Abstract:
An electric motor has a rotor ( 36 ) and a stator ( 28 ) having a lamination stack formed with slots ( 126 ). The slots have a predetermined slot pitch (τ S ), and a multi-phase stator winding is arranged in them. The rotor ( 36 ) has salient poles having pole shoes ( 136 ) and a magnetic return path ( 130 ). Between the return path ( 130 ) and each pole shoe ( 136 ), a recess ( 138, 140 ) is formed for receiving a permanent magnet ( 38 ). On each side of such a permanent magnet ( 38 ), a region ( 146   a   , 146   b ) of poor magnetic conductivity is arranged, in order to make the flux distribution in the air gap more sinusoidal. Measuring in a circumferential direction, the width (β) of a pole shoe ( 136 ) decreases with increasing distance from an interface ( 138 ) between said return path and said permanent magnet ( 38 ), and, at a place of lowest width, the pole shoe has an angular extent (β C) which has, with respect to the slot pitch (τ S ) between said stator slots ( 126 ), the following relationship: 
 
β C   =n*τ   S *(1−0.02) . . .  n*τ   S *(1−0.2), 
where n=1, 2, 3 . . . and 
 
β C  and τ S  are measured in mechanical degrees.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of my U.S. Ser. No. 10/390,824, filed 18 Mar. 2003. This application claims priority of German application DE 203 17 021.0, filed 5 Nov. 2003 and of European application 04 020 696.3, filed 1 Sep. 2004. The parent US application claims priority of German application DE 202 04 6605, filed 22 Mar. 2002. The contents of all of these priority applications are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to an electric motor, which preferably is configured as an internal rotor motor.  
       BACKGROUND  
       [0003]     Such motors, preferably employing electronic commutation, are used, because of their low axial moment of inertia, for jobs where an electric motor&#39;s RPM must very quickly respond to electrical commands, e.g. for fast positioning of parts, or servo-assistance to movements. For this purpose, one desires such a motor to have a very uniform torque. One generally achieves this by a three-phase configuration of the motor, in which each of the phases has an essentially sinusoidal current applied to it, and the motor is so designed that, in the phases or strands of the multi-phase stator winding, sinusoidal voltages are induced. One also calls such a motor a “sine motor.” 
         [0004]     In such motors, the phenomenon occurs that the boundaries between the individual rotor poles, the so-called “pole boundaries,” seek the positions of the largest air gaps. For the observer, this has the appearance as if the pole boundaries were attracted by the slots of the stator. This effect is called “cogging.” The torque created thereby is called “cogging torque” because it seeks to hold the rotor in particular rotational positions.  
         [0005]     This effect is generated by a so-called “reluctance torque,” i.e. during the rotation of the rotor, relative to the stator, magnetic energy is stored in the magnetic circuit of the motor in certain rotation angle ranges, and, in other rotation angle ranges, this magnetic energy is released, analogous to when one alternately tensions a spring and releases it. For the storing, energy must be supplied to the rotor, i.e. the rotor is being braked thereby, and conversely, where the stored energy is being released, the rotor is being driven. If one turns the rotor of such a motor by hand, one has the impression that one “feels every slot.” 
         [0006]     In the context of many drive applications, this reluctance torque is disruptive, so that there one is forced to use so-called “ironless” stator windings in which no reluctance torque arises. However, the power of such motors with ironless stators is generally insufficient because their air gap is very large. This leads to a high “specific weight” (weight/power ratio), i.e. the relationship of motor power to motor volume or motor power to motor weight is unfavorable with them.  
         [0007]     Some have tried to overcome this problem by giving the pole shoes of the rotor a particular form, but that has led, thus far, to a structure in which the specific weight was unfavorable.  
       SUMMARY OF THE INVENTION  
       [0008]     It is therefore an object of the invention to provide a sufficiently powerful internal rotor motor with minimized cogging torque. According to the invention, this object is achieved by careful shaping of cavities, formed in the rotor adjacent each circumferential end of the permanent magnets, and extending into undersides of the pole shoes, thereby giving each pole shoe a kind of mortarboard shape. One thereby obtains a multi-phase electric motor, in whose rotor one can use magnets whose angular span is not much smaller than a pole pitch of the rotor, and which nevertheless results in an induced voltage having a good sinusoidal form and an acceptable cogging torque. This makes it possible to improve the specific weight of such motors, i.e., for the same power, it suffices to use a motor which is smaller and lighter than before.  
         [0009]     Further details and advantageous refinements of the invention are set forth in the following description and accompanying drawings, which are to be understood as preferred embodiments but not as any limitation of the invention.  
       BRIEF FIGURE DESCRIPTION  
       [0010]      FIG. 1  is a longitudinal sectional view of a preferred embodiment of a motor according to the present invention;  
         [0011]      FIG. 2  is a section, looking along line II-II of  FIG. 1 , at a scale enlarged with respect to  FIG. 1 ;  
         [0012]      FIG. 3  shows a detail of part of  FIG. 2 , at a further enlarged scale;  
         [0013]      FIG. 4  is a view analogous to  FIGS. 2 and 3 , in which the pattern of magnetic flux lines, for a particular rotor position, is shown, in order to explain the generation of a sinusoidal induced voltage;  
         [0014]      FIG. 5  is a view analogous to  FIG. 3 , namely the illustration of a section of a symmetrical rotor lamination; and  
         [0015]      FIG. 6  is a view analogous to  FIGS. 3 and 5 , namely an illustration of a rotor lamination, in which the poles are formed slightly asymmetrically. 
     
