Electric heating – Metal heating – By arc
Reexamination Certificate
2000-12-06
2002-12-03
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S1370PS
Reexamination Certificate
active
06489592
ABSTRACT:
The present invention relates to the art of electric arc welding and more particularly to a welding apparatus using two consumable electrodes and the method of welding with tandem consumable electrodes.
INCORPORATION BY REFERENCE
The present invention utilizes a high frequency inverter of the transistor switching type for converting a three phase input power source to a load coupling transformer from which the AC output of the inverter may be rectified to create current flow between the electrode and workpiece of a welding operation. Such welders employ a pulse width modulator operated at over 18 kHz for controlling the magnitude of the welding current flowing through the welding operation. These inverters are well known in the art and are generally shown in Blankenship U.S. Pat. No. 5,349,157 and Blankenship U.S. Pat. No. 5,351,175, which patents are incorporated by reference herein as background information. They illustrate a three phase inverter with current controlled by a high frequency pulse width modulator directing current pulses to the output transformer of the inverter. The three phase inverter has a pulse width modulator operated by an error amplifier for controlling the current at the output of the inverter. Output switching networks in arc welders for creating AC welding current from DC terminals are disclosed in Stava U.S. Pat. No. 4,861,965 and Stava U.S. Pat. No. 4,947,021, also incorporated by reference herein. Shutt U.S. Pat. No. 4,246,463 and Fratiello U.S. Pat. No. 5,155,330 show tandem mounted consumable electrodes used for welding the joint between two spaced plates. These patents are also incorporated by reference as background information, since they relate to the field to which the present invention is directed.
BACKGROUND OF INVENTION
The present invention relates to an electric arc welder of the type using two consumable electrodes for welding the joint between two edges of relatively thick plates, which plates may be in the form of the cylindrical ends of two pipe sections being welded together in a pipe welding operation or, in practice, seam welding of a formed plate into a pipe. In such welding procedures, two tandem mounted consumable electrodes are moved in unison along the joint so two layers of molten metal are deposited first from the front electrode and then from the trailing electrode to fill the joint between the two beveled edges of the adjacent edges of the plate forming a pipe. These edges are hereinafter referred to as adjacent plates for global application. The invention is applicable for seam welding of pipe; however, pipe welding will be described in a welding operation where the ends of the pipe sections are welded together. The invention is much broader in application and may be used to butt weld two adjacent heavy plates, such as the plates forming sections of gantries for oil rigs, armor of plates used in ship building or seam welding of pipe. Tandem consumable electrodes deposit large amounts of molten metal and cause the metal to be fused in the joint between the plates at high rates as necessary in welding heavy plates. In the past, a single phase power supply was normally used to produce the welding current for both of the tandem electrodes. The frequencies of the welding operation for the adjacent electrodes were the same causing extreme arc generated interference. Such systems required elaborate connections, for instance a Scott connection that produce an electrical phase shift. The frequencies of the welding procedures were dictated by the line frequency of the single phase input power supply. Thus, the frequencies of the pulsating welding current for the tandem mounted electrodes was generally the same fixed value determined by the 50 Hz or 60 Hz input voltage. The prior art welders using tandem consumable electrodes had generally caused an unbalance in the three phase power system and had welding frequencies controlled by the line frequency of the input voltage. This limitation was extremely detrimental when the high currents of pipe welding were used, which high currents exceed about 200 amperes and were often as high as about 1000-1200 amperes, or more. When tandem mounted electrodes are subject to relatively fixed low frequencies, determined by the line frequency, and conduct extremely high currents, used in pipe welding and welding thick plates, arc interference presents a serious problem requiring complicated connections and shielding. In the prior devices, it was somewhat normal practice for each of the electrodes to be driven by current having the same frequency, such as 50 Hz or 60 Hz. The only way to reduce arc interference was to shift the phase of the line current being directed to each of the tandem electrodes. This procedure was extremely complicated. The process was dictated by the line frequency and the phase shifting did not fully alleviate arc interference. Increasing the frequency of the welding current above 100-200 Hz to reduce interference was not practical in the prior systems. Thus, there is a substantial need for an improved electric arc welder using two or more consumable electrodes which do not have the problem of arc interference and do not utilize high current pulse frequencies.
THE INVENTION
The present invention overcomes the problems experienced in efforts to provide an electric arc welder that can effectively utilize two consumable electrodes, while reducing or generally eliminating arc interference without requiring high frequencies for the welding current. Although the invention can be used in a DC mode, preferably it involves an electric arc welder that directs AC welding currents to two separate consumable electrodes, which welder can be driven by a single three phase power supply while the welding operation at each electrode is independently controlled. The frequency of the welding current for each of the two electrodes is independently controlled and does not depend upon the input line frequency. By using the present invention, a three phase input voltage is used for a tandem electrode welder. Thus, there is a balanced input power, not an unbalanced single phase as in the prior art. The three phase voltage input has a line frequency of 50 Hz or 60 Hz; however, this frequency does not dictate the frequency of the welding current at each consumable electrode. This welder develops high welding current for heavy plates with the current exceeding about 200 amperes and is normally at least about 1000 amperes. Although two tandem mounted consumable electrodes are used in the preferred embodiment, it is possible to use three or more electrodes in the electric arc welder of the present invention.
In accordance with the invention, the electric arc welder includes a first and second consumable electrode, which electrodes are moved in unison along a welding path between the edges of the two adjacent mutually grounded plates, such as the seam in a pipe manufacturing operation. There are two separate power supplies for passing welding currents between the individual electrodes and the plates forming the grounded workpiece of the welding operation. The power supplies each are constructed to provide low frequency current pulses for the welding operation performed by the individual electrodes. In the preferred embodiment, the power supplies include a three phase voltage input, which input is operated at line frequency, such as 50 Hz or 60 Hz. Since the line frequency is isolated and does not control the output frequency at the individual electrodes, the same three phase voltage source can be used for both power supplies separately controlling the tandem mounted electrodes. The three phase power supply is rectified to convert the input voltage to a DC voltage link and a high frequency switching type inverter converts the DC voltage link to a high frequency AC current. The high frequency switching type inverter is controlled by a pulse width modulator operated at a frequency generally greater than 20 kHz with a duty cycle that is adjusted to control the magnitude of the output current at the electro
Peters Steven R.
Stava Elliott K.
Fay Sharpe Fagan Minnich & McKee
Lincoln Global Inc.
Shaw Clifford C.
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