Abstract:
The invention relates to an electric machine, in particular to a generator for supplying the electrical system of a motor vehicle, comprising a laminated rotor ( 20 ) with an energiser winding ( 29 ), which provides the energisation for the machine preferably in conjunction with permanent magnets ( 24, 25 ) arranged on the rotor periphery. According to the invention, the rotor laminate bundle ( 21 ) is made up of at least two part bundles ( 21   a, b ) in the axial direction with grooves ( 40 ) running into each other and with poles offset relative to each other ( 32  to  38 ). A machine with reduced detent torque is thus obtained with grooves ( 40 ) aligned parallel to the axis, in which the energiser windings ( 29 ) can be placed with a high filling factor and without production difficulties and without the risk of damaging the winding insulation.

Description:
BACKGROUND OF THE INVENTION 
     The invention is based on an electric machine, such as is described in the older German patent application 10 2007 025 971.0. Such a hybrid-excited synchronous machine is suitable, in particular, for feeding the electrical system of motor vehicles, wherein in generator mode said synchronous machine is used with regulated induced voltage in a multi-phase stator winding system, and the poles of the rotor are excited permanently magnetically and/or electrically. 
     SUMMARY OF THE INVENTION 
     The electric machine according to the invention has the advantage that the proposed configuration of the rotor causes fluctuations in torque and noises of the machine caused by the variable forces acting on the stator teeth to be significantly reduced, wherein at the same time the advantage of simple insertion of the exciter winding into the grooves of the rotor by virtue of a direct winding method with a high filling factor of the grooves is retained. In contrast to a purely permanently magnetically excited machine with component magnets which are arranged in an offset fashion, such as is known, for example, from EP 1 447 901 A2, the combination of the permanently magnetic excitation with an exciter winding provides the significant advantage that the output power of the machine can be regulated with simple means with a comparatively small exciter current, which can easily be regulated, in the rotor winding and at the same time the high power density of a permanently magnetically excited machine can be used. 
     In order to reduce the detent torques, the electrically excited poles which are adjacent to a groove opening are preferably embodied each with different pole pitches here, while the remaining electrically or permanently magnetically excited poles on the rotor circumference are expediently embodied with the same pole pitch. In this context it is also advantageous that the aimed-at offset in the laminated rotor packet is achieved with a single sheet-metal cut for the laminations of the laminated rotor packet, solely by virtue of mirror-symmetrical layering of the pieces of sheet metal, while the aligned groove profile remains the same. In this way, with two different sheet-metal cuts it is possible to obtain four component packets of the laminated rotor packet which are offset with respect to one another, and with three different sheet-metal cuts it is possible to bring about a division into six component packets which are rotated with respect to one another, in each case with an unchanged, axis-parallel groove profile. 
     The different pole pitches for the electrically excited poles which adjoin the groove openings are advantageously selected here in such a way that the difference between the pole pitches is smaller than the distance between the detent positions of the rotor. The pole pitch of the permanently magnetically excited poles and of the electrically excited subsequent poles arranged therebetween is expediently located here in the region between the pole pitches of the electrically excited poles, with different widths, at the groove openings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention is illustrated in the drawings and will be explained in more detail in the following description. 
       In the drawings: 
         FIG. 1  shows a longitudinal section through an alternating current generator for motor vehicles with a hybrid-excited rotor in an alternating pole arrangement, and 
         FIGS. 2   a  and  2   b  show views of two component packets of the laminated rotor packet for a machine with two-pole electrical excitation, with a total of 14 poles and with six permanently magnetically excited poles, wherein in the illustrations  2   a  and  2   b  the sheet-metal cut is the same in each case but the sheet-metal laminations are turned. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic illustration of a section through an electric machine  10  in an embodiment as an alternating current generator for motor vehicles. Said alternating current generator has a two-component housing  13 , which is composed of a first end plate  13 . 1  and a second end plate  13 . 2 . The end plate  13 . 1  and the end plate  13 . 2  hold a stator  16 , with a circular-ring-shaped laminated stator packet  17 , in whose inwardly open and axially extending grooves  19  a stator winding  18  is inserted. The annular stator  16  surrounds, with its radially inwardly directed surface, a rotor  20  which is embodied as a hybrid-excited rotor. The stator  16  interacts here, across a working air gap, with the rotor  20  which is rotatably mounted in the stator  16 . 
