JPH03205796A - Transition type plasma torch - Google Patents

Abstract

PURPOSE:To improve the controllability of a plasma jet, miniaturize a plasma torch, and reduce the breakage of a nozzle or the like by applying the rotary magnetic field to a plasma column from the outside. CONSTITUTION:Multiple pairs of coils 10 are arranged into the structure like the stator winding of an inductor toward a carbon black anode from the tip of a cathode 2 on the outside of a nozzle 3 so that the rotary magnetic field symmetrical with respect to a torch shaft. Multiple pairs of coils 12 are arranged centering the torch shaft, currents with phase differences are fed to them, and the rotary magnetic field is applied to a plasma column. The rotating speed of the rotary magnetic field is controlled by an inverter 11 via the coils 10 and a rotary magnetic field generating power source 12. The high controllability of the plasma column is obtained, a plasma torch is miniaturized, and the operating cost can be reduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a transfer type plasma torch that generates a plasma jet and is used for heating, melting, reactions, etc. Thermal plasma is an ultra-high-temperature, high-energy source that is widely applied to heating and melting, as well as reactions and surface treatments. In particular, thermal plasma has a high energy density, so the equipment can be made compact, and the atmosphere can be freely controlled by the plasma working gas, and the input power can be adjusted freely and responsively by controlling the current and plasma length. It has characteristics. Taking advantage of these characteristics, in recent years in the steelmaking process, it has been considered to heat the molten steel in a ladle or tundish with plasma to control the molten steel temperature and perform refining reactions. In these applications, transition type plasma torches are used in which the object to be heated is the opposite electrode due to heating efficiency. [Prior art] As shown in Fig. 5, in the conventional transfer type plasma torch, from the viewpoint of directivity and stability, the flow velocity of the plasma working gas in the axial direction of the torch is increased to create a stable linear plasma jet. However, the applicant invented a transfer type plasma torch that takes advantage of the inherent magnetic instability of plasma, and previously filed patent application No. 1-28735.
The application was filed as No. This transfer type plasma torch makes the plasma jet fluidly unstable, allows it to rotate at high speed, and forms plasma that spreads radially toward the object to be heated. This method not only increases the actual plasma but also increases the cooling effect on the plasma column.
Effects such as an increase in generated voltage and an increase in heating area can be obtained.
Methods for forming such a plasma include a method of imparting a velocity component in the circumferential direction around the torch axis to the plasma working gas and a method of applying a magnetic field to the plasma column from the outside. In the latter method, the electromagnetic force generated by the interlinkage between the applied magnetic field and the current flowing through the plasma column causes the plasma column to rotate or reverse, causing and promoting magnetic instability.According to this method, , the gas consumption is smaller than the above method using plasma working gas,
It is advantageous in that it is highly controllable. Specifically, there is a method of applying a DC magnetic field that is rotationally symmetrical to the torch axis (Japanese Patent Application No. 1-121483), and a method of applying an AC magnetic field that also makes it possible to prevent cathode oxidation. [Problem to be solved by the invention J] However, in the case of making a plasma jet fluidly unstable, rotating it at high speed, and forming a plasma that spreads radially toward an object to be heated, it is difficult to apply a DC magnetic field or an AC magnetic field. If this method is adopted, a coil must be installed around the outside of the nozzle in the direction of the torch axis. At this time, a problem arises in that most of the magnetic flux is in the same direction as the current flowing in the plasma column and does not contribute effectively to the formation of electromagnetic force. In order to solve this problem, it would be possible to increase the number of turns in the coil or increase the current flowing through it, but this would lead to an increase in power consumption and an increase in the size of the torch, resulting in a decrease in overall thermal efficiency and ancillary equipment such as coils. Cooling system W)
Increase in size. Problems such as physical restrictions on the installation location may arise. On the other hand, by installing the above-mentioned coil in a direction perpendicular to the torch axis direction, it is possible to increase the magnetic flux that effectively contributes to the formation of electromagnetic force and to downsize the coil. A decrease in symmetry cannot be avoided. This decrease in axial symmetry is not desirable in practice, as it increases the risk of damage to surrounding components such as the nozzle, and reduces controllability of heating, melting, etc. Furthermore, when a swirling plasma is obtained, a region with lower pressure than the surrounding area occurs near the cathode. This is particularly noticeable when swirling plasma is obtained by applying a DC magnetic field. In such cases, outside air is drawn into this region, causing oxidation and wear and tear of the cathode. Furthermore, in order to form a plasma that rotates at high speed and spreads radially toward the object to be heated without damaging the nozzle or oxidizing the cathode due to the entrainment of outside air, it is necessary to It is desirable that the plasma be stable at a fixed end and fluidly unstable outside the torch. The present invention was made to solve the above problems, and achieves high controllability of plasma flow, miniaturization of plasma torch, and reduction in operating cost, and at the same time prevents oxidation consumption of the cathode. The purpose is to obtain a transfer-type plasma torch that makes it possible. [Means for Solving the Problems] In order to achieve the above object, in the transfer type plasma torch according to the present invention, a rotating magnetic field is applied to the plasma column. Also, the cathode tip was moved back from the nozzle tip.
