BEARING

The embodiments disclosed relate generally to magnetic bearings for rotary machines having a rotor. In particular, the embodiments relate to magnetic bearings for rotary machines in which the rotor and the bearing are, in use, in contact with a fluid, for instance a gaseous atmosphere, that is corrosive, acid, or carrying particles. Some exemplary embodiments specifically relate to a rotary machine comprising such a magnetic bearing.

Use of magnetic bearings to rotary machines is becoming more and more widespread, in particular in case of corrosive fluid. When the operating fluid of the machine in which the bearing operates is either acid, or corrosive or carrying particles, it is essential to protect the coils of the magnetic bearing and the associated means, by using anti-corrosion protection technologies. An example of such a technology is the jacketed bearing in which the stator portion of the bearing is protected by a metal jacket made of a material that does not oxidize or corrode and in general does not suffer any phenomena related to the aggressiveness of the environment.

A jacket may be in the form of a plate welded to an annular support in which is placed a stator magnetic circuit comprising at least one coil and a ferromagnetic body. The annular support may be made of a stainless material, such as magnetic stainless steel. The jacket of closure plate may be made out of the same material as the annular support, or it may be of a different metallic material, such as nickel base alloy for example.

In order to withstand operating conditions (pressure, fast variations in pressure, temperature, ability to withstand corrosion and abrasion), the jacket generally presents thickness lying in the range of 0.3 to 1mm or more, for instance in the range of 0.3 to 0.5mm, i.e. similar to that of the airgap of the magnetic bearing (which is the distance between the stator magnetic circuit and a rotor armature of the bearing). The presence of such a jacket of non-magnetic material thus amounts to increasing the thickness of the airgap of the bearing, which leads to a significant limit on the load available from said bearing. Moreover, it does not totally ensure the lack of contacts between the jacket and the rotor armature of the magnetic bearing, in any conditions.

It is therefore desirable for the thickness of the jacket to be reduced and for the jacket to be constituted by a fine metal sheet. Nevertheless, it requires the use of specific materials having high mechanical properties and anti-corrosion properties, in order to ensure the protection of the stator magnetic circuit against corrosion and to keep the same shape and dimensions in operation.

Furthermore, the production of the jacket, after welding and final re-machining, does not allow to check the exact thickness of the jacket, and thus the actual airgap of the bearing.

An object of the present invention is to remedy the above drawbacks while keeping the benefit of the principle of jacketing bearings. In particular, one aim of the invention is to provide a magnetic bearing which is simpler to be built and having a higher loading capacity.

According to a first aspect, in an exemplary embodiment, a magnetic bearing for a rotary machine having a rotor comprises a stator magnetic circuit secured to a stationary support device. The stator magnetic circuit comprises at least one coil and a ferromagnetic body placed in a protective annular support, the protective annular support leaving uncovered a surface of the ferromagnetic body and a surface of the at least one coil. The bearing may also comprise at least one annular plug placed on the surface of the at least one coil which is left uncovered by the protective annular support, and the annular plug and the surface of the ferromagnetic body which is left uncovered by the protective annular support are coated by a protective layer. Thanks to the protective layer, the material of the plug may be chosen for its magnetic and mechanical properties: the properties against corrosion are no more relevant. The material of the plug and the material of the stator magnetic circuit are protected by the protective layer, against corrosion. In particular, the protective layer avoids wet C0 ₂ corrosion damages on carbon and low alloy steels, and avoids chlorides pitting corrosion damages on stainless steel. It is then possible to choose these materials (having the wanted magnetic and mechanical properties) for the plug. Moreover, when the material of the plug is a ferromagnetic material, it is no more necessary to have a thin plug to protect the coils: the plug may have a larger thickness than a standard jacket, which reduces the requirements of the material regarding the mechanical properties and the deformation of the plug in use, leading to a longer lifetime of the bearing and to a smaller airgap.

Therefore, thanks to the lack of a jacket and thus to the reduction of the airgap, the capabilities of the magnetic bearing of the invention are increased. Moreover, the protective layer can be easily refurbished, during a maintenance step, which allows to improve and to ease the serviceability of the bearing. Furthermore, the protective layer is cheaper than a standard jacket.

In some embodiments, the protective layer may comprise a layer of nickel.

Said layer of nickel may be formed by electroless-nickel plating. Said layer of nickel may comprise nickel and phosphorus.

In some embodiments, the annular plug may comprise magnetic material chosen in the group of ferromagnetic material, magnetic stainless steel and nickel base alloy.

According to an embodiment, the bearing is an axial magnetic bearing. The bearing may comprise a rotor armature in the form of a disk secured to the rotor, and the stator magnetic circuit may be facing said rotor armature.

The rotor and the rotor armature may be, in use, in contact with a fluid, for instance a gaseous atmosphere, that is corrosive, acid, or carrying particles.

In some embodiments, the annular plug is brazed to the at least one coil, for instance by low temperature brazing. In some embodiments, the annular plug may have a U section with a radial web and two axial flanges. According to a further aspect, a rotary machine, for example a turbomachinery, may comprise a rotor and a bearing as previously defined.

