Abstract:
A rotor for an electric machine includes a rotor core constructed of magnetically-permeable material and a permanent-magnet cluster that creates a magnetic pole of the rotor. The permanent-magnet cluster may be composed of permanent magnets mounted to the rotor core, including a first permanent magnet, a second permanent magnet, a third permanent magnet, and a fourth permanent magnet. The second permanent magnet may have a first end disposed adjacent a first end of the first permanent magnet, with a first portion of the rotor core disposed therebetween. Additionally, the third permanent magnet may have a first end disposed adjacent a second end of the first permanent magnet, with a second portion of the rotor core disposed therebetween. The fourth permanent magnet may form at least a portion of an outer perimeter of the rotor core and may be disposed radially outward of at least a portion of at least one of the first permanent magnet, the second permanent magnet, and the third permanent magnet.

Description:
TECHNICAL FIELD  
       [0001]     The present disclosure relates to electric machines having a stator and a rotor and, more particularly, to electric machines having a rotor that includes permanent magnets.  
       BACKGROUND  
       [0002]     Many electric machines, such as electric motors and electric generators, include a stator that is held stationary and a rotor that rotates adjacent the stator. The stator and rotor may be configured to transfer power between one another through one or more rotating magnetic fields. Some electric machines may include a permanent-magnet type rotor with permanent magnets mounted on or inside a rotor core of the rotor. Each permanent magnet of the rotor may individually create a north or south magnetic pole of the rotor. A permanent-magnet type rotor having only a single permanent magnet creating each of its magnetic poles may, however, limit the performance potential of the associated electric machine.  
         [0003]     U.S. Pat. No. 6,664,688 to Naito et al. (“the &#39;688 patent”) shows a rotor with each of its magnetic poles created by a group of permanent magnets. Each group of permanent magnets of the &#39;688 patent includes an outer permanent magnet disposed in a recess in an outer surface of a rotor core. Additionally, each group of permanent magnets of the rotor disclosed by the &#39;688 patent includes two arc-shaped inner permanent magnets mounted in cavities in the rotor core. Inner ends of the two inner permanent magnets are disposed adjacent one another, radially inward of the outer permanent magnet of the group. Outer ends of the two inner permanent magnets of each group are disposed at the outer surface of the rotor on opposite sides of the outer permanent magnet. A relatively thin portion of the rotor core between the inner ends of the two inner permanent magnets provides the only connection between a portion of the rotor core disposed radially outward of the two inner permanent magnets and other portions of the rotor core.  
         [0004]     Although each of the magnetic poles of the rotor of the &#39;688 patent is created by multiple permanent magnets, certain disadvantages persist. For example, the relatively narrow portion of the rotor core that extends between the inner ends of the two inner permanent magnets may be subjected to undesirably high stresses during rotation of the rotor. Rotation of the rotor may create centrifugal force on the portion of the rotor core disposed radially outward the inner permanent magnets. In order to keep the rotor core intact, the relatively thin portion of the rotor core between inner ends of the two inner permanent magnets must counteract all of the centrifugal force on the portion of the rotor core disposed radially outward of the two inner permanent magnets. Additionally, the shape of the outer permanent magnet and the cavity in which it is mounted may make the outer permanent magnet susceptible to detachment from the rotor core during high-speed rotation of the rotor. Furthermore, because they are arc-shaped, the two inner permanent magnets may be expensive.  
         [0005]     The electric machine and rotor of the present disclosure solve one or more of the problems set forth above.  
       SUMMARY OF THE INVENTION  
       [0006]     One disclosed embodiment relates to a rotor for an electric machine. The rotor may include a rotor core constructed of magnetically-permeable material and a permanent-magnet cluster that creates a magnetic pole of the rotor. The permanent-magnet cluster may be composed of permanent magnets mounted to the rotor core, including a first permanent magnet, a second permanent magnet, a third permanent magnet, and a fourth permanent magnet. The second permanent magnet may have a first end disposed adjacent a first end of the first permanent magnet, with a first portion of the rotor core disposed therebetween. Additionally, the third permanent magnet may have a first end disposed adjacent a second end of the first permanent magnet, with a second portion of the rotor core disposed therebetween. The fourth permanent magnet may form at least a portion of an outer perimeter of the rotor core and may be disposed radially outward of at least a portion of at least one of the first permanent magnet, the second permanent magnet, and the third permanent magnet.  
