EP0372500A2 - Plasma arc welding apparatus - Google Patents

Abstract

Plasma arc welding device with an overload test circuit which detects a short circuit between the electrode (20) and the torch nozzle (30) and switches off the power supply if the short circuit is detected. A sensor circuit (138, 140) is provided in the burner cable (130), which switches on the overload test circuit (82) if the cable (130) is seriously broken through. The sensor circuit includes a shielding film (138) embedded in the cable (130), which surrounds the main conductor (132) and to which an earth conductor (140) is connected. The grounding conductor (140) detects a short circuit between the shielding film (138) and the main conductor (132) caused by an intruding foreign body (150) and actuates the overload test circuit (82). In addition, the overload test circuit (82) operates in conjunction with a single continuity shutdown circuit (121) to ensure the safety of the burner. A rectified control circuit is also provided in order to maintain a stable arc and to prevent erosion of the nozzle and the associated exposure of the electrode (20).

Description

The invention relates to a plasma arc welding device and is more specifically the subject of shutdown devices associated with the torch nozzle, such as are used in plasma arc welding devices.

The invention is particularly applicable to a plasma arc cutting torch that initially creates an auxiliary or pilot arc between the torch nozzle and the electrode and is described with particular reference thereto. However, the invention has a further field of application and can also be used in plasma arc cutting systems which generate a plasma arc without an ignition or auxiliary arc.

In burners of the type to which the invention relates, an ionizable gas is generally supplied to the burner nozzle in front of a negatively charged electrode. A high frequency, high voltage, alternating current signal is superimposed on the direct current signal, causing a spark to jump between the electrode and the torch nozzle that creates an initial ignition arc. The pilot arc heats and ionizes the gas flowing through the nozzle and generates a plasma flow. If the nozzle of the torch is brought close to the workpiece, which has a lower impedance than the torch nozzle, jumps the pilot arc from the nozzle to the workpiece and creates a plasma arc between the electrode and the workpiece. The plasma arc allows the torch to perform its function properly, either cutting, welding or depositing metal.

In all plasma arc welding devices, a nozzle is provided which encloses the electrode which carries a high voltage and protects it from occasional grounding, in order to direct the current of the ionizable gases as a plasma against the workpiece and to constrict the plasma jet and to give it a very high plasma temperature to lend. Burner parts, especially the burner nozzle, consume "something" and may need to be replaced. In particular, the pilot arc between the nozzle and the electrode can cause wear on the nozzle. The heat of the ionized gases also contributes to the wear of the nozzle, and to a certain extent the unfavorable effect of the heat occasionally requires the electrode to be replaced. Accordingly, the nozzle and the electrode are usually formed as interchangeable parts which are screwed into the torch body so that the nozzle protects and surrounds the electrode except for a narrow opening through which the plasma jet passes. Indeed, when the parts are to be replaced, they must be reassembled to ensure that the electrode is properly protected to avoid occasional grounding, which could occur if the torch is poorly assembled without the nozzle has been.

This problem has been recognized earlier and various solutions have been proposed. For example, Kneeland et al, U.S. Patent 4,663,515, proposes a nozzle that causes the gas pressure to increase when it is not in place. The rise in gas pressure is felt and the power source is switched off. The U.S. patents 4,590,354 and 4,682,005 from Marhic disclose protective nozzles which surround the electrode and can slide into the working contact due to the pressure of the plasma gas. Although such devices turn off the burner, in practice an arc can discharge in the period when the gas pressure is sensed or activated. In addition, gas pressure sensing devices are expensive.

In U.S. Patent 4,585,921 to Wilkins et al, incorporated herein by reference, a switch contact is made as soon as a cone surrounding the nozzle is positioned. As long as the switch is not activated, the power source is switched off. In this known device, however, the cone could be in place and the nozzle could be inadvertently removed from the device. The switch could then be activated because the cone is in position, but the electrode could be exposed. Therefore, there is still a need to avoid any accidental arc discharge in a plasma arc welding torch as long as the parts of the torch are not assembled properly.

In all plasma arc welding devices, the main electrical current of the electrode in the torch body is supplied through a main conductor which is located in a cable which extends from the torch body to the energy source. This cable can be punctured or damaged by any of a variety of different metallic objects that are always present in the work area of a commercial activity. Such metallic objects can touch the main conductor and bring about the same type of electrically dangerous state that is also present with an exposed electrode. The previous attempts to deal with these problems have been directed towards being tough, lasting and through to create shock-resistant cable. However, such cables make the equipment more expensive and require periodic inspection and occasional replacement.

Common to all plasma arc welding devices which use a pilot arc is that the current in the arc increases at the moment when the pilot arc is transferred as a plasma arc from the nozzle to the workpiece. When transferring the arc, the arc instantaneously changes from the electrode to the nozzle opening next to the electrode and from the nozzle opening to the workpiece. This double arc may destroy the nozzle, which leads to it. that the nozzle opening enlarges and the electrode is exposed. This problem has already been recognized and various circuits have been proposed in the prior art in which the electric current in the arc is reduced at the moment of transition. An example of such a circuit is shown in U.S. Patent 3,745,321 to Shapiro et al, which is incorporated herein by reference. Although such circuits react somewhat easily to the problem, they either have stability problems or are associated with disadvantages in energy efficiency.

