Abstract:
A rotor for a rotary electric machine, the rotor including first and second pole pieces each having a respective magnetic hub arranged for rotation about an axis along which they are spaced. Pluralities of magnetic first and second pole fingers are spaced from each other and extend between the hubs. Each pole finger has a proximal end connected to its respective hub, and an axially opposite distal end. The first and second pole fingers circumferentially alternate about the axis, and each pole finger has a respective radially inner surface defining a cavity that extends axially from the distal end to a cavity terminus. Relative to each pole finger, at a respective axial position between the distal end and the cavity terminus the radial distance between the axis and the radially inner surface is substantially greater inside of the cavity than outside of the cavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit under Title 35, U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/808,991, entitled ROTARY ELECTRIC MACHINE ROTOR POLE CONFIGURATION, filed Apr. 5, 2013, the entire disclosure of which is expressly incorporated herein by reference. 
       BACKGROUND 
       [0002]    The present disclosure relates to rotary electric machines, and particularly to rotors therefor, and more particularly to rotor types which include permanent magnets. 
         [0003]    An example of a prior rotary electric machine to which the teachings of the present disclosure may be applied, an alternator for use in a vehicle, is depicted in  FIG. 1 . Alternator  20  has a housing  24  and a rotor shaft  28  supported within the housing  24  by front and rear rolling element bearings  32  and  36 , respectively. A belt driven pulley  40  is fastened to a protruding front end of the rotor shaft  28 . The rotor  56  of the depicted rotary electric machine  20  includes front and rear alternator pole pieces  44  and  48 , respectively, which are mounted to and rotate with the shaft  28 . Alternator  20  generally includes a stator  52  which surrounds the rotor  56  and is affixed to the housing  24 . Rotation of the rotor  56  about its axis of rotation, the machine central axis  60 , causes an alternating current to be induced in the stator winding  68 . 
         [0004]    The stator  52  generally includes a stator core  64  about which stator winding  68  is coiled. As is known in the art, the stator core  64  generally includes a lamina stack  72  formed by a plurality of laminae stacked axially relative to the rotational axis  60  of the rotor shaft  28 . Each lamina may be made of electrical steel or another suitable ferromagnetic material. Referring to  FIG. 2 , the stator winding  68  typically includes a plurality of conductors  76 , and the stator core  64  defines a plurality of slots  80  leaving a plurality of stator teeth  84  therebetween; the stator slots  80  and teeth  84  are also shown in  FIG. 10 . The plurality of conductors  76  extend axially through the slots  80  and are looped in a conventional fashion such that the loops are distributed around the circumference of the stator  52 . As shown in  FIG. 2 , the plurality of stator winding conductors  76  are namely a first conductor  76   a,  a second conductor  76   b,  and a third conductor  76   c,  the conductors defining three phases of electrical power generated by the alternator. 
         [0005]    The rotor  56  is a type well-known as a claw-pole rotor, and includes the pair of opposing claw-pole pieces  44 ,  48  and an excitation field coil  88  disposed about the central axis  60 . Pole pieces  44  and  48  are made of a magnetic material such as steel, and are substantially identical to each other, having respective hub portions  92 ,  96  and a plurality of respective, elongate pole segments or fingers  100 ,  104 . The pole fingers of each pole piece  44 ,  48  are distributed about the circumference of the respective hub portion  92 ,  96  and are spaced by voids  108  in the respective hub portion. The pole fingers  100 ,  104  of each pole piece  44 ,  48  extend axially away from their respective hub portion, and axially towards the hub portion  92 ,  96  of the other pole piece. Further, the pole fingers  100 ,  104  of each pole piece  44 ,  48  are symmetrically spaced around the perimeter of the respective hub portion  92 ,  96  and, with the rotor  56  configured as assembled onto the shaft  28 , are interleaved in a non-contacting, spaced relationship with the pole fingers of the other pole piece, as shown in  FIG. 2 . Air gaps or channels are thus defined between adjacent pole fingers  100 , 104  and are distributed circumferentially about the rotor  56 . 
         [0006]    Referring to  FIG. 3 , the excitation field coil  88  of the rotor  56  is wound upon an electrically insulative bobbin  112  and the coil  88  and bobbin  112  are sandwiched between the pair of opposing, axially inwardly facing surfaces  116 ,  120  of the pole piece hub portions  92 ,  96 . Pole pieces  44 ,  48  may have axially-extending portions  121  about which the field coil  88  and its bobbin  112  are disposed, as shown in  FIG. 1 , or the field coil  88  and its bobbin  112  may be disposed about a cylindrical rotor core member  122  disposed about the central axis  60  and located between the pole pieces  44 ,  48 , as shown in  FIG. 3 . Referring again to  FIG. 1 , DC excitation current is applied to the excitation winding  88  through a pair of slip rings  124  and associated contact brushes  128 . The slip rings  124  are secured to the shaft  28  and in operation couple the field coil  88  to a regulated DC current source via the contact brushes  128 . A control system known as a voltage regulator (not shown) is used to apply an appropriate level of DC voltage to the excitation windings  88 . 
         [0007]    The pole pieces  44 ,  48  and the energized field winding  88  produce an alternating polarity magnetic field that rotates with the rotor  56  about the central axis  60 . Although a DC excitation current is applied to the field winding  88 , the interlacing of the alternating pole fingers  100 ,  104  creates an alternating polarity magnetic flux linkage. This magnetic flux linkage is presented to the winding conductors  76  of the stationary stator  52  that surrounds the rotor  56 . The movement of the alternating polarity magnetic flux linkage presented by the rotor  56  across the stator winding conductors  76   a,    76   b,    76   c  generates three-phase AC electrical power in a well-known manner. 
