Plasma Hand Burner With Contact Protection

Abstract

A circuit arrangement for plasma hand torches for fusion cutting having a contact protection which comprises a housing, a nozzle disposed in the housing and having an exit opening. A nozzle cap is in electrically conducting connection with the housing. Two conduits ground the housing at two points. A current-feeding source is electrically connected with the two conduits. One of the conduits constitutes a protection conduit and the other of the conduits constitutes an auxiliary conduit. The one of the conduits includes a current relay with first contact and the other of the conduits includes an auxiliary relay and a secondary winding of a low voltage transformer. The auxiliary relay includes a working contact disposed in series with the first contact. The plasma hand torch has an operating relay, and the operating relay is capable of being either switched off or not switched on respectively, by means of said first contact upon occurrence of a short circuit or by the working contact upon interruption of the protection conduit or of the auxiliary conduit.

Description

The present invention relates to a plasma hand torch and a circuit arrangement therefor. More particularly, the present invention relates to a plasma hand torch protected against accidental contact, and which can be used for cutting operations.

Plasma torches of various constructions are known, but they all have the disadvantage of not being protected against contacting, so that their use as hand torches is restricted. Even in the known torches which are specially constructed as hand torches, the voltage-carrying nozzle or nozzle cap is not protected against accidental contact. Consequently, such plasma torches are only suitable for low operating voltages and consequently are only of low power as for as the plasma jet is concerned.

In other constructions, the nozzle or nozzle cap is protected from contact by a metallic housing which does not carry a voltage. However, there are circumstances under which the housing becomes live, for example due to insulation breakdown, ionization of the insulating air space between the nozzle cap and the housing, or passage of the arc between the nozzle and the housing.

In one of the known arrangements, the nozzle is mounted by two rubber seals in a nozzle carrier which extends almost as far as the nozzle opening. The rubber seals have the function of sealing the cooling water chamber, which extends as far as the nozzle, on the one hand relative to the supplied gas and on the other hand relative to the atmosphere. For the electric insulation of the nozzle cap relative to the nozzle or the nozzle carrier, a suitably formed ceramic ring is provided. This arrangement has the disadvantage that due to the physical requirements, the nozzle, the nozzle carrier, the nozzle cap and the ceramic ring have a relatively complicated shape.

In addition, the nozzle and the nozzle cap constitute members which are subject to wear, so that they are required in large numbers. Due to this, a complicated shape for these members is particularly disadvantageous.

Moreover, there is the disadvantage that due to the interposition of the ceramic ring and the shape of the nozzle and nozzle cap which result from this, as well as the necessary insulation in the region of the nozzle, the hand torch has relative large dimensions. This has the disadvantage that it can readily obstruct the vision of the operator, and the torch can only be used at low inclination angles relative to the vertical, this being particularly disadvantageous in view of the frequent necessity for making angular cuts. Finally, it should be mentioned that as a result of the large dimension of the torch, there is a large surface present above the workpiece and the torch is thereby vulnerable to be soiled or damaged by liquid material splashing upwards from the workpiece.

It is one object of the present invention to provide a plasma hand torch which avoids the above stated disadvantages.

It is another object of the present invention to provide a plasma hand torch wherein the insulation is arranged between voltage conducting parts and the housing of the torch in such a manner that while retaining easy exchangeability of the nozzle, and effective cooling of the same, a danger-free operation is enabled.

It is still another object of the present invention to provide a circuit arrangement for a plasma hand torch, wherein a housing of the torch is electrically connected with a nozzle cap of the torch, is protectively grounded at two places through two lines which are connected to the housing of a current supply unit, a current relay with a normally closed contact being provided in one of the lines, an auxiliary relay and the secondary winding of a low-voltage transformer being provided in the other auxiliary line, the auxiliary relay having a normally open contact which is connected in series with the said normally closed contact. An operating relay of the plasma torch is made releasable or nonengageable as a result of opening of the normally closed contact in the event of a short circuit, or as a result of closure of the normally open contact in the event of an interruption in either of the two lines.

Preferably, the nozzle of the torch is connected to the nozzle cap with with the interposition of an elastic electrically insulating layer. A protective ground connection may be made to the housing of the plasma torch.

It is still another object of the present invention to provide a ceramic insulator at the nozzle end of the torch, disposed concentrically relative to the nozzle and the nozzle cap, and extending beyond the nozzle and nozzle cap at the one end. The nozzle, and especially the outlet opening for the plasma jet, is surrounded by the ceramic insulator. For the purpose of an easy exchangeability of the nozzle and nozzle cap, the ceramic insulator and the nozzle cap are provided with mutually engaging collars.

