Patent Publication Number: US-2021194301-A1

Title: Rotary electrical machine with a ratio of dimensions which minimises the noise

Description:
The present invention relates to a rotor of a single-lip rotary electrical machine which is used in particular in motor vehicles. It also relates to a rotary electrical machine comprising the rotor according to the present invention. 
     The invention has a particularly advantageous, but non-exclusive application with high-power electrical machines which can operate in alternator mode and in motor mode. 
     In a known manner, rotary electrical machines comprise a stator and a rotor integral with the shaft. The rotor can be integral with a driving and/or driven shaft, and can belong to a rotary electrical machine in the form of an alternator, an electrical machine, or a reversible machine which can operate in both modes. 
     The stator is fitted in a housing which is configured to rotate the shaft, for example by means of roller bearings. The stator comprises a body provided with a plurality of teeth defining notches, and a winding which is inserted in the notches of the stator. 
     The winding is obtained for example from continuous wires covered with enamel, or from conductive elements in the form of pins which are connected to one another by welding. Alternatively, the phases of the machine are formed from individual coils which are each wound around a stator tooth. The phases, which are connected in the form of a star or a triangle, comprise outputs which are connected to an electrical control module. In addition, the rotor comprises a body formed by a stack of metal plate sheets, which are retained in the form of a set by means of an appropriate securing system. The rotor comprises poles, which are formed for example by permanent magnets accommodated in cavities each delimited by two adjacent rotor teeth. 
     A certain number of machines according to the prior art, such as synchronous machines with permanent magnets, have optimal performance levels such as the torque and the output, but they also have a problem with noise. A plurality of works have been tested by various means for reduction of the noise, for example by optimising the geometric form of each stator tooth, by working on the form of the magnet wheel, or on the control algorithm. 
     The objective of the invention is to propose an optimal definition of the rotor, in order to reduce the noise of electromagnetic origin of the machine, without detracting from the performance levels. 
     For this purpose, the subject of the invention is a rotary electrical machine, in particular for a motor vehicle, having an axis of rotation, a rotor radius, a polar step, and comprising:
         a body comprising a plurality of rotor teeth defining cavities in which permanent magnets are accommodated, each rotor tooth comprising a single lip for radial retention of a permanent magnet; and   an assembly of magnetic poles, each pole having a curvature in the form of an arc of a circle, the said arc of a circle being defined by a radius of curvature, a centre of curvature, and a polar angle,
 
wherein the rotor has a ratio between the radius of curvature and the rotor radius contained between 0.52 and 0.63, and a ratio between the polar half-angle and the polar step contained between 0.62 and 0.80.
       

     The definition of a ratio of this type makes it possible to obtain an optimum compromise between the torque supplied by the machine, and the corresponding level of torque undulation. The invention thus makes it possible to improve the magnetic and acoustic performance levels of a rotary electrical machine comprising a rotor of this type. In fact, the radius of curvature of the magnetic pole and the polar angle are the two geometric parameters which affect the performance levels of the electrical machine most. 
     In addition, each rotor tooth comprising a single lip for radial retention of a permanent magnet thus makes it possible, by eliminating one of the retention lips, to reduce the magnetic leakages between the poles of the rotor. This therefore improves the magnetic performance levels of the rotor, which makes it possible to reduce the mass of permanent magnets used, in order to make savings on the cost price of the machine. 
     Within the context of the present invention, the “polar step” is defined as being equal to 360° divided by the number of poles. “Arc of a circle” designates the osculating arc of a circle of the pole. The centre of this circle is known as the centre of curvature of the curve at the point M, and its radius is the radius of curvature. 
     In the description and the claims, the terms “outer” and “inner” will be used, as well as the orientations “axial” and “radial” in order, according to the definitions given in the description, to designate elements of the rotor, the stator and/or of the electrical machine. By convention, the “radial” orientation is directed orthogonally to the axial orientation. According to the context, the axial orientation relates to the axis of rotation of the rotor, the stator and/or the electrical machine. The “circumferential” orientation is directed orthogonally to the axial direction and orthogonally to the radial direction. The terms “outer” and “inner” are used to define the relative position of one element in relation to another, in relation to the reference axis, and an element close to the axis is thus classified as inner as opposed to an outer element which is situated radially on the periphery. 
     In addition, any interval of values which is designated by the expression “between a and b” signifies the field of values going up to b (i.e. including the strict limits a and b). 
     The radius of curvature and the rotor radius are expressed in millimetres [mm], and the polar half-angle and the polar step are expressed in degrees [° ]. 
     According to other advantageous embodiments, a rotor of this type can have one or a plurality of characteristics alone or in combination. 
     According to one embodiment, the rotor has a ratio between the radius of curvature and the rotor radius contained between 0.54 and 0.61, and a ratio between the polar half-angle and the polar step contained between 0.64 and 0.78. This ratio makes it possible to reduce the torque undulations even more efficiently, whilst maintaining a very satisfactory level of the mean torque supplied by the machine comprising the rotor according to the present invention. The term “contained” means that the limits of the intervals are included in the interval. 
     According to one embodiment, each rotor tooth comprises a radially outer surface, the said surface being defined by the said arc of a circle with a centre of curvature, with one of the two ends comprising the said lip. 
     According to one embodiment, the two ends of a radially outer surface of a rotor tooth have a curvature different from the curvature of the arc of a circle with a centre of curvature of this same surface. In other words, the two ends, with one of the two ends comprising the lip, are not inscribed in the circle of curvature defining the arc of a circle of each magnetic pole. 
     As a variant, the two ends of a radially outer surface of a rotor tooth are not inscribed in the circle of curvature defining the arc of a circle of each magnetic pole. The ends and the arc of a circle of each pole thus have the same curvature. 
     According to one embodiment, the two ends of a single radially outer surface of the rotor tooth are asymmetrical. This is asymmetry (which is also known as dis-symmetry) makes it possible to reduce the torque undulations even more efficiently, whilst maintaining a very satisfactory level of the mean torque supplied by the machine comprising the rotor according to the present invention. 
     As a variant, the two ends of a single radially outer surface of a rotor tooth are symmetrical. 
     According to one embodiment, the rotor is of the type with concentration of flux. 
     According to one embodiment, the rotor comprises added-on strips which are each fitted between a lip and an outer periphery of a corresponding permanent magnet. 
     According to one embodiment, the permanent magnets are made of rare earth. 
     According to one embodiment, the rotor is formed by a set of metal plates. 
     According to one embodiment, the rotor comprises a core provided with a central opening for the passage of a shaft, the said rotor teeth being connected to the said core by one of their ends. 
     According to another aspect, the invention relates to a rotary electrical machine comprising a rotor as previously defined. 
    
