Abstract:
A rotor assembly for use in an electric motor or generator where the mass of the rotor assembly is reduced with respect to conventional rotor assemblies. In addition, the rotor assembly is configured to be scalable to different sized electric motors. Within the rotor assembly, the rotor hub, the shaft, and the permanent magnets can independently or collectively be modified to have a reduced mass. In one aspect, a portion of the rotor hub adjacent to the shaft is configured with passages and spokes. In another aspect, an intermediate hub with lightening holes is provided between the shaft and the rotor hub. In yet another aspect, a large diameter hollow shaft replaces a portion of the rotor hub. In yet another aspect, the permanent magnets are configured to have an arc-shape, which permits the thickness of the magnets to be reduced without reducing the efficiency of the magnets.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation application U.S. patent application Ser. No. 11/192,321, filed Jul. 28, 2005, which claims benefit to U.S. Provisional Patent Application No. 60/608,930, filed Jul. 30, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     The present disclosure relates generally to electric machines, for example, permanent magnet motors and generators.  
         [0004]     2. Description of the Related Art  
         [0005]     Electric machines, for example, electric motors and generators, are used in many applications, including those ranging from electric vehicles to domestic appliances. Improvements in machine performance, reliability, efficiency, and power density for all types of electric motors are desirable.  
         [0006]     An electric machine converts electrical or electromagnetic energy into mechanical energy or conversely converts mechanical energy into electrical or electromagnetic energy.  
         [0007]     The permanent magnets used in rotor assemblies are disposed within pockets. The pockets are typically formed near the outer perimeter of the rotor hub, which is built up from laminations made from electric grade steel. Electric grade steel is used on rotor assemblies because it has a greater permeability for conducting the magnetic lines of force. The process of building up a rotor with laminations is done to reduce eddy current losses in the rotor hub, especially during higher rotation speeds. The rotor extends from its outer perimeter to an inner diameter that interfaces with a shaft. The total mass of the rotor assembly is one of the parameters that affects the acceleration characteristics of the electric motor, the cost of the rotor assembly, and the amount of stress experienced by the various components of the rotor assembly, among other things.  
         [0008]     Shafts used in electric machine are typically made from structural steel, which is slightly more dense and certainly stronger than electric grade steel. In one application, an electric motor of the Toyota Prius, which is a hybrid vehicle, utilizes a hollow shaft with an integrated carriage. The carriage includes a central web having one end connected to the main shaft and the other end connected to a carriage support that extends axially in either direction away from the central web. A laminated rotor hub with permanent magnets is retained within the carriage support. The inclusion of the central web extending radially from the shaft creates unique balancing issues with respect to vibration modes. The bearing positions on the Toyota Prius shaft must be positioned to minimize the bending stress arising from the central web. Thus, although the Toyota Prius shaft provides some marginal weight reduction benefits, the configuration of the rotor assembly is not readily convertible to other types or sizes of motors.  
         [0009]     Conventional rotor assemblies include rectangular-shaped rotor pockets in which the rectangular-shaped permanent magnets are disposed. In these conventional rotor assemblies, the stress concentrations in the magnet pockets and in the rotor laminations exacerbate the localized stresses as the operating speeds increase. When the rotor rotates at high speeds, the permanent magnets exert an outward radial force on the magnet pockets, which results in the centrifugal forces being reacted at the outer corners of the pockets. These localized stresses in conventional rotor assemblies are one reason for providing more material in the rotor.  
         [0010]     It would be desirable to reduce the mass of the rotor hub, the shaft, and the permanent magnets either individually or collectively while maintaining a rotor assembly configuration that could be easily manufactured and scaled to different size electric machines.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The assemblies and components described herein provide a variety of ways to reduce the weight of a rotor assembly for an electric machine. Reducing the weight of the rotor assembly permits the rotor to rotate at higher speeds while meeting specific mass targets for electric machines in the automotive industry, as well as other industries.  
         [0012]     In one embodiment, a rotor assembly includes a rotor hub comprising a first portion and a second portion, the first portion comprising an outer diameter and an inner diameter, the first portion comprising a plurality of uniformly, circumferentially spaced magnet pockets, the second portion comprising an inner diameter and an outer diameter, the outer diameter of the second portion abutting with the inner diameter of the first portion, the second portion comprising a plurality of passages, each adjacent passage separated by spokes, each spoke comprising a uniform thickness with respect to an adjacent spoke, the spokes connecting the outer diameter of the second portion with a shaft attachment region, the region integrally and proximately formed with the inner diameter of the second portion; a first set of permanent magnets, a respective one of the permanent magnets of the first set of permanent magnets received in a respective one of the magnet pockets; and a shaft comprising an outer diameter sized to closely receive the inner diameter of the second portion of the rotor hub.  
