EP0461263A1

EP0461263A1 - Plasma torch with instable plasma arc

Info

Publication number: EP0461263A1
Application number: EP91900350A
Authority: EP
Inventors: Kazushige Nkk Corporation Inokuchi; Akio Nkk Corporation Nagamune; Isamu Nkk Corporation Komine; Norio Nkk Corporation Ao; Hiroshi Nkk Corporation Sekine; Yoshiyuki Nkk Corporation Kanao; Yoshiyuki Nkk Corporation Kitano
Original assignee: NKK Corp; Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1990-01-04
Filing date: 1990-11-22
Publication date: 1991-12-18
Anticipated expiration: 2010-11-22
Also published as: DE69032205T2; CA2048654A1; WO1991010342A1; EP0461263B1; EP0461263A4; DE69032205D1; ATE164721T1

Abstract

A moving plasma torch which generates a plasma arc between a nozzle and an object to be heated. A thermionic cathode is used as an electrode to secure a cathode point, the speed of plasma arc in the axial direction is suppressed to a degree to generate magnetically unstable condition, and the plasma torch is turned, reversely rotated or vibrated, in order to increase the energy density and to increase the capacity.

Description

(Technical Field)

The present invention relates to a transferred plasma-arc torch which produces and uses a plasma arc for heating or melting purposes.

(Background Art)

A hot plasma is widely used as a source of very high temperature and high energy for such applications a heating and melting as well as reaction, surface treatment and other purposes. Particularly, it features that it high energy density has the effect of making the equipment compact and that the power input can be adjusted freely with improved response through the control of the atmosphere by a plasma working gas and the control of the current and the plasma length. Studies have recently been made to make the best use of these features in the still making processes such that the molten steel in a ladle or tundish is heated by a plasma so as to control the molten steel temperature or effect a refining reaction.
Basic types of plasma arc torches include the transferred arc type and nontransferred arc type. The transferred arc type is such that a voltage is applied between the electrode within the plasma-arc torch and an object to be heated thereby generating a plasma between the two, and generally a heating efficiency of 60 to 80% is obtained. On the other hand, the nontransferred arc type is such that the plasma torch includes a negative electrode and a positive electrode so that a plasma is generated between the electrodes thereby blowing a hot gas like a combustion burner against an object to be heated, and the heating efficiency is 20 to 40%. Where the object to be heated has an electric conductivity as in the case of a metal, the transferred plasma-arc torch is used form the heat efficiency point of view.
A typical construction of the conventional transferred plasma-arc torches is shown in Fig. 1. In the Figure, numeral (1) designates a plasma-arc torch, (2) an electrode, and (3) a nozzle. Generally, tungsten is used for the electrode (2) and it is enclosed by the nozzle (3). There are many instances where the nozzle (3) is a water-cooled copper nozzle or a ceramic nozzle. Numeral (4) designates a plasma a working gas such as argon or hydrogen and it is caused to flow between the electrode (2) and the nozzle (3). Numeral 5 designates an object to be heated, (6) a plasma arc, (7) insulating spacers, (8) a cooling water, and (9) electric terminals. Then, while the plasma is generated between the forward end of the electrode (2) and the object (5) to be heated, the stable plasma arc (6) having a strong axial directivity is formed in the vicinity of the electrode (2) by the plasma working gas (4) whose flow is constricted by the nozzle (3)
In the conventional plasma-arc torch (1), in order to enhance the directivity and stability of the plasma arc (6) as mentioned above, the shape and arrangement of the electrode (2) and the nozzle (3) and the manner of flow of the plasma working gas (4) are determined. For this purpose, the forward end of the electrode (2) is pointed to have an acute angle so that the plasma arc (6) extending along the axial direction is formed by a magnetic, pumping effect caused by a pinch force due to the current of the plasma itself at the plasma spot area where the plasma contacts with the electrode (2) or alternatively the gap the nozzle (3) and the electrode (2) is reduced to less than several mm so that the flow velocity of the plasma working gas (4) is increased, thereby maintaining the plasma arc (5) which is stable in the axial direction.
As mentioned above, the conventional transferred plasma-arc torch takes notice of only the directivity and stability of the plasma arc (6) and it is in no way considered to make the plasma arc (6) unstable. Also, due to the fact that according to the conventional design, if the plasma arc (6) is caused to become unstable, there is the danger of producing a plasma between the nozzle (3) and the electrode (2) and causing a burning loss of the nozzle (3) and therefore importance is placed on the stabilization.
The conventional transferred plasma-arc torch (1) is constructed so as to generate a plasma arc (6) which is stable and having a strong axial directivity. Such stable plasma-arc (6) is naturally reduced in arc spreading and thus the object (5) to be heated is locally heated. As a result, where molten steel, for example, is heated, a localized temperature rise and evaporation of the molten steel are caused thereby giving rise to such problems as the nonuniformity of the temperature and the reduction in the heating efficiency and the yield. Also, in order to prevent the entry of nitrogen and hydrogen into the motlen steel, an inert argon is used for the plasma working gas (4), and further there are cases where an argon atmosphere is maintained in the heating chamber. At this time, as shown in the following Table 1,argon is low in applied voltage per unit plasma lenght as compared with the other gases and therefore the power input is decreased.

For instance, in a high-temperature argon atmosphere the voltage applied to the plasma is as low as between 0.1 and 0.2 V/mm and therefore the applied voltage becomes about 100 to 150 V at the most even if the plasma length is 500mm. Since the power input is the product of the voltage and the current, the only way to input a large power is to increase the current or to increase the number of plasma-arc torches. In particular, if the current is increased, there is a problem that not only the consumption of the electrode (2) is increased at a rate which is about the square of the current but also the equipment cost of the power source and the power supply circuit is increased. Also, if the length of the plasma is increased in order to increase the voltage, the heating efficiency is deteriorated on the other hand, there is a limit to the length of the plasma due to the restriction on the equipment. In view of these points, it is desirable that the applied voltage per unit plasma length as high as possible.

