Rotor for permanent magnet electromachines

Abstract

A rotor (2) for a permanent magnet electric machine includes a hub (10) having a central rotational reference axis, and first and second sets (4, 6) of rotor segments (60). Each segment includes at least one permanent magnet (20), with the permanent magnets of the first set being circumferentially offset relative to the permanent magnets of the second set, and an engagement portion (16) configured to mechanically engage with the hub to resist radial forces and having a central axis (70) extending radially in a plane extending perpendicular to the central rotational reference axis. In each rotor segment of the first set, the center of mass of its magnet is circumferentially offset from the central axis of its respective engagement portion, and the segments are shaped such that the center of mass (68) of the segment is substantially aligned with the central axis (70) of its respective engagement portion.

Description

Technical Field

[0001] This disclosure relates to a rotor for a permanent magnet electric machine. More specifically, it relates to the configuration of rotor segments of the rotor.

Background Art

[0002] A rotor for a permanent magnet electric machine such as a motor or a generator may include a central hub with a set of permanent magnets and rotor segments disposed around its outer circumference. Such a rotor needs to be designed to securely hold the magnets and segments in position against high centrifugal and magnetic forces, as it may operate at very high rotational speeds.

Summary of the Invention

Means for Solving the Problems

[0003] This disclosure provides a rotor for a permanent magnet electric machine, the rotor comprising a hub having a central rotation reference axis, first and second sets of rotor segments, the first set extending around the circumferential surface of the hub at an axial position different from that of the second set, each segment comprising at least one permanent magnet, the permanent magnets of the first set being circumferentially offset with respect to the permanent magnets of the second set, an engagement portion formed to mechanically engage with the hub so as to resist radial forces and having a central axis extending radially in a plane perpendicular to the central rotation reference axis, in each rotor segment of the first set, the center of mass of its magnet being circumferentially offset from the central axis of its respective engagement portion, the segments being shaped such that the center of mass of each segment is substantially aligned with the central axis of its respective engagement portion.

[0004] Preferably, the segments are shaped such that the center of mass of each segment lies on the central axis of each engaging portion.

[0005] The permanent magnets of the first set of segments may be circumferentially offset from the permanent magnets of the second set in order to minimize cogging torque. However, the inventors have found that this results in the first set of segments having a force acting on them that causes each segment to rotate relative to the hub (during rotor rotation), resulting in a circumferential offset between the radial centerline of the engagement portion and the center of mass of the segment. This leads to a relatively non-uniform distribution of stress on the segment, thereby increasing the risk of mechanical failure.

[0006] According to this disclosure, each segment of the set is shaped such that the effect of the circumferential offset of its magnet relative to the engagement portion on the circumferential weight distribution of the segment is compensated by shaping the segment such that its center of mass is substantially aligned with the radial central axis of each engagement portion. This arrangement functions to minimize any rotational force experienced by each segment relative to the hub due to the circumferential offset of at least one of its magnets.

[0007] The radial central axis of the engaging portion may extend through the center of mass of the engaging portion. Preferably, the engaging portion is symmetric with respect to its radial central axis in a plane extending perpendicular to the central rotation reference axis of the rotor hub. The engaging portion may also be symmetric with respect to a plane containing its radial central axis and the central rotation reference axis of the rotor hub.

[0008] The radially extending sidewalls of each rotor segment of the first set (or both the first and second sets) may be asymmetric with respect to the radial line extending through the center of mass of the magnet in a cross section perpendicular to the central rotation reference axis, such that the center of mass of the segment is substantially aligned with the central axis of its respective engagement portion.

[0009] The segments of the first set (or both the first and second sets) may be asymmetrical in a plane extending perpendicular to the central rotation reference axis of the rotor hub. Each of the segments of the first set (or both the first and second sets) may have the same circumferential profile in a cross-section extending perpendicular to the central rotation reference axis of the rotor hub.

[0010] In some implementations, each rotor segment may contain two permanent magnets. In other embodiments, each rotor segment may contain a single magnet or three or more magnets.

[0011] At least one magnet in each segment may be held within its own segment. The magnet may be surrounded by the body of the segment in a plane extending perpendicular to the central rotation reference axis of the rotor hub.

[0012] In a preferred configuration, the engaging portion of each segment is in the form of a projection that is received by the rotor hub. Each engaging portion may be received by a region of the hub having a substantially complementary shape. The projection may be received by an axially extending groove defined on the outer circumferential surface of the hub. The projection may have a dovetail shape in a cross section extending perpendicular to the central rotation reference axis of the rotor hub.

[0013] Preferably, each rotor segment of the first set (or both the first and second sets) overlaps with an adjacent segment of the first set in the circumferential direction. More specifically, each rotor segment of the first set (or both the first and second sets) may extend partway down the adjacent segment of the first set in the circumferential direction.

[0014] In some preferred implementations, each rotor segment of the first set (or both the first and second sets) includes a pair of permanent magnets. Each of the pair of permanent magnets has an elongated cross-section extending perpendicular to the central rotation reference axis, with an inner end closer to the center of mass of the segment than the outer end, and the inner end is inclined to be closer to the central rotation reference axis. Each rotor segment of the first set (or both the first and second sets) extends at least partway under the permanent magnet of the adjacent segment in the circumferential direction.