         [0013]     In another embodiment, an electric machine includes a rotor assembly comprising a rotor hub and a shaft, the rotor hub comprising a first portion and a second portion, the first portion comprising an outer diameter and an inner diameter, the first portion comprising a plurality of uniformly, circumferentially spaced magnet pockets, the second portion comprising an inner diameter and an outer diameter, the outer diameter of the second portion abutting with the inner diameter of the first portion, the second portion comprising a plurality of passages, each adjacent passage separated by spokes, each spoke comprising a uniform thickness with respect to an adjacent spoke, the spokes connecting the outer diameter of the second portion with a shaft attachment region, the region integrally and proximately formed with the inner diameter of the second portion; a first set of permanent magnets, a respective one of the permanent magnets of the first set of permanent magnets received in a respective one of the magnet pockets; and a stator comprising a plurality of windings, the windings positioned to electromagnetically cause rotation of the rotor assembly.  
         [0014]     In another embodiment, a rotor assembly includes a rotor hub comprising an outer diameter and an inner diameter, a plurality of uniformly, circumferentially spaced magnet pockets located between the outer diameter and the inner diameter; a first set of permanent magnets, a respective one of the permanent magnets of the first set of permanent magnets received in a respective one of the magnet pockets; an intermediate hub comprising an outer diameter and an inner diameter, the intermediate hub further comprising a plurality of lightening holes axisymmetrically arranged between a region bordered by the outer diameter and the inner diameter of the intermediate hub, the outer diameter of the intermediate hub being sized to closely receive the inner diameter of the rotor hub; and a shaft comprising an outer diameter sized to closely receive the inner diameter of the intermediate hub.  
         [0015]     In another embodiment, an electric machine includes a rotor assembly comprising a rotor hub, a shaft, and an intermediate hub, the rotor hub comprising an outer diameter and an inner diameter, a plurality of uniformly, circumferentially spaced magnet pockets located between the outer diameter and the inner diameter; a first set of permanent magnets, a respective one of the permanent magnets of the first set of permanent magnets received in a respective one of the magnet pockets; an intermediate hub comprising an outer diameter and an inner diameter, the intermediate hub further comprising a plurality of lightening holes axisymmetrically arranged between a region bordered by the outer diameter and the inner diameter of the intermediate hub, the outer diameter of the intermediate hub being sized to closely receive the inner diameter of the rotor hub; and a stator comprising a plurality of windings, the windings positioned to electromagnetically cause rotation of the rotor assembly.  
         [0016]     In yet another embodiment, a rotor hub includes an outer diameter and an inner diameter; a plurality of magnet pockets, the pockets formed in a region proximate to and slightly radially inward from the outer diameter of the rotor hub; and at least a first permanent magnet comprising a pole arc to pole pitch ratio of about 0.9 arranged within each magnet pocket.  
         [0017]     In still yet another embodiment, a rotor hub having an outer periphery for an electric machine includes a plurality of elongated slots proximate the outer periphery of the rotor hub, the elongated slots each having a respective major axis, the major axis being non-perpendicular to a respective radial axis extending from an axisymmetric centerline of the rotor hub. Additionally or alternatively, the rotor hub includes a plurality of passages formed in the rotor hub, at least one of the passages cooperating with an orientation of at least one of the elongated slots to minimize rotor hub weight while maintaining operational integrity of the rotor hub.  
         [0018]     The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth herein. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings.  
         [0020]      FIG. 1  is a cross-sectional view of an electric machine according to one illustrated embodiment.  
         [0021]      FIG. 2  is a front, left isometric view of a rotor assembly for an electric motor according to one illustrated embodiment.  
         [0022]      FIG. 3  is a cross-sectional view of the rotor assembly of  FIG. 2 .  
         [0023]      FIG. 4  is a cross-sectional view of the rotor assembly of  FIG. 2  along line  4 - 4  of  FIG. 3  showing the rotor hub configured with circumferentially spaced passages and spokes.  
         [0024]      FIG. 5A  is a cross-sectional view of another rotor assembly having reduced thickness spokes according to another illustrated embodiment.  
         [0025]      FIG. 5B  is a cross-sectional view of another rotor assembly having a reduced number of passages and spokes according to another illustrated embodiment  FIG. 6  is a front, left isometric view of a rotor assembly having an intermediate hub according to another illustrated embodiment.  
