Patent Publication Number: US-2017353074-A1

Title: Rotor for rotating electric machine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority from Japanese Patent Application No. 2016-112283, filed on Jun. 3, 2016, the content of which is hereby incorporated by reference in its entirety into this application. 
     BACKGROUND 
     1 Technical Field 
     The present invention relates to rotors for rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators. 
     2 Description of Related Art 
     There are known rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators. These rotating electric machines include a stator having a stator coil wound on a stator core and a rotor that is rotatably disposed so as to radially face the stator through a predetermined air gap formed therebetween. 
     Moreover, there are also known Lundell-type rotors which include a field core and a field coil. The field core has a cylindrical boss portion fixed on a rotating shaft and a plurality of claw-shaped magnetic pole portions located radially outward of the boss portion. The field coil is wound on a radially outer periphery of the boss portion of the field core. In operation, upon energization of the file coil, the claw-shaped magnetic pole portions of the field core are magnetized to respectively form a plurality of magnetic poles whose polarities alternate between north and south in a circumferential direction of the rotating shaft. 
     Japanese Patent Application Publication No. JP2009148057A discloses a hollow cylindrical (or annular) core member employed in a Lundell-type rotor. The core member is disposed to cover (or surround) radially outer peripheries of the claw-shaped magnetic pole portions of the field core. Consequently, with the core member, it is possible to reduce fluctuation in magnetic flux transferred between the rotor and the stator during rotation of the rotor. As a result, it is possible to reduce magnetic noise caused by the fluctuation in the magnetic flux. Moreover, since the claw-shaped magnetic pole portions of the field core are connected with each other by the core member, it is possible to suppress the radially outward deformation of the claw-shaped magnetic pole portions due to the centrifugal force during rotation of the rotor. 
     Furthermore, the core member disclosed in the above patent document is formed of a laminate obtained by laminating a plurality of soft-magnetic sheets in an axial direction of the core member. Consequently, it is possible to reduce eddy current loss in the core member. 
     However, the inventors of the present application have found that the core member disclosed in the above patent document involves the following problem. 
     In terms of preventing occurrence of a magnetic short circuit in the core member and improving the magnetic performance of the core member, it is preferable to set the radial thickness of the core member as small as possible. On the other hand, the smaller the radial thickness of the core comber, the less the achievable reduction in the eddy current loss in the core member. Therefore, if the radial thickness of the core comber is too small, it may be difficult to sufficiently reduce the total eddy current loss in the rotor. 
     SUMMARY 
     According to an exemplary embodiment, there is provided a rotor for a rotating electric machine. The rotor includes a field core having a plurality of claw-shaped magnetic pole portions, a field coil wound on the field core, and a hollow cylindrical core member disposed to cover radially outer peripheries of the claw-shaped magnetic pole portions of the field core. The core member has a plurality of first electrically-insulating portions that are spaced at first intervals in an axial direction of the core member. Each of the claw-shaped magnetic pole portions of the field core has a radially outer peripheral surface abutting the core member and a plurality of second electrically-insulating portions provided on the radially outer peripheral surface. The second electrically-insulating portions are spaced at second intervals in the axial direction of the core member. 
     With the above configuration, eddy current generated in the core member is fragmented (or divided) by the first electrically-insulating portions of the core member; thus the eddy current loss in the core member can be reduced. Moreover, for each of the claw-shaped magnetic pole portions of the field core, eddy current generated in the claw-shaped magnetic pole portion is fragmented by the second electrically-insulating portions provided on the radially outer peripheral surface of the claw-shaped magnetic pole portion; thus the eddy current loss in the claw-shaped magnetic pole portion can be reduced. Hence, it is unnecessary to increase the radial thickness of the core member for the purpose of reducing the total eddy current loss in the rotor. Consequently, it becomes possible to effectively reduce the total eddy current loss in the rotor with the radial thickness of the core member kept small. 
     It is preferable that the first intervals are unequal to the second intervals. 
     The core member may be formed of a plurality of soft-magnetic bodies that are laminated in the axial direction of the core member. The first electrically-insulating portions may include a plurality of insulating layers each of which is interposed between one axially-adjacent pair of the soft-magnetic bodies to electrically insulate the pair of the soft-magnetic bodies from each other. 
     The second electrically-insulating portions may include a plurality of grooves formed in the radially outer peripheral surface of the claw-shaped magnetic pole portion. 
     In a further implementation, the claw-shaped magnetic pole portions of the field core are spaced in a circumferential direction of the core member with gaps formed therebetween. The field coil is wound on the field core so that upon energization of the field coil, the claw-shaped magnetic pole portions are magnetized to respectively form a plurality of magnetic poles whose polarities alternate between north and south in the circumferential direction. The rotor is rotatably disposed radially inside a stator in a rotating electric machine. The core member is preferably formed so that an air gap between the core member and the stator is greater at portions of the core member which respectively face the gaps between the claw-shaped magnetic pole portions than at portions of the core member which respectively abut the claw-shaped magnetic pole portions. 
