Plasma arc ignition using a unipolar pulse
A starting circuit for use with a plasma torch is provided including circuitry for initiating a pilot arc using a unipolar voltage impulse. A transformer is selectively coupled to a DC source so that an impulse is introduced using the same DC source used to maintain an established pilot arc. A method is provided wherein an arc can be initiated while at the same time the DC source is pre-loaded so that surge injection circuitry is not needed to sustain the arc while ramping to the full pilot arc current level.
1. A method of initiating a plasma arc in an apparatus comprising an arc gap defined by a first gap side and second gap side, the method comprising:
generating a DC pilot current from a DC power supply, the power supply comprising a pilot current output and a return current input;
generating a voltage impulse by directing current along an initial current path from the pilot current output and back to the return current input;
injecting the voltage impulse in series with the pilot current output and the first gap side to generate an arc across the first and second gap sides;
directing current flow through a second current path from the pilot output to the first gap side, across the gap to the second gap side, and back to the return current input without injecting current from any components outside the initial or second current flow paths; and
discontinuing current flow along the initial current path at or shortly after current flow begins through the second current path;
whereby both the initial and the second current flow paths comprise the DC power supply.
2. The method as set forth in claim 1 , wherein:
directing current flow along an initial path comprises directing current flow through the primary winding of a transformer, the transformer comprising a primary winding and a secondary winding; and
injecting the voltage impulse comprises injecting the voltage pulse in series via a connection between the secondary winding of the transformer and the first gap side.
3. The method as set forth in claim 2 , wherein directing current flow through a second current path comprises directing current flow through a diode biased for current flow from the pilot current output to the first gap side.
4. The method as set forth in claim 2 , wherein directing current flow through the primary of a transformer comprises directing current flow through the primary of an autotransformer and wherein directing current flow through a second current path comprises directing current flow through the secondary winding of the transformer.
5. The method as set forth in claim 4 , further comprising:
shunting current from the secondary winding of the transformer to the first side gap side via a shunt path parallel to the secondary winding of the transformer.
6. The method as set forth in claim 5 , wherein shunting current comprises providing a diode biased for current flow from the pilot current output to the first gap side.
7. The method as set forth in claim 1 , wherein directing current along an initial current path comprises changing the conductivity of at least one switching component in the initial current path.
8. The method as set forth in claim 7 , wherein directing current comprises changing the conductivity of a transistor connected between a winding of the transformer and the return current input of the DC power supply.
9. The method as set forth in claim 1 , further comprising:
providing a plasma gas flow through the area defined by the first gap side and the second gap side is in the range of 0-30 CFH.
10. An arc initiation circuit comprising:
a transformer having a primary winding coupled between a first node and second node and a secondary winding coupled between a third node and a fourth node;
a first diode connected between the first node and the second node, the first diode connected so as to be forward-biased for current flowing through the diode from the second node toward the first node;
a transistor connected between the second node and a fifth node; and
a DC power supply connected to the first node and the fifth node;
wherein a plasma torch nozzle at a sixth node is in electrical communication with the third node and a plasma torch electrode is connected to the fifth node.
11. The arc initiation circuit as set forth in claim 10 , wherein the third node and sixth node are the same node and the fourth node and the fifth node are the same node.
12. The arc initiation circuit as set forth in claim 10 , wherein the third node and sixth node are the same node and the fourth node and the first node are the same node.
13. The arc initiation circuit as set forth in claim 10 , wherein the third node is connected to the sixth node through a series resistance and the fourth node and the fifth node are the same node.
14. The arc initiation circuit as set forth in claim 10 , wherein the third node is connected to the sixth node through a series resistance and the fourth node and the first node are the same node.
15. The arc initiation circuit as set forth in claim 11 , further comprising a second diode connected between the first node and the sixth node, the second diode connected so as to be forward-biased for current flowing through the diode from the first node towards the sixth node.
16. The arc initiation circuit as set forth in claim 12 , further comprising a second diode connected between the first node and the sixth node, the second diode connected so as to be forward-biased for current flowing through the diode from the first node towards the sixth node.
17. A plasma cutting system comprising the arc initiation circuit as set forth in claim 15 and a gas manifold assembly, wherein the second diode is positioned for cooling by the gas manifold assembly.