Protection device against unwanted electric arcs

Abstract

Arc Flash Protection Device This description relates to an arc flash protection device (50) comprising a first electrically conductive rod (60) having a first pointed end (61), an electrically conductive part (40), the first pointed end (61) being separated from the electrically conductive part (40) by a gap (62), a first electrical conductor (54) connected to the first electrically conductive rod (60), and a sensor (58) configured to detect the passage of an electric current in the first electrical conductor (54). Figure for the abstract: Fig. 1

Description

Title of the invention: Device for protection against unwanted electric arcs. Technical field

[0001] This description relates generally to devices for protection against unwanted electric arcs, particularly for an arc melting furnace. Prior art

[0002] An arc melting furnace comprises, within a chamber, an electrode which is brought close to a load, for example, metal pieces arranged in an ingot mold, in order to generate an electric arc between the electrode and the load, causing the load to melt. Depending on the type of arc melting furnace, the electrode corresponds to the cathode and the ingot mold to the anode. However, depending on the type of load, the cathode and anode may be reversed in order to limit contamination of the furnace by the dispersion of the molten pool. To create the electric arc between the electrode and the load, an arc-starting voltage must be applied between the electrode and the load to reach the disruptive field of the atmosphere between the electrode and the load, resulting in the generation of a spark between the electrode and the load, thus closing the circuit electrically.The starting voltage is applied by an ignition device, which is generally a very low-power generator that creates a high voltage, for example a few kilovolts, at high frequency between the electrode and the metal pieces. Once the arc is created, the system can then switch to a high-power supply providing the power needed to melt the charge under a lower voltage, generally between 10 V and 100 V, sufficient to maintain the arc, but which would have been insufficient to generate the arc.

[0003] Several adaptations can be implemented to facilitate arc formation and thus lower the arc ignition voltage. One example of an adaptation is to place the furnace chamber under partial pressure of a few hundred millibars of a gas, for example argon. Another example of an adaptation is to use an electrode with a pointed tip.

[0004] One drawback is that the electric arc may not form between the electrode and the load, but rather between the electrode and another conductive part of the furnace, for example, between the electrode and the furnace walls, which are generally cooled, particularly with water. This occurs when the conditions for arc formation are more favorable elsewhere in the furnace than between the electrode and the load. Typical causes include, for example, an excessive distance between the electrode and the load, or electrode wear causing it to lose its geometry, thus promoting arc formation. Arcing can be caused by the formation of an electrical insulating deposit, such as an oxide, on the electrode or charge. This deposit can be due to factors such as repeated furnace use or inadequate charge cleaning; electrical conductivity defects in the lines connecting the generator and the electrode / ingot mold containing the charge; insulation defects in the furnace chamber, such as the deposition of metallic particles from the melting process on insulating components; and / or a geometry that promotes arcing at the current passages in the chamber, such as a capacitive effect due to the cylindrical geometry of the passages. The formation of such an unwanted electric arc can lead to furnace damage. In the worst-case scenario, this can result in a loss of furnace seal, such as water leaking into the chamber (potentially causing an explosion). It is desirable to be able to detect and prevent the formation of such unwanted electric arcs. Summary of the invention

[0005] One embodiment overcomes all or part of the disadvantages of devices for protection against unwanted electric arcs.

[0006] One embodiment provides a device for protection against unwanted electric arcs comprising: - a first electrically conductive rod having a first pointed end; - an electrically conductive part, the first pointed end being separated from the electrically conductive part by a gap; - a first electrical conductor connected to the first electrically conductive rod; - a sensor configured to detect the passage of an electric current in the first electrical conductor.

[0007] According to one embodiment, the device includes means for modifying the distance between the first pointed end and the electrically conductive part.

[0008] According to one embodiment, the device comprises an airtight enclosure containing the first pointed end, the gap, and at least a portion of the electrically conductive part.

[0009] According to one embodiment, the enclosure contains an atmosphere consisting of a neutral gas or a mixture of neutral gases.

[0010] According to one embodiment, the device includes a support having a threaded opening, and the first electrically conductive rod includes a threaded portion cooperating with the threaded opening.

[0011] According to one embodiment, the electrically conductive part comprises a second electrically conductive rod having a second pointed end, the the first pointed end being separated from the second pointed end by said gap.

[0012] According to one embodiment, the device further comprises a second electrical conductor connected to the second electrically conductive rod.