         [0008]    Typically, AC electrical output by the alternator  20  is directed to a rectifier  132 , which may be located at the rear of the housing  24  as shown in  FIG. 1 . The alternator may also include further filtering and power conditioning devices through which the electrical output is directed before it is conducted as DC electrical power to the positive terminal of the vehicle battery (not shown) or an electric distribution bus (also not shown). The desired RMS value of the outputted alternating current from the alternator  20  is dependent upon the level of DC voltage applied by the voltage regulator to the excitation windings  88 . Additionally, front and rear air circulation fans  136  and  140  are located at opposite axially outward sides of the pole pieces  44 ,  48 . The fans  136 ,  140  are coupled to the rotor  56  and rotate in unison therewith. Cooling airflow is typically drawn axially inwardly of the housing  24 , and is expelled radially outwardly of the housing  24 , by the fans  136 ,  140 . The rear fan  140  typically directs cooling airflow across the rectifier  132  and other electronic components of the alternator  20 . If an airflow path is provided, the fans  136 ,  140  may also direct some amount of cooling airflow around the pole fingers  100 ,  104  and the excitation coil  88 . 
         [0009]    The direction of rotation of the rotor  56  relative to the stator  52 , and thus the direction of movement of the rotor pole fingers  100 ,  104  relative to the stator teeth  84  is shown by arrow  144 . Upon energization of the field coil  88  with a regulated DC current the rotor  56  is magnetized, with the adjacent pole fingers  100 ,  104  alternating circumferentially between north (N) and south (S) magnetic polarities. In other words, all pole fingers  100  have N magnetic polarity and all pole fingers  104  have S magnetic polarity. Accordingly, it will be recognized that upon rotation of the rotor  56 , the alternating magnetic polarities of the pole fingers  100 ,  104  pass sequentially around the stator  52 , thereby inducing an output current in the stator winding  68 . Those of ordinary skill in the art will recognize that the respective N and S magnetic polarities of the front and rear pole pieces  44 ,  48  are determined as a function of the chosen direction of DC current flow through the excitation field coil  88 . 
         [0010]      FIGS. 5A-5H  show an example of a prior claw-pole piece  44  or  48  including a plurality of pole fingers or segments  100 ,  104  each having a base or proximal end  148  connected to the respective pole piece hub portion  92 ,  96  at locations between the voids  108 . Each pole finger  100 ,  104  also has a tip or distal end  152  opposite is respective base  148 , and the tips  152  of the pole fingers  100 ,  104  of one pole piece  44 ,  48  are located near the base  148  of the pole fingers  100 ,  104  of the other pole piece  44 ,  48 , as shown in  FIG. 3 . 
         [0011]    Each pole finger  100 ,  104  also has a leading edge  156  and an opposite trailing edge  160 , each of which extends between the base  148  and the tip  152  of the pole finger. The designation of an edge  156 ,  160  as leading or trailing is related to the direction of pole finger travel relative to the stator core teeth  84 , as indicated by arrow  144 . The leading and trailing edges  156 ,  160  of each pole finger  100  of front pole piece  44  respectively define leading edge side surface  164  and trailing edge side surface  168 ; the leading and trailing edges  156 ,  160  of each pole finger  104  of rear pole piece  48  respectively define leading edge side surface  172  and trailing edge side surface  176 . 
         [0012]    Each pole finger  100  also defines a radially outer surface  180  and a radially inner surface  184 , each of which extends circumferentially between its opposite leading and trailing edge side surfaces  164 ,  168 . Each pole finger  104  also defines a radially outer surface  188  and a radially inner surface  192 , each of which extends circumferentially between its opposite leading and trailing edge side surfaces  172 ,  176 . As shown in  FIGS. 2 and 5A , each radially outer surface  180 ,  188  lies along a respective surface line  196  that is substantially parallel with central axis  60 , such that a cylinder may be defined by the arranged plurality of surface lines  196 . Thus, the radially outer surfaces  180 ,  188  of the plurality of alternating pole fingers  100 ,  104  define the substantially cylindrical outer circumferential surface of the rotor  56 . 
         [0013]    Relative to each pole finger  100 ,  104  shown in  FIGS. 1-8  and  10 , which depict them as having a generally pyramidal shape, the respective radially inner surface  184 ,  192  is closer to the central axis  60  near its base or proximal end  148 , and further from the central axis  60  near its tip or distal end  152 , which may be flattened, as shown. Thus, each pyramidal pole finger  100 ,  104  is thicker radially, relative to the axis  60 , between its radially outer surface  180 ,  188  and its radially inner surface  184 ,  192 , at its proximal end or base  148  than at its distal end or tip  152 . Additionally, when viewed in a radial direction each pyramidal pole finger  100 ,  104  is tapered as the pole finger extends away from its respective hub portion  92 ,  96  and therefore is circumferentially wider between its leading and trailing edges  156 ,  160  at its proximal end  148  and narrower at its distal end  152 . It can therefore be understood that each pole finger  100 ,  104  may be generally V-shaped as viewed in both a radial direction relative to the central axis  60 , and in a direction normal to an imaginary plane in which the respective surface line  196  and the central axis  60  both lie. In other words, each generally pyramidal pole finger  100 ,  104 , if sectioned at its base  148  by an imaginary plane oriented perpendicular to the central axis  60  and flattened at its tip, is substantially hexahedral. 
