Design of rotor barriers to reduce demagnetization in a machine with internal permanent magnets

Abstract

Machine (20) with internal permanent magnets (32), comprising: a wound stator (22); a rotor core (28) which defines and is designed to interact magnetically with the wound stator (22) in order to rotate about a rotational axis (26); and several magnets (32), wherein one of the several magnets (32) is arranged in each of the several cavities (30); wherein at least one of the several cavities (30) comprises an end cavity (40) arranged along a curved section which is convexly oriented outwards relative to the axis of rotation (26) and which defines a curved end (42) for the curved section; wherein the end cavity (40) defines an air groove (52) which is arranged at the curved end (42) of the end cavity (40) adjacent to a radially outermost edge (48) of the magnet (32) arranged in the end cavity (40) relative to the axis of rotation (26); and wherein the rotor core (28) includes a leakage flux path (54) extending across the air groove (52) and spaced from the radially outermost edge (48) of the magnet (32) arranged in the end cavity (40) and designed to direct a flux (55) away from the magnet (32); wherein the leakage flow path (54) contains a bridge (56) which is oriented in an approximately tangential relationship relative to the axis of rotation (26) and connects opposite sides of the end cavity (40); wherein the bridge (56) is angled relative to the radial outermost edge (48) of the magnet (32) which is arranged in the end cavity (40) in order to direct the flux (55) away from the magnet (32).

Description

TECHNICAL FIELD

[0001] The invention relates generally to a machine with internal permanent magnets and more particularly to a rotor assembly for the machine with internal permanent magnets. BACKGROUND

[0002] Internal permanent magnet (IPM) machines are preferred for the operation of fuel cell and hybrid electric vehicles due to their desirable properties, such as good torque density, high overall efficiency, and a relatively constant power range. The rotor field in a permanent magnet machine is maintained due to its structure, unlike other machines such as induction machines or switched or synchronous reluctance machines, where the field is generated by a stator current supplied from a source. As a result, permanent magnet machines exhibit better efficiency compared to other such machines.

[0003] An IPM machine can use either ferrite magnets or rare earth magnets, such as NdFeB. However, ferrite magnets often carry the risk of demagnetization under certain operating conditions (e.g., low temperature and high d-axis current). To allow the use of less expensive ferrite magnets, some IPM machines may incorporate one or more rotor barriers containing permanent magnets and / or air gaps (which may be filled with a non-magnetic material). These rotor layers act as barriers to the permanent magnetic field and reduce the d-axis or magnet axis inductance, thereby improving the saliency of the motor. Increasing the motor saliency improves motor efficiency and torque output.

[0004] JP 2009-296685 A discloses a rotor of a permanent magnet machine with an outer and an inner barrier, each divided by radial bridges. The divided sections of the inner barrier are further circumferentially divided by bridges, with the distance of the radial bridge of the outer barrier from a rotational axis being approximately equal to the distance of the bridges in the circumferential direction from the rotational axis in order to reduce mechanical stress on peripheral bridges of the rotor.

[0005] The document DE 10 2008 008 054 A1 discloses a rotor for a permanent magnet excited electrical synchronous machine, in which permanent magnets can be arranged in magnetic pockets in such a way that both a two-pole and a four-pole rotor can be realized.

[0006] The document DE 10 2007 046 997 A1 discloses a rotor of an electric motor in which the arrangement of magnets determines a number of poles of the rotor, wherein the magnets are divided into groups with a group number that corresponds to the number of poles of the rotor.

[0007] JP 2007 - 174 790 A discloses a mechanical structure of a rotor with 40 slots on a rotor surface, each of which houses a permanent magnet. The permanent magnets are magnetized so that the rotor has 2, 4, 8, 10, 20, or 40 poles. SUMMARY

[0008] An internal permanent magnet (IPM) machine is provided. The IPM machine includes a wound stator and a rotor core. The rotor core defines a plurality of cavities and is magnetically configured to interact with the wound stator to rotate about a rotational axis. The IPM machine further includes a plurality of magnets, one of the plurality of magnets being disposed in each of the plurality of cavities. At least one of the plurality of cavities includes an end cavity. The end cavity is disposed along an arcuate path that is convexly oriented outward relative to the rotational axis. The end cavity defines a arcuate end for the arcuate path. The end cavity defines an air groove disposed at the arcuate end of the end cavity. The air groove is adjacent to a radially outermost edge of the magnet disposed within the end cavity, relative to the rotational axis.The rotor core includes a leakage flux path extending across the air groove and spaced from the radially outermost edge of the magnet located in the end cavity. The leakage flux path is designed to direct the flux away from the magnet.