     The rotor  20  has, in a predefined sequence over its circumference, multiple north poles N and south poles S which are embodied by divided permanent magnets  24 ,  25  and by means of the exciter winding  29 . In this context, the pole number of the rotor  20  can be changed as a function of the strength and direction of an exciter current in the exciter winding  29  and by the number of inserted permanent magnets. 
     The rotor  20  has a magnetically permeable body which is embodied as a divided laminated packet  21 . The laminated rotor packet is laminated in the axial direction with a sheet-metal thickness between 0.1 mm and 2.0 mm. Below 0.1 mm, the resistance capability of the laminated packet  21  to centrifugal forces is too low. Above 2.0 mm, the reduction in the eddy current losses on the outer surface of the rotor  20  is no longer sufficient, which means that the installed permanent magnets  24 ,  25  can be damaged or demagnetized. 
     The axial length of the laminated rotor packet  21  preferably corresponds to the axial length of the circular-ring-shaped laminated stator packet  17 , or up to 2 mm longer or shorter than the laminated stator packet  17  for tolerance compensation, and is preferably held together by weld seams. Instead of welds, it is also possible to use rivets or buttoned connections. 
     In the two-pole variant, the exciter winding  29  is preferably embodied as a diameter coil and is located in grooves which are punched out of the laminated packet  21 . The exciter winding  29  can be inserted, for example as a flyer winding (double flyer) directly into the laminated rotor packet  21 . Furthermore, regions  41  into which permanent magnets  24 ,  25  can be inserted are hollowed out in the laminated rotor packet. 
     According to the invention, the magnets  24 ,  25  are preferably inserted into punched-out pockets in the laminated rotor packet. This makes it possible to take up the centrifugal forces occurring during operation and as a result to ensure that the magnets are held reliably on the rotor. A material with a remanence induction of greater than 1 T proves to be particularly advantageous as a magnetic material. In particular, permanent magnets made of rare-earth material have these magnetic properties. The magnets are installed here in the rotor in such a way that they generate a substantially radial field. This field then crosses from the rotor via the air gap into the laminated stator packet and induces a voltage in the windings of the stator as the rotor rotates. 
     The rotor  20  is mounted rotatably in the respective end plates  13 . 1  and  13 . 2  by means of a shaft  27  and in each case a roller bearing  28  which is located on a rotor side. Said rotor  20  has two axial end faces, to each of which a fan  30  is attached. These fans are essentially composed of a plate-shaped or disk-shaped section, from which fan blades project in a known fashion. The fans  30  serve to permit air to be exchanged between the outside and the interior of the electric machine  10  via openings  48  in the end plates  13 . 1  and  13 . 2 . For this purpose, openings  48  are provided at the axial ends of the end plates  13 . 1  and  13 . 2 , via which openings  48  cooling air is sucked into the interior of the electric machine  10  by means of the fans  30 . This cooling air is accelerated radially outward by the rotation of the fans  30 , with the result that said cooling air can pass through the cooling-air-transmissive winding heads  50  on the drive side and cooling-air-transmissive winding heads  51  on the electronics side. The winding heads  50 ,  51  are cooled by this effect. After the cooling air has passed through the winding heads  50 ,  51  or after it has flowed around the winding heads, said cooling air adopts a radially outward path through openings (not illustrated). 
     On the right-hand side of  FIG. 1  there is a protective cap  47  which protects various components against ambient influences. For example, this protective cap  47  covers a slip ring assembly  49  which supplies the exciter winding  29  with exciter current. A heat sink  53 , which acts here as a positive heat sink, along which positive diodes  59  are mounted, is arranged around this slip ring assembly  49 . The end plate  13 . 2  acts as what is referred to as a negative heat sink. A connecting plate  56 , which connects the negative diodes  58  and positive diodes  59 , attached in the bearing plate  13 . 2 , to one another in the form of a bridge circuit  69  is arranged between the end plate  13 . 2  and the heat sink  53 . 
       FIGS. 2   a  and  2   b  each show the same laminated rotor section, rotated through 180°, of an electric machine  10  with a total of 14 poles. The illustrations correspond here to the views of two component packets  21   a  and  21   b  of the laminated rotor packet  21 , wherein the pole orientations are respectively rotated with respect to one another through the angle α, with the position of the groove center line  22  unchanged. 