     It is preferable that the core member has a lower magnetic permeability than the field core. 
     It is also preferable that the core member has a higher saturation flux density than the field core. 
     The field core may consist of a pair of first and second pole cores. Each of the first and second pole cores has a boss portion, a disc portion and a plurality of claw-shaped magnetic pole portions. The boss portion is cylindrical in shape. The disc portion extends radially outward from an axially outer part of the boss portion. Each of the claw-shaped magnetic pole portions protrudes axially inward from a radially outer part of the disc portion. The claw-shaped magnetic pole portions of the first and second pole cores constitute the claw-shaped magnetic pole portions of the field core. The claw-shaped magnetic pole portions of the first pole core are interleaved with the claw-shaped magnetic pole portions of the second pole core. The field coil is wound on radially outer peripheries of the boss portions of the first and second pole cores so that upon energization of the field coil, the claw-shaped magnetic pole portions of the first and second pole cores are magnetized to respectively form a plurality of magnetic poles whose polarities alternate between north and south in the circumferential direction of the core member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description given hereinafter and from the accompanying drawings of one exemplary embodiment, which, however, should not be taken to limit the present invention to the specific embodiment but are for the purpose of explanation and understanding only. 
       In the accompanying drawings: 
         FIG. 1  is a schematic cross-sectional view, along an axial direction, of a rotating electric machine which includes a rotor according to an exemplary embodiment; 
         FIG. 2  is a perspective view of the rotor according to the embodiment; 
         FIG. 3  is a perspective view of the rotor according to the embodiment omitting a hollow cylindrical core member of the rotor; 
         FIG. 4  is a side view, from the radially outside, of the rotor according to the embodiment; 
         FIG. 5  is a perspective view of the rotor according to the embodiment omitting the core member and illustrating the shape of radially outer peripheral surfaces of claw-shaped magnetic pole portions of a field core of the rotor; 
         FIG. 6  is a partially cross-sectional view of part of the rotor according to the embodiment; 
         FIG. 7  is a schematic view illustrating eddy current loops generated in the rotor according to the embodiment; 
         FIG. 8  is a side view, from the radially outside, of a rotor according to a modification; and 
         FIG. 9  is a cross-sectional view of part of the rotor according to the modification taken along the line IX-IX in  FIG. 8 . 
     
    
    
     DESCRIPTION OF EMBODIMENT 
       FIG. 1  shows the overall configuration of a rotating electric machine  22  which includes a rotor  20  according to an exemplary embodiment. 
     In the present embodiment, the rotating electric machine  22  is configured as a motor-generator for use in a motor vehicle. Specifically, upon being supplied with electric power from a battery (not shown) of the vehicle, the rotating electric machine  22  functions as an electric motor to generate torque (or driving force) for driving the vehicle. Otherwise, upon being supplied with torque from an engine (not shown) of the vehicle, the rotating electric machine  22  functions as an electric generator to generate electric power for charging the battery. 
     As shown in  FIG. 1 , the rotating electric machine  22  includes the rotor  20 , a stator  24 , a housing  26 , a brush device  28 , a rectifier  30 , a voltage regulator  32  and a pulley  34 . 
     As shown in  FIGS. 1-3 , the rotor  20  includes a field core, a hollow cylindrical (or annular) core member  46 , a field coil  48  and a plurality of permanent magnets  49 . 
     The field core consists of first and second pole cores that are made of a soft-magnetic material. Each of the first and second pole cores includes a boss portion  40 , a disc portion  42  and a plurality of claw-shaped magnetic pole portions  44 . 
     The boss portion  40  is cylindrical in shape and has a shaft hole  52  formed along its central axis. In the shaft hole  52 , there is fixedly fitted a rotating shaft  50  (see  FIG. 1 ). In other words, the boss portion  40  is fixedly fitted on an outer periphery of the rotating shaft  50 . 
     The disc portion  42  is disc-shaped and extends radially outer ward from an axially outer part of the boss portion  40 . 
     Each of the claw-shaped magnetic pole portions  44  protrudes axially inward in the shape of a claw from a radially outer part of the disc portion  42 . That is, each of the claw-shaped magnetic pole portions  44  is located radially outward of the boss portion  40 . Moreover, each of the claw-shaped magnetic pole portions  44  has a radially outer peripheral surface that is shaped in a circular arc having its center located in the vicinity of a central axis O of the rotating shaft  50  (more specifically, located on the central axis O of the rotating shaft  50  or slightly offset toward the claw-shaped magnetic pole portion  44  from the central axis O). 
