Rotor assembly

Abstract

A rotor assembly for an electric motor includes a shaft, a lamination stack, a plurality of magnets including first magnets, and a plurality of pole shoes including first pole shoes. A first magnet is disposed between the lamination stack and the first pole shoe, and each pole shoe has an outer pole surface. The rotor assembly also includes a sleeve including: an outer sleeve body disposed around an outer pole surface of each pole shoe to retain each of the pole shoes relative to the stack of laminations; and a plurality of spacers extending radially inward from the outer sleeve body toward the shaft. The plurality of spacers includes a first spacer and a second spacer, and the first spacer and the second spacer are disposed between the first pole shoe and the stack of laminations to reduce flux leakage of the first magnet.

Description

Cross - Reference to Related Applications

[0001] This application claims the benefit of priority and all advantages of U.S. Non - Provisional Patent Application No. 17 / 957,019, filed on September 30, 2022, the entire content of which is incorporated herein by reference. Technical Field

[0002] The present invention generally relates to a rotor assembly for an electric motor and to an electric motor including the rotor assembly. Background Art

[0003] An electric motor includes a rotor assembly and a stator disposed around the rotor assembly. The rotor assembly typically includes a shaft, a stack of laminations disposed around the shaft, and a plurality of pole shoes radially spaced apart from the stack of laminations. Typically, permanent magnets are embedded in the rotor assembly, usually between the stack of laminations and the plurality of pole shoes, and generate magnetic flux. The stator includes a coil winding, and energization of the coil winding in the stator generates magnetic flux that interacts with the magnetic flux generated by the permanent magnets to apply a force that causes the rotor assembly to rotate.

[0004] Typically, the stack of laminations and the plurality of pole shoes are physically connected to each other by a plurality of bridge members, thus forming what is commonly referred to as a bridged rotor assembly. Each bridge member is integral with both the stack of laminations and one of the plurality of pole shoes and is typically made of the same material as the stack of laminations and the plurality of pole shoes. The bridge member prevents the plurality of pole shoes from moving away from the stack of laminations due to the centrifugal force applied to the plurality of pole shoes during rotation of the rotor assembly.

[0005] However, the magnetic flux generated by the permanent magnets is disturbed or leaked by the bridge members such that a portion of the magnetic flux is directed towards the stack of laminations and connects the north and south poles of each permanent magnet. The leaked magnetic flux limits the amount of magnetic flux from the permanent magnets available to interact with the magnetic flux generated by the coil winding of the stator, thus reducing the force applied to the rotor assembly and, consequently, also reducing the efficiency of the electric motor. Flux leakage will occur until the bridge members reach magnetic saturation. To minimize the flux leakage through the bridge members, the bridge members are typically designed to be as thin as possible to quickly reach magnetic saturation within the stack of laminations and thus reduce the magnetic flux leakage from the permanent magnets. However, the centrifugal force applied to the plurality of pole shoes during rotation of the rotor assembly is also applied to the bridge members. These centrifugal forces applied to the bridge members result in stress concentration on the bridge members and a risk of bridge member structural failure, especially when the bridge members are designed to be as thin as possible.

[0006] Thus, there is still a need to provide an improved rotor assembly for an electric motor. Summary of the Invention

[0007] A rotor assembly for an electric motor is disclosed. The rotor assembly includes a shaft extending along an axis and rotatable about the axis. The rotor assembly further includes a stack of laminations coupled to the shaft, a plurality of magnets including a first magnet coupled to the stack of laminations, and a plurality of pole shoes radially outwardly spaced from the shaft. The plurality of pole shoes includes a first pole shoe, and the first magnet is disposed between the stack of laminations and the first pole shoe. Each of the plurality of pole shoes has an outer pole surface facing away from the axis.

