Abstract:
A method of making a motor stator in which the stator includes an inner stator part and an outer stator part, the method including placing the inner stator part inside the outer stator part, and rotating the inner stator part with respect to the outer stator part such that the inner stator part is retained with respect to the outer stator part by means of an interference fit.

Description:
BACKGROUND TO THE INVENTION  
         [0001]    This invention relates to a method of making a stator for a motor, of the kind including a plurality of stacked laminations.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    Stators are known, for example for brushless motors, which include a plurality of generally radially extending pole pieces upon which coils are wound. Conventionally such pole pieces extend radially inwardly from an outer stator ring and the coils are wound onto the pole pieces from the interior of the stator ring.  
           [0003]    In U.S. Pat. No. 5,134,327 there is disclosed a stator which has an inner stator part, the stator pole pieces extend radially outwardly from an inner stator ring, so that the coils can be wound onto the pole pieces exteriorly. The stator includes an external stator part, also in the form of a ring, which is assembled and connected to the inner stator part upon relative sliding, the inner and outer stator parts being a push fit.  
           [0004]    Whereas this arrangement provides advantages over the conventional arrangement described above, assembling the inner and outer stator parts as a push fit can be problematical.  
         SUMMARY OF THE INVENTION  
         [0005]    According to a first aspect of the invention we provide a method of making a motor stator including an inner stator part and an outer stator part, the method including placing the inner stator part inside the outer stator part, and rotating the inner stator part with respect to the outer stator part such that the inner stator part is retained with respect to the outer stator part by means of an interference fit.  
           [0006]    Thus by making the stator by the method of the invention, the difficulties of achieving a satisfactory push fit by simple relative sliding, as in U.S. Pat. No. 5,134,327 are overcome. Moreover, a clearance may be provided between the inner and outer stator parts prior to relatively rotating and so placing the inner stator part within the outer stator part may readily be achieved by relative sliding without there being any resistance to sliding. Because of this it is possible to form the inner and outer stator parts from laminations economically from a common area of metal sheet material.  
           [0007]    Thus the method may include the steps of forming from metal sheet material a plurality of laminations, each lamination including a first lamination part and a second lamination part, either at least one of the first lamination parts being formed from material surrounding one of the second lamination parts or at least one of the second lamination parts being formed from material removed from the interior of one of the first lamination parts, stacking a plurality of first lamination parts to form the outer stator part, and stacking a plurality of second lamination parts to form the inner stator part.  
           [0008]    Desirably the inner stator part includes an inner stator ring from which extend radially outwardly a plurality of stator pole pieces onto which coils are wound prior to placing the inner stator part in the outer stator part. The inner stator ring may simply be thin walled so as to provide a substantial resistance to transmission of magnetic flux from one pole to the next, but alternatively or additionally, the inner stator ring wall may be provided with thinned regions between the poles to provide localised high magnetic resistance locations. In another example, the pole pieces are provided by separate components, and are assembled to provide an inner stator part with an inner ring wall by nonmetallic material such as an insulating resin.  
           [0009]    In each case, the inner stator ring primarily provides support for the pole pieces whilst the coils are wound onto the pole pieces, the mechanical strength of the inner stator part being maximised upon placing the inner stator part within the outer stator part.  
           [0010]    To permit the inner stator part to be placed within the outer stator part, the outer stator part may include a plurality of axially extending recesses, one for each pole piece of the inner stator part, end surfaces of the pole pieces upon relative rotation of the inner and outer stator parts, engaging with areas of an inner wall of the outer stator part as an interference fit. If desired, the end surfaces of the pole pieces and/or the areas of the inner wall with which they engage, may be shaped so that as the inner and outer stator parts are relatively rotated upon assembly, the interference between the end surfaces and the areas of the inner wall of the outer stator part increases. If desired, the end surfaces of the pole pieces and/or the areas of the inner wall with which they engage, may instead or additionally each include a protuberance which enhances the interference fit between the pole piece end surfaces and the inner wall areas.  
           [0011]    According to a second aspect of the invention we provide a method of making a motor stator including a plurality of laminations, the method including the steps of forming from metal sheet material a plurality of laminations, each lamination including a first lamination part and a second lamination part, either at least one of the first lamination parts being formed from material surrounding one of the second lamination parts or at least one of the second lamination parts being formed from material removed from the interior of one of the first lamination parts, stacking a plurality of first lamination parts to form an outer stator part, stacking a plurality of second lamination parts to form an inner stator part, and assembling the inner and outer stator parts to form the stator.  