     Hereinafter, for the sake of ease of explanation, the claw-shaped magnetic pole portions  44  of the first pole core will be referred to as first claw-shaped magnetic pole portions  44   a  while the claw-shaped magnetic pole portions  44  of the second pole core will be referred to as second claw-shaped magnetic pole portions  44   b.    
     The first claw-shaped magnetic pole portions  44   a  have the same shape as the second claw-shaped magnetic pole portions  44   b . Moreover, the number of the first claw-shaped magnetic pole portions  44   a  is equal to the number of the second claw-shaped magnetic pole portions  44   b . More particularly, in the present embodiment, both the number of the first claw-shaped magnetic pole portions  44   a  and the number of the second claw-shaped magnetic pole portions  44   b  are set to  8 . Consequently, the total number of the claw-shaped magnetic pole portions  44  of the field core is equal to  16  (i.e., 8 north poles and 8 south poles). 
     The first and second pole cores are assembled together so that the first claw-shaped magnetic pole portions  44   a  are interleaved with the second claw-shaped magnetic pole portions  44   b . Consequently, the first claw-shaped magnetic pole portions  44   a  are arranged alternately with the second claw-shaped magnetic pole portions  44   b  in a circumferential direction of the rotor  20 . Moreover, between each circumferentially-adjacent pair of the first and second claw-shaped magnetic pole portions  44   a  and  44   b , there is formed a gap  54 . 
     In addition, it should be noted that the boss portions  40  of the first and second pole cores, which abut each other in an axial direction of the rotor  20 , may be integrally formed to make up a common boss portion to the first and second pole cores. 
     Each of the claw-shaped magnetic pole portions  44  of the field core (i.e., the first and second claw-shaped magnetic pole portions  44   a  and  44   b ) has both a predetermined width in the circumferential direction of the rotor  20  and a predetermined thickness in a radial direction of the rotor  20 . Moreover, for each of the claw-shaped magnetic pole portions  44 , both the circumferential width and the radial thickness of the claw-shaped magnetic pole portion  44  are gradually decreased from a proximal end part (or root part) to a distal end part of the claw-shaped magnetic pole portion  44 . That is, each of the claw-shaped magnetic pole portions  44  is gradually decreased in size in both the circumferential and radial directions of the rotor  20  from the proximal end part to the distal end part thereof. 
     All the gaps  54  formed between circumferentially-adjacent first and second claw-shaped magnetic pole  44   a  and  44   b  have the same shape. Moreover, each of the gaps  54  has a substantially constant circumferential width in the axial direction of the rotor  20 . 
     In addition, in terms of preventing occurrence of magnetic unbalance in the rotor  20 , it is preferable for all the gaps  54  to have the same shape. However, in the case where the rotor  20  is designed to rotate only in one direction, to reduce the iron loss in the rotor  20 , the claw-shaped magnetic pole portions  44  may be modified to have an asymmetrical shape with respect to a reference line that radially extends through the central axis  0  of the rotating shaft  50 , thereby making the circumferential width of each of the claw-shaped magnetic pole portions  44  vary in the axial direction of the rotor  20 . 
     The core member  46  has a hollow cylindrical (or annular) shape and is disposed radially outside the claw-shaped magnetic pole portions  44  of the field core (or the first and second claw-shaped magnetic pole portions  44   a  and  44   b  of the first and second pole cores) so as to cover (or surround) the radially outer peripheries of the claw-shaped magnetic pole portions  44 . The core member  46  has a radial thickness that is set to be in the range of, for example, 0.6 mm-1.0 m so as to ensure both the mechanical strength and the magnetic performance of the core member  46  in the rotor  20 . Moreover, the core member  46  is provided in contact with the radially outer peripheral surfaces of the claw-shaped magnetic pole portions  44 . Consequently, each circumferentially-adjacent pair of the claw-shaped magnetic pole portions  44  are connected with each other by the core member  46  with the gap  54  formed therebetween covered by the core member  46  from the radially outside. 
     The core member  46  is made of a soft-magnetic material such as magnetic steel. As shown in  FIG. 4 , the core member  46  is formed by laminating a plurality of soft-magnetic sheets (e.g., magnetic steel sheets)  56  in an axial direction thereof. Each of the soft-magnetic sheets  56  has both a predetermined thickness in a radial direction of the core member  46  and a predetermined width in the lamination direction (or the axial direction of the core member  46 ). 
     Moreover, as shown in  FIG. 7 , for suppressing eddy current loss in the core member  46 , between each axially-adjacent pair of the soft-magnetic sheets  56 , there is interposed an insulating layer  58  to electrically insulate the pair of the soft-magnetic sheets  56  from each other. 
