Patent Publication Number: US-2023135655-A1

Title: Vehicle drive unit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a vehicle drive unit including a motor and an inverter. 
     2. Description of the Related Art 
     A drive unit that drives a motor by converting direct current from a battery into alternating current by an inverter is known. The inverter has a power module including a switching element. An inverter disclosed in Japanese Unexamined Patent Application Publication No. 2021-112011 includes a power module unit including a U-phase power module, a V-phase power module, and a W-phase power module. The phase power modules correspond to respective phase coils of the motor. 
     Since an inverter of this type handles high electric power, a high voltage is applied and a large current flows. Therefore, the heat generation amount at the time of operation is large, and thus cooling is required. A high surge voltage is also generated. Therefore, individual electronic components of the inverter become large, and also tend to be heavy. Consequently, the inverter of the related art is a factor that prevents fuel costs and electricity costs from being reduced. 
     In order to reduce a power transmission distance, the inverter is ordinarily disposed near a drive motor. However, in the case of a car, there are many devices to be installed and thus a space in which the inverter can be disposed is limited. It is also necessary to consider the balance of a car body. Therefore, it is difficult to adequately dispose a large, heavy inverter in a car. 
     With regard to such a problem, in Japanese Unexamined Patent Application Publication No. 2021-112011, the inverter is made small and light by integrating the inverter with the motor. 
     Each phase power module of the inverter is connected to a corresponding one of the phase coils of the motor via an electrical connection member. Since a large current flows in the electrical connection member, the electrical connection member is large and heavy. When a wiring length of the electrical connection member increases, the electrical resistance increases correspondingly, and thus copper loss occurs at the time of energization. The electrical connection member generates a large amount of heat. Moreover, in the inverter, since the large current is turned on and off at a high speed by switching control, a large change in magnetic field occurs in the electrical connection member. 
     Therefore, when the inverter operates, noise, vibration, electromagnetic interference, and the like occur at the electrical connection member due to the changing magnetic field. The noise, vibration, electromagnetic interference, and the like result in energy loss and variously adversely affect the performance of the car, as a result of which measures need to be taken. 
     Therefore, from the viewpoint of motor control, it is important to reduce inductances of electrical paths extending from the phase power modules to the respective phase coils of the motor, and to equalize the inductances between the U phase, the V phase, and the W phase. In particular, when the output of the motor is large, it is important to reduce and equalize the inductances of the electrical paths. 
     On the other hand, in order to increase the output of the motor, it is important to increase the number of power modules of the inverter. However, an increase in the size of the inverter caused by an increase in the number of power modules is to be avoided. 
     SUMMARY OF THE INVENTION 
     The present disclosure has been made in view of such points, and it is an object of the present disclosure to, while suppressing a drive unit including a motor and an inverter from increasing in size, increase an output of a motor, and reduce and equalize inductances of electrical paths extending from phase power modules to respective phase coils. 
     A vehicle drive unit according to a first aspect of the present disclosure includes a motor and an inverter disposed adjacent to each other in an axial direction of the motor, the inverter having a plurality of power modules. In the vehicle drive unit, each power module is placed on a placement surface orthogonal to the axial direction. In the motor, at least a first coil group and a second coil group are provided, each including one U-phase coil, one V-phase coil, and one W-phase coil. The plurality of power modules constitute at least a first power module group and a second power module group that are connected in parallel. The first power module group and the second power module group each include one U-phase power module, one V-phase power module, and one W-phase power module corresponding to the U-phase coil, the V-phase coil, and the W-phase coil, respectively, of the first coil group or the second coil group. Each power module of the first power module group is connected to a corresponding one of the coils of the first coil group, and each power module of the second power module group is connected to a corresponding one of the coils of the second coil group. When seen in the axial direction, a distance between the U-phase power module of the first power module group and the U-phase coil of the first coil group, a distance between the V-phase power module of the first power module group and the V-phase coil of the first coil group, and a distance between the W-phase power module of the first power module group and the W-phase coil of the first coil group are equal to each other. 