         [0007]     Another embodiment relates to a rotor for an electric machine. The rotor may include a rotor core constructed of magnetically-permeable material and a plurality of permanent magnets mounted to the rotor core. The plurality of permanent magnets mounted to the rotor core may include a permanent-magnet cluster that creates a magnetic pole of the rotor. The permanent-magnet cluster may include at least one outer permanent magnet that forms at least a portion of an outer perimeter of the rotor. At least a portion of the rotor core may overlap at least a portion of the outer permanent magnet. Additionally, the permanent-magnet cluster may include at least one inner permanent magnet, wherein at least a portion of the inner permanent magnet is disposed radially inward of the outer permanent magnet.  
         [0008]     A further disclosed embodiment relates to an electric machine. The electric machine may include a rotor that is rotatable about a rotor rotation axis. The rotor may include a rotor core constructed of a magnetically-permeable material. The rotor core may include a first cavity, the first cavity having a curved first end and a second end. The second end of the first cavity may be disposed closer to an outer perimeter of the rotor than the first curved end, and the first curved end may be wider than the second end. Additionally, the rotor core may include a second cavity, at least a portion of the second cavity being disposed adjacent the first curved end of the first cavity. The rotor may also include a first permanent magnet disposed in the first cavity and a second permanent magnet disposed in the second cavity.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a sectional illustration of one embodiment of an electric machine according to the present invention;  
         [0010]      FIG. 2  is an enlarged view of the portion of  FIG. 1  shown in rectangle  2  of  FIG. 1 ; and  
         [0011]      FIG. 3  is a sectional illustration of another embodiment of an electric machine according to the present disclosure. 
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  illustrates one embodiment of an electric machine  10  according to the present disclosure. Electric machine  10  may be configured to operate as an electric motor and/or an electric generator. Electric machine  10  may include a housing  12 , a stator  14 , and a rotor  16 .  
         [0013]     Housing  12  may provide support for stator  14  and rotor  16 . Rotor  16  may be supported by housing  12  in such a manner that rotor  16  may rotate about a rotor rotation axis  18 . Housing  12  may support stator  14  in a stationary position adjacent rotor  16 . As  FIG. 1  shows, in some embodiments, stator  14  may extend around rotor rotation axis  18  and rotor  16 , with an annular air gap  66  between an outer perimeter  30  of rotor  16  and stator  14 .  
         [0014]     Stator  14  may include windings of an electrical conductor (not shown), such as wire. Such windings of an electrical conductor may be operable to receive electricity from an electrical power source to produce a rotating magnetic field adjacent rotor  16 .  
         [0015]     Rotor  16  may include a rotor shaft  20 , a rotor hub  22 , and a rotor core  24 . Rotor hub  22  may be constructed of a material with a relatively low permeability to magnetic flux. Rotor hub  22  may extend around rotor shaft  20  at a shaft/hub interface  23 . Rotor core  24  may be constructed of a material having a relatively high permeability to magnetic flux, such as a ferrous metal. Rotor core  24  may extend around rotor hub  22  at a hub/core interface  25 .  
         [0016]     Rotor  16  may also include permanent magnets mounted to rotor core  24 , and some or all of these permanent magnets may be arranged in permanent-magnet clusters  26 ,  28 . Permanent-magnet clusters  26  and permanent-magnet clusters  28  may be arranged in alternating positions around outer perimeter  30  of rotor  16 . As will be described in greater detail below, permanent-magnet clusters  26  may create north magnetic poles of rotor  16 , and permanent-magnet clusters  28  may create south magnetic poles of rotor  16 .  
         [0017]      FIG. 2  shows a pair of permanent-magnet clusters  26 ,  28  in greater detail. Permanent-magnet clusters  26  may include permanent magnets  32 - 35  each of which may be disposed in one of cavities  40 - 43  in rotor core  24 . Permanent-magnet cluster  28  may include permanent magnets  36 - 39  disposed in cavities  44 - 47 .  
         [0018]     Permanent magnets  32 - 35  and permanent magnets  36 - 39  may form multiple permanent-magnet layers  48 - 50  and  51 - 53  of permanent-magnet cluster  26  and permanent-magnet cluster  28 , respectively. As used herein, the term permanent-magnet layer refers to multiple permanent magnets arranged generally end-to-end or a single permanent magnet that is not arranged end-to-end with other permanent magnets. An inner permanent-magnet layer  50  of permanent-magnet cluster  26  may include permanent magnets  34 ,  35 . Ends  58  of permanent magnets  35  and cavities  43  may be disposed adjacent opposite ends  63  of permanent magnet  34  and cavity  42 . Between each end  63  of permanent magnet  34  and an adjacent end  58  of one of permanent magnets  35 , a portion  64  of rotor core  24  may extend in a direction  67 . From ends  58 , permanent magnets  35  and cavities  43  may extend away from one another as they extend to ends  59  disposed adjacent portions  80 ,  82  of outer perimeter  30 . Between each end  59  of one of permanent magnets  35  and outer perimeter  30  of rotor  16 , a portion  65  of rotor core  24  may extend in a direction  69 . An inner permanent-magnet layer  53  of permanent-magnet cluster  28 , may include permanent magnets  38 ,  39  arranged similar to permanent magnets  34 ,  35 .  