     In addition, the core member  46  is fixed to the claw-shaped magnetic pole portions  44  of the field core by one or a combination of shrinkage fitting, press fitting and welding. 
     Referring back to  FIG. 1 , the field coil  48  is wound on both the radially outer peripheries of the boss portions  40  of the first and second pole cores. Consequently, the field coil  48  is surrounded by the boss portions  40 , the disc portions  42  and the claw-shaped magnetic pole portions  44  of the first and second pole cores. 
     The field coil  48  generates magnetic flux upon being supplied with DC field current. The generated magnetic flux then flows to the claw-shaped magnetic pole portions  44  via the boss portions  40  and the disc portions  42  of the first and second pole cores. Consequently, the claw-shaped magnetic pole portions  44  are magnetized to respectively form a plurality (e.g., 16 in the present embodiment) of magnetic poles whose polarities alternate between north and south in the circumferential direction of the rotor  20 . For example, each of the first claw-shaped magnetic pole portions  44   a  is magnetized to form a north pole while each of the second claw-shaped magnetic pole portions  44   b  is magnetized to form a south pole. 
     As shown in  FIG. 3 , each of the permanent magnets  49  is arranged in one of the gaps  54  formed between circumferentially-adjacent first and second claw-shaped magnetic pole portions  44   a  and  44   b . That is, each of the permanent magnets  49  is provided as an inter-pole magnet which is arranged radially inside the core member  46  and between one circumferentially-adjacent pair of the first and second claw-shaped magnetic pole portions  44   a  and  44   b . Moreover, the permanent magnets  49  are held by a magnet holder (not shown) so that the centrifugal force acting on the permanent magnets  49  during rotation of the rotor  20  is transmitted to the first and second claw-shaped magnetic pole portions  44   a  and  44   b  via the magnet holder. 
     Each of the permanent magnets  49 , which is arranged between one circumferentially-adjacent pair of the first and second claw-shaped magnetic pole portions  44   a  and  44   b , is magnetized so as to reduce magnetic flux leakage between the circumferentially-adjacent pair of the first and second claw-shaped magnetic pole portions  44   a  and  44   b , thereby strengthening magnetic flux transferred between the rotor  20  and the stator  24 . Specifically, each of the permanent magnets  49  is magnetized so that: the north pole of the permanent magnet  49  faces one of the circumferentially-adjacent pair of the first and second claw-shaped magnetic pole portions  44   a  and  44   b  which is magnetized to form a north pole upon energization of the field coil  48 ; and the south pole of the permanent magnet  49  faces the other of the circumferentially-adjacent pair of the first and second claw-shaped magnetic pole portions  44   a  and  44   b  which is magnetized to form a south pole upon energization of the field coil  48 . That is, each of the permanent magnets  49  is magnetized so as to generate a magnetomotive force in the circumferential direction of the rotor  20 . In addition, the permanent magnets  49  may be magnetized either before or after being assembled into the rotor  20 . 
     Referring again to  FIG. 1 , the stator  24  includes a hollow cylindrical (or annular) stator core  60  and a three-phase stator coil (or armature coil)  62 . The stator core  60  is disposed radially outside the rotor  20  so as to radially face the rotor  20  through a predetermined air gap formed therebetween. The stator core  60  has a plurality of teeth (not shown) and a plurality of slots (not shown) formed therein. The teeth each radially extend and are circumferentially spaced at a predetermined pitch. Each of the slots is formed between one circumferentially-adjacent pair of the teeth. The stator coil  62  is comprised of three phase windings (e.g., U-phase, V-phase and W-phase windings) that are wound on the stator core  60  so as to be received in the slots of the stator core  60 . 
     In addition, in the present embodiment, the stator  24  functions as an armature while the rotor  20  functions as a field. 
     The housing  26  receives both the rotor  20  and the stator  24  therein. The housing  26  supports, via a pair of bearings, the rotating shaft  50  so that the rotating shaft  50  can rotate together with the rotor  20 . Moreover, the housing  26  has the stator  24  fixed therein. 
     The brush device  28  includes a pair of slip rings  64  and a pair of brushes  66 . The slip rings  64  are provided on a rear end portion (i.e., a right end portion in  FIG. 1 ) of the rotating shaft  50  and respectively electrically connected with opposite ends of the field coil  48 . The brushes  66  are held by a brush holder that is fixed to the housing  26 . Moreover, the brushes  66  are respectively spring-loaded on the slip rings  64  to establish sliding contacts with them during rotation of the rotor  20 . 
     The rectifier  30  is electrically connected with the three-phase stator coil  62  of the stator  24 . The rectifier  30  is configured to rectify three-phase AC power outputted from the stator coil  62  into DC power. 
