JPS6347549B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to starting a plasma arc in a plasma arc device used for heating, processing, melting, refining, etc. metals. In this type of plasma arc equipment, a pilot arc is started between the arc electrode and the conductor with the arc torch close to the object to be processed (object to be processed), and then the main arc is transferred between the arc electrode and the object to be processed. After the arc occurs and a main arc is formed, the arc torch is separated from the object to be processed at a predetermined distance, and the main arc is used to perform required processing. Generating the main arc is not easy, and the main arc cannot be generated unless the arc torch is brought very close to the object to be processed. Conventionally, when starting the arc, the arc torch is brought close to the object to be processed to generate the main arc, and then the arc torch is moved away from the object to be processed and set at a predetermined distance. Therefore, it is necessary to make at least one of the arc torch and the object to be processed movable, and to accurately set the distance between the two at the time of starting the arc and after starting the arc, which requires time and effort for mechanical positioning or There is a problem that the mechanism becomes complicated. In addition, since the arc torch is close to the object to be processed when the arc is started, there is a risk that the tip of the arc torch may be damaged by splashes from the object to be processed, and arc torches generally have a water-cooled structure. It is also dangerous if this is damaged. A first object of the present invention is to reduce the required forward and backward movement of the arc torch before and after arc activation, and a second object is to make the forward and backward movement substantially unnecessary. In order to achieve the above object, in the present invention,
When a pilot arc is generated, an inert gas flow is formed in the arc electrode section, and the inert gas supply amount and pilot arc current are determined so that the pilot arc becomes a long laminar plasma flame. When the inert gas supply amount is below a predetermined critical value Q ₀ , the plasma flame becomes a laminar flow, and as shown in Figure 2, the critical value Q ₀
The length of the plasma flame is several times that of the turbulent flow when the gas supply amount exceeds 5.
In laminar flow, the length is 30 to 40 cm, and it becomes even longer when the arc current is increased. In other words, a long laminar plasma flame is formed by keeping the inert gas supply amount below a critical value and increasing the arc current. As a result, a pilot arc (long plasma flame) is formed while the arc torch is placed at a distance required for machining by the main arc from the object to be processed, and then, after waiting for the main arc to occur, the arc torch and the object are separated. Machining can be performed using the main arc without changing the distance between the objects to be processed. Since the arc torch is separated from the object to be processed even when the pilot arc is started, the probability of damage to the arc torch due to splash etc. is reduced, and the arc torch can be set to the required processing distance before starting the arc. Setting up the arc torch becomes easier. FIG. 1 schematically shows an apparatus configuration for carrying out one embodiment of the present invention. The arc torch includes an arc electrode (cathode) 10, an insulating casing 12 surrounding it, and a conductor 14 that forms a part of the casing 12 and has a nozzle formed in a portion facing the lower end of the arc electrode 10. It is made up of. A pilot arc power source 16 including a high frequency voltage generator is connected between the arc electrode 10 and the conductor 14, and a main arc power source 18 is connected between the arc electrode 10 and the object 20 to be treated. Inert gas such as argon is introduced into the housing 12 from the gas introduction pipe 12a, and the introduced gas is
It flows out from the nozzle of the conductor 14 toward the object to be processed 20 . First, the distance between the arc electrode 10 and the object to be processed 20 is set to a distance required for processing by the main arc.
Next, the arc electrode 10 is
A starting voltage is applied between the conductor 14 and the conductor 14 by the power supply 16. Then, the pilot arc 22 is ignited, and a pilot plasma flame flows out from the nozzle of the conductor 14. The housing 12 is supplied with an inert gas at a low flow rate to make the pilot plasma a laminar flow, and the power supply 16 is supplied with a relatively large current sufficient to extend the laminar plasma flame to the object 20 to be treated. The electrode 10
- flowing between the conductors 14; As a result, arc electrode 1
A laminar plasma flame is created that extends from zero to the object 20 to be treated. Before and after the generation of this laminar plasma flame, a main arc generation voltage is applied between the arc electrode 10 and the object to be treated 20 by the power source 18. When the main arc generation voltage is applied before the generation of the laminar plasma flame, the main arc 24 is generated simultaneously with the generation of the laminar plasma flame. When the main arc generating voltage is applied after the laminar plasma flame is generated, the main arc 24 is generated simultaneously with the application of the main arc generating voltage. When a main arc occurs, the pilot power supply 16 is cut off,
The main arc 24 performs the required processing of the object to be processed. When the plasma gas formed by the pilot arc 22 is ejected from the nozzle as a pilot plasma flame, the flow rate of the gas supplied from the gas inlet 12a is throttled to make the plasma flame ejected from the nozzle of the conductor 14 into a laminar flow state. Significantly extends the arc starting limit length. FIG. 2 shows the length of the pilot plasma flame with respect to the plasma gas flow rate. When the gas flow rate falls below a certain value _Q0 , the plasma gas flame changes from turbulent flow to laminar flow, and its length increases rapidly. For example, when the flow is turbulent, the length is 5 cm, but when the flow is laminar, the length is 5 cm.
Changes to 30-40cm. This effect is further enhanced by increasing the current of the pilot arc power supply 16. As described above, the present invention makes it possible to keep the arc torch separated from the arc starting time by utilizing the length of the laminar plasma flame. Next, the specific arc starting procedure will be shown. The pilot power supply 16 is turned on and the pilot arc 22 is ignited. Reduce the plasma gas flow rate and extend the pilot plasma flame. The degree of elongation depends on the nozzle shape,
It also changes depending on the gas composition, pilot arc current, etc., but the first two are fixed; the gas flow rate is easy to adjust, and the pilot current is controlled as necessary (this is a laminar flow through thermal expansion of the gas inside the torch).
(involved in turbulence) to perform plasma flame extension. The main arc power supply 18 is turned on, and a no-load voltage (main arc starting voltage) is applied to the arc electrode 10 and the material to be heated 20. Increase the pilot arc current until the main arc starts. Pilot arc power supply 16
is an adjustable current power supply that includes, for example, a thyristor, and changes the firing angle of the thyristor according to the correlation between the set value and the detected current value to set the output current value as the set value. Increase the setting value to increase the pilot arc current. When a main arc occurs, the power supply 16 is cut off,
Increase and adjust the gas flow rate to the required value during processing. When a main arc occurs, a large current flows through the circuit of the main arc power supply 18, and the output voltage decreases. This is detected to detect the occurrence of the main arc. As described above, the present invention utilizes a long pilot plasma flame in a laminar flow state, starts the main arc with the plasma arc torch at the same distance as when processing, and performs processing at the same distance. Can be put into operation. A non-transferable plasma device, that is, a type that does not have a power source 18 and only uses a power source 16, and the gas supplied into the housing 12 is ionized by an arc between the arc electrode 10 and the conductor 12 and is ejected from a nozzle as a plasma jet. In this case, the plasma jet is made into a laminar flow to produce a long flame, but the nozzle is seriously damaged. In other words, if the gas supply is sufficient to create a laminar flow,
The inside of the nozzle is easily overloaded, causing overheating and damage.
In this regard, in a transfer type plasma device, the gas supply is increased and the pilot arc is extinguished after the main arc is ignited, so the nozzle is prevented from being overloaded and its life is extended. However, as mentioned above, there was a problem with the ignition of the main arc. However, in the present invention, laminar flow is used only at the time of starting, and arc starting can be performed which has the advantages of a transfer type plasma device and has no problems in starting. Next, examples will be given. Working gas is argon, nozzle diameter is 8mmφ
The no-load voltage of the main arc power supply is 400V,
The starting gas flow rate Q (N/min) and the output current A (A) of the pilot arc power source were selected as shown in Table 1 below. The state of the pilot plasma flame and the starting limit length L of the main arc were as shown in the table under each condition.

