Abstract:
The invention relates to a permanent magnet synchronous motor, in particular an electric three phase motor, comprising a stator in which stator teeth with interposed stator grooves are arranged, at least one winding made of an electrically conductive material being provided on each stator tooth, further comprising a rotor with permanent magnets which are arranged radially in a spoke-like manner in the rotor. Said synchronous motor has a predetermined and defined maximum brake torque based on a predetermined diameter-length-ratio of the rotor, a number of rotor poles and a number of stator grooves. The present invention further relates to an electric power-assisted steering system.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a permanent magnet synchronous motor, in particular a three-phase electric motor. The present invention also relates to an electric power-assisted steering system of a motor vehicle. 
       TECHNICAL BACKGROUND 
       [0002]    Permanent magnet synchronous motors of this type are, for example, used in drive devices of motor vehicles, in particular in power-assisted steering systems and the like. 
         [0003]      FIG. 1  schematically shows the structure and the mode of functioning of a generally known power-assisted steering system  100  for a motor vehicle. The power-assisted steering system shown there comprises a steering gear  101 , a steering shaft  102  with a steering wheel, not designated in more detail, and a servo drive  103  with a control unit  104 . A track rod  107  cooperates with the steering gear  101  by means of a pinion  105 . The steering gear  101  has a gear rack (only indicated symbolically by a double arrow) here, which is connected to the pinion  105 . The pinion  105  is rotatably coupled to the steering wheel by the steering shaft  102 . The track rod  107  cooperates with steerable wheels  106  of the motor vehicle. 
         [0004]    The servo drive  103  is equipped, for example, with a three-phase electric motor and is attached here with the control unit  104  between the steering wheel and the steering gear  101 . The servo drive  103  is used as assistance during steering operations in that it cooperates with the gear rack. The servo drive  103  can be attached on a steering column for this purpose. 
         [0005]    A motor currently used as a servo drive  103  may be a brushless motor, for example a permanent magnet synchronous motor or an asynchronous motor. For example, a permanent magnet synchronous motor has a stator with stator grooves, in which windings are arranged, and a rotor with permanent magnets. The permanent magnets are, for example, produced from a rare earth material and arranged in a spoke-like manner in the rotor. 
         [0006]    A motor of this type has to satisfy at least the following two main requirements: 1) performance and 2) safety. In relation to the performance, parameters such as, for example, power density, maximum output torque, torque fluctuation or ripple and cogging torque or pulsating torque are important. In the case of safety, the parameter of maximum brake torque is, inter alia, critical at a phase short circuit and, for example, has to be limited to a value of about 0.6 Nm at a complete short circuit of a phase. 
         [0007]    WO 2002/060740 A2 and EP 1028047 B1 provide examples or solution proposals for the above problems. 
         [0008]    The increased requirements of the use conditions in a motor vehicle, in particular in relation to as small an installation volume as possible, low weight, low number of individual parts used and a simultaneously high efficiency, and the above-mentioned parameters result in the constantly present requirement of providing a correspondingly improved permanent magnet synchronous motor. 
       SUMMARY OF THE INVENTION 
       [0009]    Against this background, the present invention is based on the object of providing an improved permanent magnet synchronous motor. 
         [0010]    According to the invention, this object is achieved by a permanent magnet synchronous motor having the features of claim  1  and/or by a power-assisted steering system having the features of claim  10 . 
         [0011]    Accordingly, there is provided
       a permanent magnet synchronous motor, in particular a three-phase electric motor, with a stator, in which stator teeth with interposed stator grooves are arranged, at least one winding made of an electrically conductive material being provided on each stator tooth, with a rotor with permanent magnets, which are arranged radially in a spoke-like manner in the rotor, wherein the synchronous motor is configured with a predetermined and limited maximum brake torque, based on a predetermined diameter/length ratio of the rotor, a rotor pole number and a number of stator grooves.   a power-assisted steering arrangement of a motor vehicle has a permanent magnet synchronous motor according to the invention.       
 
         [0014]    The idea on which the present invention is based consists in that a maximum brake torque of the permanent magnet synchronous motor, based on a previously established diameter/length ratio, a rotor pole number and a number of stator grooves, can be established beforehand. A permanent magnet synchronous motor of this type thus has the advantages of a very small cogging torque linked to a high power density and simultaneously low torque ripple and high fault tolerance. 
         [0015]    A further advantage is that the permanent magnet synchronous motor according to the invention has, in comparison with conventional permanent magnet synchronous motors, a relatively low brake torque, which may be up to 40% lower. 
         [0016]    In comparison with a conventional permanent magnet synchronous motor, the permanent magnet synchronous motor according to the invention also has the following advantages:
       increased performance of about 10%;   brake torque that is lower by about 40%;   reduced cogging torque;   reduced torque ripple;   no relay necessary;   a delta connection can be used;   simple stator construction.       
 