[0008] The rotor assembly further includes a sleeve, which includes an outer sleeve body and a plurality of spacers. The outer sleeve body of the sleeve is disposed around the outer pole surface of each of the plurality of pole shoes to hold each of the pole shoes relative to the stack of laminations. The plurality of spacers extend radially inwardly from the outer sleeve body toward the shaft. The plurality of spacers includes a first spacer and a second spacer, and the first spacer and the second spacer are disposed between the first pole shoe and the stack of laminations to reduce flux leakage of the first magnet.

[0009] The outer sleeve body holds the first pole shoe relative to the stack of laminations, thereby preventing the first pole shoe from moving away from the stack of laminations when a centrifugal force is applied to the first pole shoe by rotation of the rotor assembly. In addition, the outer sleeve body, the first spacer, and the second spacer prevent interruption or leakage of the magnetic flux of the first magnet. Thus, it is ensured that the magnetic flux of the first magnet can be used to effectively apply a force to the rotor assembly to rotate the rotor assembly. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Other advantages of the present invention will be readily appreciated, as the present invention will become better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which: Figure 1 is a perspective view of a rotor assembly according to the present invention; Figure 2 is Figure 1 a cross-sectional view of the rotor assembly of [reference to previous figure], with the rotor assembly including a first end cap, a second end cap, and a plurality of rods extending therebetween; Figure 3 is Figure 1 another cross-sectional view of the rotor assembly of [reference to previous figure], with the rotor assembly including a stack of laminations, a plurality of pole shoes, a plurality of magnets, and a sleeve including an outer sleeve body and a plurality of spacers; Figure 4 is a perspective view of the first end cap, the second end cap, and the plurality of rods extending therebetween; Figure 5 is including Figure 1 a perspective view of an electric motor of the rotor assembly of [reference to previous figure]; and Figure 6 is including Figure 5 a partially dashed perspective view of an electric drive unit of the electric motor of [reference to previous figure]. DETAILED DESCRIPTION

[0011] Refer to the accompanying drawings, in which like numerals represent like parts throughout several views, throughout Figures 1-6 shows the rotor assembly 10 of the electric motor 12. The rotor assembly 10 includes a shaft 14 that extends along and is rotatable about an axis A1. The rotor assembly 10 further includes a stack of laminations 16 coupled to the shaft 14, a plurality of magnets 18 including a first magnet 20 coupled to the stack of laminations 16, and a plurality of pole shoes 24 spaced radially outward from the shaft 14. Although not required, the plurality of magnets 18 may further include a second magnet 22 coupled to the stack of laminations 16. The plurality of pole shoes 24 includes a first pole shoe 26. It should be appreciated that the plurality of pole shoes 24 may be referred to as a plurality of pole shoes. The first magnet 20 is disposed between the stack of laminations 16 and the first pole shoe 26. In embodiments having a second magnet 22, the second magnet 22 is disposed between the stack of laminations 16 and the first pole shoe 26. Each of the plurality of pole shoes 24 has an outer pole surface 28 facing away from the axis A1.

[0012] The rotor assembly 10 further includes a sleeve 30 that includes an outer sleeve body 32 and a plurality of spacers 34. The outer sleeve body 32 of the sleeve 30 is disposed about the outer pole surface 28 of each of the plurality of pole shoes 24 to hold each of the pole shoes 24 relative to the stack of laminations 16. The plurality of spacers 34 extend radially inward from the outer sleeve body 32 toward the shaft 14. The plurality of spacers 34 includes a first spacer 36 and a second spacer 38, and the first spacer 36 and the second spacer 38 are disposed between the first pole shoe 26 and the stack of laminations 16 to reduce flux leakage of the first magnet 20.

[0013] The outer sleeve body 32 holds the first pole shoe 26 relative to the stack of laminations 16, thereby preventing the first pole shoe 26 from moving away from the stack of laminations 16 when a centrifugal force is applied to the first pole shoe 26 by the rotation of the rotor assembly 10. In addition, the outer sleeve body 32, the first spacer 36, and the second spacer 38 prevent interruption or leakage of the magnetic flux of the first magnet 20. Thus, it is ensured that the magnetic flux of the first magnet 20 can be used to effectively apply a force to the rotor assembly 10 to rotate the rotor assembly 10.

