Plasma torch with integrated independent electromagnetic coil for moving the arc foot

The present invention concerns a plasma torch with an integrated independent electromagnetic coil for moving the arc foot. The torch includes a bare torch portion housing a single upstream electrode or a pair of coaxial upstream and downstream electrodes. The electrodes are tubular and are cooled by an appropriate cooling circuit. An injection mechanism injects plasma gas downstream of the upstream electrode or between the upstream electrode and the downstream electrode. A starter mechanism ensures starting of the torch. A field coil moves the arc foot. The field coil has a generally cylindrical shape and is formed of a set of three supports made of an electrically non-conducting material with the shape of coaxial cylindrical tubes connected at one extremity of the unit to a crown provided with an electric connection mechanism. The electric connection mechanism is connected to at least one helical winding provided on at least one of the faces of one of the three cylindrical supports. The unit is moveable and able to be inserted in the bare torch around the upstream electrode.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally concerns industrial plasma torches and more 
particularly an electromagnetic coil for moving the arc foot of these 
torches. 
2. Discussion of Background 
The principle of this coil, known as a field coil, is well known. In a 
transferred arc plasma torch with a single upstream electrode, the other 
downstream electrode being outside the torch, as well as in a 
non-transferred arc plasma torch with two upstream and downstream 
electrodes disposed within the torch, a field coil is used to compel the 
electric arc foot opposite the upstream electrode to move in order to 
reduce and distribute wear of the electrode caused by erosion due to the 
arc. 
In non-transferred arc type plasma torches, this coil is generally provided 
in two ways. 
As described in FR-A-2.609.358, the field coil is independent of the plasma 
torch and engaged externally on the casing of the torch containing the 
upstream electrode. This embodiment proves to be costly and takes up a 
large amount of space. 
As described in FR-A-2.654.295, the field coil is firmly integrated in the 
upstream electrode it tightly surrounds so that it is totally dependent on 
this electrode as regards the diameter and axial position and as regards 
the series type electrical connection. 
SUMMARY OF THE INVENTION 
The present invention seeks to avoid these various drawbacks by offering a 
new field coil conception ensuring both independence and integration with 
regard to the upstream electrode and offering modularity which being 
associated with the movable nature of the coil, makes it possible to equip 
a given torch with field coils with different characteristics. 
To this effect, the invention concerns a plasma torch with an integrated 
independent electromagnetic coil for moving the arc foot. The torch 
includes a bare torch portion housing a single upstream electrode or a 
pair of coaxial upstream and downstream electrodes, the electrodes being 
tubular and cooled by a suitable cooling circuit. A field coil moves the 
arc foot and means to inject a plasma gas downstream of the upstream 
electrode or between the upstream electrode and the downstream electrode, 
means further being provided to ensure starting of the torch. The field 
coil has the general shape of a cylinder and is formed of coaxial 
cylindrical tubes connected to one extremity of said unit, to a crown 
provided with electric connection means connected to at least one helical 
winding provided on at least one of the faces of one of the three 
above-mentioned cylindrical supports, said unit being movable and able to 
be inserted on the bare torch around said upstream electrode. 
According to one application of the invention, the torch is of the type 
comprising a bare torch whose bearing structure is formed of three coaxial 
casings partially overlapping, that is, an external casing, an 
intermediate casing defining with the external casing the return circuit 
of the cooling fluid of the hot portion of the torch, and an internal 
casing defining with the intermediate casing the plasma gas intake circuit 
and channeling via its internal face the entering flow of the cooling 
fluid in the direction of the upstream electrode, the field coil and 
subsequently the downstream electrode. The coil is inserted between the 
upstream electrode and a portion or extension of the internal casing and 
wherein the electric connection means of the coil is formed of two 
connection pins integral with said crown and disposed parallel to the axis 
of the torch and able to receive via plugging electric connection rods 
extending inside the internal casing and connected at their extremity to 
electric terminals provided on said external structure. 
According to a further characteristic of the invention, the central 
cylindrical support of said unit is disposed so as to establish a 
communication on the side of its extremity orientated towards said crown 
between the spaces delimited between the central support and the two 
adjacent supports. The coil is inserted in the torch or disposed so that 
the fluid, once it has cooled the outer face of the upstream electrode, 
penetrates between the central support and the internal support and then 
between the central support and the external support. 
According to one embodiment, the coil comprises two helical windings placed 
respectively on the two opposing faces of the central cylindrical support. 