The applicant has used an overload test circuit for its arc welding power sources, but not for plasma arc welding devices as such, which detects a short circuit between the electrode and the nozzle and turns off the power source in response to a sensed voltage exceeding a predetermined value.

The object of the invention is to provide a plasma arc welding device in which the durability and reliability of the plasma arc welding is improved and which has a current interruption and / or shutdown device which improves the operation of the torch.

This object is achieved with the invention by the features specified in the claims.

Here, according to the invention, a plasma arc welding device is used which is suitable for carrying out various welding processes on a workpiece and which includes, in a manner known per se, an electrical power source, a supply source for gas for generating a plasma and a torch body which has an electrode and a Has nozzle device which surrounds the electrode at a predetermined distance from it. A conventional overload test circuit is provided which detects a short circuit between the nozzle device and the electrode and switches off the electrical power source if a short circuit is detected. In a cable which is connected at one end to the torch body and at its opposite end to the electrical power source, an electrical main conductor is arranged which supplies the electrical current from the power source to the electrode in the torch body. A sensor device in the cable detects a breakdown or break in the cable which is severe enough to make contact with the main conductor and which, on the basis of this contact, actuates the overload test circuit to switch off the electrical power source.

More specifically, the overload detector circuit includes an ignition arc conductor, one end of which is connected to the nozzle device and the other end of which is connected to the earth potential (workpiece line). When the electrical power source is activated, an ignition arc is generated between the electrode and the nozzle device, and the voltage between the ignition arc conductor and ground is measured by the overload detector circuit. The overload test circuit turns off the electrical power source when the voltage exceeds a predetermined value. The sensor or sensing circuit has a continuous, electrically conductive shield which is embedded in the cable and surrounds the main conductor and extends over the entire length of the cable. A bare wire or an earth wire is attached to the shielding film and is connected to the ignition arc conductor. When an electrically conductive object breaks through the shielding foil and comes into contact with the main conductor, a short circuit is generated which is detected by the overload test circuit, which switches off the electrical power source.

According to a further feature of the invention, the nozzle device has a substantially cylindrical nozzle sleeve, which is constantly embedded in the burner body, and an exchangeable, cup-shaped nozzle tip, which can be screwed firmly into the nozzle sleeve. The pilot arc wire is connected to the nozzle sleeve. An electrically insulated spring wire trailing contact is embedded in the burner body and a continuity test conductor connected to an electrical power source is connected to the trailing contact. When the nozzle tip is properly secured in the nozzle sleeve, an electrical connection is made between the continuity test line and the pilot arc conductor and a circuit for measuring electrical continuity is established between these parts such that the electrical power source is turned off in the event of a power failure. In conjunction with the overload test circuit, the continuity test circuit ensures a safe welding torch at all times.

The cup-shaped nozzle tip completely surrounds and protects the electrode and is always at a relatively low potential, as described above, so that the torch is relatively safe as long as the cup-shaped nozzle tip is in position. Should the electrode be damaged or inserted incorrectly against the bowl-shaped nozzle tip or If a foreign body has found its way to the electrode so that contact is made with the cup-shaped nozzle tip, the overload detector circuit is actuated.

According to a further feature of the invention, the electrical current source generates a plurality of DC electrical pulses through a thyristor bridge, which are supplied to the electrode and generate an ignition arc when the ignition arc switch is connected to ground. The amplitude of each current pulse is sampled and the output circuit that generates the DC electrical pulses is phased back by a triggering circuit or delayed when a predetermined current level is exceeded. In practice, the amplitude of the last pulse in the pilot arc exceeds the predetermined current level, causing the circuit to operate to change the phase angle and, consequently, the current as soon as the pilot arc changes to the plasma arc.

According to another feature of the invention, a capacitor circuit is provided which maintains the plasma arc when the output circuit is reset in phase to reduce any pendulum tendency of the output circuit. The capacitor circuit is operated in a similar manner when the circuit is brought forward in phase when the pilot arc is ignited, i.e. is accelerated.

The invention has the advantage that, in a plasma arc welding torch, the torch is switched off when the cable is broken through. Another advantage of the invention is a forced power cut in a plasma arc welding torch which ensures that the torch will not be usable if it is not properly assembled.

Another advantage is the control device for a plasma arc cutting torch, which prevents excessive arcing and wear of the nozzle.

A particular advantage of the invention can be seen in a control device for a plasma arc cutting device, which allows the use of the electrical power source to generate a plasma arc with the highest efficiency.

Further advantages of the invention lie in the inexpensive but reliable current interruption and / or overload test devices and in the structural measures which make the use of the welding device impossible as long as the electrode is not fully protected by the nozzle.