[0014] Although the benefits of the outer sleeve body 32, the first spacer 36, and the second spacer 38 have been described with reference to the first pole shoe 26, the first magnet 20, the optional second magnet 22, and the lamination stack 16, it should be appreciated that these benefits may equally apply to all other spacers 34 among the plurality of spacers 34, all other pole shoes 24 among the plurality of pole shoes 24, all other magnets 18 among the plurality of magnets 18, and the lamination stack 16. Each spacer 34 and pole shoe 24 may have the characteristics of the first spacer 36, the second spacer 38, and the first pole shoe 26 as described herein. More specifically, the outer sleeve body 32 may hold each of the pole shoes 24 relative to the lamination stack 16, thereby preventing the plurality of pole shoes 24 from moving away from the lamination stack 16 when a centrifugal force is applied to each pole shoe 24 by the rotation of the rotor assembly 10. In addition, the outer sleeve body 32 and the plurality of spacers 34 prevent interruption or leakage of the magnetic flux between the plurality of pole shoes 24 and the lamination stack 16. Thus, it is ensured that the magnetic flux can be used to effectively apply a force to the rotor assembly 10 to rotate the rotor assembly 10.

[0015] In a non-limiting example, the lamination stack 16 may be made of a plurality of laminations fixed to each other by interlocking, welding, clamping, and / or bonding. To improve the manufacturability of the lamination stack 16, each lamination in the lamination stack 16 may be the same as each other lamination in the lamination stack 16.

[0016] Each magnet among the plurality of magnets 18 (including the first magnet 20 and the second magnet 22) may be a permanent magnet. As Figure 3 shown, the first magnet 20 and the second magnet 22 may be configured to form a V shape. Each pole shoe among the plurality of pole shoes 24 (including the first pole shoe 26) may be made of metal and / or metalloid, including but not limited to iron, cobalt, nickel, silicon, manganese, aluminum, steel (including electrical steel), and combinations thereof. The lamination stack 16 may be made of metal and / or metalloid, including but not limited to iron, cobalt, nickel, silicon, manganese, aluminum, steel (including electrical steel), and combinations thereof. Although not necessary, it should be appreciated that the plurality of pole shoes 24 may be made of the same metal and / or metalloid as that constituting the lamination stack 16.

[0017] Although not required, sleeve 30 may include a polymeric material. In non-limiting examples, the polymeric material of sleeve 30 may be a thermoplastic, a thermoset, or an elastomer. The polymeric material of sleeve 30 may be an engineering plastic. More specifically, the polymeric material of sleeve 30 may be, but is not limited to, any polyalkene or polyolefin, including its copolymers and terpolymers, such as polyethylene (including high-density polyethylene (HDPE) and low-density polyethylene (LDPE)), polypropylene (PP), polybutene, and polybutylene terephthalate (PBTR), acrylic resins, such as acrylonitrile butadiene styrene (ABS) or polymethyl methacrylate (PMMA), polyoxymethylene (POM) or any acetal copolymer or acetal terpolymer, polyketone, polyether ketone, and / or polyarylether ketone, such as polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), polyetherimide (PEI), polyimide, polyvinyl chloride (PVC), polyphenylene sulfide (PPS), polyphenylene ether (PPO), polysulfone (PSU), polytetrafluoroethylene (PTFE), polyamide (including polyphthalamide), polycarbonate, polyurethane, epoxy resin, and thermoplastic elastomer (TPE). Although not required, the polymeric material of sleeve 30 may be molded, such as overmolding, insert molding, injection molding, compression molding, and thermoforming.

