Electric machine rotor comprising a recess with a lug or a groove

Abstract

The invention relates to a rotor and an electric machine. The rotor comprises a rotor body and a plurality of pairs of magnetic poles. Each magnetic pole comprises at least one flux barrier, and each flux barrier comprises at least one recess (2a, 2b, 3, 4a, 4b, 11, 5). Each magnetic pole comprises at least one permanent magnet in a recess (2a, 2b, 3, 4a, 4b). Each recess (2a, 2b, 3, 4a, 4b, 11, 5) is formed between two lines (17, 18). Each recess (2a, 2b, 3, 4a, 4b) in which a permanent magnet is inserted comprises a lug (12, 13, 14, 15, 16) or a groove, which is defined by a rounded profile with a predefined radius in direct connection with one of said two lines (17, 18), the permanent magnet having a shape complementing the lug (12, 13, 14, 15, 16) or the groove.

Description

[0001] ELECTRIC MACHINE ROTOR COMPRISING A RECESS WITH A LUG OR GROOVE

[0002] Technical field

[0003] The present invention relates to a rotating synchro-reluctant electrical machine (assisted by permanent magnets) or with buried magnets and more particularly concerns the particular architecture of a rotor of such a machine.

[0004] Typically, such an electrical machine has a stator and a rotor arranged coaxially within each other.

[0005] The rotor consists of a rotor body with a stack of laminations placed on a rotor shaft. These laminations include housings for permanent magnets and perforations to create flux barriers to direct the magnetic flux from the magnets radially towards the stator and to promote the creation of a reluctant torque, and to lighten this rotor to reduce the centrifugal forces that the stack of laminations must withstand.

[0006] This rotor is generally housed inside a stator which carries electrical windings to generate a magnetic field to drive the rotor in rotation.

[0007] Prior art

[0008] As better described in particular in patent application WO2020 / 020580, the rotor of a synchro-reluctant machine comprises a plurality of axial recesses which pass right through the sheets.

[0009] For the rotor design described in this patent application, a first series of axial recesses, arranged radially above each other and at a distance from each other, form housings for magnetic flux generators, here permanent magnets in the form of a rectangular bar.

[0010] The other series of recesses consists of perforations of inclined radial direction, which start from these housings to arrive in the vicinity of the edge of the sheets, in the vicinity of the air gap.

[0011] The inclined perforations are arranged symmetrically with respect to the magnet housings so as to form each time a geometric figure substantially in the shape of a V with a flattened bottom with the flat bottom formed by the magnet housing and with the inclined arms of this V formed by the perforations. This creates flux barriers formed by the perforations. The magnetic flux coming from the permanent magnets can then only pass through the solid parts between the perforations. These solid parts are made of a ferromagnetic material.

[0012] Patent application WO2022 / 128541 relates to an electrical machine with a rotor comprising flux barriers adapted to rectangular permanent magnets.

[0013] Figure 1 illustrates the rotor of this prior art electrical machine. The rotor 1 is made of a plurality of rolled sheets. Figure 1 is a partial view of a rotor in a cross-section, which represents a single pole of the rotor of the electrical machine. The rotor 1 comprises several magnetic poles and each magnetic pole here comprises three flux barriers positioned radially above each other.

[0014] The first flow barrier, the internal flow barrier (closest to the rotor axis 1) comprises a central recess 3 and two lateral recesses 2a and 2b.

[0015] The second flow barrier comprises two lateral recesses 4a and 4b and the third flow barrier comprises a central recess 5.

[0016] In all the recesses 2a, 2b, 3, 4a and 4b of the first and second flux barriers, rectangular permanent magnets are placed. The permanent magnets are guided, positioned and held in this position in the recesses by means of stops 6a, 6b, 7a, 7b, 8a and 8b. The stops 6a, 6b, 7a, 7b, 8a and 8b center the magnet in the recess in question by pressing on the lateral end faces of the magnets (on each end of the short sides of the rectangular magnet).

[0017] The stops 6a, 6b, 7a, 7b, 8a and 8b are formed by local reductions in the width of the recesses 2a, 2b, 3, 4a, 4b

[0018] Each stop 6a, 6b, 7a, 7b, 8a and 8b thus divides the space of the recesses 2a, 2b, 3, 4a and 4b into three volumes: a central volume located between the two stops of the recess in question, the central volume being occupied by a permanent magnet and two peripheral volumes 9, 10, on each side of the central volume. The peripheral volumes 9, 10 allow an air flow and therefore do not include permanent magnets.