           [0012]    According to a third aspect of the invention we provide a method of making a motor stator including a plurality of laminations, the method including the steps of forming from metal sheet material a plurality of laminations, each lamination including a first lamination part and a second lamination part, either at least one of the first lamination parts being formed from material surrounding one of the second lamination parts or at least one of the second lamination parts being formed from material removed from the interior of one of the first lamination parts, stacking a plurality of first lamination parts to form an outer stator part, stacking a plurality of second lamination parts to form an inner stator part, placing the inner stator part inside the outer stator part, and rotating the inner stator part with respect to the outer stator part such that the inner stator part is retained with respect to the outer stator part by means of an interference fit.  
           [0013]    According to a fourth aspect of the invention we provide a motor including a stator made by a method according to any one of the preceding claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Embodiments of the invention will now be described with the aid of the accompanying drawings in which:  
         [0015]    [0015]FIG. 1 is an end view of a stator made by a method in accordance with the present invention, during assembly of inner and outer stator parts;  
         [0016]    [0016]FIG. 2 is an end view of the stator of FIG. 1 with the inner and outer stator parts more fully assembled;  
         [0017]    [0017]FIG. 3 is an enlarged view of part of the stator of FIGS. 1 and 2;  
         [0018]    [0018]FIG. 4 is an illustrative view showing how laminations of the stator or FIGS. 1 and 2 are formed.  
         [0019]    [0019]FIG. 5 is a view similar to FIG. 3, of an alternative embodiment.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    Referring to FIGS. 1 and 2 there is shown a stator  10  which includes an inner stator part  12  and an outer stator part  14 . The inner stator part  12  includes an inner stator ring  15  which is generally cylindrical, and from which extend radially outwardly a plurality of axially extending stator pole pieces  16   a ,  16   b  etc. Each pole piece  16   a ,  16   b  etc. has in use, a coil of wire wound onto the pole piece although in the drawings for clarity, a single coil  17  is shown, wound about pole piece  16   a.    
         [0021]    In use the coils  17  are energised so that a rotor (not shown) of the motor is caused to rotate within the stator  10 , in the inner space indicated at  19 . The invention is particularly but not exclusively applicable where the stator  10  is for a brushless motor such as a DC brushless motor.  
         [0022]    The inner stator ring  15  is thin walled so as to provide a substantial resistance to the transmission of magnetic flux generated at one pole, to the next pole. If necessary the inner ring  15  wall may include thinned regions between the poles, to provide local high magnetic resistance locations.  
         [0023]    Because the pole pieces  16   a ,  16   b  etc. extend radially outwardly, the coils  17  may be wound onto the pole pieces  16   a ,  16   b  etc. from exteriorly of the inner stator part  12 , prior to assembly with the outer stator part  14 .  
         [0024]    The outer stator part  14  is generally ring shaped in this example, and includes an outer stator ring  20  with a generally cylindrical inner wall  21 . However the inner wall  21  includes a plurality of axially extending recesses or grooves  22   a ,  22   b  etc., there being a groove  22   a ,  22   b  etc. for each inner stator part pole piece  16   a ,  16   b  etc. Moreover the grooves  22   a ,  22   b  etc. and the pole pieces  16   a ,  16   b  etc. are generally evenly spaced around the stator axis A so that each pole piece  16   a ,  16   b  etc. may be aligned with a respective groove  22   a ,  22   b  etc. as shown in FIG. 1.  
         [0025]    It can be seen that in this condition there is a clearance between the inner  12  and outer  14  stator parts so that the inner stator part  12 , complete with coils  17  wound onto the pole pieces  16   a ,  16   b  etc. may be placed within the outer stator part  14  by relative axial sliding, without any frictional resistance or the like.  
         [0026]    Referring particularly to FIG. 3, when the inner and outer stator parts  12 ,  14  are in their correct relative axial positions, the inner stator part  12  may be rotated relative to the outer stator part  14  so that end surfaces  25   a ,  25   b  etc. of the pole pieces  16   a ,  16   b  etc. move out of their respective grooves  22   a ,  22   b  etc., and engage adjacent areas  26   a ,  26   b  etc. of the inner wall  21  of the outer stator part  14 .  
         [0027]    It can be seen from FIG. 3 that the end surfaces  25   a ,  25   b  etc. of the pole pieces  16   a ,  16   b  etc. are shaped so that as the stator parts  12 ,  14  are relatively rotated (the inner stator part  12  being rotated clockwise relative to the outer stator part  14  in this example) an increasingly tight interference fit between the end surfaces  25   a ,  25   b  etc. and the areas  26   a ,  26   b  of the inner wall  21  is achieved. Thus upon sufficient relative rotation, a substantially tight fit between the inner and outer stator parts  12 ,  14  is achieved.  