         [0024]    The previously established diameter/length ratio of the rotor designates a quotient of a diameter of the rotor and a length of the rotor. Thus, simple geometric parameters, which are easy to master, are decisive for establishing the maximum brake torque. 
         [0025]    The described permanent magnet synchronous motor is preferably suitable for the drive device of a motor vehicle power-assisted steering system. However, other applications in other drive devices, such as, for example, in electric parking brakes, are also conceivable. 
         [0026]    Advantageous configurations and developments of the invention emerge from the subordinate claims and from the description viewed together with the figures of the drawings. The permanent magnet synchronous motor has nine or twelve stator grooves. The rotor pole number of the rotor is preferably six, eight, ten or fourteen. In this case, a conventional standard design of a stator can be used. 
         [0027]    It has surprisingly turned out that the previously established diameter/length ratio of the rotor, the rotor pole number and the number of stator grooves are related in accordance with the following table: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Rotor pole number 
                 Number of stator 
                   
               
               
                 210-n 
                 grooves 202 
                 DLV 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 6 
                 9 
                 &gt;1.63 
               
               
                 8 
                 12 
                 &gt;1.55 
               
               
                 10 
                 12 
                 &gt;1.50 
               
               
                 14 
                 12 
                 &gt;1.42 
               
               
                   
               
             
          
         
       
     
         [0028]    The rotor may have at least one rotor packet in a further embodiment, a rotor packet number depending on the rotor length in accordance with the following table: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Rotor length L (mm) 
                 Rotor packet number 209-n 
               
               
                   
                   
               
             
             
               
                   
                 L ≦ 20 
                 1 
               
               
                   
                 20 ≦ L ≦ 40 
                 1 or 2 
               
               
                   
                 40 ≦ L ≦ 60 
                 2 or 3 
               
               
                   
                  60 ≦ L ≦ 100 
                 3 or 4 
               
               
                   
                   
               
             
          
         
       
     
         [0029]    Moreover, in the case of a rotor packet number greater than one, the rotor packets may be arranged rotated in relation to one another by an interconnection angle about a rotor axis. In this case, the interconnection angle may depend on the diameter/length ratio of the rotor in accordance with the following table: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Rotor pole number 
                 Number of stator 
                 Interconnection 
                   
               
               
                 210-n 
                 grooves 202 
                 angle γ (°) 
                 DLV 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 6 
                 9 
                 9 ± 2 
                 &gt;1.63 
               
               
                 8 
                 12 
                 7.5 ± 2     
                 &gt;1.55 
               
               
                 10 
                 12 
                 6 ± 2 
                 &gt;1.50 
               
               
                 14 
                 12 
                 2 ± 1 
                 &gt;1.42 
               
               
                   
               
             
          
         
       
     
         [0030]    In a further embodiment, the stator has a three-phase winding in a delta connection. This is advantageous as, on the one hand, a so-called star relay can be dispensed with for separating a phase in the event of a short circuit. On the other hand, manufacturing of the windings is thereby facilitated. 
         [0031]    The stator may, however, also have a three-phase winding in a star connection. The star relay for separating a phase in the event of a short circuit may also be dispensed with here. 
         [0032]    In a further embodiment, the permanent magnets of the rotor may contain ferrite magnets or/and materials made of rare earths. Because of the diameter/length ratio that can be established beforehand and the other parameters it may be possible here for the permanent magnets to be able to be used in a low power stage, which significantly reduces the costs for the permanent magnet synchronous motors according to the invention. 
         [0033]    The above configurations and developments of the invention can be combined with one another in any desired manner. 
     
