Current unit for arc welding

Abstract

An a.c. operated arc welding current supply unit comprises a frequency converter of the series capacitor type and operating with a half period which is substantially less than the average duration of the current and voltage transients caused by short circuits through droplets of weld material during welding. The frequency converter is connected to welding electrodes through a transformer in series with a rectifier to provide direct current for the welding electrodes, and the frequency converter is associated with a control device for controlling the operating frequency of the converter in a manner such that it is substantially inversely proportional to the square of the input voltage of the current supply unit, thereby maintaining the power output of said unit substantially unchanged irrespective of changes in load caused by the welding operation and irrespective of variations in said input voltage.

I claim: 
     
       1. An arc welding current supply unit arranged to be fed with alternating current voltage and to provide direct current for welding electrodes, said unit including a controlled frequency converter operating with a half period which is less than the average duration of the current and voltage transients caused by short circuits through droplets of the weld material and adapted to be connected to the welding electrodes through a transformer in series with a rectifier, a control device for controlling said converter in a manner such that the arc power remains substantially unchanged irrespective of changes in load caused by the welding operation, wherein current pulses are formed in the secondary winding of the transformer by alternatingly charging and discharging at least one capacitor which is connected in series with said primary winding of said transformer and which is discharged therethrough, and said at least one capacitor being charged by a direct current voltage intermediate source and being connected to said source by a number of thyristors equal in number to the number of said capacitors, said thyristors being connected to said control device, and wherein said control device further comprises means for alternatingly making said thyristors conductive at a frequency which is substantially inversely proportional to the square of said voltage applied to said current supply unit. .Iadd. 2. An arc welding current supply unit arranged to be fed with alternating current voltage and to provide direct current for welding electrodes, said unit including a controlled frequency converter operating with a half period which is less than the average duration of the current and voltage transients caused by short circuits through droplets of the weld material and adapted to be connected to the welding electrodes through a transformer, a control device for controlling said converter in a manner such that the arc power remains substantially unchanged irrespective of changes in load caused by the welding operation, wherein current pulses are formed in the secondary winding of the transformer by alternatingly charging and discharging at least one capacitor which is connected in series with said primary winding of said transformer and which is discharged therethrough, a direct current voltage intermediate source, and switching means responsive to said control device for connecting said at least one capacitor to said intermediate source for charging and discharging said capacitor; and wherein said control device further comprises means for controlling said switching means to altenatingly charge and discharge said at least one capacitor at a frequency which is substantially inversely proportional to the square of said voltage applied to said current supply unit. .Iaddend..Iadd. 3. An arc welding current supply unit as claimed in claim 2 further comprising a rectifier connected between said transformer and said welding electrodes. .Iaddend. .Iadd. 4. An arc welding current supply unit as claimed in claim 3 wherein said control device comprises, a voltage controlled oscillator for providing a pulse train output at a rate dependent upon a voltage input thereto, said pulse train output determining the frequency of switching of said switching means, and   means connected between said input alternating current voltage and said oscillator for supplying to said oscillator a voltage input substantially inversely proportional to the square of the voltage applied to said current supply unit. .Iaddend..Iadd. 5. An arc welding current supply unit as claimed in claim 4 wherein said switching means comprises first and second thyristors and wherein said control device further comprises,   thyristor ignition circuitry for altenately igniting said thyristors into their conducting states in response to triggering pulses applied thereto, and   gating means connected to said voltage controlled oscillator and said thyristor ignition circuitry for applying pulses in said pulse train output to said thyristor ignition circuitry as said triggering pulses when said gating means is not otherwise blocked. .Iaddend. .Iadd. 6. An arc welding current supply unit as claimed in claim 5 wherein said control device means further comprises, a first blocking means connected between said thyristors and said gating means for blocking said gating means at all times other than when a reverse bias appears across the anode cathode path of either of said thyristors, whereby neither thyristor can be turned on until the previously conducting thyristor is turned off. .Iaddend..Iadd. 7. An arc welding current supply unit as claimed in claim 6 wherein said control device means further comprises, a second blocking means for blocking said gating means when the voltage to said welding electrodes exceeds a predetermined maximum. .Iaddend..Iadd. 8. An arc welding current supply unit as claimed in any of claims 5, 6, or 7 further comprising first and second capacitors alternately charged and discharged through said primary winding by the alternate conduction of the second and first of said two thyristors, wherein said first and second thyristors are in series with one another and connected across said d.c. voltage intermediate source, said one and second capacitors are in series with one another and connected across said d.c. voltage intermediate source, and said primary winding is connected between the junction of said first and second thyristors and the junction of said one and second capacitors. .Iaddend. .Iadd. 9. An arc welding current supply unit as claimed in claim 2 wherein said switching means comprises first and second thyristors and wherein said at least one capacitor comprises first and second capacitors, said thyristors, capacitors and primary winding being connected such that said first capacitor is charged via said second thyristor and said primary winding and is discharged via said first thyristor and primary winding, and wherein said second capacitor is charged through said first thyristor and primary winding and is discharged through said second thyristor and primary winding. .Iaddend. .Iadd. 10. An arc welding current supply unit as claimed in any of claims 2, 3 or 4 wherein said switching means comprises first and second thyristors and wherein said at least one capacitor comprises first and second capacitors, said first and second capacitors being alternately charged and discharged through said primary winding by the alternate conduction of the second and first of said two thyristors, wherein said first and second thyristors are in series with one another and connected across said d.c. voltage intermediate source, said one and second capacitors are in series with one another and connected across said d.c. voltage intermediate source, and said primary winding is connected between the junction of said first and second thyristors and the junction of said one and second capacitors. .Iaddend.