TABLE 1

Comparison of potential gradients of arc columns in various gases and air (where the potential gradient of air = 1)
Gas	potential gradient ratio
Argon	0.5
Air	1.0
Nitrogen	1.1
Carbon dioxide gas	1.5
Oxygen	2.0
Steam	4.0
Hydrogen	10.0

In addition, in view of the construction of the plasma-arc torch (1), if the electrode (2) is pointed to form an arc, in the event that the electrode (2) is consumed so that its forward end shape is deformed, the directivity of the plasma arc (6) is lost and the plasma jumps to the nozzle (3) thereby causing a burning loss of the nozzle (3). Also, the construction of the nozzle (3) must be adjusted so that the gap between the nozzle (3) and the electrode (2) becomes several mm and their central axes coincide with each other. Due to such sophisticated construction, there is a problem that damages tend to be caused to the nozzle (3) due to a change in the shape of the electrode (2) and a deflection of the plasma arc (6) caused by an external magnetic field.

(Disclosure of Invention)

It is an object of the present invention to provide a transferred plasma-arc torch having an increased energy density.
It is another object of the present invention to provide a transferred plasma-arc torch capable, of heating an object to be heated over the wide area thereof.
It is another object of the present invention to provide a transferred plasma-arc torch capable of reducing the consumption of the electrode thereby increasing its life.
It is another object of the present invention to provide a transferred plasma-arc torch capable of obtaining a high degree of controllability on the plasma arc.
It is another object of the present invention to provide a transferred plasma-arc torch which ensures a reduction in the size of the plasma-arc torch and a reduction in the operating cost.
In accordance with one aspect of the present invention, the transferred plasma-arc torch utilizes a magnetic instability inherent to a plasma such that a plasma arc is made unstable fluidly to whirl it at a high velocity and also the plasma arc is radially generated from the electrode of the plasma-arc torch. as a result, not only the applied voltage is increased but also the heating area is increased as compared with the conventional stable plasma arc.
As shown in Fig. 2, once a plasm column (P) is bent, the magnetic field produced by the plasma current is intensified on the concave side but weakened on the convex side so that the bend of the plasma column (P) is further promoted by the magnetic pressure. Such magnetic instability of the plasma column (P) is called as a kink instability. At this time, if one end is made a fixed end and the other end is made a free end, the other end is oscillated by an electromagnetic force F produced by the previously mentioned magnetic field. While the description is made to dimensionally in connection with the Figure, in a three-dimensional space the similar magnetic instability generates an electromagnetic force F of a turning direction (Θ direction) about the electrode axis and the plasma column (P) starts a whirling motion.
The deforming force due to the kink instability is given by the following equation (1)
Here, Fd represents the deforming force per unit length, µ_o the permeability in vacuum, I the current, λ the wave length of the disturbance, R the radius of curvature of the deformation, and Rc the radius of the plasma column (P). On the other hand, to correct this requires that the axial flow velocity of the plasma column (P) is increased to obtain the correcting force given by equation (2)
Here, Fc represents the correcting force per unit length, ρ the density of the plasma, v the axial velocity of the plasma arc, and S the cross-sectional area of the plasma column (P).
The conventional plasma-arc torch (1) increases the axial velocity v to strengthen the correcting force Fc. On the contrary, in accordance with the present invention a magnetic instability (kink instability) is caused in the plasma arc so that Fd>Fc. In addition, in order that a radial plasma arc may be generated, a thermionic emission-type cathode having a stable cathode spot is used so that the end of the plasma on the plasma-arc torch side becomes a fixed end. Tungsten and carbon may be cited as typical thermionic emission-type cathode materials. Also, since molten steel or the like becomes a free end if it serves as the anode, the plasma arc is allowed to move around at a high speed over the surface of the molten steel, owing to the kink instability.
As described hereinabove, in accordance with the plasma-arc torch of the present invention the thermionic emission type cathode is used and the deforming force and the correcting force are selected in a manner that Fd > Fc, thus decreasing the axial velocity of the plasma working gas and thereby causing a motion in the ϑ direction. As a result, the plasma arc whirls at a high speed in the ϑ direction and also it takes the form of one spreading radially from the cathode.
The shapes of the conventional stable plasma arc (6) and the unstable plasma arc (16) according to the present invention are comparatively shown in Fig. 3a and 3b, respectively. As will be seen from the Figures, the spread of the plasma arc (16) on an object (15) to be heated is greater than that of the conventional plasma arc (6) by more than 10 times thus ensuring a large area heating. Also, Fig. 4 shows the variations of the voltages (V) applied to the plasma arcs and (16), respectively, in cases where the distance S(L) between the plasma-arc torches (1) and (11) and the objects (5) and (15), respectively, are varied. It will be seen that the voltage (VII) is applied which is 1.5 to 2 times the voltage (VI) applied in the case of the stable plasma arc (6) is twisted and bent thus increasing the substantial path and that the radiation of heat from the plasma arc (16) is increased due to its high velocity whirling motion thereby increasing the energy input so as to be in equilibrium therewith. Therefore, as compared with the conventional plasma-arc torch, the power input (VII) of 1.5 to 2 times is obtained even for improved same the (L) and both the improved energy density and the greatly increased capacity are attained.
In addition, since the object (15) to be heated is heated extensively and uniformly with the improved efficiency, not only the considerable evaporation due to the conventional, localized heating is reduced but also the yield is improved and the heating efficiency is enhanced. Also, due to the fact that the forward end shape of the electrode need not take the form of a pointed end shape, even if its shape is changed, there is no danger of the plasma arc jumping to the nozzle and the life of the electrode is increased. Moreover, the control of the forward end shape of the electrode and the gap between the nozzle and the electrode is made easy.
Further, in accordance with another aspect of the present invention, the transferred plasma-arc torch is designed so that the plasma working gas is caused to flow as a whirling flow between the nozzle and the electrode so as to decrease the axial velocity of the plasma jet. In this way, the magnitude of Fd is increased.
Further in accordance with another aspect of the present invention the transferred plasma-arc torch is designed so that two gas streams, i.e., a gas stream which seals the cathode to reduce its oxidation loss and a gas stream which causes the plasma arc to whirl at a high velocity are combined. Thus, the transferred plasma-arc torch causes a shielding gas to flow around the electrode in the axial direction and it also includes a supply nozzle so that as for example, the plasma working gas containing a radial flow component is supplied to the outer side of the shielding gas.
