Tandem electrode welder and method of welding with two electrodes

A method of electric arc welding the joint between two plates, said method comprising:

• (a) moving first and second consumable electrodes in unison along said joint;
• (b) passing a first pulsating welding current between said first electrode and said plates with a first low frequency;
• (c) passing a second pulsating welding current between said second electrode and said plates with a second low frequency;
• and,
• (d) varying at least one of said low frequencies as a function of time.

An electric arc welding apparatus comprising at least a first consumable electrode and a second consumable electrode movable in unison along a welding path between the edges of two adjacent, mutually grounded plates, a first power supply for passing a first welding current between said first electrode and said plates, a second power supplv for passing a second welding current between said second electrode and said plates, each of said power supplies including a three phase voltage input operated at line frequency, a rectifier to convert said input voltage to a DC voltage link and a high frequency switching type inverter converting said DC voltage link to a high frequency AC current, an output rectifier circuit to provide a positive voltage terminal and a negative voltage terminal, and an output switching network for directing a welding current from said terminals across one of said electrodes and said plates, and a circuit for independently adjusting said output switching network so the value of said first welding current of said first power supply is different from said second welding current of said second power supply, wherein said output switching network of each power supply includes a first switch to create positive current across one of said electrodes and said plate and a second switch to create a negative current across one of said electrodes and said plate and a circuit to operate said first and second switches to control said welding current.

An electric arc welding apparatus comprising at least a first consumable electrode and a second consumable electrode movable in unison along a welding path between the edges of two adjacent, mutually grounded plates, a first power supply for passing a first low frequency welding current between said first electrode and said plates, a second power supply for passing a second low frequency welding current between said second electrode and said plates, each of said power supplies including a three phase voltage input operated at line frequency, a rectifier to convert said input voltage to a DC voltage link and a high frequency switching type inverter converting said DC voltage link to a high frequency AC current, an output rectifier circuit to provide a positive voltage terminal and a negative voltage terminal, and an output switching network operated at a given low frequency for directing a pulsating welding current at said given low frequency from said terminals across one of said electrodes and said plates, a master controller for creating a synchronizing signal alternating between a positive command and a negative command at a selected frequency, means for driving said first power supply by said synchronizing signal whereby said frequency of said first current is a function of said selected frequency, means for driving said second power supply by said synchronizing signal whereby said frequency of said second current is a function of said selected frequency, means for delaying said synchronizing signal to said second power supply to phase shift said second current from said first current, and means for operating one of said power supplies at a frequency different from said selected frequency upon response to one of said commands from said synchronizing signal.