Abstract:
A novel electric motor rotor structure, particularly desirable for use with brittle rare-earth-magnets, offers improved resistance to rattling and axial shifting. This is achieved by forming the rotor with an annular central yoke connecting to a plurality of pole shoes along the periphery of the rotor and defining a magnet-receiving recess or pocket  160  between each pole shoe and the central yoke. Spaced circumferentially between adjacent magnets  38  are regions  146  of reduced magnetic conductivity, which include relatively thin metallic holding segments, which connect adjacent pole shoes to each other and to the central yoke. During manufacturing, tools are applied to upset or crimp the holding segments, and thereby form spring elements, to hold the magnets in stable positions and resist any tendency of the magnets to rattle or axially shift during motor operation. One obtains the same power level from a smaller, and therefore lighter, motor than was previously possible.

Description:
CROSS-REFERENCE This application claims priority of my German application DE 10 2010 023 159.2, filed 4 Jun. 2010, the content of which is incorporated by reference. 
     FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to an electric motor whose rotor includes permanent magnets and, more particularly, to an improved structure for mounting the magnets within the rotor. This motor is preferably implemented as an internal-rotor motor. 
       BACKGROUND 
       [0002]    Because of their low axial moment of inertia, such motors are used for drive situations in which the motions of an electric motor must follow electrical instructions very quickly, for example for fast displacement of parts or for servo-assistance of motions. In a motor of this kind, the permanent magnets must not rattle, and also must not shift in an axial direction. 
       SUMMARY OF THE INVENTION 
       [0003]    It is therefore an object of the invention to make available a novel electric motor structure with improved resistance to rattling and axial shifting. 
         [0004]    According to the invention, this is achieved by forming the rotor with an annular central yoke connecting to a plurality of pole shoes along the periphery of the rotor and defining a magnet-receiving recess or pocket between each pole shoe and the central yoke. Spaced circumferentially between adjacent magnets are regions of reduced magnetic conductivity, which include relatively thin metallic holding segments which connect adjacent pole shoes to each other, and to the central yoke. During manufacturing, tools are applied radially inward toward the rotor central axis, to upset or crimp the holding segments and thereby form spring elements, to hold the magnets in stable positions and resist any tendency of the magnets to rattle or axially shift during motor operation. This yields an electric motor in which it is possible to use, in the rotor, ceramic magnets whose angular extent is not much narrower than a pole pitch of the rotor, and in which one obtains a rotationally induced voltage with a good form. This allows the power-to-weight ratio of such motors to be improved; in other words, one obtains the same power level from a smaller, and therefore lighter, motor than was previously possible. The permanent magnets are retained in the rotor, in such a way that they cannot rattle or fall out. This is achieved by the invention in a simple and reliable manner, and also very economically. The invention also permits the use of ceramic magnets having constituents made of rare earths, e.g. neodymium. Such ceramic magnets are brittle and might easily break under mechanical stress, such as stress due to rattling within the rotor. This stress is largely prevented by the invention. 
     
    
     
       BRIEF FIGURE DESCRIPTION 
         [0005]    Further details and advantageous refinements of the invention are evident from the exemplifying embodiments, in no way to be understood as a limitation of the invention, that are described below and shown in the drawings. 
           [0006]      FIG. 1  is a longitudinal section through an internal-rotor motor with permanent-magnet excitation, according to the prior art, 
           [0007]      FIG. 2  is a section looking along line II-II of  FIG. 1  at a scale enlarged by comparison with  FIG. 1 , also according to the prior art, 
           [0008]      FIG. 3  is a schematic perspective view, according to the present invention, showing permanent magnets  38  of a rotor  36  before assembly by insertion of the magnets into recesses or pockets  160  of the rotor, 
           [0009]      FIG. 4  shows a variant of  FIG. 2 , 
           [0010]      FIG. 5  is a side view of rotor lamination stack  130 , looking in the direction of arrow V of  FIG. 3 , 
           [0011]      FIG. 6  is a section looking along line VI-VI of  FIG. 5 , in which the permanent magnets are depicted in their assembled position;  FIG. 6  is enlarged by comparison with  FIG. 5 , and also shows a crimping or notching tool  166 , 
           [0012]      FIG. 7  is an enlarged view of detail VII of  FIG. 6 , 
           [0013]      FIG. 8  is an enlarged view of an example of a tool, and 
           [0014]      FIG. 9  is a schematic view to explain the mounting of the rotor magnets. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In the drawings that follow, identical or identically-functioning parts are labeled with the same reference characters and are each described only once. Terms such as “upper,” “lower,” “left,” and “right” refer to the particular Figure of the drawings. Angles, for example β, are depicted in simplified fashion using arrows and straight lines. 