Furthermore, a member made of magnetic material was placed around the cathode. [Operation] As shown in Figure 6, a so-called plasma current (I) flows inside the plasma column from the anode (and the object to be heated) to the cathode of the torch. Once a plasma column bends, the magnetic field created by the plasma current weakens in convex parts and strengthens in concave parts, creating magnetic pressure, which inherently causes it to bend further (kink instability). ing. In order to make the plasma jet fluidly unstable, rotate it at high speed, and form a plasma that spreads radially toward the object to be heated, it is necessary to reduce the correction force of the plasma jet that restrains the kink instability of the plasma. All you have to do is weaken it in an appropriate way and make it into a swirling plasma. Now, as shown in Figure 2, a set of coils, such as stator windings in an induction machine, are placed around the outside of the nozzle. !
do. By passing current through this set of coils, we obtain the rotating magnetic field ( B (t)) at time t. This rotating magnetic field interlinks with the plasma current substantially perpendicularly,
Generates an electromagnetic force (F) that bends the plasma column. The electromagnetic force expressed as F=IXB(t) works while rotating around the axis of the plasma column because the rotating magnetic field changes with time. If the plasma column deviates even slightly from its axial symmetry, it becomes fluidly unstable due to the superimposed effect of its own King instability and the electromagnetic force, as shown in Figure 2 (Cb). The shape and generated voltage of such fluidically unstable plasma can be changed by adjusting the coil current and frequency. Using this property, the third
From the stable plasma state shown in Figure 18l, to the fluidly unstable plasma state shown in Figure 3, etc., in which the plasma jet rotates at high speed and spreads radially toward the heated object This makes it possible to control operations according to operating conditions. It is also easy to create a rotating plasma that is axially symmetrical about the torch axis. Moreover, it is possible to operate with a constant amount of plasma working gas. Furthermore, since the rotating magnetic field acts approximately orthogonally to the plasma current, even if the coil current is small, or at least the number of turns of the coil, it can sufficiently contribute to fluid destabilization of the plasma column. In cases such as when swirling plasma occurs, the pressure near the cathode is lower than the surrounding area, causing outside air to be drawn in and the false cathode to be oxidized and consumed. To prevent this, the cathode tip is set back from the nozzle tip so that the nozzle surrounds the cathode. This makes it possible to minimize oxidative consumption of the cathode. Plasma torches are often made of materials with high thermal and electrical conductivity, such as copper. When a rotating magnetic field is applied from around the nozzle, eddy currents are generated in the nozzle. The magnetic field generated by this eddy current and the rotating magnetic field cancel each other out near the axis (near the cathode) inside the torch, and are significantly smaller than outside the torch. For this reason, a stable plasma is formed near the cathode spot, and there is a strong tendency for the plasma to become fixed-end.
This tendency is particularly strong when the cathode tip is set back from the nozzle tip and the nozzle is structured to surround the cathode. However, depending on the rotation speed of the rotating magnetic field, the structure of the torch, the material of the torch, etc., the magnetic field near the cathode may not be weakened sufficiently. In such cases, and not only in such cases, a member made of magnetic material is installed around the cathode to shield the magnetic field from the outside. Then, a more stable plasma inside the torch,
A fluidically unstable plasma is formed outside the torch. [Embodiment of the Invention] FIG. 1 is a configuration explanatory diagram showing an embodiment of the present invention. The cathode 2 connected to the electric terminal 9 is a tungsten electrode, which is a type of thermionic emission type cathode, and has a diameter of 20 mm.
