Output choke for D.C. welder and method of using same

Abstract

An output choke for a D.C. arc welder comprising a high permeability core with an inductance controlling air gap defined by first and second pole pieces terminating in first and second surfaces facing each other and each having two spaced edges with an intermediate area, said surfaces converging from said intermediate area toward each of said edges to generate a specific cross sectional shape for said gap wherein said choke is large enough to carry at least about 100 amperes of weld current.

Description

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 of 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 or the large range of currents needed for arc welding. An arc welder involves currents exceeding 50 amperes. Indeed, the choke of the present invention is a choke that can handle currents of 100–500 amperes while still maintaining an unsaturated core. The invention handles at least about 100 amperes. This clearly distinguishes the output choke of the present invention from other inductors used in power supplies.

The present invention is directed to the arc welding field where the optimum operation involves an inverse relationship between the inductance and weld current. Small inductors are usually used where the optimum operating characteristic between current and inductance is linear. To provide operation in an inverse relationship between current and inductance, such small inductors are operated on the knee of the saturation curve. This provides an inductance that is maximum for small current and swings to a lower value as the current increases. Such inductors are referred to as “swinging reactors”; however, they operate over a relatively small current range at the knee of the magnetic saturation curve and normally are sized to handle small currents less than 10 amperes. Such small swinging reactor could not be successful for the output choke of a D.C. welder since the current range is quite large and the weld currents are extremely large, over about 50 amperes.

The primary object of the present invention is the provision of an output choke for a D.C. arc welder, which choke has a gradually varying inductance over a wide current range and is capable of handling currents exceeding about 50 amperes and normally in the range of 100–500 amperes.

Still a farther object of the present invention is the provision of an output choke for a D.C. arc welder, as defined above, which choke produces no inflection points and does not cause the power supply to oscillate as the wire feed speed is changed or as the arc length is changed.

Still a further object of the present invention is the provision of an output choke for a D.C. arc welder, as defined above, which choke has no areas of non-linearity and can operate over a wide weld current range without saturation.

Yet another object of the present invention is the provision of an output choke for a D.C. arc welder which has a generally straight line relationship between current and inductance over a wide range of welding currents and the method of controlling the inductance in the output circuit of a D.C. electric arc welder using this choke.

Still a further object of the present invention is the provision of an output choke for a D.C. arc welder and method of using same, as defined above, which allows for high inductance at low wire feed speed and low inductance at high wire feed speeds without transition from one saturation curve to another saturation curve for the choke.

Another object of the present invention is the provision of an output choke for a D.C. arc welder which has a diamond shape air gap to control the current-inductance relationship.

These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic wiring diagram of a D.C. arc welder having an output circuit using the present invention;

FIG. 2 is a pictorial view showing schematically a standard, prior art output choke for a D.C. welder;

FIG. 3 is a current-inductance graph showing the saturation curves for various air gaps used in the prior art choke schematically illustrated in FIG. 2;

FIG. 4 is a pictorial view showing schematically an output choke for a D.C. welder which has been suggested for correcting the problems of the prior art choke illustrated schematically in FIG. 2;

FIG. 5 is a current-inductance graph showing the saturation curve for the choke illustrated schematically in FIG. 4;

FIG. 6 is a pictorial view of an output choke for a D.C. welder constructed in accordance with the preferred embodiment of the present invention;

FIG. 7 is a current-induction graph for the preferred embodiment of the present invention as illustrated in FIG. 6;

FIGS. 8, 9 and 10 are partial views of the core and air gaps having shapes using the preferred embodiment of the present invention;

FIG. 11 is a current-inductance graph similar to FIG. 7 showing the operating curve for the embodiments of the invention shown in FIG. 8-10;

FIGS. 12 and 13 are a partial view of the core of the choke showing air gaps having shapes which are modifications of the preferred embodiments of the present invention as shown in FIGS. 8–10;

FIG. 12A is a modification of FIG. 12 wherein the air gap shape is obtained by two core pieces which touch each other;

FIG. 14A is a partial view of the core of an electrode constructed in accordance with the present invention wherein the preferred diamond air gap shape is obtained by two core pieces which touch each other and are affixed, and a low permeable material fills the air gap.

FIG. 14 is a partial view of the core of an electrode constructed in accordance with the present invention wherein the preferred diamond air gap shape is obtained by two core pieces which touch each other and are affixed.