    
     
       CONTENTS OF THE DRAWINGS 
         [0034]    The present invention will be described in more detail below with the aid of the embodiments disclosed in the schematic figures of the drawings, in which: 
           [0035]      FIG. 1  is a schematic view of a power-assisted steering system according to the prior art; 
           [0036]      FIG. 2  is a schematic plan view of an embodiment of a permanent magnet synchronous motor according to the invention; 
           [0037]      FIG. 3  is a schematic perspective view of a rotor of the permanent magnet synchronous motor according to the invention according to  FIG. 2 ; 
           [0038]      FIG. 4  is a schematic partial side view of the rotor of the permanent magnet synchronous motor according to the invention according to  FIG. 2 ; 
           [0039]      FIGS. 5-6  are circuit diagrams of windings of the permanent magnet synchronous motor according to the invention according to  FIG. 2 ; and 
           [0040]      FIG. 7  is a graphic view of a brake torque depending on a diameter/length ratio of rotors of the permanent magnet synchronous motor according to the invention according to  FIG. 2 . 
       
    
    
       [0041]    The accompanying drawings are to convey a further understanding of the embodiments of the invention. They illustrate embodiments and are used in conjunction with the description to explain principles and concepts of the invention. Other embodiments and many of the advantages mentioned emerge in view of the drawings. The elements of the drawings are not necessarily shown true-to-scale in relation to one another. 
         [0042]    Elements, features and components that are the same, functionally the same and act the same are in each case provided with the same reference numerals in the figures of the drawings, as long as nothing else is stated. 
       DESCRIPTION OF EMBODIMENTS 
       [0043]      FIG. 2  is a schematic plan view of an embodiment of a permanent magnet synchronous motor  200  according to the invention. 
         [0044]    The permanent magnet synchronous motor  200  in  FIG. 2  has a stator  201  with stator grooves  202 , in which windings (not shown) are arranged, and a rotor  208  with a rotor core  203  and permanent magnets  204 , which are arranged here in the radial direction of the electric motor within the rotor  208  in a spoke-like manner between portions of the rotor core  203 . The rotor  208  is located within the stator  201 . The portions of the rotor core  203  and the permanent magnets  204  are fastened on a rotor body  206 , through which a rotor shaft  206  with a rotor axis  207  runs. The rotor shaft  206  with the rotor axis  207  is perpendicular here to the plane of the drawing. The rotor  208  is located within the stator  201  and with the latter has the common rotor axis, i.e. the rotor  208  and stator  201  are coaxial. The permanents magnets  204  are arranged in such a way that like poles always oppose one another, i.e. north poles N oppose north poles N and south poles S oppose south poles S. 
         [0045]    The stator  201  is configured with twelve stator grooves  202  and twelve stator poles and may be straight or oblique, i.e. the stator grooves  202  run linearly parallel to the rotor axis  207  or obliquely with respect to the latter. The rotor  208 , in contrast, is a ten-pole rotor here, i.e. it has ten permanent magnets  204 . The permanent magnets  204  may be ferrite magnets or/and have materials of rare earths, for example. 
         [0046]      FIG. 3  shows a schematic perspective view of an embodiment of the rotor  208  of the permanent magnet synchronous motor  200  according to  FIG. 2 . 
         [0047]    Two rotor packets  209  are attached on the rotor shaft  206  here. The permanent magnets  204  are arranged radially in a spoke-like manner between portions of the respective rotor cores  203 . The rotor cores  203  and the permanent magnets  204  are connected in a manner not shown in more detail to the rotor body  205 , which is fastened to the rotor shaft  206 . 
         [0048]    The rotor  208  has an external diameter, which is given as the rotor diameter D. In the longitudinal direction parallel to the rotor axis  207 , the rotor has a longitudinal dimension designated the rotor length L. The rotor length L is to be taken to mean the entire length of the rotor packets  209  here in the direction of the rotor axis  207 . The two rotor packets  209  are arranged rotated in relation to one another by an interconnection angle γ about the rotor axis  207  on the rotor shaft  206 . 
         [0049]    It has surprisingly been found that a specific ratio of geometric dimensions of the rotor  208  in combination with the permanent magnets  204 , for example as ferrite permanent magnets, and further parameters of the rotor  208  and stator  201  is decisive for limiting a maximum brake torque of the permanent magnet synchronous motor  200 . A geometric ratio of this type is a diameter/length ratio DLV between the rotor diameter D and the rotor length L, namely the dimensionless quotient: 
         [0000]        DLV=D/L   (1)
 