• Fig. 1 einen Plasmalichtbogenschweißbrennerkörper im Längsschnitt und die zugehörige Plasma­lichtbogenschweißvorrichtung in einer sche­matischen Darstellung;
• Fig. 2 und 3 den Brennerkörper nach Fig. 1 in einem Quer­schnitt nach den Linien 2-2 bzw. 3-3 der Fig. 1,
• Fig. 4 einen Teil des Schweißbrenners in einer Seitenansicht und teilweise im Schnitt,
• Fig. 5 den Schweißbrennerkörper in einer ausein­andergezogenen, perspektivischen Darstellung,
• Fig. 6 einen perspektivischen Querschnitt durch ein Kabel, das in Verbindung mit dem Brenner nach der Erfindung verwendet wird,
• Fig. 7 die Befestigung des Leiters an der Abschirmfolie bei dem Kabel nach Fig. 6,
• Fig. 8 einen Teilquerschnitt der Fig. 7 nach Linie 8-8 der Fig. 7,
• Fig. 9 eine graphische Darstellung der in dem in den Fig. 7 und 8 gezeigten Leiter gemessenen Spannung,
• Fig. 10 und 11 schematische Diagramme, die verschiedene an den Zündlichtbogenleiter angeschlossene Schaltkreise zum Abschalten der elektri­schen Stromquelle zeigen,
• Fig. 12 eine schematische Darstellung der Prinzip­schaltung, wie sie bei der Schweißvorrich­tung nach der Erfindung verwendet wird,
• Fig. 13 ein Diagramm, das den von dem Brenner gelieferten Strom und seine Spannung wiedergibt, wenn er einen Plasmalicht­bogen entwickelt, und
• Fig. 14 einen Teil der Brennerspitze in einer Seitenansicht und teilweise im Schnitt, welcher den bei der Erfindung verwende­ten federnden Kontakt zeigt.

Further features and advantages of the invention result from the following description and the drawings, in which a preferred embodiment of the invention is explained in more detail with examples. It shows:

• Figure 1 shows a plasma arc torch body in longitudinal section and the associated plasma arc welding device in a schematic representation.
• 2 and 3, the burner body of FIG. 1 in a cross section along the lines 2-2 and 3-3 of FIG. 1,
• 4 a part of the welding torch in a side view and partly in section,
• 5 shows the welding torch body in an exploded perspective view,
• 6 is a perspective cross section through a cable used in connection with the burner according to the invention,
• 7 shows the attachment of the conductor to the shielding foil in the cable according to FIG. 6,
• 8 is a partial cross section of FIG. 7 along line 8-8 of FIG. 7,
• 9 is a graphical representation of the voltage measured in the conductor shown in FIGS. 7 and 8;
• 10 and 11 are schematic diagrams showing various circuits connected to the ignition arc conductor for switching off the electrical power source,
• 12 is a schematic representation of the basic circuit as used in the welding device according to the invention,
• 13 is a graph showing the current and voltage supplied by the torch when it develops a plasma arc, and
• 14 shows a part of the burner tip in a side view and partly in section, which shows the resilient contact used in the invention.

1 to 5, the plasma arc torch is designated in its entirety by "A", which has an electrically insulated torch body 10 made of hard plastic, which is brought into a suitable shape, for example a handle, but which is not shown. A suitable plasma gas line 12 is arranged in the torch body 10, through which a suitable, ionizable gas, for example air, is fed to the plasma arc welding torch A. The gas line 12 is in gas-tight connection with a gas distributor 13 made of metal, which has a central throughflow opening 15 passing through it. One end of the central flow opening 15 is provided with an internal thread 16. The internal thread 16 continues to the end of the gas distributor 13, which forms a flat seat 17 there. A plurality of radially extending gas distribution channels 18, which distribute the plasma gas onto the outer sides of the gas distributor 13, open into the central throughflow opening 15, as is indicated by the arrows in FIG. 1.

An electrode 20 in the form of a hafnium wire 21 is embedded in a cylindrical metal housing 23. The cylindrical housing 23 has a rounded end 24 and an annular shoulder 26 at its opposite end, on which there is a pin 27 with an external thread. The pin 27 is screwed into the gas distributor 13 so far that the shoulder 26 bears tightly against the flat seat surface 17 of the gas distributor 13.

The gas distributor 13 and the electrode 20 are surrounded by a nozzle device or a nozzle body 30, which consists of a nozzle sleeve 31 and a bowl-shaped nozzle tip 32. 1, 4 and 5, the nozzle sleeve 31 has a cylindrical base part 34 which surrounds the gas distributor 13 and is embedded in the plastic burner body 10. The base part 34 ends in an annular contact ring part 35.

The ring portion 35 has a flat, annular contact surface 37 and a central opening 38 with an internal thread, which extends inward from the annular contact surface 37, and an outwardly widening, frustoconical outer surface 39, which extends from the outer edge of the annular contact surface 37 and with which cylindrical base part 34 forms an outer shoulder 40. In addition to the intersection of the ring part 35 with the cylindrical base part 34, an inner ring shoulder 42 is formed in the ring part 35.

The ring part 35 penetrates at least one gas passage 44, which creates a gas connection between the inside and the outside of the nozzle sleeve 31. An annular, electrically insulating seat ring 45 made of plastic rests with one end on the inner ring shoulder 42 and with its other end on the flat seat 17 of the metal gas distributor 13. As a result, the annular seat ring 45 positions the gas distributor 13 and the electrode 20 at an electrically insulating, fixed spatial distance from the nozzle sleeve 31.

Radial gas channels 46 extend from the cylindrical outer wall to the cylindrical inner wall of the insulating seat ring 45. An annular space is formed between the cylindrical base part of the nozzle sleeve 31 and the gas distributor 13. The gas flows from the radial passage openings 18 in the gas distributor 13 into the annular space 48 and from here through the gas channels 46 extending in the radial direction against the cylindrical electrode housing 23, where the gas is ionized. Alternatively, the gas flows through the gas outlet openings 44 in the ring part 35 for metal removal, while this gas flow simultaneously cools the nozzle body 30.