[0019] This technology is not optimal. Indeed, the presence of these stops in the form of "ears" is bulky, has complex shapes, constrains the cost of the tool for swaging rolled sheets and can present concentrations of mechanical stresses limiting the maximum admissible speed. The blocking shape composed of the ears of the stops 6a, 6b, 7a, 7b, 8a and 8b and the peripheral volumes 9 and 10 means that part of the space of the flux barrier is lost while this space could be optimized for the magnetic circuit. In addition, this shape impacts the geometry and the positioning of the magnetic bridges which mechanically hold the sheet and the magnetic bridges have a significant negative impact on the performance of the rotor. Thus, these shapes limit the performance of the machine.

[0020] Additionally, installing magnets in the recesses requires specific tools. Therefore, the recesses are designed to allow the passage of these tools (tools that are then removed once the magnets are positioned in the recesses). These tools also require precise design and manufacturing, implementation in an industrial environment and recurring verification.

[0021] US patent application 2015 / 137646 is also known, which relates to an electrical machine rotor. In this patent application, the permanent magnets are positioned and centered by male studs formed by the rotor body, the permanent magnets having female orifices complementary to the male studs of the rotor body. However, the male studs include a straight portion, before ending in a rounded profile. This straight portion of the studs prevents any free rotational movement between the magnet and the rotor body. Positioning the permanent magnets in the rotor is therefore complicated and risks stressing the permanent magnet and / or the rotor body.

[0022] The technical problem that we propose to solve thus consists of designing a simple and easy solution for guiding and positioning the magnets in the recesses of the flux barriers, while increasing the electromagnetic and mechanical performance of the machine, and reducing its cost.

[0023] Summary of the invention

[0024] The invention relates to a rotor for an electrical machine comprising:

[0025] - a rotor body, formed by a stack of sheets, preferably configured to be positioned on a rotor shaft, and

[0026] - a plurality of pairs of magnetic poles distributed circumferentially on the rotor body, each magnetic pole comprising at least one flux barrier, preferably at least two flux barriers located radially above each other, each flux barrier comprising at least one recess, each magnetic pole comprising at least one permanent magnet in a recess.

[0027] In addition, each recess is formed between two lines, each permanent magnet being adapted to be inserted into at least one recess, and each recess into which a permanent magnet is inserted comprises a lug or a groove, the lug or the groove being defined by a rounded profile with a predefined radius in direct connection with one of said two lines, the permanent magnet comprising a shape complementary to the lug or to the groove.

[0028] Advantageously, each permanent magnet has a non-prismatic or prismatic shape to fit the shape of the recess into which it is inserted.

[0029] Preferably, the predefined radius is between 0.5 and 2 mm, preferably between 0.8 and 1.2 mm.

[0030] According to a variant of the invention, the lug or groove is positioned on the line of the recess closest to the axis of rotation of the rotor.

[0031] Advantageously, the lug or groove does not have any flat or straight part.

[0032] Preferably, the lug or groove is open at an angle of between 60° and 150°, preferably between 80° and 120° and more preferably between 90° and 100°.

[0033] Advantageously, the rotor body comprises three or four flow barriers.

[0034] Preferably the first and second flow barriers, starting from the center outward, each have three recesses, one of said three recesses being a central recess and the other two being lateral recesses, the third flow barrier having two recesses and the optional fourth flow barrier having two recesses.

[0035] Advantageously, the first barrier comprises three permanent magnets, one in each recess, and the second flux barrier comprises two permanent magnets, one in each lateral recess and the third and optionally fourth flux barriers do not comprise permanent magnets.

[0036] According to one configuration of the invention, the width of the flow barriers may be constant or decreasing from the center toward the radial ends.

[0037] The invention also relates to an electrical machine comprising a stator and a rotor as described previously, said rotor being housed inside said stator.

[0038] Preferably, said electric machine is of the synchro-reluctant electric machine type.

[0039] List of figures

[0040] Other characteristics and advantages of the rotor and the electrical machine according to the invention will appear on reading the following description of non-limiting examples of embodiments, with reference to the figures appended and described below.

[0041] Figure 1 shows an example of a rotor according to the prior art.

[0042] Figure 2 illustrates an electric machine rotor according to the invention.