     According to such a structure, since the first power module group and the second power module group of the inverter are connected in parallel, the output of the motor can be increased. 
     Since each power module is placed side by side on the placement surface orthogonal to the axial direction of the motor, the axial-direction length of the inverter can be suppressed from being increased even though the number of power modules has been increased. 
     Since the motor and the inverter are disposed adjacent to each other in the axial direction of the motor, the distances between the phase power modules and the respective phase coils can be reduced. Therefore, the inductances of the electrical paths extending from the phase power modules to the respective phase coils can be reduced. 
     Further, in the first power module group and the first coil group, when seen in the axial direction of the motor, the distances between the phase power modules and the respective phase coils are equal to each other between the U phase, the V phase, and the W phase. Therefore, in the first power module group and the first coil group, the inductances of the electrical paths extending from the phase power modules to the respective phase coils can be equalized between the U phase, the V phase, and the W phase. 
     Accordingly, while suppressing the drive unit including a motor and an inverter from increasing in size, the output of the motor is increased, and the inductances of the electrical paths extending from the phase power modules to the respective phase coils can be reduced and equalized. 
     In one embodiment, when seen in the axial direction, a distance between the U-phase power module of the second power module group and the U-phase coil of the second coil group, a distance between the V-phase power module of the second power module group and the V-phase coil of the second coil group, and a distance between the W-phase power module of the second power module group and the W-phase coil of the second coil group are equal to each other. 
     According to such a structure, even in the second power module group and the second coil group, as in the first power module group and the first coil group, the inductances of the electrical paths extending from the phase power modules to the respective phase coils can be equalized between the U phase, the V phase, and the W phase. 
     In one embodiment, when seen in the axial direction, the distance between the U-phase power module of the first power module group and the U-phase coil of the first coil group and the distance between the U-phase power module of the second power module group and the U-phase coil of the second coil group are equal to each other, the distance between the V-phase power module of the first power module group and the V-phase coil of the first coil group and the distance between the V-phase power module of the second power module group and the V-phase coil of the second coil group are equal to each other, and the distance between the W-phase power module of the first power module group and the W-phase coil of the first coil group and the distance between the W-phase power module of the second power module group and the W-phase coil of the second coil group are equal to each other. 
     According to such a structure, the inductances of the electrical paths extending from the phase power modules to the respective phase coils can be equalized between the first power module group and the first coil group and the second power module group and the second coil group. 
     In one embodiment, in the motor, the U-phase coils, the V-phase coils, and the W-phase coils of each of the first and second coil groups are each wound in a concentrated manner. When seen in the axial direction, the U-phase power module of the first power module group is disposed at a position overlapping the U-phase coils of the first coil group, the V-phase power module of the first power module group is disposed at a position overlapping the V-phase coils of the first coil group, and the W-phase power module of the first power module group is disposed at a position overlapping the W-phase coils of the first coil group. 
     According to such a structure, in the first power module group and the first coil group, the distances between the phase power modules and the corresponding phase coils can be reduced. Therefore, in the first power module group and the first coil group, the inductances of the electrical paths extending from the phase power modules to the corresponding phase coils can be reduced. 
     In one embodiment, when seen in the axial direction, the U-phase power module of the second power module group is disposed at a position overlapping the U-phase coils of the second coil group, the V-phase power module of the second power module group is disposed at a position overlapping the V-phase coils of the second coil group, and the W-phase power module of the second power module group is disposed at a position overlapping the W-phase coils of the second coil group. 
     According to such a structure, even in the second power module group and the second coil group, as in the first power module group and the first coil group, the inductances of the electrical paths extending from the phase power modules to the corresponding phase coils can be reduced. 
     In one embodiment, each of the power modules of the first power module group is disposed on one side in a radial direction of the motor, each of the power modules of the second power module group is disposed on an opposite side in the radial direction, the U-phase power module of the first power module group and the U-phase power module of the second power module group oppose each other in the radial direction, the V-phase power module of the first power module group and the V-phase power module of the second power module group oppose each other in the radial direction, and the W-phase power module of the first power module group and the W-phase power module of the second power module group oppose each other in the radial direction. 