[0009] A rotor assembly for a machine with internal permanent magnets is also provided. The rotor assembly includes a rotor core defining a plurality of cavities. The rotor core is magnetically configured to interact with a wound stator to rotate about a rotational axis. The rotor assembly further includes a plurality of ferrite magnets, one of the plurality of ferrite magnets being disposed in each of the plurality of cavities. Each of the plurality of ferrite magnets includes a radially inner surface extending along the rotational axis and a radially outer surface extending along the rotational axis. The radially outer surface is offset from the radially inner surface, and the radially inner surface is disposed closer to the rotational axis than the radially outer surface. At least one of the plurality of cavities includes an end cavity.The end cavity is disposed along a curved path that is oriented convexly outward relative to the axis of rotation. The end cavity defines a curved end for the curved path. The end cavity defines an air groove disposed at the curved end of the end cavity, adjacent to a radially outermost edge of the magnet disposed within the end cavity. The rotor includes a leakage flux path extending across the air groove and spaced from the radially outermost edge of the magnet disposed within the end cavity. The leakage flux path is configured to direct flux away from the ferrite magnet. The end cavity further defines an air pocket disposed along the radially inner surface of the magnet disposed within the end cavity, adjacent to the air groove at the curved end of the end cavity.

[0010] Accordingly, the rotor core is designed via the air pocket and the leakage flux path to direct the flux away from a convex inner corner of the magnet, i.e., the corner of the magnet located at the interface between the radially inner surface and the radially outermost edge of the magnet. Directing the flux away from the convex inner corner of the magnet prevents the flux from penetrating the magnet, thereby preventing demagnetization of the magnet and enabling the use of ferrite magnets in the rotor assembly.

[0011] The foregoing features and advantages and other features and advantages of the present invention will be readily apparent from the following detailed description of the best modes for carrying out the invention when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic partial cross-sectional view of an internal permanent magnet (IPM) machine. Fig. Figure 2 is an enlarged schematic partial cross-sectional view of the IPM machine showing a flow path through a rotor core. DETAILED DESCRIPTION

[0012] Those skilled in the art will recognize that terms such as "above", "below", "upward", "downward", "top", "bottom", etc., are used to describe the figures and do not represent limitations on the scope of the invention as defined by the appended claims.

[0013] Referring to the figures, wherein like reference numerals designate like parts throughout the several views, an internal permanent magnet machine is shown generally at 20. The IPM machine 20 may include, but is not limited to, an electric motor or other similar device.

[0014] The IPM machine 20 includes a wound stator 22 and a rotor assembly 24. The rotor assembly 24 is magnetically configured to interact with the wound stator 22 to rotate relative to the wound stator 22 about a rotational axis 26. The rotor assembly 24 includes a rotor core 28 defining a plurality of cavities 30. The cavities 30 may include recesses, grooves, etc. The rotor assembly 24 further includes a plurality of magnets 32, with one of the plurality of magnets 32 disposed within each of the plurality of cavities 30. It should be noted that the magnets 32 and the cavities 30 include corresponding cross-sectional shapes perpendicular to the rotational axis 26, such as the arcuate shapes shown, or alternatively, rectangular shapes.The wound stator 22 includes a plurality of windings 34 that magnetically interact with the plurality of magnets 32 disposed in the cavities 30 of the rotor core 28 to generate torque to rotate the rotor core 28 about the rotation axis 26 relative to the wound stator 22 during operation.

[0015] The permanent magnets 32 may include any type of magnetic material suitable for use in the IPM machine 20. For example, each of the plurality of magnets 32 may be made from, but not limited to, a ferrite magnet, an alnico magnet, or alternatively, a rare earth magnet such as neodymium iron boron (NdFeB).

[0016] The cavities 30 defined by the rotor core 28 are arranged in one or more layers 36, 38, each layer shaped to define a cross-section perpendicular to the axis of rotation 26 and forming a generally curved path oriented convexly outward from the axis of rotation 26. As shown in the figures, the cavities 30 define two layers, i.e., a first or radially inner layer 36 and a second or radially outer layer 38. However, it should be noted that the cavities 30 may define only a single layer or more than the two layers shown. Furthermore, each layer may contain any number of cavities 30 therein. As shown, the radially inner layer 36 contains three cavities 30 and the radially outer layer 38 contains two cavities 30. However, the number of cavities 30 in each layer may differ from the exemplary embodiment shown.