[0043] Figure 3 illustrates an example of a lug or groove of an electric machine rotor recess according to the invention. Description of the embodiments

[0044] The invention relates to a rotor for an electrical machine, in particular an electrical machine, for example of the buried magnet type and more particularly of the synchro-reluctant type assisted by permanent magnets. Furthermore, the present invention relates to an electrical machine, preferably of the synchro-reluctant type, comprising a rotor according to the invention and a stator, the rotor being arranged within the stator coaxially therewith.

[0045] "Buried magnets" are magnets that are inserted within the rotor itself, unlike surface magnets which are only arranged on the surface of the rotor and which are only separated from the stator by an air gap corresponding to the functional clearance between the stator and the rotor (this clearance is generally called the "air gap").

[0046] According to the invention, the rotor for an electric machine comprises:

[0047] - a rotor body, formed by a stack of sheets, preferably configured to be positioned on a rotor shaft, and

[0048] - a plurality of pairs of magnetic poles distributed circumferentially on the rotor body.

[0049] Each magnetic pole comprises at least one flux barrier, preferably at least two flux barriers located radially above each other and each flux barrier comprises at least one recess. In addition, each magnetic pole comprises at least one permanent magnet in a recess.

[0050] The recesses of the flux barriers are axial. In other words, the recesses extend in the axial (longitudinal) direction, i.e. along the rotor axis, crossing the rotor along its entire axial length.

[0051] Permanent magnets generate a magnetic flux, allowing the rotor to rotate by creating a rotating magnetic field that can also be generated by the stator. Flux barriers guide the magnetic field generated by the rotor and by the at least one permanent magnet towards the air gap (the air gap being the space formed between the periphery of the rotor and the stator), so as to limit magnetic flux leakage and increase the performance (particularly torque and power) of the electrical machine. A pair of magnetic poles comprises two magnetic poles of opposite polarity.

[0052] According to the invention, each recess is formed between two lines, for example between an upper line and a lower line. The upper and lower lines delimit the upper and lower edges of the flow barriers. The terms "upper" and "lower" are understood radially relative to the rotor axis. In other words, the upper line (the upper edge) of a recess is further from the rotor axis than the lower line (the lower edge) of the same recess.

[0053] The lines can be straight lines or include straight lines, for example for placing prismatic shaped magnets.

[0054] Alternatively or additionally, the lines may be curved or include curved lines (whether or not associated with straight lines), for example for the installation of non-prismatic shaped magnets. This shape of non-prismatic magnets makes it possible to maximize the electromagnetic performance of the electrical machine.

[0055] Furthermore, each permanent magnet is adapted to be inserted into at least one recess: the width of the permanent magnet is substantially equal to the width of the recess.

[0056] Preferably, at least one permanent magnet (preferably several of them) can be adapted to be inserted into several recesses of a magnetic pole of the electric machine, so as to limit the diversity of geometries and tools. In other words, the rotor can comprise several identical permanent magnets (in particular for the shape and dimensions).

[0057] Additionally, each recess into which a permanent magnet is inserted has a lug or groove. A lug is a male portion of the rotor body that projects toward the permanent magnet (projecting toward the permanent magnet). Conversely, a groove is a female portion of the rotor body that projects away from the magnet. In other words, the lug forms a bump on the rotor body while the groove forms a hollow.

[0058] Advantageously, the lug or groove may be arranged on the lower line or the upper line of the flow barrier recess.

[0059] According to one implementation of the invention, each permanent magnet inserted into a recess that includes a lug or a groove includes a complementary shape. In other words, when the recess includes a lug, the permanent magnet may include a groove of a shape similar to the lug, and conversely when the recess includes a groove, the permanent magnet may include a lug of a shape similar to the groove. Thus, the permanent magnet is adapted to the recess.

[0060] According to the invention, the lug or groove is defined by a rounded profile with a predefined radius in direct connection with one of said two lines and the permanent magnet has a shape complementary to the lug or groove. In other words, the lug or groove of the recess is formed by an arc of a circle of predefined radius and the lug or groove has no straight part. By straight part, it is meant that the lug or groove has no part extending on a straight line. The lug forms a convex arc of a circle and the groove forms a concave arc of a circle.