In accordance with the present invention, a plasma hand torch is provided which, while being suitable for a high current density in the cutting jet, can be used without danger even for such workpieces requiring a low cutting width and a high cutting speed, such applications being not particularly suitable for machine cutting as a result of the shape and/or the dimensions.

With these and other objects in view, which will become apparent in the following detailed description, the present invention, which is shown by example only, will be clearly understood in connection with the accompanying drawings, in which:

FIG. 1 is a circuit diagram of the arrangement of the present invention;

FIG. 2 is a fragmentary sectional view of a part of a plasma hand torch according to the present invention; and

FIG. 3 is a perspective view of a complete plasma hand torch.

Referring now to the drawings and in particular to FIG. 1, the protective circuit of a plasma hand torch designed in accordance with the present invention is disclosed. A housing 6 of a plasma hand torch, and therewith also the nozzle cap 5, are connected through a protecting line 11 to the protectively grounded housing of a current supply unit 12. The protecting line 11 is connected through the winding of a current relay 13.

An auxiliary protecting line 14 connected at a second region of the housing 6 is connected through the winding of an auxiliary relay 15 and the secondary winding of a low voltage transformer 16, likewise with ground.

The normally open contact 17 and the normally closed contact 18 of the respective relays 13 and 15 are connected in series and interlocked with an operating relay 23 of the current supply unit 12, so that the operating relay 23 is switched "off" or is prevented from being switched "on," respectively if the protecting line 11 or the auxiliary line 14 is interrupted, that is, if the excitation current circuit of the auxiliary relay 15 is not closed, so that the auxiliary relay 15 is not excited and as a result thereof its normally open contact 17 stays open, as hereinafter described.

The connection of the two lines 11 and 14 to different regions of the torch housing 6 excludes the case that the lines 11 and 14 are only connected to each other but not to the housing 6. The operation of the protective circuit is as follows:

Upon switching on the main switch (not shown) of the current supply unit 12, the operating voltage appears on the terminals 20 and 21 and the control voltage on the terminals 24 and 25. The supervising current circuit which is fed from the transformer 16 is closed (the supervising current circuit consisting of the secondary winding of the transformer, the winding of the auxiliary relay 15, the auxiliary line 14, the housing 6 of the plasma torch, the protective line 11, the first winding 26 of the current relay 13 and the housing of the current supply unit 12), that is, the housing 6 of the plasma torch is connected with the housing of the current supply unit 12 and thereby protectively grounded, thus the auxiliary relay 15 responds to the current fed by the actuated transformer 16 and closes its working contact 17, the latter being disposed in the control circuit comprising the contacts 17 and 18, the relay 23, and a control switch 22.1.

By the operation of the control switch 22.1 (i.e. closing the switch 22.1 in the control circuit), the operating relay 23 is rendered operative, and its contact 28 disposed in the torch operating circuit closes, and the operating voltage appears over the working contact 28 on the cathode 19. The operating relay 23 switches off in known manner also the working gas and the ignition thereof, and thereby the burner is always in operation.

Ordinarily, housing 6 and nozzle cap 5 are insulated from the high voltage from terminal 20 by the ceramic insulator 7.

When the control circuit comprising the contacts 18 and 17 is open-circuited (due to either contact 17 or 18 or switch 22.1 being open), then the operating relay 23 is inoperative and its contact 28 is open in the electrode or torch-operating circuit which prevents terminals 20 and 21 from supplying the operating voltage to the torch.

In normal operation, the low-voltage transformer 16 is initially actuated upon closing the main switch (not shown), which places the control voltage on terminals 24 and 25, thereby causing relay 15 to close its normally open contact 17 in the control circuit. Switch 22 (FIG. 3) is then actuated to close contact 22.1 which now closes the control circuit and thereby and causes the operating relay 23 to be actuated by the voltage at terminals 24 and 25, thereby closing contact 28 in the torch operating circuit and permitting the high voltage on terminals 20 and 21 to provide a high voltage across the electrode 19 and workpiece, causing the torch to be in operation. Since the housing 6 is insulated from the high voltage during normal operating, relay 13 in line 11 receives no actuating current (the current from the low-voltage transformer 16 being insufficient to open contact 18 in the control circuit, and this contact 18 remains closed and the torch may be operated continuously.)

