Abstract:
A component to generate a magnetic field and a method of designing windings, comprised of a plurality of strands, within each slot of the component, are described. The component includes a plurality of slots. The component also includes sets of a plurality of strands that form windings, each set of the plurality of strands being enclosed in a respective one of the plurality of slots, each set of the plurality of strands being divided into a plurality of a collection of strands configured to be twisted over a portion of a length of the respective one of the plurality of slots. Each of the collection of strands changes cross sectional shape over the portion.

Description:
STATEMENT OF FEDERAL SUPPORT 
       [0001]    This invention was made with Government support under contract number DE-AR0000308 awarded by the Department of Energy. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Exemplary embodiments pertain to the art of devices using magnetic field generation. 
         [0003]    Magnetic field generation is part of many systems. For example, various systems include a rotor rotating within a stationary stator to interact with or produce a rotating magnetic field. Exemplary ones of these electromagnetic devices or systems include electric machines (motors, generators, brakes, actuators), transformers, electromagnetic coils, and inductors. The stator comprises windings, which often comprise smaller strands. The use of small strands is intended to reduce the impact of skin and proximity effects that together effectively limit current to only a portion of a conductor carrying alternating current in the presence of external magnetic fields. These combined detrimental effects lead to what is commonly referred to as AC winding losses. Having many strands, each of which has relatively small cross-sectional area, and appropriately twisting these strands, maximizes the effective current-carrying cross section and reduces localized current density for a given total winding current and effectively reduces the AC winding losses and thereby increases machine efficiency. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    Disclosed is a component to generate a magnetic field including a plurality of slots; and sets of a plurality of strands that form windings, each set of the plurality of strands being enclosed in a respective one of the plurality of slots, each set of the plurality of strands being divided into a plurality of a collection of strands configured to be twisted over a portion of a length of the respective one of the plurality of slots, wherein each of the collection of strands is configured to change cross sectional shape over the portion. 
         [0005]    Also disclosed is a method of designing strand layout of windings, comprised of a plurality of strands, within each slot of a component configured to generate a magnetic field. The method includes twisting two or more of the plurality of strands over a portion of a length of the respective slot; and changing a cross sectional shape of the two or more of the plurality of strands over the portion of the length. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0007]      FIG. 1  illustrates the stator  110  and rotor  120  of an electric motor; 
           [0008]      FIG. 2  illustrates strands according to one embodiment of the invention; 
           [0009]      FIG. 3  illustrates strand layout according to one embodiment of the invention; 
           [0010]      FIG. 4  illustrates strand layout according to another embodiment of the invention; 
           [0011]      FIG. 5  illustrates strand layout according to another embodiment of the invention; 
           [0012]      FIG. 6  illustrates strand layout according to another embodiment of the invention; 
           [0013]      FIG. 7  illustrates strand layout according to another embodiment of the invention; 
           [0014]      FIG. 8  illustrates strand layout according to another embodiment of the invention; 
           [0015]      FIG. 9  illustrates an exemplary strand layout along a stator slot according to embodiments of the invention; and 
           [0016]      FIG. 10  is a process flow of a method of designing strand layout of a stator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
         [0018]    At noted above, an electric machine or other electromagnetic device includes a stator with windings of strands. The windings may be comprised of copper or other conducting material (e.g., aluminum, copper clad aluminum) wound around a core, in the case of electrical machines with a core, or in a support structure, in the case of a coreless machine). Each group of strands forming a winding passes through a slot, which may be supported by the core for mechanical support, for example. The performance of an electric machine is based in large part on the total current through the windings and the magnetic reluctance of the flux path through stator and rotor. The reluctance is driven by path length and cross section and can be reduced by increasing the current density. Increased current density requires an increased conductor fill factor within the slot. Slot fill factor is maximized when all of the space in a slot is filled either with useful thickness of conductor or the minimum amount of insulator needed to isolate adjacent windings. Another consideration is higher efficiency of the electric machine, which requires lower copper and core losses. Copper losses include direct current (DC) copper losses which are proportional to a square of the current and resistance, and alternating current (AC) copper losses produced due to skin and proximity effects. The AC copper losses can reach magnitudes that are  8  to  10  times or even higher (depending on the excitation frequency and strand cross sectional area and other associated factors) the magnitude of DC copper losses. In order to achieve reduced AC winding copper losses, several insulated copper strands are twisted together about an axis. 