The shielding as flows around the electrode in the axial direction and the plasma working gas containing a raidal flow component is blown out to the outer side of the shielding gas from the supply nozzle. As a result, while the electrode is enclosed and shielded by a gas curtain of the shielding gas, a plasma arc is formed which whirls at a high velocity in such a manner that its radius of whirling is increased as it moves away from the electrode due to the plasma working gas blown out to the outer side of the shielding gas from the supply nozzle. Thus, despite the fact that the generated voltage is high and the heating area is increased, the oxidation loss of the electrode can be decreased.
Further, in acocrdance with another aspect of the present invention the transferred plasma-arc torch is designed so that the nozzle is provided with an electro magnetic coil for controlling the plasma arc. As the electromagnetic coil, an electromagnetic coil which generates for example a dc magnetic field in the axial direction of the nozzle is mounted so that the deflection of the plasma arc is reduced and also a turning force by the electro magnetic force is generated, thereby causing the plasma arc to whirl and spread toward the object to be heated.
The current flows within the plasma arc from the anode toward the cathode. By supplying a dc current to the electromagnetic coil, a dc magnetic field of a magnetic flux emerging from one end of the coil and entering the other coil end is generated and this dc magnetic field links the current within the plasma arc. As a result, an electromagnetic force is generated in a direction tending to rotate about the axis of the plasma arc and the plasma arc is rotated. When the applied external magnetic field is decreased, the axial flow of the plasma working gas causes the action of a correcting force to effect a transition from the whirling plasma to a stable linear plasma. In this way, the intensity of the external magnetic field is adjusted thereby ensuring the control of the change-over between the whirling plasma and the stable plasma, the whirling velocity, etc.
Further, in accordance with another aspect of the present invention the transferred plasma-arc torch is designed so that an electromagnetic coil for generating an ac magnetic field axially is mounted on the outer side of the nozzle or within its body whereby the deflection of the plasma arc is reduced and a turning force due to an electromagnetic force is generated, thereby causing the plasma arc to whirl and spread toward the object to be heated then. The frequency of the ac magnetic field is set to a range from 0.5 to 300 Hz.
In accordance with this transferred plasma-arc torch, the plasma arc is not whirled but reversed or vibrated so as to prevent to the central portion from becoming a negative pressure. In other words, an ac magnetic field is externally applied to the plasma column so as to link the current flowing in the plasma column thus forming within the plasma column a magnetic field which reverses at a period equal to the ac magnetic field about the torch axis and the plasma itself is reversed to cause a magnetic instability and thereby to prevent the occurrence of a low pressure portion within the plasma column.
The ac magnetic field is generated by winding a coil around the torch and it is caused to link the current within the plasma column. The magnitude and the period of the electromagnetic force can be handled by adjusting the magnitude and frequency of the coil current.
Generally, a stable plasma is formed when the coil current is small and it initiates its reversion due to the electromagnetic force as the coil current increases, thereby for, the plasma which is fluidly instable, that is, the plasma having a large spread and a high generated voltage. Also, by varying the frequency, it is possible to vary the plasma; particularly, in order to prevent the entrapping of the air and to efficiently generated a plasma which is high in generated power and large in spreading, it is very effective to effect the operation within a range from 0.5 to 300 Hz. Since the optimum frequency differs depending on the coil current, it is determined by adjusting the coil current and the frequency in a well balanced manner so that the power consumption of the coil is reduced.
Further, in accordance with another aspect of the present invention the transferred plasma-arc torch comprises magnetic coils for applying a rotating magnetic field to the plasma column. These magnetic coils comprise a set of coils which are similar for example to the stator windings in an induction machine and are arranged about the outer peripery of the nozzle. A current is supplied to the set of coils thereby generating a rotating magnetic field. This rotating magnetic field links the plasma current so as to cross with the latter substantially at right angles thereby generating an electromagnetic force tending to bend the plasma column. Since the rotating magnetic field varies with time, this electromagnetic force acts while rotating about the axis of the plasma column so that if the plasma column deviates from the axially symmetric form even slightly, the plasma column is brought into a fluidly instable condition due to a superposition effect of the kink instability of the plasma column itself and the electromagnetic force.
With such fluidly instable plasma, its shape and generated voltage are varied in any manner in response to the adjustment of the coil current and the frequency. By utilizing this property, the plasma can be controlled throughout a wide range of conditions from a stable plasma condition as shown in Fig. 3a to a fluidly instable condition as shown in Fig. 3b, that is, a plasma condition in which the plasma arc is whirled high velocity and radially spread toward an object to be heated in accordance with operating conditions. It is easy to produce a whirling plasma of the axially symmetric form with respect to the torch axis. Moreover, it is possible to realize an operation in which the plasma working gas is reduced and maintained constant.
In addition, the rotating magnetic field acts to substantially cross the plasma current at right angles so that even if the coil current is small or the number of turns of the coil is small, the rotating magnetic field can satisfactorily contribute toward making the plasma column instable fluidly.
In other words, the forming efficiency of the electromagnetic force is improved and it is possible to decrease the size of the magnetic field generating section is reduced and hence the plasma torch is reduced in size. Then, by virtue of the reduction in power consumption, it is possible to ensure the improved overall heat efficiency, the prevention of increase in the size of the peripheral equipment and the solution of physical problems such as the place of installation and hence the improved economy is ensured.
Further, in accordance with another aspect of the present invention the transferred plasma-arc torch is designed so that a member made of a magnetic material is arranged at least on one of the outer and inner sides of the electromagnetic coil so that the magnetic field is caused to effectively link the plasma column, and also the cathode forward end is arranged at a position receded from the magnetic material member thereby reducing the effect of the magnetic field on the cathode spot.
Due to the arrangement of the magnetic material member on the electromagnetic coil, there is the effect of strengthening the magnetic field at the forward end of the nozzle and increasing the number of lines of magnetic flux which link the plasma current. In other words, by arranging the magnetic material member, it is possible to decrease the number of turns in the coil and the coil current considerably.
Moreover, the arrangement of a cathode spot at the position receded from the magnetic material member has the effect of forming the stable cathod spot and stably forming plasma column in the vicinity of the cathode, thereby making the entrapping of the air difficult and effectively preventing oxidation of the cathode.