     In the motor (in particular, a concentrated winding type in which the number of coils is even), coils of the same phase are often disposed so as to oppose each other in the radial direction of the motor. According to such a structure, as with the coils of the same phase, power modules of the same phase are disposed so as to oppose each other in the radial direction of the motor. Therefore, the phase power modules are easily positioned with respect to the respective phase coils. 
     In one embodiment, at least an output busbar is interposed between each of the power modules and each of the coils, and the output busbar is formed with a width greater than a thickness thereof so as to extend widthwise along a peripheral direction of the motor. 
     According to such a structure, since the output busbar is easily formed with a wide width, the inductance of the output busbar is easily reduced. 
     In one embodiment, the inverter has a smoothing capacitor placed on the placement surface. 
     Such a structure is advantageous in terms of suppressing an increase in the axial-direction length of the inverter. 
     In one embodiment, the smoothing capacitor and each power module are connected to each other by a negative-electrode-side busbar and a positive-electrode-side busbar, serving as input busbars, one end portion of each input busbar is connected to the smoothing capacitor, the other end portion of each input busbar is connected to each power module, an inductance of each input busbar is a function of a length extending from the one end portion to the other end portion of each input busbar, the function has a minimum value so that the inductances become the same at a first length and a second length that differ from each other, the length of one of the negative-electrode-side busbar and the positive-electrode-side busbar is the first length, and the length of the other of the negative-electrode-side busbar and the positive-electrode-side busbar is the second length. 
     According to such a structure, in each input busbar that connects the smoothing capacitor and each power module to each other, the inductance of the negative-electrode-side busbar and the inductance of the positive-electrode-side busbar can be equalized with respect to each other even though the length of the negative-electrode-side busbar and the length of the positive-electrode-side busbar differ from each other. 
     A vehicle drive unit according to a second aspect of the present disclosure includes a motor and an inverter disposed adjacent to each other in an axial direction of the motor, the inverter having a plurality of power modules. In the vehicle drive unit, each power module is placed on a placement surface orthogonal to the axial direction. In the motor, at least a first coil group and a second coil group are provided, each including one U-phase coil, one V-phase coil, and one W-phase coil. The plurality of power modules constitute at least a first power module group and a second power module group that are connected in parallel. The first power module group and the second power module group each include one U-phase power module, one V-phase power module, and one W-phase power module corresponding to the U-phase coil, the V-phase coil, and the W-phase coil, respectively, of the first coil group or the second coil group. Each power module of the first power module group is connected to a corresponding one of the coils of the first coil group, and each power module of the second power module group is connected to a corresponding one of the coils of the second coil group. When seen in the axial direction, the U-phase power module of the first power module group is disposed at a position overlapping the U-phase coil of the first coil group, the V-phase power module of the first power module group is disposed at a position overlapping the V-phase coil of the first coil group, and the W-phase power module of the first power module group is disposed at a position overlapping the W-phase coil of the first coil group. 
     According to such a structure, since the first power module group and the second power module group of the inverter are connected in parallel, the output of the motor can be increased. 
     Since each power module is placed side by side on the placement surface orthogonal to the axial direction of the motor, the axial-direction length of the inverter can be suppressed from being increased even though the number of power modules has been increased. 
     Since the motor and the inverter are disposed adjacent to each other in the axial direction of the motor, the distances between the phase power modules and the respective phase coils can be reduced. Therefore, the inductances of the electrical paths extending from the phase power modules to the respective phase coils can be reduced. 
     Further, in the first power module group and the first coil group, when seen in the axial direction of the motor, the phase power modules are disposed at positions overlapping the respective phase coils. Therefore, in the first power module group and the first coil group, the inductances of the electrical paths extending from the phase power modules to the respective phase coils can be equalized between the U phase, the V phase, and the W phase. 