As already indicated, the nozzle body 30 has a cup-shaped nozzle tip body 32, from whose annular contact base 50 a bush 51 with an external thread extends. which can be screwed into the internal thread of the central opening 38 of the nozzle sleeve 31. The lower part 53 of the bowl-shaped nozzle tip 32 has an opening 54 which extends in the axial direction and which is slightly smaller than the electrode wire 21, but is flush with it. The inner surface 56 of the bowl-shaped nozzle tip 32 is designed approximately the same as the shape of the cylindrical electrode housing 23, so that between the inner surface 56 of the bowl-shaped nozzle tip 32 and the outer surface of the cylindrical electrode housing 23, a sparkover space 57 is formed, which is gently curved toward the tip, as this is apparent from Fig. 1. A spark is formed in the arcing space 57, which migrates very quickly to the ignition arc space 59, which is located near the rounded end 24 of the cylindrical electrode housing 23, where the ignition arc is generated. In fact, the pilot arc space 59 is a maximum space in the spark flashover space 57, and it has been found that such an arrangement improves the cutting ability of the torch.

A gas cooling sleeve 60 is pushed over the nozzle body 30 and bears tightly with the aid of an O-ring 62, which is pressed together between the shoulder 40 of the contact ring part 35 of the nozzle sleeve 31, the burner body 10 and the inner surface of the cooling sleeve 60. The gas cooling sleeve 60 interacts with the nozzle body 30 in such a way that a fine gas flow is directed onto the cut in the workpiece or is focused on this cut, which is generated by the plasma arc to remove the metal. In addition, the configuration of the frustoconical surface 39, the outer surface of the cup-shaped nozzle tip 32, the arrangement of the gas outlet openings 44 and the inner shape of the gas cooling sleeve 60 are such that when the gas flows out through the gas outlet openings 44 and the space between the cooling sleeve 60 and the nozzle body 30 a somewhat turbulent gas flow is generated which improves the cooling of the nozzle body 30, whereby this Improved gas cooling continues by supplying the gas as a nozzle stream tangential to the end face of the cup-shaped nozzle tip 32 when the gas leaves the cooling space 63, which primarily serves to direct the gas stream precisely onto the workpiece for the purpose of metal removal.

As shown in Fig. 1, a 250-350 V DC power source is connected to the torch A and the workpiece W. A cathode conductor 70 connects the negative pole of the direct current source to the electrode 20 via the metal gas distributor 13, although for the sake of simplicity the cathode conductor 70 is shown as connected to the plasma gas line 12. A conductor 71 coming from the workpiece is connected to the positive terminal of the electrical energy source. A main energy switch 73 for switching the main energy source on and off and a high-frequency transformer 74 which supports the ignition of the pilot arc are installed in the cathode conductor 70. An ignition arcing contact wire 75 is embedded in the burner body 10 in an electrically insulated manner and fastened in an electrically conductive manner to the cylindrical base part 34 of the nozzle sleeve 31. An ignition arc conductor 76 connects the ignition arc contact wire 75 to the positive ground via a 3-ohm resistor 78 and an ignition arc switch 79. A capacitor 80 is connected in parallel between the electrode 20 and the high frequency transformer 74 to the cathode conductor 70 and the ignition arc conductor 76, and an 80 V overload test circuit 82 is connected in parallel between the ignition arc conductor 76 and the grounding or workpiece conductor 71.

So far, the burner circuitry is somewhat conventional. A switch, not shown, on the burner A is actuated in order to switch the ignition arc switch 79 and the main energy switch 30. ("Switching" is used here in a functional sense. In reality, these are "Switches" 73 and 79 contacts that are opened or closed by the switch or trigger on the torch handle.) If the welder wants to operate switches 79 and 73 one after the other, the trigger could alternatively be operated twice; first to operate the pilot arc switch 79 and then to operate the main power switch 73. When both switches are closed, the potential of approximately 300 V of an open circuit is applied to the electrode 20 and an ignition arc is generated in the flashover space 57, which quickly migrates to the ignition arc space 59. The capacitor 80 is sized relative to the high frequency transformer 74 so that it charges and discharges rapidly to maintain the pilot arc. The voltage tapped at the pilot arc conductor 76 when a pilot arc is formed is approximately 66 V. When the plasma arc welding torch A is in its pilot arc or initial phase, the gas exiting the nozzle opening 54 is ionized by the pilot arc and heats and develops a plasma. The plasma arc torch A is then lowered onto the workpiece W by the welder. As soon as the cup-shaped nozzle tip 32 reaches the workpiece W, the pilot arc jumps from the nozzle tip 32 to the workpiece W, which has a lower impedance than the nozzle tip 32 as a result of the resistance 78. When the pilot arc passes from the cup-shaped nozzle tip 32 to the workpiece W. , a plasma arc is formed.

As mentioned, the pilot arc voltage which is tapped in the pilot arc conductor during the normal existence of the pilot arc is always lower than the plasma arc voltage for the reasons explained in more detail below and is approximately 66 V for the particular torch shown here, if for any reason between the electrode and Nozzle short circuit occurs, the full potential of 250 - 350 V of the open circuit is applied to the nozzle body 30, which can be determined on the ignition arc conductor 76. As a result, an overload test circuit or fault detector circuit 82 is provided which is designed to switch at about 80 V in order to avoid a disturbance in the overload detection which is caused by the current formation of a double arc or a switching surge. When operated, the overload test circuit 32 turns off the main power source.