         [0014]    Moreover, as can be clearly understood from the various views of  FIGS. 1-8 , in imaginary planes perpendicular to the central axis  60 , at varying distances axially along each pyramidal pole finger  100 ,  104  (that is, at various axial locations in directions generally parallel with the central axis  60 , the respective thickness of each pole finger between its radially outer surface  180 ,  188  and its radially inner surface  184 ,  192  is substantially uniform between its leading and tailing edges  156 ,  160 . Additionally, but for radially inner and outer surfaces  180 ,  188 ,  184 , and  192  presenting slight curvatures about the central axis  60  corresponding to the cylindrical shape of the rotor  56  (convex in the case of radially outer surface  180 ,  188 , and concave in the case of radially inner surface  184 ,  192 ), these surfaces  180 ,  188 ,  184 , and  192  are generally flat and featureless between their respective pole finger leading and trailing edges  156 ,  160 . 
         [0015]    In some prior machines  20  the pole fingers  100 ,  104 , rather than being generally pyramidal in shape as discussed above, instead have a different geometry. For example, referring to  FIG. 9 , the pole pieces  44 ,  48  may instead define pole fingers or segments  100 ,  104  that are generally rectangular in shape when viewed radially relative to the central axis  60 . As in the case of the generally pyramidal pole segments described above, the pole fingers or segments  100 ,  104  of the prior claw-pole pieces  44  or  48  shown in  FIG. 9  each have: a base or proximal end  148  connected to the respective pole piece hub portion  92 ,  96  at locations between the voids  108 ; a tip or distal end  152  opposite its respective base  148 , with the tip  152  of the pole finger of one pole piece  44 ,  48  being located near the base  148  of the pole finger of the other pole piece  44 ,  48 ; a leading edge  156 ; and an opposite trailing edge  160 , the leading and trailing edges  156 ,  160  extending between the pole finger base  148  and tip  152 . The generally parallel leading and trailing edges  156 ,  160  of each pole finger  100  respectively define the leading edge side surface  164  and the trailing edge side surface  168 , whereas the leading and trailing edges  156 ,  160  of each pole finger  104  respectively define the leading edge side surface  172  and trailing edge side surface  176 . As discussed above, the designation of an edge  156 ,  160  as leading or trailing is related to the direction of pole finger travel relative to the stator core teeth  84 , as indicated by arrow  144 . 
         [0016]    Unlike the generally pyramidal pole segments described above, however, in the example of  FIG. 9  the leading and trailing edges  156 ,  160  are generally parallel to each other and to the central axis  60 . Here, the depicted pole finger tips or distal ends  152  are flat, and each pole finger  100 ,  104 , if sectioned at its base  148  by an imaginary plane oriented perpendicular to the central axis  60 , may be substantially hexahedral. In the example depicted in  FIG. 9 , each pole finger  100 ,  104  respectively defines a radially outer surface  180 ,  188  and a radially inner surface  184 ,  192  (not shown in  FIG. 9 ). As in the case of the generally pyramidal pole fingers, each radially outer, rectangular surface  180 ,  188  lies along a respective surface line  196  that is substantially parallel with the central axis  60 , whereby the cylindrical rotor shape may be defined by the arranged plurality of surface lines  196 . Relative to each pole finger  100 ,  104 , its respective radially outer surface  180 ,  188  extends a substantially uniform distance between the circumferentially opposite leading edge  156  and trailing edge  160 ; similarly, its respective radially inner surface  184 ,  192  extends a substantially uniform distance between the circumferentially opposite leading edge  156  and trailing edge  160 . Thus, each pole finger  100 ,  104  has a generally rectangular shape when viewed in a radial direction, as mentioned above. 
         [0017]    Furthermore the pole fingers  100 ,  104  depicted in  FIG. 9  may each be substantially configured as a rectangular parallelepiped or cuboid, wherein, as viewed in a direction normal to an imaginary plane in which the respective surface line  196  and the central axis  60  both lie, the thickness of each pole finger  100 ,  104  between its respective radially outer surface  180 ,  188  and radially inner surface  184 ,  192 , is substantially uniform along its axial direction, i.e., in a direction generally parallel with the surface line  196 . Thus, each pole finger  100 ,  104  has a generally rectangular shape when viewed in a tangential direction, perpendicular to the central axis  60 . Furthermore, but for surfaces  180 ,  188 ,  184 ,  192  presenting slight curvatures about the central axis  60  corresponding to the cylindrical shape of the rotor  56  (convex in the case of radially outer surface  180 ,  188 , and concave in the case of radially inner surface  184 ,  192 ), the surfaces  180 ,  188 ,  184 , and  192  of the generally cuboid pole fingers  100 ,  104  are generally flat between their respective pole finger leading and trailing edges  156 ,  160 . Moreover, the opposed radially outer surface  180 ,  188  and radially inner surface  184 ,  192  of each generally cuboid pole finger  100 ,  104  are substantially parallel. In other words, in imaginary planes perpendicular to the central axis  60 , at varying distances axially along each pyramidal pole finger  100 ,  104  (that is, at distances in directions generally parallel with surface lines  196 ), the respective thickness of each pole finger between its radially outer surface  180 ,  188  and radially inner surface  184 ,  192  is substantially uniform. A prior electrical machine including pole fingers or pole segments having leading and trailing edges substantially parallel with each other and the machine central axis is also disclosed in U.S. Pat. No. 7,973,444 entitled ELECTRIC MACHINE AND ROTOR FOR THE SAME and assigned to the assignee of the present application, the entire disclosure of which is expressly incorporated herein by reference. 