     Accordingly, while suppressing the drive unit including a motor and an inverter from increasing in size, the output of the motor is increased, and the inductances of the electrical paths extending from the phase power modules to the respective phase coils can be reduced and equalized. 
     In one embodiment, when seen in the axial direction, the U-phase power module of the second power module group is disposed at a position overlapping the U-phase coil of the second coil group, the V-phase power module of the second power module group is disposed at a position overlapping the V-phase coil of the second coil group, and the W-phase power module of the second power module group is disposed at a position overlapping the W-phase coil of the second coil group. 
     According to such a structure, even in the second power module group and the second coil group, as in the first power module group and the first coil group, the inductances of the electrical paths extending from the phase power modules to the respective phase coils can be equalized between the U phase, the V phase, and the W phase. 
     According to the present disclosure, while suppressing the drive unit including a motor and an inverter from increasing in size, the output of the motor is increased, and the inductances of the electrical paths extending from the phase power modules to the respective phase coils can be reduced and equalized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic structural view of a vehicle system including a drive unit according to a first embodiment. 
         FIG.  2    is a perspective view of the drive unit including a motor and an inverter. 
         FIG.  3    is a cross sectional view of the motor when seen from the inverter. 
         FIG.  4    is a circuit diagram of the inverter. 
         FIG.  5    shows a comparison between a SiC-MOSFET and an IGBT. 
         FIG.  6    is a perspective view and a circuit diagram of a detailed structure of a power module. 
         FIG.  7    is a cross sectional view of the inverter when seen from a side opposite to the motor. 
         FIG.  8    is a vertical sectional view of the inverter. 
         FIG.  9    is a perspective view of a busbar. 
         FIG.  10    shows graphs showing the relationship between size and inductance sensitivity of busbar. 
         FIG.  11    is a view, corresponding to  FIG.  7   , of Modification 1 of the first embodiment, and is a cross sectional view of an inverter when seen from a side opposite to a motor. 
         FIG.  12    is a view, corresponding to  FIG.  7   , of Modification 2 of the first embodiment, and is a cross sectional view of an inverter when seen from a side opposite to a motor. 
         FIG.  13 A  is a view, corresponding to  FIG.  7   , of Modification 3 of the first embodiment, and is a cross sectional view of an inverter when seen from a side opposite to a motor. 
         FIG.  13 B  shows an output busbar according to Modification 4 of the first embodiment. 
         FIG.  14    is a view, corresponding to  FIG.  7   , of a second embodiment, and is a cross sectional view of an inverter when seen from a side opposite to a motor. 
         FIG.  15    is a view, corresponding to  FIG.  8   , of the second embodiment, and is a vertical sectional view of the inverter. 
         FIG.  16    is a view, corresponding to  FIG.  7   , of a third embodiment, and is a cross sectional view of an inverter when seen from a side opposite to a motor. 
         FIG.  17    is a view, corresponding to  FIG.  8   , of the third embodiment, and is a vertical sectional view of the inverter. 
         FIG.  18    is a view, corresponding to  FIG.  8   , of Modification 1 of the third embodiment, and is a vertical sectional view of an inverter. 
         FIG.  19    is a perspective view of power modules and busbars according to Modification 2 of the third embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present disclosure are described in detail below based on the drawings. The description of preferable embodiments below merely substantially gives exemplifications, and is not intended at all to limit the present disclosure, applicable matters thereof, or use thereof. 
     First Embodiment 
     Vehicle Structure 
       FIG.  1    shows a vehicle  1  including drive units A according to a first embodiment when seen from below the vehicle. In the vehicle  1 , power from at least one of an engine  2  and a drive motor  3  disposed at the front of the vehicle is transmitted to rear wheels  4  disposed at the rear of the vehicle. That is, the vehicle  1  is a hybrid vehicle of a front engine/rear drive (FR) type. 