[0061] Thanks to this rounded profile lug or groove directly linked to the line on which it is installed, the magnet can be guided for its installation in the recess and it benefits from a degree of freedom in rotation to facilitate its installation: thus, the assembly of the magnet can be carried out without constraining the magnet. In addition, this lug or groove ensures that the magnet remains in position over time.

[0062] This particular shape also allows the entire cross-section of the recess to be used to insert a permanent magnet: it is no longer necessary to provide air volumes on either side of the permanent magnet. This way, the electromagnetic performance of the electric machine can be increased and this configuration allows for easier mechanical optimization. This also reduces the cost of the tooling for manufacturing the lamination on an industrial scale.

[0063] According to one configuration of the invention, the grooves or lugs of different recesses or of the same recess may be defined by rounded arc profiles with different predefined radius centers. For example, one of the grooves or lugs may have a first predefined radius that is different from a second predefined radius of another groove or lug.

[0064] Advantageously, each permanent magnet has a non-prismatic or prismatic shape to adapt to the shape of the recess in which it is inserted. The non-prismatic shape can be adapted in particular when the recess extends between two curved lines. These shapes make it possible to increase the performance of the electrical machine. The prismatic shape of the magnets allows a more conventional production and is suitable for recesses extending in particular between two straight lines.

[0065] Prismatic or non-prismatic permanent magnets can be produced by machining for example or by powder sintering techniques or by injection.

[0066] Advantageously, the predefined radius can be between 0.5 and 2 mm, preferably between 0.8 and 1.2 mm. Such a radius allows both the guiding and the holding in position of the magnet while ensuring easy installation of the permanent magnet in the recess. It therefore offers an excellent compromise.

[0067] According to one configuration of the invention, the lug or groove may be positioned on the line of the recess closest to the axis of rotation of the rotor (on the lower line of the recess).

[0068] Alternatively, the lug or groove may be positioned on the recess line furthest from the rotor rotation axis (on the top recess line).

[0069] Preferably, each recess into which a permanent magnet is inserted has a single lug or a single groove. The single lug or groove may be on the top line or the bottom line, but if one line has a lug or a groove, the other line of the recess has neither a lug nor a groove. Indeed, the positioning of the magnet is facilitated by maintaining a degree of rotational freedom, which would not be the case if the recess had at least two lugs, at least two grooves, or at least one lug and one groove.

[0070] Advantageously, the lug or groove may not have any flat or straight part. Indeed, by not having any flat or straight part, the magnet retains a degree of freedom in rotation around the lug or groove to facilitate its insertion into the recess.

[0071] According to one configuration of the invention, the lug or groove may be open at an angle of between 60° and 150°, preferably between 80° and 120° and more preferably between 90° and 100°. This opening makes it possible to maintain the degree of freedom in rotation to facilitate the positioning of the magnet, while ensuring its guidance in the recess and its maintenance in position. The opening is defined by the opening of the arc of a circle formed by the radius of curvature of the lug or groove.

[0072] According to a variant of the invention, the rotor body may comprise three or four flux barriers. Indeed, the more flux barriers the rotor body comprises, the better the magnetic field is guided. The number of three or four barriers offers an excellent compromise between the expected performance and manufacturing complexity.

[0073] Preferably, the first and second flow barriers, starting from the center outward, each comprise three recesses. One of the three recesses of these first and second flow barriers is a central recess: it is positioned on a radial axis of symmetry of the flow barriers and the central recess is symmetrical on either side of this radial axis of symmetry. The other two recesses of the first and second flow barriers are lateral (peripheral) recesses, one lateral recess on each side of the central recess. The lateral recesses are symmetrical with respect to each other with respect to the radial axis of symmetry.

[0074] The two side recesses are thus positioned and spaced on either side of a central recess.

[0075] For this configuration, the recesses of each internal flow barrier substantially form a U (according to a cross-section to the axis of the rotor, which corresponds to the plane of a sheet constituting the rotor body), the bottom of the U being formed by the central recess, and the opposite segments of the U being formed by the lateral recesses.

[0076] The rotor body laminations can be made of ferromagnetic material so as to guide the magnetic flux created by permanent magnets and possibly the stator winding. The recesses for the flux barriers can be obtained by perforations in the stacked laminations forming the rotor body, and the magnetic bridges are formed by the lamination itself.