         [0018]    As noted above, regardless of whether their pole fingers  100 ,  104  are generally pyramidal or generally cuboid, in prior rotary electrical machines such as an alternator  20  the pole finger radially inner surfaces  184 ,  192  are substantially flat or provided with only a very minor concave curvature about the central axis  60  between their respective leading and trailing edges  156 ,  160 , at various locations along the axial length of the pole finger, i.e., in directions parallel with surface lines  196 . The curvature of the radially inner surface  184 ,  192 , where present, is more pronounced near the pole finger base or proximal end  148  than it is near the pole finger tip or distal end  152 , as revealed by comparisons between  FIGS. 5F-5H , and between  FIGS. 7B-7E . 
         [0019]    It is also known to employ permanent magnets in the rotors of rotary electrical machines such as alternators. In some prior alternators, high-magnetic-strength permanent magnets  200  are disposed between the adjacent claw-pole fingers  100 ,  104  to supplement the magnetic field generated by the excitation coil  88 . Such magnets  200 , which are optional, are shown in  FIGS. 6-9 . Any of a variety of permanent magnet material may be used for permanent magnets  200  such as neodymium-iron-boron, samarium-cobalt, or ferrite. Alternators utilizing both field coil and permanent magnet fluxes coupled to a stator coil are referred to as hybrid alternators. Referring to  FIG. 10 , in a hybrid alternator  20 , permanent magnets  200  maintain a permanent magnet flux across channels  204  that would otherwise be air gaps between the claw-pole segments  100 ,  104 , which in a hybrid alternator are magnetically linked to the permanent magnets  200  disposed in channels  204  and carried by the rotor  56 , and a portion of the stator structure  52 , thereby coupling significant magnetic flux through the stator structure. The magnetic flux path  208  is shown in dashed lines in  FIG. 10 . When the field coil  88  is not energized, the magnetic flux developed by the permanent magnets  200  is shunted through the rotor  56 . However, when the field coil  88  is energized, the magnetic flux developed by the permanent magnets  200  additively contributes to the electromagnetically generated magnetic flux resulting from field coil excitation, across the stator/rotor air gap  212 . Depending on the desired output of the hybrid alternator  20 , the effect of the permanent magnets  200  on the flux across the radial stator/rotor air gap  212  may supplement, or boost, the electromagnetic flux generated by the DC current being passed in one direction through the field effect coil  88 ; the effect of the permanent magnets on the flux across the stator/rotor air gap may also be reduced, or bucked, by electromagnetic flux that is generated by DC current being passed in the opposite direction through the field effect coil  88 . Alternator buck/boost control circuits are known in the art and may be of various designs, one of which is disclosed in above-mentioned U.S. Pat. No. 7,973,444. 
         [0020]    Channels  204  may be oriented as described above; typically, the orientation and shape of the permanent magnets  200  is similar. Thus, permanent magnets  200  are generally prism-shaped with six substantially flat faces. The permanent magnets  200  being substantially prism-shaped provides substantially symmetrical abutting surfaces at their respective interfaces with the leading and trailing edge side surfaces  164 ,  168 ,  172 ,  176 . The prism-shaped permanent magnets  200  are illustrated herein as an exemplary shape, it being understood that other shapes for the permanent magnets will be apparent to the skilled artisan. As shown herein, each permanent magnet  200  has a pair of circumferentially opposing pole faces  216 , with the polarized faces  216 N and  216 S corresponding to N and S magnetic polarities, respectively. The polarities of the permanent magnets alternate such that adjacent magnets are of opposite polarity. Therefore, it can be appreciated that claw-pole fingers  100  abut permanent magnet pole faces  216 N and have a first common polarity (i.e., N), and claw-pole fingers  104  abut permanent magnet pole faces  216 S and have a second common polarity (i.e., S). The pole faces  216 N,  216 S of magnets  200  are immediately adjacent respective leading and trailing edge side surfaces  164 ,  168 ,  172 ,  176  on pole fingers  100  and  104 . As mentioned above, all pole fingers  100  have N magnetic polarity and all pole fingers  104  have S magnetic polarity. All permanent magnet pole faces  216 N are adjacent the side surfaces  164 ,  168  of each N pole finger  100 . Likewise, all permanent magnet pole faces  216 S are adjacent the side surfaces  172 ,  176  of each S pole finger  104 . The foregoing arrangement is generally well known to those skilled in the art. 
         [0021]    Typically, when permanent magnets  200  are added between the claw poles  100 ,  104  of an alternator  20  to boost machine performance, the air gap channel  204  is machined or otherwise adapted to provide a constant width between the opposing pole finger leading and trailing edge side surfaces  164 ,  168 ,  172 ,  176  to contain the magnets. However, the shape of the claw-pole pieces  44 ,  48  used in prior hybrid alternators is not optimized to maximize the use of the permanent magnets  200 . Rather, the pole piece designs of such hybrid machines, and particularly the designs of their pole segments or fingers  100 ,  104  are “carried over” from a conventional, non-permanent magnet-equipped claw-pole rotor design, which had already evolved to maximize machine performance without the addition of permanent magnets to the claw-pole rotor. To simply adhere to this practice does not take full advantage of the benefit of adding magnets to claw-pole rotors. 
         [0022]    A rotary electric machine configured to maximize the beneficial aspects of a permanent magnet-equipped rotor would provide a desirable improvement in the art. 