         [0016]      FIG. 1  shows an electronically commutated three-phase internal-rotor motor  20  having a housing  22  that comprises a cylindrical housing part  24 , an A-side bell  26 , and a mounting flange  29 . 
         [0017]    Arranged in cylindrical housing part  24  is a lamination stack  27  ( FIG. 2 ) of an external stator  28  whose end windings are indicated at  30  and  32 . Stator  28  has an internal recess  34  in which an eight-pole internal rotor  36 , having a lamination stack  37  made up of laminations  41  (indicated schematically in  FIG. 5 ) and having a total of eight permanent magnets  38 A to  38 H (see  FIG. 2 ), is arranged on a shaft  40  whose drive end is labeled  42  and whose internal shaft end is labeled  44 . A magnetically effective air gap  39  ( FIG. 4 ) separates stator  28  from rotor  36 . A motor  20  of this kind can be referred to in various ways, for example as a “permanently-excited synchronous internal-rotor machine” or an “electronically commutated sine-wave motor” or a “three-phase motor with permanent-magnet excitation.” 
         [0018]    A seal  46  for shaft  40  is provided in A-side bell  26 . Also located therein is a recess  48  in which is mounted a guide member  50  for outer ring  55  of a rolling bearing  54 . Inner ring  60  of rolling bearing  54  is pressed onto shaft  40 . 
         [0019]    A B-side bell  66  is mounted in the open end of cylindrical housing part  24 . This bell has a recess  68 , equipped with an annular shoulder  67 , for outer ring  70  of a rolling bearing  72  whose inner ring  74  is mounted on shaft end  44 . Shaft  40  has for this purpose an annular collar  78  with which it abuts against the left side of inner ring  74 . Abutting against its right side is a shaped part  80  made of brass that is pressed by flat head  81  of a flat-head screw  82  toward shaft  40 , and is shaped approximately annularly. Screw  82  is threaded into an internal thread  84  of shaft end  44 , and thereby presses shaped part  80  toward inner ring  74 . 
         [0020]    Secure clamping of outer ring  70  is provided by a part  90  that is mounted, by means of three uniformly distributed screws  92  on its periphery, onto bearing bell  66 , and abuts with its radially inner part against outer ring  70  and presses it to the left against shoulder  67 . 
         [0021]    Once shaped part  80  has been mounted by means of screw  82  on shaft end  44 , a control magnet  110  is mounted in a recess of shaped part  80 . Said magnet is equipped on its right side (in  FIG. 1 ) with a magnetization pattern, and serves to actuate magnetoresistive sensors (not shown) that are arranged on a housing cover  112  on the B side of motor  20 , and serve to sense the rotational position of rotor  36 , so that the shape and commutation of the currents in stator  28  can be exactly controlled. 
         [0022]      FIG. 2  shows, in enlarged fashion, a section looking along line II-II of  FIG. 1 . Magnets  38 A to  38 H are radially polarized. Magnet  38 A has a south pole S on the outside and a north pole N on the inside. The next magnet  38 B in the clockwise direction has a north pole N on the outside and a south pole S on the inside, and so on, as is evident from the drawing. 
         [0023]    Stator lamination stack  27  has on the outside a magnetic yoke  120  from which twelve teeth  122 A to  122 L protrude radially inward; as depicted, they are equipped with enlarged tooth heads  124  between which slots  126  are located. In this example, the value of the slot pitch τ_S between two adjacent stator slots  126  is 
         [0000]      τ —   S= 360°/12=30° mech.   (1).
 
         [0000]    Stator  28  can also, for example, be implemented with nine stator poles, and rotor  37  with six rotor poles  144  (embodiment not shown). 
         [0024]    Teeth  122  are wound with concentrated windings. This is shown, by way of example, for phase U. This begins with a concentrated winding  128 G on tooth  122 G, continues into a concentrated winding  128 D on tooth  122 D, then into a winding  128 A on tooth  122 A and a winding  128 J on tooth  122 J. From there, strand U goes back to neutral point O if a star-configured winding is being used. A delta circuit configuration is, of course, also possible. 
         [0025]    The sub-windings  128 G,  128 D,  128 A, and  128 J can also be connected in parallel, for example if motor  20  is being operated from a low-voltage DC source, since winding strands having a low inductance and low ohmic resistance are then obtained. Winding strands V and W are merely indicated in  FIG. 2 . 
         [0026]    Rotor  36  is arranged on a shaft  40  (made of ferromagnetic material). Shaft  40  is mounted in a yoke part  130 , and is part of the magnetic circuit. 
