Electric heating – Metal heating – By arc
Reexamination Certificate
2002-09-06
2003-12-09
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S132000
Reexamination Certificate
active
06660966
ABSTRACT:
The present invention relates to the art of electric arc welding and more particularly to an electric arc welding system for combining power supplies to operate tandem electrodes.
INCORPORATION BY REFERENCE
The present invention is directed to an electric arc welding system utilizing high capacity alternating circuit power supplies for driving two or more tandem electrodes of the type used in seam welding of large metal blanks. Although the invention can be used with any standard AC power supply with switches for changing the output polarity, it is preferred that the power supplies use the switching concept disclosed in Stava U.S. Pat. No. 6,111,216 wherein the power supply is an inverter having two large output polarity switches with the arc current being reduced before the switches reverse the polarity. Consequently, the term “switching point” is a complex procedure whereby the power supply is first turned off awaiting a current less than a preselected value, such as 100 amperes. Upon reaching the 100 ampere threshold, the output switches of the power supply are reversed to reverse the polarity from the D.C. output link of the inverter. Thus, the “switching point” is an off output command, known as a “kill” command, to the power supply inverter followed by a switching command to reverse the output polarity. The kill output can be a drop to a decreased current level. This procedure is duplicated at each successive polarity reversal so the AC power supply reverses polarity only at a low current. In this manner, snubbing circuits for the output polarity controlling switches are reduced in size or eliminated. Since this switching concept is preferred to define the switching points as used in the present invention, Stava U.S. Pat. No. 6,111,216 is incorporated by reference. The concept of an AC current for tandem electrodes is well known in the art. Prior application Ser. No. 09/336,804 filed Jun. 12, 1999 discloses a system whereby tandem electrodes are each powered by a separate inverter type power supply. The frequency is varied to reduce the interference between alternating current in the adjacent tandem electrodes. Indeed, this application relates to single power sources for driving either a DC powered electrode followed by an AC electrode or two or more AC driven electrodes. In each instance, a separate inverter type power supply is used for each electrode and, in the alternating current high capacity power supplies, the switching point concept of Stava U.S. Pat. No. 6,111,216 is employed. This system for separately driving each of the tandem electrodes by a separate high capacity power supply is background information to the present invention and is incorporated herein as such background. In a like manner, U.S. application Ser. No. 09/406,406 filed Sep. 27, 1999 discloses a further arc welding system wherein each electrode in a tandem welding operation is driven by two or more independent power supplies connected in parallel with a single electrode arc. The system involves a single set of switches having two or more accurately balanced power supplies forming the input to the polarity reversing switch network operated in accordance with Stava U.S. Pat. No. 6,111,216. Each of the power supplies is driven by a single command signal and, therefore, shares the identical current value combined and directed through the polarity reversing switches. This type system requires large polarity reversing switches since all of the current to the electrode is passed through a single set of switches. This application does show a master and slave combination of power supplies for a single electrode and discloses general background information to which the invention is directed. For that reason, this application is also incorporated by reference.
BACKGROUND OF INVENTION
Welding applications, such as pipe welding, often require high currents and use several arcs created by tandem electrodes. Such welding systems are quite prone to certain inconsistencies caused by arc disturbances due to magnetic interaction between two adjacent tandem electrodes. A system for correcting the disadvantages caused by adjacent AC driven tandem electrodes is disclosed in prior application Ser. No. 09/336,804 filed Jun. 21, 1999 by assignee of this invention. In that prior application, each of the AC driven electrodes has its own inverter based power supply. The output frequency of each power supply is varied so as to prevent magnetic interference between adjacent electrodes. This system requires a separate power supply for each electrode. As the current demand for a given electrode exceeds the current rating of the inverter based power supply, a new power supply must be designed, engineered and manufactured. Thus, such system for operating tandem welding electrodes require high capacity or high rated power supplies to obtain high current as required for pipe welding. To decrease the need for special high current rated power supplies for tandem operated electrodes, assignee developed the system disclosed in application Ser. No. 09/406,406 wherein each AC electrode is driven by two or more inverter power supplies connected in parallel. These parallel power supplies have their output current combined at the input side of a polarity switching network. Thus, as higher currents are required for a given electrode, two or more parallel power supplies are used. In this system, each of the power supplies are operated in unison and share equally the output current. Thus, the current required by changes in the welding conditions can be provided only by the over current rating of a single unit. A current balanced system did allow for the combination of several smaller power supplies; however, the power supplies had to be connected in parallel on the input side of the polarity reversing switching network. As such, large switches were required for each electrode. Consequently, such system overcame the disadvantage of requiring special power supplies for each electrode in a tandem welding operation of the type used in pipe welding; but, there is still the disadvantage that the switches must be quite large and the input, paralleled power supplies must be accurately matched by being driven from a single current command signal. This prior application does utilize the concept of a synchronizing signal for each welding cell directing current to each tandem electrode. However, the system still required large switches. This type of system was available for operation in an ethernet network interconnecting the welding cells. In ethernet interconnections, the timing cannot be accurately controlled. In the system described, the switch timing for a given electrode need only be shifted on a time basis, but need not be accurately identified for a specific time. Thus, the described system requiring balancing the current and a single switch network has been the manner of obtaining high capacity current for use in tandem arc welding operations when using an ethernet network or an internet and ethernet control system. There is a desire to control welders by an ethernet network, with or without an internet link. Due to timing limitation, these networks dictated use of tandem electrode systems of the type using only general synchronizing techniques.
THE INVENTION
It is advantageous in high current systems for arc welding to drive one electrode with several paralleled inverter type power supplies while accommodating network control. The disadvantage has been that such systems required the current to be accurately balanced and required a single high capacity output switching network. Such systems could be controlled by a network; however, the parameter to each paralleled power supply could not be varied. Each of the cells could only be offset from each other by a synchronizing signal. Such systems were not suitable for central control by the internet and/or local area network control because an elaborate network to merely provide offset between cells was not advantageous.
The present invention utilizes the concept of a single AC arc welding cell for
Houston William S.
Myers Russell K.
Stava Elliott K.
Fay Sharpe Fagan Minnich & McKee
Lincoln Global Inc.
Shaw Clifford C.
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