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 1  shows an electronically commutated internal rotor motor  20  with a housing  22  having a cylindrical housing portion  24 , an A-side bell  26  and a securing flange  29 .  
         [0017]     In the cylindrical housing portion  24 , there is arranged the lamination stack  27  ( FIG. 2 ) of an external stator  28  whose coil ends are designated  30  and  32 . Stator  28  has an internal recess  34  in which is arranged an eight-pole internal rotor  36  having a lamination stack  37  made of an alloy according to German Industrial Standard DIN 46400, sheet 1, preferably alloy V400. Preferably, eight permanent magnets  38 A through  38 H (see  FIGS. 2-3 ) are arranged on a shaft  40 , whose drive end is designated  42  and whose inside shaft end is designated  44 . An air gap  39  separates stator  28  from rotor  36 . Such a motor can be called a “permanently excited synchronous internal rotor machine” or a “three-phase motor with impressed sinusoidal currents.” 
         [0018]     In the A-side bell  26 , in the usual manner, a seal  46  is provided for the shaft  40 . Also there is a recess  48 , into which is placed a guide element  50  for the outer race  55  of a ball bearing  54 . The inner race  60  of ball bearing  54  is pressed onto shaft  40 .  
         [0019]     In the open end of cylindrical housing portion  24 , a B-side bell  66  is secured. It has a recess  68  provided with an annular shoulder  67  for the outer race  70  of a ball bearing  72 , whose inner race  74  is secured to shaft end  44 . Shaft  40  has a collar  78 , with which it rests against the left side of inner race  74 . Against its right side rests a fitting  80  made of brass which is pressed by the countersunk head  31  of a countersunk screw  82  in the direction of shaft  40 , and which has an essentially annular shape. Screw  82  engages in an internal thread  84  in shaft end  44 , and thereby presses the fitting  80  in the direction of inner race  74 .  
         [0020]     For secure holding-in of outer race  70 , there is provided a flat, annular part or washer  90 , which is secured at its outer periphery to bell  66  using three evenly spaced screws  92 . Part  90  rests, with its radial inner portion against outer race  70 , which it presses leftwards against shoulder  67 . The recess  68  is somewhat shallower than the outer race  70 .  
         [0021]     The screw  82  is a flathead screw with a hexagonal recess. After fitting  60  is secured, by means of screw  82 , onto shaft end  44 , a control magnet  110  is secured in a cylindrical recess of fitting  80 , e.g. by gluing. Control magnet  110  is provided, on its right side as shown in  FIG. 1 , with a magnetization pattern, and serves for control of magnetoresistive resistors (not shown) which are arranged inside a housing cover  112  on the B-side of motor  20 , and serve for detection of the rotational position of rotor  36 , in order to exactly control the form and commutation of the currents in stator  28 . Commutation by means of such rotor position sensors controlled by a control magnet  110  is known, in many variations, to those of ordinary skill in the art, and therefore requires no further explanation.  
         [0022]      FIG. 2  is an enlarged section looking along line II-II of  FIG. 1 . As can be seen from  FIG. 2 , magnets  38 A through  38 H are arranged with alternating respective radial polarities. Magnet  38 A has a south pole S outward, and a north pole N inward. Looking clockwise around the rotor, the next magnet  38 B has a north pole N outward and a south pole S inward, etc, as shown in the drawing.  
         [0023]     Stator lamination stack  37  has outward an armature  120 , from which twelve teeth  122 A through  122 L project radially inward, and which are formed, in the manner shown, with widened heads  124 , defining between them slots  126 . The slot pitch between two adjacent slots  126  is designated τ S  and amounts here to: 
 
τ S =36/12=30 ° mech.   (1) 
 
 It was found, surprisingly, that the form of the rotor poles should have a specified relationship to τ S . This is further explained below, with reference to  FIGS. 3-4  and equations (4) and (5). 
 
