Patent Publication Number: US-2021167654-A1

Title: Stator winding arrangement

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This specification is based upon and claims the benefit of priority from United Kingdom Patent Application Number 1917313.7, filed on 28 Nov. 2019, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE DISCLOSURE 
     The present disclosure concerns winding arrangements for electrical machine stators. 
     BACKGROUND 
     The stator of an electrical machine, which may operate as a motor and/or generator, comprises a ferritic core with a plurality of radially aligned teeth, the teeth surrounded by windings of electrically conductive wire to form the electrical phases of the machine. 
     A known manufacturing technique for electrical machine windings uses components commonly known as “hairpin windings”. In this, the coils around each stator tooth are formed from multiple hairpin-like metal bars, which are typically copper-based. Each hairpin is inserted through adjacent slots either side of a tooth, and the free ends are bent and electrically connected to form continuous windings. 
     The coils on a stator form the (usually three) phases of the electrical machine, which may be done by connecting the coils to each other in series or in parallel. Forming interconnections between coils can result in a significant amount of additional interconnecting wiring, which consumes space, adds weight and adds to electrical losses, particularly if electrical connections are required from inner radial positions to outer radial positions. 
     SUMMARY OF THE DISCLOSURE 
     According to a first aspect there is provided a coil for a tooth element of an electrical machine stator, the coil formed from a plurality of hairpin wires having pairs of legs passing through first and second slots on opposing radial faces of the tooth element, the coil having n full turns passing around the tooth element, the first and second slots each having 2n+2 slot positions having a depth for accommodating a single hairpin wire, the slots each having a first slot position at a first common end and a 2n+2 th  slot position at a second opposing end, the coil having first and second terminal connections connected to hairpin wires extending through the first and second slots at respective first and second slot positions. 
     The winding arrangement provided, formed using hairpin wires, allows for improvements relating to interconnecting coils such as reduced consumed volume, weight and losses due to input and output terminals of the coil being on a common end of a tooth. 
     The coil may be formed from n+1 hairpin wires, with n electrical connections made between legs of adjacent pairs of hairpins across the first axial face of the stator. 
     In typical examples, half of the slot positions in each slot are filled with a wire of the coil. The other half of the slot positions are thereby available to be filled with wires from adjacent coils. 
     The coil may be configured for a radial or axial flux arrangement for an electrical machine. For radial flux arrangements, the first slot position of each slot may be at a first common radial end and the 2n+2 th  slot position may be at a second opposing radial end. The first and second radial ends of each slot may be at respective outer and inner radial ends of each slot. In alternative radial flux arrangement examples the arrangement may be reversed. For axial flux arrangements, the first slot position of each slot may be at a first common axial end and the 2n+2 th  slot position at a second opposing axial end. 
     In accordance with a second aspect there is provided a stator of an electrical machine, comprising a plurality of tooth elements separated by a plurality of slots, each tooth element surrounded by a respective coil according to the first aspect. 
     Each slot position may have a width that is suitable for accommodating a single hairpin wire. In some arrangements the width of each slot position may be suitable for accommodating more than one hairpin wire, for example two wires side by side. Being able to accommodate two wires may allow for two concentric coils to be provided around a common tooth element. 
     Alternatively expressed, the second aspect may be defined as a stator of an electrical machine, the stator comprising a plurality of tooth elements separated by a plurality of slots, each tooth element surrounded by a respective coil, each coil formed from a plurality of hairpin wires having pairs of legs passing through first and second slots on opposing radial faces of the tooth element, the coil having n full turns passing around the tooth element, the first and second slots each having 2n+2 slot positions having a depth for accommodating a single hairpin wire, the slots each having a first slot position at a first common end and a 2n+2 th  slot position at a second opposing end, the coil having first and second terminal connections connected to hairpin wires extending through the first and second slots at respective first and second slot positions. 
     The coils around alternate tooth elements of the stator may be connected in series. 
     Each tooth element of the stator may comprise a plurality of teeth, for example up to three teeth for each tooth element. 
     In accordance with a third aspect there is provided an electrical machine comprising a rotor and a stator according to the second aspect. 
     The skilled person will appreciate that, except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the Figures, in which: 
         FIG. 1  is a schematic perspective view of an example helically wound coil for a stator tooth of an electrical machine; 
         FIG. 2  is a schematic perspective view of an example portion of a coil for a stator tooth of an electrical machine; 
         FIG. 3  is a schematic perspective view of an example winding arrangement of a coil; 
         FIG. 4  is a schematic side view of the winding arrangement of  FIG. 3 ; 
         FIG. 5 a    is a schematic side view of an example winding arrangement of a coil with one full turn; 
         FIG. 5 b    is a schematic side view of an example winding arrangement of a coil with two full turns; 
         FIG. 5 c    is a schematic side view of an example winding arrangement of a coil with three full turns; 
         FIG. 5 d    is a schematic side view of an example winding arrangement of a coil with four full turns; 
         FIG. 5 e    is a schematic side view of an example winding arrangement of a coil with five full turns; 
         FIG. 6  is a schematic linearized representation of a portion of a stator for an electrical machine having a plurality of coils; and 
         FIG. 7  is a schematic axial view of an example electrical machine. 
     