         [0050]    The further parameters are the pole number or number of teeth of the stator, a rotor pole number  210 - n  (see  FIG. 7 ) of the rotor  208  and the interconnection angle γ of the rotor packets  209  from a rotor packet number  209 - n  greater than one. This is dealt with in more detail below. 
         [0051]    This specific diameter/length ratio DLV with the corresponding boundary conditions based on the further parameters makes possible a permanent magnet synchronous motor  200  with a high power with permanent magnets  204  in a low power stage. Moreover, a small brake torque is produced at a short circuit of a phase. 
         [0052]      FIG. 4  shows a schematic partial side view of the rotor  208  of the permanent magnet synchronous motor  200  according to the invention according to  FIG. 2 . 
         [0053]    The rotor  208  may have a rotor packet  209  or else more than the two rotor packets  209  shown here. A plurality of rotor packets  209  are shown here with a respective rotor packet number  209 - 1 ,  209 - 2  . . .  209 - n . The rotor packets  209  have the interconnection angle γ in relation to one another and are arranged on the rotor axis  207  in the axial direction of the rotor  208 . 
         [0054]    The stator  201  may be configured with two different types of winding.  FIGS. 5 and 6  for this purpose are circuit diagrams of windings of the permanent magnet synchronous motor  200  according to the invention according to  FIG. 2 . 
         [0055]      FIG. 5  shows a conventional star connection  300  with terminal points U, V, W and a centre point or star point  301 . However, the stator may also be provided with a delta connection  302  according to  FIG. 6 . The delta connection provides the advantage here that only three terminals U, V, W are necessary as a star terminal  301  or star connecting of the star connection  300  is dispensed with. Moreover, a winding of the delta connection  302  may be simpler in terms of manufacturing depending on the stator  201  or/and may take place more quickly. 
         [0056]    In a permanent magnet synchronous motor according to the prior art, in the case of the star connection  300 , a star relay (not shown but easily imaginable) is necessary, which, in the event of a short circuit of a phase of the star winding  300  in the star point  301 , brings about an interruption of the respective short-circuited phase in order to limit the brake torque in the event of the short circuit or to prevent it. A relay of this type is not necessary in the permanent magnet synchronous motor  200  according to the invention as, because of the special diameter/length ratio DLV with the further parameters, a short circuit of a phase can only cause a limited brake torque of the permanent magnet synchronous motor  200 . For this reason, the delta connection  302  according to  FIG. 6  can also be implemented as the latter cannot be used without a relay in the prior art or higher outlay with a relay for separating short-circuited phases would be necessary. 
         [0057]      FIG. 7  is a graphic view of a brake torque of the permanent magnet synchronous motor  200  according to the invention depending on a diameter/length ratio DLV of rotors  208  of the permanent magnet synchronous motor  200  according to the invention according to  FIG. 2 . 
         [0058]    A brake torque M BT  in Nm is plotted on the ordinate for a phase of the permanent magnet synchronous motor  200 . The abscissa shows values for the diameter/length ratio DLV of rotors  208 . A maximum brake torque M BTMAX , with a reliable value here of 0.6 Nm, is entered as a dotted line parallel to the abscissa. The maximum brake torque M BTMAX  indicates the brake torque, at which a power-assisted steering system, which is equipped with the permanent magnet synchronous motor  200  as the servo motor, is not yet blocked or is still capable of functioning, for example during a short circuit of a phase. 
         [0059]    Four different curves are plotted, which represent different rotor pole numbers  210 - n  of the rotor  208 . These four curves are intersected by the dotted straight line of the maximum brake torque M BTMAX  parallel to the abscissa. Respective parallels to the ordinate through these intersection points in each case indicate on the abscissa an associated diameter/length ratio DLV 1  . . .  4  of the rotor  208 . 
         [0060]    It has been found that the optimum diameter/length ratio DLV for a permanent magnet synchronous motor  200  with a rotor  208  with permanent magnets  204  arranged in a spoke-like manner according to  FIGS. 2 and 3  depends on the rotor pole number  210 - n , the rotor diameter D and the rotor length L. 
         [0061]    Thus, for example, the optimum diameter/length ratio DLV for a rotor  208  with a rotor pole number  210 - 6  (n=6) of six rotor poles and a stator  201  with nine stator grooves  202  is greater than the numerical value DLV 1 =1.63. 
         [0062]    The interconnections of the optimal diameter/length ratio DLV and the parameter rotor number  210 - n  are clearly shown in Table 1 below. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 DLV and parameter rotor pole number 210-n 
               
             
          
           
               
                   
                   
                 Rotor pole number 
                 Number of stator 
                   
               
               
                   
                 No. 
                 210-n 
                 grooves 202 
                 DLV 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 6 
                 9 
                 &gt;1.63 
               
               
                   