[0077] For this configuration, preferably, the third flow barrier may comprise (or consist of) two recesses (which are two lateral recesses symmetrical to each other with respect to the radial axis of symmetry) and the possible fourth flow barrier may also comprise (or consist of) two recesses (which are two lateral recesses symmetrical to each other with respect to the radial axis of symmetry).

[0078] Such a rotor thus makes it possible to better guide the magnetic field towards the stator and to increase the reluctant torque.

[0079] Advantageously, the first flux barrier may comprise three (and preferably only three) permanent magnets, one in each recess, and the second flux barrier may comprise two (and preferably only two) permanent magnets, one in each lateral recess. In addition, the third and the possible fourth flux barriers do not comprise permanent magnets. This configuration makes it possible to maximize the torque and power density of the machine, by limiting the quantity of permanent magnets required. Limiting the number of permanent magnets also makes it possible to reduce industrial constraints on the dimensions of the permanent magnets as well as the manufacturing cost.

[0080] Advantageously, the width of the flux barriers (in particular the recesses of the flux barriers) can decrease from the center towards the radial ends. In other words, the width of the different recesses of the different flux barriers decreases from the center towards the radial ends of the flux barriers. This configuration makes it possible to maximize the torque and power density of the electric machine.

[0081] Preferably, the magnets may then have a suitable shape with decreasing thickness, corresponding to the width of the recesses in which they are inserted, from the center towards the outside of the rotor body.

[0082] Alternatively, the width of the flux barriers can be constant from the center to the radial ends, so as to simplify the fabrication of laminations and permanent magnets for example.

[0083] Preferably, the magnets can then have a suitable shape with a constant thickness, corresponding to the width of the recesses in which they are inserted, from the center towards the outside of the rotor body. Figure 2 illustrates, in a schematic and non-limiting manner, a rotor for an electrical machine according to the invention. Figure 2 is a partial view of a rotor according to a cross-section, which represents a single pole of the rotor of the electrical machine.

[0084] The rotor has multiple magnetic poles, and each magnetic pole includes at least one flux barrier. Here, the magnetic pole shown includes three flux barriers.

[0085] The first flow barrier, starting from the center outwards, comprises a central recess 3 and two lateral recesses 2a and 2b; the second flow barrier, starting from the center outwards, comprises two lateral recesses 4a and 4b and a central recess 11 and the third flow barrier comprises two lateral recesses 5.

[0086] The lateral recesses 5 of the third flux barrier do not include any permanent magnets. The lateral recesses 2a, 2b, 4a, 4b of the first and second flux barriers and the central recess 3 of the first flux barrier include permanent magnets whose shape is substantially identical to the shape of the recesses in which they are inserted. Thus, there is no loss of volume of the recesses where part of the volume would only consist of air and thus generate an electromagnetic loss.

[0087] The recesses shown are formed between lines, an upper line and a lower line. For example, the lateral recess 4a is formed between the lower line 17 and the upper line 18. As shown, the lower 17 and upper 18 lines are curved lines but they could be or include straight lines.

[0088] Each recess 2a, 2b, 3, 4a and 4b in which a permanent magnet is inserted here comprises one (and only one) lug 12, 13, 14, 15, 16 (alternatively it could comprise a groove) forming a male part oriented towards the permanent magnet inside the recess. The lug 12, 13, 14, 15, 16 forms a rounded part in an arc of a circle starting directly from the lower line and joining the lower line directly. The lug 12, 13, 14, 15, 16 therefore does not comprise any straight (or flat) part and is only constituted by the rounded profile in an arc of a circle of predefined radius.

[0089] The lugs 12, 13, 14, 15, 16 are shown on the lower lines of the various recesses 2a, 2b, 3, 4a and 4b but they could just as well be shown on the upper lines of these various recesses.

[0090] When the lugs are replaced by grooves, these form female parts in the opposite direction to the permanent magnet. The groove forms a rounded part in an arc of a circle starting directly from the lower line and joining directly to the lower line. The groove therefore does not have any straight (or flat) part here and consists only of the rounded profile in an arc of a circle with a predefined radius.

[0091] Each permanent magnet has a shape adapted to the shape of the recess in which it is inserted. In particular, each permanent magnet has a shape complementary to the lug 12, 13, 14, 15, 16 or to the groove of the recess. When the recess comprises a lug, the permanent magnet then comprises a complementary shape in the form of a groove in the shape of an arc (or a groove) whose radius is substantially equal to the predefined radius of the lug. When the recess comprises a groove, the permanent magnet then comprises a complementary shape in the form of a protrusion in the shape of an arc (or a lug) whose radius is substantially equal to the predefined radius of the groove.

