Improved rotor for an electric motor

Abstract

A rotor (4) for an electric motor comprising a crankshaft (8) extending along a main extension axis (X-X) coinciding with the axis of rotation of the crankshaft (8); and comprising at least one steel sheet (12) provided with a slot (16) for housing a magnet and with a central seat (20) keyed on an outer lateral wall (24) of the crankshaft (8) according to an interference coupling, wherein the steel sheet (12) is coupled to the crankshaft (8) at said central seat (20) by means of a first tooth (36) obtained on the steel sheet (12) projecting towards the associated crankshaft (8) and a first radial seat (40) obtained on said outer wall (24) of the crankshaft (8). The first radial seat (40) has an isosceles trapezoidal cross section delimited laterally by a pair of inclined sides (44) converging towards the axis of rotation; the first tooth (36) has a "V" shaped cross section with a pair of curved lateral walls (48) suitable to engage said inclined sides (44) of the first radial seat (40).

Description

Technical Field

[0001] This invention relates to an improved rotor for electric motors, particularly for traction, and related electric motors. Background Technology

[0002] In the automotive industry, particularly in traction motors, it is well known that the rotor comprises a crankshaft with stacks of ferromagnetic steel sheets bonded to it. These steel sheets interact with magnets arranged on the stator of the motor to generate torque on the crankshaft typically used for traction.

[0003] The mechanical coupling between the crankshaft and the steel plates is an extremely critical component of the electric motor.

[0004] In fact, in the field of traction, the crankshaft of an electric motor can reach extremely high speeds, approaching 20,000 rpm, and typically transmits considerable torque.

[0005] Clearly, such rotational speed and torque (and therefore power transmission) exert tension on all components, especially on the mechanical coupling between the shaft and the steel sheet.

[0006] It must be pointed out that in high-performance electric motors, any related angular deviation between the crankshaft and the lamination stack must be avoided, as the performance of such motors (especially in terms of torque) will rapidly decrease, especially when the motor design has a certain "skew" angle, i.e., angular misalignment of the laminations, moving between the front or front wall towards the rear of the rotor.

[0007] Therefore, it is necessary to provide a mechanical coupling between the shaft and the steel plate that is repeatable on the assembly line and always ensures an integral connection of rotation between the shaft and the steel plate (i.e., without mutual angular misalignment) under all motor operating conditions (which may vary greatly in torque, angular velocity and temperature).

[0008] For this purpose, a forced connection, namely an interference (press fit) connection, is known between the steel sheet stack and the crankshaft center seat. This forced connection can only ensure that no angular misalignment occurs when the crankshaft speed is not too high.

[0009] However, when the rotational speed increases and / or high torque is involved, the use of a press-fit connection between the shaft and the steel plate stack may no longer prevent significant (and destructive) angular misalignment between the shaft and the steel plate stack.

[0010] Furthermore, the operation of the motor is particularly difficult in the case of torque reversal (which is extremely frequent in traction applications): in the case of torque reversal, it is necessary to maintain continuous contact between the shaft and the rotor stack, and even the smallest angular movement may cause the corresponding speed sensor (resolver) to read incorrectly, resulting in suboptimal operation of the motor.

[0011] For this reason, there is a solution in the prior art that involves a shape connector between the steel sheet stack and the motor rotor; however, the known shape connector cannot always ensure sufficient performance, especially when thermal stress is increased due to motor overheating (in addition to high speed and high torque). Summary of the Invention

[0012] Therefore, there is a need in the art to address the aforementioned drawbacks and limitations cited in the prior art.

[0013] The improved rotor for an electric motor according to claim 1 and the related electric motor according to claim 14 satisfy this requirement.

