Patent Publication Number: US-2007108863-A1

Title: Stator for an electrical machine

Description:
PRIOR ART  
      The invention relates to a stator for an electrical machine, in particular for a three-phase generator for motor vehicles, having an annular laminated stator core that has a multitude of grooves parallel to one another into which the phase windings are inserted.  
      Three-phase generators of this kind are sufficiently well-known from the prior art. They are used in particular as generators in motor vehicles. So-called claw-pole generators have gained acceptance due to their advantages with regard to size, manufacture costs, and ruggedness.  
      A claw-pole generator usually includes a rotor that accommodates the excitation winding and an annular stator encompassing it, which accommodates the three-phase windings. For this purpose, the stator, which is embodied in the form of a laminated core, is provided with a multitude of grooves that extend axially parallel to one another and are spaced uniformly apart from one another. The windings for the three phases are then inserted into the grooves in a particular winding scheme in which only windings of the same phase are contained in one groove.  
      When used in motor vehicles, the problem of noise generation plays an important role in the development of three-phase generators. In particular, the changing magnetic fields in the air gap contribute to this problem; the air gap field is generated by the superposition of the main rotor field and the armature reaction field of the stator.  
      Various countermeasures are taken against this magnetic noise, for example enlargement of the air gap or tightening of production tolerances. An effective measure for reducing noise is the so-called claw cutting method, a beveling of the trailing pole tips of the rotor.  
      This change in the claw pole shape reduces the effects of armature reaction of the stator currents, which, with electrical loading of the generator, causes powerful field distortion in the air gap and thus generates noise.  
      Other changes in the claw pole shape are also executed to reduce noise generation.  
      All of these measures, however, mean that different claw-pole rotors must be manufactured and stored for generators with a different current/speed characteristics and for generators that are of the same type, but are destined for use in different applications. This entails high production and storage costs.  
     ADVANTAGES OF THE INVENTION  
      The stator according to the invention with the defining characteristics of claim  1  has the advantage that a noise reduction is achieved through an intervention in the stator winding. Furthermore, achieving this noise reduction does not require any change to the claw poles or any change to their shape, which reduces production and storage costs.  
      The fact that out of all the conductors of a phase winding, at least one conductor is shifted by at least one groove in relation to the conventional winding scheme makes it possible to influence the shape of the air gap field so that a noise reduction is achieved.  
      Advantageous modifications and improvements of the three-phase generator disclosed in claim  1  are possible by means of the measures disclosed in the dependent claims.  
    
    
     DRAWINGS  
      Two exemplary embodiments of the invention will be explained in greater detail below in conjunction with the drawings.  
       FIG. 1  shows a winding scheme of an offset wave winding according to a first exemplary embodiment of the invention,  
       FIG. 2  shows a winding scheme of an offset wave winding according to a second exemplary embodiment of the invention,  
       FIG. 3  shows a noise level graph for three different winding schemes. 
    
    
     DESCRIPTION  
       FIG. 1  is a top view of an essentially flat stator iron  10 , which is constituted by a packet of individual strip-shaped laminae  13  placed against one another. In a three-phase machine, a total of three phase windings  19  of a stator winding  21  are inserted into the flat stator iron  10 , for example equipped with 36 or 48 grooves  16 ; only one phase winding  19 . 1  with the phase winding end U is shown in this case. The beginnings V and W of the two other phase windings are also depicted.  
      The phase winding  19 . 1  is comprised of a multiple of a group  22  of several coils, a first coil  24  and a second coil  27 . The first coil  24  has first coil sides  28  and second coil sides  29 , which are inserted into grooves  16  that are spaced apart from one another by 180° electrically. The first coil  24  has a certain number of turns z w ; in the example, z w =5. The second coil  27  likewise has first coil sides  30  and second coil sides  31 , which are, in turn, inserted into grooves  16  that are spaced apart from one another by 180° electrically. The second coil  27  has a certain number of turns z w ; in the example, z w =1. This therefore yields a winding ratio of 5:1. The second coil  27  is offset from the first coil  24  in a first direction R 1  by 180°/m electrically. In a three-phase generator with three phase windings, m=3, which means that the offset between the first coil  24  and the second coil  27  is 60° electrically. In accordance with the number of pole pairs predetermined for an electrical machine, a corresponding number of groups  22  that are offset from one another by 360° electrically are arranged one after another in the stator. If the electrical machine has six or eight pole pairs, then the stator is correspondingly provided with six or eight groups  22 . This therefore yields the number six or eight as the previously mentioned multiple.  
