Abstract:
Disclosed herein is an apparatus that relates to a bobbin of a dynamoelectric machine. The bobbin comprising, a central hub with radial surfaces on axial ends thereof receivable of end caps attachable to the radial surfaces. The central hub is receivable of a wire winding, and at least one radial protrusion extending radially outwardly from one of the radial surfaces is receivable of a wrapped end of the wire winding.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Currently, the majority of 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 dynamoelectric machine. The rotor includes a field coil or coil assembly made up of a number of insulated copper wires wrapped around an electrically insulated bobbin. The bobbin surrounds a steel hub, and also 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]    In such claw pole rotors, it is preferable to incorporate the steel core or hub into the pole pieces. Stated another way, each pole piece includes one half of the steel center hub, thereby forming a single face-to-face contact region. This design is preferred because by reducing the number of contact regions or surfaces, the magnetic field strength of the rotor increases, which is proportional to the amount of power the alternator can provide to the vehicle system. In these designs, the insulating bobbin needs to be sturdy in order to support winding of the field coil directly onto the bobbin, which is not supported by the hub. The bobbin of a typical alternator is therefore made relatively thick. 
         [0003]    The end caps of a traditional bobbin are made thick as well, typically in the range of 0.6 to 1.5 mm, to allow the integration of wire tie-offs in the end caps. Tying off one or both ends of the wire to the end caps maintains tension in the wound wire. The thick end caps are required to prevent the end caps from yielding under the tension in the wire. Such tension is important to minimize air gaps between adjacent layers of wound wire to thereby maximize the packing density of copper in the available volume. Another reason to keep the field coil winding tight is to prevent movement of one layer of the wire relative to other layers of the wire. Such movement, if allowed to occur, could result in failure of the insulation, due to rubbing, which could result in electrical shorting and malfunction of the machine. Additionally, having the tie-offs on the bobbin allow the wound bobbin to be removed from the winding machine without the winding unraveling. Commonality of bobbins with tie-offs integrated into the end caps has driven the design of conventional automated winding machines such that many automatically wrap one or both ends of the wire around the tie-offs in the end caps. 
         [0004]    Unfortunately, thick bobbin end caps reduce the dissipation of heat in the rotor and occupy space that could be better used for additional field coil or steel. Recent advancements in rotor design have reduced the thickness of the bobbin end caps by using thin laminated layers of material that are attached to the ends of a cylindrical portion of the bobbin by adhesives, welding or heat staking tabs that protrude through slots in the end caps for example. Such thin and flexible end caps are not rigid enough to maintain wire tension by incorporation of tie-offs integrated therein and thus such bobbins are not compatible with conventional winding machines. 
         [0005]    Accordingly, there exists a need for bobbins with thin end caps that are compatible with conventional winding machines. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    Disclosed herein is an apparatus that relates to a bobbin of a dynamoelectric machine. The bobbin comprising, a central hub with radial surfaces on axial ends thereof receivable of end caps attachable to the radial surfaces. The central hub is receivable of a wire winding, and at least one radial protrusion extending radially outwardly from one of the radial surfaces is receivable of a wrapped end of the wire winding. 
         [0007]    Further disclosed herein is a method that relates to maintaining tension in a wound wire. The method comprising, forming a bobbin without end caps, and attaching end caps to axial ends of the bobbin. Winding wire circumferentially around the bobbin between the end caps a plurality of complete rotations to form a winding, and wrapping an end of the wound wire around a radial protrusion of the bobbin to thereby maintain tension in the winding. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0009]      FIG. 1  depicts a perspective view of a field coil disclosed herein; and 
           [0010]      FIG. 2  depicts a perspective view of a bobbin disclosed herein. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Referring to  FIG. 1 , a field coil also referred to herein as a coil assembly according to an embodiment of the present invention is generally depicted at  10 . The coil assembly  10  comprises a bobbin  14  substantially shaped as a hollow cylinder to which a first end cap  18  and a second end cap  20  are attached to opposing axial ends. Wire  22  is wound around the bobbin  14  between the end caps  18 ,  20  to form a winding  24 . An optional metal sleeve, not shown, may line the inner circumferential surface of the bobbin  14  for additional structural support. 