Two common control circuits that can be used as overload test and shutdown circuits 82 are shown in FIGS. 10 and 11. The overload sensing circuit shown in Fig. 10 uses an integrator 84 which calculates the magnitude of a voltage change over a period of time, for example over half a second. The integrated function dv / dt (or alternatively di / dt) is then compared to a limit value in a comparator circuit 85, and if the limit value of the comparator 85 is exceeded, a suitable switch-off circuit 87 switches off the main energy source. Examples of various integration and comparator circuits that measure dt / dt (or di / dt) and turn off the power source after being compared to predetermined maximum limits are described in applicant's U.S. Patent 4,717,807, which is incorporated by reference becomes.

This shutdown circuit becomes effective because the voltage change is faster when a short circuit is sensed than when the ignition arc is generated.

A simpler overload detector circuit is shown in FIG. 11, in which a capacitor 88 is charged by a resistor 89 which is connected between the ground or workpiece conductor 71 and the ignition arc conductor 76. If a short circuit is sensed, there will be a greater voltage applied to the capacitor and discharged the capacitor. During the discharge process, the capacitor 88 actuates a circuit 90.

The general circuit for the plasma arc control system is shown in FIG. 12 and has a thyristor- or silicone-controlled rectifier bridge 92 which is connected to the secondary part of a transformer, not shown, of a three-phase AC source, not shown. The rectified output of the silicon controlled rectifier bridge 92 is passed through a shunt line 93 and then through the high frequency transformer 74 to the electrode 20. Parallel to the silicon-controlled rectifier bridge 92 is a stabilizer circuit 95, which has a capacitor 96 which is charged by a diode 97 and discharged by a resistor 98 which is connected in parallel to the diode 97. The gates of the silicon controlled rectifier bridge 92 are controlled by a conventional switching or trigger circuit 100 which opens the gates of the silicon controlled rectifiers in the bridge 92 or phases them forward or backward in order to change the energy content of the electrical pulses (and accordingly the current) in the usual way Way to change. The time during which the gates are kept open or brought back in phase by the circuit 100 is controlled by a phase back circuit 101. The delay circuit 101 in turn is actuated by the ignition or initial arc or by the current or voltage differential sensed in the shunt line 93.

13, this diagram shows schematically that the energy source has a pulsating output when the main energy switch 73 of the open voltage circuit of approximately 300 V is actuated. If When the pilot arc switch 79 is closed, a pilot or pilot arc is generated which has a potential of approximately 160-170 V and a current draw of approximately 22.5 A. If the ignition arc is transferred to the workpiece and a plasma arc is created, the plasma arc has a potential of approximately 110-120 V and a related increase in current. When the pilot arc is established, the thyristors must be brought in phase through delay circuit 101 to approximately maintain their full conductivity and generate a voltage high enough to ignite the arc. If the arc is transmitted as a plasma arc, the current flowing through the arc increases and the voltage of the plasma arc drops to approximately 110-120 V. This means that the thyristors must be brought back in phase by the delay circuit 101, ie typically 25 degrees for the burner under consideration here (but this depends on the setting point of the machine) immediately after a current has been detected in the pulse that occurs. when the pilot arc passes to the workpiece W and is higher than that expected to operate the pilot arc. If the phase feedback does not take place, the current in the torch rises, the arc is not constricted and damages the torch nozzle tip 32 when it passes through the opening 54.

Generally speaking, for any given torch, there is a limit to the amount to which an arc can be restricted, which is generally determined by air pressure, air flow, the size of the nozzle opening, and the amperage. The current is the only variable that can be controlled by the energy source. If an arc of a certain size, which is given by a predetermined current value, is constricted by an excessively small nozzle opening 54 in the torch, the current in the arc passes first to the bowl-shaped nozzle tip part 32 at the edge of the nozzle opening 54 and then to the workpiece W on the outside of the nozzle opening. This process erodes the nozzle opening until it is large enough to let the arc through. At the same time, the electrode is exposed and the bowl-shaped nozzle tip 32 must be replaced. For this reason, the circuit shown in Fig. 12 accelerates the trigger circuit 100 when the torch is in its pilot arc cutting phase, and at the moment the pilot arc changes to a plasma arc, the current rise through the shunt line 93 in this electrical has Pulse is sensed, the effect that the thyristors are delayed immediately. When this delay occurs, the control system becomes somewhat unstable and tends to oscillate until the current reaches its regulated level, as shown in the plasma arc current portion of the curve in FIG. 13.

To stabilize the system and prevent the arc from being extinguished, capacitor 96 is discharged through resistor 98 for a sufficient time (generally about 12 milliseconds) to maintain the arc. To a lesser extent, this also occurs when the pilot arc begins to form. In this way, the circuit shown in Fig. 12 minimizes damage to the nozzle tip 32, stabilizes the electrical system both at the time the pilot arc is formed and at the time when the pilot arc is a plasma arc on that Workpiece is transferred, while at the same time the energy available from the energy source is optimized by reducing the degree of phase delay of the silicon-controlled rectifier to a minimum.