[0014] Other embodiments of the invention are described in the dependent claims. Attached Figure Description

[0015] Other features and advantages of the invention will become more apparent from the following detailed description of its preferred, non-limiting embodiments, wherein:

[0016] Figure 1 This is an exploded perspective view of the motor rotor according to an embodiment of the present invention;

[0017] Figure 2 yes Figure 1 A plan view of the bonded steel sheets on the rotor crankshaft;

[0018] Figure 3 yes Figure 1 A three-dimensional view of the motor rotor in its assembled configuration;

[0019] Figure 4 yes Figure 1 Cross-sectional view of the connector between the steel plate and the crankshaft;

[0020] Figures 5 to 6 This is an enlarged detail of the geometry of the connecting teeth of the steel plates on the crankshaft according to an embodiment of the present invention.

[0021] The same reference numerals will be used to identify common elements or parts of elements in the embodiments described below. Detailed Implementation

[0022] Referring to the aforementioned figure, 4 generally refers to the electric motor rotor, especially in the automotive field.

[0023] The rotor is also coaxially housed, at least partially, relative to the stator (not shown), in a known manner.

[0024] It should be noted that the application of this invention in the automotive field is preferred, but not exclusive, because electric motors can have a variety of applications.

[0025] The rotor 4 includes a crankshaft 8 and at least one steel plate 12, the crankshaft 8 extending along a main extension axis XX that coincides with the axis of rotation of the crankshaft 8, and the steel plate 12 having a slot 16 for receiving a magnet and a center seat 20 that is bonded to the outer wall 24 of the crankshaft 8 by an interference fit.

[0026] For example, an interference fit involves a pair of angled sections 28 that contact each other between the outer sidewall 24 of the crankshaft 8 and the center seat 20, the angled sections 28 being arranged in diametrically opposed positions relative to the axis of rotation of the rotor 4.

[0027] Preferably, the rotor 4 comprises a plurality of steel plates 12, preferably having the same axial thickness, and arranged axially (i.e. along the main extension axis XX).

[0028] Preferably, the steel sheets 12 are identical to each other and have the same number of slots 16, which are arranged with the same geometry and angles relative to the axis of rotation. In this way, magnets can be accommodated in the corresponding slots 16 of adjacent steel sheets 12.

[0029] Each steel sheet 12 is connected to the crankshaft 8 at the center seat 20 via a shape-connecting device 32.

[0030] Advantageously, the shape-connecting device 32 includes a first tooth 36 formed on the steel sheet 12, which protrudes toward the associated crankshaft 8, and the shape-connecting device 32 includes a first radial seat 40 formed on the outer sidewall 24 of the crankshaft 8 to at least partially radially accommodate the first tooth 36.

[0031] Preferably, the first radial seat 40 extends continuously on the outer sidewall 24 of the crankshaft 8 to intercept the first tooth 36 of a plurality of steel plates 12 arranged side by side along the main extension axis XX.

[0032] Advantageously, the first radial seat 40 has an isosceles trapezoidal cross-section relative to a cross-sectional plane perpendicular to the axis of rotation; the isosceles trapezoid is laterally defined by a pair of hypotenuses 44 converging toward the axis of rotation (but not necessarily having a directrix coinciding with the axis).

[0033] Preferably, the first radial seat 40 is defined in height and in the radial direction by a pair of flat portions 46, which reduce the outer diameter of the crankshaft 8 to avoid direct contact with the associated steel sheet 12.

[0034] Similarly, the first tooth 36 has a “V” shaped cross-section with a pair of curved sidewalls 48 adapted to engage with the inclined side 44 of the first radial seat 40.

[0035] Preferably, the inclined side 44 of the first radial seat 40 is flat, and the curved sidewall 48 of the first tooth 36 is round.

[0036] Preferably, the circular wall of the first tooth has a radius of curvature of at least 10 mm.

[0037] According to an embodiment, the "V"-shaped cross-section of the first tooth 36 has a radial chamfer 52 at the cantilevered radial end of the first tooth 36 on one side of the rotation axis; therefore, the cantilevered radial end defines a radial clearance with the bottom 56 of the first radial seat 40.

[0038] According to a possible embodiment, the first tooth 36 has a radial recess 60 at each connection region between the steel sheet 12 and one of the curved sidewalls 48, the radial recess 60 extending from the opposite side of the associated crankshaft 8.

