Abstract:
A device for positioning a planar array of magnets within a permanent magnet electrical machine of the type having a rotor and stator with an air gap there between. The device includes a body made of non-ferrous material and having a first side which is attachable to the rotor and a second side which, in an assembled machine, faces the air gap. The first side of the body has a plurality of recesses therein for receiving a corresponding plurality of magnets. The recesses are shaped and arranged to separate the magnets from each other and maintain a consistent spacing between them. When the device is attached to the rotor the magnets are held in a fixed position against the rotor. The electrical machine may be a motor or a generator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of and claims priority to U.S. patent application Ser. No. 12/598,652, filed on Mar. 1, 2010, which is a 35 U.S.C. §371 National Stage Application of PCT/AU2008/000591, filed Apr. 29, 2008, which claims priority to Australian Patent Application No. 2007902348, filed May 3, 2007, the entire content of each application is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to permanent magnet electrical machines including a planar array of magnets. In particular, the invention relates to a device for positioning magnets on a rotor of a permanent magnet electrical machine. The device is especially suited for use in axial flux electric motors and it will be convenient to describe the invention in relation to that example application. It should be understood however that the device is also suitable for other forms of electrical machine that incorporate a planar array of magnets, such as linear motors or generators. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a permanent magnet electric machine the magnets are often secured to the rotor in a planar array by an adhesive. Positioning of the magnets on the rotor is a difficult and time consuming exercise because the magnets tend to be attracted to each other or repel each other, depending upon the orientation of the magnets. It is therefore important that the magnets be precisely placed in a position in which the magnetic forces are balanced. For example, the magnets may be oriented at an equal distance from one another in an alternating pole configuration, i.e. North, South, North, South, etc. This arrangement would produce an attractive force between the magnets, which results in a net force of zero on each magnet if the equal distance between magnets is carefully maintained in a precarious equilibrium. 
         [0004]    Given this issue, it is also important that the magnets be held in that position at least until the adhesive has properly cured. The curing time of the adhesive thus makes handling of partially assembled rotors a delicate exercise because any movement of the magnets outside of their ideal positions will cause them to move further, until they eventually come together. It is therefore undesirable to handle partially assembled rotors before the adhesive is completely cured. This slows down the production process and/or requires a large amount of valuable work space to be dedicated to the storage of rotors whilst the adhesive cures. 
         [0005]    In a rotating motor having magnets attached to a rotor, centrifugal forces on the magnets during operation of the motor tend to pull the magnets radially outward and away from the rotor. If the sole means by which the magnets are secured to the rotor is an adhesive, the speed of the motor is thus limited by the strength of the adhesive. In this type of construction it is thus essential that the operating speed of the motor be kept well below a speed at which the centrifugal forces generated upon the magnets would overcome the adhesive bond holding the magnets to the rotor. Failure of the adhesive bonds could cause the magnets to be dislodged and subsequently damage the motor. 
         [0006]    Magnets used in electric motors may be coated or plated to prevent corrosion. Thus, any adhesive which is used to secure a magnet to a rotor is in fact merely securing the anticorrosion coating or plating to the rotor. The strength of the bond between the actual magnet and the rotor is thus limited to the strength of the bond between the anticorrosion coating/plating and the magnet material. The use of adhesive alone is therefore not ideal for securing magnets to a rotor but it is still the norm. 
         [0007]    With the foregoing difficulties in assembly and operation of permanent magnet electrical machines in mind, there is a need for a device which could facilitate the positioning of magnets within a permanent magnet electrical machine, such as a motor. It would also be desirable, although not essential, for the device to be able to remain within the assembled machine to assist in retaining the magnets to the rotor in their correct positions, with or without the use of adhesive, during operation of the motor. 
       SUMMARY OF THE INVENTION 
       [0008]    One aspect of the present invention accordingly provides a device for positioning magnets in a planar array within a permanent magnet electrical machine of the type having a rotor and stator with an air gap there between. The device includes a body made of non-ferrous material and has a first side which is attachable to the rotor and a second side which, in an assembled machine, faces the air gap. The first side of the body has a plurality of recesses therein for receiving a corresponding plurality of magnets. The recesses are shaped and arranged to separate the magnets from each other and maintain a consistent spacing between them. When the device is attached to the rotor the magnets are held in a fixed position against the rotor. 
