EP2297757B1 - Use of an electrical contact material for blowing an electric arc - Google Patents

Abstract

The material has an array of conductor metal and non-magnetized magnetic entities, where the entities are magnetized with an average orientation defined by a direction of a magnetic field applied on the material for blowing an electrical arc between electrical contact studs. Each electrical contact stud has an upper layer that comprises material such as silver or copper, or stable refractory compound. The magnetic entities are rare earth based hard magnetic compounds such as nanostructured alloy of neodymium/praseodymium, iron and boron. An independent claim is also included for a method for fabricating an electrical contact stud.

Description

Technical area

The present invention relates to the field of electrical contacts. It relates, more particularly, to the use of an electrical contact material with arc extinguishing effect.

State of the art

Such a type of material finds its application mainly for the realization of so-called "low voltage" contacts, that is to say whose operating range is approximately between 10 and 1000V and between 1 and 10000A. These contacts are generally used in the domestic, industrial and automotive fields, both in direct current and AC, for switches, relays, contactors and circuit breakers, etc.

When a pair of pads of electrical contacts under tension is opened, the current continues to pass from one pad to the other by ionizing the gas through which it passes. This column of ionized gas, commonly called "electric arc", has a maximum length that depends on various parameters such as the nature and pressure of the gas, the terminal voltage, the contact material, the geometry of the device, the impedance of the circuit, etc.

The energy released by the electric arc is sufficient to melt the material constituting the pads, which entails not only the degradation of the metal parts but also, sometimes, their welding, with the consequence of blocking the device.

In AC applications, the zero-crossing of the voltage facilitates the breaking of the arc. Nevertheless, some protective devices have to cut very high currents, which cause arcs sufficiently energetic to damage the contacts.

On the other hand, for DC applications, arcing is very stable, especially when the voltage is well above 10V. A solution to cut the arc is to increase its length so that it becomes unstable and disappears on its own. For a voltage of 14V, a distance of the order of a millimeter is sufficient whereas for a voltage of 42V, particularly when in the presence of an inductive load, this distance can be several centimeters. This seriously complicates the construction of the cut-off devices and the duration of the arcs created greatly reduces their service life.

The problem arises particularly in the automobile industry which is considering the use of continuous 42V circuits or more to adapt to the ever increasing number of electrical devices present in cars (up to one hundred engines in one high-end vehicle). At such tensions, the interest of limiting the problems related to arcs becomes paramount.

• une résistance de contact faible et stable pour éviter un échauffement excessif lors du passage du courant ;
• bonne résistance au soudage en présence d'un arc électrique ; et
• faible érosion sous l'effet de l'arc.

Thus, the materials of the electrical contacts must satisfy the following three requirements:

• a low and stable contact resistance to prevent overheating during current flow;
• good resistance to welding in the presence of an electric arc; and
• weak erosion under the effect of the arc.

In order to satisfy these partially contradictory requirements, one solution consists in using pseudoalloys comprising a silver or copper matrix and, inserted in this matrix, a fraction consisting of approximately 10 to 50% by volume of refractory particles (for example , Ni, C, W, WC, CdO, SnO ₂ ) of a size generally between 1 and 5 microns. The material thus obtained is more resistant to the energy released by the electric arc. Although an interesting solution, this method does not allow to limit the mergers and, because of their repetition, problems of erosion and welding of the studs can occur in the short or medium term.

Another solution, described in the US patent US 3,626,124 consists in using a material comprising single-domain magnetic particles. Such particles are spontaneously magnetized in a random orientation in the absence of an applied external field. These particles are therefore initially magnetized and do not need an external source of magnetization. The field generated by each magnetized particle acts on the arc of cutoff, facilitating its blowing. The particles described remain monodomain even after a heating beyond their Curie temperature so that the blowing efficiency is not affected by the heating due to the arc of cutoff, during openings previous contacts. However, each particle acts individually on the breaking arc so that the magnetic blow effect is very small. This solution is therefore not satisfactory.

Furthermore, when it comes, in alternating current, to make protection devices (circuit breakers) capable of cutting very high currents, it has been proposed to use auxiliary means to facilitate the extinction of the arc or avoid re-ignition: electromagnetic or pneumatic blow.