[0039] The radial recess 60 forms an additional radial clearance relative to the flat portion 46 on the outer sidewall 24 of the crankshaft 8.

[0040] According to a possible embodiment, the first tooth 36 includes a central recess 64, which further divides each first tooth 36 into a pair of fins 68, the pair of fins being arranged symmetrically with respect to the central recess 64.

[0041] Preferably, the central recess (64) has an elliptical cross-section relative to a cross-sectional plane perpendicular to the axis of rotation, wherein the major axis “a” of the ellipse is oriented along the tangent (i.e., perpendicular to the radial direction passing through the axis of rotation), and the minor axis “b” of the ellipse is oriented along the radial direction.

[0042] Preferably, the ratio of the major axis "a" to the minor axis "b" of the ellipse is greater than or equal to 2.

[0043] Preferably, the central recess 64 is open to or leads to the associated first radial seat 40. This increases the controlled bending of the fins 68 separated by the central recess during torque transmission.

[0044] Preferably, the connection between the first tooth 36 and the first radial seat 40 is interference fit.

[0045] Specifically, the interference fit occurs at the contact point between the curved sidewall 48 of the first tooth 36 and the inclined side 44 of the first radial seat 40.

[0046] Preferably, the shape-connecting device 32 includes a second tooth 72 formed on the steel sheet 12, the second tooth 72 protruding toward the associated crankshaft 8, and the shape-connecting device 32 includes a second radial seat 76 formed on the outer sidewall 24 of the crankshaft 8 to at least partially radially receive the second tooth 72.

[0047] Advantageously, the second tooth 72 and the second radial seat 76 are arranged diametrically opposite the first tooth 36 and the first radial seat 40 relative to the axis of rotation of the crankshaft 8.

[0048] Preferably, the second tooth 72 has the same geometry and size as the first tooth 36, and the second radial seat 76 has the same geometry and size as the first radial seat 40.

[0049] The shape connection between each tooth 36, 72 and the associated radial seat 40, 76 is interference-type, wherein the connection between the second tooth 72 and the second radial seat 76 provides less interference or no interference than the connection between the first tooth 36 and the first radial seat 40.

[0050] According to an embodiment, the steel sheet 12 has an angular misalignment between the assembly axis of symmetry SS passing through the first tooth 36 and the second tooth 72 and the magnetic axis of symmetry WW of the steel sheet 12. The angular misalignment allows for a total double misalignment or "skew" SK between the steel sheets 12 that are identical to each other but mounted on the crankshaft 8 after a 180° rotation.

[0051] In the configuration, the major axis “a” of the central recess 64 is defined at an angle to the radial direction passing through the axis of rotation, the angle being equal to 90 degrees ± angular misalignment between the assembly symmetry axis SS and the magnetic symmetry axis WW of the steel sheet 12.

[0052] The improved rotor connection method and related operations according to the present invention will now be described.

[0053] Specifically, since the interference in diameter between the crankshaft 8 and the center seat 20 of the disc is not too large, the rotor 4 can be assembled in the following manner:

[0054] By cooling only the crankshaft 8;

[0055] By heating only the steel sheet laminate 12;

[0056] Direct compression fitting at room temperature.

[0057] It should be noted that the shape of the radial seats 40,76, through cooling the crankshaft 8, allows for a larger “clearance” or play to achieve more precise gear assembly 36,72.

[0058] In terms of operation, at high speeds (e.g., 20,000 rpm), the diameter of crankshaft 8 is “separated” from the steel plate 12: therefore, the only contact that prevents crankshaft 8 from rotating in opposite directions with the steel plate stack is teeth 36 and 72.

[0059] Therefore, at low speeds, torque is transmitted via interference fit on the diameter of crankshaft 8, while at medium to high speeds it is transmitted via one or more teeth and their respective radial seats.

[0060] When the crankshaft 8 rotates from high to low, the "V" geometry of the teeth allows for automatic alignment.