         [0012]    Referring to  FIGS. 1 and 2 , the bobbin  14  has a central hub  26  that includes a first flange  28  at a first end with a first radial surface  29  and a second flange  30  at a second end with a second radial surface  31 , opposite the first end, that extend radially beyond a central cylindrical portion. The first end cap  18  is attached to the first radial surface  29  by adhesive bonding, ultrasonic welding, spin welding or the like. By similar methods the second end cap  20  is attached to the second radial surface  31 . Each end cap  18 ,  20  includes a plurality of flaps  34 ,  38  that project radially outwardly and are equidistantly spaced about the end caps  18 ,  20 . The flaps  34 ,  38  are numbered, sized and structured to correspond with fingers of pole pieces, not shown, of a rotor to which the coil assembly  10  is installed. Each of the end caps  18 ,  20  include a cutout center portion  42 ,  46  which is sized and shaped to correspond with an inner diameter  50  of the bobbin  14 . 
         [0013]    The end caps  18 ,  20  are made from different materials than is used to make the bobbin  14 , such as a laminate sheet structure consisting of a combination of Mylar™ and Nomex™ materials, for example. However, a number of other laminates are also acceptable, such as paper laminates or single layer sheets of Mylar or other materials, for example. In addition, stamped or molded polymer end caps may also be used. The laminate structure is preferred because of its ability to resist tearing and puncture. That is, laminates can be designed to exhibit higher tear strength than its plastic polymer counterparts. This allows the field coil  10  to be “crushed” between pole pieces with greater force, increasing the heat transfer by virtue of increased contact area and contact force. Additionally, more wire  22  can be wound into the field coil  10 , since the field coil  10  decreases in size when it is sandwiched or crushed between pole pieces. 
         [0014]    The thin, in the range of 0.1 to 0.5 mm, flexible nature of the end caps  18 ,  20 , however, prevents using the end caps  18 ,  20  as anchor points to which the wire  22  may be fastened to maintain tension in the winding  24 . In an embodiment of the present invention, therefore, the bobbin  14  includes a tie-off  54  that extends both axially and radially from the first flange  28 . By extending axially the tie-off  54  does not encroach into the volume that the winding  24  occupies and it allows the tie-off  54  to fit through an opening, not shown, in the second end cap  20 . By extending radially, the tie-off  54  forms a protrusion around which a wire end  56  can be wound upon completion of the winding process. The rigidity of the bobbin  14  provides adequate stability for the tie-off  54  to maintain tension in the winding  24  after completion of the winding operation. The tie-off  54  has a head  58 , which has a larger cross sectional area than a central portion  62  to prevent the wire  22 , wound around the tie-off  54 , from inadvertently slipping off of the tie-off  54 . 
         [0015]    The tie-off  54  can be injection molded as an integral part of the bobbin  14  itself or may be a separate component that is attached by means of welding, adhesive bonding, etc. Integrally molding the tie-off  54  as part of the bobbin  14  eliminates additional processing steps for forming and attaching a separate tie-off  54  component and may therefore be advantageous for economic reasons. 
         [0016]    The presence of the tie-off  54  permits the automatic conventional winding machine to wrap an end of the wire  22  around the tie-off  54  for a few rotations prior to the wire  22  being cut. The elasticity and memory of the wire  22  prevents the end from unwrapping from the tie-off  54  and thereby maintains the tension of the winding  24  around the hub  26  of the bobbin  14 . 
         [0017]    A first end  66  of the wire  22  is automatically positioned, by the winding machine, in an axial orientation relative to the bobbin  14 , through a slit  70  and into a hole  74  formed in the second end cap  20 , prior to the beginning of the winding process. The hole  74  is located radially in line with an outer diameter  78  of the second flange  30  to properly position the wire  22  for initiation of the winding process. The winding machine holds the first end  66  fixed relative to the first end cap  20  and hub  26  throughout the winding process. Upon completion of the winding process the first end  66  is sandwiched between the hub  26  and the outer layers of the winding  24  thereby frictionally locking the first end  66  to the bobbin  14 . 
         [0018]    Alternately, the first end  66  could be wrapped around a second tie-off, not shown, that could be attached to either of the end caps  18 ,  20 , to maintain tension in the winding  24  from the beginning of the winding process while not deviating from the spirit and scope of the present invention. 
         [0019]    After the wire  22  has been wound onto the bobbin  14  and anchored around the tie-off  54 , the flaps  34 ,  38  of the end caps  18 ,  20  can be folded down over the field coil  10 , and then held in place with tape, not shown. By connecting the end caps  18 ,  20  together via the flaps  34 ,  38 , by way of the tape, the winding  24  is prevented from overcoming the thin end cap sidewalls causing bulging or sagging of the coil  10 . Alternatively, the tape could be replaced with an adhesive, mechanically interlocking flaps  34 ,  38  or other fastening means to secure the flaps  34 ,  38  of opposing end caps  18 ,  20  together. The tape, however, could also be eliminated in applications that can afford to allow the coil to sag before assembly. 
         [0020]    While the invention has been described with reference to an exemplary embodiment or 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 invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.