It has a pointed shape. The cathode material may be carbon, and the cathode shape may be hemispherical or semicylindrical in cross section. The back surface of the cathode 2 is made of copper and is cooled by cooling water 8. The nozzle 3 is made of cylindrical copper and is cooled by cooling water 8. The nozzle 3 and the cathode 2 are separated by an insulating spacer 7. Plasma working gas 4 is supplied to nozzle 3
It is supplied from inside. Note that the nozzle 3 is a conventional plasma torch 1 shown in FIG.
It does not have a structure like the nozzle 13 seen in 1. That is,
The nozzle l3 has a structure that converges the plasma working gas 14 to form a stable plasma jet 16 and increases its speed in the torch axial direction. In order to prevent wear and tear, the cathode 2 is surrounded by inert gas (argon gas), so the distance between the tip of the cathode 2 and the nozzle 3 is widened. The inner wall at the tip of the nozzle 3 has a width of 15 mm. Thickness 1
A steel plate (not shown) with a diameter of mm is embedded all around the circumference. The tip of the cathode 3 is set back 5 mm from the tip of the nozzle 3. On the outside of the nozzle 3, a rotating magnetic field symmetrical about the torch axis is obtained from the tip of the cathode 3 toward the carbon block anode (not shown). A plurality of sets of coils 10 are arranged in a structure similar to the stator winding of an induction machine. That is, a set of multiple coils is arranged around the torch axis, currents with phase differences are passed through them, and a rotating magnetic field is applied to the plasma column. In this example, the structure is similar to the stator winding of a three-phase induction machine (Fig. 1(b)).
), and each coil has 3 turns. still,
When applying a rotating magnetic field by arranging multiple sets of coils as described above, the rotating magnetic field does not need to be perpendicular to the torch axis. The purpose of the present invention can be achieved as long as there is a perpendicular magnetic field component that is large to a certain extent, and the purpose of this invention is not to exclude cases where the rotating magnetic field is present. The rotation speed of the rotating magnetic field is controlled by the inverter 11 via the coil 10 and the rotating magnetic field generating tllW12.
It was controlled by The distance between the tip of torch 1 and the carbon anode is 200 mm.
．． A plasma jet 6 was generated with a plasma current of 900 A DC and a plasma working gas 4 of argon gas at a flow rate of 30 n/sin. First, when no current is applied to the coil, a stable plasma is formed and the generated voltage is 1.
52V. The input power was 137kW. Next, when a three-phase alternating current was applied to the u, V, and W terminals of the coil, it was observed that the plasma was disturbed so as to spread from the cathode 2 toward the anode (therefore, toward the heating object 5). At this time, the rotation speed of the rotating magnetic field was 1500 rpm. Also, the plasma column started to be disturbed at about 15 minutes from the tip of nozzle 3. mm on the anode side, and the generated voltage at that time is 290V. The input power was 261kW. By the way, when the tip position of cathode 3 was made the same as the tip position of nozzle 3, the plasma column already began to be disturbed near the cathode point. Furthermore, when the rotation speed of the rotating magnetic field was changed using the inverter 11, it was found that as the rotation speed increased, the generated voltage decreased, and conversely, as the rotation speed decreased, the generated voltage tended to increase. Furthermore, as the coil current increased, the generated voltage increased, and as the coil iJ current increased, the generated voltage decreased, resulting in a stable plasma shape. In the above embodiments, the coil current and its frequency (several H
By appropriately selecting and controlling the frequency (from z to several hundred kHz), it was possible to form a fluidically unstable plasma freely and with good reproducibility. In particular, a wide plasma jet with high torch axis symmetry can be obtained. We also compared the case where an iron plate was embedded in the inner wall of the tip of nozzle 3 and the case where it was not. After driving for 100 hours,
