Patent Publication Number: US-9887596-B2

Title: Rotating electric machine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority from Japanese Patent Application No. 2015-30649 filed on Feb. 19, 2015, the content of which is hereby incorporated by reference in its entirety into this application. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to 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 motor vehicles as electric motors and electric generators. These rotating electric machines generally include a rotor and a stator. The stator includes an annular stator core and a stator coil. The stator core is disposed in radial opposition to the rotor and has a plurality of slots arranged in a circumferential direction thereof. The stator coil is comprised of a plurality of phase windings that an mounted on the stator core so as to be received in the slots of the stator core and be different in electrical phase from each other. 
     Japanese Patent Application Publication No. JP2011109894A (to be referred to as Patent Document 1 hereinafter) discloses a rotating electric machine which includes a three-phase stator coil  140  as shown in  FIG. 16 . The stator coil  140  is comprised of a plurality of phase windings mounted on a stator core  130 . Each of the phase windings is formed of a plurality of substantially wave-shaped electric wires; each of the electric wires is configured with an electric conductor having a substantially rectangular cross-sectional shape and an insulating coat that covers the outer surface of the electric conductor. Moreover, each of the phase windings includes a plurality of in-slot portions  151 C and a plurality of turn portions  152 . Each of the in-slot portions  151 C is received in one of slots  131  formed in the stator core  130 . Each of the turn portions  152  is located outside the slots  131  of the stator core  130  to connect one pair of the in-slot portions  151 C respectively received in two different ones of the slots  131 . Moreover, each of the turn portions  152  includes an apex part  153  that is positioned at the center of the turn portion  152  in the extending direction of the turn portion  152  and extends in the circumferential direction of the stator core  130 . Further, at the center of the apex part  153 , there is formed a crank-shaped part  154  that is bent to radially offset the turn portion  152 . Moreover, those turn portions  152  of the phase windings of the stator coil  140  which are located on a first axial side of the stator core  130  together constitute a first coil end part of the stator coil  140 ; those turn portions  152  of the phase windings of the stator coil  140  which are located on a second axial side of the stator core  130  together constitute a second coil end part of the stator coil  140 . 
     More specifically, as shown in  FIG. 16 , the stator coil  140  is wave-wound on the stator core  130  so that each of the turn portions  152  connects a first in-slot portion  151 C arranged at the Nth layer counting from the radially inside in a first slot  131  and a second in-slot portion  151 C arranged at the (N−1)th layer counting from the radially inside in a second slot  131  that is away from the first slot  131  by six slot-pitches, where N is a natural number greater than or equal to 2. 
     However, in the rotating electric machine disclosed in Patent Document 1, the stator coil  140  generates heat upon being supplied with electric current, thereby lowering the efficiency of the rotating electric machine. Therefore, it is necessary to suitably cool the stator coil  140 . To this end, liquid coolant may be supplied to drop onto the first and second coil end parts of the stator coil  140  which protrude respectively from opposite axial end faces of the stator core  130 . In this case, if the liquid coolant dropped on the first and second coil end parts of the stator coil  140  could not effectively flow through the first and second coil end parts, it would be impossible to effectively cool the stator coil  140 . Consequently, with an excessive increase in the temperature of the stator coil  140 , the insulating coats of the electric wires forming the phase windings of the stator coil  140  might be fused, thus resulting in an insulation failure. 
     Japanese Patent Application Publication No. JPH11164506A (to be referred to as Patent Document 2 hereinafter) discloses a rotating electric machine which includes a three-phase stator coil  240  as shown in  FIGS. 17-19 . The stator coil  240  is comprised of a plurality of phase windings mounted on a stator core  130 . Each of the phase windings is formed of a plurality of substantially U-shaped electric conductor segments; each of the electric conductor segments is configured with an electric conductor having a substantially rectangular cross-sectional shape and an insulating coat that covers the outer surface of the electric conductor. Moreover each of the phase windings includes a plurality of in-slot portions  151 C and a plurality of turn portions  152 . Each of the in-slot portions  151 C is received in one of slots  131  formed in the stator core  130 . Each of the turn portions  152  is located outside the slots  131  of the stator core  130  to connect one pair of the in-slot portions  151 C respectively received in two different ones of the slots  131 . Moreover each of the turn portions  152  includes an apex part  153  that is positioned at the center of the turn portion  152  in the extending direction of the turn portion  152  and extends in the circumferential direction of the stator core  130 . Further, at the center of the apex part  153 , there is formed a crank-shaped part  154  that is beat to radially offset the turn portion  152 . Moreover, all the turn portions  152  of the phase windings of the stator coil  240  are located on the same axial side (i.e., the upper side in  FIG. 17 ) of the stator core  130  and together constitute a coil end part of the stator coil  240 . 
     More specifically, as shown in  FIGS. 18-19 , before being mounted to the stator core  130 , each of the electric conductor segments is substantially U-shaped to have a pair of straight portions extending parallel to each other and a turn portion that connects ends of the straight portions on the same side. In forming the stator coil  240 , the straight portions are axially inserted, from a first axial side (i.e., the upper side in  FIG. 17 ) of the stator core  130 , respectively into two slots  131  of the stator core  130  which are away from each other by six slot-pitches. Then, free end parts of the straight portions, which protrude outside the slots  131  on a second axial side (i.e., the lower side in  FIG. 17 ) of the stator core  130 , are twisted respectively toward opposite circumferential sides. Thereafter, each corresponding pair of distal ends of the twisted free end parts of the electric conductor segments are joined by, for example, welding. Consequently, in the resultant stator coil  240 , those parts of the straight portions of the electric conductor segments which are received in the slots  131  of the stator core  130  respectively constitute the in-slot portions  151 C of the phase windings of the stator coil  240 ; the turn portions of the electric conductor segments respectively constitute the turn portions  152  of the phase windings of the stator coil  240 . 
     Moreover, according to Patent Document 2, the electric conductor segments forming the phase windings of the stator coil  240  are comprised of a plurality of large electric conductor segments  150 A as shown in  FIG. 18  and a plurality of small electric conductor segments  150 B as shown in  FIG. 19 . The turn portions  152  of the large electric conductor segments  150 A are larger than the turn portions  152  of the small electric conductor segments  150 B. In each of the slots  131  of the stator core  130 , there received four of the in-lot portions  151 C of the electric conductor segments  150 A and  150 B in radial alignment with each other. 
     More specifically, as shown in  FIG. 18 , each of the large electric conductor segments  150 A includes a first in-slot portion  151 C arranged at the first layer (i.e., the innermost layer) in a first slot  131 , a second in-slot portion  151 C arranged at the fourth layer (i.e., the outermost layer) in a second slot  131  that is away from the first slot  131  by six slot-pitches, and one turn portion  152  that connects the first and second in-slot portions  151 C. On the other hand, as shown in  FIG. 19 , each of the small electric conductor segments  150 B includes a first in-slot portion  151 C arranged at the second layer in a first slot  131 , a second in-slot portion  151 C arranged at the third layer in a second slot  131  that is away from the first slot  131  by six slot-pitches, and one turn portion  152  that connects the first and second in-slot portions  151 C. Moreover, each joined-pair of the free and parts of the large and small electric conductor segments  150 A and  150 B respectively protrude from two slots  131  that are away from each other by six slot-pitches. 