         [0026]      FIG. 7  is a cross-sectional view of the rotor assembly of  FIG. 6 .  
         [0027]      FIG. 8A  is a cross-sectional view of the rotor assembly of  FIG. 6  along line  8 - 8  of  FIG. 7  showing the rotor hub configured with an intermediate hub that includes lightening holes therein.  
         [0028]      FIG. 8B  is a cross-sectional view of another rotor assembly having a different configuration of lightening holes in the intermediate hub.  
         [0029]      FIG. 9  is a cross-sectional view of a rotor assembly having a shaft torsionally coupled with a full-thickness rotor hub according to one illustrated embodiment.  
         [0030]      FIG. 10  is a cross-sectional view of a rotor assembly having an enlarged diameter hollow shaft according to one illustrated embodiment.  
         [0031]      FIG. 11  is a cross-sectional view of another rotor assembly having an enlarged diameter hollow shaft with a generally tapered region between an end plate and bearing according to one illustrated embodiment.  
         [0032]      FIG. 12  is a cross-sectional view of a rotor hub having a number of angled, elongated slots arranged with a number of passages according to the illustrated embodiment.  
         [0033]      FIG. 13  is an enlarged view of a pair of the elongated slots of the rotor hub of  FIG. 12 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the present assemblies, devices and systems. However, one skilled in the relevant art will recognize that the present assemblies, devices and systems may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with electric machines have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the present assemblies, devices and systems.  
         [0035]     Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
         [0036]     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present assemblies, devices and systems. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Further more, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.  
         [0037]     The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed invention.  
         [0000]     Rotor Assembly  
         [0038]      FIG. 1  illustrates an electric machine  2  according to one embodiment of the present assemblies, devices and systems. The electric machine  2  of the illustrated embodiment comprises a housing  4 , a stator  6 , and a rotor assembly  10 . The stator  6  includes electrical windings, which are not shown, but are well known in the art.  
         [0039]      FIGS. 2 and 3  show the rotor assembly  10  comprising a rotor hub  12 , a shaft  14 , a number of permanent magnets  16 , and a banding layer  18 . The rotor assembly  10  further comprises a pair of end plates  20 . The shaft  14  is mounted on roller bearings  22 . The rotor assembly  10  is mass balanced to rotate about a centerline  24 . The mass balancing can be accomplished by removing or adding material to the end plates  20 .  
         [0040]     The rotor hub  12  includes a first portion  30  and a second portion  32 . The rotor hub  12  is built up from laminations, which is a process well known in the art to reduce the eddy current effect in the rotor hub  12 . The laminations are thin steel layers or sheets, which are stacked and fastened together by cleats, rivets or welds. The first portion  30  of the rotor hub  12 , often referred to as the “active” portion of the rotor hub  12 , conducts the lines of magnetic flux. Thus, the dimensions of a cross-sectional area of the first portion  30  affect the efficiency of the device. As the cross-sectional area of the first portion  30  decreases, the reluctance (e.g., resistance) increases. Accordingly, one way to reduce the weight of the rotor assembly  10  is to reduce the cross sectional area of the second portion  32  of the rotor hub  12 .  
         [0041]     The first portion  30  and the second portion  32  can be integrally formed to achieve a monolithic or one-piece rotor hub  12 . However, one skilled in the art will understand and appreciate that the first portion  30  and the second portion  32  can also be separate components that are mechanically joined, for example by an interference fit-up process.  
         [0042]      FIG. 4  shows the rotor assembly  10  of  FIG. 2 . A dashed line  34  represents the demarcation between the first portion  30  and the second portion  32  of the rotor hub  12 . The shaft  14  is torsionally coupled with the second portion  32  of rotor hub  12  by complementary formed keyways  26 . The torsional coupling strength between the shaft  14  and the rotor hub  12  can be increased by providing an interference fit between the shaft  14  and the rotor hub  12 . The interference fit can be in addition to the keyways  26  or it can be the sole means of torsionally coupling the shaft  14  to the rotor hub  12 . In the illustrated embodiment, only two keyways  26  are shown, however one skilled in the art will understand and appreciate that the rotor assembly  10  may employ a greater or a lesser number of keyways  26 .  
