NZ244352A - Arc strike sensing and power control of plasma torch - Google Patents

Description

.: "a i ■; ;H D.5>.1:1 J y3.t> ;- .1 - ' ;2 7 APR TO ;f P.O ;NEW ZEALAND PATENTS ACT, 1953 ;15 S:r 1i;2 ;REC. , ;COMPLETE SPECIFICATION PLASMA TORCH ELECTRONIC CIRCUIT ;We, THERMAL DYNAMICS CORPORATION, a corporation of the State of Delaware, United States of America, of 101 South Hanley Road, St. Louis, Missouri 63105, United States of America, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- ;- 1 -(followed by page la) ;Description ;Technical Field ;The present invention is in the field of plasma torches and in particular is directed to a plasma torch having an improved pilot and main arc generating circuit. ;5 Background Art ;Plasma torches, otherwise known as electric arc torches, are known in the art for performing operations, such as cutting, welding, etc., on workpieces, and operate by directing a plasma consisting of ionized gas 10 particles towards a workpiece. An example of the conventional single gas plasma torch is illustrated in Hatch, U.S. patent No. 3,813,510, assigned to the assignee herein. Other patents disclosing such torches are U.S. patent Nos. 4,225,769; 4,663,512; and 4,663,515. 15 The disclosures of all of the above-mentioned patents are incorporated herein by reference thereto. As these patents illustrate, a gas to be ionized, such as nitrogen, is fed through channels in the torch mechanism in such a manner as to swirl in front of the end of a 20 negatively charged electrode. The welding tip which is adjacent the end of the electrode has a sufficiently high voltage applied thereto to cause a spark gap to jump between the electrode and the welding tip, thereby heating the gas and causing it to ionize. A pilot DC 25 voltage between the electrode and the welding tip maintains the pilot arc. The ionized gas in the gap appears as a flame and extends externally of the tip where it can be seen by the operator. The extension of the pilot arc and the flame, which for practical pur-30 poses, may be considered as being co-extensive depends upon the power in the gap — i.e., the arc current— ;-2- y - - K 9 ;(followed by page 2a) ^ i 1 3 J t of light which enables the operator to see the proper position for the torch before starting the welding or cutting operation. In actual practice, when the pilot arc is on, a loop-shaped arc extending out of the torch 5 can be seen. As the torch head is brought down towards the workpiece, the pilot arc jumps from the electrode to the workpiece due to the fact that the impedance of the workpiece current path is lower than the impedance of the welding tip current path. ;10 Conventional single gas plasma torches include pilot arc circuits which provide a 20-40 amp. pilot arc current at 100-200 volts across the electrode-tip gap, resulting in an extension of the arc about 1/4-1/2 inch past the welding tip. As a consequence, the torch must 15 be brought to within about 1/4-1/2 inch of the workpiece before the transfer arc jumps to the workpiece. This creates difficulties in the starting of cutting or welding operations. ;Disclosure of Invention 20 It is an object of the invention to provide a plasma arc torch circuit which is more efficient than prior art circuits and which regulate the power source in response to sensing of the torch arch transferring between the pilot electrode and the work member. 25 In one aspect the invention may be said to consist in a plasma torch unit of the type having a torch assembly adapted to operate on a workpiece having a torch electrode, a pilot electrode, and means for providing an ionized gas therebetween, the torch unit characterized 30 by: ;a current regulated power supply coupled between the electrodes and the workpiece to supply regulated current to the torch electrode and the pilot electrode for the pilot arc, and to the torch electrode and the workpiece 35 for the main arc; and a pulsing circuit coupled to the power supply and operable to pulse the current for the pilot arc. ;■7 r3 ft ;53£ ;- 2a - ;In another aspect the invention may be said to consist in a plasma torch unit of the type having a torch assembly adapted to operate on a workpiece, and having a torch electrode, a pilot electrode, and means for providing an