No. 4,897,522 issued to Bilczo. The boost power supply has a higher voltage and lower current than the welding power supply. Such power supplies may be more expensive and may not be suitable for maintaining the arc at lower output voltages.
Accordingly, a CV power source that is relatively inexpensive, reliable, and easily controlled, but that also provides easy starts and high quality welding at lower output voltages is desirable.
SUMMARY OF THE PRESENT INVENTION
According to a first aspect of the invention a CV welding power supply includes first and second power sources, connected in parallel across a first and a second output line. The first CV source provides an output sufficient for welding. The second CV source also has an output sufficient for welding, but at a voltage less than the voltage of the output of the first CV source.
According to a second aspect of the invention a CV welding power supply includes a background CV source, having a lower voltage output and a lower current output to first and second output lines. A welding CV source, having a higher voltage output and a higher current output sufficient for welding, is also connected across the first and second output lines.
Voltage from the first CV source is blocked from being applied to the second CV source in one embodiment.
The second CV source is a background source with a background secondary and a background rectifier in another embodiment. The background secondary may be a center tapped secondary and the background rectifier may be a full wave rectifier having two background rectifying elements. The two background rectifying elements may be SCRs or diodes, which also block the first voltage.
A background inductor is disposed between the background secondary and at least one of the first and second output lines and/or a background capacitor disposed such that current can flow from one of the first and second output lines, to the background capacitor, to the background inductor, and to the other of the first and second output lines in various alternatives.
The first CV source is a welding source with a welding secondary and a welding rectifier in another embodiment. The welding secondary may be a center tapped secondary and the welding rectifier may be a full wave rectifier having two welding rectifying elements. The two welding rectifying elements may be SCRs or diodes.
The background secondary and the welding secondary are wound about a first core and the background secondary may also be wound about a second core in another alternative.
A welding inductor is disposed between the welding secondary and at least one of the first and second output lines and/or a welding capacitor disposed such that current can flow from one of the first and second output lines, to the welding capacitor, to the welding inductor, and to the other of the first and second output lines in various alternatives.
The welding inductor and the background inductor share a common inductor core in another alternative.
According to a third aspect of the invention a method of providing CV welding power includes providing power to a pair of output lines at a first CV voltage and a first current sufficient for welding. Power is al

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