GB2047509A

GB2047509A - High-power plasmatron

Info

Publication number: GB2047509A
Application number: GB7944200A
Authority: GB
Original assignee: Edelstahlwerk 8 Mai 1945 Freital VEB
Current assignee: Edelstahlwerk 8 Mai 1945 Freital VEB
Priority date: 1978-12-21
Filing date: 1979-12-21
Publication date: 1980-11-26
Also published as: SE7910466L; GB2047509B; FR2445089B1; ATA756779A; CH643417A5; SE432335B; FR2445089A1; DE2951121A1; YU302579A; AT374276B; DD151248A1; JPS5595300A; DE2951121C2

Abstract

To prevent nozzle destruction in a high-power plasmatron such as is used in plasma melting furnaces photodetectors (5) are provided which can recognize heavy-current subsidiary arcs and supply control signals to a protective control circuit. The photodetectors (5) are disposed in the gap (6) between the cathode (3) and the nozzle (4), behind the nozzle outlet aperture so that they overlook adjoining zones of the outlet aperture. <IMAGE>

Description

SPECIFICATION High-power plasmatron The invention relates to a high-power plasmatron, more particularly for use in plasma melting furnaces.

Whereas in plasmatrons for cutting, the cathode is disposed downstream of the nozzle, which is formed as a constricted duct to narrow the plasma-gas discharge, in highpower plasmatrons with an unpowered, watercooled nozzle, such as are mainly used in plasma melting furnaces, the cathode is disposed in the nozzle, and the arc is formed in front of the nozzle. Moreover, in high-power plasmatrons, in the interests of saving gas and to facilitate furnace construction, the specific gas flow rates, related to arc power and the nozzle duct cross-section are lower by a factor of about 10 than in the case of low-power plasmatrons used, for instance, for cutting.

Consequently, the arc-stabilizing and arc-insulating effect provided by the gas jacket established by the flowing gas flow is extremely low. High-power plasmatrons are also operated in furnaces with a hot-i.e., ionized furnace atmosphere. It is very probable that subsidiary arcs will occur and burn via the nozzle to the material being treated. If such subsidiary arcs are of very heavy current, for example one kA or more, the nozzles become molten in as short a time as even less than 0.1 second, whereupon the cooling water emerges from the nozzle, and the plasmatron ceases to function.The known electric monitoring devices for the nozzles of high-power plasmatrons are unsatisfactory, since either due to the small cool gas jacket no subsidiary arc can be recognized by a change in the electric parameters, or low-power subsidiary arcs, which are not dangerous to the nozzle, are not distinguished from high-power subsidiary arcs.

It is an object of the invention to provide a high-power plasmatron which is protected against nozzle destruction due to subsidiary arcs.

Another object of the invention is to enable the recognition of heavy-current subsidiary arcs, which quickly destroy the nozzles of high-power plasmatrons, so as to enable the taking of suitable steps to limit the effect of such arcs.

The invention consists in a high-power plasmatron comprising a pin-type cathode enclosed by a nozzle providing a gap for the passage of a plasma-forming gas, a plurality of photodetectors being disposed behind the nozzle outlet aperture in the gap between the cathode and the nozzle and so arranged that they overlook adjoining zones of the outlet aperture, the photodetectors serving for providing control signals for a protective control circuit of the plasmatron.

The photodetectors are preferably disposed so as far behind the cathode that a small angle of aperture is ensured for monitoring a partial zone of the nozzle outlet aperture by each photodetector. There should be a low surrounding temperature in the zones where the photodetectors are disposed. Although the photodetectors are facing the main arc, if subsidiary arcs occur between the cathode and the nozzle an additional light radiation is produced which can be detected by the photodetectors: This additional light radiation depends on the current strength of the subsidiary arc and may be utilized for further switching actions from a subsidiary arc power of 10% of the main arc power onwards.

When a heavy-current subsidiary arc is detected by one of the photodetectors, a control circuit in the power circuit of the plasmatron may be brought into operation in known manner, for immediately causing interruption of the subsidiary arcs by reducing the power of the main arc.

Advantageously, the photodetectors are disposed in the supply zone of the plasmaforming gas, and light-conducting cables are provided which extend from the nozzle gap to the photodetectors.

In order to make the invention clearly understood, reference will now be made to the accompanying drawing which is given by way of example and which is a diagrammatic sectional view of a plasmatron of the invention, shown disposed in a furnace.

A high-power plasmatron is disposed in a furnace 1. The plasmatron comprises a pintype cathode 3 mounted in a water-cooled retaining means 2 and surrounded by a watercooled nozzle 4. A nozzle gap is defined between the nozzle 4 and the cathode 3.

Photodetectors 5 are provided in the nozzle gap, their optical axes being directed towards the nozzle outlet aperture 6.

When a subsidiary arc occurs in the nozzle gap, an increase in light radiation is detected by at least one of the photodetectors 5. The electric signal from the said one photodetector is supplied in a suitable manner by known switching elements to a control circuit, which acts accordingly on the power source of the plasmatron to reduce the main arc current.

The material for melting in the furnace is denoted by the reference numeral 7 and the main arc by the reference numeral 8.