     However, in the rotating electric machine disclosed in Patent Document 2, the stator coil  240  generates heat upon being supplied with electric current, thereby lowering the efficiency of the rotating electric machine. Therefore, it is necessary to suitably cool the stator coil  240 . To this end, liquid coolant may be supplied to drop onto the coil end part of the stator coil  240  which protrudes from an axial end face of the stator core  130 . In this case, if the liquid coolant dropped on the coil end part of the stator coil  240  could not effectively flow through the coil end part, it would be impossible to effectively cool the stator coil  240 . Consequently, with an excessive increase in the temperature of the stator coil  240 , the insulating coats of the electric conductor segments forming the phase windings of the stator coil  240  might be fused, thus resulting in an insulation failure. 
     SUMMARY 
     According to one exemplary embodiment, there is provided a rotating electric machine which includes a rotor, a stator and a cooling mechanism. The stator includes an annular stator core and a stator coil. The stator core is disposed in radial opposition to the rotor and has a plurality of slots arranged in a circumferential direction of the stator core. The stator coil is comprised of a plurality of phase windings mounted on the stator core. Each of the phase windings includes a plurality of in-slot portions and a plurality of turn portions. Each of the in-slot portions is received in one of the slots of the stator core. Each of the turn portions is located outside the slots of the stator core to connect one pair of the in-slot portions respectively received in two different ones of the slots. The turn portions together constitute a coil end part of the stator coil on at least one axial side of the stator core. The cooling mechanism is configured to drop liquid coolant onto the coil end part, thereby cooling the stator coil. The turn portions of the phase windings of the stator coil include, at least, a plurality of first-type turn portions and a plurality of second-type turn portions that have a smaller length than the first-type turn portions. In the coil end part of the stator coil, there are axially-overlapping pairs of the first-type and second-type turn portions over an entire circumferential range of the stator coil. For each axially-overlapping pair of the first-type and second-type turn portions, the second-type turn portion is located axially inside the first-type turn portion and faces the first-type turn portion through a void space formed therebetween over entire lengths of the first-type and second-type turn portions. 
     With the above configuration, the liquid coolant dropped on the coil and part will flow through the void space between each axially-overlapping pair of the first-type and second-type turn portions, thereby cooling the first-typo and second-type turn portions over the entire lengths thereof. Consequently, it is possible to effectively cool the stator coil. 
     In further implementations, each of the phase windings of the stator coil may have a substantially rectangular cross section and an insulating coat formed on its outer surface. In this case, it is preferable that for each axially-overlapping pair of the first-type and second-type turn portions, corresponding pairs of flat side surfaces of the first-type and second-type turn portions extend parallel to and face each other. Further, it is also preferable that each of the first-type and second-type turn portions is rounded at four corners of the rectangular cross section to have arc-shaped corners. Furthermore, it is also preferable that each of the first-type and second-type turn portions has four recesses each of which is formed at a central portion of one of four sides of the rectangular cross section so as to be recessed inward from other portions of the side. 
     Each of the first-type and second-type turn portions may have an apex part and a pair of oblique parts. The apex part is furthest in the turn portion from an axial end face of the stator core and extends in the circumferential direction of the stator core. The oblique parts are respectively formed on opposite sides of the apex part so as to extend obliquely with respect to the axial end face of the stator core at a predetermined oblique angle. In this case, it is preferable that the predetermined oblique angle is set to be the same for all the first-type and second-type turn portions. 
     In each of the slots of the stator core, there may be received at least four of the in-slot portions of the phase windings of the stator coil in radial alignment with each other. In this case, it is preferable that each of the turn portions of the phase windings of the stator coil connects the in-slot portion arranged at the Kth layer counting from the radially inside in one of the slots of the stator core and the in-slot portion arranged at the (K−1)th layer counting from the radially inside in another one of the slots, where K is an even number not less than 2. 
     It is also preferable that each of the first-type turn portions has a circumferential length of (M+1) slot-pitches while each of the second-type turn portions has a circumferential length of (M−1) slot-pitches, where M is a natural number not less than 2. 
    
    
     
       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 partially cross-sectional view of a rotating electric machine according to the exemplary embodiment; 
         FIG. 2  is a perspective view of part of a stator of the rotating electric machine; 
         FIG. 3  is a partially cross-sectional view of part of the stator; 
         FIG. 4  is a perspective view of a pair of large and small electric conductor segments used for forming a stator coil of the stator; 
         FIG. 5  is a perspective view of part of the small electric conductor segment; 
         FIG. 6  is a cross-sectional view illustrating the configuration of the electric conductor segments used for forming the stator coil; 
         FIG. 7  is a circumferential development view illustrating the arrangement of the electric conductor segments in slots of a stator come of the stator; 
         FIG. 8  is a schematic circuit diagram of the stator coil; 
         FIG. 9  is a schematic view illustrating the arrangement of the pair of large and small electric conductor segments in the stator; 
         FIG. 10  is a cross-sectional view taken along the line X-X in  FIG. 9 ; 
         FIG. 11  is a schematic view illustrating the flow of liquid coolant dropped on a first coil end part of the stator coil; 
         FIG. 12  is a schematic view illustrating the flow of the liquid coolant through turn portions of a U-phase winding of the stator coil; 
         FIG. 13  is a schematic view illustrating the flow of the liquid coolant through one axially-overlapping pair of the turn portions; 
         FIG. 14  is a schematic view illustrating the flow of liquid coolant through one pair of turn portions according to a comparative example; 
         FIG. 15  is a cross-sectional view illustrating a modification of the configuration of the electric conductor segments used for forming the stator coil; 
         FIG. 16  is a perspective view of part of a first conventional stator; 
         FIG. 17  is a perspective view of part of a second conventional stator; 
         FIG. 18  is a perspective view of large electric conductor segments used for forming a stator coil of the second conventional stator; and 
         FIG. 19  is a perspective view of small electric conductor segments used for forming the stator coil of the second conventional stator. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
       FIG. 1  shows the overall configuration of a rotating electric machine  1  according to an exemplary embodiment. 
     The rotating electric machine  1  is designed to be used in a motor vehicle, such as a passenger car or truck, as an electric motor. 
     As shown in  FIG. 1 , the rotating electric machine  1  includes a housing  10 , a rotor  14  and a stator  20 . The housing  10  is comprised of a pair of cup-shaped housing pieces  10   a  and  10   b  which are jointed together at the open ends thereof. The housing  10  has a pair of bearings  11  and  12  mounted therein, via which a rotating shaft  13  is rotatably supported by the housing  10 . The rotor  14  is received in the housing  10  and fixed on the rotating shaft  13 . The stator  20  is fixed in the housing  10  so as to surround the radially outer periphery of the rotor  14 . 
