Abstract:
A stator for a multi-phase electric motor comprises a plurality of teeth for each phase. The windings on two teeth from one phase are formed from a single length of conductor. The windings on a first one of the teeth being formed at least in part from two sections of the conductor spaced apart along the length of the conductor, and the windings on a second one of the teeth being formed from an intermediate section of the conductor between the spaced apart sections.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to International Patent Application No. PCT/GB2007/001639 filed May 4, 2007, the disclosures of which are incorporated herein by reference in entirety, and which claimed priority to Great Britain Patent Application No. 0609045.0 filed May 6, 2006, the disclosures of which are incorporated herein by reference in entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to electric motors and in particular to the windings of stators in electric motors. 
         [0003]    In many forms of electric motor the stator has a number of phases each of which is made up of a number of teeth. Each tooth has a length of conductor wound round it for a number of turns, and the windings on the teeth making up any one of the phases are connected together electrically so that the magnetic fields generated in all of the teeth of the phase are controlled together. 
         [0004]    There are a number of ways of forming the windings in such a motor. Some methods result in balanced windings, in which the number of turns is the same for each tooth. Other methods result in unbalanced windings, in which there are different numbers of windings on different teeth. 
         [0005]    There are a number of factors which determine the most appropriate winding method for any motor, and these include whether balanced windings are required, packaging limitations due to the small amount of space into which the motor generally needs to fit, and ease of manufacture. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a stator for a multi-phase electric motor comprising a plurality of teeth for each phase, wherein the windings on two teeth from one phase are formed from a single length of conductor, the windings on a first one of the teeth being formed at least in part from two sections of the conductor spaced apart along the length of the conductor, and the windings on a second one of the teeth being formed from an intermediate section of the conductor between the spaced apart sections. 
         [0007]    The present invention further provides a method of winding a stator for a multi phase electric motor, the stator comprising a plurality of teeth for each phase, the method comprising forming the windings on two teeth of one phase from a single length of conductor, wherein the windings on a first one of the teeth are formed at least in part from two sections of the conductor spaced apart along the length of the conductor, and the windings on a second one of the teeth are formed from an intermediate section of the conductor between the spaced apart sections. 
         [0008]    Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a bottom view of a stator of a motor according to a first embodiment of the invention; 
           [0010]      FIG. 2  is a schematic representation of the winding arrangement of the stator of  FIG. 1 ; 
           [0011]      FIG. 3  is a bottom view of a stator of a motor according to a second embodiment of the invention; and 
           [0012]      FIG. 4  is a schematic representation of the winding arrangement of the stator of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Referring to  FIG. 1  an electric motor, which in this case is a synchronous, three phase, sinusoidal AC, permanent magnet, brushless motor for a power steering system, includes a stator  10  having nine teeth  12 ,  14 ,  16  three for each phase. The three teeth of each phase are equally spaced around the stator axis at 120° intervals. The rotor is a fully round design with all of the teeth fixed in position relative to each other. Although the other parts of the motor are not shown in the figures, the top of the stator as seen in the figures corresponds to the top end as installed, which will therefore be at an open end of the motor housing. The bottom of the stator as seen in the figures corresponds to the bottom end as installed and will generally be at the closed bottom end of the motor housing. 
         [0014]    Referring to  FIG. 2 , the method of winding one of the phases  12  will now be described. As the rotor is of a one-piece fully round design the winding is carried out with the teeth in their final relative positions. Appropriate automated tools for carrying out this winding are known and it will be appreciated that the skilled man can modify these to carry out this winding method. To produce a winding having n turns on each tooth, during winding of the first phase  12 , a single length of conductor  18  is used to wind all of the teeth  12   a ,  12   b ,  12   c  of that phase. The winding starts from the top end  20  of the first tooth  12   a  and the conductor is passed down one side  22  of the first tooth  12   a . It is then passed right around the first tooth for a number n−1 of complete turns, one less than the final required number n of turns per tooth, each turn passing under the bottom end  24 , up the second side  26 , across the top  28 , and back down the first side  22  of the first tooth  12   a . It is then passed beneath the bottom ends of the first tooth  12   a , and the next two teeth  14   a ,  16   a  which are in the second and third phases, before being passed through a number of complete turns round the second tooth  12   b  of the first phase. Each of these turns starts and ends at the bottom of the tooth  12   b . The conductor  18  is then passed under the second tooth  12   b  of the first phase and on under the second teeth  14   b ,  16   b  of each of the second and third phases, before being passed through a number of complete turns around the third tooth  12   c  of the first phase. Finally the conductor  18  is passed under the bottom of the third tooth  12   c  of the first phase, and under the third teeth  14   c ,  16   c  of the second and third phases back to the first tooth  12   a  of the first phase, where it is passed under the first tooth  12   a , and up the second side  26  of the first tooth. The conductor therefore extends, in a loop, completely around the stator axis, with the windings round the respective teeth spaced around the loop as the teeth are spaced around the stator  10 . 