For example, such an electromagnetic extinction solution by devices external to the contact itself is described in the document EP 1 482 525 . The latter discloses a magnetic device placed at a distance from the contact and which generates a magnetic field extending an electric arc that would occur between the pads, in order to extinguish it. However, the extra cost, size and overweight caused by this solution make it problematic, especially for automotive applications.

Among other things, it has been proposed to replace the gas present in the space separating the two contacts with a gas that is very stable and therefore difficult to ionize, such as SF ₆ . However, this solution is complex to implement.

An object of the present invention is therefore to overcome these disadvantages, by proposing to use an electrical contact material to make contact pads whose operation is not impaired in the short term or in the long term by the energy an electric arc.

Disclosure of the invention

The invention relates to the use of a material comprising a matrix of conductive metal and magnetic entities representing between 8 and 80% by weight of the material and comprising hard magnetic phases, said magnetic entities initially not magnetized having been made magnetized with a mean orientation, defined by the direction of a magnetic field applied to said material, for blowing an electric arc between two pads of electrical contacts, at least one of which comprises said material, and thus reduce the duration of the arc.

Alternatively, the material may further comprise a stable refractory fraction at a temperature greater than 900 ° C.

Advantageously, at least one of the phases of the magnetic entities is a hard magnetic compound based on rare earths.

To enable use according to the invention, said material is capable of generating a magnetic induction field, measured at its surface, greater than 20 mT, preferably greater than 60 mT, and more preferably greater than 100 mT.

Particularly remarkable effects on the extinction of an electric arc have been observed for use according to the invention, according to which said pads define between them an axis, at least one of said pads being made in said material and having a magnetization generating a magnetic field perpendicular to said axis.

Advantageously, at least one of said pads which comprises said material with the magnetic entities, has an overlayer comprising a material selected from silver and copper.

The present invention also relates to a constituent material of an electrical contact pad comprising a matrix of conductive metal and magnetic entities representing between 8 and 80% by weight of the material and comprising hard magnetic phases, said magnetic entities initially not magnetized having been rendered magnetized with a mean orientation, defined by the direction of a magnetic field applied to said material, at least one of the magnetic phases being a rare earth compound, with the exception of the samarium.

• élaboration d'un matériau à partir d'argent ou de cuivre pour former la matrice dudit matériau et d'entités magnétiques comprenant des phases magnétiques dures, lesdites entités magnétiques étant non aimantées, l'une au moins des phases magnétiques étant un composé à base de terres rares,
• mise en forme du plot,
• assemblage sur un support, et
• aimantation du plot.

According to another aspect, the present invention relates to a method of manufacturing an electrical contact pad comprising the following steps:

• developing a material from silver or copper to form the matrix of said material and magnetic entities comprising hard magnetic phases, said magnetic entities being non-magnetized, at least one of the magnetic phases being a compound based on rare earths,
• layout of the plot,
• assembly on a support, and
• magnetization of the stud.

According to another aspect, the invention relates to a pair of pads of electrical contacts, said pads defining between them an axis, in which that at least one of said pads is made of a material as defined above and has a magnetization generating a magnetic field perpendicular to said axis.

In the case of direct current, very good results have also been observed for a pair of electrical contact pads comprising, at the cathode, a contact pad made of a material defined above.

Brief description of the drawings

The invention will be better understood on reading the description which follows, made with reference to the appended figure, illustrating different orientations of the magnetic field presented by the pads of an electrical contact.

Mode (s) of realization of the invention

• une matrice en métal conducteur, généralement en argent ou en cuivre ; et
• des entités magnétiques représentant entre 8 et 80% en poids du matériau et comprenant des phases magnétiques dures, lesdites entités magnétiques étant initialement non aimantées et susceptibles d'être aimantées, avec une orientation moyenne, définie par la direction d'un champ magnétique appliqué sur ledit matériau.

The contact material used in the present invention consists essentially of:

• a matrix of conductive metal, usually silver or copper; and
• magnetic entities representing between 8 and 80% by weight of the material and comprising hard magnetic phases, said magnetic entities being initially non-magnetized and capable of being magnetized, with a mean orientation, defined by the direction of a magnetic field applied to said material.