ionized gas therebetween the torch unit, characterized by: ;a current regulated power supply electrically coupled between the electrodes and the workpiece to supply regulated current to the torch electrode and the pilot electrode for a pilot arc, and to the torch electrode and the workpiece for a main arc; ;a pilot arc control circuit coupled between said power supply and the pilot electrode; ;a disconnect circuit coupled to said pilot arc control circuit and operable to open circuit said pilot arc control circuit to generate an induced voltage in a first inductor coupled between said power supply and said torch electrode to help initiate the transfer of the ;\ FEB S35 \ ;-3- ;244352 ;Advantages of the invention will become more apparent upon a reading of a detailed description of the preferred embodiment in conjunction with the drawings, wherein: ;Brief Description of the Drawings ;Fig. 1 is a schematic wiring diagram of one prior art plasma arc torch operating circuit; ;Fig. 2 is a schematic wiring diagram of another prior art plasma arc torch operating circuit; ;Fig. 2A is a schematic wiring diagram of a prior art pilot arc regulating circuit as used within Fig. 2; ;Fig. 2B is a schematic wiring diagram of another prior art pilot arc regulating circuit as used within Fig. 2; ;Fig. 3 is a schematic wiring diagram depicting a plasma arc torch operating circuit according to the principles of the present invention; ;Fig. 4 is a schematic wiring diagram of a modified embodiment of the present invention as it would be configured within the circuit of Fig. 3; and ;Figs. 5A and B, in combination, are a more detailed schematic wiring diagram of the present invention. ;Best Mode for Carrying out the Invention ;With reference to the drawings, and Fig. 1 in particular, there is shown a schematic wiring diagram for one prior art plasma arc torch, generally referred to by the reference numeral 10. Torch 10 includes torch tip electrode 12 and annular torch pilot electrode ;: ."f. '995 1 ;electrode 14 provides an electric potential between electrodes 12 and 14 to create a pilot arc which heats a supplied gas such as nitrogen causing it to ionize as is well known in the art. Fig. 1 shows prior art 5 circuit C which using a resistively regulated pilot arc having a current regulated power means 16 and a pilot regulator means 18 including a disconnect means 20 in series with a resistor 22. A high frequenty pilot initiation means 24 is positioned in series with pilot 10 regulator means 18 and may be inserted in the circuit adjacent either electrode 12 or electrode 14 as shown in Fig. 1 to initiate investigation of plasma gas to commence pilot operations. ;A current sensing means 26 is connected in 15 parallel with pilot regulator means 18 and connects with the metal to be worked 28 in main circuit M. When tip electrode 12 is placed sufficiently close to the metal work 28 the arc will transfer the work 28 causing current to flow through main circuit M and current 20 sensing means 26 will sense the current differential and act to disconnect pilot regulator means 18 by opening pilot regulator means 18 by opening disconnect means 20. ;One problem associate with the prior art circuit 25 10 of Fig. 1 is that the circuit voltage of current regulated power means 16 must be large compared with the torch piloting voltage between tip electrode 12 and pilot electrode 14 to allow the pilot regulator means 18 to perform the function of a current source during 30 pilot operation. This causes circuit 10 to be inefficient, power being dissipated as heat in pilot regulator means 18. ;Fig. 2 shows another prior art circuit 10' similar to that of Fig. 1 in that it also contains 35 electronically controlled pilot regulator means 18* in pilot circuit P1 and a parallel main circuit M1. The electrode 14', current sensing means 26', pilot initiation means 24' (alternatively positioned as shown" and work 28'. The difference between circuit 10' of Fig. 2 and circuit 10 of Fig. 1 is the provision of 5 circuit connection on the opposite side of power means 1 from current means 26' connected to pilot regulator means 18' in order to provide a second current regulated control loop, one for pilot arc operation and one for transferred main arc cutting.