[0017] At least one of the cavities 30 contains or is defined as an end cavity 40. The end cavity 40 is arranged along the curved path of one of the layers 36, 38 and defines a curved end 42 for the curved path. The curved end 42 of the end cavity 40 is arranged at the radially outermost end of the curved path relative to the rotation axis 26, ie, the curved end 42 is the outermost end of the end cavity 40 that is arranged furthest from the rotation axis 26. The Fig.The illustrated embodiment shown in Figure 1 shows two end cavities 40 disposed along the arcuate path defined by the radially inner layer 36 of cavities 30, and no or zero end cavities 40 disposed along the arcuate path defined by the radially outer layer 38 of cavities 30. However, it should be appreciated that the radially outer layer 38 may alternatively include one or more end cavities 40.

[0018] Each of the magnets 32 includes a radially inner surface 44 extending longitudinally along the rotation axis 26 and a radially outer surface 46 extending longitudinally along the rotation axis 26. The radially outer surface 46 is offset from the radially inner surface 44 by a distance equal to the thickness of the magnet 32. The radially inner surface 44 is located closer to the rotation axis 26 than the radially outer surface 46. The magnet(s) 32 disposed in the end cavities 40 include a radially outermost edge 48 relative to the rotation axis 26 and a radially innermost edge 50 relative to the rotation axis 26. The radially outermost edge 48 is located adjacent the curved end 42 of the end cavity 40 and is located farther from the rotation axis 26 than the radially innermost edge 50.

[0019] The end cavity 40 defines an air groove 52 located at the curved end 42 of the end cavity 40. The air groove 52 is located adjacent the radially outermost edge 48 of the magnet 32 ​​located within the end cavity 40. The air groove 52 extends into and out of the side shown in the figures along and parallel to the axis of rotation 26. The air groove 52 allows the magnet 32 ​​to be moved away from the rotor surface and also prevents excessive leakage flux through a leakage flux path 54. Furthermore, the air groove 52 shields the rotor magnet 32 ​​from demagnetization.

[0020] The rotor core 28 includes and / or defines a leakage flux path 54. As shown in the figures, the leakage flux path 54 extends beyond the air groove 52 and is spaced from the radially outermost edge 48 of the magnet 32 ​​disposed within the end cavity 40. It should be noted that the leakage flux path 54 extends longitudinally along and parallel to the axis of rotation 26 in and out of the side shown in the figures. The leakage flux path 54 directs flux, indicated generally by arrows 55, away from the magnet 32, thereby preventing the flux from entering and demagnetizing the magnet 32. The leakage flux path 54 may be integrally formed with the rotor core 28 or, alternatively, it may be mechanically attached to the rotor core 28 within the end cavity 40.

[0021] The leakage flux path 54 may, for example, include a bridge 56 oriented in an approximately tangential relationship relative to the rotation axis 26 and connecting opposite longitudinal sides of the end cavity 40. However, it should be appreciated that the leakage flux path 54 may be configured in a different manner not shown or described herein. The bridge 56 is angled relative to the radially outermost edge 48 of the magnet 32 ​​disposed in the end cavity 40 to define an angle 58 therebetween. The angle 58 preferably comprises a value in the range of 10° to 20°. The bridge 56 is angled relative to the radially outermost edge 48 of the magnet 32 ​​to direct the flux away from the magnet 32. Accordingly, the bridge 56 directs the flux away from a convex inner corner 60 of the magnet 32, i.e.the corner of the magnet 32 ​​located at the intersection of the radially inner surface 44 and the radially outermost edge 48 of the magnet 32, to a convex outer corner 62 of the magnet 32, i.e., the corner of the magnet 32 ​​located at the intersection of the radially outer surface 46 and the radially outermost edge 48 of the magnet 32. The bridge 56 preferably includes a width 64 measured perpendicular to the axis of rotation 26 that is in the range of 0.5 mm to 1.0 mm. The bridge 56 and the magnet 32 ​​located in the end cavity 40 are separated by a distance in the range of 0.1 mm to 1.5 mm. In particular, the convex inner corner 60 of the magnet 32 ​​is preferably separated from the bridge 56 by a distance 66 in the range of 0.1 mm to 0.2 mm and the convex outer corner 62 of the magnet 32 ​​is preferably separated from the bridge 56 by a distance 68 in the range of 0.75 mm to 1.5 mm.