[0018] Sleeve 30 may also include a first composition that includes any of the polymeric materials detailed herein and optional fillers and / or additives, such as plasticizers, carbon (including carbon fiber), ceramic materials, and / or minerals (including calcium carbonate, silica, clay, and kaolin), fibers (including glass fiber, carbon fiber, aramid fiber, basalt fiber, and paper fiber), stabilizers (including oxidation stabilizers, ultraviolet (UV) stabilizers, heat stabilizers, light absorbers, reinforcing agents, acid scavengers, metal deactivators), and flame retardants (including aluminum hydroxide, phosphorus compounds, and bromine compounds).

[0019] The plurality of pole shoes 24 may be separated from the stack 16. In other words, each of the plurality of pole shoes 24 may be a component separated from the stack 16 and thus form a non-bridged arrangement of the pole shoes 24 and the stack 16. In other words, in the non-bridged arrangement, the pole shoes 24 and the stack 16 are not connected to each other by a bridging material. In this embodiment, the centrifugal force applied to the plurality of pole shoes 24 by the rotation of the rotor assembly 10 is also not applied to any bridging member connecting the plurality of pole shoes 24 and the stack 16. Thus, the non-bridged arrangement of the pole shoes 24 and the stack 16 eliminates the concern of the centrifugal force applying a force on any bridging member, resulting in stress concentration on any bridging member and potentially leading to a structural failure of any bridging member.

[0020] The outer sleeve body 32 of the sleeve 30 and the plurality of spacers 34 of the sleeve 30 can be integral. In other words, the outer sleeve body 32 of the sleeve 30 and the plurality of spacers 34 of the sleeve 30 can be formed as one piece with each other (i.e., a single piece). The outer sleeve body 32 of the sleeve 30 and the plurality of spacers 34 of the sleeve 30 can be integrally formed together as a whole, or can be formed separately first and then joined to become a whole.

[0021] Although not necessary, the first spacer 36 can extend at least to the first magnet 20, and the second spacer 38 can extend at least to the second magnet 22. The first spacer 36 and the second spacer 38 can also be disposed between the first magnet 20 and the second magnet 22. It should be appreciated that the first spacer 36 can extend radially inward from the outer sleeve body 32 toward the first magnet 20, and can also extend beyond the first magnet 20, optionally encapsulating at least a portion of the first magnet 20, and then disposed between the first magnet 20 and the second magnet 22. Similarly, it should be appreciated that the second spacer 38 can extend radially inward from the outer sleeve body 32 toward the second magnet 22, and can also extend beyond the second magnet 22, optionally encapsulating at least a portion of the second magnet 22, and then disposed between the first magnet 20 and the second magnet 22. It should also be understood that both the first spacer 36 and the second spacer 38 can be disposed between the first magnet 20 and the second magnet 22, and can be fixed to each other, formed integrally, or otherwise joined between the first magnet 20 and the second magnet 22.

[0022] The first spacer 36 can extend radially inward from the outer sleeve body 32 toward the shaft 14. In addition, as Figure 3 shown, the first pole shoe 26 can define a first recess 40, and the first spacer 36 can be at least partially disposed in the first recess 40 to further couple the sleeve 30 and the first pole shoe 26. The first spacer 36 can have a hook-like configuration and can extend radially away from the shaft 14 to be disposed in the first recess 40 of the first pole shoe 26. The first recess 40 can be disposed adjacent to the first magnet 20. The second spacer 38 can extend radially inward from the outer sleeve body 32 toward the shaft 14. In addition, as also Figure 3 shown, the first pole shoe 26 can define a second recess 42, and the second spacer 38 can be at least partially disposed in the second recess 42 to further couple the sleeve 30 and the first pole shoe 26. The second spacer 38 can have a hook-like configuration and can extend radially away from the shaft 14 to be disposed in the second recess 42 of the first pole shoe 26. The second recess 42 can be disposed adjacent to the second magnet 22.