[0015] Herein, embodiments of the present disclosure will be described as examples with reference to the attached schematic diagrams. [Brief explanation of the drawing]

[0016] [Figure 1] This is a top perspective view of a rotor for a permanent magnet electromachine. [Figure 2] Figure 1 is a cross-sectional view of the upper part of the rotor. [Figure 3] Figure 1 is a cross-sectional view of the upper part of the rotor. [Figure 4] Figure 1 is a cross-sectional view of the upper part of the rotor. [Figure 5] This is a cross-sectional view of the upper part of a rotor according to one embodiment of the present disclosure. [Figure 6] This is a cross-sectional view of the upper part of a rotor according to one embodiment of the present disclosure. [Modes for carrying out the invention]

[0017] Figures 1 to 4 show the upper part of rotor 2, including two adjacent sets 4 and 6 of rotor segments. The rotor segments are supported on the outer circumferential surface 8 of rotor hub 10.

[0018] The hub 10 is formed by machining a solid piece of material, by an additive manufacturing process, or preferably by casting. The hub 10 may be made of, for example, steel or aluminum.

[0019] Each set of segments is arranged in a ring around the hub, and in the direction of the rotation reference axis of the hub, one set is axially displaced with respect to the other set. The segments can be formed of, for example, laminated steel plates.

[0020] Each rotor segment 12 includes a body portion 14 and an engagement portion 16. The engagement portion is shaped to mechanically engage with the hub so as to resist displacement of the segment with respect to the hub due to a radial force acting on the segment.

[0021] In the embodiments shown in FIGS. 1 to 4, each rotor segment includes an engagement portion in the form of a protrusion 16 extending radially inward. In other implementations, the engagement portion may alternatively be in the form of a groove for receiving an outwardly extending protrusion forming part of the hub 10.

[0022] In FIGS. 1 to 4, the protrusion 16 is substantially in a double-tail shape in a plane extending perpendicular to the rotation reference axis of the hub. The protrusion is received in an axially extending groove 18 defined by a circumferential surface facing radially outward of the rotor hub 10. The cross-sectional shape of the groove is substantially complementary to the cross-sectional shape of the protrusion 16. The groove extends linearly across the width of the hub in the axial direction and also receives the protrusions of the other set (6) of segments.

[0023] Each rotor segment may include a pair of magnets 20. Each magnet is held in a respective notch 22 defined by the body portion 14 of its rotor segment. The magnets are spaced apart in the circumferential direction and have the mass centers of the magnets located at the same distance from the central axis of the hub.

[0024] Each magnet may be elongated when viewed in a cross-section perpendicular to the central rotation reference axis of the hub (as shown in FIGS. 1 and 2). Each magnet is inclined in this plane such that its inner end 21 (closer to the mass center of the segment than its outer end 23) is closer to the center of the rotor hub than its outer end 23.

[0025] In other implementations, each rotor segment may contain a single magnet or three or more magnets.

[0026] As can be seen in Figure 2, each segment of one set 4 is configured such that the radial center reference line or axis 30 of the projection 16 of each segment is offset 34 circumferentially from the radial center reference line or axis 32 of the body portion 14 of the segment. The radial axis 32 extends between a pair of magnets 20. The projection is symmetrical in a plane perpendicular to the rotor's axis of rotation with respect to its central axis 30. The body portion 14 is symmetrical in that plane with respect to its central axis 32. This offset may also exist in the other set (6) of rotor segments shown in Figure 1, but on opposite sides circumferentially. In that configuration, the rotor segments of both sets may be identical, and in the assembled rotor, one set is reversed relative to the other. Each set is circumferentially offset or skewed relative to the other. The circumferential offset between the two sets of rotor segments serves to minimize the effect of cogging torque.

[0027] However, in the rotor configured according to Figures 1 to 4, when the rotor rotates around its central axis, the segments of set 4 experience a force acting to rotate each segment relative to the rotor hub because the mass centers of the segments are not aligned circumferentially with the central axis 30 of the projection 16. This can lead to the formation of undesirable gaps between the segments and the rotor hub. This effect can be reduced by improving the dimensional accuracy of the surfaces of the interlocking segments and rotor hub, but doing so would increase manufacturing costs.

[0028] Furthermore, these rotational forces tend to result in a non-uniform stress distribution on the segments and rotor hub. This is shown in the simulation results depicted in Figure 3, where different shades indicate the differences in stress experienced across the cross-sections of these components during rotor rotation, with darker shades indicating higher stress. The stress is non-uniformly distributed, and it can be seen that higher stress occurs near the corners 40 of the groove 18 compared to the corners 42. Similarly, higher stress is experienced around the joint 44 between the projection 16 on one side and the main body portion 14 compared to the joint 46 on the other side of the projection.