(Brief Description of Drawings)

Fig. 1 is a diagram showing the construction of a conventional plasma torch.
Fig. 2 is a diagram showing the principle of a kink instability.
Figs. 3a and 3b are diagrams for explaining the behaviors and shapes of plasma arcs.
Fig. 4 is a graph showing the electric characteristics of a plasma.
Fig.5 is a diagram showing the construction of a transferred plasma-arc torch according to an embodiment of the present invention.
Fig. 6 is a diagram showing the construction of a transferred plasma-arc torch according to another embodiment.
Fig. 7 is a diagram showing the construction of the insulating space in the transferred plasma-arc torch of Fig. 5.
Fig. 8 is a diagram showing the construction of another example of the insulating spacer.
Fig. 9 is a diagram showing the construction of a transferred plasma-arc torch according to another embodiment of the present invention.
Fig. 10 is a diagram showing the construction of the insulation spacer in the transferred plasma-arc torch of Fig. 9.
Figs. 11, 12A and 12B are diagrams respectively showing the constructions of the principal parts of transferred plasma arc torches according to another embodiments of the present invention.
Fig. 13 is a diagram showing the construction of a transferred plasma-arc torch according to another embodiment of the present invention.
Figs. 14 and 14b are diagrams for explaining a stable plasma and a whirling plasma, respectively.
Fig. 15 is a diagram showing the construction of a transferred plasma-arc torch according to another embodiment of the present invention.
Figs. 16a and 16b are diagrams respectively showing the magnetic field distribution of a magnetic field by a coil along the coil axis in cases where no steel conduit is used and where it is used.
Fig. 17 is a diagram showing the construction of a transferred plasma torch according to another embodiment of the present invention .
Figs. 18a and 18b are diagrams for explaining the operation of the transferred plasma-arc torch of Fig. 17.
Fig. 19 is a diagram showing the construction of a transferred plasma-arc torch according to another embodiment of the present invention.
Fig. 20 is a diagram showing the construction of a transferred plasma-arc torch according to another embodiment of the present invention
Fig. 21 is a plane sectional view showing the arrangement of the set of rotating magnetic field generating coils in the embodiment of Fig. 20.
Figs. 22a and 22b and Figs. 23a and 23b are diagrams respectively showing specific examples of the arrangement of the rotating magnetic field generating section and the corresponding rotating magnetic field generating method, respectively.

(Best Mode for Carrying Out the Invention)