[0061] Through geometrical planar / spherical contact, the stress on the first and / or second teeth 36,72 in torque transmission remains low: the planar contact is given by the inclined surface of the bevel 44 that defines the radial seat 40,76 of the crankshaft 8, while the spherical contact is given by the radius of the curved sidewall 48 of each tooth 36,72 of the steel sheet 12.

[0062] It is evident that favorable conditions require that the radius R of the sidewall 48 of this curve be greater than or equal to 10 mm.

[0063] In this way, the contact is precise, but the mechanical stress is not too great.

[0064] The central recess 64 on the teeth 36, 72 is designed to provide greater flexibility to the teeth, and its geometry is elliptical so as to perform better in centrifugal force during the rotation of the crankshaft 8.

[0065] Preferably, the ratio of the semi-axes of the ellipse should be a / b>=2.

[0066] As can be appreciated from the above description, the present invention overcomes the disadvantages of the prior art.

[0067] Specifically, even under heavy-load operating conditions (i.e., high speed / high torque and frequent reversal of rotation direction), the rotor makes it possible to avoid any associated angular deviations between the steel sheet stack and the crankshaft.

[0068] More specifically, at low speeds, torque transmission occurs through friction between the outer wall of the crankshaft and the center seat of the steel plate stack, while at high speeds it occurs due to the action of teeth meshing in their respective radial seats.

[0069] The specific geometry of the teeth avoids any angular deviation at high speeds and allows for automatic centering when the rotational speed decreases.

[0070] Furthermore, the geometry of the teeth and the walls of the radial seat enable geometric contact that reduces specific contact pressure to an acceptable range to ensure a long service life for the component.

[0071] Using two identical but diametrically opposed teeth allows for quick and economical achievement of the desired skew between the (rotated 180°) stacks of steel plates bonded to the crankshaft.

[0072] Those skilled in the art can make many modifications and variations to the above solutions to meet specific and particular requirements, and all such modifications and variations are included within the scope of protection of the present invention as defined in the following claims.

Claims

1. A rotor (4) for an electric motor, said rotor (4) comprising: - Crankshaft (8), the crankshaft (8) extending along a main extension axis (XX) that coincides with the axis of rotation of the crankshaft (8), - At least one steel sheet (12) having a groove (16) for receiving a magnet, the steel sheet (12) having a center seat (20) which is keyed to the outer wall (24) of the crankshaft (8) by an interference fit. -Among them, steel sheets (12) The crankshaft (8) is connected at the center seat (20) by a shape-connecting device (32). -The shape-connecting device (32) includes a first tooth (36) and a first radial seat (40), the first tooth (36) being formed on a steel sheet (12) and protruding toward an associable crankshaft (8), and the first radial seat (40) being formed on the outer sidewall (24) of the crankshaft (8) to at least partially radially accommodate the first tooth (36). - Wherein, relative to the cross-sectional plane perpendicular to the axis of rotation, the first radial seat (40) has an isosceles trapezoidal cross-section, the isosceles trapezoid being laterally defined by a pair of hypotenuses (44) converging toward the axis of rotation. -The first tooth (36) has a "V" shaped cross section with a pair of curved sidewalls (48) adapted to engage the inclined side (44) of the first radial seat (40). -The first tooth (36) includes a central recess (64) that divides the first tooth (36) into a pair of fins (68) that are symmetrically arranged with respect to the central recess (64).

2. The rotor (4) for an electric motor according to claim 1, wherein, The hypotenuse of the first radial seat is flat, and the curved sidewall of the first tooth is round.

3. The rotor (4) for an electric motor according to claim 2, wherein, The circular wall of the first tooth has a radius of curvature of at least 10 mm.

4. The rotor (4) for an electric motor according to claim 1, 2 or 3, wherein, The first radial seat (40) is defined in height and in the radial direction by a pair of flat portions (46) that reduce the outer diameter of the crankshaft (8) to avoid direct contact with the associated steel sheet (12).

5. The rotor (4) for an electric motor according to claim 1, 2 or 3, wherein, The "V"-shaped cross section of the first tooth (36) has a radial chamfer (52) on one side of the rotation axis at the cantilevered radial end of the first tooth (36), which defines the radial clearance with the bottom (56) of the first radial seat (40).

