Reversible mounting of a permanent magnet in an electric machine

Abstract

The invention relates to an electric machine with a permanent magnet rotor, the rotor comprising a magnetic circuit (22) and at least one permanent magnet (20), the permanent magnet (20) extending along the axis of rotation of the electric machine and having an overall shape of radial section, the magnetic circuit (22) comprising a cavity (26) configured to contain the permanent magnet (20), the magnetic circuit comprising at least one lumen (28, 30) opening onto the cavity (26), in the lumen (28, 30) an outer surface (38, 40) of the permanent magnet (20) is distant from the magnetic circuit (22) so as to form a barrier to the magnetic flux generated by the permanent magnet (20). The magnetic circuit (22) comprises, in the lumen (28, 30), an elastic part (32, 34) bearing against the permanent magnet (20) in order to limit the displacement of the permanent magnet (20), in translation parallel to the axis of rotation.

Description

DESCRIPTION Title of the invention: Reversible mounting of a permanent magnet in an electrical machine

[0001] The invention concerns electrical machines used in a wide variety of fields such as transport, industry, electricity generation, etc. More and more permanent magnet electrical machines are being implemented. These machines make it possible, in particular, to avoid the use of wound rotors requiring an electrical connection.

[0002] Permanent magnets are made of so-called hard ferromagnetic materials, i.e., those with a wide hysteresis cycle. In the mid-20th century, iron-nickel-aluminum alloys, to which cobalt can be added, contributed to the development of permanent magnets. More recently, the use of certain rare earths, particularly those from the lanthanide family such as samarium, cerium, and neodymium, has significantly increased the demagnetizing coercive fields of magnets. Today, neodymium-iron-boron (NdFeB) alloys are widely used.

[0003] In order to limit the use of natural resources, we aim to recycle as much complex equipment as possible at the end of its life. To do this, it is necessary to separate the different components of the equipment in order to process these components separately, each through its own recycling process.

[0004] The recycling of computer hard drives containing NdFeB magnets has been extensively studied. A hydrogen decrepitation technique leading to the fragmentation of the hard drives makes it possible to separate and recover the magnets in powder form.

[0005] Electric traction motors have recently made significant progress, particularly in the automotive industry. The increase in the rotational speed of electrical machines used, particularly as motors in automobiles, poses problems in the mechanical retention of the magnets in the rotors equipped with them. To ensure this mechanical retention, surface mounting of the magnets is often replaced by buried mounting inside stacks of metal sheets forming a magnetic circuit ensuring the guidance of the magnetic flux. generated by the magnets. For the recycling of these new electric traction machines, the approach implemented for computer hard drives is not applicable due to the much greater mass of the order of several kg of NdFeB type magnets and due to the fixing of the magnets in the sheet metal packs by an adhesive attachment (using resins) and by a mechanical attachment (pins, punches). For these motors, the removal of the magnets requires compensating for the adhesion and mechanical retention of the magnets inserted in the sheet metal packs. The magnets must therefore be extracted by mechanical means combined with heat treatments to reduce the attachment of the resins.Heat treatments are restrictive because they are carried out on entire motors with the aim of degrading the adhesives used to fix the magnets and demagnetizing the magnets to reduce the magnetic forces that block the magnets or tend to agglomerate them. In certain electrical machines where the magnets are embedded, during manufacturing, within the internal sheets of the rotors, to extract the magnets, it is necessary to implement significant mechanical means such as ejectors driven by a hydraulic press. These mechanical means require significant investments that are difficult to support by recycling channels. In addition, this type of investment can only be profitable with standardization in the topology of electrical machines and the establishment of an upstream channel to increase volumes.

[0006] Furthermore, the mechanical forces used can impact the quality of the recovered magnets given the fragility of their materials.

[0007] Finally, the presence of residual bonding resin on the surface of the magnets after extraction reduces the value of the magnets because it constitutes a source of pollution during the magnet recycling stages. A magnet surfacing stage is therefore necessary after extraction, but this operation is complicated if the magnets are damaged or fragmented. For all these technical reasons and the absence of binding regulations, the recycling of end-of-life magnets, particularly those from buried magnet motors, is not developed on a large scale despite the potential in terms of recovery of critical materials.