[0013] One embodiment also provides for an arc melting furnace comprising an electrode holder, an electrode mounted on the electrode holder, and a hearth comprising a cavity for holding a charge, the arc melting furnace being configured to generate an electric arc for melting the charge and further comprising a device for protection against unwanted electric arcs as defined above, wherein the electrically conductive part corresponds to said electrode holder or is connected to said electrode holder.

[0014] According to one embodiment, the first electrical conductor is connected to earth or to the sole.

[0015] According to one embodiment, the electrically conductive part comprises a second electrically conductive rod having a second pointed end, the first pointed end being separated from the second pointed end by said gap, the device for protection against unwanted electric arcs further comprising a second electrical conductor connected to the second electrically conductive rod and to the electrode holder. Brief description of the drawings

[0016] These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the accompanying figures, among which:

[0017] [Fig.1] is a partial and schematic cross-sectional view of an embodiment of an arc melting furnace comprising a device for protection against unwanted electric arcs;

[0018] [Fig.2] and [Fig.3] are respectively a perspective view with section and a perspective view of the furnace of [Fig.1] comprising an embodiment of the controlled arc generation device of the protection device;

[0019] [Fig.4] and [Fig.5] are respectively a perspective view with hidden edges in dotted lines and a perspective view with partial section and without hidden edges of the embodiment of the controlled arc generation device of figures 2 and 3;

[0020] the [Fig.6] is a timing diagram of the signal provided by the protection device of the [Fig.1];

[0021] [Fig. 7] represents, in a partial and schematic way, another embodiment of the controlled arc generation device of the protection device of [Fig. 1]; and

[0022] Figure 8 shows curves of the evolution of the device's ignition voltage controlled arc generation of the protection device as a function of the product of the gap width and the pressure of the controlled atmosphere; controlled arc generation device of the protection device for different controlled atmospheres. Description of the implementation methods

[0023] The same elements have been designated by the same reference numerals in the different figures. In particular, structural and / or functional elements common to the different embodiments may have the same reference numerals and may have identical structural, dimensional and material properties.

[0024] For the sake of clarity, only the steps and elements necessary for understanding the described embodiments have been shown and are detailed. In particular, the operating principle of an arc furnace is well known to those skilled in the art and is not described in detail thereafter.

[0025] Unless otherwise specified, when referring to two interconnected elements, this means directly connected without any intervening elements other than conductors, and when referring to two coupled elements, this means that these two elements can be connected or linked via one or more other elements. Furthermore, the terms "insulator" and "conductor" are taken to mean "electrically insulating" and "electrically conductive," respectively.

[0026] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures or to an electric arc furnace in a normal operating position.

[0027] Unless otherwise specified, the expressions "approximately", "roughly", and "on the order of" mean to within 10% or 10°, preferably to within 5% or 5°.

[0028] Embodiments of a device for protection against unwanted electric arcs will now be described for an arc melting furnace. However, the device for protection against unwanted electric arcs can be used in any machine comprising live parts between which unwanted electric arcs are likely to occur. This is the case, for example, with other furnace technologies, such as a resistive furnace where an arc can be created between the resistor and the chamber, or an induction furnace where an arc can be created between the inductor and the enclosure. This is also the case with an electric motor in which an unwanted electric arc is likely to form between the rotor and the stator.

[0029] Figure 1 represents an embodiment of an adapted arc melting furnace 10 to melt a charge 11, for example small pieces of refractory metals, such as titanium, zirconium or other reactive metals and alloys. As an example, the charge 11 may weigh from a few hundred grams to a few hundred kilograms.

[0030] The arc melting furnace 10 comprises: - a cooled sole 12, also called a mold or crucible, containing the charge 11; - a cooled and mobile electrode holder 40, on which is mounted an electrode 41, the base 12 acting as an anode and the electrode 41 acting as a cathode or the base 12 acting as a cathode and the electrode 41 acting as an anode depending on the application envisaged; - a containment chamber 30 containing the sole 12 and the electrode and allowing control of the melting conditions of the charge 11, and in particular the atmosphere in which the melting of the charge 11 takes place, the chamber 30 being able to be cooled and electrically isolated from the anode and the cathode; - sealed openings in enclosure 30 allowing cooling and electrical energy to be brought to the anode and cathode; - a generator 45 which may include a starting stage and a power stage - a control system, not shown, configured to control the elements of the furnace, including the generator 45, the cooling devices, the pressure regulation elements in the containment chamber 30, etc.