         [0009]    The electrical machine may be a motor or a generator. The invention is applicable to both types of machine. Suitable motors may include rotary motors, in which a rotor rotates about an axis, or linear motors, in which a “rotor” moves linearly with respect to an elongate stationary stator. The term “rotor” is thus used more broadly in connection with this type of motor because the rotor does not actually rotate. Rotary motors may include axial flux motors, in which the flux in the air gap extends in a direction parallel to the axis of rotation. It is suggested that the more common radial flux machine in which the flux extends radially from the axis of rotation does not benefit as highly from the invention. In that case the magnets are not configured in a planar array but are configured in a circular array. This means that a part produced according to the invention would need to mate to a circular rotor and project a circular face into the air gap, which leads to production tolerance issues that are difficult to overcome. In the case of a planar array of magnets the invention results in a simple, flat shape which is substantially easier to produce and use. Similarly, the invention may be applicable to all types of generator that incorporate planar arrays of magnets. 
         [0010]    The magnet positioning device in accordance with the invention provides significant advantages over prior art methods used for securing magnets to a rotor. The device includes recesses which may be specifically shaped to closely conform to the shape the magnets. This prevents movement of the magnets within the recesses and maintains a consistent spacing between the magnets, both during assembly of the rotor and during subsequent operation of the machine. 
         [0011]    In the latter regard, it is to be noted that if adhesive is also used to secure the magnets to the rotor, the device may be removed from the rotor after the adhesive has cured. Given that the device maintains a consistent spacing between the magnets, there is now no danger of the magnets moving if the rotor is handled before the adhesive is properly cured. The device will retain the magnets in their correct positions. 
         [0012]    Whilst the device may be removed from the assembled rotor, it is however preferably that it remains in place after the motor is fully assembled. In this way, the device assists in securing the magnets to the rotor during subsequent operation of the machine. 
         [0013]    In a preferred embodiment, the device is designed for use in an axial flux rotary motor having a disc shaped rotor which is rotatable about an axis. In this type of motor the rotor disc is spaced from the stator in a direction extending along the axis with an air gap then remaining between the stator and the rotor disc. In this embodiment, the body of the device may have an annular shape and the recesses for the magnets may be spaced about the annulus such that, in operation of the motor, the magnets are spaced around the axis of rotation. 
         [0014]    Advantageously, connecting means are provided for attaching the body of the device to the rotor. In the case of an axial flux motor, the connecting means may include a plurality of clips spaced around the body of the device so as to attach the body to the rotor disc. Preferably, the clips are positioned adjacent a peripheral edge of the body and are shaped to engage a peripheral edge of the rotor disc. 
         [0015]    Preferably, the device includes locating means for aligning the body of the device relative to the rotor. If the machine is an axial flux rotary motor, the locating means preferably includes at least one pin projecting in an axial direction from the first side of the body for engaging with a corresponding aperture in the rotor disc. This arrangement thereby facilitates alignment of the body of the device with the rotor disc. More preferably, the locating means includes at least three pins projecting in the axial direction from the first side of the body for engaging with at least three corresponding apertures in the rotor disc. In this embodiment, the pins may be configured to hold the first side of the body away from the rotor disc until the pins and corresponding apertures are aligned. This also ensures accurate concentricity of the magnet array with respect to the rotor disc. 
         [0016]    In a preferred embodiment, the device also includes balancing means for enabling weights to be secured to the device and thereby allow any imbalance in the rotor to be corrected. In the case of a radial flux rotary motor, the balancing means may include an annular grove located around a peripheral edge of the body of the device. The groove is preferably provided with a cross sectional shape which is configured to enable weights to be inserted and retained within the annular groove. 
         [0017]    Another aspect of the present invention provides a permanent magnet electrical machine including a rotor, a stator, a plurality of magnets and a device as described above for positioning the magnets on the rotor. In such a machine the device may also serve to assist in securing the magnets to the rotor during operation. 
         [0018]    A further aspect of the invention provides a method of assembling a rotor for a permanent magnet electrical machine. The method includes the steps of 
         [0019]    providing a rotor; 
         [0020]    providing a magnet positioning device as described above; 
         [0021]    providing a plurality of magnets; 
         [0022]    inserting the magnets into the recesses in the first side of the body of the device; 
         [0023]    bringing a ferrous object into contact with the second side of the body of the device such that the ferrous object is adhered to the device by magnetic forces generated by the magnets and the magnets are held within the recesses by the magnetic forces; 
         [0024]    attaching the first side of the body of the device to the rotor such that the magnets are held in a fixed position against the rotor; 
         [0025]    and removing the ferrous object from the second side of the body. 