However, at the instant of an insulation break, etc., and a jumping of the arc (for instance, as a result of a defective ceramic insulator 7) from the cathode 19 over the nozzle 1 to the nozzle cap 5, the operating voltage which appears at the terminals 20 and 21 causes the housing 6 to momentarily have a high voltage and sends a high actuating current through the protective line 11, and the winding 26 of the current relay 13. This immediately actuates the current relay 13 sufficiently to cause contact 18 in the control circuit to open, thereby opening or breaking the control circuit in which the operating relay 23 is disposed and thereby switching off the latter and causing the operating relay 23 to open its contact 28 in the control circuit and prevent terminals 20 and 21 from supplying the high voltage to the electrode 19 and thus to the defective insulator and torch housing 6. The opening of contact 18 in the relay 23 circuit i.e., the control circuit, however, is designed such that the contact 18 closes a self-holding or self-locking circuit in which is disposed the second winding 27 of the current relay 13, i.e., the second winding 27 of the current relay 13 now becomes actuated by the voltage across terminals 24 and 25, in this self-locking circuit, thus maintaining the contact 18 open in the control circuit of the operating relay 23, but held closed in the self-locking circuit, so that even though the high operating voltage to the electrode 19, and thus to winding 26 of the current relay 13 in the insulation-damaged condition is momentarily removed, the contact 18 is assured of not returning to again actuate the operating relay 23, before the main switch is turned off to remove the voltages across the terminals 20 and 21 as well as terminals 24 and 25.

The nozzle 1, the defective insulator, the nozzle cap 5, the protective line 11 and the winding 26 of the current relay 13 only momentarily conduct the high current and consequently are protected before destruction.

If without the self-holding feature of the current relay 13 over the contact 18 and the second winding 27, the response of the current relay 13 otherwise would thereafter cause the operating voltage of the cathode 19 to be switched off, but the current relay 13 would again fall off and close its contact 18 and with the still-actuated control switch 22.1, the operating relay 28 would repeat to switch the operating voltage on again the to the cathode 19 and hence through the defective insulation. This is avoided by the self-maintaining operation of the relay 13 by the second winding 27 in the self-locking circuit of the relay 13.

After the switching out of the main switch of the current supply unit 12, the current relay 13 falls off and the contact 18 closes again in the control circuit and the switching arrangement is again in the control circuit and the switching arrangement is again operatively ready after removal of the cause of damage and switching on of the main switch.

The switching out in accordance with the working results by means of release of the actuating element 22 of the control switch 22.1, whereby the latter is opened and the operating relay 23 falls off.

If the supervising current circuit, i.e., lines 11 and 14 is interrupted, at some point, for instance, in the path of the protecting line 11, by which the protection device is no longer assured through faults of the protective ground of the housing 6 of the plasma torch, thus the auxiliary relay 15 does not respond and falls away, respectively, when such interruption during the working occurs. Thereby the working contact 17 of the auxiliary relay 15 opens in the control circuit and the operating relay 23 no longer responds (the actuated control switch 22.1 still being actuated), and falls off, respectively, whereby again the operating voltage to the electrode 19 of the plasma torch is switched off.

By this em exemplified interruption of the protective line 11 the secondary voltage of the transformer 16 appears over the winding of the auxiliary relay 15 and the auxiliary line 14 at the housing 6 of the plasma torch. Therefore this voltage is a harmless auxiliary voltage. The transformer 16 serves moreover for the disconnecting of the ground-connected supervising current circuit of the remaining control current circuit.

On the other hand, during normal operation, if the low-voltage transformer 16 is turned off, the auxiliary relay 15 is shut off and contact 17 opens its contact 28 in the control circuit, thereby turning off the operating relay 23 and also cutting off the operating voltage to the electrode 19. The transformer 16 may be used as a safety device preventing the turning on of the torch, even by switch 22, unless transformer 16 is operating.

The operation of transformer 16 which is sufficient to actuate the auxiliary relay 15 to close its contact 17 is insufficient to cause the current relay 13 to open its contact 18, since it is a low-voltage transformer, the relay 13 requiring a higher current through the winding 26 in order to open its contact 18.

That is, in the event of a possible establishment of the arc from the cathode 19 over the nozzle 1 to the nozzle cap 5, the operating voltage which is applied to the terminals 20 and 21 causes a high current to flow through the protecting line 11. This causes the current relay 13 to respond so as to open its normally closed contact 18, which results in the operating relay 23 interrupting the operating voltage, whereupon the nozzle 1, the nozzle cap 5 and the protecting line 11 are isolated from the voltage. In this manner these parts carry the high current for only a short period of time, and they are thus protected from damage.