         [0015]    The two ends  18   a ,  18   b  of the conductor are connected to respective connectors  30 ,  32  which are located near to, and in this case above, the top of the stator  10 . Since both ends  18   a ,  18   b  of the conductor  18  are close to the same tooth  12   a , the two connectors  30 ,  32  are close together. This simplifies the design of the lead frame that provides electrical power to the winding. As can be seen from  FIG. 2 , a set of adjacent teeth,  12   a ,  14   a ,  16   a , one from each phase, can be used as the starting and ending point for the windings for the phases, so that the connectors for all of the phases are adjacent to that group of adjacent teeth. Again, this helps to simplify the design of the lead frame which provides power to the windings. 
         [0016]    It will also be appreciated that the winding arrangement of this embodiment has a number of advantageous features. Each tooth except the first one  12   a  has the same number n of complete turns of conductor around it. The conductor extends down each side of each tooth, including the first, the same number of times n. The conductor  18  extends under each tooth of the first phase the same number times n+1. If the position of the connectors  30 ,  32  is modified, the conductor can be arranged to pass one more time over the top end  20  of the first tooth, so that each tooth, including the first, has the same number n of complete turns of conductor around it. This arrangement therefore allows for a balanced winding. This is achieved because the sections  18   c ,  18   d  of the conductor  18  adjacent to the ends  18   a ,  18   b  of the conductor extend along opposite sides of the same tooth  12   a , with the windings round the other teeth  12   b ,  12   c  of the phase being formed from intermediate sections  18   e ,  18   f  of the conductor. Also all of the interconnecting sections  18   g ,  18   h ,  18   i  of the conductor that extend between the teeth  12   a ,  12   b ,  12   c  of the phase extend under the bottom end of the rotor  10 , below any intervening teeth, which is advantageous from a packaging point of view as there is generally little room at the top end of the rotor. 
         [0017]    A number of modifications can be made to the embodiment of  FIGS. 1 and 2 . For example, the number of phases and the number of teeth per phase can clearly vary. Also, whereas in this example all of the teeth in each phase are wound from a single conductor, it is also possible where there are higher numbers of teeth per phase, to wind each phase as two separate groups of teeth, each group being wound with a single length of conductor. Furthermore, while in this embodiment the teeth are wound in the order in which they are located around the stator, which is clearly simple and convenient, there is no need for this always to be the case, and the winding could proceed in any suitable order. 
         [0018]    Because, in the embodiment of the  FIG. 1 , the conductor extends in a loop around the stator axis, the conductor has to be wound onto the stator in situ, i.e. with the teeth in the final position relative to each other. However, referring to  FIGS. 3 and 4 , in a second embodiment the stator  110  is segmented, with the teeth  112   a ,  112   b ,  112   c  of the first phase and the teeth of the other two phases formed separately so that they can be inserted into the stator  110  separately or in groups. This allows the teeth making up each phase to be wound with a single length of conductor  118  to form a pre-wound group of teeth before they are inserted into the stator  110 . Many of the features of the second embodiment are the same as those of the first, and corresponding parts are indicated by corresponding reference numerals increased by 100. 
         [0019]    In order to allow installation of the pre-wound groups of teeth, the closed loop arrangement of the first embodiment needs to be avoided. This is done by, during the winding, passing the conductor  118  from a first tooth  112   a  in the phase in one direction to a last tooth  112   c  in that phase, passing all other teeth in that phase in between, and then passing the conductor back in the opposite direction from the last tooth  112   c  to the first. Obviously the windings round each tooth  112   a ,  112   b ,  112   c  need to be completed during this winding process. In this example, the winding of the three teeth  112   a ,  112   b ,  112   c  is carried out in the same order as the first embodiment, except that between winding the third tooth  112   c  and the last half winding of the first tooth  112   a , the conductor is passed back under the second tooth. 
         [0020]    The teeth  112   a ,  112   b ,  112   c  can be wound while mounted on a jig which gives easy access to the teeth for the winding. For example the teeth may be mounted on the jig so that the ends which will be their inner ends in the assembled stator, face outwards away from each other. This can allow precision winding in which the position of each turn of the winding is carefully controlled so as to be in a specific predetermined position on the tooth. The teeth  112   a ,  112   b ,  112   c  with the conductor  118  are then inserted into the rotor  110 , and the ends of the conductor  118  connected to connectors  130 ,  132 . For the first phase winding, this leaves one tooth of each of the other phases that the conductor  118  does not pass. The teeth of each of the other two phases can be pre-wound into respective sub-assemblies, which can then be assembled into the stator  110  separately. Since all of the interconnecting sections  118   g ,  118   h ,  118   i  of the conductor extend between the bottom ends of the respective teeth, when the second and third phases are installed these interconnecting sections can be simply inserted below the teeth of the previously installed phases. 
         [0021]    When the windings have all been inserted, the two ends  118   a ,  118   b  of each conductor are connected to respective connectors  130 ,  132 , in the same manner as in the first embodiment. 
         [0022]    In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.