PREFERRED EMBODIMENTS

Referring now to the drawings, wherein the showings are for the purpose of illustrating preferred embodiments of the invention only and not for the purpose of limiting same, FIG. 1 shows a D.C. electric arc welder 10 capable of creating a welding current of at least about 50 amperes and up to 200–1,000 amperes. Power source 12, shown as a single phase line voltage, is directed through transformer 14 to rectifier 16. Of course, the rectifier could be driven by a three phase power source to create a D.C. voltage. In accordance with standard practice, a capacitor 20 having a size of about 20 K–150 K micro farads is charged by inductor 22 having a size of approximately 20 mH. Rectifier 16 charges capacitor 20 through inductor 22, which inductor may be replaced by inductance of the transformer. Output voltage from rectifier 16 at terminals 24, 26 is the voltage across capacitor 20 that maintains a voltage across arc gap a between electrode 30 from a wire feeder 32 and workpiece 34. To maintain an even flow of current across arc a, a relatively large output choke 50 is provided in the output circuit between capacitor 20 and gap or arc a. The invention involves the construction and operation of current control output choke 50, as best shown in FIG. 6. In the past, the output choke was a large choke as schematically shown in FIG. 2 wherein choke 100 has a high dependability core 102 with an air gap g defined between two facing surfaces 104, 106. The high currents demand large wires for winding 110. To obtain high inductance, the number of turns is high. To prevent saturation, the cross section of core 102 is large. Thus, choke 100 is large, heavy and expensive. By changing the width of gap g between surfaces 104, 106, core 102 is saturated by high weld currents in winding 110 by saturation curves, as shown in the graphs of FIG. 3. When air gap g is relatively small for a given choke, a high inductance is created; however, at low weld currents the core is saturated. This is shown in saturation curve 120. As the width of gap g is increased, the inductance is decreased and saturation current is increased. This relationship of an increased gap size is indicated by saturation curves 122, 124 and 126. Each of the saturation curves has saturation knees or points 120a, 122a, 124a and 126a, respectively. When operating arc welder 10 with a fixed air gap, as shown in FIG. 2, a saturation curve must be selected to accommodate the desired welding currents. To produce both a high inductance and a large current range, the windings 110 must be increased and the core size must be increased. This drastically increases the size and weight of the choke. By decreasing the weight and size of the choke the saturation curve has a reduced saturation current which causes erratic operation of the D.C. welder. In order to correct the problems associated with an output choke having a fixed gap for controlling the current in the output circuit of a D.C. arc welder, it has been suggested to use a choke as shown schematically in FIG. 4. Choke 200 includes a high permeability core 202 having an air gap 210. In this choke, the air gap is stepped with a large gap 212 and a small gap 214 created by adding a small pole piece 216. When currents exceeding 100–500 amperes are passed through winding 220, the inductance follows a two part saturation curve as shown in FIG. 5. This non-linear curve includes a first portion 230 employed until gap 214 is saturated and then a second portion 232 employed until larger gap 212 is saturated. These two sections create an effective current-inductance relationship illustrated by dashed line 240. This inverse current-inductance is extremely beneficial in electric arc welding. The two part curve accommodates both low current and high current operation. However, there is an abrupt saturation knee 232a causing an inflection point 242. As the arc welder operates along line 240, inflection point 242 causes oscillation as the wire feed speed is changed or the arc length or arc voltage is changed. Thus, there is a hunting action in the area of the inflection point 242 which reduces the effectiveness of the suggested stepped air gap approach shown schematically in FIG. 4.

Choke 50 of FIG. 1 incorporates the preferred embodiment of the present invention as illustrated in FIGS. 6–8. Core 52 of high permeability material has a cross section large enough to prevent saturation at over 50 amperes and preferably over 100–500 amperes. Facing surfaces 54, 56 of core 52 are between spaced edges 54a, 54b and 56a, 56b. The respective transversely spaced edges face each other and provide a relatively small air gap, if any. The center area 58 between surfaces 54, 56 constitutes a large air gap. This diamond shape air gap is between the spaced edges of faces 54, 56 and is defined by portions 54c, 54d of surface 54 and 56c, 56d of surface 56. These portions diverge together from a maximum air gap at apex 54e and apex 56e of the diamond shaped air gap. A winding 60, having a size to carry the weld current and a turn number to obtain the desired inductance, conducts the welding current around core 52. By using the diamond shaped air gap as shown in FIG. 6, with the selected core size and turn number, current-inductance curve 70 in FIG. 7 is obtained. Curve 70 represents an ideal current-inductance relationship for electric arc welding when the current progresses from 20 amperes to a high level exceeding about 200 amperes and often exceeding 500–1,000 amperes. As shown in FIG. 8, the small air gap at edges 54a, 56a and 54b, 56b tends to saturate at low currents. As the current increases, the diamond shaped air gap in choke 50 cannot saturate. At high levels the choke attempts to saturate an extremely large air gap. As indicated by the arrows, the saturation of the core by flux through the diamond shaped air gap would saturate the smaller gaps at position a, but not progressing upward from points b, c, d. The apex of the diamond shaped air gap is selected to prevent saturation even at maximum weld current. Thus, there is a straight line relationship between current and inductance, which relationship is gradual and continuous by the use of the diamond shaped air gap.

