Abstract:
An alternator rotor prevents the field coil from moving within the rotor assembly in a manner that increases the power density of the alternator. The structure maximizes the available space for the field coil and provides increased dissipation of heat to accomplish the same. Preferably, the outer diameter of the field coil is greater than or equal to the inner diameter of the first and second pole pieces for frictional engagement of the coil assembly to the first and second pole pieces. In this manner, the field coil is compressed to form depressions corresponding to the pole pieces to prevent rotation.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to alternators for automotive vehicles, and more particularly relates to rotors used in such alternators. 
     BACKGROUND OF THE INVENTION 
     Currently, the majority of all vehicles driven today use front-end accessory drive alternators that contain Lundell style rotors, also known as “claw pole” rotors. The rotor provides the alternator&#39;s 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 hub, 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. 
     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 failure including wire fatigue fractures, insulation abrasion, and bobbin insulator wear. 
     Therefore, it is critical 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. 
     Unfortunately, these locking features remove steel from the pole pieces, leading to high 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. 
     Accordingly, there exists a need to provide an alternator rotor that prevents the field coil from moving within the rotor assembly while maximizing the available space for the field coil and providing increased dissipation of heat to increase the power density of the alternator. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides an alternator rotor that prevents the field coil from moving within the rotor assembly in a manner that increases the power density of the alternator. The structure maximizes the available space for the field coil and provides increased dissipation of heat to accomplish the same. Briefly, the outer diameter of the field coil is preferably greater than or equal to the inner diameter of the first and second pole pieces for frictional engagement of the coil assembly to the first and second pole pieces. This prevents rotation of the field coil. Stated another way, the field coil defines depressions corresponding to pole fingers of the first and second pole pieces. The pole fingers are positioned within the depressions to prevent rotation of the of the coil assembly relative to the first and second pole pieces. 
     Preferably, the first and second pole pieces compress the field coil of the coil assembly. The field coil may be compressed radially and/or axially by the first and second pole pieces. Generally, the first and second poles include pole fingers and a pole hub, the pole fingers including a first portion extending radially from the pole hub and a second portion extending axially relative to the pole hub. The first portion and/or the second portion of the pole fingers may be positioned within the depressions, thereby preventing rotation. 
     The lack of any extra locking features and the compression of the field coil increases the amount of copper wire within the rotor and improves heat transfer from the field coil to the pole pieces. The improved heat transfer is a result of both increased contact pressure and increased contact area between the coil assembly and the pole pieces. All of these features improve power density of the alternator, while at the same time securely connecting the field coil to the pole pieces to prevent unwanted movement of the field coil within the rotor assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings: 
     FIG. 1 is a perspective view of an embodiment of a rotor constructed in accordance with the teachings of the present invention; 
     FIG. 2 is an exploded view showing the construction. of the rotor depicted in FIG. 1; 
     FIG. 3 is a perspective view of the coil assembly; 
     FIG. 4 is a perspective view of the coil assembly of FIG. 3 fitted on a pole piece; and 
     FIG. 5 is an enlarged front view of the rotor depicted in FIG.  1 . 
    
    
     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the figures, FIGS. 1 and 2 show a rotor assembly or rotor  20  constructed in accordance with the teachings of the invention. Generally, the rotor  20  includes a shaft  22  defining a central axis passing through the center of the rotor 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 plurality of pole fingers  27 ,  29 , respectively, which are equidistantly spaced about the periphery of the poles  26 ,  28 . The shaft  22  is inserted through the center of the pole pieces  26 ,  28 , and a stop washer  21  and retaining ring  23  are used in combination to connect the pole pieces  26 ,  28  to the shaft  22 , although other connection mechanisms may be used as is known in the art. 
     The poles  26 ,  28  are used to encase a coil assembly  25  therebetween. The coil assembly  25  is best seen in FIGS. 3 and 4. 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 . The leads  32  include a wire  31  (shown in FIGS. 2 and 3) which is covered with insulation  33  to prevent electrical shorts from the leads  32  to the steel pole pieces  26 ,  28 . 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 shaft  22  within the alternator. 
     The bobbin  50  is utilized to electrically insulate the field coil  30  from the pole pieces  26 ,  28 . A single piece construction may be employed, although a multiple piece construction is preferred. The construction of the bobbin  50  will not be described in detail here, however one exemplary bobbin is described in co-pending U.S. application Ser. No. 10/264,778 filed Oct. 4, 2002, the disclosure of which is hereby incorporated by reference in its entirety. Suffice it to say, and with reference to FIGS. 3 and 4, the bobbin  50  includes a first end cap  52  and a second end cap  54  attached to opposing ends of a cylinder  60 . Each end cap  52 ,  54  includes a plurality of flaps  53 ,  55  which project radially outwardly and are equidistantly spaced about the end caps  52 ,  54 . The flaps  53 ,  55  are numbered, sized, and structured to correspond with the fingers  27 ,  29  of the pole pieces  26 ,  28 . Each of the plurality of flaps  53 ,  55  are folded over the outer surface of the field coil  30 , and a layer of tape  40  is applied, as shown in FIG.  3 . At least one of the end caps  52 ,  54  includes a small slot  36  for guiding the leads  32  of the field coil  30 . 
