Electric heating – Metal heating – By arc
Reexamination Certificate
2000-05-03
2001-02-27
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130100
Reexamination Certificate
active
06194684
ABSTRACT:
The present invention relates to an output choke for a D.C. arc welder and a method of controlling the inductance in the output circuit of a D.C. electric welder using such choke.
BACKGROUND OF INVENTION
In D.C. electric arc welders, the output circuit normally includes a capacitor in parallel across the electrode and workpiece with a relatively small inductance for charging the capacitor as the rectifier or power supply provides D.C. current. This inductance removes the ripple from the welding current. In series with the arc gap of the welder there is provided a large choke capable of handling high currents over about 50 amperes and used to control current flow for stabilizing the arc. As the feeding speed of the electrode toward the workpiece and the length of the arc change, the welding current varies. In the past, the large output choke in series with the arc had a fixed air gap in the core to control the inductance at a fixed value as current changes. However, when the choke experienced high weld currents, the core saturated and reduced the inductance drastically. For this reason, the width of the air gap in the core was enlarged to provide constant inductance over the operating current range of the welder. The choke was selected for a particular operating current range. However, this range would vary for different welding operations. Thus, the air gap of the choke was selected for the majority of welding operations. In a standard choke, a small air gap provided high inductance, but would saturate at relatively low currents. To increase the current capacity of the choke, the air gap was enlarged to reduce the amount of inductance for a particular size of the choke. For these reasons, the choke was made quite large with large wires to carry the weld current and a large cross sectioned core to prevent saturation. The gap was large to accommodate a wide range of welding currents. Such chokes were expensive and drastically increased the weight of the welder. Further, the choke produced a constant inductance until the saturation point or knee, even though ideal arc welding is realized with an inductance that is inversely proportional to the weld current. To alleviate these problems, it has been suggested that the air gap could include two or three different widths. This suggestion produced a high inductance until the small air gap saturated. Thereafter, a lower inductance would be realized until the larger air gap saturated. By using this concept of two, or possibly three, stepped air gaps, the size of the choke could be reduced and the range of current controlled by the choke could be increased. Further, the relationship of current to inductance was inverse. The concept of using a stepped air gap in the core of the output choke allowed a smaller choke; however, one or more inflection points existed. When the feed speed of the electrode or arc length changed to operate in the area of the inflection points, the D.C. welder would oscillate about the saturation or inflection points causing unstable operation. A standard swinging choke was not the solution because the weld current varied too much to operate on the saturation knee. In addition, such swinging chokes were for small current applications.
The use of a fixed output choke for a D.C. arc welder is now standard. Such choke is large and the operating point is in the linear portion of the inductance preventing drastic reductions in the output inductance of the welder. Such choke is expensive and heavy. By the procedure of having a stepped air gap, the size of the choke could be reduced and the current operating range increased; however, the inflection point at the saturation of one gap, made the welder less robust and susceptible to oscillation at certain arc lengths and feed speeds. Consequently, this suggested modification was not commercially acceptable.
THE INVENTION
The present invention relates to an output choke for a D.C. arc welder which solved the problems of weight, cost and welding inconsistencies experienced by a large choke having a fixed air gap or a smaller choke having a stepped air gap. In accordance with the invention, the output choke for the D.C. arc welder comprises a high permeability core with an area having a cross sectional shape with two spaced edges and an air gap wherein the air gap has a gradually converging width for at least a portion of the distance between the two edges. Thus, the air gap gradually increases from the edges. In the preferred embodiment, the air gap is a diamond shape, gradually increasing from the edges to the center portion of the core. This diamond core technology for the output choke of a D.C. welder produces an inductance in the output circuit which gradually varies over the current range in an inverse relationship with the weld current. As the welding current increases, the inductance decreases in a continuous manner without any discontinuity or steps. Thus, the weld current is never at a saturation point for the output choke or operating on the saturation knee. There is no oscillation of the power to the weld. This invention produces a robust welder which can handle changes and up to 5-10 volts with arc length changes without causing instability of the arc. Thus, the choke provides current control over a wide range of weld currents without oscillating or without the need for a large output choke.
In accordance with another aspect of the present invention the output choke includes a high permeability core with an air gap defined by first and second pole pieces terminating in first and second surfaces facing each other. Each of these surfaces has two spaced apart edges with an intermediate area with the facing surfaces converging from the intermediate area toward the respective edges of the surfaces to generate a specific cross sectional shape for the air gap. This cross sectional shape is preferably a diamond; however, it may be an oval or other curvilinear shape so long as there is gradual changes in the inductance with changes in weld current. In the preferred diamond shape air gap, the intermediate area is in the center of the pole pieces; however, the intermediate area may be closer to one edge of the facing surfaces. This provides a non-equilateral diamond. In accordance with another aspect of the invention, the gap may have a shape which converges from one edge of the facing surfaces toward the other edge of the facing surfaces. This provides an air gap having the shape of a triangle. All of these configurations result in a choke where the inductance gradually changes with the output current of the welder without saturation between adjacent areas causing inflection points that can result in hunting or oscillation of the welder at certain wire speeds and arc lengths.
Another aspect of the present invention is the provision of a method of controlling the inductance in the output circuit of a D.C. electric arc welder operated over a given current range to weld by passing a weld current in the gap between an electrode and a workpiece. This method comprises: providing an inductor with a generally constant inductance over the current range for charging a capacitor connected in parallel with the welding gap or arc; providing an output choke with an inductance gradually varying over the current range; and, connecting the choke in series with the gap or arc and between the arc and the capacitor. In this method, the inductance varies in a generally straight line inversely proportional to the weld current so that as current increases the inductance gradually decreases along a generally straight line. This is an optimum relationship for arc welding. Generally straight includes concave or convex linear relationship so long as there is no inflection points along the curve as are caused by stepped air gaps.
The present invention relates to an arc welder which requires a relatively large output choke. This field is distinguished from power supplies used for low power appliances, such as lights, sound or video equipment. Such miniature power supplies do not have the large currents
Clark Keith Leon
Housour Brian Keith
Lincoln Global Inc.
Shaw Clifford C.
Vickers Daniels & Young
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