       SUMMARY 
       [0023]    The present disclosure is aimed at providing such a rotary electric machine and rotor. The present disclosure teaches a pole piece configuration that maximizes the performance of its claw-pole segments when used with permanent magnets by modifying the pole geometry. The shape of the radially inner underside surface of the pole finger near the area of the tip is configured to be significantly concave, which minimizes the flux leakage from the tip or distal end of a first pole finger or segment, to the base or proximal end of a circumferentially adjacent second pole finger or segment, where the first pole segment tip overlaps the second pole segment base. Yet, the first pole finger or segment may also have its full radial thickness or depth along the sides of the permanent magnet disposed between the first and second pole fingers, which facilitates full utilization of the magnet&#39;s flux production. 
         [0024]    The sides of the pole finger against which the permanent magnets rest are essentially flat and of constant radial depth or thickness for the full axial length of the pole finger. Although the exemplary embodiment of the improved pole finger geometry is shown as having a generally cuboid envelope having a generally square axial cross section, it is to be understood that the teachings of the present disclosure apply equally well to a conventional, generally pyramidal alternator claw-pole segment or finger. The teachings of the present disclosure also apply equally well to brushed or brushless alternators. 
         [0025]    The benefit of this geometry is that, for a given magnetically active axial length of the rotor, it allows the magnetic utilization of a much longer permanent magnet in the rotor. This greatly increases electrical machine performance by generating much higher levels of flux linkage between the stator and the rotor for a given rotor axial stack length and/or a given stator axial stack length, vis-à-vis prior alternators. Second, by virtue of the concave radially inner surface shape of the pole finger, a natural air passage is formed that allows axial air flow through the rotor assembly for improved cooling. 
         [0026]    The pole finger geometry according to the present disclosure was arrived at through the use of three dimensional Finite Element Analysis (3D FEA) magnetic modeling and a lengthy design process. Measured performance with actual alternator prototypes yielded output current levels twice that of conventional claw-pole alternators of comparable size, which represents a significant improvement over the prior art. 
         [0027]    The present disclosure provides a rotor for a rotary electric machine, the rotor including a first pole piece and a second pole piece each having a respective magnetic hub arranged for rotation about an axis, the first and second pole piece hubs spaced along the axis. The rotor also includes a plurality of magnetic first pole fingers and a plurality of magnetic second pole fingers spaced from each other and extending between the first and second pole piece hubs. Each pole finger has a proximal end and an axially opposite distal end, the first and second pole finger proximal ends connected to the respective one of the first and second pole piece hubs. The first and second pole fingers circumferentially alternate about the axis, and each pole finger has a respective radially inner surface defining a cavity that extends axially from the distal end to a cavity terminus. Relative to each pole finger, at a respective axial position between the distal end and the cavity terminus the radial distance between the axis and the radially inner surface is substantially greater inside of the cavity than outside of the cavity. 
         [0028]    A further aspect of the rotor is that relative to each pole finger, the cavity terminus is located between the proximal end and the distal end. 
         [0029]    A further aspect of the rotor is that relative to each pole finger, the radial distance between the axis and the radially inner surface inside of the cavity is greater at a first axial location which is between the distal end and the cavity terminus than at a second axial location which is between the first axial location and the cavity terminus. 
         [0030]    A further aspect of the rotor is that each pole finger has circumferentially opposite leading and trailing edges, and the respective cavity is located between the leading and trailing edges. 
         [0031]    A further aspect of the rotor is that relative to each pole finger, the cavity has a width that varies in a direction perpendicular to the axis, the width being greater at a first axial location which is between the distal end and the cavity terminus than the width at a second axial location which is between the first axial location and the cavity terminus. 
         [0032]    A further aspect of the rotor is that relative to each pole finger, the cavity has a generally triangular shape in an imaginary plane perpendicular to the axis. 
         [0033]    A further aspect of the rotor is that relative to each pole finger, the cavity has a generally triangular shape in an imaginary plane parallel to the axis. 
         [0034]    A further aspect of the rotor is that the cavity terminus of each pole finger defines a cavity apex. 
         [0035]    A further aspect of the rotor is that relative to each pole finger, at the respective cavity terminus and distal end, the radially inner surface at locations outside of the cavity are radially equidistant from the axis. 
         [0036]    A further aspect of the rotor is that each pole finger defines a radially outer surface, and the respective pole finger has a radial thickness between the radially inner surface and the radially outer surface. The radial thickness at a first location outside of the cavity is greater than the radial thickness at a second location inside of the cavity. 
         [0037]    An additional aspect of the rotor is that each pole finger has circumferentially opposite leading and trailing edges, and the first location is circumferentially between the cavity and one of the leading and trailing edges of the respective pole finger. 
         [0038]    An additional aspect of the rotor is that the first location is between the proximal end and the cavity terminus of the respective pole finger. 
         [0039]    A further aspect of the rotor is that each pole finger has circumferentially opposite leading and trailing edges. The respective radially inner surface extends circumferentially between the leading and trailing edges, and the leading and trailing edges are substantially parallel with the axis. 
         [0040]    A further aspect of the rotor is that, magnetically, the first pole fingers are N pole fingers and the second pole fingers are S pole fingers. The rotor also includes at least one magnet disposed between a circumferentially adjacent pair of N and S pole fingers. The magnet has opposite N and S pole faces, with the magnet N pole face interfacing the N pole finger, and the magnet S pole face interfacing the S pole finger. 
         [0041]    A further aspect of the rotor is that each pole finger has circumferentially opposite leading and trailing edge side surfaces and the respective radially inner surface extends circumferentially between the leading and trailing edge side surfaces. The rotor also includes at least one magnet disposed between the interfacing leading and trailing edge side surfaces of a pair of circumferentially adjacent first and second pole fingers, and having magnetically opposite pole side surfaces. Each magnetically opposite pole side surface of the magnet(s) abut one of the pole finger leading and trailing edge side surfaces substantially along the entire length of the respective pole finger between the pole finger&#39;s proximal and distal ends. 
