Abstract:
The apparatus disclosed herein relates to a pole piece for an electric machine comprising, a pole piece with a finger support structure, a plurality of fingers having an axial component of extension relative to the finger support structure, a coil contact surface and at least one of a protrusion or a depression having an axial component of extension relative to the coil contact surface. Further disclosed is a method that relates to assembling a coil to pole pieces comprising, axially compressing a coil between coil contact surfaces of a pair of opposing pole pieces and deforming the coil axially and without radial deformation with at least one of protrusions and depressions in the coil contact surfaces of the pole pieces thereby rotationally fixing the coil to the pole pieces.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Currently, the majority of all vehicles driven today use front-end accessory drive electric machines also referred to herein as alternators or starter alternators. These electric machines are typically driven by belt and contain Lundell style rotors, also known as “claw pole” rotors. The rotor provides the electric machine with a magnetic field and rotates within the machine. The rotor includes a coil assembly having a field coil made up of an insulated copper wire or wires wrapped around an electrically insulating bobbin. The bobbin surrounds a steel core, and insulates the field coil from the steel pole pieces which sandwich the field coil to form north and south poles. The magnetic field is generated when the field coil is energized and a current flows through the wires. 
         [0002]    One problem with conventional rotors is preventing rotational movement of the field coil within the rotor assembly. The rotor is driven via a belt by the engine of the vehicle. The engine is constantly changing speeds during operation leading to accelerations and decelerations of the rotor speed. Typical vehicles experience acceleration and deceleration rates of approximately 15,000 RPM/sec with transit excursions as high as 30,000 RPM/sec. Movement of the field coil wires leads to a variety of coil failures including wire fatigue fractures, insulation abrasion, and bobbin insulator wear. 
         [0003]    Therefore, it is important in the rotor design to prevent the field coil from moving within the rotor assembly. Conventional solutions to this problem include locking features formed into the coil assembly and the pole pieces, as well as the use of epoxy fillers or other adhesives to attach the coil assembly to the pole pieces. For example, projections may be formed into the outside face of the bobbin that mate with indented features in the poles to help lock the bobbin and hence coil assembly in place. 
         [0004]    Unfortunately, these locking features remove steel from the pole pieces, leading to higher magnetic saturation in the poles and reducing power density. In addition, the thick locking protrusions created on the bobbin are made of plastic bobbin material that is a poor conductor of heat, preventing good heat transfer from the coil to the cooler poles and leading to an increase in field coil temperature. Likewise, the use of epoxy filler takes up space that could otherwise be filled by the field coil and prevents good heat transfer, both of which decrease the power density of the alternator. In sum, current methods of locking the field coil in position create unwanted performance tradeoffs. 
         [0005]    More recent advancements rely on interference between the pole pieces and the coil windings themselves to prevent rotation of the coils. U.S. Pat. No. 6,707,227 discloses such a structure, specifically, the coil interference with axially extending portions of the fingers of the pole pieces. The inner diameter formed by the axial extensions of the pole fingers contact the outer diameter of the coil assembly resulting in deformation of the coils into a zigzag shape as viewed axially and radially. Drawbacks of this design include the requirement for the coil assembly diameter to be matched closely with the inner diameter of the pole fingers, which means that many applications would have either, more wire in the coil than they need, or would require customized pole pieces to accommodate the diameters of smaller coils. Both conditions incur cost penalties, the first for the excess wire and the second for the custom tooling and lower volumes for the customized pole pieces. Additionally, in the condition with the excessive coils the extra wire in the coil adds inertia to the rotor slowing its acceleration and deceleration response times. 
         [0006]    Accordingly, there exists a need to provide alternator rotor pole pieces that prevent field coil movement, within the rotor assembly, while allowing for increased heat dissipation and improved cost efficiency of the alternator. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    The apparatus disclosed herein relates to a pole piece for an electric machine comprising, a pole piece with a finger support structure, a plurality of fingers having an axial component of extension relative to the finger support structure, a coil contact surface of the finger support structure radially inward of the plurality of fingers, and at least one of a protrusion or a depression having an axial component of extension relative to the coil contact surface. 