         [0019]     Permanent magnets  33 ,  37  may form intermediate permanent magnet-layers  49 ,  52  of permanent-magnet clusters  26 ,  28 , respectively. Intermediate permanent-magnet layer  49  may be configured similar to inner permanent-magnet layer  50 , and intermediate permanent-magnet layer  49  may be disposed radially outward of inner permanent-magnet layer  50 . Intermediate permanent-magnet layer  52  may be similarly configured and arranged within permanent-magnet cluster  28 .  
         [0020]     Permanent magnets  32 ,  36  may form outer permanent-magnet layers  48 ,  51  of permanent-magnet clusters  26 ,  28  respectively. Permanent magnet  32  may be disposed radially outward of at least a portion of inner permanent-magnet layer  50  and intermediate permanent-magnet layer  49 . For example, permanent magnet  32  may be disposed radially outward of permanent magnet  34  of inner permanent-magnet layer  50  and radially outward of a middle one of permanent magnets  33  of intermediate permanent-magnet layer  49 . Similarly, permanent magnet  36  may be disposed radially outward of at least a portion of inner permanent-magnet layer  53  and intermediate permanent-magnet layer  52 . Additionally, each of cavities  40 ,  44  may be open on an outer radial side of rotor  16  such that permanent magnets  32 ,  36  disposed therein may each form a portion of outer perimeter  30  of rotor  16 .  
         [0021]     Rotor core  24  may include portions  56 ,  57  that overlap end portions  60 ,  61  of permanent magnets  32 ,  36 . Within this disclosure, a portion of rotor core  24  is considered to overlap a portion of a permanent magnet if a radius of rotor  16  crosses both the portion of rotor core  24  and the portion of permanent magnet and the portion of rotor core  24  is disposed radially outside of the portion of the permanent magnet. End portions  60  of permanent magnet  32  may have end surfaces  73  that extend away from one another as they extend inward of outer perimeter  30  of rotor  16  into rotor core  24 . End portions  61  of permanent magnet  36  may have end surfaces  75  similarly configured. Portions  56 ,  57  of rotor core  24  may be disposed directly adjacent end surfaces  73 ,  75  respectively. In addition to, or in place of, portions  56 ,  57  of rotor core  24  overlapping end portions  60 ,  61  of permanent magnets  32 ,  36 , one or more portions of rotor core  24  may overlap other portions of permanent magnets  32 ,  36 , such as middle portions thereof.  
         [0022]     Each of permanent magnets  33 - 35 ,  37 - 39  may have a same shape and size as their host cavities  41 - 43 ,  44 - 47 . As is shown in  FIG. 2 , permanent magnets  33 - 35 ,  37 - 39  and cavities  41 - 43 ,  44 - 47  may have straight sides and curved ends. Permanent magnets  33 ,  34 ,  37 ,  38  may have substantially constant width. In contrast, ends  58  of each of permanent magnets  35 ,  39  and cavities  43 ,  47  may be wider than ends  59  thereof.  
         [0023]     As mentioned above, permanent-magnet cluster  26  may create a north magnetic pole of rotor  16 . Permanent magnet  32  may have its north magnetic pole directed radially outward, and permanent magnets  33 - 35  may have their north magnetic poles generally facing outer perimeter  30  of rotor  16 . Additionally, portions  62  of rotor core  24  located inside permanent-magnet cluster  26  may be magnetically isolated from other portions of rotor core  24  by inner permanent-magnet layer  50 . Because permanent magnets  34 ,  35  have a low permeability to magnetic flux, permanent magnets  34 ,  35  greatly impede magnetic flux from flowing across them to enter or exit portions  62  of rotor core  18 . Additionally, portions  64 ,  65  of rotor core  24  adjacent ends  58 ,  59  of permanent magnets  35  may be sufficiently narrow that they are highly saturated with magnetic flux from permanent magnets  34 ,  35 . When highly saturated with magnetic flux, portions  64 ,  65  of rotor core  18  also have a low permeability to magnetic flux and, therefore, greatly impede magnetic flux from flowing through them to enter or exit portions  62  of rotor core  24 . As a result, very little of the magnetic flux generated by the north magnetic poles of permanent magnets  34 ,  35  may leave permanent-magnet cluster  26  by flowing across permanent magnets  34 ,  35  or through portions  64 ,  65  of rotor core  24 . So, nearly all of the magnetic flux generated by the north magnetic poles of permanent magnets  34 ,  35  may be forced to leave permanent-magnet cluster  26  by flowing substantially radially across air gap  66 , into stator  14 .  