         [0019]    Embodiments of the systems and methods described herein relate to increased fill factor within each slot (increased current density) as well as increased control over twisting of the strands in order to reduce AC winding copper loss. Specifically, the embodiments relate to changing the cross sectional shape of the individual strands during a twist such that an overall (polygonal) shape for a given set of strands can be maintained and the polygonal shapes may be selected to fill the slot space. This ability to change the cross sectional shape of the strands is facilitated by one of several different techniques according to several embodiments. Several types of additive manufacturing technologies (e.g., powder deposition, powder bed fusion) may be used, for example. As another example, casting may also be used to cast the cross sectional shapes on each side of a twist. According to an extrusion technique, each portion of a strand with a uniform cross sectional shape (e.g., circular) on each side of a twist may be pushed through a different mold to reshape it. According to a molding or cold working technique, each portion of a strand with a uniform cross sectional shape on each side of a twist may have a mold shaped around it to reshape it differently. The embodiments described below are not limited to any particular technique for shaping the cross sectional shape of the strand. 
         [0020]      FIG. 1  depicts the stator  110  and rotor  120  of an electric machine  100 . As noted above, the electric machine  100  is an electromagnetic device such as, for example, an electric motor, a generator, brake, or an actuator. While a stator  110  and rotor  120  are specifically illustrated and discussed, the strand layout embodiments discussed herein relate, as well, to other components that generate a magnetic field. The stator  110  includes slots  115  that run along the length of the electric machine  100  and support windings of conductor material that are formed from strands  117 . The slots  115  traverse the length of the electric machine  100  and follow a path that may or may not be straight according to various embodiments. For example, the slots  115  may be skewed to achieve lower torque ripple. The slots  115  may follow helical paths. The stator  110  and rotor  120  may be conical sections. The approach also applies to radial slot designs. The embodiments discussed below are not limited based on an alignment of the slot  115  with the stator  110 . A cross section of the stator  110  is shown in  FIG. 1 . The strands  117  whose cross section is shown in  FIG. 1  have a circular cross sectional shape that does not change over the length of the strand  117 . That is, regardless of where along the strand  117  the cross section was taken, the shape would look like the strand  117  cross sectional shape shown in  FIG. 1 . Each slot  115  includes slot insulation  119  and each strand may be surrounded by conductor insulation  118 . The slot  115  may have a different cross sectional shape than the exemplary one shown in  FIG. 1 . A group of strands  117  (e.g., five strands  117 ) within the slot  115  is twisted together to reduce AC winding loss. As noted, the cross sectional shape of the conventional strands  117  does not change such that the shape before the twist is substantially the same (barring slight deformation due to the twisting) after the twist. As a result, as  FIG. 1  illustrates, even if the strands  117  were packed in more tightly, free space is needed within the slot  115  to accommodate the strands  117  and the twist. The embodiments detailed below relate to strands  210  ( FIG. 2 ) whose cross sectional shape changes, as needed, such that a group of strands  210  that is twisted together occupy the same space within the slot  115  before and after the twist. As detailed below, this facilitates a high fill factor within the slot  115 . While the discussion below of the exemplary embodiments shown in  FIGS. 3-8  relates to a specific degree of twist of the strands  117 , the embodiments are not limited to any particular value. That is, for example, the twist of the strands  117  shown and discussed below for  FIG. 3  is  45  degrees but could be any other value. The degree of twist may be based on design requirements or the impact of the twist on AC winding losses. 
         [0021]      FIG. 2  illustrates strands  210  according to one embodiment of the invention. A slot  115  is shown with collections  205  of strands  210  that are twisted together. A grouping of strands  210  that is twisted together (a coil) is referred to herein as a collection  205  for explanatory purposes. In the embodiment shown in  FIG. 2 , each collection  205  includes two strands  210 , and the polygon formed by each collection  205  is a rectangle. Each of the collection  205  of strands  210  may be identical or, in alternate embodiments, could have a different size and shape for the polygon formed by each collection  205  and the number of strands  210  in each collection  205 . The polygon shape is not limited to a rectangle but could instead be another shape (e.g., triangle, hexagon) and could have rounded corners (see e.g.,  FIG. 5 ) or rounded faces. As a result, it should be clear that a polygon size and shape for each collection  205  of strands  210  could be chosen to fill the slot  115  with little or no gaps. This is facilitated because the polygon size and shape of the collection  205  may be maintained even as the strands  210  that make up the collection  205  are twisted, based on the fact that each strand  210  may change cross sectional shape. This is illustrated with reference to  FIGS. 3-8  below. 