         [0008]    Further disclosed is a method that relates to assembling a coil to pole pieces comprising, axially compressing a coil between coil contact surfaces of finger support structures of a pair of opposing pole pieces and deforming the coil axially and without radial deformation with at least one of protrusions and depressions in the coil contact surfaces of the pole pieces thereby rotationally fixing the coil to the pole pieces. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0010]      FIG. 1  depicts a perspective view of a rotor disclosed herein; 
           [0011]      FIG. 2  depicts an exploded view of the rotor of  FIG. 1 ; 
           [0012]      FIG. 3  depicts a coil assembly from the rotor of  FIGS. 1 and 2 ; 
           [0013]      FIG. 4  depicts a partial cross sectional view of the rotor or  FIGS. 1 and 2 ; 
           [0014]      FIG. 5  depicts a partial cross sectional view of a pole piece disclosed herein; and 
           [0015]      FIG. 6  depicts a partial cross sectional view of a rotor disclosed herein. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Referring to  FIGS. 1 and 2 , a rotor assembly or rotor  20  according to embodiments of the invention is depicted. Generally, the rotor  20  includes a shaft  22  defining a central axis passing through the center of the rotor  20  and including a slip ring assembly  24  for providing power to the rotor  20 . The rotor  20  further includes a first or front pole piece  26  and a second or rear pole piece  28 . These opposing pole pieces  26 ,  28  each include a finger support structure  36 ,  38  that have a plurality of pole fingers  27 ,  29  extending therefrom, respectively. The fingers  27 ,  29  are equidistantly spaced about the periphery of the finger support structures  36 ,  38 . The opposing finger support structures  36 ,  38  also include a plurality of recesses  31 ,  33 , respectively, which are defined by the spaces, positioned substantially midway between adjacent fingers  27 ,  29 . The shaft  22  is inserted through the center of the pole pieces  26 ,  28 , and the shaft  22  is connected to the pole pieces  26 ,  28  by means of a press fit between the shaft  22  and the pole pieces  26 ,  28 , although other connection mechanisms may be used as is known in the art. For example, poles  26 ,  28  may be connected to the shaft  22  by staking the pole material into grooves on the shaft (not shown). 
         [0017]    The poles  26 ,  28  are used to encase a coil assembly  25  therebetween. Referring to  FIG. 3 , the coil assembly  25  generally includes a field coil  30  wound onto an insulating bobbin  50 . The field coil  30  includes two leads  32  which extend in a slot along the outer surface of the rear pole  28  for internal connection to the slip ring assembly  24 . When the field coil  30  is powered via the slip ring assembly  24  and leads  32 , a magnetic field is generated which flows through the pole pieces  26 ,  28 , while the entire rotor assembly  20  is rotated via the shaft  22  within the alternator. 
         [0018]    The bobbin  50  is utilized to electrically insulate the field coil  30  from the pole pieces  26 ,  28 . A single piece or a multiple piece bobbin construction may be employed. The bobbin  50  includes a first end cap  52  and a second end cap  54  attached to opposing ends of a cylinder  58 . Each end cap  52 ,  54  includes a plurality of flaps  53 ,  55 , which project radially outward and are equidistantly spaced about the end caps  52 ,  54 . The flaps  53 ,  55  are folded over the outer surface of the field coil  30 , and a layer of electrically insulating tape  40  is applied thereover. 
         [0019]    The coil assembly  25  is pressed onto an optional integrated core  46  of one pole ( FIG. 2 ), such as pole  26 . The opposing pole  28  and its optional integrated core  46  is then pressed into the opposing end of the coil assembly  25  such that the faces of each pole core  46  come in contact with each other. The shaft  22  is press-fit through bores defined in the poles  26 ,  28  keeping them in contact with each other. It will be recognized that for some rotors, the core  46  may be a separate piece and not integrally formed with the pole pieces, and hence the coil assembly  25  will be attached to the core in a manner dictated by the particular rotor structure. 
         [0020]    Referring to  FIG. 4 , a partial cross sectional view of a rotor  20 , with an embodiment of the invention, is shown. Rotor  20  is shown with pole pieces  26  and  28  abutted such that center cores  46  of both pole pieces  26 ,  28  are mated flush with one another. The nominal width  56  of the coil  30  is designed to be substantially equal to the nominal width  57  created by the pole pieces  26 ,  28 . The nominal width  57  is defined by the distance between a coil contact surface  60  of finger support structure  36  of pole piece  26  and, a coil contact surface  62  of finger support structure  38  of pole piece  28 . In accordance with an embodiment of the present invention protrusions  64  are formed on the coil contact surfaces  60 ,  62  substantially in circumferential alignment with the fingers  27  and  29 , respectively. The protrusions  64  extend axially inboard from the nominal coil contact surfaces  60 ,  62  a distance  66 . The distance  66  may be determined for each particular rotor  20  to assure that the coil  30  is adequately deformed by the protrusions  64  to rotationally fix the coil  30  to the pole pieces  26 ,  28 . Referring to  FIG. 5 , the protrusions  64  are shown extending the distance  66  beyond the nominal surfaces  60 ,  62 . The protrusions  64  may take the form of various shapes, however, care should be taken to prevent damage to the end caps  52 ,  54 , and therefore gradual transitions between the protrusions  64  and the surfaces  60 ,  62  may be desirable. 