         [0024]     Additionally, as is mentioned above, permanent-magnet cluster  28  may create a south magnetic pole of rotor  16 . Permanent-magnet cluster  28  may be configured similar to permanent-magnet cluster  26 , except permanent magnets  36 - 39  may have their south magnetic poles, rather than their north magnetic poles, directed generally radially outward. Additionally, like inner permanent-magnet layer  50 , inner permanent-magnet layer  53  may magnetically isolate portions  68  of rotor core  24  located inside permanent-magnet cluster  28  from other portions of rotor core  24 .  
         [0025]     In addition to creating north and south magnetic poles of rotor  16 , permanent-magnet clusters  26 ,  28  may define the location of “d”axes  70 ,  71  of rotor  16 , which are radial axes along which rotor  16  has its highest reluctance. Permanent magnets  32 - 39  may greatly impede magnetic flux created by other sources, such as stator  14 , from flowing radially between outer perimeter  30  and hub/core interface  25  in portions of rotor core  24  occupied by permanent-magnet clusters  26 ,  28 . On the other hand, a portion  72  of rotor core  24  located between permanent-magnet clusters  26  and  28  may provide a path through which magnetic flux may more readily flow in radial directions. As a result, “d”axes  70 ,  71  of rotor  16  may extend through permanent-magnet clusters  26 ,  28 , and a “d”axis  74  of rotor  16 , which is a radial axis along which rotor  16  has its lowest reluctance, may extend through portion  72  of rotor core  24 . As is shown in  FIG. 2 , “d”axis  70  may extend across three permanent magnets  32 - 34 , and “d”axis  71  may also extend across three permanent magnets  36 - 38 .  
         [0026]     In some embodiments, permanent-magnet clusters  26 ,  28  may be disposed at a sufficient distance from one another sufficient to prevent permanent-magnet clusters  26 ,  28  from saturating portion  72  of rotor core  24  with magnetic flux. For example, the spacing of permanent-magnet clusters  26 ,  28  shown in  FIG. 2  prevents permanent-magnet clusters  26 ,  28  from saturating portion  72  of rotor core  24 . For purposes of this disclosure, portion  72  of rotor core  24  may be considered not saturated with magnetic flux if the magnetic flux density therein is less than approximately 2 tesla.  
         [0027]      FIG. 3  shows another embodiment of rotor  16  in electric machine  10 . In the embodiment of rotor  16  shown in  FIG. 3 , permanent-magnet clusters  26 ,  28  may include permanent magnets  76 ,  78  forming outer permanent-magnet layers  48 ,  51  in place of permanent magnets  32 ,  36  of the embodiment shown in  FIG. 2 . In contrast to permanent magnets  32 ,  36  of  FIG. 2 , permanent magnets  76 ,  78  may be disposed entirely within rotor core  24 . In other respects, the embodiment of rotor  16  shown in  FIG. 3  may be the same as the embodiment of rotor  16  shown in  FIG. 2 .  
         [0028]     Electric machine  10  is not limited to the configurations shown in  FIGS. 1-3 . For example, one or more of outer permanent-magnet layers  48 ,  51 , intermediate permanent-magnet layers  49 ,  52 , and inner permanent-magnet layers  50 ,  53  may be formed by more or less permanent magnets than shown in  FIGS. 1-3 . Additionally, one or more of permanent magnets  32 - 39  and/or cavities  40 - 47  may have different shapes. For example, one or more of permanent magnets  33  and  37  and cavities  41  and  45  may taper like permanent magnets  35 ,  39  and cavities  43 ,  47 . Furthermore, permanent-magnet clusters  26 ,  28  may omit intermediate permanent-magnet layers  49 ,  52 . Alternatively, permanent-magnet clusters  26 ,  28  may include additional permanent-magnet layers.  
       INDUSTRIAL APPLICABILITY  
       [0029]     Rotor  16  may have application in any electric machine  10  configured to operate as an electric motor and/or an electric generator. The operation of an electric machine  10  as an electric motor is described below.  