     As shown in  FIG.  1   , the vehicle  1  includes the engine  2 , a transmission  5  that is connected to the engine  2 , the drive motor  3  that is disposed between the engine  2  and the transmission  5 , a propeller shaft  6  that is connected to the transmission  5  and that transmits power from the engine  2  and the drive motor  3  to the rear wheels, and a differential  7  that is connected to the propeller shaft  6  and that transmits power from the engine  2  and the drive motor  3  to the left and right rear wheels  4 . 
     The propeller shaft  6  extends below a floor panel  8  in a vehicle front-rear direction. A tunnel portion  9  is provided on a central side in a vehicle width direction of the floor panel  8 . The propeller shaft  6  is disposed on an inner side of the tunnel portion  9 . 
     The vehicle  1  includes an exhaust pipe  10  extending in the vehicle front-rear direction from the engine  2 . A catalytic device  11  is disposed on an upstream side of the exhaust pipe  10 . Although not shown, a silencer is disposed on a downstream side of the exhaust pipe  10 . 
     The vehicle  1  includes a fuel tank (not shown) that stores fuel to be supplied to the engine  2 , and a battery  12  that stores electric power to be supplied to the motor  3 . The drive motor  3  transmits power to the rear wheels  4 , and is rotationally driven by the propeller shaft  6  and generates regenerative power when the speed of the vehicle is reduced to supply the generated electric power to the battery  12 . The battery  12  includes a first battery unit  12   a  and a second battery unit  12   b , which are disposed one on each side in the vehicle width direction. The second battery unit  12   b  is longer than the first battery unit  12   a  in the vehicle front-rear direction. Each of the battery units  12   a  and  12   b  includes a plurality of battery cells. The battery cells are, for example, lithium-ion batteries. 
     An in-wheel motor  14  is connected to each of left and right front wheels  13 . Each in-wheel motor  14  functions as an assist motor that generates power and transmits the power to the front wheels  13  when the vehicle  1  is started. Each in-wheel motor  14  functions as a regenerative brake that generates electric power when the speed of the vehicle is reduced. Similarly to the drive motor  3 , each in-wheel motor  14  has electric power supplied thereto from the battery  12 . 
     As shown in  FIG.  1   , an inverter  15  is interposed between the drive motor  3  and the transmission  5 . The drive motor  3  and the inverter  15  are disposed adjacent to each other in an axial direction of the drive motor  3  (vehicle front-rear direction). Inverters  16  are disposed on inner sides of the respective in-wheel motors  14  in the vehicle width direction. The in-wheel motors  14  and the respective inverters  16  are disposed adjacent to each other in an axial direction of each in-wheel motor  14  (vehicle width direction) . The drive motor  3  and the inverter  15  constitute the drive unit A. Similarly, the in-wheel motors  14  and the respective inverters  16  constitute the drive units A. 
     The inverter  15  and the inverters  16  each convert direct-current electric power stored in the battery  12  into alternating-current electric power and supply the converted electric power to a corresponding one of the motor  3  and the motors  14 , and each convert the alternating-current electric power generated at the corresponding one of the motor  3  and the motors  14  into direct-current electric power to charge the battery when the speed of the vehicle is reduced. 
     Drive Units 
     The drive units A of the vehicle  1  are described by taking as an example, the drive unit A including the drive motor  3  and the inverter  15 .  FIG.  2    is a perspective view of the drive unit A. As described above, the drive unit A includes the motor  3  and the inverter  15 . The motor  3  and the inverter  15  are disposed adjacent and coaxially to each other in the axial direction of the motor  3 . Specifically, a center axis 0 of the motor  3  and a center axis 0 of the inverter  15  coincide with each other. The motor  3  (specifically, a casing of the motor  3 ) is formed with a cylindrical shape. The inverter  15  (specifically, a casing of the inverter  15 ) is formed with a cylindrical shape corresponding to that of the motor  3 . A rotation shaft  3   a  of the motor  3  extends through the inverter  15  in the axial direction. A thickness Wiv of the inverter  15  is small, and is, for example, 50 mm or less (preferably, 30 mm or less). A cooling path  61  (described later) is provided inside the inverter  15 . A cooling inlet pipe  62  and a cooling outlet pipe  63  that communicate with the cooling path  61  are connected to an upper portion of the inverter. 