    
    
     DETAILED DESCRIPTION 
     Stator coils formed using hairpin windings are generally either in the form of helical or wave form windings. Both forms of windings result in the start and end of each coil being at opposing ends of the slots between adjacent teeth, one being positioned at the top of a slot and the other at the bottom. An example single helically wound coil  100  is illustrated in  FIG. 1 . The coil  100  is formed from four hairpin sections  101   a - d , the sections being bent and electrically connected, for example by welding or braising, to provide connections  102   a - c  to form a continuous helix around a tooth (not shown) from a first connection  103  to a second connection  104 . In such an arrangement, each section  101   a - d  is similar in form, with a single step radially inwards being provided by the addition of each subsequent section. This arrangement requires interconnections to be provided between the top and bottom of adjacent slots. Space at the bottom of each slot is typically more restricted due to the reduced radius, and may not be suitable for bus bars, leaving little room for additional interconnections. 
     For wave form windings, a pattern may be required that results in the correct amount of turns, coils and parallel paths only by requiring terminals far away from each other across the stator radius, requiring lengthy interconnections. 
       FIG. 2  illustrates an example of a single hairpin  200  of a coil for an electrical machine. The hairpin  200  is formed from a wire having a cross-sectional width w, a cross-sectional height h and a radial drop distance d. The view in  FIG. 2  shows the hairpin  200  after a bending operation following insertion through slots either side of one or more teeth of a stator, with end sections  201 ,  202  bent through around 90 degrees. 
     A current I passes along the wire  200  in the direction shown. Wires of this form can be used to construct a helically wound coil, an example of which is illustrated in  FIG. 3 . The coil  300  is formed in this example from three hairpins having pairs of legs that are bent and electrically connected together with electrical connections  321 ,  322 . An electrical current I passes from a first terminal connection  323  to a second terminal connection  324 , and in this example passes fully twice around one or more teeth (not shown) of an electrical machine stator. In the completed coil, a full three turns are provided by the addition of a further section passing across the front axial face of the coil between the first and second terminal connections  321 ,  322  to connect adjacent coils on either side of the coil  300 , as described in more detail below. 
     Passage of the current through the coil  300  is illustrated in  FIG. 3  and also schematically in  FIG. 4 , which shows a simplified schematic view of the coil around a stator tooth element  400  of an electrical machine from an axial face of the stator. First, second and third portions  301 ,  302 ,  303  of the coil extend through a first slot  401  adjacent a first radial face  403  of the tooth element  400 , while fourth, fifth and sixth portions  304 ,  305 ,  306  extend through a second slot  402  adjacent an opposing radial face  404  of the tooth element  400 . The direction of current is indicated as travelling into the page for the first, second and third portions  301 ,  302 ,  303 , and out of the page for the fourth, fifth and sixth portions  304 ,  305 ,  306 . The first terminal connection  323  is connected to the first portion  301 , while the second terminal connection  324  is connected to the fourth portion  304 . 
     The first, second and third portions  301 ,  302 ,  303  are arranged sequentially from outer to inner radial positions of the first slot  401 . Similarly, the fourth, fifth and sixth portions  304 ,  305 ,  306  are arranged sequentially from outer to inner radial positions of the second slot  402 . Due to this arrangement, the first and second terminal connections  323 ,  324  are both provided at an outer radial position, enabling easier and shorter connections to adjacent coils. 