Since the transferred plasma-arc torches of the embodiments shown in Figs. 5 and 6 are somewhat different in construction from the conventional apparatus of Fig. 1 and are also different in function and effect from the latter, each of the component parts is designated by a two-place numeral in which the first place is in common and the second-place 1 is added. Each of these embodiments shows a transferred plasma-arc torch (11) of 1 KA.
Tungsten which is one of thermionic emission-type cathodes is used for the material of an electrode (12) in the embodiments shown in Figs. 5 and 6. The electrode (12) has a diameter of 20mm and its forward end is formed into a hemispherical shape, thereby deteriorating the formation of an arc due to the magnetic pumping effect of a plasma. In Fig. 6, the electrode (12) has a slightly smaller diameter and it is formed into a semi-circular sectional shape. Also, a nozzle (13) is not constructed so as to converge the plasma working gas (4) to increase its axial velocity as will be seen from the conventional plasma-arc torch (1) which forms the stable plasma arc (6). The principal purpose of the nozzle (13) is to cause an inert gas or argon to surround the tungsten electrode (12) thereby protecting it from being consumed by oxidation. As a result, the gap between the forward end of the electrode (12) and the nozzle (13) is widened. Then, while supplying a plasma working gas (14) at a rate of 20 to 30ℓ/min to flow it between the forward end of the electrode (12) and the nozzle (13), the plasma working gas (14) is caused to whirl in the circumferential direction (ϑ direction) about the axis of the plasma-arc torch (11).
Since the plasma working gas (14) is whirled about the axis of the plasma-arc torch (11), as shown in Fig. 7, each of insulating spacers (20) includes holes (20a) for passing the gas and the holes are skewed with respect to the axis of the plasma-arc torch (11). By skewing the holes (17a) in this way, the gas stream is directed obliquely and a whirling motion is imparted to to the plasma working gas (14). In order to impart a whirling motion to the plasma working gas (14), any other construction may be used so that as shown in Fig. 8, for example, the plasma working gas (14) may be directly supplied to a header 20b of each insulating spacer (20) so as to direct the gas from the header to the skewed holes (20a).
In the manner described above, while reducing the velocity component of a plasma arc (16) in the axial direction of the plasma-arc torch (11), a ϑ-direction velocity component is imparted to the plasma working gas (14) so that a magnetic instability is caused the plasma arc (16) and thus the plasma arc (16) whirling at high velocity is formed. Also, the thermionic emission-type cathode is used for the electrode (12) forming the cathode thereby forming the cathode spot stably and therefore the plasma arc (16) is generated which radially spreads toward an object (15) to be heated.
Further, since the gap between the forward end of the electrode (12) and the nozzle (13) is wide, even if the plasma arc (16) is instable, there is no danger of the plasma arc (16) jumping to the nozzle (13).
Then, the spreading toward the object (15) of the plasma arc (16) generated by the plasma-arc torch (11) is as great as about 200mm⌀ when the height of the plasma-arc torch (11) is 200mm, and the applied voltage increased to over 300V as compared with the applied voltage of as low as 170V in the case of the conventional stable plasma arc (6). Also, it has been confirmed that while the plasma arc (16) is fluidly instable, the voltage variation is so small that there is no problem from the practical point of view.
It is to be noted that while, in the above-described embodiments, tungsten is used for the electrode (12), any other thermionic emission-type cathode such as carbon may be used.
In the transferred plasma-arc torch of the embodiment shown in Fig. 9, holes (22a) of each insulating spacer (22) are parallel to the axis of the plasma-arc torch (11) as shown in Fig. 10 and the plasma working gas (4) is not whirled or disturbed by any obstruction but it is caused to flow in the axial direction of the plasma-arc torch (11) as in the conventional torch. Also, in the embodiment another nozzle (24) for whirling purposes is additionally attached to near the forward end of the nozzle (13). The outer nozzle (24) is formed into an annular shape and its discharge jet is formed circumferentially with an inward inclination downward thereby causiong jet streams to cross one another at a position somewhat apart from the electrode (12). Then, the plasma working gas (14) containing gradial flow components is shoot out from the nozzle (24).
With the transferred plasma-arc torch (11) constructed as described above, while a plasma arc (16) is generated between the forward end of the electrode (12) and the object to be heated in the condition where the nozzle (24) is closed, the plasma working gas (14) forms a linear stable plasma arc having a strong axial directivity in the vicinity of the electrode (12) (see Fig. 3a).
If the plasma working gas (14) containing the radial flow components is shoot out from the outer nozzle (24), however, the flow of the whirling gas shoot out from the nozzle (24) strikes on the plasma arc (16) in the vicinity of the crossing position, thereby promoting its magnetic instability. As a result, from near this position is generated a whirling stream tending to rotate about the axis of the plasma arc (16) and a whirling plasma is generated. The resulting condition is the same as shown in Fig. 3b. In other words, the plasma arc (16) is produced which is cylindrical in the vicinity of the electrode (12) to surround the latter and which increases in radius of whirling as it is moved away from the electrode (12).
Thus, by flowing the plasma working gas (14) from the nozzle (13) while surrounding the electrode (12), a cylindrical gas curtain is formed and the electrode (12) is shielded from the whirling plasma arc (16). As a result, even if the whirling plasma arc (16) entraps the oxygen in the air, no oxidation of the electrode (12) is caused and its consumption is reduced.
The transferred plasma-arc torch of the embodiment shown in Fig. 11 includes, in place of the annular nozzle (24) shown in Fig. 9, a nozzle (26) including one or more simple tubes whose forward ends are radially bent so as to cause a disturbance of the plasma.
On he other hand, in the transferred plasma-arc, torch of the embodiment shown in Figs. 12A and 12B, a cylindrical flow rectifying device (28) whose outer diameter is intermediary between the outer diameter of the electrode (12) and the inner diameter of the nozzle (13) is disposed in the flow path of the plasma working gas (14) near the electrode (12). Then, a part of the plasma working gas (14) whirled by the insulating spacers (20) of such skew type as shown in Fig. 7 and flowing out from the nozzle (13) is rectified and shoot out from the flow rectifying device (28) while surrounding the electrode (12). Then, the axial flow of the plasma working gas (14) rectified by the flow rectifying device (28) encloses the electrode (12), and at a position apart from the electrode (12) is formed a plasma arc (16) which spreads downward.
In the transferred plasma-arc torch of the embodiment shown in Fig. 13, a coil 30 is mounted on the forward end of the nozzle (13) so as to produce a magnetic field in the vicinity of the forward end of the cathode (12) and also a a coil power source (32) is disposed for dc energization of the coil (30).
On this embodiment, while a plasma arc (16) is generated between the forward end of the cathode (12) and an object to be heated, in the deenergized condition of the coil (30), the plasma working gas (14) generated a stable plasma arc having a strong axial directivity in the vicinity of the cathode (12) (see Fig. 3a).
On the other hand, as shown in Fig. 14a, a current is flowing from the object (15) to be heated toward the cathode (12) within the plasma arc (16). When a dc current J is supplied to the coil (30) wound on the outer periphery of the nozzle (13), the magnetic flux of a dc magnetic field B links the current within the plasma arc (16). As a result, an electromagnetic force F, is generated within the plasma-arc (16).
The electromagnetic force F, which is given by $F = J X B$
, is generated in a direction tending to rotate about the axis of the plasma arc (16) so that the body of the plasma arc (16) starts rotating as shown by the arrows in Fig. 14b. Then, if the current path slightly deviates from the axially symmetric form, a whirling plasma is formed due to the superposition of a kink instability and the electromagnetic force F resulting from the linking of the external magnetic field B and the current in the plasma arc. The resulting condition is similar to the condition of Fig.3b.
Also, if the applied external magnetic field is weakened, due to the axial flow to the plasma working gas, a correcting force tending to form an axially stable plasma-arc comes into action and the transition from the whirling plasma to the stable plasma takes place again.
In this way, by adjusting the strength of the external magnetic field, it is impossible to effect the switching between the whirling plasma and the stable plasma, the control of the whirling velocity and so on without changing the flow rate or the manner of flowing of the plasma working gas.
In this connection, the diameter of the cathode (12) was set to 20mm and the flow rate of the plasma working gas (14) was set to 30 ℓ/min. Also, the experimental generation of plasma was performed by selecting the number of turns of the coil (30) to be 300 and the plasma current to be dc 900 A, using carbon black for the anode and selecting the gap between the anode (not shown) and the cathode (12) to be in the range from 100 to 200mm.
As a result, when the excitation current was lower than dc 3A, a straight and stable plasma arc (16) was formed, whereas when it was higher than dc 4A, a plasma arc was formed which spread from near the cathode (12) toward the anode and whirled at a high velocity. Also, with the electrode gap of 200mm, the generated voltage was increased to 340V in the case of the whirling plasma as compared with the generated voltage of 180^oV in the case of the stable plasma. In addition, where the excitation current of the coil (30) was between 3 and 4A, there was also the generation of a plasma arc (16) whose downstream portion (anode side) alone was spread and which was low whirling. In this way, by utilizing the continuously varying excitation current of the coil (30), it was possible to control the whirling plasma freely with an improved repeatability. Further, it had been known that there was the effect of reducing the deflection of the plasma arc due to the external magnetic field produced by the power supply circuit or the like.
It is to be noted that while, in the above-described embodiment, the coil (30) is wound on the outer side of the nozzle (13), it may be incorporated in the body portion of the nozzle (13). Also, the mounting position of the coil (30) may be off the forward end of the cathode (12), that is, it is only necessary that the coil (30) is mounted in a position so that the magnetic field produced by the coil (30) links the plasma (16), and the shape of the cathode (12) need not be formed into a hemispherical shape as shown in the Figure. In short, it is only necessary that the magnetic flux produced by the coil (30) arranged to center on the torch axis is caused to link the axially radiated plasma arc and that the strength of the magnetic field generates an electro magnetic force which overcomes a correcting force generated in the axial direction of the plasma-arc in accordance with the shape of the electrode, the gas quantity, the shape of the nozzle, etc.
In relation to the embodiment of Fig. 13, the transferred plasma-arc torch of the embodiment shown in Fig. 15 differs in that a steel conduit(34) is placed between the nozzle (13) and the coil (30) and the forward end of the cathode (12)is retracted as compared with the forward end of the nozzle (13).
As in the above-described embodiment of Fig. 13, when a current is supplied to the coil (30) wound on the outer periphery of the nozzle (13), a magnetic field B links the plasma current and thus an electromagnetic force F is generated.