         [0043]     In addition to the second portion  32  providing a mechanical interface between the first portion  30  of the rotor hub  12  and the shaft  14 , the second portion  32  can further be configured with a reduced-weight cross-sectional profile that is capable of withstanding the operating stresses of the electric machine, for example stresses due to thermal cycling, centrifugal forces, and other forces. In one embodiment, the rotor hub  12  may be operable between speeds of about 13,500-18,000 rpm. In addition, the rotor hub  12  can operate at temperatures up to about 120 degrees Celsius. In an alternate embodiment, the rotor hub  12  can operate at temperatures up to about 180 degrees Celsius.  
         [0044]     The lamination sheets that are used to build up the rotor hub  12  are typically made from an electrical steel, which has a lower strength than a structural steel. By way of example, electrical steel, which is sometimes referred to as “lamination steel,” can have a tensile strength/density ratio that is about 50% less than the tensile strength/density ratio of structural steel. In the present embodiment, the lamination steel may have a density of 7.6 g/cm 3  and a tensile strength of 550 MPa. Structural steel, like that used for the shaft  14 , can have a density of 7.9 g/cm 3  and a tensile strength of 850 MPa.  
         [0045]     Because weaker lamination steel is typically used for building up rotor hubs, it has been common in the industry to have both the first portion  30  and the second portion  32  be solid. As explained, earlier, the first portion  30  needs to be substantially solid to efficiently conduct sufficient lines of magnetic flux. However, a solid second portion  32  adds a significant amount of material and attributes excess weight to the rotor hub  12 .  
         [0046]     Still referring to  FIG. 4 , the illustrated embodiment depicts the second portion  32  of the rotor hub  12  configured with a number of circumferentially spaced passages  36  separated by spokes  38 . The passages  36  and spokes  38  are adjacently located and connected to a shaft attachment region  40 . The shaft attachment region  40  provides sufficient material to form the keyways  26  and withstand the torsional stresses resulting from the interaction between the shaft  14  and the rotor hub  12 . The passages  36  extend axially through the second portion  32  of the rotor hub  12  as shown in  FIG. 3 . Although eight passages  36  are shown in the illustrated embodiment, one skilled in the art will understand and appreciate that second portion  32  can be configured with a greater or lesser number of passages  36 .  
         [0047]     Now referring back to the first portion  30  of the rotor hub  12 , the illustrated embodiment includes eight magnet pockets  42 , each pocket configured to receive sixteen permanent magnets  16 . The permanent magnets  16  can be made from sintered neodymium iron boron, which is suitable for operation up to a temperature of at least 180 degrees Celsius. One skilled in the art will understand and appreciate that the first portion  30  of the rotor hub  12  can include a greater or a lesser number of permanent magnets  16 .  
         [0048]     Further shown in the illustrated embodiment is the banding layer  18 , which is formed around an outer diameter  28  of the first portion  30  of the rotor hub  12 . A plurality of ribs  44  separate the circumferentially spaced magnet pockets  42 . An epoxy is used to fill the space  46  remaining in the magnet pockets  46  that is not otherwise filled by the permanent magnets  16 . One epoxy that can be used to fill the remaining space  46  is a glass filled epoxy. The permanent magnets  16  can additionally or alternatively be bonded within the magnet pockets  42  with a magnetic adhesive such as a cyanoacrylate adhesive. In the illustrated embodiment, the permanent magnets  16  are provided with straight sides and a thickness of about 9.0 mm.  
         [0049]     One advantage of forming the banding layer  18  around the rotor hub  12  is that the banding layer  18  provides radial reinforcement for the rotor hub  12  and the permanent magnets  16 . In addition, the banding layer  18  can protect the permanent magnets  16  against corrosion. The banding layer  18  is composed of a carbon/epoxy matrix. In one embodiment, the banding layer  18  is composed of a 65% carbon/epoxy matrix. The carbon/epoxy composite material is wet laid onto the rotor hub  12  where a bond is formed between an inner diameter of the banding layer  18  and the outer diameter  28  of the rotor hub  12 . A banding layer thickness in the range of about 1.00 mm to 2.00 mm is adequate for most electric machine applications.  