      Accordingly, the first group  22  of the phase winding  19 . 1  is arranged as follows in the grooves  16 : the first coil sides  28  are contained in the first groove  16 . 1 , the second coil sides  29  are contained in the fourth groove  16 . 4 . The first coil sides  30  are contained in the second groove  16 . 2 , the second coil sides  31  are contained in the fifth groove  16 . 5 .  
      Starting from the phase winding beginning U, the first group  22  is wound as follows: the first coil  24  with the number of turns z w =5 is placed into the grooves  16 . 1  and  16 . 4 . After the last second coil side  29 , a coil side connector  35  leads from it to the first coil side  30  in the groove  16 . 2  of the second coil  27 . In the example, this coil side  30  is followed by an additional coil side connector  35  that leads to the second coil side  31  of the second coil  27 . The second coil side  31  of the second coil  27  is inserted into the groove  16 . 5 . A group connector  40  leads from this second coil side  31 , extends to the groove  16 . 7 , and then transitions there into a first coil side  28  of the first coil  24  of the second group  22 .  
      A second phase winding  19 . 2  is situated with its coil sides, coil side connectors, and group connector in precisely the same manner, but with the difference that all the corresponding phase winding regions are offset by 360°/m electrically in the direction R 1 . The second phase winding  19 . 2  thus starts offset by 120° electrically, with the phase winding beginning V in the groove  16 . 3 , the third phase winding  19 . 3  starts with the phase winding beginning W in the groove  16 . 5 , and so forth.  
       FIG. 2  also shows a top view of an essentially flat stator iron  10 . A total of three phase windings  19  of a stator winding  21  are inserted into the flat stator iron  10 , for example equipped with  36  or  48  grooves  16 ; in this case, too, only the phase winding  19 . 1  with the phase winding end U is shown.  
      The phase winding  19 . 1  is likewise comprised of a group  22  of several coils, a first coil  24 , a second coil  27 , and a third coil  50 . The first coil  24  has first coil sides  28  and second coil sides  29  that are inserted into grooves  16 , which are spaced apart from one another by 180° electrically. The first coil  24  has a certain number of turns z w ; in the example, z w =4. The second coil  27  likewise has first coil sides  30  and second coil sides  31 , which are, in turn, inserted into grooves  16  spaced apart from one another by 180° electrically. The second coil  27  has a certain number of turns z w ; in the example, z w =1. The second coil  27  is offset from the first coil  24  in a first direction R 1  by 180°/m electrically. The third coil  50  likewise has first coil sides  51  and second coil sides  52 , which are inserted into grooves  16  that are spaced apart from one another by 180° electrically. The third coil  50  has a certain number of turns z w ; in the example, z w =1. The third coil  50  is offset from the first coil  24  in a second direction R 2  by 180°/m electrically. The second direction R 2  is opposite from the first direction R 1 . The third coil  50  has fewer turns than the first coil  24 .  
      In a three-phase generator with three phase windings, m=3, which means that the offset between the first coil  24  and the second coil  27  is 60° electrically. The offset between the first coil and the third coil is −60° electrically. In accordance with the number of pole pairs predetermined for an electrical machine, a corresponding number of groups  22  that are offset from one another by 360° electrically are arranged one after another in the stator. If the electrical machine has six or eight pole pairs, then the stator is correspondingly provided with six or eight groups  22 .  
      Accordingly, the first group  22  of the phase winding  19 . 1  is arranged as follows in the grooves  16 : the first coil sides  28  are contained in the second groove  16 . 2 , the second coil sides  29  are contained in the fifth groove  16 . 5 . The first coil sides  30  are contained in the third groove  16 . 3 , the second coil sides  31  are contained in the sixth groove  16 . 6 . The first coil sides  51  are contained in the first groove  16 . 1 , the second coil sides  52  are contained in the fourth groove  16 . 4 .  