As a result, the material used according to the invention initially contains multidomain magnetic entities forming a set initially initially unmagnetized, and then to be magnetized by the application of a field. Preferably, the material used according to the invention does not initially contain spontaneously magnetized monodomain entities.

Preferably, the magnetic entities represent between 10 and 50% by weight of the material, preferably between 12 and 30% by weight, and more preferably between 18 and 22% by weight of said material.

Magnetic entities include magnetic phases that can be obtained from one or more ferromagnetic or ferrimagnetic hard compounds. Advantageously, they are chosen from compounds based on rare earths, among which mention may be made of the so-called RE-Fe-B type compounds (RE being the abbreviation of Rare Earth, Rare Earth). Preferably, RE is neodymium or praseodymium. Other compounds of the RE-M type can be used, RE being preferably La, Gd, Y or Lu, of the 1/5, 1/7 or 2/17 type, and M being predominantly Co or Fe and possibly containing Cu, Zr, Al and other minority elements. The compounds of the RE-Fe-N type can also be used.

Other compounds such as those of the family Pt (Fe, Co) may also be suitable, or compounds of the ferrite type of barium or strontium.

Other materials can still be envisaged, the essential thing being that the magnetic entities have a coercive field and a residual induction sufficient to allow their use in the targeted applications, these two parameters can be evaluated by simple experimental tests. Indeed, as will be explained below, it is necessary that the contact generates a certain magnetic field so as to destabilize a possible electric arc occurring between the pads. In particular, it must be that after having been exposed to a magnetic field, the material has a sufficient and stable long-term residual induction for long-term use. This induction, obtained by magnetization of the pads under an external magnetic field, can be characterized by the magnetic field generated on the surface of the pads, and persistent after removal of the applied magnetic field. As an indication, the field generated on the surface must be greater than 20 mT, preferably 60 mT, and more preferably greater than 100 mT, measured using a Hall effect probe distributed by Lakeshore.

Optionally, the matrix comprises a refractory fraction, stable at a temperature above 900 ° C. The refractory fraction may comprise one or more of the elements selected from the following group: CdO, SnO ₂ , ZnO, Bi ₂ O ₃ , C, WC, MgO, In ₂ O ₃ , as well as Ni, Fe, Mo, Zr , W or their oxides.

The refractory fraction is added in an amount so that the percentage of magnetic entities is at least 8% and the amount of conductive metal is at least 20%.

Advantageously, the magnetic entities are dispersed in the matrix, either regularly, or according to a concentration gradient, or again in localized blocks.

In addition, the material may also contain dopants or minor additives, facilitating the use of the material, which may be, for example, Ni, Co, Fe, Bi, Re, Zr and their oxides.

The material described above is used to make pads of electrical contacts. The first steps of the process of developing the material and forming the contact pads are common and are known to those skilled in the art who can choose between several techniques. In addition, the method comprises an additional step of magnetizing the material on the already developed pads.

• métallurgie des poudres,
• voie chimique de précipitation de sels en solution,
• atomisation,
• dépôt de couche mince ou épaisse, ou
• extrusion à partir d'une billette ou d'un mélange de poudres.

In particular, and without it being necessary to detail it further for the person skilled in the art who will be able, by means of a few tests of current practice, to implement the following techniques, the elaboration of the material used according to the invention may be carried out by:

• powder metallurgy,
• chemical way of precipitation of salts in solution,
• atomization,
• Thin or thick layer deposition, or
• extrusion from a billet or a mixture of powders.

Advantageously, the material preparation step may be carried out by powder metallurgy, one of the magnetic entities being nanostructured RE-Fe-B, where RE is a rare earth element.

A preferred direction of the magnetic entities may be obtained by applying a suitable method during the development of the pads (pressure, magnetic field, heat treatment). This operation is not essential but it makes it possible to increase the magnetization of the studs induced by the field, applied after elaboration of the pads.