    
     
       The invention will be better understood by reading the following description and examining the figures which accompany it. These figures are provided purely by way of illustration and in no way limit the invention. 
         FIG. 1  represents a view in transverse cross-section of the rotary electrical machine according to the present invention. 
         FIG. 2  represents a detailed view in transverse cross-section of a rotor according to a first embodiment. 
         FIG. 3  represents a detailed view in transverse cross-section of a rotor according to a second embodiment. 
     
    
    
     Elements which are identical, similar or analogous retain the same reference from one figure to another. 
       FIG. 1  shows a rotary electrical machine  10  comprising a rotor  11  with an axis of rotation X, a rotor radius Rr, a polar step τ, and which is designed to be fitted on a shaft  12 . A wound stator  15 , which can be polyphase, surrounds the rotor  11  with the presence of an air gap between the outer periphery of the rotor  11  and the inner periphery of the stator  15 . This stator  15  is secured on a housing which is configured to rotate the shaft  12  via ball bearings and/or needle bearings. 
     More specifically, the stator  15  comprises a body  16  and a winding  17 . The stator body  16  consists of an axial stack of flat metal plates. The body  16  comprises stator teeth  20  which are distributed angularly regularly on an inner periphery of a yoke  21 . These teeth  20  delimit notches  24 , such that each notch  24  is delimited by two successive teeth  20 . The yoke  21  thus corresponds to the solid outer annular portion of the body  16  which extends between the base of the notches  24  and the outer periphery of the stator  15 . 
     The notches  24  open axially into the axial end faces of the body  16 . The notches  24  are also open radially towards the interior of the body  16 . 
     The stator  15  is provided with tooth roots  25  on the side of the free ends of the teeth  20 . Each tooth root  25  extends circumferentially on both sides of a corresponding tooth  20 . Each tooth root  25  extends circumferentially according to an arc of a circle centred on the axis of rotation X. 
     In order to obtain the winding  17 , a plurality of phases are formed by coils which are each wound around a stator tooth  20 . Each coil is formed from an electrically conductive wire covered with a layer of electrically insulating material such as enamel. A coil insulator can be interposed between each coil and the corresponding tooth  20 . 
     In addition, the rotor  11  comprises a body  31  formed by an axial stack of flat metal plates, in order to reduce the Foucault currents. The body  31  is made of ferromagnetic material. The body  31  can be connected in rotation to the shaft  12  of the rotary electrical machine in different ways, for example by forcing of the ribbed shaft  12  into the interior of the central opening  32  of the rotor, which opening is provided in the core  33 . 
     The rotor  11  comprises a plurality of cavities  36 , in the interior of which there are positioned permanent magnets  37  forming magnetic poles  50 . A cavity  36  can contain a single magnet  37  with a parallelepiped form. As a variant, a cavity  36  can contain a plurality of magnets  37  stacked axially or radially relative to one another. The magnets  37  can have bevelled angles. 
     The magnets  37  are preferably made of rare earth, in order to maximise the power of the machine  10 . As a variant, the magnets  37  can be of a different grade in order to reduce the costs. For example, in each cavity  36 , there is association of a magnet made of rare earth and a magnet made of ferrite, which is less powerful but less costly. 
     The rotor  11  has concentration of flux, i.e. the lateral faces facing one another of the magnets  37  situated in two consecutive cavities have the same polarity. 
     Each cavity  36  passes through the body  31  from one axial end face to the other, and is open on the side of the outer periphery of the rotor  11 . As a variant, the cavities  36  can be blind. 
     In this case, each cavity  36  is delimited by two adjacent rotor teeth  40  of the rotor  11 . The rotor teeth  40  are spaced angularly regularly from one another. The rotor teeth  40  are connected by one of their ends to the core  33  provided with the opening  32 . 
     In order to ensure axial retention of the magnets  37 , flanges (not represented) are placed against the axial ends of the rotor body  31 . These flanges can also have a function of balancing of the rotor  11 . 
     According to a particular embodiment not represented, it is possible to provide added-on strips which are each fitted between a lip  41  and an outer periphery of a corresponding permanent magnet  37 . Each strip is in the form of a small plate made of a material which is less hard than the material of the magnets  37 . This can for example be glass fibres embedded in a pre-impregnated plastic material. The strip is flat, rectangular, and has substantially the same dimensions and the same form as the outer circumferential face of the magnet  37 , which it covers with its coinciding edges. According to a different embodiment, the dimensions can be different. A layer of adhesive more flexible than the magnet  37  can be interposed between the magnet  37  and the strip. 
     Within the context of the present invention, each rotor tooth  40  comprises a single lip  41 . In the examples considered, the cavity  36  is partly closed at the outer periphery of the rotor by a single lip  41  for radial retention of the magnet. Thus, all the lips  41  extend circumferentially from the same face of a corresponding rotor tooth  40 . 