         [0026]    This method facilitates the assembly of a rotor for a permanent magnet electrical machine. The ferrous object serves to hold the magnets within the recesses of the positioning device whilst that device is being aligned with, and ultimately attached to, the rotor. The ferrous object may be of any suitable form and could, for example, have an annular shape with an outer diameter similar to the outer diameter of the positioning device. Alternatively, the ferrous object may be circular with a diameter similar to that of the outer diameter of the positioning device. As a further alternative, if the assembly method is implemented by an automated machine, the ferrous object may be a component of the machine. 
         [0027]    The magnets used in the electrical machines for which the present invention is applicable may be of any known type. There are however advantages in using high power magnets made of rare-earth metals such as neodymium. These provide a magnetic force which is much greater than conventional magnets for the same size and weight. It is for this reason that precise positioning of the magnets, especially during assembly of a rotor, becomes important. The magnets tend to stick together when being handled and may be very difficult to separate by hand. 
         [0028]    Having said that, it is also possible to use magnets which are initially provided in an unmagnetised state and then subsequently magnetise them after they have been placed into the positioning device, or possibly after the rotor has been assembled. In this instance a slightly different assembly method may be employed to hold the magnets within the positioning device as it is brought into contact with the rotor. For example, the ferrous object used in the assembly method described above may be replaced with a vacuum holding device. 
         [0029]    To assist the further understanding of the invention, reference is now made to the accompanying drawings which illustrate preferred embodiments. It is to be appreciated that these embodiments are given by way of illustration only and the invention is not to be limited by this illustration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    In the drawings: 
           [0031]      FIG. 1  shows a bottom perspective view of a magnet positioning device for an axial flux rotary motor in accordance with a preferred embodiment of the present invention; 
           [0032]      FIG. 2  shows a top perspective view of the magnet positioning device of  FIG. 1 ; 
           [0033]      FIG. 3  shows a bottom perspective view of the magnet positioning device shown in  FIG. 1  together with magnets inserted therein; 
           [0034]      FIG. 4  shows a bottom perspective view of the magnet positioning device of  FIG. 1  as attached to a rotor; 
           [0035]      FIG. 5  shows a top perspective view of the magnet positioning device and rotor shown in  FIG. 4 ; 
           [0036]      FIG. 6  shows a cross sectional side view of an axial flux rotary motor incorporating the magnet positioning device of  FIG. 1 ; 
           [0037]      FIG. 7  shows an exploded perspective view of the motor shown in  FIG. 6 ; 
           [0038]      FIG. 8  shows an top perspective view of an alternative rotor for an axial flux motor; 
           [0039]      FIG. 9  shows a bottom perspective view of an alternative magnet positioning device for use with the rotor shown in  FIG. 8 ; 
           [0040]      FIG. 10  shows a top perspective view of the rotor of  FIG. 8  together with the magnet positioning device of  FIG. 9 ; 
           [0041]      FIG. 11  shows a top perspective view of a further alternative magnet positioning device together with a rotor for an axial flux motor; 
           [0042]      FIG. 12  shows a bottom perspective view of the magnet positioning device and rotor of  FIG. 11 ; 
           [0043]      FIG. 13  shows a top view of the magnet positioning device and rotor shown in  FIGS. 11 and 12 ; 
           [0044]      FIGS. 13A and 13B  show cross sectional side views taken along lines A-A and  8 - 8 , respectively, in  FIG. 13  (but excluding the shaft which is shown in the previous figures); 
           [0045]      FIG. 14  shows a cross sectional side view of the magnet positioning device and rotor shown in  FIG. 11 ; 
           [0046]      FIG. 15  shows a cut away top perspective view of the magnet positioning device and rotor shown in  FIG. 11 ; 
           [0047]      FIG. 16  shows a perspective view of a rotor for a linear motor; 
           [0048]      FIG. 17  shows a top perspective view of a magnet positioning device for the linear rotor shown in  FIG. 16 ; 
           [0049]      FIG. 18  shows a bottom perspective view of the magnet positioning device of  FIG. 17 ; and 
           [0050]      FIG. 19  shows a top perspective view of the linear rotor shown in  FIG. 16  together with the magnet positioning device shown in  FIGS. 17 and 18 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0051]    Initially, it should be understood that throughout this description the terms “top” and “bottom” are used merely for convenience to refer to the embodiments shown in the accompanying drawings. It should accordingly be understood that these terms have no relevance to the orientation of the actual apparatus as it may be manufactured or used. 