Fig. 2A shows one prior art pilot regulating circuit 181 a wherein the pilot is linearly regulated; that is, the pilot current is regulated against a set demand means 3d by varying the conductance of a linear element 3a.

Fig. 2B shows another prior art pilot regulating 18'b wherein the pilot is switch regulated. That is, the pilot is regulated against a set demand 3d' to vary the duty cycle of a switching element 3g within a feedback loop. Either scheme can tightly regulate the 20 pilot arc against AC line variations and against plasma gas in use, however, both add parts count and cost to the torch and arc relatively inefficient.

Fig. 3 shows the preferred plasma torch circuit 100 in accordance with the principles of the present 25 invention. Circuit 100 includes torch tip electrode 112, pilot electrode 114, current regulated power means 116, pilot regulator means 118, alternatively position-able pilot initiation means 124, current sensing means 126, and metal work 128. Pilot regulator means 118 3 0 comprises an electronic disconnect 120 in series with a current smoothing and energy storage inductor 130, and a free-wheeling diode 132 connected in parallel with disconnect 120 and in series with inductor 130.

According to the invention, current sensing 35 means 126 not only controls disconnect 120 through line 134, but also sends a current signal to comparator 136 During torch piloting, disconnect means 120 is "on" and is in its saturated state. The voltage seen between the metal work piece 128 and torch tip electrode 112 is essentially the voltage at which the torch 5 maintains the pilot arc determined by torch geometry and the plasma gas used. This voltage is considerably lower than the open circuit voltage used in prior art torch circuits, when the torch is brought sufficiently close to metal work piece 128, ionization current is 10 detected by current sensing means 126. In response to sensing the working current, sensing means 126 acts through line 134 to force disconnect means 120 to its "off" or high impedance state. At the moment of arc transfer to work piece 128, the pilot arc is maintained 15 by current flowing through energy storage inductor means 130 and the free-wheeling diode means 132. At the same instant, the current flowing in through the smoothing inductor means 140 of power regulator 116 is forced to flow between the workpiece 128 and torch tip 20 electrode 112, thereby maintaining the transferred plasma arc. When the energy is dissipated in the storage inductor means 130, the pilot arc between torch tip electrode 112 and pilot electrode 114 self extinguishes. When transfer is detected in current sensing 25 means 126 the pilot demand means le is changed and the power means 116 changes the power to that demanded for the torch operation on work piece 128.

A further embodiment of the present invention resides in additionally pulsing the pilot current. 30 Instead of maintaining a constant pilot demand means (le), the demand may be pulsed between two (or more levels) at various frequencies and duty cycles. During this pulsing the pilot arc is maintained throughout and no high frequency arc initiation means 124 is 35 required, as would be the case for a 'blown-out' pilot.

This pulsing feature offers several advantages. fer. Second, a tip cleaning action is observed, i.e., during plasma cutting molten metal is blown onto the tip face where it adheres in particulate form. At the same time, electrode material is removed from the torch 5 electrode and adheres to the inside tip. Both forms of contamination can cause the tip orifice to become distorted. When the pilot arc is pulsed following each cut significantly more power is dissipated in the tip electrode 112 for the pulse duration. This thermal 10 modulation is believed to be responsible for dislodging metal particles from the inner and outer tip surfaces.

With reference to Fig. 4, there is shown an alternative circuit wherein a small resistor 142 is added in series with the pilot means 118. This modifi-15 cation can further improve the obtainable standoff on some plasma torch designs. The pilot current (Ip) flows through resistor 14 2 to generate a voltage drop (Ip x R) which is in series with the pilot voltage measured between torch tip 112 and pilot electrode 114. 20 Thus the open circuit voltage between the metal work piece means 128 and the pilot electrode means 114 is increased, assisting the standoff at transfer. The power dissipated in this resistor is then a function of the pilot demand and pulse duration. 25 A further alternative circuit provides a pulsing cutting or main arc. From the invention pulsing the pilot arc before transfer it is clear that it is possible to pulse the current demand means le, after the arc has transferred and while the transferred plasma arc is 30 cutting the work metal means 128. This provision of pulsing the main arc offers several potential advantages. First, by selecting the appropriate pulse rate and duty cycle in relation to the cutting variables, it will offer a proportionally greater arc cutting capacity 35 /penetration for a small increase in power consumption. Second, it allows the tip orifice size to be reduced in allow a smaller focussed plasma column and result in smaller kerk widths. PLasma arc stability may also improve as a result of pulsing.

Figs. 5A and B are a more specific electronic circuit schematic diagram embodying some of the concepts of the invention as enumerated above. Like reference numerals appearing in Fig. 5 refer to like circuit components or group of components as appear in Figs. 3 and 4. Reference numeral 120c depicts the control circuity for disconnecting means 120. The power supply means, is not shown in Fig. 5.

It can therefore be seen that the novel circuity shown in Figs. 3 through 5 fulfills the objects and provides the advantages set forth above. Inasmuch as numerous changes could be made to the circuitry without departying from the spirit and scope of this invention, the scope of the invention is to be determined solely by the language of the following claims as interpreted by the patent laws and in particular the doctrine of equivalents.