[0008] For this purpose, the subject of the invention is an electric machine with a permanent magnet rotor, the rotor being able to rotate around an axis of rotation relative to a stator of the electric machine, the rotor comprising a magnetic circuit and at least one permanent magnet, the magnetic circuit being configured to guide the magnetic flux generated by the permanent magnet, the permanent magnet extending along the axis of rotation and having a general radial cross-sectional shape suitable for generating the magnetic flux, the magnetic circuit comprising a cavity configured to contain the permanent magnet, the cavity being in contact with the permanent magnet, in which the magnetic circuit comprises at least one lumen opening onto the cavity, in the lumen an outer surface of the permanent magnet is at a distance from the magnetic circuit so as to form a barrier to the magnetic flux generated by the permanent magnet, and in which the magnetic circuit comprises, in the lumen,an elastic part bearing against the permanent magnet in order to limit the displacement of the permanent magnet, in translation parallel to the axis of rotation, relative to the cavity.,

[0009] Advantageously, the limitation of the displacement of the permanent magnet, in translation parallel to the axis of rotation, relative to the cavity is achieved by an obstacle.

[0010] Advantageously, the permanent magnet comprises a recess in which the elastic part comes into abutment to prevent translational movement of the permanent magnet relative to the cavity.

[0011] Advantageously, the recess has at least one wall extending perpendicular to the axis of rotation, the wall forming a firm stop preventing any movement of the permanent magnet in translation parallel to the axis of rotation.

[0012] Advantageously, the recess has at least one chamfer allowing the permanent magnet to be extracted from the magnetic circuit by applying a force parallel to the axis of rotation.

[0013] Advantageously, the limitation of the displacement of the permanent magnet, in translation parallel to the axis of rotation, relative to the cavity is achieved by adhesion.

[0014] Advantageously, the elastic portion is configured to have a curved neutral fiber when subjected to a force during insertion of the permanent magnet into the cavity.

[0015] Advantageously, the magnetic circuit comprises sheets stacked along the axis of rotation and the elastic part is formed in at least one of the sheets.

[0016] Advantageously, the elastic part is not formed in all the sheets of the stack.

[0017] The invention will be better understood and other advantages will appear on reading the detailed description of an embodiment given by way of example, a description illustrated by the attached drawing in which:

[0018] [Fig.1] Figure 1 schematically represents in perspective an electric machine according to the invention;

[0019] [Fig.2] Figure 2 represents in partial section a rotor of the electric machine of Figure 1;

[0020] [Fig.3] Figure 3 represents a variant of the rotor of the electric machine in partial section in a plane containing its axis of rotation;

[0021] [Fig.4] Figure 4 represents another variant of the rotor of the electric machine in partial section in a plane containing its axis of rotation;

[0022] [Fig.5] Figure 5 also represents a variant of the rotor of the electric machine in partial section in a plane containing its axis of rotation;

[0023] Figure 1 shows in perspective an electrical machine 10 comprising a stator 12 and a rotor 14 capable of rotating relative to the stator around an axis of rotation 16. In the example shown, the rotor 14 is an internal rotor and the stator 12 is external. It is also possible to implement the invention in an electrical machine with an external rotor and an internal stator.

[0024] The stator 12 comprises a winding that can be supplied with multi-phase current, typically three-phase, and producing a rotating magnetic field at the supply frequency. The rotor 14 comprises one or more permanent magnets producing a rotating magnetic field with the magnetic field of the stator when the rotor 14 rotates around its axis of rotation 16.

[0025] The electrical machine 10 can operate as a motor by transforming electrical energy into mechanical energy or as a generator by transforming mechanical energy into electrical energy.

[0026] Figure 2 illustrates the mounting of a permanent magnet 20 in the rotor 14 of the electrical machine 10. Figure 2 shows a partial section of the rotor 14 in a plane perpendicular to the axis of rotation 16.

[0027] The electrical machine 10 comprises a magnetic circuit 22 in which one or more permanent magnets 20 are arranged distributed radially around the axis 16. In the case of a plurality of permanent magnets 20 these are identical and only one is shown in FIG. 2. During operation of the electrical machine 10, the magnetic field generated by the different permanent magnets 20 clings to the rotating magnetic field generated at the stator 12. The magnetic circuit 22 channels the magnetic field generated by the permanent magnets 20.