         [0052]      FIGS. 1 to 7  of the drawings show a preferred embodiment of the present invention. In particular,  FIGS. 1 and 2  show bottom and top perspective views, respectively, of a magnet positioning device  1  for use in a permanent magnet axial flux electric motor  3 , as shown in  FIGS. 6 and 7 . 
         [0053]    The components of the motor can be best seen in the exploded view shown in  FIG. 7 . These components include a housing, incorporating end shields  5  and  7  and a side wall  9 , a stator  11  (although the windings are not shown in the drawings) mounted within the housing. A rotor disc  13  is mounted on a shaft  15  which is rotatable within the housing by means of bearings  17  and  19 . A wave washer  21  is also included between the bearing  17  and the end shield  5  so as to reduce noise produced by the bearing and promote quieter operation of the motor  3 . 
         [0054]    As can be seen in the cross sectional side view shown in  FIG. 6 , the rotor disc  13  includes a plurality of permanent magnets  23 , which are preferably neodymium magnets. As can also be seen in  FIG. 6 , an air gap exists between the top face of the magnet positioning device  1  (attached to the rotor disc  13 ) and a lower face of the stator  11 . 
         [0055]    Referring again to  FIGS. 1 and 2 ,  FIG. 1  shows a first side of the body  25  of the magnet positioning device  1 , which is attachable to the rotor  13 . A bottom perspective view of the rotor disc  13  attached to the positioning device  1  is shown in  FIG. 4  and a top perspective view is shown in  FIG. 5 . 
         [0056]      FIG. 2  shows a second side of the body  25  of the magnet positioning device  1  which, in the assembled motor shown in  FIG. 6 , faces the air gap. 
         [0057]    The body  25  of the magnet positioning device  1  is made of a non-ferrous and non-electrically conductive material, such as a plastic of a suitable type. Examples may include Nylon, which might be glass fiber filled for increased strength or mineral filled for reduced cost. The body  25  of the device  1  is of an annular shape and the first side of the annulus includes a plurality of recesses  27 . These recesses  27  are shaped so as to closely conform to the shape of the magnets  23 .  FIG. 3  shows a bottom perspective view of the magnet positioning device  1  together with magnets  23  inserted within the recesses  27 . The close conformity of the shape of the recesses  27  to the shape of the magnets  23  serves to prevent the magnets  23  moving within the recesses  27 . The magnets  23  are therefore separated from each other and a uniform spacing is maintained between them so as to prevent the magnets  23  being drawn together or being repelled. 
         [0058]    As can be best seen in  FIG. 1 , the recesses are formed by a plurality of ribs  29  which extend between inner and outer support rings  31  and  33  respectively. Together, these support rings  31  and  33  and separating ribs  29  form for a spider-like cage structure. 
         [0059]    In the preferred embodiment shown in  FIGS. 1 to 7 , the body  25  of the magnet positioning device  1  includes connecting means for attaching the body  25  to the rotor disc  13 . The connecting means is provided in the form of a plurality of clips  35  spaced around a peripheral edge of the body  25 . 
         [0060]    The body  25  of the magnet positioning device  1  also includes locating means for aligning the body relative to the rotor disc  13 . In the embodiment shown, the locating means includes four pins  37  projecting in an axial direction from the first side of the body  25 . These pins  37  engage with corresponding apertures  39  in the rotor disc  13  so as to facilitate alignment of the body  25  with the rotor disc  13 . 
         [0061]    The body  25  of the device  1  also includes balancing means in the form of an annular groove  41  extending around a peripheral edge of the body  25 . The cross sectional shape of the groove  41  is configured to enable weights, such as metal balls or wire, to be inserted and retained within the groove  41 . Any imbalance in the rotor  13  can thereby be corrected. 
         [0062]    In the embodiment shown in  FIGS. 1 to 7 , the recesses  27  extend all the way through the body  25 . However, the side wall of the recesses  27  includes a projecting flange, or shelf formation,  43  which prevents the magnets  23  passing all the way through the body  25 . 
         [0063]    In an alternative embodiment, not shown, it is possible that the recesses may not extend all the way through the body so that the second side of the body, facing the air gap, is fully covered. 
         [0064]    A method of assembling a permanent magnet axial flux motor  3  will now be described. Initially, the magnets  23  are inserted into the recesses  27  from the first side of the body  25 . Once all of the magnets  23  have been inserted the positioning device  1  will appear as shown in  FIG. 3 . 