         [0022]      FIG. 3  illustrates strand layout according to one embodiment of the invention. The strand layout or winding design refers to the formation and arrangement of the strands  210  that make up the windings. The exemplary collection  205  of two strands  210  shown in  FIG. 2  is used to illustrate a twist. Four cross sectional images (A, B, C, D) of the same collection  205  of the two strands  210  are shown at four places along the length of the strands  210 . The four different images A, B, C, D show the cross sectional shape of the strands  210  during a twist. That is, a twist of some number of strands  210  involves or occurs over a portion of those strands  210  rather than at one particular point, and the images A, B, C, D show the cross sectional shape of the strands  210  within that portion. Initially, as shown in image A, the cross sectional shape of the two strands  210  is rectangular. As indicated by image B, the cross sectional shape of the collection  205  (polygon formed by or enclosing the two strands  210 ) at a subsequent place along the length of the strands  210  is the same but the cross sectional shape of each of the strands  210  is now non-rectangular. In image C, the same is true. The collection  205  has the same cross sectional shape but the two strands  210  have a different cross sectional shape than at the other place along the strands  210  shown in image B. In image D, taken at a spot along the strands  210  after the twist (at a place where the strands  210  are twisted or displaced  45  degrees from their original orientation shown in image A), the same is true again. The cross sectional shape of the strands  210  has changed yet again but the cross sectional shape of the polygon representing the collection  205  of the strands  210  is unchanged. 
         [0023]    In the embodiment shown in  FIG. 3 , the area of the cross section of each of the two strands  210  remains the same at each of the places along the strands  210  (shown in images A, B, C, and D) even though the cross sectional shape changes at each of those places. As noted above, the twist is used to reduce AC winding losses. Based on using one of the several exemplary manufacturing techniques noted above or other techniques, the strands  210  may be fabricated, as shown in images A, B, C, D, such that the degree of twist is strictly controlled while maintaining the cross sectional shape of the polygon enclosing the collection  205 . Consequently, a high fill factor can be maintained in the slot  115 . 
         [0024]      FIG. 4  illustrates strand layout according to another embodiment of the invention. In the embodiment shown in  FIG. 4 , each collection  205  includes three strands  210 . Each of the cross sectional images A, B, C, D, E shows that the size and shape of the polygon formed by the collection  205  of strands  210  remains unchanged over the five different points along the length of the strands  210  shown by the five cross sectional images A, B, C, D, E. However, the cross sectional shape of each of the strands  210  in the collection  205  changes among the images A, B, C, D, E. Thus, the slot  115  could be filled up with collections  205  of the three strands  210  and the cross sectional size and shape of each collection  205  would remain unchanged over the length of a twist made to reduce AC winding losses. The total twist shown by the images A, B, C, D, E is a 90 degree twist. 
         [0025]      FIG. 5  illustrates strand layout according to another embodiment of the invention. In the embodiment shown in  FIG. 5 , each collection  205  includes four strands  210 . Like  FIGS. 3 and 4 ,  FIG. 5  shows different cross sectional images A, B, C, D corresponding with different points along the length of the strands  210 . The different points correspond with an area of the strands  210  undergoing a twist to reduce AC winding losses. The polygonal cross sectional shape of the collection  205  includes rounded corners according to the exemplary embodiment shown in  FIG. 5 . As noted above, the cross sectional shape of the collection  205  may be any polygon (e.g., rectangle, triangle, hexagon) with straight or rounded corners and straight or rounded faces. In addition, while the lines  510  delineating the strands  210  are shown as straight lines in  FIGS. 3-8 , the lines  510  may be curved lines in alternate embodiments. Whether straight or curved, the lines  510  are positioned to ensure that the cross sectional area of each strand  210  within the collection  205  is the same according to one embodiment. According to alternate embodiments, the cross sectional areas of the strands  210  may be different or may be maintained within a defined range. A defined range for the cross sectional areas of the strands  210  (with regard to the current embodiment and other embodiments discussed below) may be determined based on, for example, current density, localized heat loads, capacity for heat removal, temperatures of the materials involved, and ambient conditions. 
         [0026]      FIG. 6  illustrates strand layout according to another embodiment of the invention. In the embodiment shown in  FIG. 6 , each collection  205  includes five strands  210 . Like  FIGS. 3 ,  4 , and  5 ,  FIG. 6  shows different cross sectional images A, B, C, D corresponding with different points along the length of the strands  210 . The different points correspond with an area of the strands  210  undergoing a twist to reduce AC winding losses. As the position of line  610   a  in image A versus in image D indicates, the strands  210  shown in  FIG. 6  undergo a twist of greater than 45 degrees but less than 90 degrees. The twist can be precisely controlled based on the formation of the strands  210 . 