[0022] The end cavity 40 may further define an air pocket 70. The air pocket 70 is disposed along the radially inner surface 44 of the magnet 32 ​​disposed within the end cavity 40, adjacent the air groove 52 and near the bent end 42 of the end cavity 40, i.e., adjacent the convex outer corner 62 of the magnet 32. The air pocket 70 extends longitudinally along and parallel to the axis of rotation 26 into and out of the side as shown in the figures. The air pocket 70 further prevents or increases the resistance to the rotor field from penetrating the magnet 32 ​​near the outer corner 62, thereby preventing the outer corner 62 from becoming demagnetized.

[0023] The air pocket 70 defines a gap distance 72 between the rotor core 28 and the radially inner surface 44 of the magnet 32 ​​disposed in the end cavity 40, which is measured perpendicular to the rotation axis 26. The gap distance 72 can vary in the range of 0.35 mm to 0.75 mm. Furthermore, the air pocket 70 defines a gap length 74 perpendicular to the rotation axis 26 and tangential or parallel to the radially inner surface 44 of the magnet 32 ​​disposed in the end cavity 40. The gap length 74 can vary in the range of 1.0 mm to 20.0 mm.

[0024] The detailed description and the drawings or figures support and describe the invention, but the scope of the invention is defined only by the claims. While some of the best modes and other embodiments for carrying out the claimed invention have been described in detail, various alternative designs and embodiments exist for practicing the invention defined in the appended claims.

Claims

[1] Machine (20) with internal permanent magnets (32), comprising: a wound stator (22); a rotor core (28) defining a plurality of cavities (30) and configured to magnetically interact with the wound stator (22) to rotate about a rotational axis (26); and a plurality of magnets (32), wherein one of the plurality of magnets (32) is arranged in each of the plurality of cavities (30); wherein at least one of the plurality of cavities (30) comprises an end cavity (40) arranged along a curved path that is convexly oriented outwardly relative to the axis of rotation (26) and that defines a curved end (42) for the curved path; wherein the end cavity (40) defines an air groove (52) disposed at the curved end (42) of the end cavity (40) adjacent to a radially outermost edge (48) of the magnet (32) disposed in the end cavity (40) relative to the rotation axis (26); and wherein the rotor core (28) includes a leakage flux path (54) extending across the air groove (52) and spaced from the radially outermost edge (48) of the magnet (32) disposed in the end cavity (40) and configured to direct a flux (55) away from the magnet (32); wherein the leakage flow path (54) includes a bridge (56) oriented in an approximately tangential relationship relative to the axis of rotation (26) and connecting opposite sides of the end cavity (40); wherein the bridge (56) is angled relative to the radially outermost edge (48) of the magnet (32) disposed in the end cavity (40) to direct the flux (55) away from the magnet (32). [2] The machine (20) with internal permanent magnets (32) according to claim 1, wherein the bridge (56) includes a width (64) perpendicular to the axis of rotation (26), the width (64) of the bridge (56) being in the range of 0.5 mm to 1.0 mm. [3] The internal permanent magnet (32) machine (20) of claim 2, wherein the bridge (56) and the magnet (32) disposed in the end cavity (40) are separated by a distance in the range of 0.1 mm to 1.5 mm. [4] The internal permanent magnet (32) machine (20) of claim 1, wherein each of said plurality of magnets (32) includes a radially inner surface (44) extending along said rotational axis (26) and a radially outer surface (46) extending along said rotational axis (26), said radially outer surface (46) being offset from said radially inner surface (44), and said radially inner surface (44) being located closer to said rotational axis (26) than said radially outer surface (46). [5] The internal permanent magnet machine (20) of claim 4, wherein the end cavity (40) defines an air pocket (70) disposed along the radially inner surface (44) adjacent the air groove (52) at the curved end (42) of the end cavity (40). [6] The machine (20) with internal permanent magnets (32) of claim 5, wherein the air pocket (70) defines a gap distance (72) perpendicular to the axis of rotation (26) and between the rotor core (28) and the radially inner surface (44) of the magnet (32) disposed in the end cavity (40). [7] Machine (20) with internal permanent magnets (32) according to claim 6, wherein the gap distance (72) is in the range of 0.35 mm to 0.75 mm. [8] The machine (20) with internal permanent magnets (32) according to claim 5, wherein the air pocket (70) defines a gap length (74) perpendicular to the axis of rotation (26) and tangential to the radially inner surface (44) of the magnet (32) disposed in the end cavity (40).

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