Two other preferred embodiments using the diamond air gap concept are illustrated in FIGS. 9 and 10. In FIG. 9, pole pieces 300, 302 of the core 52 have facing surfaces 304, 306 which are arcuate in shape to create an oval or elliptical air gap. This air gap includes small air gaps 310, 312 and a large center air gap at area 314. This preferred embodiment of the invention provides a linear curve 72 which is slightly concave, as shown schematically in FIG. 11. A generally linear, but convex, curve 74 is created by the preferred embodiment of the invention illustrated generally in FIG. 10 wherein core 52 includes pole pieces 320, 322 with facing surfaces 324, 326, respectively. These surfaces are curvilinear with small air gaps 330, 332 separated by an enlarged air gap at center portion 334. As can be seen, the preferred embodiments of the invention gradually change the width of the air gap from the center of the core to the outside edges of the core. The optimum application of the preferred embodiment is the diamond shaped air gap, as best shown in FIGS. 6 and 8. The oval air gap of FIG. 9 and the curvilinear air gap of FIG. 10 also provide a relatively straight, inversely proportional curve for the relationship between the current and inductance of the large current controlled by choke 50 used in a D.C. arc welder as illustrated in FIG. 1.

In the preferred embodiments, the air gap is gradually converging and is symmetrical with respect to the core. It is possible to provide an asymmetrical air gap configuration as shown in FIGS. 12 and 13. In FIG. 12, core 52a of choke 50 includes pole pieces 350, 352 with facing surfaces having converging portions 360, 362 and 364, 366. These portions define a large air gap area 338, which area is slightly offset from the center of the core. FIG. 12A is a modification of FIG. 12 wherein the air gap shape is obtained by the two core pieces 350, 352 touching each other. Another asymmetric air gap configuration is shown in FIG. 13 wherein core 52b includes pole pieces 370, 372 with an angled surface 374 and a straight surface 376. The air gap shown in FIG. 13 is also accomplished by forming pole piece 370 with a flat perpendicular surface, but tilting it with respect to pole piece 372. These structures produce an air gap with a small portion on the left and a large portion on the right. These two asymmetric air gaps produce better results than the stepped air gap 210 in FIG. 4; however, they do not obtain the desirable effects shown in FIG. 11 as accomplished by the symmetric air gap configurations shown in the preferred embodiments of FIGS. 8–10.

In practice, choke 50 has a core 52c as illustrated in FIG. 14. A diamond shaped symmetrical air gap 400 is provided between pole pieces 402, 404 with the abutting edge portions 406, 408 touching each other to define the intermediate air gap 400 with small gap portions 410, 422 gradually increasing to a large gap portion 414. Pole pieces 402, 404 are joined by a strap 420 using appropriate pins 422, 424. Air gap 400 is a diamond shaped air gap, which air gap is large at the apex or center and decreases toward both edges of the core 52c. This diamond shaped air gap provides a generally straight line, inversely proportional relationship between current and inductance, which relationship is optimum for electric arc welding. A low permeability potting material 416 can fill air gap 400 as illustrated in FIG. 14A when the choke is packaged for use in the field.

Claims

1. An output choke for a D.C. arc welder having an inductance comprising a high permeability core having first and second pole pieces and an inductance controlling air gap, said air gap defined between at least a portion of an end surface on said first and second pole pieces, said end surfaces facing one another, each end surface including two outer edges and a middle portion positioned therebetween, at least a portion of said middle portion on said end surface of said first pole piece being spaced apart at a varying distance from at least a portion of said middle portion on said end surface on said second pole piece to vary the inductance of said choke over a current rage, said middle portions forming an air gap to reduce inflection points along at least a majority of a saturation curve of said choke.

2. The choke as defined in claim 1, wherein said inductance of said choke at least partially varies linearly to said weld current.

3. The choke as defined in claim 1, wherein said inductance of said choke at least partially varies non-linearly to said weld current.

4. The choke as defined in claim 3, wherein said inductance of said choke at least partially varies curvilinearly to said weld current.

5. The choke as defined in claim 1, wherein said core has a size to prevent saturation at a weld current of at least about 100 amperes.

6. The choke as defined in claim 1, wherein said air gap includes a converging width that at least partially converges toward at least one set of said inner and outer edges of said first and second pole pieces, at least a portion of said air gap having a width that is greater than the spacing between at least one set of said inner and outer edges.

7. The choke as defined in claim 1, wherein said middle portions of said end surfaces of said first and second pole pieces have substantially the same shape.

8. The choke as defined in claim 1, wherein said outer edges of said end surfaces of said first and second pole pieces at least partially engage one another.

9. The choke as defined in claim 1, wherein both of said end surfaces are spaced from one another.

10. The choke as defined in claim 9, wherein said outer edges of said end surfaces of said first and second pole pieces are spaced apart at substantially the same distance.

11. The choke as defined in claim 1, wherein said air gap is at least partially filled with a low permeability material.

12. The choke as defined in claim 1, wherein said at least a portion of said air gap has a varying width to cause substantially gradual varying of the inductance of said choke over a majority of a current range of said choke.

13. The choke as defined in claim 12, wherein said air gap has a varying shape and width to substantially gradually vary the inductance of said choke over substantially a complete current range of said choke.

14. The choke as defined in claim 1, wherein said inductance of said choke at least partially varies inversely to said weld current.

15. The output choke as defined in claim 14, wherein said inductance of said choke at least partially varies generally inversely proportional to said weld current.

16. The choke as defined in claim 1, wherein said middle portions have substantially non-perpendicular oriented surfaces.