      Starting from the phase winding beginning U, the first group  22  is wound as follows: first, the third coil  50  with the number of turns z w =1 is placed into the grooves  16 . 1  and  16 . 4 . After the last second coil side  52 , a coil side connector  35  leads from it to the first coil side  28  of the first coil  24  and thus transitions into the first coil side  28 . The first coil  24  with the number of turns z w =4 is placed into the grooves  16 . 2  and  16 . 5 . After the last second coil side  29 , a coil side connector  35  leads from it to the first coil side  30  in the groove  16 . 3  of the second coil  27 . In the example, this coil side  30  is followed by an additional coil side connector  35  that leads to the second coil side  31  of the second coil  27 . The second coil side  31  of the second coil  27  is inserted into the groove  16 . 6 . A group connector  40  leads from this second coil side  31  to the groove  16 . 7 , and then transitions there into a first coil side  28  of the third coil  50  of the second group  22 .  
      A second phase winding  19 . 2  is situated with its coil sides, coil side connectors, and group connector in precisely the same manner, but with the difference that all the corresponding phase winding regions are offset by 360°/m electrically in the direction R 1 . The second phase winding  19 . 2  thus starts with the phase winding beginning V in the groove  16 . 3 , the third phase winding  19 . 3  starts with the phase winding beginning W in the groove  16 . 5 , and so forth.  
      Generally speaking, the phase windings  19  can be wound either with a single-strand wire or with a multi-strand wire. The term multi-strand wire means that two or more parallel wires are wound at the same time during the winding process.  
      With regard to the number of turns, the following ratios have turned out to be favorable for 14 V generators:  
      z w  of the first coil/z w  of the second coil: 4:1; 5:1; 6:1; 7:1; 8:1; 2:4; 4:2; 2:5; 5:2; 2:6; 6:2; 3:6; 6:3; 2:7; 7:2; 2:8; 8:2; 6:4; 4:6  
      z w  of the first coil/z w  of the second coil/z w  of the third coil: 1:4:1; 1:5:1; 1:6:1  
      The stator is a so-called flat-packet stator. This means that the stator is manufactured according to a particular manufacturing process. In this process, an essentially flat stator iron  10  that is constituted by a packet of individual strip-shaped laminae  13  placed against one another; a winding is inserted into the grooves  16  and then the stator iron  10  is bent into a circle along with the winding so that its electrical properties essentially correspond to those of conventional annular stators. Before being inserted, the coil sides of the winding, i.e. of the phase windings  19 , are shaped in a die so that the coil sides provided for a groove  16  are adapted to a groove contour after being bent into a circle. The stator is intended for use as the stator of a three-phase machine, in particular a three-phase generator.  
      Through appropriate selection of the offset ratio of the conductors, the magnetic field in the air gap can be shaped by changing the armature field in such a way as to reduce the magnetic noise.  
      Measurements have shown that starting from a certain speed, the generator current of a generator with an offset winding is greater than that of a conventionally designed generator. The current supplied by the conventional generator is only greater at low speeds. However, this can be compensated for very easily by increasing the total number of conductors or by elongating the stator iron, as long as the current supplied to the partially offset winding at low generator speeds is not equivalent to the minimum requirements.  
      It is clear from the noise generation measurement graphs depicted in  FIG. 3  that particularly in the lower speed range of 2000 rpm (generator speed), with winding ratios of 5:1 (b) and 4:2 (c), the airborne noise level L (db(A)) is sharply reduced in comparison to the non-offset winding arrangement according to  FIG. 1   a.  Since a generator speed of 2000 rpm with a conventional turns ratio of approximately 3:1 corresponds to a motor speed of 600 to 700 rpm, the achieved noise reduction for vehicle occupants is markedly perceptible since the motor is still relatively quiet at idling speeds.  
      Another advantage of the offset winding is that it improves generator efficiency. This is due to the fact that the reduced harmonic content of the air gap magnetic field generates lower iron losses. A triangular arrangement of the three phase windings reduces the circular currents in the stator windings caused by the third harmonic, also reducing the associated losses. Furthermore, this reduces the ripple in the d.c. current supplied.