         [0050]      FIGS. 5A and 5B  illustrate two alternative embodiments where each of the alternative embodiments differs from the previous embodiment only by the configuration of the passages  36  and spokes  38 .  FIG. 5A  illustrates one alternate embodiment of a rotor assembly  100 . The rotor assembly  100  has a rotor hub  112 , a shaft  114 , permanent magnets  116 , and a banding layer  118 . The passages  120  are widened, or stating this alternatively, the thickness of each spoke  122  is reduced. Such a reduction can be verified through the use of finite element analysis or prototype testing to insure that the spokes  122  retain enough cross-sectional area to support the first portion  124  of the rotor hub  112 . Now referring to  FIG. 5B , the rotor assembly  200  is similar to the previous embodiment in that it has a rotor hub  212 , a shaft  214 , magnets  216 , and a banding layer  218 . The rotor hub  212  is configured with a fewer number of passages  220  and likewise a fewer number of spokes  222 . In short, the relative weight reduction in a range of about 25%-35% may be achieved with any of the above embodiments. The stated weight reduction is in comparison to a solid rotor hub, specifically a solid second portion of a rotor hub.  
         [0051]      FIGS. 6, 7  and  8 A illustrate a rotor assembly  300  according to another embodiment of the present assemblies, devices and systems. The rotor assembly  300  is similar to the previous embodiment in that it has a rotor hub  312 , a shaft  314 , magnets  316 , and a banding layer  318 . However, the rotor hub  312  differs from that of  FIGS. 2 through 5 B in that an intermediate hub  320  is substituted for the second portion  32  of the embodiment depicted in e.g.  FIG. 3 .  
         [0052]      FIG. 8A  shows the intermediate hub  320  located between the rotor hub  312  and the shaft  314 . In addition, the intermediate hub  320  is made from aluminum in the present embodiment. The tensile strength of aluminum in comparison to its low density makes aluminum a good component for the intermediate hub  320 . The intermediate hub  320  can be interference fit with the shaft  314 . Due to the range of operating temperatures of the rotor assembly  300 , the interface pressure developed during the interference fit generation between the intermediate hub  320  and the shaft  314  can be increased. One method of developing a high interference fit between the intermediate hub  320  and the shaft  314  is to heat up the intermediate hub  320 , assemble it with the shaft  314 , and then allow the assembly to cool.  
         [0053]     The intermediate hub  320  also physically interfaces with the rotor hub  312 . In the illustrated embodiment, the torsional coupling of the intermediate hub  320  with the rotor hub  312  can be accomplished with keyways  322 . Alternatively, the torsional coupling of the intermediate hub  320  with the rotor hub  312  can be mechanically accomplished with an interference fit, bonding, welding, or some other process.  
         [0054]     The weight of the intermediate hub  320  can be further reduced by the addition of lightening holes  324 , which can extend all the way through the axial length of the intermediate hub  320 .  
         [0055]      FIG. 8B  illustrates a rotor assembly  400 , which is similar to the rotor assembly  300  of  FIG. 8A  except that an intermediate hub  420  includes a number of larger lightening holes  424 . One skilled in the art will understand and appreciate that the size, shape, and orientation of the lightening holes  424  can vary depending on any number of factors. In one embodiment, the lightening holes  424  can be configured to augment the mass balancing of the rotor assembly  400 . Consequently, the relative weight reduction of the embodiments shown in  FIGS. 6, 7 ,  8 A, and  8 B, when compared to a solid rotor hub, specifically a solid second portion of a rotor hub, is in the range of about 15%-25%.  
         [0000]     Arc-Shaped Magnets in the Rotor Hub  
         [0056]      FIG. 9  illustrates a cross-sectional view of a rotor assembly  500  according to one embodiment of the present assemblies, devices and systems. Only significant differences between the present embodiment and the above embodiments will be identified. In the illustrated embodiment, a number of permanent magnets  502  are arranged around an outer portion  504  of a rotor hub  506 . Each of the permanent magnets  502  has an annular shape with an inner arc  508  and an outer arc  510 . The permanent magnets  502  can be recessed into the rotor hub  506  and retained with the rotor hub  506  by a banding layer  512 . A magnet adhesive (not shown), such as a cyanoacrylate adhesive, can be used to bond the permanent magnets  502  with the rotor hub  506  and/or the banding layer  512 .  
         [0057]     In the illustrated embodiment, the permanent magnets  502  are configured to have an arc measurement  514 . When the arc measurement  514  is in the range of about 35.5-45.5 degrees, the thickness and thus the weight of the permanent magnets  502  can be reduced. In one embodiment, the arc measurement  514  is about 40.5 degrees, which correlates to a pole arc to pole pitch ratio of 0.9. The magnet thickness can be reduced to about 7.5 mm when the arc measurement  514  is about 40.5. Testing has indicated that magnetic loading and electromotive force (EMF) begin to fall off at pole arc to pole pitch ratios below 0.9. In order to counter this phenomenon, additional electrical loading would be required, but in turn, this results in greater copper losses (i.e., I 2 R losses).  