According to another embodiment, the coil comprises four helical windings 
placed respectively on the two opposing faces of the central cylindrical 
support and on the two faces opposite the other two cylindrical supports. 
This design of the field coil makes it possible to render the upstream 
electrode independent and change the coil so as to subsequently implement 
different electric configuration, that is, with a different number of 
windings and having a different power and spatial disposition. 
The coil exhibits remarkable compactness and is easily integrated in the 
torch and especially in the cooling circuit of the electrode(s). 
Several electric connecting modes, individual, series or parallel with a 
common connecting point between the coil and the upstream electrode, are 
possible. 
Similarly, it is possible to position the coil axially with respect to the 
upstream electrode by adapting the length of the electric connection rods. 
Other characteristics and advantages shall appear more readily from a 
reading of the following description of one embodiment of a plasma torch 
provided with a field coil conforming to the invention, said description 
being given solely by way of example.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a non-transferred arc type plasma torch. The torch shown in 
FIG. 1 is also described in co-pending U.S. application Ser. No. 
08/666,790, pending. 
This torch, able to be used in particular in waste vitrification ovens, 
includes a bare torch portion 1 formed of three coaxial casings partially 
overlapping and connected at its rear extremity to an external structure 2 
ensuring connections, that is, the fluid and electric links with the 
outside of the torch. 
Disposed in the bare torch 1, which includes an external cylindrical 
metallic casing 3, an intermediate cylindrical metallic casing 4 and an 
electrically nonconducting cylindrical casing 5, are disposed an annular 
upstream electrode 6, an annular downstream electrode 7, a plasma gas 
injection device 8, between the electrodes 6 and 7, and an annular field 
coil 9 surrounding the upstream electrode 6. 
The upstream electrode 6 can be moved in the direction of the downstream 
electrode 7 so as to start the torch with the aid of a starter jack 10 
disposed outside the cylindrical body 11 of the external structure 2. 
The jack 10 acts on the upstream electrode 6 via a set of rods 12, 13 
coaxial to the torch, traversing the body 11 and extending inside the 
internal casing 5. 
The cooling of the hot portions of the torch is effected with a fluid, such 
as demineralised water, penetrating into the body 11 via an intake 14 
opening into a space delimited by the body 11 and the internal casing 5 
and housing the set of rods 12, 13. The cooling fluid then flows by 
sweeping the external face of the upstream electrode 6 while being 
laminated by an annular separator 15 inserted between the electrode 6 and 
the field coil 9, and then traverses via passages 16 an annular element 17 
of the plasma gas injection device 8, sweeps the outer face of the 
downstream electrode 7 and finally returns to the body 11 via the annular 
space delimited between the outer 3 and intermediate 4 casings. The fluid 
comes out of the body 11 via an outlet 18. 
The injecting device 8 is fed with plasma gas, air for example, introduced 
into the body 11 via an intake 19 and routed to the injector by means of 
the annular space delimited between the intermediate casing 4 and internal 
casing 5. 
For more disclosure of the details of this structure, reference can be made 
to U.S. application Ser. No. 08/666,790. 
In accordance with the present invention, the field coil 9 is formed of an 
annular cylindrical unit formed of three coaxial cylindrical tubes 
rendered integral with one another with a specific spacing between the 
central tube 20 and the internal 22 and external 21 tubes. 
The tubes 20, 21 and 22 are made of an electrically nonconducting material 
with a given gap between the central tube 20 and the lateral tubes 21,22. 
The linking between the tubes is effected by nonconducting braces 23. 
According to the embodiment shown, the central tube 20 extends by a certain 
length 24 past the adjacent tubes 21, 22 to the distal extremity of the 
coil, whereas the proximal extremity of the tube 20 is located at a 
certain distance from a crown 25 connecting the two external 21 and 
internal 22 tubes. Thus, the gap between the central tube 20 and the 
external tube 21 communicates with the gap between the central tube 20 and 
the internal tube 22 via an annular passage 26 at the bottom of the coil 
9. 
The crown 25 is provided with two electric connection pins 27 disposed 
diametrically opposite and orientated towards the internal structure 2. 
In the embodiment of FIG. 2, the pins 27 are connected to two series 
helical windings, namely an internal winding 28a placed on the internal 
face of the central tube 20 and an external winding 28b placed on the 
outer face of said tube 20. This disposition of the windings having a 
small section and emitting heat enable them to be cooled, as shall be seen 
subsequently. 