         [0042]    A further aspect of the rotor is that it also includes an excitation coil disposed about the axis and located between the first and second pole piece hubs. The N and S magnetic polarity designations of the first and second pole piece hubs are selectively determined by a chosen electric current flow direction through the excitation coil. 
         [0043]    The present disclosure also provides a rotary electric machine including a rotor as described above, a stator surrounding the rotor, and a housing connected to the stator. The rotor is supported by the housing for rotation relative to the stator. 
         [0044]    The present disclosure also provides a rotor for a rotary electric machine, the rotor including a pair of magnetic, first and second pole pieces each having a respective hub. The first and second pole piece are hubs spaced along an axis and have first and second pluralities of pole fingers, respectively. Each of the first and second pluralities of pole fingers are spaced from the other and distributed about the axis to define a substantially cylindrical outer rotor surface. Each pole finger has a base attached to its respective first or second pole piece hub and extends towards the other pole piece hub. Each pole finger of one of the first and second pluralities of pole fingers terminates at a tip positioned proximate the bases of a pair of pole fingers included in the other of the first and second pluralities of pole fingers. Each pole finger tip is provided with a radially inwardly open cavity. The cavity has a length that extends in an axially inward direction from the tip towards the base of the respective pole finger to a cavity terminus. The cavity has a width dimension extending between opposite edges of the respective pole finger in a direction generally perpendicular to the axis, and a depth dimension extending generally radially into the respective pole finger. At least one of the cavity width and depth dimensions diminishes along the cavity length in the axially inward direction. 
         [0045]    A further aspect of the rotor is that, magnetically, the first pole fingers are N pole fingers and the second pole fingers are S pole fingers. The rotor also includes at least one magnet disposed between a circumferentially adjacent pair of N and S pole fingers, the magnet having opposite N and S pole faces. The magnet N pole face(s) interface the N pole finger, and the magnet S pole face(s) interface the S pole finger. 
         [0046]    The present disclosure also provides a rotary electric machine including a rotor as described above, a stator surrounding the rotor, and a housing connected to the stator. The rotor is supported by the housing for rotation relative to the stator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]    The various objects, features and attendant advantages of the present invention will become fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings. It is to be noted that the accompanying drawings are not necessarily drawn to scale or to the same scale; in particular, the scale of some of the elements of the drawings may be exaggerated to emphasize characteristics of the elements. Moreover, like reference characters designate the same, similar or corresponding parts throughout the several views, wherein: 
           [0048]      FIG. 1  is a sectional side view of a prior alternator assembly to which the teachings of the present disclosure may be adapted; 
           [0049]      FIG. 2  is a partially sectioned front perspective view of a prior alternator stator and rotor to which the teachings of the present disclosure may be adapted; 
           [0050]      FIG. 3  is a sectional side view of a prior alternator rotor along line  3 - 3  of  FIG. 4 ; 
           [0051]      FIG. 4  is a fragmented front view of the prior alternator rotor; 
           [0052]      FIG. 5A  is perspective view of a prior pole piece having pole fingers that are generally pyramidal in shape; 
           [0053]      FIG. 5B  is an axial end view of the pole piece of  FIG. 5A ; 
           [0054]      FIG. 5C  is an opposite axial end view of the pole piece of  FIG. 5B ; 
           [0055]      FIG. 5D  is a side view of the pole piece of  FIG. 5C ; 
           [0056]      FIG. 5E  is a sectional view of the pole piece of  FIG. 5C  along line  5 E- 5 E; 
           [0057]      FIG. 5F  is a sectional view of a pole finger of the pole piece of  FIG. 5D  along line  5 F- 5 F; 
           [0058]      FIG. 5G  is a sectional view of a pole finger of the pole piece of  FIG. 5D  along line  5 G- 5 G; 
           [0059]      FIG. 5H  is a sectional view of a pole finger of the pole piece of  FIG. 5D  along line  5 H- 5 H; 
           [0060]      FIG. 6  is a partial, fragmented, exploded view of a prior rotor having generally pyramidal pole fingers and optional permanent magnets; 
           [0061]      FIG. 7A  is a sectional side view of a prior alternator rotor having permanent magnets along line  7 A- 7 A of  FIG. 8 ; 
           [0062]      FIG. 7B  is an axial end view of the rotor of  FIG. 7A  along line  7 B- 7 B, showing the pole finger tip; 
           [0063]      FIG. 7C  is a sectional view of the rotor of  FIG. 7A  along line  7 C- 7 C, showing an axial cross section of the pole finger; 
           [0064]      FIG. 7D  is a sectional view of the rotor of  FIG. 7A  along line  7 D- 7 D, showing an axial cross section of the pole finger and permanent magnets; 
           [0065]      FIG. 7E  is a sectional view of the rotor of  FIG. 7A  along line  7 E- 7 E, showing an axial cross section of the pole finger and permanent magnets; 
           [0066]      FIG. 8  is a fragmented front view of a prior alternator rotor having permanent magnets; 
           [0067]      FIG. 9  is a partial, fragmented, exploded view of a prior rotor having generally cuboid pole fingers and optional permanent magnets; 
           [0068]      FIG. 10  is an enlarged partial sectional view of a prior rotor having permanent magnets and a stator core (with their excitation field coil and stator windings removed), showing the magnetic flux path therebetween; 
           [0069]      FIG. 11  is a partial, sectional, exploded view of an embodiment of a rotor according to the present disclosure; 
           [0070]      FIG. 12  is a sectional side view of an alternator rotor according to the present disclosure along line  12 - 12  of  FIG. 13 ; 
           [0071]      FIG. 13  is a fragmented front view of an alternator rotor according to the present disclosure; 
           [0072]      FIG. 14A  is a sectional side view of an alternator according to the present disclosure along line  14 A- 14 A of  FIG. 15 ; 
           [0073]      FIG. 14B  is a fragmented, partial sectional view of the rotor of  FIG. 14A  along line  14 B- 14 B, showing an axial end view of a pole finger and permanent magnets; 
           [0074]      FIG. 14C  is a fragmented, sectional view of the rotor of  FIG. 14A  along line  14 C- 14 C, showing an axial cross section of the pole finger and magnets; 
           [0075]      FIG. 14D  is a fragmented, sectional view of the rotor of  FIG. 14A  along line  14 D- 14 D, showing an axial cross section of the pole finger and magnets; 
           [0076]      FIG. 15  is a fragmented front view of an alternator rotor according to the present disclosure; 
           [0077]      FIG. 16  is an enlarged, fragmented view of an embodiment of a pole finger tip and permanent magnets according to the present disclosure; 
           [0078]      FIG. 17  is a sectional side view of an alternator rotor and stator according to the prior art, for comparison with  FIG. 18 ; 
           [0079]      FIG. 18  is a sectional side view of an alternator rotor and stator according to the present disclosure, for comparison with  FIG. 17 ; and 
           [0080]      FIG. 19  is a graph illustrating the performance improvement of a hybrid alternator according to the present disclosure over a non-hybrid alternator according to the prior art. 