         [0025]     Teeth  122  are wound with concentrated or “lumped” windings. Phase U is illustrated, as an example. This phase begins with a concentrated winding  128 G on tooth  122 G, continues in a concentrated winding  128 G on tooth  122 G, continues in a concentrated winding  126 D on tooth  122 D, further in a winding  128 A on tooth  122 A, and a winding  128 J on tooth  122 J. From there, phase U goes back to neutral point 0, assuming that a winding in a star or Y configuration is used. Naturally, a Δ configuration is also possible.  
         [0026]     The partial windings  126 G,  128 D,  128 A and  128 J can also be connected in parallel, e.g. in case motor  20  is driven from a DC source with a low voltage such as in a car, since then one obtains winding phases having a low inductance and a low ohmic resistance.  
         [0027]     Winding phases V and W are only shown schematically in  FIG. 2 , since it is clear to those skilled in the art, that they have the same topology, but are displaced, in the counterclockwise direction, by 22.5° mech. (phase V) or 45° mech. (phase W) with respect to phase U.  
         [0028]     In the present invention, one tries to cause sinusoidal voltages to be induced in the individual phases U, V and W as rotor  36  turns. One thus also speaks of a “sine motor.” In phases U, V, and W, sinusoidal currents are then impressed.  
         [0029]     The structure of rotor  36  will now be explained with reference to  FIG. 3  which is an enlargement of a detail of  FIG. 2 . Shaft  40  is not shown there, since it consists of a ferromagnetic material and forms a portion of the magnetic circuit in rotor  36 .  
         [0030]     Rotor  36  has, in its center, a magnetic core or armature  130  which is composed, in the usual manner, of a stack of stamped laminations. This stack is preferably constructed in the same manner as the one which is thoroughly shown and described in WO 03/081748-A1,  FIGS. 2-9 , filed 10 Jan. 2003, corresponding to my earlier U.S. patent application Ser. No. 10/390,824, filed 18 Mar. 2003. For the sake of brevity, the entire content of these applications is hereby incorporated by reference.  
         [0031]     A plurality of pole pieces  136 A,  136 B,  136 C, etc. are connected with core  130  via radially-oriented narrow connecting parts  132  and attached circumferentially-oriented connecting parts  134   a ,  134   b . The axis of symmetry of pole shoe  136 B is designated  137 .  
         [0032]     The pole shoes  136  each have, on their core-adjacent side, a respective interfacing surface  138 A,  138 B,  138 C, hereinafter designated as “magnet/pole shoe boundary” and which is parallel to, and spaced by a distance D from, respective opposing surfaces  140 A,  140 B,  140 C etc. Instead of a single magnet  38 , one could assemble it from multiple parts, as is known to those skilled in the art.  
         [0033]     Between these interfacing surfaces  138 ,  140 , the already-described permanent magnets  38 A,  38 B,  38 C are inserted. They each have a rectangular cross-section and a magnetization which is illustrated in  FIG. 4 .  
         [0034]     As shown in  FIG. 3 , each magnet  38  has, at its magnet/pole shoe boundary, an angular extent β M , and this angular extent corresponds to that of the contiguous pole shoe  136 . If one goes radially outward from this magnet/pole shoe boundary  138 , the width β of pole shoe  136  tapers down within a substantially continuous transition zone  139  and reaches, at a point  142 , its smallest width β C , which is smaller than β M , as shown in  FIG. 3 . Radially outward of point  142 , pole shoe  136  transitions sidewise into the peripherally extending connecting parts  134   a ,  134   b . These parts, during operation, are magnetically saturated, so in the context of the present invention, they primarily have a mechanically supporting function. As one recognizes particularly well from  FIG. 3 , pole shoes  136  have, in conjunction with the holding parts  134 , essentially the cross-sectional shape of an American mortarboard cap, and this represents a preferred form of these pole shoes.  
         [0035]     As one further recognizes from  FIG. 3 , the radially outer side  144  of a pole shoe  136  is so configured that, in air gap  39 , an approximately sinusoidal flux distribution arises, i.e. the diameter of outer side  144  diminishes, starting from the center of a rotor pole, in both sidewise directions, as shown.  
         [0036]     On both sides of permanent magnet  39 , referring to the circumferential direction, there is formed a respective cavity  146   a ,  146   b , whose cross-sectional shape approximates a right triangle. The long diagonal side of the triangle has kind of a dogleg, because a radially outer corner of permanent magnet  38  juts somewhat into this cavity  146 .  
         [0037]     In  FIG. 3 , the pole pitch τ P  of rotor pole  136 B is indicated. Since rotor  36  has eight poles  136 , the corresponding pole pitch τ P =360°/8=45°  mech.   (2)  
         [0038]     As also indicated in  FIG. 3 , a permanent magnet  38  has, on its inner boundary surface  140 , a magnetic width β Bi  of about 41° mech., i.e. about 91% of a pole width τ P . Magnet  38  extends almost to connecting part  132 , and the volume of magnets  3 B is therefore large.  
         [0039]     Normally, such a rotor topology would be undesirable and would lead to more of a rectangular flux distribution in air gap  39 , and to a high cogging torque. However, by means of the constriction  142  of pole shoes  136  having the angle β C  which is smaller than β M , one obtains a flux distribution which quite closely approaches the desired sinusoidal form.  
         [0040]     Please refer to  FIG. 4 , which shows the distribution of flux lines.  
         [0041]     As one sees, e.g. at magnet  38 B, on its two sides, referring to the circumferential direction, a portion of the flux passes through cavities  146  ( FIG. 3 ) adjacent the constrictions  142 . These cavities act like a supplemental magnetic resistance and, since a cavity broadens circumferentially in a direction away from constriction  142 , the magnetic resistance also increases as one moves circumferentially away from this point  142  ( FIG. 3 ). One thereby obtains the substantially sinusoidal flux distribution shown in  FIG. 4 , i.e. the constrictions  142  cause, first, a concentration of the magnetic flux at the pole center and, secondly, act as magnetic resistances which permit, even adjacent the pole gaps of rotor poles  136 , a small magnetic flux, as one desires for a sinusoidal flux distribution. By corresponding dimensioning of the cavities  146 , one can “titrate” (incrementally adjust) the flux at the sides.  
         [0042]     I have discovered that it is important, for the magnitude of the cogging torque, that the angle β C  ( FIG. 3 ) has, at most, the magnitude of the angle of a slot pitch β C  and preferably is smaller than that. In the embodiment illustrated, 
 