In the coiling mode of FIG. 4, the pins 27' are series connected to four 
helical windings 29a, 29b, 29c, 29d distributed respectively on the outer 
face of the internal tube 21, the internal and external faces of the 
central tube 20 and the internal face of the outer tube 22. 
As can be seen in FIGS. 2 and 4, the number, extent and disposition of the 
winding(s) on the coil 9 may vary so as to have significant flexibility of 
adaptation of the magnetic field it is desired to obtain. 
As shown in FIG. 1, each pin 27 is able to be connected to an electric 
connection rod 30 parallel to the axis of the torch and extending inside 
the internal casing 5, traversing the body 11 and connected to a 
connection terminal 32 accessible outside said body. 
In addition, another electric connection terminal 33 accessible outside the 
body 11 is connected to a rod 34 similar to the rod 30 and extending 
inside the internal casing 5 by being situated outside the plane defined 
by the two rods 30, for example by being angularly offset by 90.degree. 
with respect to these two rods 30. 
The rod 34 is connected to a conductive bush 35 mounted sliding on the rod 
13 for moving the upstream electrode 6, said rod 13 ensuring the electric 
connection with said electrode 6. 
Various mountings of the coil 9 are possible. 
If it is desired to mount the circuit of the coil 9 independent of that of 
the electrodes, as shown in FIG. 5a, the coil is fed by the two terminals 
32 connected to the two pins 27 of the coil. The upstream electrode 6 is 
fed from the terminal 33 by the rods 13, 34 and the element 35, the return 
of current from the downstream electrode 7 being carried out by the 
external casing 3, the body 11 and the terminal 36 of the structure 2. So 
as to have a maximum distance between the rods 30 and 34 and for improved 
insulation, it is preferable for the three rods 30 34 to be angularly 
offset by 120.degree. with respect to one another, the two pins 27 then no 
longer being diametrically opposed as shown in FIGS. 2 to 4. 
If it is desired to carry out a mounting of the coil 9 in series with the 
electrodes as shown in FIG. 5b, one of the terminals 27 is connected to 
the element 35 as shown in FIG. 6 through a connecting screw V, the rod 34 
being not mounted. 
If it is desired to carry out a mounting in parallel as shown in FIG. 5c, 
one of the terminals 32 is connected to one of the pins 27 and the 
terminal 33 is connected to both the upstream electrode 6 and to the other 
pin (27) by an auxiliary T connection rod extending the rod 34 and 
traversing the element 35, as shown in FIG. 7. The advantage of this 
connection is of placing the coil 9 with the same potential as the 
upstream electrode 6 so as to avoid disruptive breakdowns while keeping a 
separate control of the coil and arc as in the independent connection. 
It is to be noted that, as shown in FIG. 1, the portion 24 extending past 
the central tube 20 is in support against one portion of the element 17 of 
the injection device so as to channel, as indicated by the arrow 38, the 
cooling fluid originating from the upstream electrode 6 into the gap 
between the tubes 20 and 22, then into the gap between the tubes 20 and 21 
so as to sweep all the winding carrier faces of the coil 9 over their 
entire length, as well as into the annular passage 26, the electric links 
between the windings and the pins 27. 
The spacing between the tubes 20, 21, 22 of the coil 9 is determined so as 
to have the desired cooling characteristics. 
The support of the extension 24 on the element 17 is sliding and allows 
free sliding of the coil 9 in its annular housing defined by the separator 
15 and an annular extension 17a of the element 17 disposed in the 
extension of the internal casing 5. 
According to another characteristic of the device of the invention, it is 
also possible to axially modify the relative position of the coil 9 with 
respect to the upstream electrode 6 by replacing the rods 30 by rods of a 
different length or even by placing at the outer extremity of the rods a 
device (not shown on the drawings) ensuring a controlled axial movement of 
said rods according to a specific program, the coil 9 as indicated above 
being able to slide freely between the separator 15 and the element 17a. 
Conversely, during movement of the upstream electrode 6 towards the 
downstream electrode 7 at the time of start up, the separator 15 is able 
to slide freely inside the coil 9 which remains stationary. 
Finally, the invention is not merely limited to the embodiments shown and 
described above, but on the contrary covers all possible variants, 
especially as regards the shapes, dimensions and disposition of the three 
tubular portions 20, 21,22 of the coil support, the disposition, number 
and mounting of the windings and the coil connection means. 
The invention can be applied to all types of plasma torches, transferred 
arc or non-transferred arc plasma torches, regardless of the type of start 
up device used, especially regardless of whether the upstream electrode 6 
is mobile or not.