Abstract:
A stator of an electric machine includes stator teeth. First windings are wound around first stator teeth and define a first cross section having a width that is narrower at a first radial edge of said stator teeth and that is wider at a second radial edge of said stator teeth. Second windings are wound around second stator teeth and define a second cross section. The first and second windings have an interleaved relationship.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to stators of electric machines, and more particularly to multiple winding coil shapes for stators of electric machines.  
       BACKGROUND OF THE INVENTION  
       [0002]     Reluctance electric machines, such as motors and generators, typically include a stator that is mounted inside a machine housing and a rotor that rotates relative to the stator. Reluctance electric machines produce torque as a result of the rotor tending to rotate to a position that minimizes the reluctance of the magnetic circuit (and maximizes the inductance of the stator windings). The reluctance of the rotor is minimized when a pair of diametrically-opposed rotor poles are aligned with a pair of energized and diametrically-opposed stator poles. In synchronous reluctance electric machines, the windings are energized at a controlled frequency. In switched reluctance electric machines, the angular position of the rotor is detected. A drive circuit energizes the stator windings as a function of the sensed rotor position.  
         [0003]     Conventional switched reluctance electric machines generally include a stator with a solid stator core and/or a laminated stator with a plurality of circular stator laminations. The laminations are usually punched from a magnetically conducting material and are stacked together. In inside-rotor electric machines, the stator plates define salient stator teeth that project radially inward and inter-tooth slots that are defined between adjacent stator teeth. Wire is wound to form windings about the stator teeth. The rotor rotates inside of the stator.  
         [0004]     There are several conventional methods for placing the winding wire on the stator of a switched reluctance electric machine. One conventional method is referred to as transfer winding. Transfer winding involves transferring pre-wound windings onto the stator teeth. In some instances however, interference between adjacent winding may inhibit the assembly process. Transfer winding can achieve slot fill up to around 65% when the stator is not segmented. Another conventional winding method includes needle winding. Needle winding employs a needle that winds the wire directly on the stator teeth. The needle, however, takes up some of the stator slot area and generally reduces slot fill to approximately 50% when the stator is not segmented.  
         [0005]     As can be appreciated, increasing the number of winding turns and the slot fill increases the torque density of the electric machine. The number of winding turns, however, is limited by the proximity of adjacent windings. More particularly, because the stator teeth extend radially, adjacent stator teeth are closer together at a radially-inner end. Therefore, the narrower end limits the number of winding turns at the wider or radially-outer end.  
       SUMMARY OF THE INVENTION  
       [0006]     Accordingly, the present invention provides a stator of an electric machine. The stator includes stator teeth. First windings are wound around first stator teeth and define a first cross section having a width that is narrower at a first radial edge of said stator teeth and that is wider at a second radial edge of said stator teeth. Second windings are wound around second stator teeth and define a second cross section. The first and second windings have an interleaved relationship.  
         [0007]     In one feature, the second cross section is rectangular-shaped. The first cross section is trapezoidal-shaped.  
         [0008]     In another feature, the first cross section is trapezoidal-shaped. The second cross section is trapezoidal-shaped that is inverted relative to the first cross section.  
         [0009]     In another feature, a width of the second cross section remains constant along a length of the stator teeth.  
         [0010]     In still another feature, the width of the first cross section is narrower at a radial inner edge of the stator teeth and is wider at a radial outer edge of the stator teeth.  
         [0011]     In yet another feature, the width of the second cross section is wider at a radial inner edge of the stator teeth and is narrower at a radial outer edge of the stator teeth.  
         [0012]     In still another feature, the first windings are assembled onto the first stator teeth before the second windings are assembled onto the second stator teeth.  
         [0013]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0015]      FIG. 1  is a cross-sectional view of an electric machine having a stator that includes multiple winding coil shapes according to the present invention; and  
         [0016]      FIG. 2  is a cross-sectional view of the electric machine having a stator that includes alternative multiple winding coil shapes according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements.  
         [0018]     Referring now to  FIG. 1 , an exemplary electric machine  10  is illustrated and includes a stator  12  and a rotor  14 . The stator  12  is attached to a housing  16  and the rotor  14  rotates relative to the stator  12 . The stator  12  includes a plurality of radially extending stator teeth  18  and slots  20  separating adjacent stator teeth  18 . Each stator tooth  18  has an arcuate inner edge  22  and a base  24 . The stator is preferably defined by a plurality of stator plates or laminations (not shown) that are stacked together. The stator plates are die cut from thin sheets of magnetically conductive material that are stacked and press fit together. Alternatively, the stator can be solid rather than including a plurality of laminations.  