Unlike the latter, no oxidation of the cathode surface was observed in the former, and it was also confirmed that the former had the effect of preventing electrode wear. By adopting a rotating magnetic field, the power consumption of the coil was reduced to about 20 W, or less than half of the conventional one, and we were able to confirm that it was highly economical and that the rotating magnetic field generator could be significantly smaller than that of conventional DC to AC magnetic field systems. ．． Regarding the installation position of the coil. It goes without saying that the position does not have to be such that the rotating magnetic field is always generated near the cathode tip, and that the same effect can be obtained as long as the magnetic field can be linked to the plasma. Multiple sets of coils shaped like the stator windings of an induction machine may be installed outside near the tip of the torch, or the 41st mTo
It is also possible to install a set of coils with a structure in which the magnetic flux concentrates outward from the tip of the torch, as shown in +. In the latter case, by curving or tilting the coil at the tip of the plasma torch toward the cathode, the position where the rotating magnetic field is orthogonal to the plasma current can be brought relatively close to the cathode. Also, in the above embodiment. Although a set of coils is placed outside the nozzle 3 to obtain a rotating magnetic field, the same effect can be obtained by incorporating the coil into the nozzle 3 body. [Effects of the Invention] As described above, according to the present invention, since a rotating magnetic field is applied to the plasma column from the outside, the plasma jet, which is fluidly unstable and rotates at high speed, can be spread radially toward the object to be heated. Of course, the magnetic field acts effectively on the plasma current and maintains the axial symmetry of the plasma jet. Therefore, it goes without saying that it is possible to obtain effects equivalent to the conventional technique of obtaining swirling (inverted) plasma by applying a DC or AC magnetic field, such as an increase in the actual plasma, an increase in the generated voltage, and an increase in the heating area. In addition, ■The efficiency of forming electromagnetic force is improved, making it possible to miniaturize the magnetic generation part and, by extension, the plasma torch. ■Less damage to nozzles, etc. ■Reducing power consumption makes it possible to improve overall thermal efficiency, prevent the size of peripheral equipment, and solve physical problems such as installation locations, which in turn makes it possible to improve economic efficiency. ■More controllability. The following effects can be obtained. In addition, the cathode tip is set back from the nozzle tip, which prevents the cathode from being consumed by oxidation. Furthermore, because a member made of magnetic material is placed around the cathode, a stable plasma is formed inside the plasma torch, and a fluidically unstable plasma is formed outside the plasma torch. Therefore, damage to the nozzle, etc. and cathode oxidation consumption can be reduced, and it also contributes to high controllability.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the configuration of one embodiment of the present invention, especially To
l indicates the arrangement of a set of coils for generating a rotating magnetic field in this example. Figure 2 is a schematic diagram showing the principle of the present invention, Figure 3 (fat) is a cross-sectional view of stable plasma obtained when no current is flowing through the coil, and Figure 3 (Fig. 3) is the rotation obtained when current is flowing through the coil. Figure 4, a cross-sectional view of fluidically unstable plasma formed by a magnetic field, shows a specific example of the arrangement of the rotating magnetic field generator and the corresponding method of generating the rotating magnetic field. Figure 5 is an explanatory diagram of the configuration of a conventional plasma torch, and Figure 6 is an explanatory diagram of the principle of kink instability. The instruction symbols in the figure are as follows. 1, 1: Plasma torch 2, 12: Electrode (cathode) 3.13: Nozzle 4.14 = Plasma working gas 5.15: Heating object 6.16: Plasma jet 7.17: Insulating spacer 8.18: Cooling water 9.19: Electrical terminal 1 O: Rotating magnetic field generating coil 1 l: Imperter 1 2: Power supply for rotating magnetic field generation. Applicant Nippon Steel Tube Co., Ltd. Figure (a) Diagram (b) Φ 0 Figure 5

Claims (3)

[Claims] (1) A transfer type plasma torch that generates a plasma jet between a nozzle and an object to be heated, the transfer type plasma torch being characterized in that a rotating magnetic field is externally applied to the plasma column. (2) The transfer type plasma torch according to claim 1, wherein the cathode tip is set back from the nozzle tip. (3) The transfer type plasma torch according to claim (1) or (2), characterized in that a member made of a magnetic material is arranged around the cathode.