[0023] The outer sleeve body 32 has an outer sleeve surface 44 that faces away from the axis A1. The outer sleeve surface 44 contacts air during rotation of the rotor assembly 10. The outer sleeve surface 44 can be smooth to reduce air friction. Alternatively, the outer sleeve body 32 includes aerodynamic features 46 that are recessed into the outer sleeve surface 44 to reduce air friction. The aerodynamic features 46 can include, but are not limited to, grooves, a series of grooves, pits, or a series of pits.

[0024] The lamination stack 16 extends along the axis A1 between a first lamination stack end 48 and a second lamination stack end 50. The rotor assembly 10 can further include a first end cap 52 adjacent to the first lamination stack end 48 and a second end cap 54 adjacent to the second lamination stack end 50. The first end cap 52 and the second end cap 54 help prevent the plurality of magnets 18 from falling off between the plurality of pole shoes 24 and the lamination stack 16.

[0025] The first end cap 52 has a first end cap outer surface 56 that faces away from the axis, and the second end cap 54 has a second end cap outer surface 58 that faces away from the axis A1. Although not required, the outer sleeve surface 44, the first end cap outer surface 56, and the second end cap outer surface 58 can be flush with each other. In other words, the outer sleeve surface 44, the first end cap outer surface 56, and the second end cap outer surface 58 can all extend the same radial distance away from the axis A1 and can form a single continuous surface. Additionally, the single continuous surface formed by the outer sleeve surface 44, the first end cap outer surface 56, and the second end cap outer surface 58 may not deviate from the same radial distance away from the axis A1 as the first end cap 52, the outer sleeve body 32, and the second end cap 54 extend along the axis A1. The single continuous surface contributes to the manufacturability and aerodynamic performance of the rotor assembly 10.

[0026] The lamination stack 16 can define a plurality of channels 60 between the first lamination stack end 48 and the second lamination stack end 50. The plurality of channels 60 can be configured to direct lubricant through the rotor assembly 10 for lubrication and / or cooling of the rotor assembly 10. As Figure 2 and Figure 3 shown, the rotor assembly 10 can further include a plurality of rods 62 disposed in the plurality of channels 60. It should be appreciated that the plurality of rods 62 can be disposed in the plurality of channels 60 and the lamination stack 16 can still define a channel 60 that is configured to direct lubricant through the rotor assembly 10 for lubrication and / or cooling of the rotor assembly 10. Alternatively, it should also be appreciated that one rod 62 of the plurality of rods 62 can correspond to and be disposed in one channel of the plurality of channels 60. Although not required, as Figure 4As shown, a plurality of rods 62 can be integral with the first end cap 52 and the second end cap 54. In other words, the plurality of rods 62, the first end cap 52, and the second end cap 54 can be integrally formed with each other (i.e., a single piece). The plurality of rods 62, the first end cap 52, and the second end cap 54 can be integrally formed with each other.

[0027] The first end cap 52, the second end cap 54, and / or the plurality of rods 62 can include a polymeric material. In embodiments where the sleeve 30 includes a polymeric material and the first end cap 52, the second end cap 54, and / or the plurality of rods include a polymeric material, it should be recognized that these polymeric materials can be the same as or different from each other. In non-limiting examples, the polymeric material of the first end cap 52, the second end cap 54, and / or the plurality of rods 62 can be a thermoplastic, a thermoset, or an elastomer. The polymeric material of the first end cap 52, the second end cap 54, and / or the plurality of rods 62 can be an engineering plastic.