                 2 
                 8 
                 12 
                 &gt;1.55 
               
               
                   
                 3 
                 10 
                 12 
                 &gt;1.50 
               
               
                   
                 4 
                 14 
                 12 
                 &gt;1.42 
               
               
                   
                   
               
             
          
         
       
     
         [0063]    It can thus be seen from these test results that the permanent magnet synchronous motor  200  according to the invention with a brake torque, which is smaller than the maximum brake torque M BTMAX , provides greater safety in relation to a conventional motor, in that in the event of a possible fault, for example in a power-assisted steering system, the steering is not blocked by a brake torque that is too high. 
         [0064]    In addition to the diameter/length ratio DLV, the following parameter has to be adhered to. This further parameter is the rotor packet number  209 - n  of small rotor packets. This is illustrated in Table 2. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Rotor length and rotor packet number 
               
             
          
           
               
                 No. 
                 Rotor length L (mm) 
                 Rotor packet number 209-n 
               
               
                   
               
               
                 1 
                 L ≦ 20 
                 1 
               
               
                 2 
                 20 ≦ L ≦ 40 
                 1 or 2 
               
               
                 3 
                 40 ≦ L ≦ 60 
                 2 or 3 
               
               
                 4 
                  60 ≦ L ≦ 100 
                 3 or 4 
               
               
                   
               
             
          
         
       
     
         [0065]    With an active axial rotor length L, which is less than 20 mm, a rotor packet number  209 - n  with the value n=1 is necessary. A further example (3) shows that with a rotor length L in the range from 40 mm to 60 mm for the rotor packet number  209 - n , the value n=2 or n=3 applies. 
         [0066]    Moreover, Table 1 can be extended by the further parameter interconnection angle γ to Table 3 below. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Interconnection angle γ 
               
             
          
           
               
                   
                 Rotor pole number 
                 Number of stator 
                 Interconnection 
                   
               
               
                 No. 
                 210-n 
                 grooves 202 
                 angle γ (°) 
                 DLV 
               
               
                   
               
             
          
           
               
                 1 
                 6 
                 9 
                 9 ± 2 
                 &gt;1.63 
               
               
                 2 
                 8 
                 12 
                 7.5 ± 2     
                 &gt;1.55 
               
               
                 3 
                 10 
                 12 
                 6 ± 2 
                 &gt;1.50 
               
               
                 4 
                 14 
                 12 
                 2 ± 1 
                 &gt;1.42 
               
               
                   
               
             
          
         
       
     
         [0067]    It can be seen that with an increasing rotor pole number  210 - n , the diameter/length ratio DLV and also the interconnection angle γ reduces. 
         [0068]    Although the present invention is described above using a preferred embodiment, it is not to be limited thereto, but can be modified in any desired manner without deviating from the subject matter of the present invention. 
         [0069]    It is conceivable that the rotor packet number  209 - n  has an n that is greater than in the embodiments shown. 
         [0070]    The use of the permanent magnet synchronous motor  200  according to the invention in a power-assisted steering system  100  ( FIG. 1 ) of a motor vehicle can not only take place on the steering gear  101  but also on the steering shaft  102 , on the pinion  105  or else at a different point with corresponding couplings. 
         [0071]    The use of the permanent magnet synchronous motor  200  according to the invention for a drive device in a motor vehicle and here, in particular, for a servo drive  103  of a power-assisted steering system  100  of a motor vehicle is also merely to be understood by way of example. Rather, the invention can be advantageously used in any electric drives. 
         [0072]    The aforementioned numerical details are indeed preferred and sometimes taken from a specific application, but these may also be varied to a greater or lesser extent in accordance with expert action and knowledge. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           100  power-assisted steering system 
           101  steering gear 
           102  steering shaft 
           103  servo drive 
           104  control unit 
           105  pinion 
           106  wheel 
           107  track rod 
           200  permanent magnet synchronous motor 
           201  stator 
           202  stator groove 
           203  rotor core 
           204  permanent magnet 
           205  rotor body 
           206  rotor shaft 
           207  rotor axis 
           208  rotor 
           209  rotor packet 
           209 - n  rotor packet number 
           210 - n  rotor pole number 
           300  star connection 
           301  star point 
           302  delta connection 
         DLV, DLV 1  . . .  4  diameter/length ratios 
         D rotor diameter 
         L rotor length 
         MBT brake torque 
         N, S magnet poles 
         U, V, W terminals 
         γ Interconnection angle