     The coil assembly  25  is pressed onto the center hub  46  of one pole, such as pole  26  as shown in FIG.  4 . The opposing pole  28  and its hub (not shown) is then pressed onto the opposing end of the coil assembly  25  such that the faces of each pole hub  46  coming 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, as shown in FIG.  1 . It will be recognized that for some rotors, the hub may be a separate piece and not integrally formed with the pole pieces, and hence the coil assembly  25  will be attached to the hub in a manner dictated by the particular rotor structure. 
     In accordance with the present invention, the coil assembly  25  and pole pieces  26 ,  28  are structured to prevent unwanted motion of the coil assembly  25  within the rotor  20 . First, a calculation is made of the cross sectional area of the pocket that will contain the field coil  30  on the rotor  20 . More particularly, and as best seen in FIG. 4, the fingers  27 ,  29  each include a first portion  61  extending radially away from the pole hub  46 . The fingers  27 ,  29  further include a second portion  62  extending axially from the first portion  61  and relative to the pole hub  46 . The second portions  62  have an inner surface  64  which defines an inner diameter of each pole piece  26 ,  28 . This inner diameter defined by the fingers  27 ,  29 , in relation to the hub  46 , determines the size, including area, of the pocket that will contain the field coil  30 . As shown in the figures, the inner diameter varies from a minimum to a maximum value since the inner surfaces  64  are sloped radially as the fingers  27 ,  29  extend axially relative to the hub  46 . 
     Once the inner diameter and area of the pocket is obtained, the area of coil wire is determined so that the outer diameter of the field coil  30  will be greater than or equal to the inner diameter of the first and second pole pieces  26 ,  28 . In addition, the field coil area may also be larger than the pocket formed by the first and second poles pieces. The excess coil area, i.e. the portion of the coil that would extend outside the pocket area, is displaced both axially and radially by the pole pieces  26 ,  28 . This provides frictional engagement between the coil assembly  30  and the first and second pole pieces  26 ,  28  that is sufficient to prevent rotation of the field coil  25 . Preferably, the pole fingers  27 ,  29  will significantly deform the field coil  30  during assembly. That is, the field coil  30  is compressed, forcing the coil to generally form a zigzag pattern around the pole fingers  27 ,  29 . When the inner surfaces  64  of the fingers  27 ,  29  are sloped as depicted, the outer diameter of the field coil  30  is set greater than or equal to smallest inner diameter of the first and second pole pieces  26 ,  28 . In accordance with these principles, the field coil  30  is wound around a bobbin  50  to a specified outer diameter or coil area. The bobbin flaps  53 ,  55  are folded over the field coil  30  and taped on the outside diameter by tape  40 , as shown in FIG.  3 . As is known, the tape  40  can be eliminated. 
     As best seen in the enlarged view of FIG. 5 (with the tape  40  removed), the coil assembly  25  is then assembled between two pole pieces  26 ,  28  by applying an appropriate force necessary on each pole  26 ,  28  to deform the field coil  30  around the pole fingers  27 ,  29  and bring the pole hubs  46  in contact with each other. The field coil  30  is deformed into a zigzag pattern around each pole finger  27 ,  29  in both an axial and a radial direction. Stated another way, the field coil  30  defines depressions  66  that are sized and structured to correspond to the fingers  27 ,  29  of the pole pieces  26 ,  28 . The depressions extend both axially and radially to correspond with the first portions  61  and second portions  62  of the fingers  27 ,  29 . The pole fingers  27 ,  29  of the first and second pole pieces  26 ,  28  are positioned within the depressions  66  to prevent rotation of the of the coil assembly  25  and field coil  30  relative to the first and second pole pieces  26 ,  28 . 
     As previously indicated, the shaft  22  is inserted through the two pole pieces  26 ,  28  while they are held together to prevent them from separating due to the resilient force of the deformed field coil  30 . If necessary, a metal stake (not shown) can be employed to retain the pole pieces  26 ,  28  in the correct position on the shaft  22 . The stake can move metal from the surface of the pole  26 ,  28  surrounding the shaft  22  into grooves machined in the shaft  22 . 
     The zigzag pattern around the pole fingers  27 ,  29  locks the field coil  30  together to the poles  26 ,  28 , preventing unwanted rotation of the coil assembly  25 . In addition, the high-pressure contact and increased contact area between the field coil  30  (on its outside diameter and end surfaces) and the pole fingers  27 ,  29 , enhances the heat transfer from the coil  30  to the pole pieces  26 ,  28 . The cooler field coil  30  has a lower electrical resistance leading to increased field current that results in increased magnetic field strength and power density of the alternator. Further, compression of the field coil  30  allows more wire to be wound into the coil, further increasing power density. At the same time, the coil assembly  25  and field coil  30  are securely attached to the pole pieces  26 ,  28 , thereby preventing unwanted rotation. 
     The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Numerous modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.