It should be noted that, without limitation, the use, as a starting material for constituting the magnetic entities of the contact, of a ribbon The nanostructured RE-Fe-B obtained by a rapid solidification technique, particularly by the technique known as melt spinning gives excellent results. It is not necessary to further describe this technique known to those skilled in the art. In summary, it consists in flowing, through a nozzle, molten metal contained in a reservoir, and in bringing a net of liquid metal in contact with a cylinder, for example copper, rotating great speed. Thanks to this technique, the RE-Fe-B cools by taking a microstructure, which allows it to present remarkable hard magnetic characteristics for the intended use.

The RE-Fe-B can be combined with other magnetic materials to optimize the magnetic properties of the assembly, the RE-Fe-B advantageously representing at least 50% by weight of the magnetic entities.

Contact pads are then shaped by cutting strips, stamping son, unit compression. They are then arranged on a suitable support, by any traditional method of assembling electrical contacts, in particular: resistance welding, resistance brazing, induction brazing, flame or oven brazing, crimping, incrustation ... view of their use as electrical contacts.

Alternatively, the material used according to the invention can be formed into a washer or a layer, forming a magnetic system, made integral with a traditional electrical contact pad by incrustation, welding, brazing or riveting, or by layer deposition (s). In the latter case, the magnetic material, the contact material or both may be in the form of one or more layers. The magnetic system can also serve as a mechanical support and supply current to the electrical contact. It is advantageously possible to adapt the magnetic system according to the variant in existing installations, while retaining the initial contact material, since it occupies only a small space in addition to the contact, unlike the electromagnetic members of the prior art.

In the shaped pads, the magnetic entities are not magnetized. The pads must then undergo the magnetization step by applying a magnetizing magnetic field to provide the magnetic non-magnetized entities global magnetization in a mean orientation defined by the applied field. The studs can then fully play their role of blower or arc extinguisher. This operation can take place in the factory, after development of the stud. It can also take place at the user, before or after the final assembly of the contact. It is carried out by exposing the pads to a magnetic field of intensity between 0.5 and 30 T, preferably between 1 and 30 T, and still more preferably between 1 and 10 T. Thus, according to the invention, the material used in the form of pads comprises magnetic entities initially not magnetized which are either capable of being made magnetized by the application of a magnetic field to the user, or already made magnetized by the application of a magnetic field in factory.

By this application of a magnetic field of appropriate direction and intensity, on the already developed pads, a global magnetization of the pads is created, whose orientation is defined by the applied field. As a result, a magnetic field is generated in the environment of the pad. This field acts on the arc of cutoff and contributes to its blowing.

The field may in particular be applied in parallel or preferably perpendicular to the longitudinal axis of a stud, so that the latter has field lines as illustrated, respectively, on the Figures 1a and 1b . The conditions of the magnetization step (duration and intensity of the field) are adapted to the magnetic material so that, after having undergone the magnetization step, the studs are source of a magnetic induction field, which, measured at their surface, is greater than 20 mT, preferably 60 mT, and more preferably greater than 100 mT.

The pads thus obtained are then implemented in electrical contacts formed of two pads defining between them a first axis. The contact may comprise a single pad obtained according to the above method, arranged in the case of a DC circuit, the anode or the cathode. It is also possible that the two studs forming the contact are made of a magnetic material used according to the invention. Various magnetic field orientations are possible and conceivable, for example, when a single magnetic pad is used, the field it generates can be oriented parallel or perpendicular to the first axis.

Alternatively, the stud may comprise an overlay deposited on the magnetic material. Such an overcoat comprises a conductive material chosen from silver and copper and optionally a refractory compound chosen from the group comprising compounds CdO, SnO ₂ , ZnO, Bi ₂ O ₃ , C, WC, MgO, In ₂ O ₃ , as well as Ni, Fe, Mo, Zr, W or their oxides.

This overlay advantageously makes it possible to isolate the magnetic entities from the stud of the contact surface and thus to reduce the risks of soldering on closing. Indeed, the blowing effect can be attenuated by the ionization of the constituent elements of the magnetic compound, the latter can increase the contact resistance and promote welding. In any case, the extreme surface of the contact is strongly heated under the effect of the arc so that the magnetic properties of the surface entities are generally destroyed in operation. The overlayer must be thin enough so that the field created by the underlying magnetic entities in the arc area remains sufficiently intense, and possibly thick enough not to be completely melted under the effect of the arc. However, it turns out that the reduction in the duration of the arc obtained according to the invention leads to very low erosion. Thus, the overlay may have a thickness of between 0.05 mm and 3 mm, preferably between 0.1 mm and 2 mm, and more preferably between 0.2 mm and 1 mm.