     Moreover, the rotating electric machine  1  further includes a coolant supplier for supplying liquid coolant to a three-phase stator coil  40  of the stator  20 . The coolant supplier includes a pair of nozzles  15  and  16  for dropping the liquid coolant onto the stator coil  40 . Specifically, the nozzle  15  is mounted to the housing piece  10   b  so as to penetrate an axial end wall (i.e., a right end wall in  FIG. 1 ) of the housing piece  10   b . The nozzle  15  has a discharge outlet  15   a  formed at its distal end. The discharge outlet  15   a  is located vertically above a first coil end part  45  of the stator coil  40 , so as to discharge the liquid coolant to a central uppermost portion of the first coil end part  45 . Similarly, the nozzle  16  is mounted to the housing piece  10   a  so as to penetrate an axial end wall (i.e., a left and wall in  FIG. 1 ) of the housing piece  10   a . The nozzle  16  has a discharge outlet  16   a  formed at its distal end. The discharge outlet  16   a  is located vertically above a second coil end part  46  of the stator coil  40 , so as to discharge the liquid coolant to a central uppermost portion of the second coil end part  46 . 
     Furthermore, in the present embodiment, the rotating electric machine  1  also includes a coolant collector a pump and a cooler, none of which are shown in the figures. The coolant collector collects the liquid coolant discharged out of the discharge outlets  15   a  and  16   a  of the nozzles  15  and  16  of the coolant supplier. The pump is provided to deliver the liquid coolant to the coolant supplier. The cooler is provided to cool the liquid coolant which has been heated when passing through the stator coil  40  and collected by the coolant collector. The coolant supplier, the coolant collector, the pump and the cooler together form a liquid coolant circulation circuit for circuiting the liquid coolant. In other words, the coolant supplier, the coolant collector, the pump and the cooler together constitute a cooling mechanism for cooling the stator coil  40 . 
     In addition, in the present embodiment, ATF (Automotive Transmission Fluid) is used as the liquid coolant. However, it should be appreciated that other conventional liquid coolants, such as cooling oil, may also be used instead of ATF. 
     The rotor  14  includes a plurality of permanent magnets that form a plurality of magnetic poles on the radially outer periphery of the rotor  14  facing the radially inner periphery of the stator  20 . The polarities of the magnetic poles alternate between north and south in the circumferential direction of the rotor  14 . The number of the magnetic poles can be suitably set according to the design specification of the rotating electric machine  1 . In the present embodiment, the number of the magnetic poles is set to be equal to, for example, eight (i.e., four north poles and four south poles). 
     Referring now to  FIGS. 2 and 3 , the stator  20  includes an annular (or hollow cylindrical) stator core  30 , which is disposed radially outside the rotor  14  so as to surround the rotor  14 , and the three-phase stator coil  40  mounted on the stator core  30 . In addition, the stator  20  may further have insulating paper interposed between the stator core  30  and the stator coil  40 . 
     In the present embodiment, the stator core  30  is formed by laminating a plurality of annular magnetic steel sheets in the axial direction of the stator core  30  and fixing them together by, for example, staking. In addition, between each adjacent pair of the magnetic steel sheets, there is interposed an insulating film. It should be appreciated that other conventional metal sheets may also be used instead of the magnetic steel sheets. 
     Moreover, as shown in  FIGS. 2 and 3 , the stator core  30  includes an annular back core  33 , a plurality of stator teeth  34  and a plurality of slots  31 . The stator teeth  34  each extend radially inward from the back core  33  and are circumferentially spaced at a predetermined pitch. Each of the slots  31  is formed between one circumferentially-adjacent pair of the stator teeth  34 . Accordingly, the slots  31  are circumferentially arranged at the same predetermined pitch as the stator teeth  34 . Moreover, each of the slots  31  extends in the axial direction of the stator core  30  so as to axially penetrate the stator core  30  and opens on the radially inner surface of the stator core  30 . In addition, for each of the slots  31 , the depth direction of the slot  31  coincides with a radial direction of the stator core  30 . 
     In the present embodiment, there are provided two slots  31  per magnetic pole of the rotor  14  that has the eight magnetic poles and per phase of the three-phase stator coil  40 . In other words, the slot multiplier number is set to 2. Accordingly, the total number of the slots  31  provided in the stator core  30  is equal to 48 (i.e., 2×8×3). In addition, as shown in  FIG. 2 , the forty-eight slots  31  are comprised of pairs of U-phase slots U 1  and U 2 , V-phase slots V 1  and V 2  and W-phase slots W 1  and W 2  which are sequentially and repeatedly arranged in the circumferential direction of the stator core  30 . 
     The stator coil  40  is comprised of a U-phase winding  41 U, a V-phase winding  41 V and a W-phase winding  41 W (see  FIG. 8 ), which are mounted on the stator core  30  so as to be received in the slots  31  of the stator corn  30  and be different in electrical phase from each other. 
     Each of the phase windings of the stator coil  40  includes a plurality of in-slot portions  51 C and a plurality of turn portions  52 A and  52 B. Each of the in-slot portions  51 C is received in one of the slots  31  of the stator core  30 . Each of the turn portions  52 A and  52 B is located outside the slots  31  of the stator core  30  to connect one pair of the in-slot portions  51 C respectively received in two different ones of the slots  31 . 
     In the present embodiment, the stator coil  40  is formed by: (1) inserting a plurality of substantially U-shaped electric conductor segments  50  into the slots  31  of the stator core  30  from a first axial side of the stator core  30 ; (2) twisting free end parts of each of the electric conductor segments  50 , which protrude outside the slots  31  of the stator core  30  on a second axial side of the stator core  30 , respectively toward opposite circumferential sides; and (3) joining each corresponding pair of distal ends of the twisted free end parts of the electric conductor segments  50  by, for example, welding. Consequently, all the electric conductor segments  50  are electrically connected in a predetermined pattern, forming the stator coil  40 . 
     Furthermore, in the present embodiment, as shown in  FIG. 4 , the electric conductor segments  50  forming the stator coil  40  are comprised of a plurality of large electric conductor segments  50 A and a plurality of small electric conductor segments  50 B that have a smaller size than the large electric conductor segments  50 A. The large and small electric conductor segments  50 A and  50 B are formed by press-shaping an electric conductor wire, which has a substantially rectangular cross section, into the substantially U-shape using shaping dies. It should be noted that the shaping dies used for forming the large electric conductor segments  50 A are different from those used for forming the small electric conductor segments  50 B. 