[0031] In the embodiment illustrated in [Fig.1], the sole 12 corresponds to a circular sole plate 12 having several hollows or cavities or alveoli 14, for example separated by equal intervals and equidistant from the center of the sole plate 12. As an alternative, the sole plate 12 may not be circular. Furthermore, the hearth plate 12 may contain only one cavity 14. The hearth plate 12 is removably attached to a circular water-cooling chamber 16 by means of a watertight seal 18. A hollow shaft 20 is mounted centrally on the lower surface of the chamber 16 and opens into the space between the hearth plate 12 and the chamber 16. A water circulation system, not shown, is arranged in the hollow shaft 20 and between the hearth plate 12 and the chamber 16. In one embodiment, the hollow shaft 20 is mounted by a rotating linkage 22 on a base 24.The hollow shaft 20 then allows the sole plate 12 to rotate relative to the base 24. A hermetic seal, not shown, prevents gas from passing through the rotating link 22. According to another embodiment, the sole 12 is fixed relative to the base 24 and the containment enclosure 30.

[0032] In the embodiment illustrated in [Fig. 1], the sole plate 12 is housed in a removable casing 31 hermetically fixed to the base 24. A turret 32 ​​is located on the upper surface of the turret. This turret may include a window 34 and is positioned in line with the path of rotation of the bowls 14. The casing 31 and the base 24 form the containment enclosure 30. The turret 32 ​​and the casing 31 include water circulation cooling devices, which are not shown. The base 24 includes tubes 36 and 38 for creating a vacuum or supplying a protective gas. The electrode holder 40 is supported on the upper surface of the turret 32 ​​by a removable linkage 42, which, while maintaining a hermetic seal, allows the electrode holder 40 to undergo vertical and angular movements. The electrode holder 40 includes a water circulation cooling device, not shown in [Fig.1].In one embodiment, the furnace 10 is housed in an airtight enclosure 44 containing a controlled atmosphere. In particular, the motors used to move the furnace components can be located within the enclosure 44. The controlled atmosphere can be a neutral gas or a mixture of neutral gases, for example, helium, argon, or neon, at a given pressure, for example, between 100 mbar absolute and 500 mbar absolute. An argon atmosphere may be preferred, in particular, for applications in the nuclear field. An argon atmosphere tends to promote the formation of undesirable electric arcs. In another embodiment, the enclosure 44 is not present. The controlled electric arc generation device 52 can then be exposed to the atmosphere, and the gap 61 then corresponds to an air film.In this embodiment, unwanted electric arcs tend to form in the parts of the furnace located in the casing 31.

[0033] According to one embodiment, to operate the furnace 10, the hearth plate 12 is rotated so as to bring the cavity 14 containing the first charge 11 to be melted in front of the electrode 41, which is then moved closer to the first charge 11. The arc-starting voltage is applied between the electrode 41, which acts as the cathode, and the hearth plate 12, which acts as the anode, so as to cause the arc to strike between the electrode 41 and the charge 11. As described previously, alternatively, the hearth plate 12 can act as the cathode and the electrode 41 can act as the anode depending on the type of alloy and the polarity of the generated particles. The objective is to keep the molten material inside the cavity 14.The progress of the melting is observed, for example through the window 34 of the turret 32 ​​and once the melting is complete, the passage of the arc is interrupted and the hearth plate 12 is rotated again to bring the next charge below the electrode 4L. In the embodiment illustrated in [Fig.1], the furnace 10 allows several charges to be melted successively, without having to open the furnace each time. This results in considerable time savings, since there is no longer a need to re-establish the vacuum or protective atmosphere between the various fusions.

[0034] According to one embodiment, the furnace 10 further comprises a protective device 50 against unwanted electric arcs. The protective device 50 comprises a controlled electric arc generation device 52, shown schematically in [Fig. 1]. The protective device 50 further comprises a conductor 54 that connects the controlled electric arc generation device 52 to a source of low reference potential 56, for example, the anode of the furnace 10 or earth. The conductor 54 may be an electrical cable. The protective device 50 further comprises a sensor 58 for detecting the passage of current in the conductor 54 between the controlled electric arc generation device 52 and the source of reference potential 56.For example, the sensor 58 includes an ammeter for detecting the current intensity in the conductor 54, a voltmeter, a fuse configured to break when a current flows through the conductor 54, or an optical sensor configured to detect the formation of an electric arc. In one embodiment, the sensor 58 is configured to send a signal to the control system of the furnace 10 upon detecting the flow of a current in the conductor 54, allowing, for example, the power supply to the furnace 10 to be shut off and thus preventing damage to it. In the embodiment illustrated in [Fig. 1], the protective device 50 is located in the enclosure 40 outside the containment enclosure 30. Alternatively, the protective device 50 is located inside the containment enclosure 30. In either case, an insulated electrical penetration must be provided through the enclosure for the passage of the conductor 54.