$F = J X B$
This electromagnetic force functions so that the plasma column whirls about the torch axis, and when the plasma colomn is deformed by this electromagnetic force, a kink instability formed by the magnetic pressure of the current itself and an electromagnetic force formed by the magnetic field and the plasma current are superposed, thereby forming a fluidly instable plasma.
Then, when a current is supplied to the coil (30), the magnetic material member disposed on the inner side of the coil (30) or the conduit (34) is magnetized and the magnetic flux produced by the conduit (34) disturbs the magnetic field of the coil (30). This point will be described in greater detail.
The magnetic field distribution of the coil (30) without the conduit (34) becomes as indicated by the characteristic diagram shown in Fig. 16b, whereas the magnetic field distribution of the coil (30) with the conduit (34) becomes as indicated by the characteristic diagram of Fig.16b. While the magnetic field produced in the central portion is increased as compared with that at the end point along the axis when the current is supplied to the coil (30) without the presence of the conduit (34), where the conduit (34) is present on the innerside of the coil (30), a magnetic field tending to cancel the magnetic field by the coil (30) is produced on the inner side of the conduit (34) and therefore the magnetic field is increased at the end portion along the axis as compared with the magnetic field in the central portion of the coil (30)
As a result, in the case of the coil (30) provided with the conduit (34) the magnetic field at the forward end of the nozzle (13) is strengthened as compared with the case of a coil without the conduit (34) and the number of flux lines linking the plasma current is increased. Thus, by arranging the conduit(34) or the magnetic material member, it is possible to greatly reduce the number of turns in the coil(30) and the coil current.
Also, where the conduit (34) is arranged on the inner side of the coil (30), as shown in Fig.16b, the magnetic field is reduced considerably as compared with the outerside of the coil (30), whereas if the cathode spot is on the inner side of the conduit (34), the stable cathode spot is formed and the plasma column near the cathode is also formed stably, thereby making the entrapment of the outside air not easy and making it effective to prevent the oxidation of the cathode.
With this embodiment, its operation was confirmed as follows. The conduit (34) was 2mm in thickness, 100mm in length and 45mm in inner diameter, and the forward end of the cathode (12) was retracted by 10mm from the forward end of the nozzle (13). Then, a plasma arc was generated by selecting the number of turns of the coil to be 1500, the gap between the forward end of the torch (11) and the carbon anode to be 100mm, the plasma current to be dc 900A and the flow rate of the plasma working gas to be 100 ℓ/min. Firstly, in a condition where no current was supplied to the coil (30), a stable plasma was generated with the generated voltage of 101V
and the power input of 91 KW.
Then, when a current was supplied to the coil (30) and the coil current was increased gradualy, it was observed that the plasma was disturbed so as to spread from the cathode (12) toward the anode (hence toward the object(15) to be heated) at dc 0.3A. The plasma column started spreading at a distance on the anode side of about 10mm from the forward end of the nozzle (13) but it was always stable in the vicinity of the cathode spot. At this time, the generated voltage was 185 V and the power input was 167 KW. After about 2 hour's operation, no oxidation was confirmed by the observation of the surface of the cathode (12).
Then, the similar test was performed by substituting SUS 304 (nonmagnetic material member) for the iron conduit (34) on the outer side of the nozzle (13). After a stable plasma had been formed, when a current was supplied to the coil (30) and the coil current was increased gradually, it was observed that the plasma was disturbed so as to spread from the cathode (12) toward the anode at dc 1.5A; at this time, the generated voltage was 187 V and the power input was 168 KW. On the other hand, after the operation had been performed for about 2 hours, from the observation of the surface of the cathode (12) it was confirmed that the forward end was consumed violently and was oxidized black.
As the result of the forgoing operation confirmation, it was confirmed that due to the use of the magnetic material member, the power consumption of the coil(30) was reduced to about 1/5 of the embodiment of Fig.1 thereby ensuring a high degree of economic effect and a great reduction the size of the coil.
It is to be noted that as regards the mounting position of the iron conduit (30) or the magnetic material member, it need note on the outerside of the nozzle (13) so that by arranging its inside or on the inner side of the nozzle (13) and the magnetic field linking the plasma current is strengthened and the magnetic field generating section is further reduced in size. Particularly, under such conditions where there is no danger of oxidation consumption even if the outside air is entrapped, the magnetic field generating section can be further reduced in size by arranging the member on the back of the cathode. Even if the magnetic material member is arranged on the outer side of the coil(30), the magnetic field at the nozzel forward end can be strengthened and its mounting place can be determined freely depending on the construction of the torch.
In the transferred plasma-arc torch of the embodiment shown in Fig. 17, a coil (38) is mounted on the forward end of the nozzle (13) to form an alternating field near the forward end of the electrode (12) and also a coil power supply (40) is provided for ac excitation of the coil (38).
In this embodiment, while a plasma arc is generated between the forward end of the cathode (12) and an object to be heated, in the non excited condition of the coil (38) the plasma working gas (14) forms a stable plasma arc (Fig. 3a) having a strong axial directivity in the vicinity of the cathode (12).
On the other hand, the current flows from te object (15) to be heated toward the cathode (12) within the plasma arc (16). At this time, if an ac current is supplied to the coil(30) wound on the outer periphery of the nozzle (13), the magnetic flux of an ac magnetic field B links the current J in the plasma arc (16). As a result, an electromagnetic force F is generated within the plasma arc (16).