     In the examples considered, a single lip  41  extends from the left face of each rotor tooth  40 . In a variant not represented, a single lip  41  could extend from the right face of each rotor tooth  40 . 
     With reference to  FIGS. 2 and 3 , each magnetic pole  50  has curvature in the form of an arc of a circle. The arc of a circle corresponds to the common part between the magnetic pole  50  and the osculating circle of this same pole  50  defined by a radius of curvature Rc, a centre of curvature Cc, and a polar angle  2 αp. The arc of a circle is thus limited at its two circumferential ends by points M 1  and M 2 , the points M 1  and M 2  being the final points which are situated both on the osculating circle of the magnetic pole  50  and on the magnetic pole  50 . 
     The polar angle  2 αp is thus equal to the angle between two straight lines D 1  and D 2  each passing via the centre of curvature Cc and an end M 1 /M 2 . 
     Each rotor tooth  40  comprises a radially outer surface  42 , situated facing the inner periphery of the stator  15 . The surface  42  is thus defined by the said arc of a circle with a centre Cc and two ends  43 . In the examples considered, one of the two ends  43  comprises the lip  41 . 
     According to a preferred embodiment, the two ends  43  of a surface  42  have a different curvature from the curvature of the arc of a circle with a centre Cc of this same surface  42 . In other words, the radially outer surface of the ends  43  does not have any point situated on the osculating circle of the magnetic pole  50  and comprising the said arc of a circle delimited by the points M 1  and M 2 . This makes it possible to obtain a machine with a variable air gap. 
     In the rotor shown in  FIG. 2 , the two ends  43  of a single surface  42  are asymmetrical, whereas in the rotor shown in  FIG. 3 , the two ends  43  of a single surface  42  are symmetrical. The symmetry corresponds to an outer surface profile which is symmetrical relative to a median straight line of the pole. 
     In the context of the present invention, the rotor has a ratio r between the radius of curvature Rc and the rotor radius Rr contained between 0.5 and 0.7, and a ratio α between the polar half-angle αp and the polar step τ contained between 0.6 and 0.8. 
     A description will be provided hereinafter of how these ratios have been determined. The objective is to attempt to minimise the noise by means of a process of optimisation of the radius of curvature and the polar half-angle, but whilst maintaining the magnetic performance levels required by the specifications. 
     A conventional machine with a magnet in the form of an I comprising two lips per pole (also known as bi-lip) with magnets made of neodymium, iron and boron (Nd2Fe14B), with 10 poles, 15 notches, a concentrated winding, a package height of 217 mm and a stator outer diameter of 93 mm was used as a reference machine. An optimal combination of the performance levels was thus obtained on this machine as far as the torque and noise are concerned by seeking the optimal combination of the radius of curvature Rc and the polar half-angle αp, with values respectively of 18 mm and 22°. By relating these values to the rotor radius Rr and the polar step τ, these optimal values become r=0.57 and α=0.61. 
     The reference machine was then modified according to a machine 1 ( FIG. 2 —asymmetrical ends  43 ) and a machine 2 ( FIG. 3 —symmetrical ends  43 ) in order to reduce the noise of electromagnetic origin, whilst providing an optimal performance level imposed by the specifications, in particular the harmonic distortion level (THD) of the electromagnetic force. The process adopted consists of finding the optimal combination of the radius of curvature Rc relative to the rotor radius Rr and of the polar half-angle αp relative to the polar step τ, by varying parameters in the permissible ranges. 
     By means of a 2-D analysis then 3-D calculations, the parameters r and α were varied in order to obtain an optimal compromise for the values of the aforementioned machines. On the basis of the results indicating the THD of the electromagnetic force (THD of the emf), and the harmonics of order  30  and  40  of the force sustained by each stator tooth (force of interaction between the rotor and the stator which give rise to the noise of the machine) it was possible to determine which were the solutions which succeeded in complying with the THD required, whilst minimising the harmonics  30  and  40  (Fy  30 , Fy  40 ) of the component and the force on each stator tooth. 
     On the basis of these results, it was possible to determine the optimal ranges of values, i.e. a ratio r between the radius of curvature Rc and the rotor radius Rr contained between 0.52 and 0.63, and a ratio α between the polar half-angle αp and the polar step τ contained between 0.62 and 0.80. 
     The performance levels can be further improved when the rotor has a ratio between the radius of curvature Rc and the rotor radius contained between 0.54 and 0.61, and a ratio α between the polar half-angle and the polar step contained between 0.64 and 0.78. 
     Six embodiments will now be presented, i.e. the reference machine (MAR), two machines according to the present invention (machine 1—MA1 and machine 2—MA2), and three comparative machines (MAC1, MAC2 and MAC3). These machines are presented with their characteristics in the following table 1. The comparative machines (MAC1, MAC2 and MAC3) are machines 1 wherein either the ratio r or the ratio α is varied, such that one of these ratios is no longer in the range of the present invention. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Characteristics 
                 MAR 
                 MA1 
                 MA2 
                 MAC1 
                 MAC2 
                 MAC3 
               