1 to 5 and 14, a spring wire trailing contact 110 is shown, which is embedded in the burner body 10 and is part of the nozzle sleeve. More specifically, the nozzle sleeve 31 has a cutout 111 in its contact ring part 35 and a cutout 112 in its cylindrical base part 34, into which a segment block 113 made of electrically insulating plastic material is inserted, the outer surface of which is shaped in such a way that there is a continuous one for the contact ring 35 and smooth outer surface results. A groove 115 is arranged in the segment block 113 and receives the bent end 117 of a spring wire trailing contact 110.

As best seen in Fig. 14, the spring wire contact 110 is shaped in relation to the groove 115 such that the bent end 117 can move by an amount x relative to the groove 115 when the annular contact surface 50 of the cup-shaped nozzle tip 32 firmly into the Nozzle sleeve 31 is screwed in. This ensures good electrical contact.

A continuity conductor 120 is connected to the continuity spring wire contact 110, which leads to a continuity test circuit 121 in FIG. 1 and has a continuity test voltage V _cc of approximately 15 V and a current of 50-100 mA. When the cup-shaped nozzle tip 32 is properly positioned, a current flows through the continuity or test conductor 120, the continuity spring wire contact 110, through the cup-shaped nozzle tip 32, the nozzle sleeve 31, the pilot arc contact wire 75 and from there through the pilot arc conductor 76. As a result, in A continuity test circuit 120 may be installed in the circuit to test continuity or to measure the voltage difference between the pilot arc conductor 76 and the continuity conductor 120. If a voltage differential occurs there or if a minimal electrical current flow is not found, the energy source can be switched off.

1 and 6 to 9, a cable 130 is shown, which is fastened with one end to the burner body in a manner not shown and the other end to the energy source in a manner also not shown. A gas line 12 for supplying the protective / cooling / cutting plasma gas and a main conductor 132 are embedded in the cable, which in turn is surrounded by a protective jacket 133 and supplies the arc current. Also embedded in the cable is a collection of control lines 132, which are similarly provided with protective coatings 135 and are used for various purposes, for example for switching the contacts, for supplying current to the continuity wire spring contact 110, etc. Each of the aforementioned conductors is embedded in a flexible, insulating plastic coating compound 137, to which a metal foil or a metal shield 138 is attached, which extends over the entire length of the cable 130 and completely surrounds at least the main conductor 132. A grounding line 140 is attached to the shield 138, which consists of a bare wire that runs along the entire length of the cable and is connected to the ignition arcing contact wire 75 in the burner body 10 on the burner body 10. On its outside, the shield 138 is surrounded by a flexible, insulating plastic jacket 41, which consists of a flexible, thermoplastic material which is "cross-linked" by an electron beam process in order to make it abrasion-resistant and tough.

If the cable 130 is pierced or cut by any electrically conductive object, such as designated 150 in FIG. 6, and that object, after penetrating the shield 138, cuts through the coating 133 and makes contact with the main conductor 132 , a short circuit is detected between the ground line 140 and the main conductor 132 and the short circuit thus discovered is ignited by the test circuit arcing contact wire 75 and from there to the ignition arc conductor 46 and the overload test circuit 82. This registers the voltage of an open circuit of approximately 300 V and switches off the energy source in the same way as the test circuit would have done if a short circuit had been detected on the nozzle body 30. The breakdown detection or the sensor circuit according to the invention is always effective, regardless of whether the ignition arc switch 79 is open or closed.

Various features have thus been described which can be used in a plasma arc welding device and which are dependent on one another and relate to one another. For example, an overload detector circuit that is able to detect short circuits between the torch nozzle and the electrode and switch off the energy source is used to detect a breakdown or a break in the torch cable. Similarly, an electrical interlock circuit in conjunction with an overload test circuit is used to provide a safe burner by combining sensing whether the nozzle is in position with sensing a nozzle voltage to provide a safe burner under all operating conditions guarantee. Finally, a silicon controlled rectifier circuit is used to disable the output when the arc is transferred so that the nozzle at its nozzle opening is not eroded and the electrode is exposed.

It is apparent that many modifications can be made to the disclosed circuits and circuits without departing from the spirit of the invention. All of these modifications and changes are within the scope of the present invention.

The essence of the invention is therefore to provide a safe plasma arc cutting device in which the durability and reliability of the plasma arc torch is improved.

Claims (18)