$F = J X B$
This electromagnetic force alternately turns clockwise and counterclockwise at a frequency f about the axis of the plasma column and the plasma column proper starts reversing. Then, when it deviates slightly from the axially symmetrical form, a kink instability formed by the magnetic pressure of the current itself as shown in Fig. 18b and an electromagnetic force formed by the ac magnetic field and the plasma current are superposed, thereby forming a reversing plasma as shown in Fig. 3b.
By adjusting the coil current and the frequency, the turning force and the period of reversal of the reversing plasma can be varied as desired so that the controls corresponding to the operating conditions ranging or the stable plasma as shown in Fig. 3a to the reversing plasma as shown in Fig. 3b are made possible with the plasma working gas being reduced in amount and maintained constant.
With the reversing plasma formed by the foregoing action, no whirl or vortex of a fixed direction is produced so that any area of a lower pressure than the surrounding portions tends to be formed less easily and the entrapment of the outside air is prevented.
Then, with the above-mentioned transferred plasma-arc torch, the diameter of the cathode (12) was selected to be 20mm and argon was used for the plasma working gas (14) whose flowrate was set to 30 ℓ/min. Also, the number of turns of the coil (38) and the plasma current were respectively selected to be 1500 and dc 900 A, carbon black was used for the anode, and the gap between the anode (not shown) and the cathode (12) was set to the range from 10 to 200mm.
Where no current was supplied to the coil (38), as shown in Fig. 32a, a stable plasma was formed with the generated voltage of 159 V and the power input of 143 KW. Then, when an ac current of 50 Hz was supplied to the coil (38), it was observed that the plasma started dusturbing so as to spread from the cathode toward the anode in the vicinity of the carbon anode as shown in Fig. 3b. The position at which the plasma column started disturbing was moved to the cathode side as the coil current was increased and the generated voltage was correspondingly increased. The generated power at the coil current of ac 3.5A and 50Hz was 248 V and the input voltage was increased to 223 KW.
Then, the frequency dependency of the plasma generated voltage at the time of the coil current of 3 A was examined. When the frequency of the coil current was selected 100 Hz, reversing plasma having a reduced fluidal disturbance was formed and the generated voltage was 185 V. Further, as the frequency was decreased, the disturbance of the plasma was increased and the generated voltage reached 250 V at 35 Hz.
As described hereinabove, by suitably selecting the current and frequency of the coil, it was possible to more freely form a reversing plasma with an improved repeatability. Also, it was confirmed that no oxidation on the electrode surface was observed and the measure was extremely effective from the electrode consumption prevention point of view.
Then, it is confirmed that the suitable range for the frequency of the ac magnetic field is from 0.5 Hz to 300Hz. The frequency of less than 0.5 Hz is unsuitable on the ground that the rate of reversion is retarded depending on the frequency and that the outside air is entrapped giving rise to a problem of oxidation consumption. On the other hand, the frequency of over 300 Hz is not suitable on the ground that the impedance and eddy current of the coil become excessively large thus increasing the power consumption.
Also, while, in the above-described embodiment, the coil (30) is wound on the outer side of the nozzle (13), it may be incorporated within the body of the nozzle (13) as in the case of the dc coil (30). Further, the mounting position of the coil (38) may be off the forward end of the cathode (12), that is, it is only necessary that the coil (38) is disposed in such position that the magnetic field produced by the coil (38) links the plasma (16), and also the shape of the cathode (12) need not be hemispherial as shown in the Figure. In short,it is only necessary that the magnetic flux of the coil (38) arranged around the torch axis links the axially radiated plasma arc, and the intensity of the magnetic field is only necessary that anelectromagnetic force is produced which overcomes a correcting for generated in the axial direction of the plasma arc in accordance with the shape of the electrode, the gas amount, the shape of the nozzle, etc.
In relation to the embodiment Fig. 17, the transferred plasma-arc torch of the embodiment shown in Fig. 19 differs in that the steel conduit (34) is interposed between the nozzle (13) and the coil (38) and the forward end of the cathode (12) is retracted relative to the forward end of the nozzle (13).
In the like manner as the previously mentioned embodiment, when a current is supplied to the coil (38) wound on the outer periphery of the nozzle (13), a magnetic field B links the plasma current and an electromagnetic force F is generated.