               
                   
               
             
            
               
                 Height [p.u.] 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 αp 
                 0.61 
                 0.76 
                 0.78 
                 0.78 
                 0.76 
                 0.56 
               
               
                 r [p.u.] 
                 0.58 
                 0.58 
                 0.58 
                 0.51 
                 0.64 
                 0.58 
               
               
                 Minimal air gap [p.u.] 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Magnet thickness [p.u.] 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
            
           
         
       
     
     The performance levels of the six machines are then summarised in the following table 2. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Characteristics 
                 SPEC 
                 MAR 
                 MA1 
                 MA2 
                 MAC1 
                 MAC2 
                 MAC3 
               
               
                   
               
             
            
               
                 emf THD 
                 &lt;8% 
                 6.64 
                 7.92 
                 6.57 
                 8.17 
                 7.36 
                 5.98 
               
               
                 Fy30 [N] 
                 To be 
                 1 
                 0.05 
                 0.30 
                 1.44 
                 1.23 
                 1.44 
               
               
                   
                 reduced 
                   
                   
                   
                   
                   
                   
               
               
                 Fy40 [N] 
                 To be 
                 1 
                 0.69 
                 1 
                 0.41 
                 1.28 
                 1.31 
               
               
                   
                 reduced 
               
               
                   
               
            
           
         
       
     
     Compared with the initial reference machine, the machine 1 has made it possible to reduce the harmonics  30  and  40  of the component and the force acting on each stator tooth, whilst maintaining an emf THD lower than the limit imposed. This makes it possible also to reduce the noise of the machine to a certain level. The machine 2 for its part reduces the harmonic F 30  significantly compared with that of the reference machine. Its harmonic F 40  remains at a level which is equivalent to that of the reference machine. Consequently, this machine has less noise than the reference machine, whilst having more torque. 
     On the other hand, when the ratio r is lower than 0.52, only the harmonic F 4  is reduced, with the harmonic F 30  having increased and the THD exceeding the minimum required (MAC 1 ). When the ratio r is greater than 0.63 or a is lower than 0.62, the THD is well below the specification, but there is no improvement in the noise level; on the contrary. 
     It will be appreciated that the foregoing description has been provided purely by way of example, and does not limit the scope of the invention, a departure from which would not be constituted by replacing the different elements by any other equivalents. 
     In addition, the different characteristics, variants, and/or embodiments of the invention can be associated with one another according to different combinations, provided that they are not incompatible or mutually exclusive.