[Table] In other words, when the gas flow rate is 20/min and the plasma flame is made turbulent, the distance between the nozzle and the object to be treated is:
The main arc will not ignite unless it is less than 40mm or 60mm, but if the gas flow rate is set to 6N/min to make the plasma flame a laminar flow, this means that the pilot arc current
Extends to 200mm at 400A. In addition, the pilot plasma flame in laminar flow state is 300
mm to 400 mm, but this does not mean that the main arc activation limit length increases by this length. That is,
For the main arc to ignite, sufficient charged particles must be present in the arc path, which is determined by the voltage of the main arc power source, the resistance of the arc path, the pilot plasma current, etc. If a bimolecular gas alone or a mixture containing the bimolecular gas is used as the working gas, the main arc can be started more easily. As explained above, the present invention is highly advantageous in that it is possible to start up a transferred plasma arc device without moving the device.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an outline of the configuration of one apparatus for implementing the present invention, and FIG. 2 is a graph showing the relationship between plasma gas flow rate and plasma flame length. 10: arc electrode, 12: housing, 14: conductor, 12a: gas introduction pipe, 16: pilot arc power supply, 18: main arc power supply, 20: object to be processed, 22: pilot arc, 24: main arc.

Claims (1)

[Claims] 1. In starting a plasma arc that starts a pilot arc between an arc electrode and a conductor, and then generates a main arc between the arc electrode and an object to be processed; between the arc electrode and an object to be processed; A method for starting a plasma arc, which is characterized in that a pilot arc is started between an arc electrode and a conductor by forming an inert gas flow, and the pilot arc is a long laminar plasma flame. 2. Inert gas is supplied to the housing surrounding the arc electrode, which has a nozzle formed at a position facing the tip of the arc electrode, and a pilot arc is started between the arc electrode and the conductor to generate a laminar plasma flame. 2. The method of starting a plasma arc according to claim 1, wherein the plasma arc is caused to flow from a nozzle and then a main arc is generated between an arc electrode and an object to be treated.