[0028] More specifically, the polymeric material of the first end cap 52, the second end cap 54, and / or the plurality of rods 62 can be, but is not limited to, any polyolefin or polyolefin, including its copolymers and terpolymers, such as polyethylene (including high-density polyethylene (HDPE) and low-density polyethylene (LDPE)), polypropylene (PP), polybutene, and polybutylene terephthalate (PBTR), acrylic resins, such as acrylonitrile butadiene styrene (ABS) or polymethyl methacrylate (PMMA), polyoxymethylene (POM) or any acetal copolymer or acetal terpolymer, polyketone, polyether ketone, and / or polyarylether ketone, such as polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), polyetherimide (PEI), polyimide, polyvinyl chloride (PVC), polyphenylene sulfide (PPS), polyphenylene ether (PPO), polysulfone (PSU), polytetrafluoroethylene (PTFE), polyamide (including polyphthalamide), polycarbonate, polyurethane, epoxy resin, and thermoplastic elastomer (TPE). Although not required, the polymeric material of the first end cap 52, the second end cap 54, and / or the plurality of rods 62 can be molded, such as overmolding, insert molding, injection molding, compression molding, and thermoforming.

[0029] More specifically, the plurality of rods 62 can be non-magnetic, can be conductive, and can further include carbon fibers. It should be recognized that the first end cap 52 and / or the second end cap 54 can also be non-magnetic, can be conductive, and can further include carbon fibers. At least one of the first end cap 52 and the second end cap 54 can include a second composition. The second composition of the first end cap 52 and / or the second end cap 54 can be the same as or different from the first composition of the sleeve 30. It should also be recognized that the plurality of rods can include a second composition such that at least one of the first end cap 52, the second end cap 54, and the plurality of rods 62 can include a second composition.

[0030] The second composition of the first end cap 52, the second end cap 54, and / or the plurality of rods 62 may include any polymer material detailed herein and optional fillers and / or additives such as plasticizers, carbon (including carbon fibers), ceramic materials, and / or minerals (including calcium carbonate, silica, clay, and kaolin), fibers (including glass fibers, carbon fibers, aramid fibers, basalt fibers, and paper fibers), stabilizers (including oxidation stabilizers, ultraviolet (UV) stabilizers, heat stabilizers, light absorbers, reinforcing agents, acid scavengers, metal deactivators), and flame retardants (including aluminum hydroxide, phosphorus compounds, and bromine compounds). In one embodiment, the second composition includes a thermoplastic and a filler encapsulated by the thermoplastic. In this embodiment, the filler may include carbon fibers.

[0031] As detailed herein, the sleeve 30 may be molded, such as overmolding, insert molding, injection molding, compression molding, and thermoforming. It should be appreciated that the first end cap 52, the second end cap 54, and / or the plurality of rods 62 may also be molded, such as overmolding, insert molding, injection molding, compression molding, and thermoforming. More specifically, the polymer material of the first end cap 52, the second end cap 54, and / or the plurality of rods 62 may be molded. It should be appreciated that the first end cap 52, the second end cap 54, and the plurality of rods 62 may be molded together. The sleeve 30, the first end cap 52, the second end cap 54, and / or the plurality of rods 62 may be molded together. However, the first end cap 52 and the second end cap 54 may be molded separately from the sleeve 30. Thus, the first end cap 52, the second end cap 54, and the plurality of rods 62 may be molded separately from the sleeve 30. The first end cap 52, the second end cap 54, and optionally the plurality of rods 62 may be molded after the sleeve 30 is molded.