     Each of the large electric conductor segments  50 A has a pair of straight portions  51 A extending parallel to each other and a turn portion  52 A that connects ends of the straight portions  51 A on the same side. On the other hand, each of the small electric conductor segments  50 B has a pair of straight portions  51 B extending parallel to each other and a turn portion  52 B that connects ends of the straight portions  51 B on the same side. The turn portions  52 B of the small electric conductor segments  50 B have a smaller length than the turn portions  52 A of the large electric conductor segments  50 A. 
     More specifically, in the present embodiment, the turn portions  52 A of the large electric conductor segments  50 A are formed to have a circumferential length of seven slot-pitches. On the other hand, the turn portions  52 B of the small electric conductor segments  50 B are formed to have a circumferential length of five slot-pitches. Accordingly, the turn portions  52 A of the large electric conductor segments  50 A may be referred to as long-pitch turn portions  52 A; the turn portions  52 B of the small electric conductor segments  50 B may be referred to as short-pitch turn portions  52 B. 
     Moreover, each of the turn portions  52 A of the large electric conductor segments  50 A includes an apex part  53 A that is positioned at the center of the turn portion  52 A in the extending direction of the turn portion  52 A (or in the circumferential direction of the stator core  30 ) and furthest in the turn portion  52 A from a first axial end face  30   a  of the stator core  30 ; the first axial end face  30   a  is on the first axial side of the stator care  30 . The apex part  53 A extends in the circumferential direction of the stator core  30  and parallel to the first axial end face  30   a  of the stator core  30 . Further, at the center of the apex part  53 A, there is formed, by press-shaping, a crank-shaped part  54 A that is bent to radially offset the turn portion  52 A. The amount of radial offset made by the crank-shaped part  54 A is set to be substantially equal to the radial thickness of the large and small electric conductor segments  50 A and  50 B. Similarly, each of the turn portions  52 B of the small electric conductor segments  50 B includes an apex part  53 B that is positioned at the center of the turn portion  52 B in the extending direction of the turn portion  52 B (or in the circumferential direction of the stator core  30 ) and furthest in the turn portion  52 B from the first axial end face  30   a  of the stator core  30 . The apex part  53 B extends in the circumferential direction of the stator core  30  and parallel to the first axial end face  30   a  of the stator core  30 . Further, at the center of the apex part  53 B, there is formed, by press-shaping, a crank-shaped part  54 B that is bent to radially offset the turn portion  52 B. The amount of radial offset made by the crank-shaped part  54 B is also set to be substantially equal to the radial thickness of the large and small electric conductor segments  50 A and  50 B. 
     Furthermore each of the turn portions  52 A of the large electric conductor segments  50 A also includes a pair of oblique parts  55 A that are respectively formed on opposite sides of the apex part  53 A so as to extend obliquely with respect to the first axial end face  30   a  of the stator core  30  at a first predetermined oblique angle θ 1  (see  FIG. 9 ). Each of the turn portions  52 A of the large electric conductor segments  50 A further includes a pair of bent parts  56 A. Each of the bent parts  56 A is formed, by press-shaping using shaping dies, substantially into the shape of a “&lt;” character between one of the oblique parts  55 A and one of the straight portions  51 A connected by the turn portion  52 A. The bent parts  56 A protrude from the first axial end face  30   a  of the stator core  30 . Similarly, each of the turn portions  52 B of the small electric conductor segments  50 B also includes a pair of oblique parts  55 B that are respectively formed on opposite sides of the apex part  53 B so as to extend obliquely with respect to the first axial and face  30   a  of the stator core  30  at a second predetermined oblique angle θ 2  (see  FIG. 9 ). Each of the turn portions  52 B of the small electric conductor segments  50 B further includes a pair of bent parts  56 B. Each of the beat parts  56 B is formed, by press-shaping using shaping dies, substantially into the shape of the “&lt;” character between one of the oblique parts  55 B and one of the straight portions  51 B connected by the turn portion  52 B. The bent parts  56 B protrude from the first axial end face  30   a  of the stator core  30  (see  FIG. 5 ). In addition, in the present embodiment, the first predetermined oblique angle θ 1  and the second predetermined oblique angle θ 2  are set to be equal to each other (see  FIG. 9 ). 
     In the present embodiment, as shown in  FIG. 6 , each of the large and small electric conductor segments  50 A and  50 B is configured with an electric conductor  58  and an insulating coat  59  that covers the outer surface of the electric conductor  58 . The electric conductor  58  has a substantially rectangular cross section. The insulating coat  59  is two-layer structured to include an inner layer  59   a  and an outer layer  59   b . In addition, at least the turn portions  52 A and  52 B of the large and small electric conductor segments  50 A and  50 B are rounded at the four corners of the rectangular cross section to have arc-shaped corners  50   c.    
     Next, the arrangement of the large and small electric conductor segments  50 A and  50 B in the slots  31  of the stator core  30  will be described with reference to  FIG. 7 . 
     In addition, for the sake of simplicity, in  FIG. 7 , there are shown only two pairs of U-phase slots U 1  and U 2  in which the U-phase winding  41 U of the stator coil  40  is received. In the present embodiment, since the slot multiplier number is set to 2, the U-phase slots U 1  are circumferentially spaced at six slot-pitches and the U-phase slots U 2  are also circumferentially spaced at six slot-pitches. 
     For one of the large electric conductor segments  50 A, one of the two straight portions  51 A of the large electric conductor segment  50 A is arranged at the first layer counting from the radially inside in one of the U-phase slots U 1 ; the other straight portion  51 A is arranged at the second layer counting from the radially inside in that one of the U-phase slots U 2  which is away from the one of the U-phase slots U 1  by seven slot-pitches in the clockwise direction (i.e., the X direction in  FIG. 7 ). On the other hand, for one of the small electric conductor segments  50 B, one of the two straight portions  51 B of the small electric conductor segment  50 B is arranged at the first layer counting from the radially inside in one of the U-phase slots U 2 ; the other straight portion  51 B is arranged at the second layer counting from the radially inside in that one of the U-phase slots U 1  which is away from the one of the U-phase slots U 2  by five slot-pitches in the clockwise direction. 
     Consequently, the long-pitch turn portion  52 A of the large electric conductor segment  50 A and the short-pitch turn portion  52 B of the small electric conductor segment  50 B are arranged to overlap each other in the axial direction of the stator core  30 , with a predetermined void space S formed therebetween (see  FIG. 9 ). Moreover, the short-pitch turn portion  52 B of the small electric conductor segment  50 B is located axially inside the long-pitch turn portion  52 A of the large electric conductor segment  50 A and faces the long-pitch turn portion  52 A through the void space S formed therebetween over the entire lengths of the long-pitch and short-pitch turn portions  52 A and  52 B. 
     The above arrangement of the large and small electric conductor segments  50 A and  50 B is repeated at the first and second layers in all the pairs of U-phase slots U 1  and U 2 . Moreover, though not shown in the figures, the large and small electric conductor segments  50 A and  50 B are also arranged at the first and second layers in the pain of V-phase slots V 1  and V 2  and at the first and second layers in the pairs of W-phase slots W 1  and W 2  in the same manner as at the first and second layers in the pairs of U-phase slots U 1  and U 2 . 
