Electric heating – Metal heating – By arc
Reexamination Certificate
2000-11-27
2002-05-14
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130400, C219S1370PS
Reexamination Certificate
active
06388232
ABSTRACT:
The present invention relates to the art of electric arc welding and particularly a starting device for use in a DC TIG welder operated by an SCR input and capable of sustaining a welding arc at a low current.
INCORPORATION BY REFERENCE
In TIG welding a tungsten electrode is spaced from a workpiece to define a gap across which an arc is created for melting a filler wire moved into the arc. This type of welding presents problems relating to the starting of the arc at the start of the TIG welding process. Several procedures are known to start the arc when the TIG welder is operated at over about 5 amperes. Starting procedures and background technology regarding arc starting is discussed in De Coster U.S. Pat. No. 6,075,224. This patent illustrates an arc starter circuit using high frequency. As background information, this patent is incorporated by reference. The present invention relates to a high current DC TIG welder which is operable at a low weld current, such as a current in the general range of 2-5 amperes, but with capability for lower minimum output. The architecture of the TIG welder employed in practicing the present invention involves a power supply with a background stage used in conjunction with a standard phase controlled constant current stage. The use of an auxiliary or background power supply is shown and discussed in De Coster. Such an architecture for a welder is also disclosed and described in Campiotti U.S. Pat. No. 4,950,864, Vogel U.S. Pat. No. 5,218,182, Terayama U.S. Pat. No. 5,645,741 and Heraly U.S. Pat. No. 6,034,350. These patents are incorporated by reference herein to describe the operation of a background, rectified power supply used in combination with a standard phase controlled power supply for TIG welding. The background power supply maintains a minimum current and the main power supply provides weld current above a minimum level for the actual welding process. The present invention utilizes a background current stage and details of such operation are not necessary in view of the well known technology set forth in these several patents.
BACKGROUND OF INVENTION
DC TIG welders powered by a single phase SCR rectified welding current have an adjustable welding current determined by the phase firing angle of the SCRs in the network of the output rectifier. For industrial use they have adjustable welding DC currents as high as 200-300 amperes. As the desired welding current is reduced to a low level, such as 5-10 amperes, the phase angle is shifted to produce a reduced amount of the current flow. Such low current operation is not conducive to arc stability because the SCRs are on for a short time; therefore, a secondary power supply creating a fixed minimum current is often used in conjunction with the phase controlled rectifier. The low current operation includes a series of small current pulses in combination with a fixed background current. This architecture presents difficulty at low welding currents because the standard procedures for starting the arc is not consistent at low currents. Consequently, in the past the lowest current which could be used in a DC TIG welder of the type capable of operating at 200-300 amperes and driven by a single phase SCR platform was about 5-10 amperes. At this low weld current, high frequency or scratch arc starting, as discussed in De Coster U.S. Pat. No. 6,075,224, was marginally acceptable. If the main power supply stage was operated at a high level for a short time to start the TIG welder, the minimum current operation was still 5 amperes or more because of the short SCR spikes during the welding operation. There is no SCR driven TIG welder which can be successfully started at low currents below 5 amperes and then operated at such level with a stable arc. Such low welding current is the optimum current for certain welding processes, such as welding thin aluminum parts. Low current for DC TIG welding means a welding current less than about 5 amperes, and in the general range of 2-5 amperes. The power supply for a DC TIG welder can be adjusted to a low current by reducing the conduction period of the SCRs; however, there is no dependable procedure for starting the DC TIG welding process at such low currents and then maintaining the arc stability since the low current involves merely short spikes from the SCRs. This is especially true when the welder is designed to operate at current levels of 200-300 amperes when the SCRs are nearly fully conductive.
SUMMARY OF INVENTION
The invention involves a DC TIG welder driven by a single phase SCR platform and having a capacity of up to 100-300 amperes. The welding current of the welder can be set to a low level, substantially less than 5 amperes and in the general range of 2-5 amperes. At such low current, the welder now provides a welding current waveform similar to an inverter based welder. The invention employs a power supply having a phase control rectifier stage and a background rectifier stage. The background rectifier is set at the desired low current to be used for low current welding. The starting device of the welder employs both stages for starting, but maintains the arc using only the low current stage of the power supply. Consequently, the SCRs are not used during minimum current operation. A primary aspect of the invention is a high capacity welder that uses the phase control stage of the power supply only during starting when the welder is set for low current operation. While providing low current from the second rectifier a novel start sequence is implemented by a start device using any software or hardware technique. A high frequency generator passes a high frequency, high voltage signal across the gap between the workpiece and electrode. A start pulse with a controlled peak current in the general range of 50-70 amperes is created by short term operation of the SCR rectifier. By combining the standard high frequency arc starting procedure with the controlled arc starting pulse, the arc is positively created and stabilized. The pulse is tailored to optimize the creation of the arc by having the controller progress to a peak current of 50-70 amperes, then tail off. The duration is generally 30-50 milliseconds. This includes about 2-4 firings by the SCRs of the first rectifier. The high frequency complements the controlled short, high current start pulse to assure ionization of the gap before the pulse is applied across the gap by the first rectifier. After the current pulse has created the arc, high frequency is terminated. The high frequency is discontinued for welding at the fixed low current established by the background stage of the power supply using no SCR currents.
If the desired weld current is adjusted above the set low current of the second rectifier, the main SCR rectifier is initiated and the SCR network is used. The invention has proven satisfactory in TIG welding to a level as low as 1 ampere with a SCR based welder having a high current capability. The welder operated at 2 amperes maintains a well-defined arc created by the high voltage start pulse of the present invention without disrupting SCR spikes associated with low current welding by such high capacity DC welder. Although starting procedures known in the prior art are successful for high welding currents in an SCR based DC TIG welder, such starting procedures are not capable of establishing an arc for low current TIG welding and then sustaining the arc without use of SCR current spikes. The present invention accomplishes the objective of a DC TIG welder of the SCR type that can be operated at a low current without SCR spikes. If the desired weld current is greater than the peak current of the start pulse, the start pulse is not required.
In accordance with the present invention, there is provided a DC TIG welder operated at a low current level, under about 5 amperes in the gap between an electrode and a workpiece. The DC TIG welder has an adjustable output welding current and is driven by a constant current power supply with a first rectifier having an output current controlled by
Nguyen Trang D.
Samodell Ralph M.
Lincoln Global Inc.
Shaw Clifford C.
Vickers Daniels & Young
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