1. Plasma arc welding device, characterized by
an electrical energy source;
a torch body (10) with an electrode (20) and a nozzle body (30) which surrounds the electrode (20) at a fixed distance therefrom;
an overload test circuit (82) for detecting a short circuit between the nozzle body (30) and the electrode (20) and which switches off the current source as a function of such a short circuit;
a cable (130) connected to the burner body (10) and having an electrical main conductor (132) inside the cable (130) which is connected at one end to the electrode (20) and at the other end to the energy source, and
Scanning means (138, 140) inside the cable (130) which detect a breakdown of the cable (130) which is sufficient to make contact with the main conductor (132) and which, upon such a contact, actuate the overload test circuit to identify the energy source switch off. 2. Device according to claim 1, characterized in that the scanning means have a continuous, electrically conductive shield (138) which is embedded in the cable (130) and surrounds the main conductor (132) and extends over the entire length of the cable (130 ) extends and that the scanning means a grounding line device (140) which is in electrical contact with the shield (138) and the nozzle body (30), the scanning means (138, 140) being actuated when any electrically conductive object (150) breaks through the shield (138) and touches the main conductor (132) and thereby causes a short circuit between the electrode (20) and the nozzle (10). 3. Apparatus according to claim 1 or 2, characterized in that the burner body (10) has an ignition arc contact wire (75) which is in electrical contact with the nozzle body (30) and the ground line (140) and in that an ignition arc switch ( 79) and an ignition arc conductor (76) are provided, which are connected to the overload test circuit (82) and the pilot arc contact wire (75) and that a main energy switch (73) is arranged between the electrode (20) and the main energy source, the overload test circuit (82) is actuated when the pilot arc switch (79) and the main current switch (73) are activated to generate an ignition arc between the nozzle body (30) and the electrode (20) and when a predetermined voltage, measured relative to the ignition arc conductor (76) is exceeded. 4. Device according to one of claims 1 to 3, characterized in that the overload test circuit (82) switches off the current source when the ignition arc switch (79) is open and the scanning means (138, 140) are activated. 5. Device according to one of claims 1 to 4, characterized in that the electrode (20) is a cathode, that the workpiece (W) the ground potential represents that the power source provides a DC voltage in the range of about 250 to 350 volts to the electrode (20), that the pilot switch (79) is connected to ground, and that the predetermined value is the voltage that exists between the pilot switch (76) compared to the earth potential. 6. Device according to one of claims 1 to 5, characterized in that the predetermined voltage value is not less than about 66 volts and not greater than about 250 volts. 7. Device according to one of claims 1 to 6, characterized in that the nozzle body (30) in the burner body (10) anchored nozzle sleeve (31) and a bowl-shaped nozzle tip (32) which are screwed together to form the nozzle body (30) and that the ground line (140) is connected to the nozzle sleeve (31) via an ignition arc contact wire (75) and that in the burner body (10) a continuity line (12) is arranged to which an electrical voltage source for a continuity circuit (V _cc ) is connected and that the cup-shaped nozzle tip (132), when properly attached to the nozzle sleeve (31), allows current to pass from the continuity conductor (120) to the ground line (140) and that the continuity or continuity test circuit (121) measures the passage between the continuity conductor (120) and the pilot arc contact wire (75) such that in the absence of a flow between this continuity conductor (120) and the ignition line contact wire (75) the main power source is switched off. 8. Device according to one of claims 1 to 7, characterized in that the nozzle sleeve (31) has an annular contact ring (35) which projects from the burner body (10) with an internally threaded central opening (38) and one in has a substantially flat end face (37) and that a cylindrical base part (34) extends from the opposite side of the ring part (35) and is anchored in the burner body (10) and that the ring part (35) has an electrically insulated section (113) having a groove (115) and that the continuity conductor (120) has the shape of a continuity contact spring wire (110) which lies in the groove (115) and protrudes over the end face (37) of the base part (34) when the nozzle device (A) is in an unassembled position and that the cup-shaped nozzle tip part (32) has an annular contact base (50) and a protruding threaded socket part ( 51), which protrudes beyond the contact surface (50) and can be screwed to the internally threaded central opening (38) of the nozzle sleeve (31), the contact base surface (50) laying against the flat end surface (37) and the continuity - Contact spring wire (117) in the groove (15) moves when the cup-shaped nozzle tip (32) is properly attached to the nozzle body (30) and thereby allows the continuity of current, which is measured by the continuity test circuit (121). 9. Device according to one of claims 1 to 8, characterized in that in the burner body (10) a gas line (12) and a gas distribution block (13) is arranged, which at one end with the outlet of the gas line (12) in Ver is standing and is arranged within the cylindrical base part (34) of the nozzle sleeve (31) and that between the gas distributor (13) and the ring part (35) of the nozzle sleeve (31) an annular insulating seat ring (45) is arranged, which the gas distributor (13 ) electrically insulated from the nozzle sleeve (31) and that the gas distributor (13) has an end face (17) in which there is a threaded opening (15) which is adjacent to the ring part (35) and that the electrode (20 ) has a cylindrical body part (23), one end of which is rounded and which has an annular shoulder (26) at its other end and an externally threaded end (27) which projects beyond this shoulder and which has the internally threaded opening ( 16) of the gas distributor (13) is screwed and that the cup-shaped nozzle tip part (32) is designed on its inside so that it closely matches the cylindrical body part (23) of the electrode (20) is correct and together with this forms an electrode spark space which gradually grows to a maximum spark gap at the point which is essentially adjacent to the rounded end (24) of the cylindrical body part (23) of the electrode (20) and in the middle of which a nozzle opening ( 54) and that the insulating seat ring (45) has gas channels (46) which are in communication with the electrode spark space and that the gas distributor block (13) has gas passage openings (18) which on the one hand connect to the gas line (12) and on the other hand to a space ( 48), which is formed between the gas distributor block (13) and the cylindrical base part (34) of the nozzle sleeve (31), an ionizable gas being injected through the opening. 10. Device according to one of claims 1 to 9, characterized in that a gas cooling cup (60) is provided which surrounds part of the cup-shaped nozzle tip part (32) and is sealed with sealing means (62) against the burner body (10) and that Ring part (35) of the nozzle sleeve (31) has at least one gas passage (44) which communicates with a space (63) between the cooling cup (60) and the cup-shaped nozzle tip (32), a part of the gas line (44) leaving gas is directed against the cup-shaped nozzle tip (32) to focus the gas in the cutting zone of the workpiece (W) and to remove molten metal, while cooling the cup-shaped nozzle tip (32). 11. The device according to one of claims 1 to 10, characterized by