1. A high-power plasmatron comprising a pin-type cathode enclosed by a nozzle providing a gap for the passage of a plasma-forming gas, a plurality of photodetectors being disposed behind the nozzle outlet aperture in the gap between the cathode and the nozzle and so arranged that they overlook adjoining zones of the outlet aperture, the photodetectors serving for providing control signals for a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION High-power plasmatron The invention relates to a high-power plasmatron, more particularly for use in plasma melting furnaces. Whereas in plasmatrons for cutting, the cathode is disposed downstream of the nozzle, which is formed as a constricted duct to narrow the plasma-gas discharge, in highpower plasmatrons with an unpowered, watercooled nozzle, such as are mainly used in plasma melting furnaces, the cathode is disposed in the nozzle, and the arc is formed in front of the nozzle. Moreover, in high-power plasmatrons, in the interests of saving gas and to facilitate furnace construction, the specific gas flow rates, related to arc power and the nozzle duct cross-section are lower by a factor of about 10 than in the case of low-power plasmatrons used, for instance, for cutting. Consequently, the arc-stabilizing and arc-insulating effect provided by the gas jacket established by the flowing gas flow is extremely low. High-power plasmatrons are also operated in furnaces with a hot-i.e., ionized furnace atmosphere. It is very probable that subsidiary arcs will occur and burn via the nozzle to the material being treated. If such subsidiary arcs are of very heavy current, for example one kA or more, the nozzles become molten in as short a time as even less than 0.1 second, whereupon the cooling water emerges from the nozzle, and the plasmatron ceases to function.The known electric monitoring devices for the nozzles of high-power plasmatrons are unsatisfactory, since either due to the small cool gas jacket no subsidiary arc can be recognized by a change in the electric parameters, or low-power subsidiary arcs, which are not dangerous to the nozzle, are not distinguished from high-power subsidiary arcs. It is an object of the invention to provide a high-power plasmatron which is protected against nozzle destruction due to subsidiary arcs. Another object of the invention is to enable the recognition of heavy-current subsidiary arcs, which quickly destroy the nozzles of high-power plasmatrons, so as to enable the taking of suitable steps to limit the effect of such arcs. The invention consists in a high-power plasmatron comprising a pin-type cathode enclosed by a nozzle providing a gap for the passage of a plasma-forming gas, a plurality of photodetectors being disposed behind the nozzle outlet aperture in the gap between the cathode and the nozzle and so arranged that they overlook adjoining zones of the outlet aperture, the photodetectors serving for providing control signals for a protective control circuit of the plasmatron. The photodetectors are preferably disposed so as far behind the cathode that a small angle of aperture is ensured for monitoring a partial zone of the nozzle outlet aperture by each photodetector. There should be a low surrounding temperature in the zones where the photodetectors are disposed. Although the photodetectors are facing the main arc, if subsidiary arcs occur between the cathode and the nozzle an additional light radiation is produced which can be detected by the photodetectors: This additional light radiation depends on the current strength of the subsidiary arc and may be utilized for further switching actions from a subsidiary arc power of 10% of the main arc power onwards. When a heavy-current subsidiary arc is detected by one of the photodetectors, a control circuit in the power circuit of the plasmatron may be brought into operation in known manner, for immediately causing interruption of the subsidiary arcs by reducing the power of the main arc. Advantageously, the photodetectors are disposed in the supply zone of the plasmaforming gas, and light-conducting cables are provided which extend from the nozzle gap to the photodetectors. In order to make the invention clearly understood, reference will now be made to the accompanying drawing which is given by way of example and which is a diagrammatic sectional view of a plasmatron of the invention, shown disposed in a furnace. A high-power plasmatron is disposed in a furnace 1. The plasmatron comprises a pintype cathode 3 mounted in a water-cooled retaining means 2 and surrounded by a watercooled nozzle 4. A nozzle gap is defined between the nozzle 4 and the cathode 3. Photodetectors 5 are provided in the nozzle gap, their optical axes being directed towards the nozzle outlet aperture 6. When a subsidiary arc occurs in the nozzle gap, an increase in light radiation is detected by at least one of the photodetectors 5. The electric signal from the said one photodetector is supplied in a suitable manner by known switching elements to a control circuit, which acts accordingly on the power source of the plasmatron to reduce the main arc current. The material for melting in the furnace is denoted by the reference numeral 7 and the main arc by the reference numeral 8. CLAIMS

1. A high-power plasmatron comprising a pin-type cathode enclosed by a nozzle providing a gap for the passage of a plasma-forming gas, a plurality of photodetectors being disposed behind the nozzle outlet aperture in the gap between the cathode and the nozzle and so arranged that they overlook adjoining zones of the outlet aperture, the photodetectors serving for providing control signals for a protective control circuit of the plasmatron.

2. A plasmatron as claimed in claim 1, wherein the photodetectors are disposed in the supply zone of the plasma-forming gas, and light-conducting cables are provided which extend from the nozzle gap to the photodetectors.

3. A high-power plasmatron constructed and arranged substantially as hereinbefore described with reference to the accompanying drawing.