     Further, for one of the large electric conductor segments  50 A, one of the two straight portions  51 A of the large electric conductor segment  50 A is arranged at the third layer counting from the radially inside in one of the U-phase slots U 1 ; the other straight portion  51 A is arranged at the fourth layer counting from the radially inside in that one of the U-phase slots U 2  which is away from the one of the U-phase slots U 1  by seven slot-pitches in the clockwise direction (i.e., the X direction in  FIG. 7 ). On the other hand, for one of the small electric conductor segments  50 B, one of the two straight portions  51 B of the small electric conductor segment  50 B is arranged at the third layer counting from the radially inside in one of the U-phase slots U 2 ; the other straight portion  51 B is arranged at the fourth layer counting from the radially inside in that one of the U-phase slots U 1  which is away from the one of the U-phase slots U 2  by five slot-pitches in the clockwise direction. 
     The above arrangement of the large and small electric conductor segments  50 A and  50 B is repeated at the third and fourth layers in all the pairs of U-phase slots U 1  and U 2 . Moreover, though not shown in the figures, the large and small electric conductor segments  50 A and  50 B are also arranged at the third and fourth layers in the pairs of V-phase slots V 1  and V 2  and at the third and fourth layers in the pairs of W-phase slots W 1  and W 2  in the same manner as at the third and fourth layers in the pairs of U-phase slots U 1  and U 2 . 
     Furthermore, for one of the large electric conductor segments  50 A, one of the two straight portions  51 A of the large electric conductor segment  50 A is arranged at the fifth layer counting from the radially inside in one of the U-phase slots U 1 ; the other straight portion  51 A is arranged at the sixth layer counting from the radially inside in that one of the U-phase slots U 2  which is away from the one of the U-phase slots U 1  by seven slot-pitches in the clockwise direction (i.e., the X direction in  FIG. 7 ). On the other hand, for one of the small electric conductor segments  50 B, one of the two straight portions  51 B of the small electric conductor segment  50 B is arranged at the fifth layer counting from the radially inside in one of the U-phase slots U 2 ; the other straight portion  51 B is arranged at the sixth layer counting from the radially inside in that one of the U-phase slots U 1  which is away from the one of the U-phase slots U 2  by five slot-pitches in the clockwise direction. 
     The above arrangement of the large and small electric conductor segments  50 A and  50 B is repeated at the fifth and sixth layers in all the pairs of U-phase slots U 1  and U 2 . Moreover, though not shown in the figures, the large and small electric conductor segments  50 A and  50 B are also arranged at the fifth and sixth layers in the pairs of V-phase slots V 1  and V 2  and at the fifth and sixth layers in the pairs of W-phase slots W 1  and W 2  in the same manner as at the fifth and sixth layers in the pairs of U-phase slots U 1  and U 2 . 
     Consequently, in each of the U-phase slots U 1  and U 2 , V-phase slots V 1  and V 2  and W-phase slots W 1  and W 2 , the are arranged a total of six straight portions  51 A and  51 B of the large and small electric conductor segments  50 A and  50 B in radial alignment with each other. More particularly, in the present embodiment, in each of the U-phase slots U 1  and U 2 , V-phase slots V 1  and V 2  and W-phase slots W 1  and W 2 , three straight portions  51 A of the large electric conductor segments  50 A are arranged alternately with three straight portions  51 B of the small electric conductor segments  50 B in the radial direction of the stator core  30 . 
     Moreover, for each of the large and small electric conductor segments  50 A and  50 B, free end parts of the straight portions of the electric conductor segment, which protrude outside the slots  31  of the stator core  30  on the second axial side (i.e., the lower side in  FIG. 2 ) of the stator core  30 , are twisted respectively toward opposite circumferential sides so as to extend obliquely at a predetermined angle with respect to a second axial end face  30   a  of the stator core  30 ; the second axial end face  30   a  is on the second axial side of the stator core  30 . Consequently, the free end parts of the straight portions are respectively transformed into a pair of oblique portions (not shown) of the electric wire segment; the oblique portions have a circumferential length corresponding to substantially half a magnetic pole pitch. Thereafter, on the second axial side of the stator care  30 , each corresponding pair of distal ends of the oblique portions of all the large and small electric conductor segments  50 A and  50 B are joined by, for example, welding. Consequently, all the large and small electric conductor segments  50 A and  50 B are electrically connected in the predetermined pattern, forming the stator coil  40 . 
     Specifically, those parts of the straight portions  51 A and  51 B of the large and small electric conductor segments  50 A and  50 B which are received in the slots  31  of the stator core  30  respectively constitute the in-slot portions  51 C of the phase windings  41 U,  41 V and  41 W of the stator coil  40 ; the turn portions  52 A and  52 B of the large and small electric conductor segments  50 A and  50 B respectively constitute the turn portions  52 A and  52 B of the phase windings  41 U,  41 V and  41 W of the stator coil  40 . Moreover, each of the phase windings  41 U,  41 V and  41 W of the stator coil  40  is formed of a predetermined number of the large and small electric conductor segments  50 A and  50 B that are electrically connected with one another. More specifically, each of the phase windings  41 U,  41 V and  41 W of the stator coil  40  is wave-wound on the stator core  30  so that the in-slot portion  51 C of the phase winding which is arranged at the Nth layer counting from the radially inside in each of the slots  31  is electrically connected to the in-slot portion  51 C of the phase winding which is arranged at the (N+1)th layer counting from the radially inside in another one of the slots  31 , where N is a natural number not less than 1. 
     Moreover in the present embodiment, each of the turn portions  52 A and  52 B that constitute the first coil end part  45  of the stator coil  40  connects the in-slot portion  51 C arranged at the Kth layer counting from the radially inside in one of the slots  31  and the in-slot portion  51 C arranged at the (K−1)th layer counting from the radially inside in another one of the slots  31 , where K is an even number not less than 2. 
     In addition, in the present embodiment, each of the phase windings  41 U,  41 V and  41 W of the stator coil  40  extends along the circumferential direction of the stator core  30  by six turns. Therefore, each of the phase windings  41 U,  41 V and  41 W also includes electric conductor segments (not shown) that are shaped differently from the above-described large and small electric conductor segments  50 A and  50 B. These differently-shaped electric conductor segments include, for example, electric conductor segments for forming output and neutral terminals of the phase winding and electric conductor segments for connecting different turns of the phase winding. 
     As shown in  FIG. 8 , in the present embodiment, the phase windings  41 U,  41 V and  41 W of the stator coil  40  are star-connected to each other. Moreover, the U-phase winding  41 U is comprised of four U-phase sub-windings U 1 , U 2 , U 3  and U 4  that am connected parallel to each other. The V-phase winding  41 V is comprised of four V-phase sub-windings V 1 , V 2 , V 3  and V 4  that are connected parallel to each other. The W-phase winding  41 W is comprised of four W-phase sub-windings W 1 , W 2 , W 3  and W 4  that are connected parallel to each other. 