Rectifying elements for generating a series of DC electrical pulses which are supplied to the electrode;
Switching means for generating an ignition arc between the nozzle and the electrode when the torch system is ignited, which arc is transferred to the workpiece as a plasma arc during normal operation of the plasma arc welding device;
Switching means for changing the phase angle of the pulses from a first value at which the pilot arc is formed to a second value at which the plasma arc is formed;
Sensor means for detecting the current in each electrical pulse during the formation of a pilot arc and in response to a pulse which exceeds a predetermined current value at the moment when the pilot arc changes into the plasma arc, by the switching means for reducing the phase angle of the pilot arcs - Activate impulses. 12. Device according to one of claims 1 to 11, characterized in that the trigger device for varying the phase angle varies the rectification of the electrical pulses in the plasma arc phase by about 25 ° degrees. 13. The device according to one of claims 1 to 12, characterized by stabilizing means for maintaining the plasma arc when the sensor means actuates the carrier device to reduce the phase angle of the pilot arc pulses. 14. Device according to one of claims 1 to 13, characterized in that the stabilizing means comprises a capacitor which is connected in series to its charge with a diode, a resistor for discharging the capacitor is connected in parallel, the capacitor being of a sufficient size has to supply a current sufficient to maintain the pilot arc for 12 milliseconds. 15. The device according to one of claims 1 to 14, characterized in that the Rectifier means which generates the electrical pulses, a three-phase AC source, a transformer connected to each of these phases and a rectifier bridge circuit having controlled rectifiers, the input of the rectifier circuit being connected to the secondary circuit of the transformer and the output of the rectifier circuit one provides immediate working voltage between the electrode and the workpiece and that the trigger device has a device for controlling the degree of rectification of the rectifier circuit as a function of the actuation of the switching means. 16. The device according to one of claims 1 to 15, characterized in that the cable (130) has a substantially cylindrical outer jacket (141) made of a flexible, insulating plastic material, that the shield (138) is formed by a metal foil which in is embedded in this outer sheath (141) that the grounding line (140) consists of a bare wire which extends over the entire length of the cable (130) and is in contact with the film and that the main conductor (132) is in an insulating Plastic jacket (133) is arranged so that first the outer jacket (141) and then the insulating jacket (133) must be pierced by an electrically conductive object (150) which extends between these parts before the sensor means is actuated. 17. Plasma arc welding device for performing various machining operations on a metal workpiece, characterized by:
an electrical energy source;
a gas supply source for producing a plasma;
a torch body (10) having an electrode (20) connected to the main energy source and a nozzle device (30) surrounding the electrode (20);
an overload detector circuit (82) for detecting a short circuit between the nozzle device (30) and the electrode (20) and for switching off the energy source in response to such a short circuit;
a pilot arc contact wire (75) in electrical contact with the nozzle means (30), a pilot arc switch (79), a pilot arc conductor (76) connected to the overload detector circuit (82) and the pilot arc switch (79) Main power switch (73) between the electrode (20) and the power source, wherein the overload detector circuit (82) is actuated when the pilot arc switch (79) and the main power switch (73) are actuated to switch between the nozzle device (30) and the electrode (20 ) produce a pilot arc and in this case a predetermined voltage relative to the pilot arc conductor (76) is exceeded; wherein the nozzle device (30) has an essentially cylindrical nozzle sleeve (31) in the burner body (10) and a cup-shaped nozzle tip (32) which is screwed together to form a nozzle body (30) can be and serve as a pilot arcing contact;
a continuity conductor (120) in the burner body (10), a continuity current source (V _cc ) for a continuity test circuit (121) which is connected to the continuity conductor (120). the cup-shaped nozzle tip (32), when properly assembled into the nozzle body (30) and making electrical connection from the continuity conductor (120) to an earth conductor (140), and the continuity circuit (121) the current passage between the continuity conductor (120 ) and the pilot arcing contact (25) so that in the absence of a continuity between the nozzle tip (32) and the grounding conductor the main energy source is switched off, making the burner safe at all times. 18. Plasma arc welding device for performing various machining operations on a metal workpiece, characterized by:
an electrical energy source;
a gas source for supplying a gas for generating a plasma;
a torch body (10) having an electrode (20) connected to the main energy source and a nozzle device (30) surrounding the electrode (20);
an overload test circuit (82) for detecting a short circuit between the nozzle device (30) and the electrode (20) and for switching off the electrical energy source when such a short circuit occurs;
Rectifying means for generating a series of DC electrical pulses applied to the electrode (20);
Switching means for initiating an ignition arc between the nozzle device (30) and the electrode (20) when the welding device is started, which arc is transferred to the workpiece (W) as a plasma arc during the normal operation of the plasma arc welding device;
carrier means for varying the phase angle and the amperage of the pulses from a first value that occurs when the pilot arc is generated to a second value that occurs when the plasma arc occurs;
sensor means for sensing the amperage in each electrical pulse during the formation of a pilot arc and when a pulse exceeds a predetermined amperage at the instant the pilot arc passes to the plasma arc to actuate the carrier and the phase angle of the Reduce pilot arc pulses.

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