[0035] The controlled electric arc generation device 52 includes at least one rod 60 connected to the conductor 54. The rod 60 includes a pointed end 61 which is separated from another conductive element, which may be the electrode holder 40 or another conductive element connected to the electrode holder 40, by a gap 62.

[0036] Figures 2 and 3 are respectively a cross-sectional perspective view and a perspective view of a more detailed embodiment of the furnace of Figure 1, including the controlled arc generation device 52, the charge to be melted not being shown in these figures. A water circulation cooling device 42 for the electrode holder 40 is visible in Figure 2.

[0037] Fig. 4 and Fig. 5 are respectively a perspective view with hidden edges shown in dotted lines and a perspective view with partial section and without hidden edges of the embodiment of the controlled arc generation device 52 of Figures 2 and 3.

[0038] The controlled electric arc generation device 52 comprises a support 63 having a base 64 fixed to the turret 32 ​​and a flange 65 which rises from the base 64. The support 63 is made of an insulating material. The controlled arc generation device 52 further includes a screen 66 attached to the support 53. In [Fig. 5], half of the screen 66 is removed. Alternatively, the screen 66 may be omitted. The rod 60 is mounted on the flange 64 of the support 53 by means of a threaded connection. According to one embodiment, the flange 64 includes a threaded opening 67 and the rod 60 includes a threaded portion 68 that cooperates with the threaded opening 67. The rod 60 extends from the threaded opening 68 on both sides of the flange 65. The portion of the rod 60 including the pointed end 61 extends between the screen 66 and the base 64. The controlled electric arc generation device 52 further includes an adjustment knob 69 fixed to the blunt end of the rod 60.Rotating the adjustment knob 69 allows the pointed rod 60 to be moved relative to the flange 65 along the axis of the rod 60, thus adjusting the triggering threshold of the protective device 50. Alternatively, the adjustment knob 69 is not present and the protective device 50 includes an actuator configured to move the rod 60 relative to the flange 65 under the action of a command issued by the user of the oven 10. The conductor 54 is fixed to the threaded portion 68 of the rod 60 by a nut 70.

[0039] In the present embodiment, the base 64 is fixed to the turret 32 ​​so that the pointed end 61 of the rod 60 is opposite the electrode holder 40 and separated from the electrode holder 40 by the gap 62. The displacement of the pointed rod 60 relative to the flange 65 modifies the distance between the pointed end 61 and the electrode holder 40, i.e. the width of the gap 62 between the pointed end 61 and the electrode holder 40.

[0040] During operation, the distance between the pointed end 61 and the electrode holder 40 is fixed so that an electric arc does not form between the pointed end 61 and the electrode holder 40 when the electrode 41 is at a normal operating distance from the load 11 during the application of the ignition voltage between the electrode 41 and the hearth plate 12, but so that an electric arc forms between the pointed end 61 and the electrode holder 40 when the electrode 41 is at a distance greater than the normal distance from the load 11 during the application of the ignition voltage. This advantageously prevents an electric arc from forming between the electrode 41 and another part of the furnace 10 without degrading the performance of the furnace 10, the operation of which requires allowing the formation of an electric arc on the load 11 at a distance.For adjusting the distance between the pointed end 61 and the electrode holder 40, a compromise is made to allow the formation of an electric arc on the load 11 without the protection device 50 being triggered (which means that the distance between the pointed end 61 and the electrode holder 40 must not be too small) but so that the protection device 50 is triggered. triggers before the formation of a parasitic electric arc (which means that the distance between the pointed end 61 and the electrode holder 40 must not be too high).

[0041] The formation of an electric arc between the pointed end 61 and the electrode 41 is detected by the sensor 58.

[0042] Figure 6 is a timing diagram of a signal S detected by the sensor 58. For example, the signal S represents the current intensity in the conductor 54, the variation in voltage across the conductor 54, the impedance of the conductor 54, or the light intensity received by the sensor. For example, when the signal S exceeds a threshold TH, this means that a current is flowing in the conductor 54 and that an electric arc has formed between the pointed end 61 and the electrode holder 40.

[0043] Upon detection of the generation of an electric arc in the controlled electric arc generation device 52, the protection device 50 can perform an action, such as issuing an alarm, or commanding the shutdown of the furnace 10.

[0044] The screen 66 serves to prevent an observer from directly looking at the electric arc formed between the pointed end 61 of the rod 60 and the electrode holder 40.