[0032] The plurality of pole shoes 24 may include two pole shoes, three pole shoes, four pole shoes, five pole shoes, six pole shoes, seven pole shoes, eight pole shoes, nine pole shoes, ten pole shoes, or more than ten pole shoes. The plurality of magnets 18 may include two magnets, three magnets, four magnets, five magnets, six magnets, seven magnets, eight magnets, nine magnets, ten magnets, eleven magnets, twelve magnets, thirteen magnets, fourteen magnets, fifteen magnets, sixteen magnets, seventeen magnets, eighteen magnets, nineteen magnets, twenty magnets, or more than twenty magnets. The plurality of magnets 18 associated with each pole shoe 24 may be a single magnet, or may be an array of multiple barrier pole magnets, including but not limited to V-shaped arrays, double V-shaped arrays, U-shaped arrays, and triangular arrays. The plurality of channels 60 and the plurality of rods 62 may include two channels and two rods, three channels and three rods, four channels and four rods, five channels and five rods, six channels and six rods, seven channels and seven rods, eight channels and eight rods, nine channels and nine rods, ten channels and ten rods, eleven channels and eleven rods, twelve channels and twelve rods, thirteen channels and thirteen rods, fourteen channels and fourteen rods, fifteen channels and fifteen rods, sixteen channels and sixteen rods, seventeen channels and seventeen rods, eighteen channels and eighteen rods, nineteen channels and nineteen rods, twenty channels and twenty rods, or more than twenty channels and more than twenty rods. Each pole shoe 24 may define one channel, may define two channels, may define three channels, or may define more than three channels. The stack 16 may also define a plurality of channels 60, and may define one channel, two channels, three channels, four channels, five channels, six channels, seven channels, eight channels, nine channels, ten channels, or more than ten channels.

[0033] The rotor assembly 10 may be configured to rotate at a speed of more than 20,000 revolutions per minute (RPM). In non-limiting examples, the rotor assembly 10 may be configured to rotate between about 20,000 RPM and about 50,000 RPM, between about 20,000 RPM and about 40,000 RPM, between about 20,000 RPM and about 30,000 RPM, and between about 20,000 RPM and about 25,000 RPM. The sleeve 30 may be capable of holding the plurality of pole shoes 24 to the stack 16 at a speed of 20,000 RPM or more than 20,000 RPM. Thus, the rotor assembly 10 may be considered a high-speed rotor assembly.

[0034] As Figure 5As shown, the rotor assembly 10 can be incorporated into the electric motor 12. The electric motor 12 includes a stator 64 that extends along an axis A1 and defines a stator interior 66, and the rotor assembly 10 can be disposed within the stator interior 66 of the stator 64. The stator 64 can include a coil winding 68 that can be energized to generate a magnetic flux detailed herein. The sleeve 30, which includes an outer sleeve body 32 and a plurality of spacers 34, improves the efficiency of the electric motor 12 by reducing the amount of magnetic flux that leaks, and thus improves the efficiency of the force applied to rotate the rotor assembly 10 relative to the amount of magnetic flux that needs to be generated.

[0035] In addition, a gap can be defined between the outer pole surface 28 and the stator interior 66 of the stator 64. Advantageously, this gap is minimized to minimize losses due to air friction and also to increase the torque on the rotor assembly 10. It should be appreciated that the sleeve 30 can be manufactured with relatively tight tolerances, thus allowing a relatively small gap to be defined between the outer sleeve surface 44 of the sleeve 30 and the stator interior 66 of the stator 64, and thus also allowing a relatively small gap to be defined between the outer pole surface 28 and the stator interior 66 of the stator 66, and thus also minimizing losses due to air friction. This gap can be as small as 0.2 millimeters and can be proportional to the tolerance of the diameter of the rotor assembly 10 and / or proportional to the rotational speed of the rotor assembly 10. This gap can also be scaled on the order of about 0.1% of the diameter of the stator 64.

[0036] As Figure 6 As shown, the electric motor 12 is incorporated into an electric drive unit 70. The electric drive unit 70 can include the electric motor 12 and a gear reduction mechanism 72 coupled to the shaft 14 of the rotor assembly 10. The gear reduction mechanism 72 can be configured to adjust the torque received from the shaft 14. The gear reduction mechanism 72 can be, but is not limited to, a planetary gear set, a reduction gearbox (such as a two-stage reduction gearbox), a worm gear reducer, a helical gear reducer, a spur gear reducer, a hypoid gear reducer, a bevel gear reducer, a gear train reducer, a cycloidal gear reducer, a magnetic gear reducer, an angular gear reducer, a skew axis gear reducer, a parallel axis gear reducer, a coaxial gear reducer, and combinations thereof.