τ S =360°/12=30 ° mech.   (3) 
 
 and the angle β C  is about 27° mech., i.e. about 90% of τ S . It has been found that, for β C  in a concentrated winding, approximately the following relation should hold: 
 
β C =τ S   *m   (4) 
 
 where m=0.8, . . . 1.0, and 
 
 all angles are measured in mechanical degrees. 
 
 In case a distributed winding is used, the equation reads; 
 
β C =τ S   *m*n   (5) 
 
 where m=0.8. . . 1.0 and 
 
 n=1, 2, 3, . . . 
 
 Preferably, m has a value between 0.8 and 0.98. 
 
         [0050]     It has been found that, in this manner, particularly when using concentrated windings, a very good sinusoidal form of the induced voltage can be obtained, in conjunction with an acceptable cogging torque. The considerable magnetic width β Mi  of magnets  38  allows a corresponding size reduction for motor  20 , compared with prior art versions. In one exemplary embodiment, there resulted a longitudinal size reduction in the motor from 68 mm down to 50 mm, with the same output power; in actual practice, differing values may be obtained.  
         [0051]      FIGS. 5 and 6  are views analogous to  FIG. 3 . The stator teeth  122 A through  122 D are not shown in  FIGS. 5-6 , but match those shown in  FIG. 3 .  
         [0052]      FIG. 5  shows a sector of a rotor lamination  236 , in which the magnets  38 ,  38 A and  38 B are shown. This rotor lamination has symmetrical poles  136 ,  136 A,  136 B since constrictions  142  are symmetrical with respect to a symmetry line  137 , as also shown in  FIGS. 2-4 .  
         [0053]      FIG. 6  shows, in a differing embodiment, a sector of a first variant of a rotor lamination  236   a , in which the recesses  138 A,  138 B for the rotor magnets  38 ,  38 A,  38 B match the corresponding recesses of  FIG. 5 , but the constrictions  142 L on the left side of a rotor pole (e.g.  136 A) are, in  FIG. 6 , shorter than the constrictions  142 R on the right side of this pole.  
         [0054]     As a result, although the value of angle β C  remains unchanged from the  FIG. 5  embodiment, the angle β CL  between symmetry line  137  and the left constriction  142 L is greater than the angle β CR  between symmetry line  137  and the right constriction  142 R, so that the following relation exists: 
 
β CL &gt;β CR   (6). 
 
         [0055]     Conversely, in a second version with laminations  236   b , one could make β CR  larger than β CL  (not shown). This could be done by simply inverting the lamination  236   a  shown in  FIG. 6 , so that its underside is now face up.  
         [0056]     If one makes a rotor lamination stack by, for example, first using a symmetrical lamination  236  according to  FIG. 5 , then placing an asymmetrical lamination  236   a  according to  FIG. 6 , then an asymmetrical lamination  236   b  (inverted lamination  236   a ), then again a lamination  236 , then  236   a , etc., one will obtain a better form of the voltage induced in the (unillustrated) stator winding and one obtains, by this simple expedient, a better smoothed torque.  
         [0057]     Openings  138 A,  138 B for magnets  38  have all the same positions in all laminations  236 ,  236   a ,  236   b , so that the symmetry lines  137  will match in all laminations.  
         [0058]     Naturally, within the scope of the present invention, many changes and modifications are possible, so the invention is not limited to the specific embodiments shown and described. Rather, the invention is defined by the following claims.