     The voltage regulator  32  is configured to regulate an output voltage of the rotating electric machine  22  by controlling the field current supplied to the field coil  48 . Consequently, with the voltage regulator  32 , it is possible to keep the output voltage of the rotating electric machine  22  substantially constant which otherwise varies according to electrical loads fed by the rotating electric machine  22  and the amount of electric power generated by the rotating electric machine  22 . 
     The pulley  34  is fixed on a front end portion (i.e., a left end portion in  FIG. 1 ) of the rotating shaft  50 , so that torque generated by the engine of the vehicle can be transmitted to the rotor  20  via the pulley  34 , thereby driving the rotor  20  to rotate. 
     As mentioned previously, in the present embodiment, the rotating electric machine  22  is configured as a motor-generator that selectively operates in either a motor mode or a generator mode. 
     In the motor mode, the DC field current is supplied from the battery of the vehicle to the field coil  48  via the brush device  28 . With the supply of the field current to the field coil  48 , the claw-shaped magnetic pole portions  44  of the field core are magnetized to respectively form the magnetic poles whose polarities alternate between north and south in the circumferential direction of the rotor  20 . Then, DC power supplied from the battery is converted into three-phase AC power by an inverter (not shown) and the obtained three-phase AC power is supplied to the stator coil  62 , thereby causing the rotor  20  to rotate in a predetermined direction and generate torque. 
     In the generator mode, upon transmission of torque from the engine of the vehicle to the rotating shaft  50  via the pulley  34 , the rotor  20  rotates in a predetermined direction. During rotation of the rotor  20 , the DC field current is supplied from the battery of the vehicle to the field coil  48  via the brush device  28 . With the supply of the field current to the field coil  48 , the claw-shaped magnetic pole portions  44  of the field core are magnetized to respectively form the magnetic poles whose polarities alternate between north and south in the circumferential direction of the rotor  20 . Consequently, a rotating magnetic field is created which causes the three-phase AC power to be generated in the stator coil  62 . The three-phase AC power is then rectified by the rectifier  30  into the DC power. The DC power is used to, for example, charge the battery. In addition, the output voltage of the rotating electric machine  22  (or the voltage of the DC power) is regulated by the voltage regulator  32 . 
     Next, the characteristic configuration of the rotor  20  according to the present embodiment will be described in detail with reference to  FIGS. 4-7 . 
     In the present embodiment, the rotor  20  includes the hollow cylindrical core member  46  that is disposed to cover (or surround) the radially outer peripheries of the claw-shaped magnetic pole portions  44  of the field core. Consequently, with the core member  46 , it becomes possible to make the radially outer periphery of the entire rotor  20  smooth, thereby reducing wind noise caused by unevenness of the radially outer periphery of the rotor  20 . 
     Moreover, in the present embodiment, the claw-shaped magnetic pole portions  44  of the field core are connected with each other by the core member  46 . Consequently, it becomes possible to suppress the radially outward deformation of the claw-shaped magnetic pole portions  44  due to the centrifugal force during rotation of the rotor  20 . In particular, in the present embodiment, the rotor  20  includes the permanent magnets  49  each of which is arranged in one of the gaps  54  formed between circumferentially-adjacent claw-shaped magnetic pole portions  44 . During rotation of the rotor  20 , the centrifugal force acting on the permanent magnets  49  is transmitted to the claw-shaped magnetic pole portions  44 ; thus the amount of radially outward deformation of the claw-shaped magnetic pole portions  44  may be increased. However, with the core member  46 , it is still possible to suppress increase in the amount of radially outward deformation of the claw-shaped magnetic pole portions  44  even though the centrifugal force acting on the permanent magnets  49  is transmitted to the claw-shaped magnetic pole portions  44 . 
     Moreover, in the present embodiment, the core member  46  is formed of the soft-magnetic sheets  56  that are laminated in the axial direction of the core member  46 . Between each axially-adjacent pair of the soft-magnetic sheets  56 , there is interposed one of the insulating layers  58  to electrically insulate the pair of the soft-magnetic sheets  56  from each other. As shown in  FIG. 7 , the insulating layers  58  are spaced at first intervals L 1  in the axial direction of the core member  46  (or the lamination direction of the soft-magnetic sheets  56 ). The size of the first intervals L 1  is equal to the predetermined width of each of the soft-magnetic sheets  56  in the axial direction of the core member  46 . 
     In addition, the insulating layers  58  may be formed of an insulating material (e.g., oxide films) provided on the axial end surfaces of the soft-magnetic sheets  56  by painting or coating. Alternatively, the insulating layers  58  may be formed of minute air gaps provided between axially-adjacent soft-magnetic sheets  56 . 