[0092] Figure 3 illustrates, in a schematic and non-limiting manner, an example of a groove or lug of a rotor recess according to the invention.

[0093] The rotor comprises at least one recess in which a permanent magnet is placed. The recess comprises a lug (in the direction of the permanent magnet, forming a male part) or a groove (in the direction opposite the permanent magnet, forming a female part) 21. The permanent magnet has a shape complementary to this lug or to this groove 21.

[0094] The lug or groove 21 is a rounded profile in the shape of an arc of a circle with a predefined radius R, preferably between 0.5 mm and 2 mm, preferably close to 1 mm.

[0095] The rounded arcuate profile starts from line 20 forming one end of the recess and joins line 20. Line 20 may be an upper line or a lower line of the recess, it may be straight or curved or include straight parts and / or curved parts.

[0096] The lug or groove 21 forms a rounded profile in an arc of a circle of predefined radius R and the center of this arc of center of which corresponds to point C. The lug or groove 21 is open at an angle 0 of between 60° and 150°, preferably between 80° and 120° and more preferably between 90° and 100°.

Claims

Claims 1. Rotor for electric machine comprising: - a rotor body, formed by a stack of sheets, preferably configured to be positioned on a rotor shaft, and - a plurality of pairs of magnetic poles distributed circumferentially on the rotor body, each magnetic pole comprising at least one flux barrier, preferably at least two flux barriers located radially above each other, each flux barrier comprising at least one recess (2a, 2b, 3, 4a, 4b, 11, 5), each magnetic pole comprising at least one permanent magnet in a recess (2a, 2b, 3, 4a, 4b), each recess (2a, 2b, 3, 4a, 4b, 11, 5) being formed between two lines (17, 18), each permanent magnet being adapted to be inserted into at least one recess (2a, 2b, 3, 4a, 4b), characterized in that each recess (2a, 2b, 3, 4a, 4b) in which a permanent magnet is inserted comprises a lug (12, 13, 14, 15, 16) or a groove (21), the lug (12, 13, 14, 15, 16) or the groove (21) being defined by a rounded profile with a predefined radius (R) in direct connection with one of said two lines (17, 18, 20),the permanent magnet having a shape complementary to the lug (12, 13, 14, 15, 16) or to the groove (21)., 2. Rotor according to claim 1, wherein each permanent magnet has a non-prismatic or prismatic shape to adapt to the shape of the recess (2a, 2b, 3, 4a, 4b) in which it is inserted.

3. Rotor according to one of the preceding claims, in which the predefined radius (R) is between 0.5 and 2 mm, preferably between 0.8 and 1.2 mm.

4. Rotor according to one of the preceding claims, in which the lug (12, 13, 14, 15, 16) or the groove (21) is positioned on the line (17) of the recess closest to the axis of rotation of the rotor.

5. Rotor according to one of the preceding claims, in which the lug (12, 13, 14, 15, 16) or the groove (21) does not have any flat or straight part.

6. Rotor according to one of the preceding claims, in which the lug (12, 13, 14, 15, 16) or the groove (21) is open over an angle of between 60° and 150°, preferably between 80° and 120° and more preferably between 90° and 100°.

7. Rotor according to one of the preceding claims, in which the rotor body comprises three or four flow barriers.

8. A rotor according to claim 7, wherein the first and second flow barriers, from the center outward, each have three recesses, one of said three recesses being a central recess (3, 11) and the other two being recesses lateral (2a, 2b, 4a, 4b), the third flow barrier having two recesses (5) and the possible fourth flow barrier having two recesses.

9. A rotor according to claim 8, wherein the first barrier comprises three permanent magnets, one in each recess (2a, 2b, 3), wherein the second flux barrier comprises two permanent magnets, one in each lateral recess (4a, 4b) and wherein the third and possible fourth flux barriers do not comprise permanent magnets.

10. Rotor according to one of the preceding claims, wherein the width of the flux barriers is constant or decreasing from the center towards the radial ends.

11. Electrical machine comprising a stator and a rotor according to one of the preceding claims, said rotor being housed inside said stator.

12. Electric machine according to claim 11, wherein said electric machine is of the synchro-reluctant electric machine type.

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