       
    
    
       [0081]    Corresponding reference characters indicated corresponding parts throughout the several views. Although the drawings represent embodiments of the disclosed apparatus, the drawings are not necessarily to scale or to the same scale and certain features may be exaggerated in order to better illustrate and explain the present disclosure. Moreover, in accompanying drawings that show sectional views, cross-hatching of various sectional elements may have been omitted for clarity. It is to be understood that this omission of cross-hatching is for the purpose of clarity in illustration only. 
       DETAILED DESCRIPTION 
       [0082]    The invention is susceptible to various modifications and alternative forms, and the specific embodiment thereof shown by way of example in the drawings is herein described in detail. The exemplary embodiment of the present disclosure is chosen and described so that others skilled in the art may appreciate and understand the principles and practices of the present disclosure. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
         [0083]    In referring below and in the drawings to a rotary electric machine or rotor according to the present disclosure, its structural elements corresponding to structural elements of the prior art discussed above are identified with a like reference numeral primed. Thus, for example, an embodiment of above-discussed rotary electric machine  20  and rotor  56  modified in accordance with the present disclosure is identified as rotary electric machine  20 ′ and rotor  56 ′. Corresponding structural elements of machine  20 ′ that are substantially unchanged relative to the prior art discussed above are identified with common respective element numerals. The magnetic flux path, though different between machines  20  and  20 ′, is nonstructural and is commonly referred to with reference numeral  208 . 
         [0084]      FIG. 11  shows a rotor  56 ′ according to the present disclosure. The depicted embodiment of rotor  56 ′ is, but for the configuration of its pole fingers  100 ′,  104 ′ and possibly the axial length of its permanent magnets  200 ′, substantially similar to prior rotor  56  having generally cuboid pole fingers  100 ,  104  and prism-shaped magnets  200 . 
         [0085]    As noted above, in prior rotary electric machines  20 , regardless of whether their pole fingers  100 ,  104  are generally pyramidal or generally cuboid, the pole finger radially inner surfaces  184 ,  192  are substantially flat or provided with only a very minor concave curvature about the central axis  60  between their respective leading and trailing edges  156 ,  160 . This surface curvature, where present, is more pronounced near the pole finger base or proximal end  148  than it is near the pole finger tip or distal end  152 , as revealed by comparisons between  FIGS. 5F-5H , and between  FIGS. 7B-7E . This characteristic of the pole finger radially inner surfaces  184 ,  192  remains common between rotors  56  that employ permanent magnets  200 , and those that do not. As explained above, it appears that while the configurations of the prior pole fingers  100 ,  104  may have been optimized to some degree for use in rotors  56  that do not include permanent magnets  200 , the pole finger configurations remain essentially unchanged when the magnets are incorporated into those rotors. In other words, although the addition of the magnets  200  in prior rotors  56  may boost the performance of the machine  20  (which may, for example, be a hybrid alternator), the pole segments or fingers  100 ,  104  of these prior machines, between which the incorporated permanent magnets  200  are disposed, remain essentially unchanged. 