         [0028]    If desired, as indicated in FIG. 3 in dotted lines, one of, or each of the end surfaces  25   a ,  25   b  etc. and the areas  26   a ,  26   b  etc. with which they engage, may be provided with a protuberance  29  which may simply increase the effectiveness of the interference fit between the end surfaces  25   a ,  25   b  etc. and the areas  26   a ,  26   b  etc. with which they engage, or the protuberance on one only of the end surfaces  25   a ,  25   b  etc. and the areas  26   a ,  26   b , may be received in a respective detent  30 , to provide a yet more positive interference fit.  
         [0029]    Preferably each of the stator parts  12 ,  14  are made from a plurality of lamination parts, stacked together with insulating bonding material such as a resin, therebetween.  
         [0030]    By virtue of the relative rotation between the inner and outer stator parts  12 ,  14  required to achieve an interference fit between the stator parts  12 ,  14 , and the clearance desirable between them prior to relative rotation to achieve the interference fit, the lamination parts may readily and most economically be made.  
         [0031]    Referring to FIG. 4, an area of a thin metal sheet  32  is shown which has a thickness equivalent to the thickness of a single lamination for the stator  10 . The metal sheet is typically a suitable grade of steel but another suitable metal sheet  32  may be provided.  
         [0032]    Each stator lamination includes a first lamination part  33  for the outer stator part  14 , and a second lamination part  34  for the inner stator part  12 . In one example a first lamination part  33  is formed from the area of the metal sheet  32 , and then subsequently, a second lamination part  34  is formed from material removed from the interior of the first lamination part  33 . Thus only a relatively small amount of the area of the metal sheet between the first  33  and second  34  lamination parts indicated at  36  in FIG. 4, will be wasted.  
         [0033]    In an alternative method, a second lamination part  34  is formed from the metal sheet  32  and then subsequently a first lamination part  33  is formed from the material surrounding the removed second lamination part  34 . Again, only small area  36  of the metal sheet  32  between the first  33  and second  34  lamination parts will be wasted.  
         [0034]    Although the metal sheet material  38  surrounding the removed first and second lamination parts  33 ,  34 , and an area  37  internal of the second lamination part  34  will also be wasted, in each case there will be far less waste than with an arrangement in which the inner and outer stator parts  12 ,  14  achieve an interference fit upon relative sliding only, as each of the lamination parts  33 ,  34  would need to provided from individual areas of the metal sheet  32 .  
         [0035]    Practically, when forming the lamination parts  33 ,  34 , these may be formed simultaneously from the metal sheet  32  using suitable tooling, and indeed a plurality of laminations may be formed simultaneously be stacking metal sheets  32 .  
         [0036]    A plurality of the first lamination parts  33  may be stacked to provide the outer stator part  14 , and a corresponding plurality of the second lamination parts  34  may be stacked to provide the inner stator part  12 , but of course the first stator part  33  need not be in the same position in the outer stator part  14  stack as the second stator part  34  formed from the same area of the metal sheet in the inner stator part  12  stack.  
         [0037]    Various modifications may be made without departing from the scope of the invention. For example, the inner and outer stator parts  12 ,  14  need not have the number of pole pieces  16   a ,  16   b  etc. and grooves  22   a ,  22   b  etc. respectively indented, but any desired number of poles for the stator  10  may be provided.  
         [0038]    Whereas it is preferred for the end surfaces  25   a ,  25   b  etc. to be shaped as described, these may be of a simple cylindrical shape corresponding generally to the shape of the inner wall  21  of the outer stator part  14 .  
         [0039]    In another example the outer stator part  14  at least need not have a generally cylindrical outer configuration as indicated in the drawings, but an outer stator part  14  with any desired outer configuration may be provided.  
         [0040]    The stator  10  may be of any desired axial extent, and a motor having a stator  10  made by the method of the invention my be used for any desired application, and may be of other than the DC brushless kind.  
         [0041]    In another example shown in FIG. 5, instead of the inner stator ring  15  being formed in metal integrally with the pole pieces  16   a ,  16   b  etc. the pole pieces  16   a ,  16   b  etc. may be formed as separate components. These may or may not include a foot formation  40  or formations  40 ,  41  to facilitate assembling the separate pole pieces  16   a ,  16   b  etc. in non-metallic material,  43  such as an insulating resin, to provide an inner stator part  12 . In such an arrangement, the poles of the stator  10  are insulated from each other by insulating parts  44  of the resin  43  or other non-metallic material between the pole pieces  16   a ,  16   b  etc. The individual pole piece components may be made from the area  33  of the metal sheet  32  as described above with reference to FIG. 4.