     With the insulating layers  58 , it becomes possible to reduce eddy current loss in the core member  46 , thereby improving the efficiency of the rotating electric machine  22 . 
     Moreover, in the present embodiment, as shown in  FIGS. 5-6 , each of the claw-shaped magnetic pole portions  44  of the field core has a plurality of grooves (or recesses)  70  formed in the radially outer peripheral surface thereof; the radially outer peripheral surface abuts a radially inner peripheral surface of the core member  46 . As shown in  FIG. 7 , the grooves  70  are spaced at second intervals L 2  in the axial direction of the core member  46 . Between each axially-adjacent pair of the grooves  70 , there is formed one abutting portion (or protrusion)  72  of the claw-shaped magnetic pole portion  44  which abuts (or makes contact with) the radially inner peripheral surface of the core member  46 . In other words, each of the grooves  70  is formed between one axially-adjacent pair of the abutting portions  72  of the claw-shaped magnetic pole portion  44  and constitutes an insulating layer (or air layer) that electrically insulates the pair of the abutting portions  72  from each other. The depth of the grooves  70  is set such that an eddy current loop can be formed in each of the abutting portions  72  without impairing the magnetic performance of the claw-shaped magnetic pole portion  44 . 
     The grooves  70  may be formed, at intervals of 0.1 mm-2 mm, by grooving using a special grooving machine. Alternatively, the grooves  70  may be formed by knurling without using a special grooving machine. Moreover, the grooves  70  may be formed of cutting traces or tool marks which remain on the radially outer peripheral surface of the claw-shaped magnetic pole portion  44  after machining the claw-shaped magnetic pole portion  44 . Alternatively, the grooves  70  may be formed of adhesive pools where an adhesive is pooled; the adhesive is used in covering the radially outer peripheries of the claw-shaped magnetic pole portions  44  with the core member  46 . 
     In the present embodiment, the second intervals L 2  are set to be unequal to the first intervals L 1 . Moreover, the second intervals L 2  may be set to a constant value; in this case, the grooves  70  are arranged in the axial direction of the core member  46  at a constant pitch. Alternatively, the second intervals L 2  may be set to a plurality of different values which may include a value equal to the first intervals L 1 ; in this case, the grooves  70  are arranged in the axial direction of the core member  46  at unequal pitches. For example, the second intervals L 2  may be set to 5 μm or 10 μm. In addition, though the second intervals L 2  are shown as being less than the first intervals L 1  in  FIG. 7 , the second intervals L 2  may also be set to be greater than the first intervals L 1 . 
     As described above, in the rotor  20  according to the present embodiment, the hollow cylindrical core member  46 , which covers the radially outer peripheries of the claw-shaped magnetic pole portions  44 , is formed of the soft-magnetic sheets  56  that are laminated in the axial direction of the core member  46 . Between each axially-adjacent pair of the soft-magnetic sheets  56 , there is interposed one of the insulating layers  58  to electrically insulate the pair of the soft-magnetic sheets  56  from each other. The insulating layers  58  are spaced at the first intervals L 1  in the axial direction of the core member  46 . Moreover, each of the claw-shaped magnetic pole portions  44  has its radially outer peripheral surface abutting the core member  46  and the grooves  70  formed in the radially outer peripheral surface. Each of the grooves  70  is formed between one axially-adjacent pair of the abutting portions  72  of the claw-shaped magnetic pole portion  44  and constitutes an insulating layer that electrically insulates the pair of the abutting portions  72  from each other. The grooves  70  are spaced at the second intervals L 2  in the axial direction of the core member  46 . 
     With the above configuration, the insulating layers  58  of the core member  46  and the grooves  70  of the claw-shaped magnetic pole portions  44  each function as an electrically-insulating portion to reduce eddy current loss in a magnetic circuit along which magnetic flux flows between the stator  24  and the field core. As shown in  FIG. 7 , in the core member  46 , there is generated eddy current that is divided for each of the soft-magnetic sheets  56  between the insulating layers  58 ; in each of the claw-shaped magnetic pole portions  44 , there is generated eddy current that is divided for each of the abutting portions  72  between the grooves  70 . In other words, eddy currents are generated separately in the soft-magnetic sheets  56  and the abutting portions  72  of the claw-shaped magnetic pole portions  44 . These eddy currents interact with each other, distorting the shapes thereof. Consequently, the shapes of eddy current loops in the rotor  20  become complicated. In particular, since the first intervals L 1  are unequal to the second intervals L 2 , the shapes of the eddy current loops become more complicated in comparison with the case where the first intervals L 1  are equal to the second intervals L 2 . 
