Arc transfer circuit

A method for providing D.C. current from a power source having a current regulating input to a plasma system including an electrode and a nozzle with a plasma arc opening exposing said electrode to a juxtapositioned workpiece, said circuit having a first circuit branch for creating a pilot arc across said electrode and nozzle, a second circuit branch for creating an operative plasma arc across said electrode and said workpiece, said method comprising the steps of:

• (a) sensing the workpiece current in said second circuit branch;
• (b) opening said first circuit branch when said sensed workpiece current exceeds a preselected current to transfer the pilot arc to the operative plasma arc:
• (c) regulating the current in said first circuit branch until said arc transfer;
• (d) said current regulating step including the steps of:
• (e) creating a first signal having a level representative of the actual current supplied from said power source to said first circuit branch;
• (f) providing a second signal having a level representative of the desired current in said first circuit branch;
• (g) comparing said first and second signals to provide an input signal to said current regulating input of said power source to regulate said actual current in said first circuit branch to said desired current;
• (h) creating a trip signal at the time when said sensed workpiece current exceeds a low level threshold current substantially less than said preselected current;
• (i) generating a time based signal with an increasing level of a selected profile in response to said trip signal;
• and,
• (j) adding said time based signal to said second signal for increasing the level of said second signal by said selected profile to correspondingly increase said actual current supplied from said power supply to said first circuit branch.

In a circuit for providing D.C. current from a power source having a current regulating input to a plasma system including an electrode and a nozzle with a plasma arc opening exposing said electrode to a juxtapositioned workpiece, said circuit having a first circuit branch for creating a pilot arc across said electrode and nozzle, a second circuit branch for creating an operative plasma arc across said electrode and said workpiece, a current sensor means in said second circuit branch for sensing the workpiece current in said second circuit branch, switch means for opening said first circuit branch when said sensed workpiece current exceeds a preselected current to transfer the pilot arc to the operative plasma arc, and means for regulating the current in said first circuit branch until said arc transfer, said current regulating means including sensing means for creating a first signal having a level representative of the actual current supplied from said power source to said first circuit branch, means for providing a second signal having a level representative of the desired current in said first circuit branch, means for comparing said first and second signals to provide an input signal to said current regulating input of said power source to regulate said actual current to said desired current, the improvement comprising:

• said second signal having a set level, means for creating a transfer signal when said sensed workpiece current exceeds a low level threshold current substantially less than said preselected current and circuit means responsive to said transfer signal for increasing the level of said second signal above said set level to correspondingly increase said actual current supplied from said power source to said first circuit branch.