     Moreover, as shown in  FIG. 1 , the stator coil  40  has the first coil end part  45  on the first axial side of the stator core  30  and the second coil end part  46  on the second axial side of the stator core  30 ; both the first and second coil end parts  45  and  46  are annular in overall shape. The first coil end part  45  is constituted of the turn portions  52 A and  52 B of the large and small electric conductor segments  50 A and  50 B which protrude from the first axial end face  30   a  of the stator core  30 . The second coil end part  46  is constituted of the oblique portions (or twisted free end parts) of the large and small electric conductor segments  50 A and  50 B which protrude from the second axial end face  30   a  of the stator core  30 . 
     In the present embodiment, as shown in  FIG. 2 , in the first coil end part  45  of the stator coil  40 , there are circumferentially-adjacent pairs of the long-pitch and short-pitch turn portions  52 A and  52 B over the entire circumferential range of the stator coil  40 . For each of the circumferentially-adjacent pairs of the long-pitch and short-pitch turn portions  52 A and  52 B, the apex parts  53 A and  53 B of the circumferentially-adjacent pair of the long-pitch and short-pitch turn portions  52 A and  52 B overlap each other in the axial direction of the stator core  30 . Further, as shown in  FIGS. 9-10 , for each of the circumferentially-adjacent pairs of the long-pitch and short-pitch turn portions  52 A and  52 B, the short-pitch turn portion  52 B is entirely located axial inside the long-pitch turn portion  52 A; an axially-outer side surface  57 B of the apex part  53 B of the short-pitch turn portion  52 B is covered by an axially-inner side surface  57 A of the apex part  53 A of the long-pitch turn portion  52 A. 
     Moreover, in the first coil end part  45  of the stator coil  40 , there are axially-overlapping pairs of the long-pitch and short-pitch turn portions  52 A and  52 B over the entire circumferential range of the stator coil  40 . For each of the axially-overlapping pairs, the long-pitch and short-pitch turn portions  52 A and  52 B of the pair are arranged to overlap each other in the axial direction of the stator core  30 , with the predetermined void space S formed therebetween (see  FIG. 9 ). More specifically, the short-pitch turn portion  52 B is located axially inside the long-pitch turn portion  52 A and faces the long-pitch turn portion  52 A through the void space S formed therebetween over the entire lengths of the long-pitch and short-pitch turn portions  52 A and  52 B. In addition, in the present embodiment, corresponding pairs of flat side surfaces of the long-pitch and short-pitch turn portions  52 A and  52 B extend parallel to and face each other. 
     Furthermore, each axially-overlapping pair of the apex parts  53 A and  53 B of the long-pitch and short-pitch turn portions  52 A and  52 B are of the same phase, in other words, are included in the same one of the U-phase, V-phase and W-phase windings  41 U,  41 V and  41 W of the stator coil  40 . More specifically, as shown in  FIG. 2 , each of the apex parts  53 A (U) of the long-pitch turn portions  52 A (U) of the U-phase winding  41 U axially overlaps one of the apex parts  53 B (U) of the short-pitch turn portions  52 B (U) of the U-phase winding  41 U. Each of the apex parts  53 A (V) of the long-pitch turn portions  52 A (V) of the V-phase winding  41 V axially overlaps one of the apex parts  53 B (V) of the short-pitch turn portions  52 B (V) of the V-phase winding  41 V. Each of the apex parts  53 A (W) of the long-pitch turn portions  52 A (W) of the W-phase winding  41 W axially overlaps one of the apex parts  53 B (W) of the short-pitch turn portions  52 B (W) of the W-phase winding  41 W. 
     Furthermore, as shown in  FIGS. 9-10  and the region A of  FIG. 2 , for each axially-overlapping pair of the apex parts  53 A and  53 B, the axially-inner side surface  57 A of the apex part  53 A and the axially-outer side surface  57 B of the apex part  53 B are arranged parallel to and axially facing each other. 
     Moreover, as shown in  FIG. 2 , each circumferentially-adjacent pair of the apex parts  53 A and  53 B of the long-pitch and short-pitch tam portions  52 A and  52 B of all the phase windings  41 U,  41 V and  41 W are circumferentially offset from each other by a distance greater than the predetermined pitch at which the slots  31  of the stator core  30  are arranged (i.e., greater than one slot-pitch). More specifically, in the present embodiment, each of the crank-shaped parts  54 A formed in the apex parts  53 A of the long-pitch turn portions  52 A is circumferentially adjacent to and offset by two slot-pitches from another one of the crank-shaped parts  54 A; each of the crank-shaped parts  54 B formed in the apex parts  53 B of the short-pitch turn portions  52 B is circumferentially adjacent to and offset by two slot-pitches from another one of the crank-shaped parts  54 B. Further, as shown in the region B of  FIG. 2 , each of the apex parts  53 B of the short-pitch turn portions  52 B is arranged to circumferentially neighbor on one of the straight-extending oblique parts  55 A of the long-pitch turn portions  52 A. 
     In addition, the circumferential length of the long-pitch turn portions  52 A is set to (M+1) slot-pitches while the circumferential length of the short-pitch turn portions  52 B is set to (M−1) slot-pitches, where M is a natural number greater than or equal to 2. More particularly, in the present embodiment, with M=6, the circumferential length of the long-pitch turn portions  52 A is set to seven slot-pitches and the circumferential length of the short-pitch turn portions  52 B is set to five slot-pitches. 
     Next, operation of the rotating electric machine  1  according to the present embodiment will be described. 
     In normal use, the rotating electric machine  1  is mounted at a predetermined position in the vehicle so that, the axial direction of the rotating shaft  13  coincides with a horizontal direction; and the discharge outlets  15   a  and  16   a  of the nozzles  15  and  16  of the coolant supplier are respectively located vertically above the first and second coil end parts  45  and  46  of the stator coil  40 . 
     Upon supply of electric current to the stator coil  40  of the stator  20 , the rotor  14  rotates in a predetermined direction. Moreover, with the rotation of the rotor  14 , the rotating shaft  13  also rotates in the predetermined direction, driving other devices or components mechanically connected to the rotating shaft  13 . 
     At the same time, the cooling mechanism for cooling the stator coil  40  starts operation, delivering the liquid coolant to the nozzles  15  and  16 . Then, the liquid coolant is discharged from the discharge outlets  15   a  and  16   a  of the nozzles  15  and  16  to the first and second coil end parts  45  and  46  of the stator coil  40 . 
     Hereinafter, the cooling of the first coil end part  45  by the liquid coolant will be described in detail. As described previously, the first coil end part  45  is constituted of the turn portions  52 A and  52 B of the large and small electric conductor segments  50 A and  50 B. 