[0028] The permanent magnets 20 may be arranged in notches of the magnetic circuit 22 and flush with the outer surface of the rotor 14 opposite the stator. Alternatively, the permanent magnets 20 may be buried in the magnetic circuit 22. This arrangement makes it possible in particular to increase the rotational speed of the electrical machine 10 while avoiding the risk of the permanent magnets 20 becoming detached from the magnetic circuit 22. The arrangement with buried magnets also makes it possible to create a reluctant torque by a salience effect which makes it possible to assist the torque generated by the magnets and contributes to increasing the speed of the electrical machine. Between these two alternatives, it is also possible to provide permanent magnets 20 that are partly buried and partly flush.

[0029] The permanent magnet 20 shown in Figure 2 is not flush with the outer surface 24 of the rotor 14 and has a section of which a concave-shaped part opens in the direction of the stator 12, that is to say towards the top of Figure 2, this in order to create a concentration of magnetic flux and a higher induction in the air gap between the rotor 14 and the stator 12. The section of the permanent magnet 20 is constant over the majority of the length of the rotor 14 defined along the axis of rotation 16.

[0030] The magnetic circuit 22 comprises a cavity 26 configured to contain the permanent magnet 20. The cavity 26 is in contact with the permanent magnet 20. More precisely, the shape of the cavity 26 is complementary at least in part to the shape of the permanent magnet 20. As for the permanent magnet 20, the section of the cavity perpendicular to the axis 16 is constant over the majority of the length of the magnetic circuit 22, a length defined according to the axis of rotation 16.

[0031] the magnetic circuit 22 comprises one or more slots opening onto the cavity 26. In the example shown, the magnetic circuit 22 comprises two slots 28 and 30. In each of the slots 28 and 30, the outer surface of the permanent magnet 20 is at a distance from the magnetic circuit 22. The slots 28 form a barrier to the magnetic flux generated by the permanent magnet 20. In other words, the magnetic flux generated by the permanent magnet 20 tends to bypass the slots for the most part to concentrate in the magnetic circuit 22, the permeability of which is much greater than that of the vacuum or air contained in the slots 28 and 30.

[0032] The general shape of the permanent magnet 20 and that of the magnetic circuit 22 are defined in combination with the stator 12 to optimize the path of the magnetic fluxes in the electrical machine. The partly concave shape of the section of the permanent magnet 20 and the possible presence of the slots 28 and 30 are particularly adapted to this optimization.

[0033] The magnetic circuit 22 comprises in each of the slots 28 and 30 an elastic part, respectively 32 and 34, coming to bear against the permanent magnet 20 in order to limit the displacement of the permanent magnet 20, in translation parallel to the axis of rotation 16, relative to the cavity 26. Alternatively, it is possible to equip the magnetic circuit 22 with only one elastic part, either 32 or 34. In practice, we take advantage of the presence of lights in the magnetic circuit to place at least one elastic part there.

[0034] In the example shown in Figure 2, the section of the permanent magnet 20 has an axis of symmetry which is a radial axis 36 of the rotor, axis, perpendicular to the axis of rotation 16. The lights 28 and 30 are also symmetrical with respect to the radial axis 36. The fact of placing the two elastic parts 32 and 34 each bearing against an opposite face, respectively 38 and 40, of the permanent magnet 20 makes it possible to avoid a risk of radial displacement of the permanent magnet 20 in the cavity 26. In other words, in addition to a limitation of the axial displacement of the permanent magnet 20 parallel to the axis of rotation 16, the two elastic parts limit the radial displacement of the permanent magnet 20 in the two opposite directions by balancing the forces exerted on the permanent magnet 20 by the supports of the two elastic parts 28 and 32. The permanent magnet 20 is pinched between the two elastic parts 28 and 32.