         [0021]    Also shown in  FIGS. 4 and 5 , are depressions  74  that are formed into the coil contact surfaces  60 ,  62 . The number of depressions  74  and the spacing of them relative to one another is similar to that of the protrusions  64 . The depressions  74  are positioned substantially in alignment with the recesses  31  and  33 , which are midway between the fingers  27  and  29  respectively. However, alternate embodiments may have the depressions  74  positioned substantially in alignment with the fingers  27 ,  29 . The depressions  74  are formed a depth  76  into the coil contact surfaces  60 ,  62 . The depth  76  may be determined for each particular rotor  20  to assure that the coil  30  adequately deforms into the depressions  74  to rotationally fix the coil  30  to the pole pieces  26 ,  28 . The depressions  74  are also contoured to facilitate deformation of the coil  30  into the depressions  74  in such a manner as to maintain substantial surface contact between the coil  30  and the coil contact surfaces  60 ,  62  throughout the depressions  74 . Referring to  FIG. 5 , the shape and depth  66  of the depressions  74  relative to the coil contact surfaces  60 ,  62  are shown, as well as the shapes and extension  76  of the protrusions  64 . The depressions  74  may take the form of various shapes, however, care should be taken to prevent damage to the coil end caps  52 ,  54 , and therefore gradual transitions from the depressions  74  to the coil contact surfaces  60 ,  62  may be desirable. 
         [0022]    It should be noted that although  FIGS. 4 and 5  depict an embodiment incorporating both protrusions  64  and depressions  74 , alternate embodiments employing only the protrusions  64  or only the depressions  74  are still within the scope of the present invention. Additionally, for embodiments employing both protrusions  64  and depressions  74 , it may be desirable to make the protrusions  64  and depressions  74  with a similar shape and distance  66 ,  76 , to facilitate the deformation of the coil  30 . Stated another way, since the protrusions  64  of one pole piece  28 ,  26  are in circumferential alignment with the depressions  74  of the other pole piece  26 ,  28 , by using a similar shape and distance  66 ,  76  for both the protrusions  64  and the depressions  74 , the coil  30  may deform around one protrusion  64  and into the depression  74  positioned directly opposite of the protrusion  64 . 
         [0023]    Referring to  FIG. 6 , an alternate embodiment of the invention shows protrusions and depressions with alternate shapes. A protrusion  84  formed on surfaces  60 ,  62  includes an angle  86  relative to the surfaces  60 ,  62 . Specifically, the angle  86  results in a protrusion  84  with a variable distance from the surface  60 ,  62  such that the distance increases as the radius at which it is measured increases. Similarly, a depression  94  in surfaces  60 ,  62  is formed at an angle  96  relative to the surfaces  60 ,  62 . The angle  96  results in a depression  94  with a variable distance from the surfaces  60 ,  62  such that the distance increases as the radius at which it is measured increases. 
         [0024]    Although embodiments described herein have incorporated both protrusions  64 ,  84  and depressions  74 ,  94  simultaneously, it should be noted that incorporation of protrusions  64 ,  84  without the use of depressions  74 ,  94 , and alternately, the incorporation of depressions  74 ,  94  without the use of protrusions  64 ,  84 , is fully within the scoped and spirit of embodiments of the present invention. 
         [0025]    It should be noted that embodiments of the invention allow for the use of coil assemblies  25  of various sizes with a single set of pole pieces  26 ,  28  since rotationally fixing the coil assembly  25  to the pole pieces  26 ,  28  does not rely on contact with an inner surface  97  of an axially extending finger portion  99 . Use of a single set of pole pieces  26 ,  28  with various coil assemblies  25  allows for cost savings since customized tooling and pole pieces  26 ,  28  are not required as would be if the coil assembly  25  was to be compressed by the inner surface  97  of the axially extending finger portion  99 . Additionally, the material added to the pole pieces  26 ,  28  by the addition of the protrusions  64 ,  84  increases the strength of the fingers  27 ,  29  resulting in less finger deflection due to centrifugal force at high rotor speeds, and better heat transfer, due to the increased contact area between the poles  26 ,  28  and the coil  30 , as well as more magnetic flux carrying capacity of the pole pieces  26 ,  28 . 
         [0026]    Some embodiments of the invention may have some of the following advantages: coil assemblies that are rotationally fixed to the pole pieces, fewer durability failures due to movement of the coil within the rotor assembly, less magnetic flux saturation of the pole pieces, improved heat transfer through the pole pieces, increased interchangeability of pole pieces between various rotor assemblies, and less finger deflection during high rotor speeds. 
         [0027]    While the embodiments of the disclosed apparatus and method have been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the embodiments of the disclosed apparatus and method. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the embodiments of the disclosed apparatus and method without departing from the essential scope thereof. Therefore, it is intended that the embodiments of the disclosed apparatus and method not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the embodiments of the disclosed apparatus and method, but that the embodiments of the disclosed apparatus and method will include all embodiments falling within the scope of the appended claims.