     The liquid coolant, which is discharged from the discharge outlet  15   a  of the nozzle  15 , drops onto the central uppermost portion of the first coil end part  45 , branches to both circumferential sides of the central uppermost portion, and flows downward. 
     Specifically, as shown in  FIG. 11 , the liquid coolant enters the void space S between each axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A and  52 B in the first coil end part  45 . Then, the liquid coolant flows through the void space S while cooling the long-pitch and short-pitch turn portions  52 A and  52 B; the void space S is formed over the entire lengths of the long-pitch and short-pitch turn portions  52 A and  52 B. 
     For example, as shown in  FIG. 12 , the liquid coolant dropped on the outermost layer (i.e., the sixth layer) of the U-phase winding  41 U first flows through the void space S formed between the upper halves (i.e., the left halves in  FIG. 12 ) of a first axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A(U) and  52 B(U) of the U-phase winding  41 ; the upper halves are located on the outermost layer. Then, the liquid coolant flows through the void space S formed between the lower halves (i.e., the right halves in  FIG. 12 ) of the first axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A(U) and  52 B(U); the lower halves are located on the fifth layer. That is, a layer change from the outermost layer to the fifth layer is made at the apex parts  53 A and  53 B of the first axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A(U) and  52 B(U). Thereafter, the liquid coolant moves to and flows through the void space S formed between the upper halves of a second axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A(U) and  52 B(U) of the U-phase winding  41 U; the upper halves are located on the fourth layer. Then, the liquid coolant flows through the void space S formed between the lower halves of the second axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A(U) and  52 B(U); the lower halves are located on the third layer. That is, a layer change from the fourth layer to the third layer is made at the apex parts  53 A and  53 B of the second axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A(U) and  52 B(U). 
     In the above manner, in the upper half of the first coil end part  45 , the liquid coolant flows through the void spaces S between axially-overlapping pairs of the long-pitch and short-pitch turn portions  52 A(U) and  52 B(U) of the U-phase winding  41 U, moving layer by layer from the outermost layer to the innermost layer (i.e., the first layer). 
     Moreover, in the lower half of the first coil end part  45 , the liquid coolant flows through the void spaces S between axially-overlapping pairs of the long-pitch and short-pitch turn portions  52 A(U) and  52 B(U) of the U-phase winding  41 U, moving layer by layer from the innermost layer to the outermost layer. Finally, the liquid coolant drops down from the first coil end part  45  to the bottom of the housing  10 . 
     In addition, though not shown in the figures, the V-phase and W-phase windings  41 V and  41 W of the stator coil  40  are also cooled by the liquid coolant in the same manner as the U-phase winding  41 U. The liquid coolant dropping down to the bottom of the housing  10  is then collected by the coolant collector, cooled by the cooler, and delivered by the pump again to the nozzles  15  and  16 . 
     As shown in  FIG. 13 , in the present embodiment, the first coil end part  45  is constituted of the long-pitch turn portions  52 A and the short-pitch turn portions  52 B that have a smaller length than the long-pitch turn portions  52 A. Consequently, the liquid coolant flows through the void space S between each axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A and  52 B; the void space S is formed over the entire lengths of the long-pitch and short-pitch turn portions  52 A and  52 B. As a result, it is possible to effectively cool the long-pitch and short-pitch turn portions  52 A and  52 B over a wide range. 
     In contrast, in a comparative example as shown in  FIG. 14 , all of turn portions  152 , which together constitute a coil end part protruding from an axial end face of a stator core  130 , have the same length. Therefore, the turn portions  152  cannot be arranged in axially-overlapping pairs. Consequently, for each adjacent pair of the turn portions  152 , liquid coolant flows through only a void space formed between the upper halves of the pair of the turn portions  152 , leaving the pair of the turn portions  152  from the apex parts  153  thereof. As a result, the lower halves of the pair of the turn portions  152  cannot be cooled by the liquid coolant. That is to say, it is impossible to effectively cool the pair of the turn portions  152  over a wide range. 
     The above-described rotating electric machine  1  according to the present embodiment has the following advantages. 
     In the present embodiment, the rotating electric machine  1  includes the rotor  14 , the stator  20  and the cooling mechanism. The stator  20  includes the annular stator core  30  and the three-phase coil  40 . The stator core  30  is disposed in radial opposition to the rotor  14  and has the slots  31  arranged in the circumferential direction of the stator core  30 . The stator coil  40  is comprised of the U-phase, V-phase and W-phase windings  41 U,  41 V and  41 W mounted on the stator core  30 . Each of the phase windings  41 U,  41 V and  41 W includes the in-slot portions  51 C and the turn portions  52 A and  52 B. Each of the in-slot portions  51 C is received in one of the slots  31  of the stator core  30 . Each of the turn portions  52 A and  52 B is located outside the slots  31  of the stator core  30  to connect one pair of the in-slot portions  51 C respectively received in two different ones of the slots  31 . The turn portions  52 A and  52 B together constitute the first coil end part  45  of the stator coil  40  on the first axial side of the stator core  30 . The cooling mechanism includes the nozzle  15  to drop the liquid coolant onto the first coil end part  45 , thereby cooling the stator coil  40 . Moreover, in the present embodiment, the turn portions of the phase windings  41 U,  41 V and  41 W of the stator coil  40  include the long-pitch turn portions  52 A and the short-pitch turn portions  52 B that have a smaller length than the long-pitch turn portions  52 A. In the first coil end part  45  of the stator coil  40 , there are axially-overlapping pairs of the long-pitch and short-pitch turn portions  52 A and  52 B over the entire circumferential range of the stator coil  40 . For each axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A and  52 B, the short-pitch turn portion  52 B is located axially inside the long-pitch turn portion  52 A and faces the long-pitch turn portion  52 A through the void space S formed therebetween over the entire lengths of the long-pitch and short-pitch turn portions  52 A and  52 B. 
     With the above configuration, the liquid coolant dropped on the first coil end part  45  will flow through the void space S between each axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A and  52 B, thereby cooling the long-pitch and short-pitch turn portions  52 A and  52 B over the entire lengths thereof. Consequently, it is possible to effectively cool the stator coil  40 . 
     Moreover, in the present embodiment, each of the phase windings  41 U,  41 V and  41 W of the stator coil  40  (more specifically, each of the large and small electric conductor segments  50 A and  50 B forming the phase windings  41 U,  41 V and  41 W) has a substantially rectangular cross section and the insulating coat  59  formed on its outer surface (see  FIG. 6 ). For each axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A and  52 B, corresponding pairs of the flat side surfaces of the long-pitch and short-pitch turn portions  52 A and  52 B extend parallel to and face each other (see  FIG. 9 ). 
     With the above configuration, it is possible to keep, by surface tension, the liquid coolant flowing on the flat side surfaces of the long-pitch and short-pitch turn portions  52 A and  52 B, thereby improving the cooling performance. 