     The first to sixth portions  301 - 306  of the coil  300  are connected by further portions  307 - 311 , which may be termed seventh, eighth, ninth, tenth and eleventh portions respectively. The seventh, eighth and ninth portions  307 ,  308 ,  309  extend along the back axial face of the tooth in the view in  FIG. 3 , and are indicated by dotted lines in  FIG. 4 . The tenth and eleventh portions  310 ,  311  extend along the front face of the tooth in the view in  FIG. 3 , and are indicated by solid lines in  FIG. 4 . 
     The seventh portion  307  connects the first portion  301  to the fifth portion  305 . The eighth portion  308  connects the third portion  303  to the fourth portion  304 . The ninth portion  309  connects the second portion  302  to the sixth portion  306 . The tenth portion  310  connects the fifth portion  305  to the second portion  302 . The eleventh portion  311  connects the sixth portion  306  to the third portion  303 . 
     Current flows along the coil  300  in the same sense around the tooth, which in the example shown is anti-clockwise from a view above the tooth  400 . Current flows in an inward radial direction along the seventh, tenth and eighth portions, and flows in an outward radial direction along the eleventh and ninth portions. 
     In the example shown in  FIGS. 3 and 4  the coil  300  has two full turns, but the concept can be extended to a coil having n full turns, where one full turn constitutes a complete circuit around a tooth element.  FIGS. 5 a  to 5 e    illustrate coils  501 - 505  having respectively one, two, three, four and five full turns, using a similar symbolic layout as in  FIG. 4 . The coils  501 - 505  each extend through first and second slots  521 ,  522  on opposing radial faces of a tooth element  511 . Each slot  521 ,  522  accommodates 2n+2 wires, in the form of hairpin portions extending through slots  521 ,  522 . Each of the coils  501 - 505  is formed from a total of n+1 hairpin wires, and requires n electrical connections to be made between the legs of adjacent pairs of hairpins. 
     Each slot  521 ,  522  may be considered divided into 2n+2 slot positions, each slot position having a depth d′ ( FIG. 5 c   ) that is capable of accommodating a single wire dimensioned for the slot, the depth of each slot position being measured in the radial direction. Counting from a first radial end  531  of the tooth  511  (which may be the outer radial end) to a second opposing radial end  532  of the tooth  511  (which may be inner radial end), these radial positions in each slot  521 ,  522  may be numbered from 1 to 2n+2. In  FIG. 5 a    the slot positions number from 1 to 4, and in  FIG. 5 e    the slot positions number from 1 to 12. For a single coil, any given slot position is either filled or not filled with a corresponding wire. In each case, half of the total slot positions are filled. The table below summarises the number of full turns for each coil and the corresponding number of slot positions. The table also indicates how many of these slot positions are filled with wires from the coil and the sequence of filled slot positions in alternate slots. The sequence of slot positions starts from the slot position connected to the first terminal connection  551  through to the slot position connected to the second terminal connection  552 , and alternates between a position in the first slot  521  and the second slot  522 . 
     Each slot  521 ,  522  also has a width w′ ( FIG. 5 c   ), measured in the circumferential direction. In the examples shown in  FIGS. 5 a -5 e    the width is dimensioned to accommodate a single hairpin wire. In some alternative arrangements the width may be capable of accommodating more than one hairpin wire side by side in a single slot position. This may for example allow for two coils to be provided around a common tooth element. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Summary of parameters for coils having 
               