         [0000]     A Large Diameter, Hollow Shaft in the Rotor Assembly  
         [0058]      FIG. 10  illustrates a rotor assembly  600  with a large diameter, hollow shaft  602  rotationally coupled to a rotor hub  604 . One purpose of the hollow shaft  602  is to replace the second portion  32  of the rotor hub  12  shown in  FIGS. 3 and 4 . By providing the hollow shaft  602 , the rotor hub  604  could be mounted directly to the hollow shaft  602  whether with complementary keyways, an interference fit, or some other mechanical coupling method.  
         [0059]      FIG. 11  illustrates another rotor assembly  700  with a large diameter hollow shaft  702 . A rotor hub  704  can receive the hollow shaft  702 . Unlike the previous embodiment, the hollow shaft  702  of the illustrated embodiment has a blended section  706  that blends into each journal end  708 . The blended section  706  can reduce localized stress concentrations and smooth out the load path. The embodiments with the hollow shafts  602 ,  702  illustrated in  FIGS. 10 and 11  would not only reduce the overall weight of the rotor assembly, but also reduce the part count of the rotor assemblies  600 ,  700 .  
         [0060]     One advantage of the embodiments of the rotor assemblies discussed herein is that at least a majority of any intricately shaped portions of the rotor assembly are within the laminated region of the rotor assembly. In doing such, the other rotor assembly components can have designs that are easier to manufacture, thus reducing production complexity and cost.  
         [0061]      FIG. 12  shows a rotor hub  800  for an electric machine having an outer periphery  802 . The rotor hub  800  includes a plurality of elongated slots  804 , which may be approximately rectangular and/or elliptical in shape, located proximate to the outer periphery  802  of the rotor hub  800 . The elongated slots  804  each having a respective major axis  806 . The elongated slots  804  are oriented such that the respective major axes  806  are not perpendicular to a respective radial axis  808  extending from an axis of rotation or an axisymmetric centerline  810  of the rotor hub  800 .  
         [0062]     In addition, the rotor hub  800  includes a plurality of passages  812  formed in the rotor hub  800  according to the illustrated embodiment. The arrangement of the passages  812  with respect to the elongated slots  804  allows the weight of the rotor hub to be minimized while the structural and/or operational integrity of the rotor hub  800  is maintained.  
         [0063]      FIG. 13  shows an enlarged view of the elongated slot  804  located near the periphery  802  of the rotor hub  800  according to one illustrated embodiment. The major axis of a first one of the slots  804   a  forms an acute angle  814  (i.e., greater than 0, but less than 180 degrees) with the major axis of an adjacent or next successive one of the slots  804   b.  While the term adjacent is used, such does not require the slots  804   a,    804   b  to be immediately adjacent. For example, the respective slots  804   a,    804   b  may be separated by a portion  816  of the rotor hub  800 . In addition, the arrangement and orientation of the slots  804 , specifically the slots  804   a,    804   b  forming an acute angle  814  open toward the periphery of the rotor hub  800 , can reduce the operating stress on a bridge region  818 , which is the region of the rotor hub  800  located between the slots  804  and the periphery  802  of the rotor hub  800 .  
         [0064]     One possible advantage of the embodiments described and illustrated in  FIGS. 12 and 13  is that the arrangement of the elongated slots  804  in the rotor hub  800 , which may reduce the operating stress in the bridge region  818 , may permit the rotor hub to be assembled without the banding layer  18 .  
         [0065]     Various embodiments of the present assemblies, devices, and systems have been described herein. It should be recognized, however, that these embodiments are merely illustrative of the principles of the present assemblies, devices, and systems. Numerous modifications and adaptations thereof will be apparent to those skilled in the art without departing from the spirit and scope of the present assemblies, devices, and systems.  
         [0066]     The various embodiments described above can be combined to provide further embodiments. All of the above U.S. patents, patent applications and publications referred to in this specification as well as U.S. Provisional Patent Application No. 60/432,468, filed on Dec. 10, 2002; U.S. patent application Ser. No. 10/728,715, filed on Dec. 4, 2003; U.S. Provisional Patent Application No. 60/432,727, filed on Dec. 11, 2002; U.S. patent application Ser. No. 10/730,759, filed on Dec. 8, 2003; and U.S. Provisional Application No. 60/608,930, filed on Jul. 30, 2004, are incorporated herein by reference, in their entirety. Aspects of the invention can be modified, if necessary, to employ devices, features, and concepts of the various patents, applications and publications to provide yet further embodiments of the invention.