         [0027]      FIG. 3  is a perspective depiction of elements of a novel rotor  36 , specifically in the lower part a lamination stack  130  having eight rotor pole shoes  136 A,  136 B,  136 C,  136 D,  136 E,  136 F,  136 G, and  136 H, of which only six pole shoes are visible in  FIG. 3 , and which define eight pockets or recesses  160 A to  160 H ( FIG. 3 ) that serve to receive eight permanent magnets  38 A to  38 H. The latter are depicted in the upper part of  FIG. 3  and have, for example, a rectangular cross section, and are radially magnetized (see e.g.  FIG. 7  or  FIG. 8 ). 
         [0028]    One-piece permanent magnets  38 , which are usually manufactured from magnetic ceramic material and are therefore brittle, are depicted. These can be, for example, rare-earth neodymium magnets. It would also be possible to split the magnets; for example, they could be assembled from two or three parts, although mounting in the rotor would then be somewhat more complicated. 
         [0029]    Pole shoes  136  each have, on their side facing toward yoke  130 , a boundary surface  138 A,  138 B,  138 C, etc. that is also referred to hereinafter as a magnet/pole shoe boundary. Located opposite it, at a distance D ( FIG. 4 ), is a boundary  140 A,  140 B,  140 C, etc. that extends between a magnet  38  and yoke  130 . 
         [0030]    Permanent magnets  38 A,  38 B, etc. are clamped in elastically between these boundaries  138 ,  140 , as will be described later on, with reference to  FIGS. 6 to 8 . 
         [0031]    As  FIG. 7  shows, magnet  38  has at its magnet/pole shoe boundary surface  138  an angular extent β_M, and this corresponds approximately to the size of pole shoe  136  abutting against that boundary surface. Proceeding outward in a radial direction from this magnet/pole shoe boundary surface  138 , the width β of pole shoe  136  then decreases on both sides along a flank  139 , and at a point  142  reaches its minimum width β_C which is less than β_M (see  FIG. 7 ). 
         [0032]    Approximately radially outside point  142 , pole shoe  136  is connected laterally, via the peripherally extending segments or holding parts  134   a,    134   b  (which are magnetically saturated during operation and therefore perform principally a mechanically supporting function), to carrier parts  132  that extend radially and connect holding parts  134   a,    134   b,  and, by way of them, pole shoe  136  ( FIG. 4 ), to magnetic yoke  130 . 
         [0033]    As  FIG. 4  shows, radially outer side  144  of a respective pole shoe  136  is so configured that an approximately sinusoidal flux distribution is created in magnetically effective air gap  39 , i.e. proceeding from center  135  of a rotor pole  136 , the diameter decreases to either side as depicted. This profile is usually determined empirically. 
         [0034]    Located on either side (circumferentially) of a permanent magnet  38  is a respective cavity  146   a,    146   b  whose cross-sectional shape is approximately similar to a boomerang, i.e. a right triangle whose long side bulges slightly inward, since a radially outer corner of permanent magnet  38  protrudes somewhat into said cavity  146   a  at that point. 
         [0035]    Normally, a rotor topology like this would be unfavorable, and would result in a rather rectangular flux distribution in magnetically effective air gap  39  and a high cogging torque. But, because of constriction  142  ( FIG. 7 ) of pole shoes  136 , causing angle β_C to be less than β_M, a flux distribution is obtained which gives a good approximation of a sine wave. 
         [0036]    Constrictions  142  not only produce a concentration of magnetic flux toward the center of the pole, but also act as magnetic resistors that enable a small magnetic flux, even adjacent the pole gaps between rotor poles  136 , as is desirable for a sinusoidal flux distribution. This lateral flux can be influenced by appropriate dimensioning of cavities  146 . 
         [0037]      FIGS. 3 ,  6 , and  7  illustrate the novel structure of the invention, for fastening permanent magnets  38  into pockets or recesses  160  of rotor  36 . 
         [0038]    As already explained, magnets  38  must not rattle or fall out, even at higher speeds, i.e. they must be securely fastened, both axially and radially. 
         [0039]    For this purpose, magnets  38  can be adhesively bonded into rotor  36 , or a washer (not shown) can additionally be pressed onto shaft  40  at both ends of rotor  36 . Rotor  36  could also be encapsulated in synthetic resin. All these methods, however, require additional time, as well as auxiliary materials, workstations with air extraction, etc. The present invention achieves the same objective more economically. 
         [0040]    In the case of the present motor, magnets  38  are elastically clamped, at segments  162 , between the associated pole shoe (e.g.  136 B in  FIG. 7 ) and magnetic yoke  130 . This is achieved by the fact that, on longitudinal segments  162  ( FIGS. 3 and 5 ) of rotor  36  and for some rotor laminations  41 , holding segments  134   a,    134   b,  whose original shape is evident e.g. from  FIG. 4 , are indented (or notched or crimped) radially inward, using a tool  166  that is shown schematically in  FIGS. 6 and 8 ; this produces curved segments  170 ,  172  there that deform these holding segments  134   a,    134   b  into the vicinity of the local flanks  139  of pole shoe  136 . 