     The rotor  20  has two-pole electrical excitation by means of an exciter winding  29 , which is divided into two identical component coils  29   a  and  29   b  which are arranged symmetrically on both sides of an opening  26  for the rotor shaft (not illustrated). The basic excitation of the machine is carried out via a total of eight electrically excited poles, wherein in  FIGS. 2   a  and  2   b  the four upper poles  32 ,  35  and  38  form north poles, and the four lower poles  34 ,  36  and  37  form south poles. Between the electrically excited poles, the sheet-metal cut has projections which correspond to the poles  32  and  34  and which have pocket-shaped cutouts  43  for the insertion of permanent magnets  24  and  25 , wherein the permanent magnets  24  form north poles and the permanent magnets  25  form south poles on the rotor circumference, respectively alternating with the electrically excited poles. 
     The sheet-metal cut for the component packets  21   a  and  21   b  of the laminated rotor packet in  FIGS. 2   a  and  2   b  is the same, but the laminations are rotated through 180°. In this context, the shape and the center line  22  of the grooves  40  remains unchanged, with the result that they extend in alignment over the entire axial length of the rotor  20 , while the poles  32 - 38  in the two component packets  21   a  and  21   b  are offset with respect to one another owing to different pole pitches τ 1  and τ 2  of the poles  35 - 38 . In the axial direction, an electrically excited component pole with the pole pitch τ 2  adjoins an electrically excited component pole with the pole pitch τ 1 , and vice versa, as a result of which the permanent magnet poles  24  and  25  and the electrically excited subsequent poles  32  and  34  located therebetween are respectively rotated with respect to one another through an angle α which corresponds to the different magnitudes of the pole pitches τ 1  and τ 2 . 
     The magnitude of the pole pitch τ 1  for the wider groove edge poles  35  and  37 , τ 2  for the narrower groove edge poles  36  and  38  and τ 3  for the permanent magnet poles and the electrically excited subsequent poles located therebetween is selected in such a way that the pole pitch τ 3  is in the range between the magnitude of the pole pitches τ 1  and τ 2 . In this context, the difference τ 1 −τ 2  of the pole pitches τ 1  and τ 2  of the electrically excited poles  35 - 38  is to be smaller than or at maximum of equal magnitude to the distance between the detent positions of the rotor  20 , which is itself determined by the number of rotor poles and the number of stator teeth. The detent distance here results from the quotient of the rotor circumference and of the smallest common multiple between the number of rotor poles and the stator teeth. In the exemplary embodiment illustrated in  FIGS. 1 and 2 , the stator  42  has teeth, and the rotor  14  has poles. The smallest common multiple here is the tooth number  42 , with the result that the distance between the detent positions of the rotor  20  in this exemplary embodiment is 1/42 of the rotor circumference, or of the air gap length. 
     In terms of manufacturing technology and structure, it is advantageous if the laminated rotor packet  21  has, in the axial direction, at least two component packets  21   a  and  21   b  and at most six component packets. With two component packets, a significant reduction in the torque fluctuations and the noises of the engine is already achieved, and with more than six component packets the improvements no longer justify the difficulties and costs in terms of the manufacture of the rotor. In this context, a groove shape with a substantially bell-shaped cross section has proven advantageous in particular in the case of two-pole electrical excitation of the rotor  20 , because with this groove shape it is particularly easy to wind the grooves with a high filling factor, and symmetrical apportioning of the winding on both sides of the rotor shaft can be implemented particularly easily. Together with the mirror-symmetrical design of the laminated rotor packet  21 , this also allows the unbalance to be minimized during operation of the machine. 
     In the exemplary embodiment, the rotor  20  has, in addition to the electrical excitation, six poles which are excited by permanent magnets  24  and  25  and which have the same pole pitch τ 3  as the electrically excited subsequent poles  32  and  34  located therebetween. However, instead it is also possible to provide four or eight permanent magnets  24 ,  25  in a symmetrical arrangement on the rotor circumference. Furthermore, in the case of a relatively small power requirement of a standardized machine design there is the possibility of providing, instead of individual pocket-shaped cutouts  43  or all the pocket-shaped cutouts  43 , corresponding gaps not equipped with permanent magnets, in order to be able to manufacture the machine more cost-effectively with a lower power requirement.