     Hence, with the above configuration of the rotor  20  according to the present embodiment, it becomes more difficult for eddy currents generated in the rotor  20  to flow and thus it becomes possible to further reduce the total eddy current loss in the rotor  20  in comparison with: the case where the core member  46  has the insulating layers  58  provided therein, but the claw-shaped magnetic pole portions  44  have no grooves  70  formed in their radially outer peripheral surfaces; and the case where the claw-shaped magnetic pole portions  44  have the grooves  70  formed in their radially outer peripheral surfaces, but the core member  46  has no insulating layers  58  provided therein. 
     Moreover, in the rotor  20  according to the present embodiment, since a further reduction in the total eddy current loss in the rotor  20  can be achieved as described above, it is unnecessary to increase the radial thickness of the core member  46  for the purpose of reducing the total eddy current loss in the rotor  20 . Consequently, it becomes possible to effectively reduce the total eddy current loss in the rotor  20  with the radial thickness of the core member  46  kept small. That is, it becomes possible to effectively reduce the total eddy current loss in the rotor  20  while preventing occurrence of a magnetic short circuit in the core member  46 . 
     It is only necessary for the depth of the grooves  70  to be set such that a further reduction in the total eddy current loss in the rotor  20  can be achieved in comparison with the case where the core member  46  has the insulating layers  58  provided therein, but the claw-shaped magnetic pole portions  44  have no grooves  70  formed in their radially outer peripheral surfaces. For example, the depth of the grooves  70  may be set so small as to be in the same level as the depth of a cutting trace. In this case, it is possible to form the grooves  70  during the process of machining the surfaces of the claw-shaped magnetic pole portions  44 ; thus, it is unnecessary to perform an additional special process for forming the grooves  70 . Hence, it is possible to simply and easily form the grooves  70  in the radially outer peripheral surfaces of the claw-shaped magnetic pole portions  44 , thereby achieving a further reduction in the total eddy current loss in the rotor  20 . 
     Moreover, setting the depth of the grooves  70  to be small as described above, it is possible to suppress increase in the magnetic resistance of the claw-shaped magnetic pole portions  44  due to the grooves  70 ; thus it is also possible to suppress decrease in the magnetic force due to increase in the magnetic resistance of the claw-shaped magnetic pole portions  44 . Consequently, it is possible to effectively reduce the total eddy current loss in the rotor  20  while suppressing increase in the magnetic resistance and thus decrease in the magnetic fore in the claw-shaped magnetic pole portions  44 . 
     The above-described rotor  20  according to the present embodiment has the following advantages. 
     In the present embodiment, the rotor  20  includes the field core having the claw-shaped magnetic pole portions  44 , the field coil  48  wound on the field core, and the hollow cylindrical core member  46  disposed to cover the radially outer peripheries of the claw-shaped magnetic pole portions  44  of the field core. The core member  46  has the insulating layers  58  (or first electrically-insulating portions) that are spaced at the first intervals L 1  in the axial direction of the core member  46 . Each of the claw-shaped magnetic pole portions  44  of the field core has its radially outer peripheral surface abutting the core member  46  and the grooves  70  (or second electrically-insulating portions) formed in the radially outer peripheral surface. The grooves  70  are spaced at the second intervals L 2  in the axial direction of the core member  46 . 
     With the above configuration, eddy current generated in the core member  46  is fragmented (or divided) by the insulating layers  58  of the core member  46 ; thus the eddy current loss in the core member  46  can be reduced. Moreover, for each of the claw-shaped magnetic pole portions  44 , eddy current generated in the claw-shaped magnetic pole portion  44  is fragmented by the grooves  70  formed in the radially outer peripheral surface of the claw-shaped magnetic pole portion  44 ; thus the eddy current loss in the claw-shaped magnetic pole portion  44  can be reduced. Hence, it is unnecessary to increase the radial thickness of the core member  46  for the purpose of reducing the total eddy current loss in the rotor  20 . Consequently, it becomes possible to effectively reduce the total eddy current loss in the rotor  20  with the radial thickness of the core member  46  kept small. 
     Moreover, in the present embodiment, the first intervals L 1  are unequal to the second intervals L 2 . 
     Consequently, the shapes of eddy current loops in the rotor  20  become more complicated in comparison with the case where the first intervals L 1  are equal to the second intervals L 2 . As a result, it becomes possible to further reduce the total eddy current loss in the rotor  20 . 
     In the present embodiment, the core member  46  is formed of the soft-magnetic sheets  56  that are laminated in the axial direction of the core member  46 . Each of the insulating layers  58  is interposed between one axially-adjacent pair of the soft-magnetic sheets  56  to electrically insulate the pair of the soft-magnetic sheets  56  from each other. 
     With the above configuration, it is possible to reliably reduce the eddy current loss in the core member  46 . 