[0029] The radii of joints 44 and 46 can be increased to reduce stress concentration at these locations. However, this removes material from the segment body at the locations where magnetic flux flows from one magnet to the other, which may adversely affect the electromagnetic performance of the rotor. This tends to increase the reluctance of the magnetic circuit, which in turn hinders the magnetic flux entering the gap between the rotor and the surrounding stator to generate torque.

[0030] Figure 4 shows the results of a simulation of the extent to which different parts of the segment may shift during rotor rotation. Darker shades indicate greater shift. The rotational force exerted on the segment by the offset 34 shown in Figure 2 tends to cause the side 52 of the rotor segment that is circumferentially away from the protrusion to move further away from the center of the hub than the opposite side 50.

[0031] Figures 5 and 6 are cross-sectional views of a rotor segment 60 according to one embodiment of the present disclosure. Each segment includes a body portion 62 and a projection 64. The magnet 20 is inclined similarly to the magnets of the segments shown in Figures 1 to 4.

[0032] The main body portion 62 is asymmetrical in the plane of the drawing with respect to the radial axis 66 extending between the magnets 20. The mass of the main body portion is distributed around its radial axis 66 such that the mass center 68 of the segment lies on the radial center reference axis 70 of the projection 64. The cross-sectional shape of each rotor segment may be uniform in the axial direction.

[0033] The main body portion 62 includes an extension 72 that extends in one circumferential direction to partway down an adjacent segment, preferably partway down the magnet of the adjacent segment. The extension may be tapered such that its width decreases as it moves away from the center of the segment. The upper surface 78 of the extension may be located in a plane substantially parallel to the plane defined by the inclined surface 80 facing radially inward of the magnet above it. The lower surface 82 of the extension may be in contact with the outer circumferential surface 8 of the hub 10.

[0034] The main body portion 62 may be separated in a complementary manner along the opposite side 73 in the circumferential direction to accommodate the extension 72 of the adjacent segment on the opposite side. In this way, the center of mass 68 of the segment is shifted circumferentially with respect to its magnet so as to coincide with the central reference axis 70 of the projection 64. Therefore, since the centrifugal force acting on the segment during the rotation of the rotor is aligned with the central axis of the projection 64, there is no result of a mechanical moment acting on the segment with respect to the hub.

[0035] Figure 5 shows the simulation results of stresses experienced during rotation of a rotor including a rotor segment 60 according to one embodiment of the present disclosure. Compared to those shown in Figure 3, it can be seen that the stresses are lower, more evenly distributed, and more symmetrically distributed with respect to the central radial axis 70 of the projection 64.

[0036] Figure 6 is shaded to show the simulation results of segment displacement during rotor rotation. Any displacement magnitude is significantly reduced compared to that predicted by the simulation shown in Figure 4, as indicated by thinner and more uniform shading.

[0037] The hub and segment configurations shown in Figures 5 and 6 result in lower stress, allowing for a reduction in the radius of the joints 74 and 76 compared to the corresponding sections 44 and 46 shown in Figure 3, for a given resistance of the rotor segment to centrifugal force. Furthermore, the given resistance to centrifugal force can be achieved with lower manufacturing tolerances, thereby reducing manufacturing costs.

[0038] The magnets of the rotor segments shown in Figures 5 and 6 are offset circumferentially with respect to the magnets of adjacent sets. Preferably, the rotor segments of both sets are identical, and in the assembled rotor, one set is reversed circumferentially with respect to the other set.

Claims

1. A rotor for a permanent magnet electromachine, A hub having a central rotation reference axis, A first and second set of rotor segments, wherein the first set extends around the circumferential surface of the hub at an axial position different from that of the second set, Each segment, At least one permanent magnet, wherein the permanent magnet of the first set is offset circumferentially from the permanent magnet of the second set, It includes an engaging portion formed to mechanically engage with the hub to resist radial forces and having a central axis extending radially in a plane perpendicular to the central rotation reference axis, In each rotor segment of the first set, the center of mass of the magnet is offset circumferentially from the central axis of the respective engagement portion. A rotor in which the segments are shaped such that the center of mass of each segment is substantially aligned with the central axis of each of the engaging portions.

2. The rotor according to claim 1, wherein the radially extending sidewall of each rotor segment of the first set is asymmetrical with respect to a radial line extending through the center of mass of the magnet in a cross section perpendicular to the central rotation reference axis.

3. The rotor according to claim 1 or 2, wherein each rotor segment of the first set overlaps with an adjacent segment of the first set in the circumferential direction.

4. The rotor according to any one of claims 1 to 3, wherein each rotor segment of the first set extends partway down the adjacent segment of the first set in the circumferential direction.

5. Each rotor segment of the first set includes a pair of permanent magnets, Each of the pair of permanent magnets has, in a cross-section extending perpendicular to the central rotation reference axis, an elongated shape, with an inner end closer to the center of mass of the segment than to the outer end, and the inner end is inclined to be closer to the central rotation reference axis. The rotor according to any one of claims 1 to 4, wherein each rotor segment of the first set extends at least partway down the permanent magnet of the adjacent segment in the circumferential direction.

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