$F = J X B$
This electromagnetic force functions so that the plasma column reverses about the torch axis; and when the plasma column is deformed by this electromagnetic force, a kink instability produced by the magnetic pressure of the current itself and the electromagnetic force produced by the magnetic field and the plasma current are superposed and a fluidly instable plasma is formed.
Then, the conduit (34) or the magnetic material member disposed on the inner side of the coil (38) is magnetized by the current supplied to the coil (38) and the magnetic flux produced by the conduit (34) disturbs the magnetic field of the coil (38). This point is the same as in the case of the embodiment Fig. 15 and its explanation will be omitted.
With this embodiment, its operation was confirmed as follows. The conduit (34) was 2mm in thickness, 100mm in length and 45mm in inner diameter, and the forward end of the cathode (12) was retracted relative to the forward end of the nozzle (13). A plasma arc (16) was generated by selecting the number of turns of the coil to be 1500, the plasma current to be dc 900 A and the flow rate of the plasma working gas (14) to be 100 ℓ/min. Firstly, a stable plasma was formed in the condition where no current was supplied to the coil (38) and the generated voltage and the power input were respectively 101V and 91 KW.
Then, when a current was supplied to the coil (38) so that the coil current was increased gradually, it was observed that the plasma was disturbed so as to spread from the cathode (12) toward the anode (and hence toward the object (15) to be heated) at an ac of 0.6A and 50 Hz. The plasma column started to spread at a distance of about 10mm on the anode side from the forward end of the nozzle (13) but it was always stable in the vicinity of the cathode spot. At this time, the generated voltage was 185 V and the power input was 167 KW. After the operation for about 2 hours, the surface of the cathode (12) was observed and no oxidation was confirmed.
Then, the material of the steel conduit (13) on the outer side of the nozzle (13) was replaced with SUS 304 (no magnetic material member) and the same test was performed. After a stable plasma had been formed, a current was supplied to the coil (38) so as to increase the coil current gradually, and it was observed that the plasma was disturbed to spread from the cathode (12) toward the anode by the ac of 3.0A; at this time, the generated voltage was 187 V and the input power was 168 KW. Also, after the operation for about 2 hours, the surface of the cathode (12) was observed and it was confirmed that the forward end was oxidized to become black.
By virtue of the above-mentioned confirmation, it was confirmed that the use of the magnetic material member had the effect of reducing the power consumption of the coil (38) to about 1/5 of that of the embodiment of Fig. 17 and ensuring both a high degree of economic effect and a considerable reduction in the size of the coil. It is to be noted that the mounting position of the steel conduit (34) or the magnetic material member need not be on the outer side of the nozzle (13) and it may be suitably changed according to the object of the present invention as in the case of the embodiment of Fig. 15.
In the transferred plasma-arc torch of the embodiment shown in Fig. 20, the steel conduit(34) of 15mm in width and 1mm in thickness is embedded in the inner wall of the forward end of the nozzle nozzle (13) over the whole surface thereof. The forward end of the cathode (12) is restricted by 5mm relative to the forward end of the nozzle (13). A pluarity of coils (42) are distributed and arranged in the similar construction to the stator windings of an induction machine on the outer side of the nozzle (13) in such a manner that a rotating magnetic field symmetric with the torch axis is produced toward the carbon black anode (not shown) from the forward end of the cathode (12). In other words, the plurality of coils are arranged around the torch axis and currents of different phases are respectively supplied to the coils, thereby applying a rotating magnetic field to the plasma column. In this embodiment, as shown in Fig. 21, the coils are constructed in the similar construction of the stator windings of a three-phase induction machine and the number of turns of each coils are arranged to apply a rotating magnetic field, the object of the present invention can be accomplished if the rotating magnetic field contains a vertical field component of a certain magnitude and therefore the rotating magnetic field need not absolutely be vertical with respect to the torch axis. The rotational speed of the rotating magnetic field is controlled by an inverter (44) which functions through the coils (42) and a rotating magnetic field generating power source (46).
A plasma arc (16) was formed by selecting the gap between the forward end of the torch (11) and the carbon anode to be 200mm, the plasma current to be dc of 900 A and the plasma working gas (14) to be an argon gas of 30 ℓ/min in flow rate.
Firstly, in the condition where no current was supplied to the coils (42), a stable plasma was formed with the generated voltage of 152 V and the power input of 137 KW.
Then, when three-phase ac currents were supplied to the terminals u, v, and w, of the coils (42), it was observed that the plasma was disturbed so as to spread from the cathode (12) toward the anode (and hence toward the object (15) to be heated). At this time, the rotational speed of the rotating magnetic field was 1500 rpm. Also, the plasma column started disburbing at a distance of about 15mm on the anode side from the forward end of the nozzle (13), and at this time the generated voltage and the power input were respectively 290V and 261KW. In this connection, where the forward end position of the cathode (12) was selected to be equal to the forward end position of the nozzle (13), the plasma column already started disturbing in the vicinity of the cathode spot.
Further, when the rotational speed of the rotating magnetic field was varied by the inverter (44), there was a tendency that the generated voltage was increased with increase in the rotational speed and conversely the generated voltage was decreased with decrease in the rotational speed. Also, the generated voltage was increased with increase in the coil current, and the generated voltage was decreased with decrease in the coil current thus providing a stable plasma form.
With the foregoing embodiment, by suitably selecting and controlling the coil current and its frequency (several Hz to several hundreds Hz), it was possible to freely form a fluidly instable plasma with improved repeatability. In particular, it was possible to produce a plasma arc capable of spreading with a high degree of symmetry with the torch axis.
Also, a comparison between the cases where the conduit (34) was embedded in the inner wall of the forward end of the nozzle (13) and where no conduit (34) was embedded showed that after the operation for 100 hours, in the case of the former no oxidation of the cathode surface was observed in contrast to the latter and that there was an electrode consumption preventing effect.
It was confirmed that the use of the rotating magnetic field had the effect of reducing the power consumption of the coils (42) to about 20 W or to less than half the previously and ensuring and a high degree of economic effect and a considerable decrease in the size of the rotating magnetic field generating section.
It is to be noted that the mounting positions of the coils(42) need not be such that a rotating magnetic field is always produced in the vicinity of the cathode forward end and it is needless to say that any positions can be used provided that the magnetic field links the plasma. As a result, a plurality of coils may be arranged on the outer side near the torch forward end in such configuration as the stator windings of an induction machine as shown in Figs. 22a and 22b or alternatively it is possible to arrange a set of coils of such construction that magnetic fluxes concentrate on te outer side from the torch forward end as shown in Figs. 23a and 23b. It is to be noted that in the case of the later, the coills on the plasma torch forward end side can be bent or inclined toward the cathode side such that the position at which the rotating magnetic field crosses the plasma current at right angles can be moved to one which is relatively near to the cathode.
Further, while, in the above-described embodimnent, the coils are arranged on the outerside of the nozzle (13) to produce a rotating magnetic field, the similar effect can be obtained By in corporating the coils in the body of the nozzle (13).

Claims

A transferred plasma-arc torch for generating a plasma arc between a nozzle and an object to be heated, said torch comprising:
means for whirling or reversing or vibrating said plasma arc.
A transferred plasma-arc torch as set forth in claim 1, wherein an electron emission-type cathode is used as a cathode of said nozzle.
A transferred plasma-arc torch as set forth in claim 2, wherein a forward end of said electron emission type cathode is formed into a hemispheric shape.
A transferred plasma-arc torch as set forth in claim 2, further comprising gas supply means for supplying a plasma working gas includig radial flows around said electron emission-type cathode.
A transgferred plasma-arc torch as set forth in claim 4, further comprising shielding means for shielding a surface of said electron emission-type cathode from an atmosphere gas to suppress oxidation consumption thereof.
A transferred plasma-marc torch as set forth in claim 2, further comprising gas supply means for supplying a plasma working gas around said electron-emission-type cathode in parallel to an axial direction of said nozzle.
A transferred plasma-arc torch as set forth in claim 6, further comprising an electromagnetic coil mounted on a forward end of said nozzle for generating a dc magnetic field in an axial direction of said nozzle.
A transferred plasma-arc torch as set forth in claim 7, further comprising a magnetic material member mounted at least on one of an inner and outer side of said electromagnetic coil.
A transferred plasma-arc torch as set forth in claim 8, wherein said electron emission-type cathode is restracted relative to said magnetic material member.
A transferred plasma-arc torch as set forth in claim 6, further comprising an electromagnetic coil mounted on a forward end of said nozzle for generating an ac magnetic field in an axial direction of said nozzle.
A transferred plasma-arc torch as set forth in claim 10, further comprising a magnetic material member mounted at least on one of an inner and outer side of said electromagnetic coil.
A transferred plasma-arc torch as set forth in claim 11, wherein forward end of said electron emission-type cathode is restracted relative to said magnetic material member.
A transferred plasma-arc torch as set forth in claim 12, wherein the frequency of said ac magnetic field is set to a range from 0.5 Hz to 300 Hz.
A transferred plasm-arc torch as set forth in claim 6, further comprising a plurality of electromagnetic coils for generating a rotating magnetic field perpendicular to an axial direction of said nozzle.
A transferred plasma-arc torch as set forth in claim 14, further comprising a magnetic material member mounted at least on one of an inner and outer side of said electromagnetic coils.
A transferred plasma-arc torch as set forth in claim 15, wherein said electron emission-type cathode is arranged in a position retracted relative to said magnetic material member.