         [0086]    The present disclosure provides pole pieces  44 ′,  48 ′ having modified pole finger configurations that, when used with permanent magnets  200 ′, maximize the performance of the rotor  56 ′ and its machine  20 ′. As best seen in  FIGS. 12-16 , rotor  56 ′ of machine  20 ′ includes substantially identical pole pieces  44 ′,  48 ′. The respective pole segments or fingers  100 ′,  104 ′ of pole pieces  44 ′,  48 ′ are substantially different in shape than pole fingers  100 ,  104  of prior pole pieces  44 ,  48 . Particularly, the configurations of the radially inner surfaces  184 ′,  192 ′ of the pole fingers  100 ′,  104 ′ differ significantly from those of the counterpart radially inner surfaces  184 ,  192  of prior pole fingers  100 ,  104 . A comparison of  FIGS. 14B-14D  and  FIGS. 5F-5H  and/or  FIGS. 7B-7E  best illustrates this difference, despite the depicted pole fingers  100 ′,  104 ′ according to the present disclosure being of a generally cuboid type, and depicted pole fingers  100 ,  104  according to the prior art being of a generally pyramidal type. It is to be understood that the teachings of the present disclosure apply to pole fingers  100 ′,  104 ′ of types other than those depicted, such as pole fingers that are generally pyramidal. The comparison reveals that, in accordance with the present disclosure, the respective radially inner surface  184 ′,  192 ′ of each pole finger  100 ′,  104 ′, near the region of the pole finger tip or distal end  152 ′, is configured to be significantly concave, which minimizes the flux leakage from the tip or distal end  152 ′ of a first pole finger or segment  100 ′ or  104 ′, to the base or proximal end  148 ′ of a circumferentially adjacent second pole finger or segment  100 ′,  104 ′, in the region of the rotor  56 ′ where the first pole segment tip  152 ′ overlaps the second pole segment base  148 ′. The radially inner surfaces  184 ,  192  of prior pole fingers  100 ,  104  are substantially flat surfaces, with any minor concavity that may be present being insignificant and merely corresponding to the diameter of the rotor  56 . Comparatively, any concavity defined by the radially inwardly open cavity  220  provided in radially inner surfaces  184 ′,  192 ′ is significantly greater. As shown, the cavity  220  is defined by a generally triangular pyramidal void formed in the distal end  152 ′ of the pole finger  100 ′,  104 ′; the base of that triangular void may be understood to be located at pole finger tip  152 ′, and its apex, the terminus of the cavity  220 , may be understood to be located at a location  224  axially between the pole finger proximal and distal ends  148 ′,  152 ′, as best seen in  FIGS. 12 ,  14 A, and  16 . 
         [0087]    Moreover, each pole finger or segment  100 ′,  104 ′ may also have its full radial thickness or depth between radially outer surface  180 ,  188  and radially inner surface  184 ′,  192 ′ along the leading and trailing edges  156 ,  160 , whereby the entirety of each respective circumferential face  216  of magnets  200 ′ may abut a corresponding leading or trailing edge surface  164 ′,  168 ′,  172 ′,  176 ′. The ability to mutually interface the entirety of each magnet polar face  216 N,  216 S and its respective, cooperating pole finger leading or trailing edge surface  164 ′,  168 ′,  172 ′,  176 ′ facilitates full utilization of the magnet&#39;s flux production. Notably, the leading or trailing edge surfaces  164 ′,  168 ′,  172 ′,  176 ′ are essentially flat and of constant radial depth or thickness for the full axial length of the pole finger  100 ′,  104 ′, that is, its full length in a direction generally parallel with surface line  196 . Although the exemplary embodiment of the improved pole finger geometry is shown as having a generally cuboid envelope having a generally square axial cross section, it is to be understood that the teachings of the present disclosure apply equally well to a conventional, generally pyramidal alternator claw-pole segment or finger. The teachings of the present disclosure also apply equally well to brushed or brushless alternators. 
         [0088]    Referring now to  FIGS. 17 and 18 , a benefit of the pole finger geometry in machine  20 ′ vis-à-vis a prior machine  20  is that, for a given magnetically active axial length (L1=L1) of the rotor  56 ,  56 ′, it allows the magnetic utilization of a much longer permanent magnet  200 ′ (L3&gt;&gt;L2) in the rotor  56 ′, which usefully allows a greater stator lamina stack axial length (L5&gt;L4). This greatly increases the performance of electrical machine  20 ′ vis-à-vis machine  20  by generating in machine  20 ′ much higher levels of flux linkage between the stator  52 ′ and the rotor  56 ′ for a given rotor axial stack length (L1=L1) and/or a given stator laminae stack axial length (L4=L5). 
         [0089]    A second benefit afforded by the pole finger geometry in machine  20 ′ vis-à-vis machine  20  is that, by virtue of the cavity  220  in the radially inner surface  184 ′,  192 ′ of the pole finger  100 ′,  104 ′, a natural air passage is formed that allows relatively greater axial air flow through the rotor assembly  56 ′ for comparatively improved cooling. 
         [0090]    As noted above, the pole finger geometry according to the present disclosure was arrived at through the use of three dimensional Finite Element Analysis (3D FEA) magnetic modeling and a lengthy design process, and actual alternator prototypes according to the present disclosure (i.e., prototype machines  20 ′) have yielded measured output current levels with that are 200% that of conventional claw-pole alternators of comparable size and without magnets. This performance improvement, as demonstrated with actual 14V alternators operating at 25° C., is illustrated in  FIG. 19 . In  FIG. 19 , curve  228  represents the performance of a prototype hybrid alternator  20 ′ according to the present disclosure having generally cuboid claw-pole fingers; and curve  232  represents the performance of a production non-hybrid alternator  20  (without rotor permanent magnets) according to the prior art having generally pyramidal claw-pole fingers. 
         [0091]    While an exemplary embodiment has been disclosed hereinabove, the present disclosure is not limited to the disclosed embodiment. Instead, this application is intended to cover any variations, uses, or adaptations of the present disclosure using its general principles. Thus, although the disclosed rotary electric machine is a brushed type of alternator, it is to be understood that the teachings of the present disclosure could be implemented with rotors of other types of rotary electric machines, such as electric motors or brushless alternators having rotors that employ permanent magnets. 
         [0092]    As to a further discussion of the manner of usage and operation of the present disclosure, the same should be apparent from the above description. With respect to the above description then, it is to be realized that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent and obvious to those of ordinary skill in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present disclosure.