[0045] Fig. 7 represents, in a partial and schematic way, another embodiment of the protection device 50 of the oven of Fig. 1.

[0046] The protective device 50 shown in [Fig. 7] comprises all the elements of the protective device 50 shown in [Fig. 1], the controlled arc generation device 52 further comprising an additional rod 80 having a pointed end 81. The additional rod 80 is connected to the electrode holder 40 by a conductor 82. The conductor 82 may be an electrical cable. The pointed end 81 is positioned opposite the pointed end 61, the gap 62, of width d, being located between the pointed end 81 and the pointed end 61. In one embodiment, the controlled arc generation device 52 comprises an airtight enclosure 83 containing a controlled atmosphere 84 and in which the pointed end 61 of the rod 60 and the pointed end 81 of the rod 80 are located.The controlled atmosphere 84 can correspond to a neutral gas or a mixture of gases, for example helium, argon, neon, at a given pressure P, for example between 100 mbar absolute and 500 mbar absolute. The distance d, the pressure P and the type of gas are set so as to fix the maximum arc formation voltage.

[0047] An advantage of the controlled electric arc generation device 52 shown in [Fig.7] is that it may not be located near the electrode holder 40, but for example in a general electrical cabinet.

[0048] Figure 8 shows Pashen curves of the evolution of C1, C2, C3, C4, C5 of the minimum voltage U causing the generation of an electric arc in the controlled electric arc generation device 52 as a function of the product d*P of the width d of the gap 62 and the pressure P of the controlled atmosphere 84 for different controlled atmospheres 84. The curve C1 is obtained with argon, layer C2 with neon, layer C3 with nitrogen, layer C4 with helium, and layer C5 with hydrogen. Figure 8 illustrates that the controlled electric arc generation device 52 can be configured so that electric arc formation occurs under the desired conditions.

[0049] Various embodiments and variations have been described. A person skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will become apparent to a person skilled in the art.

[0050] Finally, the practical implementation of the described embodiments and variants is within the reach of a person skilled in the art, based on the functional indications given above. For example, for an application to an electric motor, the protection device 50 can be connected to the part between the rotor and the stator, which is at the highest potential.

Claims

Demands

1. Device (50) for protection against unwanted electric arcs comprising: - a first electrically conductive rod (60) having a first pointed end (61); - an electrically conductive part (40; 80), the first pointed end (61) being separated from the electrically conductive part (40; 80) by a gap (62); - a first electrical conductor (54) connected to the first electrically conductive rod (60); - a sensor (58) configured to detect the passage of an electric current in the first electrical conductor (54).

2. Device according to claim 1, comprising means (70) for modifying the distance between the first pointed end (61) and the electrically conductive part (40; 80).

3. Device according to claim 1 or 2, comprising an airtight enclosure (44; 83) containing the first pointed end (61), the gap (62), and at least a portion of the electrically conductive part (40; 80).

4. Device according to claim 3, in which the enclosure (44; 83) contains an atmosphere consisting of a neutral gas or a mixture of neutral gases.

5. Device according to any one of claims 1 to 4, comprising a support (63) having a threaded opening (67), and in which the first electrically conductive rod (60) includes a threaded portion (68) cooperating with the threaded opening (67).

6. Device according to any one of claims 1 to 5, wherein the electrically conductive part (80) comprises a second electrically conductive rod (80) having a second pointed end (81), the first pointed end (61) being separated from the second pointed end (81) by said gap (62).

7. Device according to claim 6, further comprising a second electrical conductor (82) connected to the second electrically conductive rod (80).

8. Arc melting furnace (10) comprising an electrode holder (40), an electrode (40) mounted on the electrode holder (40), and a hearth (42) comprising a cavity (14) for holding a charge (11), the arc melting furnace being configured to generate an electric arc for melting the charge (11) and further comprising a device (50) for protection against unwanted electric arcs according to any one of claims 1 to 7, wherein the electrically conductive part (40; 80) corresponds to said electrode holder (40) or is connected to said electrode holder (40).

9. Arc melting furnace (10) according to claim 8, wherein the first electrical conductor (54) is connected to ground or to the hearth (12).

10. Arc melting furnace (10) according to claim 8 or 9, wherein the electrically conductive part (80) comprises a second electrically conductive rod (80) having a second pointed end (81), the first pointed end (61) being separated from the second pointed end (81) by said gap (62), the device (50) for protection against unwanted electric arcs further comprising a second electrical conductor (82) connected to the second electrically conductive rod (80) and to the electrode holder (40).

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