[0035] Figure 3 shows a partial section of the rotor 14 in a plane containing the axis of rotation 16. There can be seen the magnetic circuit 22 formed by a stack of sheets extending perpendicular to the axis of rotation 16. There can also be seen the permanent magnet 20. The elastic part 32 is for example formed in one of the sheets 22i of the stack. Depending on the desired stiffness for the elastic part 32, it is possible to produce it in several sheets which may or may not be contiguous. The elastic part is advantageously not produced in all the sheets of the stack.

[0036] The permanent magnet 20 comprises a recess 42 in which the elastic part 32 abuts. In practice, when inserting the permanent magnet 20 into the cavity 26 in a translational movement parallel to the axis 16, the elastic part 32 deforms by rubbing against the permanent magnet 20 on its face 38 until the elastic part 32 penetrates into the recess 42, releasing at least part of its deformation. Once in the recess 42, the elastic part 32 abuts against the recess 42, preventing the permanent magnet 20 to move in translation parallel to the axis of rotation 16.

[0037] Alternatively to the recess 42, the permanent magnet 20 may comprise several protrusions between which the elastic part 32 is wedged. More generally, the protrusions or the recess make it possible to limit the movement of the permanent magnet 20, in translation parallel to the axis of rotation 16, relative to the cavity 28.

[0038] The elastic parts 32 and 34 have, in the example shown in Figure 2, a curved shape between their root, respectively 32a and 34a, on the internal wall of their respective lumen and their free end, respectively 32b and 34b, bearing against the permanent magnet 20. A curved shape makes it possible to increase the length of the neutral fiber of the elastic part concerned compared to an elastic part which would have a rectilinear shape. The elastic part is generally subjected to a bending stress during its deformation. The neutral fiber is a line of the elastic part not subjected to any tensile / compression stress during its bending deformation. Adapting the length of the neutral fiber makes it possible to adjust the possible deformation of the elastic part. This allows in particular the elastic part to wedge itself into the recess 42.This also makes it possible to absorb dimensional dispersions due to manufacturing tolerances of the permanent magnet 20 and the magnetic circuit 22. Any other non-rectilinear shape is of course possible.

[0039] Figure 3 is shown in a plane perpendicular to that of Figure 2 and in Figure 3 the possible curved shape of the elastic part 32 is not visible.

[0040] Figure 4 shows a variant of the rotor 14 in partial section in a plane containing the axis of rotation 16. It shows the magnetic circuit 22 shown in Figure 3 equipped with the elastic part 32. It shows the permanent magnet 20. Unlike the variant shown in Figure 3, the permanent magnet 20 includes a recess 44 of slightly different shape. The recess 44 is V-shaped, the branches 44a and 44b of which extend from inclined manner relative to the axis of rotation 16. In the variant of figure 3, the recess 42 has walls 42a and 42b perpendicular to the axis of rotation 16. The walls 42a and 42b form a firm stop preventing any displacement of the permanent magnet 20 in translation parallel to the axis of rotation 16 in both directions. With a firm stop, the extraction of the permanent magnet 20 at the end of the life of the electrical machine can be done for example by breaking or permanently deforming (i.e. outside their elastic range) the elastic parts by applying a significant force on the permanent magnet 20 along an axis parallel to the axis of rotation 16. Alternatively to a break, or to a permanent deformation of the elastic parts, it is possible to provide a specific tool inserted in the corresponding slots and allowing the elastic parts to be retracted.On the contrary, a V-shape of the recess 44 allows the elastic part 32 to retract without a tool under the action of a force on the permanent magnet 20 along an axis parallel to the axis of rotation 16. It is advisable to ensure that this force is greater than the forces which may occur during the operation of the electrical machine. The inclination of the branches 44a and 44b of the V-shape makes it possible to adjust the extraction force of the permanent magnet 20. It is also possible to implement a recess having only one inclined branch, either 44a or 44b, the other branch being able to be perpendicular to the axis of rotation 16 as in the recess 42. This single inclined branch forms a chamfer. In other words, the recess may comprise one or two chamfers allowing the extraction of the permanent magnet 20 from the magnetic circuit 22 either in one direction or in both directions carried by a direction parallel to the axis of rotation 16.