         [0027]      FIG. 7  illustrates strand layout according to another embodiment of the invention. In the embodiment shown in  FIG. 7 , each collection  205  includes sixteen strands  210 . The collection  205  can be thought of as four sub-collections  710 , each including four strands  210 . The line  720  is one of the two lines delineating the four sub-collections  710 , and the line  730  is one of the lines delineating strands  210  within the four sub-collections  710 . A twist may be done of sets of four strands  210  within a sub-collection  710 , of all sixteen strands  210  of the collection  205 , or both.  FIG. 7  illustrates a twist of all sixteen strands  210  together in cross sectional images A, B, C, D. That is, the positions of lines  720  and  730  in image A compared to the positions of lines  720  and  730  in image D indicate that the lines have not moved relative to each other (no twist of the sub-collection  710  within the collection  205 ). Instead, the entire arrangement has twisted  45  degrees. The arrangement shown in  FIG. 7  maintains the lines  720  and  730  to illustrate the relative movement of the strands  210 . However, in reality, the lines  720  and  730  may be curved or the center of the arrangement may be moved to maintain the cross sectional area of each of the strands  210  to be constant or within a defined range, for example. The twist-within-a-twist configuration is possible for other sub-collections  710  of strands  210 , as well. Any non-prime number of strands  210  may be subdivided into sub-collections  710  of a collection  205 . For example, a collection  205  may include nine strands subdivided into set sub-collections  710  of three strands  210  each. As another example, twelve strands in a collection  205  may be subdivided into four sub-collections  710  of three strands  210  each or into three sub-collections  710  of four strands  210  each. When the strands  210  of the sub-collections  710  are further twisted relative to the collection  205 , the additional twist may further reduce AC winding losses. A caveat with regard to performing a twist of strands  210  to reduce AC winding losses is that increasing an effective length of the strands based on a large degree of twist can have the effect of increasing DC winding loss. Thus, the two considerations (AC winding loss and DC winding loss) must be balanced. 
         [0028]      FIG. 8  illustrates strand layout according to another embodiment of the invention. A cross section of six strands  210  is shown at nine different positions along the length of the strands  210  in nine images A through I.  FIG. 8  illustrates an interchanging of windings. That is, in a first twist, shown in images A through E, four (bottom) strands  210  are twisted by  90  degrees. In a second twist, shown in images E through I, four (top) strands  210  are twisted by  90  degrees also. Thus, strand  210   a  twists  90  degrees, along with three other (bottom) strands  210 , from the position shown in image A to the position shown in image E according to the first twist. Then, strand  210   a  twists another  90  degrees, along with three other (top) strands  210 , from the position shown in image E to the position shown in image I according to the second twist. In this case, strand  210   a  returns to its original position (shown in image A), relative to the other strands  210  shown in  FIG. 8 . While  FIG. 8  illustrates an exemplary embodiment, the interchanging shown by  FIG. 8  is not limited by the example. A different number of strands  210  may be twisted together or a different degree of twist may be implemented in alternate embodiments. 
         [0029]      FIG. 9  illustrates an exemplary strand layout along a stator  110  slot  115  according to embodiments of the invention. Four different cross sectional images A, B, C, D of four strands  210  that are twisted are shown in  FIG. 9 . As  FIG. 9  illustrates, the cross sectional area of the strands  210  does not change even though the cross sectional shape of the strands  210  certainly changes from one image A, B, C, D to the next. In an alternate embodiment, the cross sectional area of the strands  210  may change within a defined range. As noted above and illustrated in  FIGS. 3 ,  4 , and  6 - 8 , fewer or more than four strands  210  may be twisted. 
         [0030]      FIG. 10  is a process flow of a method of designing strand layout of a stator  110 . At block  1010 , selecting a cross sectional shape of the slots  115  of the stator  110  is not limited to any particular shape. At block  1020 , selecting a strand  210  design includes selecting a size, shape, and number of strands  210  by considering the skin effect. Minimizing or eliminating gaps in the slot  115  is another consideration, as well as conductor pathway planning for end turns of the electrical machine  100 . In addition, the strand  210  design (e.g., gap between strands, size) must be facilitated by the available manufacturing technique. Selecting a collection  205  of strands  210 , at block  1030 , may include a consideration of maintaining an equal cross sectional area for each strand  210  with a given turn or twist or maintaining cross sectional area within a predefined range for each strand  210 . Several different embodiments of collections  205  of different numbers of strands  210  are shown in  FIGS. 3-8 . These are only some examples of possible groupings of strands  210 . At block  1040 , controlling a twist of the strands  210  to reduce AC winding loss may be done in one of the exemplary ways discussed above. As noted above, the exemplary embodiments shown in  FIGS. 3-8  illustrate a particular degree of twist (e.g., 45 degrees, 90 degrees). However, those exemplary embodiments are not limited to any particular degree of twist. As illustrated in the figures discussed above, the cross sectional area of each strand  210  may be maintained while the shape of the cross section changes during a twist. A sub-collection  710  of strands  210  within a collection  205  of strands  210  may be twisted alternatively or additionally with the collection  205  (see e.g.,  FIG. 7 ). The grouping of strands  210  that are twisted may be interchanged (see e.g.,  FIG. 8 ). 
         [0031]    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.