Electric heating – Metal heating – By arc
Reexamination Certificate
2000-08-25
2002-05-07
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C313S231410, C361S120000
Reexamination Certificate
active
06384374
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the art of welding power supplies having high frequency arc starters and/or stabilizers. More specifically, it relates to a spark gap assembly used to produce a high frequency signal in a welding, cutting or induction heating power supply.
BACKGROUND OF THE INVENTION
It is well known to superimpose a high frequency signal on an AC welding voltage to assist in arc starting and/or arc stabilization. This involves applying a high voltage, low current signal at a high frequency across the arc.
A high-frequency voltage signal can be used to initially ignite an AC or a DC welding arc. The main advantage of this technique is that arc ignition occurs when the welding electrode is brought near the workpiece. Actual contact between the electrode and the workpiece is not needed to start the arc when this technique is used.
A high frequency voltage signal can also be used to stabilize an AC welding arc. In the event of arc rectification (e.g. extinguishment), the high frequency signal provides a voltage sufficient to maintain or restart the arc. The high frequency voltage assures a re-ignition of the welding arc every time the AC welding voltage passes through a null, thereby stabilizing the arc. The high frequency overlay may be applied only upon start-up, continuously, or as needed. When applied as needed, arc rectification is sensed and, after rectification has existed for several cycles, the high frequency component is supplied.
A variety of devices have been developed to create the desired high frequency signal. For example, switches that provide momentary ignition pulses immediately after the AC welding voltage passes through a null point have been developed. These known devices typically use ignition condensers that discharge intermittently between the electrode and workpiece through a switch in the form of a spark gap.
A spark gap is created when two conductive spark gap points (“points”) are held a fixed distance apart from each other. The spark actually arcs between two conductive spark gap surfaces, one on each point. Spark gap surface, as used herein, means the conductive surface of the spark gap point between which a spark arcs. Spark gap, as used herein, means the gap located between the spark gap surfaces across which the spark arcs.
A spark gap point or just point, as used herein, includes the spark gap surface and the rest of the body to which the spark gap surface is a part. Points are typically cylindrical in shape having a flat spark gap surface at one end. The present invention is not limited to cylindrical shaped spark gap points or flat spark gap surfaces, however, and other shapes can be used.
The assembly that holds the spark gap points in their proper position and orientation is called a spark gap assembly. A spark gap assembly, in addition to the spark gap points, can include a one or two piece plastic or ceramic housing, clamping members, heat sinks, electrical leads, retaining screws and other fasteners which hold together or hold in place the various components that make up the spark gap assembly.
One prior art spark gap assembly in common use for continuous duty cycle applications is shown in FIG.
1
. Prior art assembly
100
includes four points
101
,
102
,
103
,
104
mounted in four extruded aluminum heat sinks
105
,
106
,
107
,
108
. The points are located in holes
109
,
110
,
111
,
112
(spark gap receptacles) in the heat sinks and are axially aligned with each other in pairs. Two points are in axial alignment with each other when their longitudinal axes are substantially aligned with each other (e.g. substantially the same axis). For points having flat spark gap surfaces at right angles to the point's longitudinal axis, this provides for a substantially uniform spark gap distance at all locations between the spark gap surfaces.
The points are secured in their respective spark gap receptacles using retaining screws
113
,
114
,
115
,
116
.
The retaining screws clamp the heat sinks together around the points. Retaining fastener (bolt, screw, studs, nuts, etc . . . ), as used herein, means a fastener that is used, directly or indirectly, to tightly secure a spark gap point in a spark gap receptacle.
A jumper wire
117
electrically connects one pair of points in series with the other pair of points. Jumper wire
117
is electrically connected to the points using two of the four retaining screws
113
,
115
. Likewise, each set of points is electrically wired to power supply circuitry (not shown) using the other two retaining screws
114
,
116
which are also used to clamp heat sinks
106
,
108
around points
102
,
104
.
Heat sinks
105
,
106
,
107
,
108
are mounted on a square porcelain (ceramic) base
118
. Each heat sink is secured to base
118
from below using a pair of metallic screws
119
-
122
(only one screw from each pair is shown in FIG.
1
). These screws pass through mounting holes
123
-
126
in base
118
. The screw heads are sunk into the bottom side of ceramic base
118
to help prevent shorting to the welding power supply chassis. Nonetheless, the prior art assembly is typically mounted in a power supply chassis with a layer of insulating paper placed between the bottom side
127
of porcelain base
118
and the power supply chassis. The insulating paper is used to further reduce the possibility of a short occurring between the spark gap assembly and the power supply chassis.
This prior art spark gap assembly suffers from several problems. First, this prior art assembly is typically mounted in the welding power supply with the points oriented in the vertical direction. As such, the bottom two spark gap points
101
,
103
have a tendency to fall out when their retaining screws
113
,
115
are loosened or removed. This can occur during routine maintenance. It can also occur as a result of either thermal cycling of the heat sinks or vibrations encountered during normal power supply usage. When a point falls out, it may be lost or it may come in contact with other electrical components inside of the power supply.
Another problem with this prior art assembly is that it cannot be completely assembled until it is installed in a welding power supply. This is because the electrical leads from the power supply are attached to assembly
100
using retaining screws
114
,
116
. These same screws are used to secure points
102
,
104
in their receptacles. This means that the retaining screws holding two of the points in place cannot be adjusted and tightened until final installation in the welding power supply is completed.
Prior art spark gap assemblies, like the one described above, are used in continuous high frequency applications and have points that are completely surrounded by heat sinks. In other words, the receptacles are defined by the heat sinks. This type of arrangement hag been used in the past to dissipate the heat that is generated during continuous high frequency applications. The heat sinks are mounted to an insulating base. As a result of this mounting scheme, the points in this prior art assembly are prone to misalignment.
Misalignment can occur at the time of assembly, during initial power supply installation, or over time. Misalignment occurring at the time of assembly is due to assembly error (e.g. improper alignment of heat sinks
105
,
106
,
107
,
108
during initial assembly of spark gap assembly
100
). This misalignment can be the result of tolerances in in mounting holes
123
-
126
.
Misalignment can also result from the torque that is applied to heat sinks
106
,
108
when electrical power supply leads are attached to spark gap assembly
100
via retaining screws
114
,
116
. This torque can cause heat sinks
106
,
108
to rotate. The problem is worsened by the fact that the top surface of porcelain base
118
is an inherently slippery surface and heat sinks
105
,
106
,
107
,
108
are prone to slide on that surface. Misalignment over time results when heat sink mounting screws
119
-
122
become loosened due to the
Colling Ronald W.
Duba Richard A.
Byrne Joseph W.
Illinois Tool Works Inc.
Shaw Clifford C.
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