Electric heating – Metal heating – By arc
Reexamination Certificate
1999-09-27
2001-09-18
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06291798
ABSTRACT:
The present invention relates to the art of electric arc welding and more particularly to an improved electric arc welder constituting a plurality of separate power supplies.
BACKGROUND OF INVENTION
The present invention is particularly applicable to use in pipe welding and particularly for tandem electrodes or side-by-side electrodes used in pipe welding and it will be described with particular reference thereto; however, the invention has much broader applications and may be used for single electrode electric arc welders requiring extremely high current levels, such as welding currents exceeding about 1000-2000 amperes.
Electric arc welders for pipe welding and other similar applications often require welding currents in excess of 1000-2000 amperes. In the past, welders for developing such high welding currents were specially designed. However, when the required welding current increased beyond the design parameters of a particular power supply, a higher capacity electric arc welder had to be designed and manufactured. For instance, electric arc welders having a maximum current of 1000-1500 amperes could not be used for a welding operation requiring 2000 amperes. As soon as a higher capacity welder was manufacture and made available, it was again limited by its maximum current capacity. Consequently, as increased current levels were demanded for a welding application, such as pike welding, custom design, expensive welders were often necessitated.
Since higher welding currents required newly designed and manufactured welders, field applications could not be optimized due to the limited current capacity of the available power supply. Attempts have been made to create a high capacity welders by connecting a plurality of low capacity welders to the output welding station. Such attempts were not successful because there was difficulty balancing the dynamic current sharing capabilities of two or more separate power supplies.
SUMMARY OF INVENTION
Disadvantages of the prior attempts to provide high current power supplies, especially for pipe welding, have been overcome by the present invention which creates an electric arc welder that has an output current capability that is increased drastically by merely adding a plurality of power supplies, while still obtaining excellent static and dynamic current sharing capabilities between the paralleled power supplies. The electric arc welder of the present invention minimizes unequal current sharing between two or more power supplies. In prior schemes the power supplies each required an input command signal, its own feedback and error amplifier control circuit. They, thus suffer from lack of dynamic current shared between the power supplies due to component variations. The present invention utilizes the error amplifier of one power supply that not only controls the power supply, referred to as a master power supply, but also directs the same master current signal to the other power supplies. The other power supplies of the welder do not have their own command signal or error amplifier control circuitry. Consequently, only one error amplifier and feedback control circuit is provided in the electric arc welder. An error amplifier and feedback circuit is not used for each of the individual power supplies being parallel to increase the current capabilities.
In accordance with the present invention, there is provided an electric arc welder comprising a plurality of power supplies for creating a D.C. current of a given magnitude, which current is controlled by a single adjustable command signal. The D.C. current is used by a welding station having a D.C. input current and an output to pass an electric arc welding current between an electrode and a workpiece. A sensor, such as a shunt, senses the arc current. The plurality of power supplies includes a master power supply and one or more slave power supplies. The master power supply includes a first high speed switching type inverter having an output transformer and an output rectifier to produce a first D.C. current having a magnitude determined by a master current signal and a first pulse width modulator operated at a high frequency and having a current control voltage input controlled by the master current signal and an error amplifier for creating the master current signal based upon a comparison of the single command signal and the sensed arc current at the output of the welding station. The master power supply thus includes a command signal and an error amplifier feedback control circuit for controlling the pulse width modulator in the master power supply. The plurality of power supplies forming the welder includes at least one slave power supply which also includes a switching type inverter having an output transformer and an output rectifier to produce a second D.C. current having a magnitude determined by the same master current signal as used in the master power supply and a second pulse width modulator operated at a high frequency and having a current control voltage input connected to the master current signal of the master power supply. In this manner, the first and second D.C. currents combine to form at least a portion of the D.C. input current of the welding station. In practice, if two power supplies constitute the “plurality of power supplies” of the electric arc welder, the first D.C. current is 50% of the needed input current. The second D.C. current from the slave power supply provides the other 50% of the current. If three power supplies are used in the electric arc welder, each of the power supplies provides 33.3% of the input current for the output welding station. Consequently, the number of power supplies in the electric arc welder automatically determines the proportion of the D.C. current used to drive the welding station with the current being automatically balanced since each of the power supplies uses the same master current signal for the input of its pulse width modulator. A single command signal is used for the electric arc welder and only a single feedback circuitry is employed. By utilizing these novel concepts, a number of relatively small power supplies, such as 500 ampere power supplies, can be combined in any number to produce the desired maximum welding current. Six small power supplies, each rated for a maximum of 500 amperes, is combined to produce a 3000 ampere welding current. The ability to combine smaller power supplies to create an electric arc welder with an extremely high output welding current is obtained by the present invention. There is no need to design a power supply each time the current demand is increased.
In accordance with another aspect of the present invention, the high frequency used for the pulse width modulator of the individual power supplies is at least 18 kHz. In addition, the individual welding station driven by the plurality of power supplies includes first and second transistor type switches, such as 1 GBTs, shifted between a conductive state and a non-conductive state by the gate logic on a pair of control lines. In accordance with a secondary aspect of the invention, the switches are not shifted from the conductive state to a non-conductive state until the arc current is less than a given value. In practice, this given value is about 100-150 amperes. The invention uses a single welding station for a plurality of power supplies so large switches are needed and a single controller creates the logic signal to shift from conductive to non-conductive at the selected given value. The master power supply controls the switching and the slave or slaves merely add welding current.
In accordance with still a further aspect of the present invention, the output transformer in each of the power supplies includes a winding, specifically a primary winding, a current sensor associated with the winding and a circuit associated with the pulse width modulator of the individual power supply to hold at least one of the switches non-conductive for a time when the current in the winding exceeds a given current. This is referred to as the anti-core
Lincoln Global Inc.
Shaw Clifford C.
Vickers Daniels & Young
LandOfFree
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