Pilot arc circuit for a contact start plasma torch

1. A contact start plasma arc system, comprising:

a torch body;

an electrode having a longitudinally disposed axis and mounted in the body;

a nozzle having a longitudinally disposed axis, the nozzle axis being disposed substantially collinearly with the electrode axis;

a power supply coupled to the electrode, the nozzle and a workpiece, the power supply providing a current for operating the torch in a pilot arc mode and a transferred arc mode; and

a passive pilot arc circuit coupled between the power supply and the nozzle, wherein the pilot arc circuit controls the operation of the torch in the pilot arc mode and automatically disconnects power to the pilot arc mode to facilitate an automatic transfer from the pilot arc mode to the transferred arc mode by reducing a pilot current towards zero as a pilot capacitor in the pilot arc circuit becomes more charged. such that the pilot arc circuit opens and the pilot current stops flowing through the pilot arc circuit when the pilot capacitor becomes essentially fully charged.

2. The system of claim 1 , wherein the pilot arc circuit includes a pilot resistor.

3. The system of claim 2 , wherein the pilot capacitor and pilot resistor are coupled in series.

4. The system of claim 2 , wherein the pilot capacitor has a value ranging between 10 microfarads and 100,000 microfarads.

5. The system of claim 4 , wherein the pilot capacitor has a value of 18,000 microfarads.

6. The system of claim 2 , wherein the pilot resistor has a value ranging between 0.1 ohms and 100 ohms.

7. The system of claim 6 , wherein the resistor has a value of 5 ohms.

8. The system of claim 2 , further including a discharge circuit coupled in parallel to the pilot capacitor for the discharging the pilot capacitor.

9. The system of claim 8 , wherein the discharge circuit includes a discharge resistor and a discharge relay.

10. The system of claim 8 , wherein the discharge circuit includes a discharge resistor and a double-pole double-throw trigger switch.

11. The system of claim 1 , wherein either electrode or the nozzle is translatable.

12. The system of claim 1 , wherein the automatic transfer occurs at a predetermined time.

13. The system of claim 12 , wherein the predetermined time is determined from an RC time constant of the passive pilot arc circuit.

14. The system of claim 1 , further including a gas source coupled to the plasma chamber.

15. A method for operating of a contact start plasma system, comprising:

providing a contact start plasma arc torch having an electrode and a nozzle disposed in a torch body;

supplying electrical power to the torch for operation in a pilot arc mode and a transferred arc mode;

charging a pilot capacitor of a passive pilot arc circuit; and

automatically disconnecting power to the pilot arc mode of the torch through the passive pilot arc circuit to facilitate an automatic transfer from the pilot arc mode to the transferred arc mode by reducing a pilot arc current towards zero as the pilot capacitor in the pilot arc circuit becomes more charged, such that the pilot arc circuit opens and the pilot current stops flowing through the pilot arc circuit when the pilot capacitor becomes essentially fully charged.

16. The method of claim 15 , wherein power to the pilot arc mode is naturally disconnected at a predetermined time.

17. The method of claim 16 , wherein the predetermined time is determined by an RC time constant of the passive pilot arc circuit.

18. The method of claim 15 , wherein a magnitude of the pilot arc current is determined by an RC time constant of the pilot arc circuit.

19. The method of claim 15 , wherein the pilot arc circuit includes a pilot resistor.

20. The method of claim 19 , wherein the pilot capacitor and pilot resistor are coupled in series.

21. The method of claim 15 , further including:

removing electrical power to the torch; and

discharging the pilot arc capacitor through a discharge circuit.

22. The method of claim 21 , wherein the discharge circuit includes a discharge resistor and a discharge relay.

23. The method of claim 21 , wherein the discharge circuit includes a discharge resistor and a double-pole double-throw trigger switch.

24. The method of claim 15 , wherein disconnecting power to the pilot arc mode results in a seamless transfer of current to a workpiece.

25. The method of claim 15 , wherein either electrode or the nozzle is translatable.

26. The method of claim 15 , further including supplying gas to the torch for operation in the pilot arc mode and the transferred arc mode.

27. A contact start plasma system, comprising:

means for supplying electrical power to a contact start plasma arc torch for operation in a pilot arc mode and a transferred arc mode; and

means for automatically disconnecting power to the pilot arc mode of the torch though a passive pilot arc circuit to facilitate an automatic transfer from the pilot arc mode to the transferred arc mode by reducing a pilot current towards zero as a pilot capacitor in the pilot arc circuit becomes more charged, such that the pilot arc circuit opens and the pilot current stops flowing through the pilot arc circuit when the pilot capacitor becomes essentially fully charged.

28. The system of claim 27 , further including means for supplying gas to the contact start plasma arc torch for operation in the pilot arc mode and the transferred arc mode.

29. A power source for a contact start plasma arc system, comprising:

a power supply for supplying electrical power to a torch for operation in a pilot arc mode and a transferred arc mode; and

a passive pilot arc circuit for automatically disconnecting power to the pilot arc mode, the passive pilot arc circuit comprising a pilot capacitor that automatically disconnects the pilot arc circuit to facilitate an automatic transfer from the pilot arc mode to the transferred arc mode by reducing a pilot current towards zero as the pilot capacitor in the pilot arc circuit becomes more charged, such that the pilot arc circuit opens and the pilot current stops flowing through the pilot arc circuit when the pilot capacitor becomes essentially fully charged.