       FIG.  3    is a cross sectional view of the motor  3  when seen from the inverter  15 . The motor  3  has coils  17 . Specifically, each coil  17  is wound in a concentrated manner on a stator of the motor  3  so that two U-phase coils  17   u , two V-phase coils  17   v , and two W-phase coils  17   w  are disposed. Each coil  17  is disposed side by side uniformly in a peripheral direction of the motor  3 . 
     The U-phase coils  17   u  include a first U-phase coil  17   ul  and a second U-phase coil  17   u   2 . The V-phase coils  17   v  include a first V-phase coil  17   v   1  and a second V-phase coil  17   v   2 . The W-phase coils  17   w  include a first W-phase coil  17   w   1  and a second W-phase coil  17   w   2 . 
     In the motor  3 , a first coil group C 1  and a second coil group C 2  each including one U-phase coil  17   u , one V-phase coil  17   v , and one W-phase coil  17   w  are provided. Specifically, the first coil group C 1  includes the first U-phase coil  17   u   1 , the first V-phase coil  17   v   1 , and the first W-phase coil  17   w   1 . The second coil group C 2  includes the second U-phase coil  17   u   2 , the second V-phase coil  17   v   2 , and the second W-phase coil  17   w   2 . 
     Each of the phase coils  17   u   1 ,  17   v   1 , and  17   w   1  of the first coil group C 1  is disposed on one side in a radial direction of the motor  3 . Each of the phase coils  17   u   2 ,  17   v   2 , and  17   w   2  of the second coil group C 2  is disposed on an opposite side in the radial direction of the motor  3 . 
     The first U-phase coil  17   u   1  of the first coil group C 1  and the second U-phase coil  17   u   2  of the second coil group C 2  oppose each other in the radial direction of the motor  3 . The first V-phase coil  17   v   1  of the first coil group C 1  and the second V-phase coil  17   v   2  of the second coil group C 2  oppose each other in the radial direction of the motor  3 . The first W-phase coil  17   w   1  of the first coil group C 1  and the second W-phase coil  17   w   2  of the second coil group C 2  oppose each other in the radial direction of the motor  3 . 
     Six motor-side terminal blocks  18  are provided at an outer peripheral portion of the motor  3 . The motor-side terminal blocks  18  correspond to the respective coils  17 . Specifically, each motor-side terminal block  18  is disposed at a corresponding peripheral-direction position and at a radial-direction outer side with respect to a corresponding one of the coils  17 . A lead wire (not shown) extends from each coil  17 . Each lead wire is connected to a corresponding one of the motor-side terminal blocks  18 . As a rotor, an iron core  27  and a permanent magnet  28  having a N pole and a S pole are fixed to the rotation shaft  3   a . 
       FIG.  4    is a circuit diagram of the inverter  15 . The inverter  15  includes a smoothing capacitor  19  and a plurality of power modules  20 . The smoothing capacitor  19  smoothens a voltage that is applied to the power modules  20 . The plurality of power modules  20  constitute an inverter circuit, and convert direct-current voltage into an alternating-current voltage. 
     The plurality of power modules  20  include U-phase power modules  20   u , V-phase power modules  20   v , and W-phase power modules  20   w . The U-phase power modules  20   u  correspond to the U-phase coils  17   u . The V-phase power modules  20   v  correspond to the V-phase coils  17   v . The W-phase power modules  20   w  correspond to the W-phase coils  17   w . 
     Further, the U-phase power modules  20   u  include a first U-phase power module  20   u   1  and a second U-phase power module  20   u   2 . the V-phase power modules  20   v  include a first V-phase power module  20   v   1  and a second V-phase power module  20   v   2 . The W-phase power modules  20   w  include a first W-phase power module  20   w   1  and a second W-phase power module  20   w   2 . 