         [0041]    If, for example, rotor laminations  41  having a thickness of 0.35 mm are used on a rotor  36 , experiments have shown that deforming the holding segments  134   a,    134   b  of fewer than ten laminations  41  is sufficient to retain magnets  38  securely, i.e. segments  162  were in this case, for example, approximately two to four millimeters long. Their length of course depends on the size and power output of motor  20 . If applicable, multiple such segments  162  can also be used, for example at the beginning, middle, and end of a rotor. It is, of course, also possible to use thicker rotor laminations  41 , e.g. having a thickness of 0.5 mm or more. 
         [0042]    The indentations (or notches or crimps)  170 ,  172  produce a radially inwardly acting force F ( FIGS. 7 ,  8 ) on the relevant pole shoe  136 , and this force F securely retains each permanent magnet  38  in its recess  160 , so that it cannot either rattle or fall out. 
         [0043]    A considerable reduction in assembly time also results, and the risk of damage to magnets  38  is eliminated, since indentation  170 ,  172  acts not directly on magnets  38 , but rather on their holding segments  134   a,    134   b  which, in addition to their supporting function, now also take on the further function of a spring that generates force F. 
         [0044]    Magnets  38  are preferably inserted into recesses  160  before holding segments  134   a,    134   b  are deformed. 
         [0045]    The enlarged view of  FIG. 8  is provided for better comprehension. 
         [0046]    The left side shows, at  176 , the rotor shape according to  FIGS. 1 and 2 . Here pole shoe  136 A, below which rotor magnet  38 A is located, is connected rigidly on its left side, by way of a substantially straight holding part  134   a L, to the radially extending carrier part  132 L and, by way of that, to yoke  130 . 
         [0047]    These parts thus together constitute a part that can be regarded as a rigid shell which forms an outer wall  138 A of the cavity for magnet  38 A. 
         [0048]      FIG. 8  shows at  178 , in contrast thereto, the deformation of the middle (in  FIG. 8 ) holding parts  134   a M and  134   b M by tool  166 . 
         [0049]    Tool  166  has, for this purpose, at its working end, i.e. at the bottom in  FIG. 8 , two projections  180 ,  182  between which is a depression  184 , so that tool  166  is approximately “W”-shaped at its working end. 
         [0050]    Depression  184  forms a cavity  186  that constitutes a separation between the radially outer end of carrier part  132 M and tool  166 , so that the latter can produce no (or only a little) upsetting or crimping of carrier part  132 M. 
         [0051]    Projection  180 , on the other hand, produces an indentation of holding part  134   b M, and projection  182  produces an indentation of holding part  134   a M, as depicted in greatly enlarged fashion in  FIG. 8 . 
         [0052]    This creates, at the left end of holding part  134   b M, a torque Tcw on the right end of pole shoe  136 A, which torque presses the shoe elastically onto permanent magnet  38 B. 
         [0053]    At the right end of holding part  134   a M, a torque is likewise created on the left end of pole shoe  136 B, which torque presses the shoe elastically onto permanent magnet  38 B. 
         [0054]    The W-shaped deformation of holding parts  134   a,    134   b  thus additionally causes them to become active as springs that elastically retain magnets  38  in their recesses. 
         [0055]    Because these torques, just described, act on both ends of a permanent magnet, each in an opposite direction, the permanent magnet is securely retained in the rotor. 
         [0056]    It is useful to proceed in such a way that, during manufacture of the rotor, the necessary number of tools  166  is used in each case. 
         [0057]    In the exemplifying embodiment, rotor  36  has eight permanent magnets  38 , and the rotor is therefore placed into an apparatus having eight tools  166  that are simultaneously actuated, and thereby simultaneously deform all the holding parts  134   a,    134   b  so that all eight permanent magnets  38  are simultaneously elastically mounted, in a single working step, in rotor region  162  ( FIG. 5 ). 
         [0058]      FIG. 9  schematically shows this preferred manner of mounting magnets  38 A to  38 H in the lamination stack of rotor  36 . 
         [0059]    This entails the use of eight tools  166  that are arranged around rotor  36  and are moved simultaneously, by means of a suitable drive system (not shown), toward rotor  36 , in order to produce identical deformations of holding members  134   a,    134   b,  as shown in  FIG. 8  by way of example. 
         [0060]    Upon deformation, the holding members are bent over at the location where they are connected to the associated pole shoe (in  FIG. 8 : pole shoes  136 A and  136 B). Holding members  134   a M,  134   b M spring back slightly after this bending operation, and this rebound produces the torques Tcw (clockwise torque) and Tccw (counterclockwise torque) indicated in  FIG. 8 , which generate forces F that clamp magnets  38  in place. 
         [0061]    Many variants and modifications are, of course, possible within the scope of the present invention.