     In the present embodiment, each of the claw-shaped magnetic pole portions  44  of the field core has, as electrically-insulating portions, the grooves  70  formed in the radially outer peripheral surface thereof. In other words, the electrically-insulating portions provided on the radially outer peripheral surfaces of the claw-shaped magnetic pole portions  44  are constituted of the grooves  70 . 
     With the above configuration, it is possible to reliably reduce the eddy current loss in the claw-shaped magnetic pole portions  44 . 
     While the above particular embodiment has been shown and described, it will be understood by those skilled in the art that various modifications, changes, and improvements may be made without departing from the spirit of the present invention. 
     For example, in the above-described embodiment, the core member  46  is formed by laminating the soft-magnetic sheets (e.g., magnetic steel sheets)  56  in the axial direction. 
     However, the core member  46  may also be formed by, for example, spirally winding a soft-magnetic wire or strip on the radially outer peripheries of the claw-shaped magnetic pole portions  44  of the field core so as to have a plurality of portions of the soft-magnetic wire or strip laminated in the axial direction of the core member  46 . In this case, it is possible to reduce waste of the soft-magnetic material in forming the core member  46 . Moreover, it is also possible to keep the tension of the soft-magnetic wire or strip constant during the process of spirally winding it on the radially outer peripheries of the claw-shaped magnetic pole portions  44  of the field core. Consequently, it is possible to ensure both high quality and high productivity of the rotor  20 . In addition, in terms of ensuring mechanical strength and magnetic performance, it is preferable for the soft-magnetic wire or strip to have a rectangular cross section. However, the soft-magnetic wire or strip may also have a circular cross section or a rectangular cross section with its corners rounded. 
     In the above-described embodiment, the hollow cylindrical core member  46  has a constant radial thickness and a radially outer peripheral surface where neither protrusion nor recess is formed. That is, all the points on the radially outer peripheral surface of the core member  46  are at the same distance from the axis of the core member  46  (or from the central axis O of the rotating shaft  50 ). 
     However, as shown in  FIGS. 8 and 9 , those portions of the core member  46  which face the gaps  54  formed between circumferentially-adjacent claw-shaped magnetic pole portions  44  of the field core may be recessed radially inward, thereby forming a plurality of grooves  100  in the radially outer peripheral surface of the core member  46 . In this case, each of the grooves  100  extends along one of the gaps  54  formed between circumferentially-adjacent claw-shaped magnetic pole portions  44 . Moreover, each of the grooves  100  is located at the same circumferential position as the boundary between one circumferentially-adjacent pair of the claw-shaped magnetic pole portions  44 , i.e., located on the q axis extending between the circumferentially-adjacent pair of the claw-shaped magnetic pole portions  44 . Each of the grooves  100  is circumferentially bisected by the q axis and has substantially the same circumferential width as the gap  54  along which the groove  100  is formed. The air gap formed between the stator  24  and the core member  46  is greater at the grooves  100  than at the other portions of the core member  46 . Hence, the magnetic path between the rotor  20  and the stator  24  along the q axis is longer than that along the d axis. Consequently, it is possible to suppress magnetic flux fluctuation in the core member  46 ; it is also possible to suppress magnetic flux leakage from the rotor  20  to the stator  24  along the q-axis. As a result, it is possible to further reduce the total eddy current loss in the rotor  20 . 
     In the rotor  20  according to the above-described embodiment, the core member  46  may have a lower magnetic permeability than the field core. In this case, it is more difficult for magnetic flux fluctuation to occur in the core member  46  than in the claw-shaped magnetic pole portions  44 . Consequently, it is possible to suppress magnetic flux fluctuation in the core member  46 , thereby further reducing the total eddy current loss in the rotor  20 . 
     Moreover, the core member  46  may further have a higher saturation flux density than the field core. In this case, it is possible to suppress decrease in the maximum output of the rotating electric machine  22  while suppressing magnetic flux fluctuation in the core member  46 . Consequently, it is possible to further reduce the total eddy current loss in the rotor  20  while maintaining the maximum output of the rotating electric machine  22 . 
     In addition, the core member  46  may be made of a soft-magnetic material having a high saturation flux density, such as permendur which is a cobalt-iron alloy. On the other hand, the field core may be made of a soft-magnetic material having a higher magnetic permeability and a lower saturation flux density than the material of the core member  46 , such as permalloy which is a nickel-iron alloy, pure iron (e.g., SUY according to JIS) or cold-rolled steel (e.g., SPCC or SPCE according to JIS). 
     In the above-described embodiment, the present invention is directed to the rotor  20  of the rotating electric machine  22  which is configured as a motor-generator for use in a motor vehicle. However, the present invention can also be applied to rotors for other rotating electric machines, such as a rotor for an electric motor or a rotor for an electric generator.