According to a further aspect, a process to manufacture a bearing as previously described, may comprise the following steps of: a) welding the at least one annular plug to the at least one coil and/or to the ferromagnetic body, and b) coating the at least one annular plug and the surface of the ferromagnetic body which is left uncovered by the protective annular support with the protective layer.

In some embodiments, the process may also comprise, between steps a) and b), a step of re-machining the at least one annular plug and the surface of the ferromagnetic body which is left uncovered by the protective annular support to get a flat surface.

Other characteristics used appear on reading the following description of a particular embodiment of the invention given as a non limiting example and with reference to the accompanying drawings, in which: Fig.l is an axial section view on line I-I of Fig.2, showing a magnetic bearing according to an exemplary embodiment;

Fig.2 is a radial section view on line II-II of Fig.l .

The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale.

Figures 1 and 2 show an exemplary embodiment of an axial magnetic bearing 1 of the present invention, for a rotary machine. The magnetic bearing 1 comprises a stator armature 2 and a rotor armature 3 in the form of a disk secured to a rotary shaft 4 of the rotary machine.

The stator armature 2 comprises a stator magnetic circuit 5 including, in conventional manner, one or more annular coils 6 and a ferromagnetic body 7. The ferromagnetic body 7 may be massive or it may be laminated locally. The stator magnetic circuit 5 is placed in a metallic protective annular support 8 that is itself secured to a stationary support device 9.

The stator magnetic circuit 5 is placed facing the rotor armature 3. The stator magnetic circuit 5 and its protective annular support 8 define an airgap Δ relative to the rotor armature 3. In some embodiments, the airgap Δ may lie in the range of 0.4mm to 1.5mm, preferably in the range of 0.4mm to 1.2mm.

The protective annular support 8 of the stator magnetic circuit 5 has a U section with a radial web 10 and inner and outer axial flanges 11 and 12. The length of the flanges 1 1 and 12 in the direction of the axis of the rotary shaft 4 is at least equal to the height of the ferromagnetic body 7 of the stator magnetic circuit 5. Therefore, the protective annular support 8 leaves uncovered a surface of the ferromagnetic body 7, in particular the surface of the ferromagnetic body 7 facing the rotor armature 3, and a surface of the one or more annular coils 6, in particular the surface of the one or more annular coils 6 facing the rotor armature 3.

The magnetic bearing 1 also comprises one or more annular plugs 13. The annular plugs 13 have a U section with a radial web 14 and inner and outer axial flanges 15 and 16. The annular plugs 13 are welded to the ferromagnetic body 7 by their flanges 15 and 16. The annular plugs 13 may also be brazed to the coils 6 by the use of low temperature brazing material. The annular plugs 13 are placed on the surface of the one or more coils 6 which is left uncovered by the protective annular support 8.

The annular plugs 13 may comprise magnetic material, for instance magnetic stainless steel, nickel base alloy such as Inconel® or preferably ferromagnetic material such as carbon and low alloy steels. In particular, as the plugs are coated by a protective layer to be protected against corrosion, it is no more necessary to use materials with high properties against corrosion: the material of the plugs 13 is chosen according to its magnetic properties and its mechanical properties. Moreover, as the surface of the ferromagnetic body 7 which is left uncovered by the protective annular support 8 and the external surface of the radial web 14 of the annular plugs 13 may be re-machined to get a surface with an improved flatness, the annular plugs 13 may have a thickness similar to or greater than the one of the protective annular support 8, in order to avoid any deformation of the annular plugs

13 under pressure.

The magnetic bearing 1 also comprises a protective layer 17. The aim of the protective layer 17 is to protect the stator magnetic circuit 5 against corrosion. The protective layer 17 is present on the surface of the ferromagnetic body 7 left uncovered by the protective annular support 8, and the external surface of the web

14 of the annular plugs 13. In other words, the protective layer 17 covers the surface of the stator armature 2 facing the rotor armature 3. The protective layer 17 may also cover the external surface of the flanges 11, 12 of the protective annular support 8.

In some embodiments, the thickness of the protective layer 17 may be in the range of lnm to 1mm, preferably in the range of lOOnm to ΙΟμιη.

In some embodiments, the protective layer 17 may be a layer of nickel. The layer of nickel may be formed by electroless-nickel plating. The layer of nickel may comprise nickel and phosphorus.

Thanks to the use of the annular plugs 13 and of the protective layer 17, it is possible to get a protection of the stator magnetic circuit 5 against corrosion, while keeping an airgap Δ between the stator magnetic circuit 5 and the rotor armature 3 smaller than in the magnetic bearings of the prior art. In particular, when the annular plugs 13 comprise ferromagnetic material, the airgap Δ is the sum of the distance between the protective layer 17 and the rotor armature 3, and of the thickness of the protective layer 17.

Moreover, thanks to the re-machining of the surface of the ferromagnetic body 7 together with the surface of the radial web of the annular plugs 13, and thanks to the coating by the protective layer, it is possible to get a surface with a high flatness facing the rotor armature 3, and then to reduce the airgap. The above description is made with reference to an axial type magnetic bearing. However, it can be applied in like manner to a magnetic bearing of radial type or to a magnetic bearing of conical type combining the functions of a radial bearing and of an axial bearing.