Referring now again to the drawings and more particularly to FIG. 2, the plasma hand torch is disclosed in detail, and comprises a nozzle 1, preferably of copper, which is centralized in the nozzle carrier 2 and sealed at its rear face by a rubber seal 3. By means of an elastic voltage-resistant seal 4, the nozzle 1 is electrically insulated form the nozzle cap 5. The nozzle cap 5 is connected by means of screw threads with the protectively grounded housing 6.

By means of a ceramic insulator 7, against which the nozzle cap 5 engages, the seal 4 is pressed against the nozzle 1 and thus seals a cooling water chamber 8. The nozzle 1 is accordingly directly water cooled, and is nevertheless readily exchangeable by screwing off the nozzle cap 5.

An insulator 9 insulates the nozzle carrier 2 relative to the housing 6 of the plasma hand torch and contains channels 10 for the supply and return of the cooling water. The ceramic insulator 7, in addition to serving for thermally protecting the seal 4, serves for elongating the electrical leakage path and as a contact protection of the nozzle 1.

Referring now again to the drawings and more particularly to FIG. 3, an actuating element 22 of the control switch for the operating relay is mounted in the handgrip of the torch. This actuating element 22, when made of metal, should be included in the protective circuit. The operating relay switches the operating voltage and also the operating gas and the ignition. The remaining operation takes place automatically.

While we have disclosed one embodiment of the present invention, it is to be understood that this embodiment is given by example only and not in a limiting sense.

Claims

We claim:

1. A circuit arrangement for plasma hand torches for fusion cutting having a contact protection, comprising:

a housing;

a nozzle disposed in said housing and having an exit opening;

a nozzle cap in electrically conducting connection with said housing;

two conduits grounding said housing at two points;

a current supply unit operatively electrically connected with said two conduits;

one of said conduits constituting a protection conduit and the other of said conduits constituting an auxiliary conduit;

said one of said conduits including a current relay with a first contact;

said other of said conduits including an auxiliary relay and a secondary winding of a low-voltage transformer;

said auxiliary relay including a working contact disposed in series with said first contact;

said plasma hand torch having an operating relay; and

said operating relay capable of being rendered to switch off or not to switch on respectively, by means of said first contact upon occurrence of a short circuit or by said working contact upon breaking of said protection conduit or of said auxiliary conduit.

2. The circuit arrangement, as set forth in claim 1, which includes an elastic, electrically insulating sealing connecting said nozzle with said nozzle cap.

3. The circuit arrangement, as set forth in claim 1, which includes a ceramic insulation disposed concentrically about said nozzle and said nozzle cap, and projecting beyond said nozzle cap simultaneously enclosing said exit opening of said nozzle.

4. The circuit arrangement, as set forth in claim 3, which includes a collar each for said ceramic insulation, as well as for said nozzle cap.

5. An arrangement for a plasma hand torch including a torch housing electrically connected to a nozzle cap of the torch, comprising:

a current supply unit including a protective housing;

a protecting line connected to said torch housing at a first position and grounded to said protective housing;

an auxiliary line connected to said torch housing at a second position and grounded to said protective housing;

a control circuit;

a current relay disposed in said protecting line and including a normally closed contact disposed in said control circuit;

an auxiliary relay disposed in said auxiliary line and including a normally open contact connected in said control circuit in series with said normally closed contact;

a low-voltage transformer including a secondary winding, the latter disposed in said auxiliary line;

an operating circuit means for providing an operating voltage to said plasma torch;

means for insulating said torch housing and nozzle cap from said operating voltage during normal conditions; and

an operating relay means for said plasma torch operatively connected in said control circuit and having a closable contact disposed in said operating circuit means for preventing a letter latter from providing an operating voltage to said plasma torch upon open circuiting of said control circuit.

6. The arrangement, as set forth in claim 5, wherein:

said current supply unit selectively provides high voltage to said operating circuit means and to said low-voltage transformer:

said low-voltage transformer designed to supply sufficient current to said auxiliary relay when said transformer is actuated by said current supply unit so as to close said normally open contact in said control circuit;

said auxiliary line an said protective line constituting a supervisory circuit

so said low-voltage transformer designed to supply insufficient current to said current relay when said transformer is actuated by said current supply unit so as to maintain said normally closed contact in its closed condition is in said control circuit;

an operating switch for said control circuit; and

said current relay including a separate winding connected to said current supply unit and said normally closed contact movably disposed so as to close the circuit between said separate winding and said current supply unit when a high current passes through said current relay and said normally closed contact open circuits said control circuit.