     Further, in the present embodiment, each of the long-pitch and short-pitch turn portions  52 A and  52 B is rounded at the four corners of the rectangular cross section to have the arc-shaped corners  50   c  (see  FIG. 6 ). 
     With the above configuration, it is possible to increase the amount of the liquid coolant flowing through the void space S between each axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A and  52 B, thereby further improving the cooling performance. 
     Furthermore, in the present embodiment, each of the long-pitch turn portions  52 A has the apex part  53 A and the oblique parts  55 A. The apex part  53 A is farthest in the long-pitch turn portion  52 A from the first axial end face  30   a  of the stator core  30  and extends in the circumferential direction of the stator core  30 . The oblique parts  55 A are respectively formed on opposite sides of the apex part  53 A so as to extend obliquely with respect to the first axial end face  30   a  of the stator core  30  at the first predetermined oblique angle θ 1 . Similarly, each of the short-pitch turn portions  52 B has the apex part  53 B and the oblique parts  55 B. The apex part  53 B is furthest in the short-pitch turn portion  52 B from the first axial end face  30   a  of the stator core  30  and extends in the circumferential direction of the stator core  30 . The oblique parts  55 B are respectively formed on opposite sides of the apex part  53 B so as to extend obliquely with respect to the first axial end face  30   a  of the stator core  30  at the second predetermined oblique angle θ 1 . Further, in the present embodiment, the first predetermined oblique angle θ 1  and the second predetermined oblique angle θ 2  are set to be equal to each other (see  FIG. 9 ). 
     With the above configuration, it is possible to keep the size (or the width) of the void space S between each facing-pair of the side surfaces of the oblique parts  55 A and  55 B of the long-pitch and short-pitch turn portions  52 A and  52 B constant, thereby keeping the surface tension of the liquid coolant on the side surfaces of the oblique parts  55 A and  55 B constant. Consequently, it is possible to more reliably keep the liquid coolant flowing on the side surfaces of the oblique parts  55 A and  55 B, thereby farther improving the cooling performance. 
     In the present embodiment, in each of the slots  31  of the stator core  30 , there are received six in-slot portions  51 C of the phase windings  41 U,  41 V and  41 W of the stator coil  40  (or six straight portions  51 A and  51 B of the large and small electric conductor segments  50 A and  50 B) in radial alignment with each other. Each of the turn portions  52 A and  52 B of the phase windings  41 U,  41 V and  41 W connects the in-slot portion  51 C arranged at the Kth layer counting from the radially inside in one of the slots  31  of the stator core  30  and the in-slot portion  51 C arranged at the (K−1)th layer counting from the radially inside in another one of the slots  31 , where K is an even number not less than 2. 
     With the above configuration, it is possible to allow the liquid coolant dropped on the first coil end part  45  of the stator coil  40  to flow through the void space S between each axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A and  52 B, moving layer by layer from the outermost layer to the innermost layer. Consequently, t is possible to effectively cool the entire first coil end part  45  of the stator coil  40 . 
     In the present embodiment, each of the long-pitch turn portions  52 A has the circumferential length of seven slot-pitches (i.e., (M+1) slot-pitches with M being 6), while each of the short-pitch turn portions  52 B has the circumferential length of five slot-pitches (i.e., (M−1) slot-pitches with M being 6). Moreover, each of the apex parts  53 B of the small-pitch turn portions  52 B is located axially inside and axially overlaps one of the apex parts  53 A of the long-pitch turn portions  52 A. 
     With the above configuration, it is possible to improve the cooling performance without changing other characteristics in comparison with a conventional stator coil in which all the turn portions have the same circumferential length of M slot-pitches. 
     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, each of the phase windings  41 U,  41 V and  41 W of the stator coil  40  is formed of the large and small electric conductor segments  50 A and  50 B that have the substantially rectangular cross section (see  FIG. 6 ). 
     However, as shown in  FIG. 15 , at least the turn portions  52 A and  52 B of the large and small electric conductor segments  50 A and  50 B (or at least the turn portions  52 A and  52 B of the phase windings  41 U,  41 V and  41 W) may have four recesses  50   d  each of which is formed at a central portion of one of the four sides of the rectangular cross section so as to be recessed inward from the other portions of the side. More specifically, at least the turn portions  52 A and  52 B of the large and small electric conductor segments  50 A and  50 B may be configured with an electric conductor  58  having a substantially rectangular cross-sectional shape and an insulating coat  59  that covers the outer surface of the electric conductor  58  and has the for recesses  50   d  formed in its outer surface. In addition, the insulating coat  59  may also be rounded to have the four arc-shaped corners  50   c . In this case, it is possible to further increase the amount of the liquid coolant flowing through the void space S between each axially-overlapping pair of the long-pitch and short-pitch turn portions  52 A and  52 B, thereby further improving the cooling performance. 
     In the above-described embodiment, two types of turn portions having different circumferential lengths (i.e., the long-pitch turn portions  52 A and the short-pitch turn portions  52 B) are employed in the stator coil  40 . Moreover, the number of the apex parts of the turn portions overlapping each other in the axial direction of the stator core  30  is equal to 2. 
     However, it is also possible to employ three or more types of turn portions having different circumferential lengths in the stator coil  40 . In this case, the number of the apex parts of the turn portions overlapping each other in the axial direction of the stator core  30  would be accordingly equal to 3 or more. 
     In the above-described embodiment, each of the phase windings  41 U,  41 V and  41 W of the stator coil  40  is formed of the substantially U-shaped large and small electric conductor segments  50 A and  50 B. The first coil end part  45  of the stator coil  40  is constituted of the turn portions  52 A and  52 B of the large and small electric conductor segments  50 A and  50 B which protrude from the first axial and face  30   a  of the stator core  30 . The second coil end part  46  of the stator coil  40  is constituted of the oblique portions (or twisted free end parts) of the large and small electric conductor segments  50 A and  50 B which protrude from the second axial end face  30   a  of the stator core  30 . 
     However, each of the phase windings  41 U,  41 V and  41 W of the stator coil  40  may be alternatively formed of a plurality of substantially wave-shaped electric wires; each of the electric wires includes a plurality of in-slot portions  51 C and a plurality of turn portions  52 A and  52 B. In this case, the first coil end part  45  of the stator coil  40  would be constituted of those turn portions  52 A and  52 B of the electric wires which protrude from the first axial end face  30   a  of the stator core  30 ; the second coil end part  46  of the stator coil  40  would be constituted of those turn portions  52 A and  52 B of the electric wires which protrude from the second axial end face  30   a  of the stator care  30 . 
     In the above-described embodiment, the present invention is directed to the stator  20  of the rotating electric machine  1  that is designed to be used in a motor vehicle as an electric motor. However, the present invention can also be applied to stators of other rotating electric machines, such as a stator of an electric generator or a stator of a motor-generator that can selectively function both as an electric motor and as an electric generator.