               
                 from 1 to 5 and n full turns. 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Number of 
                 Number of 
                   
               
               
                 Number of 
                 Number of 
                 slot 
                 filled slot 
               
               
                 full turns 
                 hairpins 
                 positions 
                 positions 
                 Slot position sequence 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 2 
                 4 
                 2 
                 1, 3, 4, 2 
               
               
                 2 
                 3 
                 6 
                 3 
                 1, 3, 4, 6, 5, 2 
               
               
                 3 
                 4 
                 8 
                 4 
                 1, 3, 4, 7, 8, 6, 5, 2 
               
               
                 4 
                 5 
                 10 
                 5 
                 1, 3, 4, 7, 8, 10, 9, 6, 
               
               
                   
                   
                   
                   
                 5, 2 
               
               
                 5 
                 6 
                 12 
                 6 
                 1, 3, 4, 7, 8, 11, 12, 
               
               
                   
                   
                   
                   
                 10, 9, 6, 5, 2 
               
               
                 n 
                 n + 1 
                 2n + 2 
                 n 
                 1, 3, 4, [. . .], 2 
               
               
                   
               
            
           
         
       
     
     Wire connections across the first (or front) axial face of the stator are indicated as solid lines, while wire connections across the second (or rear) axial face of the stator are indicated as dotted lines. The dotted lines represent the top loop portion of each hairpin, and the connections  541   a - e  represent connections between legs of adjacent hairpins that have been bent inwards and joined, for example by welding or braising. The direction of current is indicated by arrows on each connection. A cross in a circle within a filled slot indicates current travelling into the page, while a circle in a circle within a filled slot indicates current travelling out of the page. The same principle will apply with the current reversed. Empty slots are indicated by crosses. 
     The number of filled slot positions is the same for the first and second slots because this allows adjacent coils to be interleaved so that all of the slot positions are filled with wires when a full set of coils is assembled, as shown further in  FIG. 6 , described below. It can be seen in each case for the coils in  FIGS. 5 a  to 5 e    that, for any given wire in a slot, there is a free slot position in the other slot. For any coil therefore, any given slot position is filled in either the first or second slots, but never in both. A consequence of this is that half of all slot positions in each slot are filled or empty. 
     In each case the number n of full turns provided by the coil equals the number of connections made across a first axial (or front) face of the stator across the tooth (or teeth)  511  around which the coil is applied. The first of these connections  541   a  is always made between the third slot position in the second slot  522  and the fourth slot position in the first slot  521 . Any subsequent connections across the first axial face of the tooth  511  are made between immediately adjacent positions on opposing slots, i.e. between positions 5 and 6, 7 and 8, 9 and 10 and 11 and 12. The direction of current through each of these connections alternates along the radial direction. In the  FIG. 5 e    example with five full turns, the current travels down the tooth from position 3 to position 4, from position 7 to position 8 and from position 11 to position 12, and travels back up the tooth from position 10 to position 9, and from position 6 to position 5. 
     For each coil  501 - 505 , first and second terminal connections  551 ,  552  are made with wire portions extending through the first and second slots  521 ,  522  at respective first and second slot positions. Typically these slot positions will be the first and second radially outermost slot positions, although in alternative arrangements the terminal connections  551 ,  552  may be arranged to be at the first and second innermost slot positions. 
     In each example, a further portion of a turn is provided once the coil is interleaved with other coils around adjacent teeth, making the total number of full turns n+1 once fully assembled, as can be seen in  FIG. 6 . 
       FIG. 6  illustrates in schematic form a portion of a stator  600  having a plurality of coils, showing three coils  601 ,  602 ,  603  of the type shown in  FIG. 5 b   , i.e. with n=2, with windings around adjacent teeth  611 ,  612 ,  613  being interwoven so that all available slot positions are filled. Coil  601  around a first tooth  611  is connected to coil  603  around a third tooth  613  with a connection  621  across a second tooth  612 . This connection  621  provides a further portion of a turn for coil  602 , making the coil  602  consist of three full turns once all the coils are assembled. This pattern may be repeated around the stator of an electrical machine, so that coils around alternate teeth, or groups of teeth, are connected in series, allowing for terminal connections to be made on the outer radius of the stator  600 . In practice, coils  601 ,  602 ,  603  will be curved to form an annular structure, the schematic representation in  FIG. 6  being a simplified linearized representation of only a portion. 
     The form of alternate coils illustrated in  FIG. 6  results in coils around alternate teeth, or sets of teeth, being connected in series. An example electrical machine  700 , comprising a stator  711  and a rotor  712  is illustrated schematically in  FIG. 7 . The stator  711  has an arrangement of coils  701 - 708  as described above in relation to  FIG. 6 , i.e. with alternate teeth, or sets of teeth, being connected in series. In alternative arrangements some or all of the coils  701 - 708  may be connected in parallel. The number of coils may be greater or fewer than in  FIG. 7 , which shows a total of eight coils to illustrate the general principle. A first set of coils is formed from coil  702  connected in series sequentially with coils  704 ,  706  and  708 . A second set of coils is formed from coil  703  connected in series sequentially with coils  705 ,  707  and  701 . The two sets of coils are connected in series, and terminal connections  713  provide connections for providing current to, or drawing current from, the coils. As can be seen, all of the connections are provided on the outer radius of the stator  711 , which enables connections to be more easily accessed and allowing connections between coils to be more simply made. 
     In each of the described above, each tooth element may comprise more than one tooth, for example to allow for multiple phases in an electrical machine. References to a tooth or tooth element may therefore encompass one or more teeth, depending on whether the coil is part of a single or multiple phase electrical machine. For multiple phase electrical machines, the number of teeth in each tooth element may typically be three for a three-phase electrical machine. 
     It will be appreciated that in the above description, the coils  300 ,  501 - 505 ,  601 - 603  and stators  600 ,  711  have been described with respect to a radial flux machine. However, the coils could be used in the stator of an axial flux electrical machine, in which the direction of the magnetic flux produced by the permanent magnets of the rotor and cut during of rotation of the rotor is aligned with the rotation axis of the machine. In such a case, the direction of winding around the stator teeth would be such that the first slot position and 2n+2 th  slot position will be at the same radial position, but at opposite axial ends of the stator. 
     It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.