[0037] The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is descriptive rather than restrictive in nature. Many modifications and variations of the present invention are possible in light of the above teachings, and the invention may be practiced otherwise than as specifically described.

Claims

1. A rotor assembly for an electric motor, the rotor assembly comprising: a shaft extending along an axis and rotatable about the axis; a stack of laminations coupled to the shaft; a plurality of magnets including a first magnet coupled to the stack of laminations; a plurality of pole shoes radially outwardly spaced from the shaft and including a first pole shoe, wherein the first magnet is disposed between the stack of laminations and the first pole shoe, and wherein each of the plurality of pole shoes has an outer pole surface facing away from the axis; and a sleeve, which includes, an outer sleeve body disposed around the outer pole surface of each of the plurality of pole shoes to hold each of the pole shoes relative to the stack of laminations, and a plurality of spacers extending radially inwardly from the outer sleeve body towards the shaft and including a first spacer and a second spacer, wherein the first spacer and the second spacer are disposed between the first pole shoe and the stack of laminations to reduce flux leakage of the first magnet.

2. The rotor assembly according to claim 1, wherein the sleeve comprises a polymeric material.

3. The rotor assembly according to claim 2, wherein the polymeric material is molded.

4. The rotor assembly according to claim 1, wherein the plurality of pole shoes are separated from the stack of laminations.

5. The rotor assembly according to claim 1, wherein the outer sleeve body and the plurality of spacers are integral.

6. The rotor assembly according to claim 1, wherein the plurality of magnets further includes a second magnet disposed between the first pole shoe and the stack of laminations, and wherein the first spacer extends at least to the first magnet, and the second spacer extends at least to the second magnet.

7. The rotor assembly according to claim 6, wherein the first spacer and the second spacer are disposed between the first magnet and the second magnet.

8. The rotor assembly according to claim 1, wherein the outer sleeve body has an outer sleeve surface facing away from the axis, and wherein the outer sleeve body includes aerodynamic features recessed into the outer sleeve surface.

9. The rotor assembly according to claim 1, wherein the stack of laminations extends along the axis between a first stack end and a second stack end, and wherein the rotor assembly further includes a first end cap adjacent to the first stack end and a second end cap adjacent to the second stack end.

10. The rotor assembly according to claim 9, wherein the outer sleeve body has an outer sleeve surface facing away from the axis, wherein the first end cap has a first end cap outer surface facing away from the axis, wherein the second end cap has a second end cap outer surface facing away from the axis, and wherein the outer sleeve surface, the first end cap outer surface and the second end cap outer surface are flush with each other.

11. The rotor assembly according to claim 9, wherein the stack of laminations defines a plurality of channels between the first stack end and the second stack end, and wherein the rotor assembly further includes a plurality of rods disposed in the plurality of channels.

12. The rotor assembly according to claim 11, wherein the plurality of rods are integral with the first end cap and the second end cap.

13. The rotor assembly according to claim 11, wherein the plurality of rods are non-magnetic and conductive.

14. The rotor assembly according to claim 11, wherein the plurality of rods comprise carbon fiber.

15. The rotor assembly according to claim 9, wherein the sleeve further comprises a first composition, and wherein at least one of the first end cap and the second end cap comprises a second composition different from the first composition.

16. The rotor assembly according to claim 15, wherein the second composition comprises a thermoplastic and a filler encapsulated by the thermoplastic.

17. The rotor assembly according to claim 16, wherein the filler comprises carbon fiber.

18. The rotor assembly according to claim 15, wherein the sleeve is molded, and wherein the first end cap and the second end cap are molded separately from the sleeve.

19. A motor, comprising: a stator extending along an axis and defining an interior of the stator; and the rotor assembly according to claim 1 disposed within the stator.

20. An electric drive unit, comprising: the motor according to claim 19, and a gear reduction mechanism coupled to the shaft and configured to condition torque received from the shaft.

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