         [0065]    The positioning device  1  (including magnets  27 ) and the rotor disc  13  are then brought together but, before doing so, a ferrous object, such as an annular metal disc  28  (shown in  FIG. 3 ), is brought into contact with the first side of the body. The magnets  23  will thus be attracted to the ferrous object and thereby be held within their respective recesses  27 . In this way, the assembly (positioning device  1 , magnets  23  and ferrous object) can then be brought into contact with the rotor disc  13  without the magnets  23  immediately being pulled out of the recesses  27  by the rotor disc  13 . As the two parts are brought together, the alignment pins  37  will contact the surface of the rotor disc  13 . This allows relative movement between the two parts until the pins  37  are aligned with the apertures  39  in the rotor disc  13 . Once they are aligned, the pins  37  will engage within the apertures  39  and the connecting clips  35  will permanently attach the magnet positioning device  1  to the rotor disc  13 . 
         [0066]    Preferably, an adhesive is applied to the magnets  23 , or to the rotor disc  13 , before the parts a brought together. The magnet positioning device  1 , together with the adhesive, would ensure that the magnets  23  do not separate from the rotor disc  13  during operation of the motor  3 . 
         [0067]    In an embodiment not shown, it is possible that the connecting clips  35  may be omitted. In this way, the magnet positioning device may be removable from the rotor disc once the adhesive has properly cured. 
         [0068]    Referring now to  FIGS. 8 to 10 , there is shown a simplified representation of an alternative embodiment of the present invention.  FIG. 8  shows a rotor  50  for an axial flux motor attached to a shaft  52 . The rotor  50  includes four permanent magnets  54 . 
         [0069]      FIG. 9  shows a magnet positioning device  56  for use with the rotor  50  shown in  FIG. 8 . The magnet positioning device  56  includes four recesses which are sized and shaped to conform to the magnets  54  shown in  FIG. 8 .  FIG. 10  shows the magnet positioning device  56  brought together with the rotor  50 . 
         [0070]    Referring now to  FIGS. 11 to 15 , there is shown a magnet positioning device  156 , in accordance with a further alternative embodiment of the invention, attached to a rotor  150  for an axial flux motor.  FIG. 11  shows a top perspective view whereas  FIG. 12  shows a bottom perspective view.  FIG. 13  shows a top view and  FIGS. 13A and 138  show cross-sectional side views taken along lines A-A and B-B, respectively, in  FIG. 13 . For the sake of clarity, however, the shaft  152 , to which the rotor  150  is attached, has been omitted in these Figures.  FIG. 14  shows a cross-sectional side view of the rotor and magnet positioning device and  FIG. 15  shows a cutaway view of the same combination. 
         [0071]    As in the embodiment shown in  FIGS. 8 to 10 , the rotor  150  is mounted on a shaft  152  and includes four magnets  154 . However, in this embodiment the magnet positioning device  156  includes connecting clips  158  spaced around the periphery of the body of the positioning device  156  to attach it to the rotor  150 . The magnet positioning device  156  also includes locating pins  160  which engage in corresponding apertures  162  in the rotor  150 . The magnet positioning device  156  further includes an annular groove  164  to enable weights to be inserted and retained therein so as to enable any imbalance in the rotor  56  to be corrected. 
         [0072]    The method of assembly of the rotors shown in  FIGS. 8 to 10  and  11  to  15  is the same as the method involved in the assembly of the embodiment shown in  FIGS. 1 to 7 . 
         [0073]    Referring now to  FIGS. 16 to 19 , there is shown a further embodiment of the present invention as applicable to a linear motor.  FIG. 16  shows a rotor for the linear motor. In this type of motor, the term “rotor” should be interpreted broadly to mean the moving part of the motor. Clearly, the rotor does not rotate in this instance but, instead, travels linearly with respect to a stator of the linear motor. The rotor  350  includes permanent magnets  354 . 
         [0074]      FIGS. 17 and 18  show top and bottom perspective views, respectively, of a magnet positioning device  356  in accordance with an embodiment of the present invention. The magnet positioning device  356  includes recesses  358  therein for receiving corresponding magnets  354  as shown in  FIG. 19 . 
         [0075]    Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims. For example, the connecting means could include clips, as shown in the embodiments described herein, but could alternatively be formed by thermal deformation, ultrasonic welding or some other form of connection between the magnet positioning device and the rotor. Similarly, the locating means may be in the form of projecting pins, as shown in the embodiments described herein, but could alternatively be corresponding formations within the magnet positioning device and rotor. Such alternatives are considered to be clearly within the scope of the appended claims.