         [0030]     During operation of electric machine  10  as an electric motor, a rotating magnetic field produced by stator  14  may interact with rotor  16  and magnetic flux flowing from rotor  16  to cause a torque on rotor  16 . The higher reluctance along “d”axes  70 ,  71  than along “q”axis  74  of rotor  16  creates a tendency for rotor  16  to align itself with the rotating magnetic field created by stator  14 . This tendency is known as a reluctance torque on rotor  16 . The magnitude of the reluctance torque may be positively correlated to a difference between the reluctance of rotor  16  along “d”axes  70 ,  71  and the reluctance of rotor  16  along “q”axis  74 . Additionally, magnetic flux flowing from permanent-magnet clusters  26  of rotor  16 , through stator  14 , to permanent-magnet clusters  28 , interacts with the rotating magnetic field created by stator  14  and causes a magnet torque on rotor  16 . The magnet torque on rotor  16  is positively correlated with the quantity of magnetic flux flowing from permanent-magnet clusters  26 , through stator  14 , to permanent-magnet clusters  28 . The total torque on rotor  16  equals the sum of the reluctance torque and the magnet torque.  
         [0031]     The disclosed embodiments of rotor  16  may cause a high reluctance torque on rotor  16  when electric machine  10  is operated as an electric motor. Each permanent-magnet layer  48 - 50  may increase the reluctance of rotor  16  along “d”axis  70 , and each permanent-magnet layer  51 - 53  may increase the reluctance of rotor  16  along “d”axis  71 . Additionally, spacing permanent-magnet clusters  26 ,  28  such that they do not saturate portion  72  of rotor core  24  with magnetic flux may contribute to rotor  16  having a low reluctance along “q”axis  74 . Thus, the disclosed embodiments of rotor  16  may have a large difference between the reluctance along each “d”axis  70 ,  71  and the reluctance along “q”axis  74 , which may cause a high reluctance torque on rotor  16 .  
         [0032]     Additionally, the disclosed embodiments of rotor  16  may provide a high magnet torque. Each of permanent-magnet layers  48 - 53  increases the quantity of magnetic flux that flows from rotor  16  through stator  14 . Additionally, placing permanent magnets  32 ,  36  on outer perimeter  30  of rotor  16  may contribute to rotor  16  having a strong magnetic field. With permanent magnets  32 ,  36  so disposed, magnetic flux may flow from the north magnetic pole of permanent magnet  32 , through stator  14 , to the south magnetic pole of permanent magnet  36 , without being diminished by rotor core  24 .  
         [0033]     Additionally, the disclosed embodiments of rotor  16  may have certain structural advantages. For example, by overlapping end portions  60 ,  61  of permanent magnets  32 ,  36 , portions  56 ,  57  of rotor core  24  may prevent centrifugal forces on permanent magnets  32 ,  36  from detaching permanent magnets  32 ,  36  from rotor core  24  during rotation of rotor  16 . Additionally, portions  64 ,  65  may share the burden of counteracting centrifugal forces on portions  62  of rotor core  24 . This may limit the amount of stress that rotation of rotor  16  causes in any one of portions  64 ,  65  of rotor core  24 .  
         [0034]     Furthermore, analysis has shown that centrifugal forces on portions  62  of rotor core  24  may create higher stresses in portions  64  of rotor core  24  than in portions  65  of rotor core  24 . This may be so at least partially because directions  67  in which portions  64  extend are more radially oriented than directions  69  in which portions  65  extend, which causes a greater amount of strain in portions  64  than in portions  65  for any given amount of radial displacement of portions  62  of rotor core  24 . Making ends  58  of cavities  43  and permanent magnets  35  relatively wide may reduce the stress concentrations created by ends  58  of cavities  43 , which may reduce the relatively high stresses in portions  64  of cavities  43 . Similar benefits accrue from making ends  58  of cavities  47  relatively wide.  
         [0035]     Moreover, the disclosed embodiments may have certain cost advantages. For instance, making ends  59  of permanent magnets  35 ,  39  relatively narrow may keep the cost of permanent magnets  35 ,  39  low. Additionally, constructing permanent magnets  33 - 35 , and  37 - 39  with straight sides may keep the costs of permanent magnets  33 - 35  and  37 - 39  low.  
         [0036]     It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed electric machine  10  and rotor  16  without departing from the scope of the disclosure. Other embodiments of the disclosed electric machine  10  and rotor  16  will be apparent to those skilled in the art from consideration of the specification and practice of the electric machine  10  and rotor  16  disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.