[0041] Figure 5 shows another variant of the rotor 14 in partial section in a plane containing the axis of rotation 16. It shows the magnetic circuit 22 shown in Figure 3 equipped with the elastic part 32. Unlike the variants shown in Figures 3 and 4, the face 38 of the permanent magnet 20 arranged opposite the elastic part 32 does not include any recess. The face 38 is parallel to the axis of rotation 16. More precisely, the face 38 can form a curve, visible in Figure 2, in a plane perpendicular to the axis of rotation 16. The face 38 forms a portion of a cylinder resting on the curve and whose generatrices are parallel to the axis of rotation 16. When inserting the permanent magnet 20 into the cavity 26, the elastic part 32 rubs against the face 38 throughout its insertion along a generatrix of the face 38 and the limitation of the displacement of the permanent magnet 20, in translation parallel to the axis of rotation 16, relative to the cavity 28 is done by adhesion. The force to be exerted on the permanent magnet 20 for its insertion into the cavity 26 is the same, in the same direction or in an opposite direction, as the force necessary for the extraction of the permanent magnet 20.

[0042] The permanent magnets 20 can be produced by injecting powder into a mold having an imprint of a shape complementary to that of the permanent magnet 20. After molding, the powder is sintered. This type of process has the advantage of allowing the production of precise details, including in particular the groove 38, in the shape of the permanent magnet 20. This manufacturing method also has the advantage of allowing the production of permanent magnets at high production rates. Any other method for producing the permanent magnets 20 can be implemented within the scope of the invention.

Claims

CLAIMS 1. Electric machine with a permanent magnet rotor, the rotor (14) being rotatable about an axis of rotation (16) relative to a stator (12) of the electric machine (10), the rotor (14) comprising a magnetic circuit (22) and at least one permanent magnet (20), the magnetic circuit (22) being configured to guide the magnetic flux generated by the permanent magnet (20), the permanent magnet (20) extending along the axis of rotation (16) and having a general radial cross-sectional shape suitable for generating the magnetic flux, the magnetic circuit (22) comprising a cavity (26) configured to contain the permanent magnet (20), the cavity (26) being in contact with the permanent magnet (20), in which the magnetic circuit comprises at least one lumen (28, 30) opening onto the cavity (26), in the lumen (28, 30) an outer surface (38,40) of the permanent magnet (20) is at a distance from the magnetic circuit (22) so as to form a barrier to the magnetic flux generated by the permanent magnet (20), and in which the magnetic circuit (22) comprises, in the lumen (28, 30), an elastic part (32, 34) bearing against the permanent magnet (20) in order to limit the displacement of the permanent magnet (20), in translation parallel to the axis of rotation (16), relative to the cavity (28, 30)., 2. Electrical machine according to claim 1, in which the limitation of the movement of the permanent magnet (20), in translation parallel to the axis of rotation (16), relative to the cavity (28, 30) is achieved by an obstacle.

3. Electrical machine according to claim 2, in which the permanent magnet (20) comprises a recess (42) in which the elastic part (32, 34) comes into abutment to prevent translational movement of the permanent magnet (20) relative to the cavity (26).

4. Electrical machine according to claim 3, in which the recess (42) has at least one wall (42a, 42b) extending perpendicular to the axis of rotation (16), the wall (42a, 42b) forming a firm stop preventing any movement of the permanent magnet (20) in translation parallel to the axis of rotation (16).

5. Electrical machine according to claim 3, in which the recess (44) has at least one chamfer (44a, 44b) allowing the extraction of the permanent magnet (20) from the magnetic circuit (22) by applying a force parallel to the axis of rotation (16).

6. Electrical machine according to claim 1, in which the limitation of the movement of the permanent magnet (20), in translation parallel to the axis of rotation (16), relative to the cavity (28, 30) is achieved by adhesion.

7. Electrical machine according to one of the preceding claims, in which the elastic part (32, 34) is configured to have a curved neutral fiber when it is subjected to a force during the insertion of the permanent magnet (20) into the cavity (26).

8. Electrical machine according to one of the preceding claims, in which the magnetic circuit comprises sheets stacked along the axis of rotation (16) and in which the elastic part (32, 34) is formed in at least one of the sheets (22i).

9. Electrical machine according to claim 8, in which the elastic part (32, 34) is not formed in all the sheets of the stack.

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