The following examples illustrate the present invention without, however, limiting its scope.

Example 1

The elaboration of the material is done by metallurgy of the powders. Thus, a ribbon of Nd-Fe-B produced by the technique known as "melt spinning" is reduced to powder, under argon, by ball milling, until a particle size of between 1 and 50 μm is obtained. . The duration of this operation is about 5h.

The powder thus obtained is mixed with powdered silver, the particles of which have a mean diameter of between 15 and 50 μm. The mixture is produced in a mass proportion of 80% of silver and 20% of EM magnetic entity powder. A magnetic material is obtained constituting an electrical contact pad.

An electrical contact pad is then shaped by unitary compression and compacted under a pressure of 700 MPa.

The pad is then sintered under vacuum at 400 ° C for about 30 minutes.

The pad is then assembled on a support according to one of the techniques mentioned above, for use in an electrical contact.

Finally, the pad is magnetized by exposing it to a magnetic field of 8 T. The pad is oriented perpendicular to the magnetic field, as illustrated on the figure 1a so that it has a magnetization perpendicular to its longitudinal axis. With the magnetization conditions above, the pad is a source of a remanent induction field of about 60 mT at the surface.

The pad obtained above is then used in a contact of a resistive-type electrical circuit, operating under a 42 V DC voltage, with an intensity of 37.5 A. For example, only a magnet pad is disposed at the cathode, the other being silver.

With this configuration (perpendicular magnetization, a single magnetic pad at the cathode), the opening arc duration is measured. Closure tests are also performed to simulate the risks of welding, under the same conditions as for opening, but with a current of 90 A. measures the percentage of welding obtained, whose breaking force is greater than 0.1 N.

The results obtained are reported in Table 1 below.

Example 2

Example 1 is reproduced by replacing 6% by weight of silver of the matrix with 6% by weight of a refractory compound (SnO ₂ ).

The same tests as for example 1 are carried out. The results obtained are reported in Table 1 below.

Example 3

An overlayer 0.6 mm thick is applied to the magnetic material of the pad obtained in Example 1. Said overlayer comprises 100% silver.

The same tests as for example 1 are carried out. The results obtained are reported in Table 1 below.

Examples 4-6

For comparison, Example 1 is reproduced by not subjecting the pad to a magnetization (Example 4) or by using other materials to make the pads of the contact (Examples 5 and 6).

The same tests as for example 1 are carried out. The composition of these materials as well as the results obtained with the elaborate pads are reported in Table 1 below: Table I Example Composition of contacts Arc duration at opening (ms) % of solder at closing 1 (inv.) Ag (80) EM (20) - Ag 3 60 2 (inv.) Ag (74) SnO _{2 (} 6) EM (20) - Ag 3 9 3 (inv.) Ag (80) EM (20) + overcoat - Ag 3 1 4 (comp.) Ag (80) EM (20) (unmagnetized) - Ag 9 60 5 (comp.) Ag-Ag 9 7 6 (comp.) AgSnO ₂ 10 / AgSnO ₂ 10 24 3

The results of Table 1 show that the use according to the invention of the magnetic material described above for making electrical contact pads makes it possible to reduce the arc duration at the opening of 9 ms, or even 24 ms at 3 ms. . Example 4 also shows the importance of the magnetization step of the stud since a contact comprising a non-magnetized stud has an arc duration at the opening of 9 ms while the contact comprising the magnetic pin has a arc duration at opening of 3 ms.

In addition, the addition of a refractory compound (Example 2) or the use of an overlayer (Example 3) makes it possible to greatly reduce the tendency of the studs to be welded, consisting of the magnetic material defined above without significantly affecting the duration from bow to opening. The use of an overlay makes it possible to obtain particularly interesting results.