6. The rotor (4) for an electric motor according to claim 1, 2 or 3, wherein, The first tooth (36) has a radial recess (60) at each connection area between the steel sheet and one of the curved sidewalls (48), the radial recess (60) extending on opposite sides of the associated crankshaft (8).

7. The rotor (4) for an electric motor according to claim 1, 2 or 3, wherein, The central recess (64) has an elliptical cross-section relative to a cross-sectional plane perpendicular to the axis of rotation, wherein the major axis (a) of the ellipse is tangentially oriented, i.e., perpendicular to the radial direction passing through the axis of rotation, and the minor axis (b) of the ellipse is oriented along the radial direction.

8. The rotor (4) for an electric motor according to claim 7, wherein, The ratio of the major axis (a) to the minor axis (b) of the ellipse is greater than or equal to 2.

9. The rotor (4) for an electric motor according to claim 1, 2 or 3, wherein, The central recess (64) is open to or leads to the associated first radial seat (40).

10. The rotor (4) for an electric motor according to claim 1, 2 or 3, wherein, The connection between the first tooth (36) and the first radial seat (40) is interference fit.

11. The rotor (4) for an electric motor according to claim 1, 2 or 3, wherein, The shape-connecting device (32) includes a second tooth (72) and a second radial seat (76), the second tooth (72) being formed on a steel sheet (12) and projecting toward an associatable crankshaft (8), and the second radial seat (76) being formed on the outer sidewall (24) of the crankshaft (8) to at least partially radially accommodate the second tooth (72), wherein the second tooth (72) and the second radial seat (76) are arranged relative to the axis of rotation at positions diametrically opposite to the first tooth (36) and the first radial seat (40).

12. The rotor (4) for an electric motor according to claim 11, wherein, The second tooth (72) has the same geometry and size as the first tooth (36), and the second radial seat (76) has the same geometry and size as the first radial seat (40).

13. The rotor (4) for an electric motor according to claim 11, wherein, The shape connection between each tooth (36, 72) and its radial seat (40, 76) is interference-fit, wherein the gap or interference provided by the connection between the second tooth (72) and the second radial seat (76) is less than the gap or interference provided between the first tooth (36) and the first radial seat (40).

14. The rotor (4) for an electric motor according to claim 11, wherein, The steel sheet (12) has an angular misalignment between the assembly symmetry axis (SS) passing through the first tooth (36) and the second tooth (72) and the magnetic symmetry axis (WW) of the steel sheet (12).

15. The rotor (4) for an electric motor according to claim 14, wherein, The central recess (64) has an elliptical cross-section relative to a cross-sectional plane perpendicular to the axis of rotation, wherein the major axis (a) of the ellipse is tangentially oriented, i.e., perpendicular to the radial direction passing through the axis of rotation, and the minor axis (b) of the ellipse is oriented along the radial direction. The major axis (a) of the central recess (64) is defined at an angle relative to the radial direction passing through the axis of rotation, the angle being equal to 90 degrees ± angular misalignment between the assembly axis of symmetry (SS) and the magnetic axis of symmetry (WW) of the steel sheet 12.

16. The rotor (4) for an electric motor according to claim 14, wherein, The central recess (64) has an elliptical cross-section relative to a cross-sectional plane perpendicular to the axis of rotation, wherein the major axis (a) of the ellipse is tangentially oriented, i.e., perpendicular to the radial direction passing through the axis of rotation, and the minor axis (b) of the ellipse is oriented along the radial direction. Wherein, the ratio of the major axis (a) to the minor axis (b) of the ellipse is greater than or equal to 2; The major axis (a) of the central recess (64) is defined at an angle relative to the radial direction passing through the axis of rotation, the angle being equal to 90 degrees ± angular misalignment between the assembly axis of symmetry (SS) and the magnetic axis of symmetry (WW) of the steel sheet 12.

17. An electric motor comprising a rotor (4) according to any one of claims 1 to 16 and a stator arranged coaxially with said rotor (4).

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