Claims (17)

WHAT WE CLAIM IS:

1. A plasma torch unit of the type having a torch assembly adapted to operate on a work member and having a torch electrode, a pilot electrode, and means for providing an ionized gas therebetween a current regulated power means electrically connected between said electrodes and work member during torch usage, pilot arc control means for controlling an electric arc between said electrodes and pulsing the same during torch usage, said pilot arc control means electrically connected between said current regulated power means, the electrodes, and the work member, first circuitry of the electrical connection provided between the power means and the work member, second circuitry of the electrical connection provided between the power means and the torch electrode, and third circuitry provided between the power means, pilot arc control means, and the pilot electrode, disconnect means in said third circuity between the power means and said pilot electrode, current sensing means for sensing current in said first circuitry and generating a signal in response to torch usage, and an inductor means in series with said disconnect means and the said pilot electrode, said current sensing means operating said disconnect means with a feedback signal, and diode means connected between said second and third circuitry to provide a path for conduction of current to the pilot electrode.

2. The torch unit of clam 1, said diode means providing for conduction of current from the current regulated power means only towards the first inductor means to temporarily sustain the pilot current when said disconnect means is open.

3. The torch unit of claim 2 and further including resistor means connected between said first and second circuity and generating a voltage drop related to the pilot demand then increasing the electrode to workpiece voltage to increase a transfer height.

4. The torch unit of claim 1, and further including a second inductor means provided within the power means and responsive to the current detected by the current sensing means to provide high frequency filtering of the power means.

5. The torch unit of claim 4, said current sensing means generating a signal to the comparator to regulate the current generated by said power means and sustaining of the pulsing of the arc between the torch electrode and the work member.

6. The torch unit of claim 5, the current generated by the current regulated power means also being pulsed.

7. The torch unit of claim 6, said current sensing means operating to provide a pulsed pilot arc.

8. The torch unit of claim 6, said current sensing means operating to provide a pulsed main arc.

9. A plasma torch unit of the type having a torch assembly adapted to operate on a workpiece, and having a torch electrode, a pilot electrode, and means for providing an ionized gas therebetween the torch unit, characterized by: a current regulated power supply electrically coupled between the electrodes and the workpiece to supply regulated current to the torch electrode and the pilot electrode for a pilot arc, and to the torch electrode and the workpiece for a main arc; a pilot arc control circuit coupled between said power supply and the pilot electrode; a disconnect circuit coupled to said pilot arc control circuit and operable to open circuit said pilot - 11 - ?.4 4 3 5 arc control circuit to generate an induced voltage in a first inductor coupled between said power supply and said torch electrode to help initiate the transfer of the pilot arc into the main arc.

10. The plasma torch unit of claim 9, wherein said pilot arc control circuit includes a pilot inductor, that temporarily maintains pilot current when said disconnect circuit is open.

11. The plasma torch unit of claim 9, further characterized by: a current sensor coupled to the workpiece and to said disconnect circuit, said current sensor generating a signal in response to current flowing in the workpiece to actuate said disconnect circuit.

12. The plasma torch unit of claim 9, wherein said regulated current for the pilot arc is pulsed.

13. The plasma torch unit of claim 9, further characterized by: a pulsing circuit coupled to said power supply for pulsing the pilot arc current.

14. The plasma torch unit of claim 13, wherein said pulsing circuit pulses the main arc current.

15. A plasma torch unit of the type having a torch 30 assembly adapted to operate on a workpiece having a torch electrode, a pilot electrode, and means for providing an ionized gas therebetween, the torch unit characterized by: a current regulated power supply coupled between the 35 electrodes and the workpiece to supply regulated current to the torch electrode and the pilot electrode for the pilot arc, and to the torch electrode and the workpiece for the main arc; and r ^ 2 1 FEB 1995 12 a pulsing circuit coupled to the power supply and operable to pulse the current for the pilot arc.

16. The plasma torch unit of claim 15, wherein said 5 pulsing circuit is operable to pulse the current for the main arc.

17. A plasma torch unit substantially as hereinbefore described with reference to Figures 3 to 5b of the 0 accompanying drawings. 2 1 FEB 1995