Claims (19)

1. The use of a material, including a matrix in conducting metal and magnetic entities representing between 8 and 80% by weight of the material, and comprising hard magnetic phases, for blowing out an electric arc between two electric contact pads, at least one comprising said material, characterized in that said magnetic entities are initially non-magnetized and they have been magnetized with an average orientation defined by the direction of a magnetic field applied on said material.
2. The use according to claim 1, characterized in that the magnetic entities represent between 10 and 50% by weight of said material, preferably between 12 and 30% by weight, and more preferentially between 18 and 22% by weight of said material
3. The use according to any of the preceding claims, characterized in that said material further includes a stable refractory fraction at a temperature above 900°C.
4. The use according to claim 3, characterized in that said refractory fraction includes one or more of the elements selected from the following group: CdO, SnO₂, ZnO, Bi₂O₃, C, WC, MgO, In₂O₃, as well as Ni, Fe, Mo, Zr, W or oxides thereof.
5. The use according to any of claims 1 to 4, characterized in that at least one of the phases of the magnetic entities is a hard magnetic compound, based on rare earths.
6. The use according to claim 5, characterized in that the magnetic entities are nanostructured RE-Fe-B alloys, wherein RE is a rare earth element.
7. The use according to claim 6, characterized in that RE is neodymium or praseodymium.
8. The use according to any of claims 1 to 7, characterized in that the material is capable of generating a magnetic induction field, measured at its surface, of greater than 20 mT, preferably greater than 60 mT and more preferentially greater than 100 mT.
9. The use according to any of the preceding claims, characterized in that said pads define between each other an axis, at least one of said pads being made in said material, and having magnetization generating a magnetic field perpendicular to said axis.
10. The use according to any of the preceding claims, characterized in that at least one of said pads comprising magnetic entities has an overlayer comprising a material selected from silver and copper.
11. The use according to claim 10, characterized in that said overlayer further comprises a refractory compound selected from the group comprising the compounds: CdO, SnO₂, ZnO, Bi₂O₃, C, WC, MgO, In₂O₃, as well as Ni, Fe, Mo, Zr, W or oxides thereof.
12. The use according to claim 10 or 11, characterized in that said overlayer has a thickness comprised between 0.05 mm, and 3 mm, preferably comprised between 0.1 mm and 2 mm, and more preferentially comprised between 0.2 mm and 1 mm.
13. A material of an electric contact pad including a matrix in conducting metal and magnetic entities representing between 8 and 80% by weight of the material, and comprising hard magnetic phases, characterized in that said magnetic entities are initially non-magnetized and they have been magnetized with an average orientation, defined by the direction of a magnetic field applied on said material, at least one of the magnetic phases being a compound, based on rare earths, excluding samarium.
14. A method for manufacturing an electric contact pad comprising the following steps:

    - elaborating a material from silver, or copper in order to form the matrix of said material, and of magnetic entities comprising hard magnetic phases, said magnetic entities being non-magnetized, at least one of the magnetic phases being a compound, based on rare earths,

    - shaping the pad,

    - assembling on a support, and

    - magnetizing the pad.
15. The manufacturing method according to claim 14, wherein said step for elaborating the material is achieved by powder metallurgy, one of the magnetic entities being nanostructured RE-Fe-B, wherein RE is a rare earth element.
16. The method according to one of claims 14 and 15, characterized in that the magnetization step is achieved so that said pad generates a magnetic induction field measured at its surface of greater than 20 mT, preferably greater than 60 mT and more preferentially greater than 100 mT.
17. A pair of electric contact pads, said pads defining between them an axis, characterized in that at least one of said pads is made in a material according to claim 13, and has magnetization generating a magnetic field perpendicular to said axis.
18. A pair of electric contact pads, comprising at the cathode a contact pad made in a material according to claim 13.
19. An electric contact pad made in a material according to claim 13, having an overlayer comprising a material selected from silver and copper, and optionally a refractory compound selected from the group comprising the compounds: CdO, SnO₂, ZnO, Bi₂O₃, C, WC, MgO, In₂O₃, as well as Ni, Fe, Mo, Zr, W or oxides thereof.

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