         [0019]     The rotor  14  includes a plurality of radially extending rotor teeth  26 . A circular bore  28  is formed in the rotor  14 . A rotor shaft (not shown) is received by and fixed to the circular bore  28  of the rotor  14 . Each rotor tooth  26  has an arcuate outer edge  30 . An air gap  32  is defined between the arcuate outer edges  30  of the rotor teeth  26  and the arcuate inner edges  22  of the stator teeth  18 . In the particular embodiment shown, the rotor  14  has eight equally-spaced rotor teeth  26  and the stator  12  has twelve equally-spaced stator teeth  18 . Other rotor teeth and stator teeth combinations are also contemplated.  
         [0020]     Windings are wound about the individual stator teeth  18  and are electrically connected to windings on other stator teeth  18 . A first set of windings  34  wound around some of the stator teeth  18  have a first cross section. The first cross section includes an approximately equal number of turns along the stator tooth  18  and is generally rectangular-shaped as illustrated by dotted lines  35 . A second set of windings  36  are formed around others of the stator teeth  18  and have a second cross section. The second cross section includes an increasing number of turns along the stator tooth  18  and is generally trapezoidal-shaped as illustrated by dotted lines  37 . The number of turns increases from the radially inner arcuate edge  22  to the radially outer base  24  of the stator tooth  18 . The first and second sets of windings  34 ,  36  are interleaved around the teeth of the stator  12 . More particularly, first windings  34  having the cross section are disposed between adjacent second windings  36  having the second cross section. Likewise, the second windings  36  having the second cross section are disposed between adjacent first windings  34  having the first cross section.  
         [0021]     The interleaved placement of the first and second sets of windings  34 , 36  enables improved assembly of the electric machine  10 . When using the transfer winding method, the second windings  36  can be assembled onto every other stator tooth  18 . The first windings  34  are assembled onto the adjacent stator teeth  18 . Because the first and second windings  34 , 36  include different cross sections, winding interference is reduced and the ease of assembly of the electric machine  10  is increased. Additionally, the second cross section includes more windings toward the base  24  of the stator tooth  18 . As a result, the slot fill between adjacent stator teeth  18  is increased.  
         [0022]     Although multiple cross sections are implemented, the electromagnetic characteristics of the electric machine  10  are symmetric. More particularly, diametrically opposed stator teeth  18  have windings with the same cross section. For example, stator teeth  18  with first windings  34  are diametrically opposed to stator teeth  18  with first windings  34 . Similarly, stator teeth  18  with second windings  36  are diametrically opposed to stator teeth  18  with second windings  36 .  
         [0023]     Referring now to  FIG. 2 , alternative winding sets are illustrated. The alternative winding sets include third windings  38  interleaved between fourth windings  40  on the stator teeth  18 . The third windings  38  have a third cross section that is similar to the second cross section of the second windings  36 . The fourth windings  40  include a fourth cross section that mirrors the third cross section. More particularly, the third cross section includes an increasing number of turns along the stator tooth  18 . The number of turns increases from the radially inner arcuate edge  22  to the radially outer base  24  of the stator tooth  18  to form a generally trapezoidal profile. The fourth cross section includes a decreasing number of turns along the stator tooth  18  so the fourth cross section is trapezoidal-shaped as illustrated by dotted lines  41 . The number of turns decreases from the radially inner arcuate edge  22  to the radially outer base  24  of the stator tooth  18  profile that is inverted as compared to the generally trapezoidal profile of the third windings  38 .  
         [0024]     While the present invention is described in conjunction with switched reluctance type electric machines, the present invention may also be implemented with other types of electric machines. Such electric machines include, but are not limited to, brushless permanent magnet electric machines. It is anticipated that the multiple shape windings of the present invention can be implemented in any type of electric machine having adjacent windings disposed on teeth that extend radially inward.  
         [0025]     As can be appreciated from the foregoing, the multiple shape windings according to the invention improves the torque density of the electric machine by allowing the stator to be precisely wound with increased slot fill. More particularly, implementation of the multiple shape windings of the present invention increases the windable area between the stator teeth by up to 23%. That is to say, the multiple shape windings can account for 23% more of the area between adjacent stator teeth than traditional winding schemes. As a result, the torque output for the electric machine can be increased. Alternately, the outer dimensions of the electric machine can be reduced for a given torque output. Further, the multiple shape windings of the present invention simplify the manufacturing process. More particularly, in the case of transfer winding, assembly of adjacent windings is more easily achieved using the multiple shape windings of the present invention.  
         [0026]     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.