     The plurality of power modules  20  constitute a first power module group P 1  and a second power module group P 2  that are connected in parallel. The first power module group P 1  and the second power module group P 2  each include one U-phase power module  20   u , one V-phase power module  20   v , and one W-phase power module  20   w . 
     The first power module group P 1  includes the first U-phase power module  20   u   1 , the first V-phase power module  20   v   1 , and the first W-phase power module  20   w   1 . The phase power modules  20   u   1 ,  20   v   1 , and  20   w   1  of the first power module group P 1  are connected to the phase coils  17   u   1 ,  17   v   1 , and  17   w   1  of the first coil group C 1 , respectively. Specifically, the first U-phase power module  20   u   1  of the first power module group P 1  is connected to the first U-phase coil  17   u   1  of the first coil group C 1 . The first V-phase power module  20   v   1  of the first power module group P 1  is connected to the first V-phase coil  17   v   1  of the first coil group C 1 . The first W-phase power module  20   w   1  of the first power module group P 1  is connected to the first W-phase coil  17   w   1  of the first coil group C 1 . 
     The second power module group P 2  includes the second U-phase power module  20   u   2 , the second V-phase power module  20   v   2 , and the second W-phase power module  20   w   2 . The phase power modules  20   u   2 ,  20   v   2 , and  20   w   2  of the second power module group P 2  are connected to the phase coils  17   u   2 ,  17   v   2 , and  17   w   2  of the second coil group C 2 , respectively. Specifically, the second U-phase power module  20   u   2  of the second power module group P 2  is connected to the second U-phase coil  17   u   2  of the second coil group C 2 . The second V-phase power module  20   v   2  of the second power module group P 2  is connected to the second V-phase coil  17   v   2  of the second coil group C 2 . The second W-phase power module  20   w   2  of the second power module group P 2  is connected to the second W-phase coil  17   w   2  of the second coil group C 2 . 
     Each power module  20  includes two arm elements, that is, a lower arm element  21  and an upper arm element  22 , serving as switching elements. In each phase power module  20 , when one of the lower arm element  21  and the upper arm element  22  is opened, the other of the lower arm element  21  and the upper arm element  22  is closed. Therefore, a three-phase alternating current is supplied to the motor  3 . 
     Here, each power module  20  includes a SiC-MOSFET.  FIG.  5    shows a comparison between a SiC-MOSFET and an IGBT. The SiC-MOSFET is a MOSFET (metal-oxide-semiconductor field-effect transistor) including silicon carbide (SiC), and constitutes a chip  24  including the lower arm element  21  and the upper arm element  22  and, for example, other control elements. A lower surface of the chip  24  is soldered and fixed to a silicon substrate. A copper block  25 , serving as a heat transfer block, is soldered and fixed to an upper surface of the chip  24 . This also applies to the IGBT (insulated gate bipolar transistor). 
     As shown in  FIG.  5   , a surface area of the chip  24  constituted by the SiC-MOSFET is smaller than a surface area of a chip  24 ′ constituted by the IGBT. Therefore, the size of the copper block  25  disposed on an upper side of the SiC-MOSFET (chip)  24  is smaller than the size of a copper block  25 ′ disposed on an upper side of the IGBT (chip)  24 ′. The SiC-MOSFET has better heat resistance than the IGBT. 
       FIG.  6    is a perspective view and a circuit diagram of a detailed structure of a power module  20 . Each power module  20  has a flat shape having a wide width. Specifically, each power module  20  is longer in a width direction W than in a thickness direction t. Each power module  20  has a substantially parallelepiped shape. The width direction W includes a first width direction W 1  and a second width direction W 2  orthogonal to each other. Hereunder, one side of the power module  20  in the thickness direction may be the lower side and an opposite side of the power module  20  in the thickness direction may be the upper side. 
     The power module  20  has a lower surface  31  on the lower side (one surface side, the one side in the thickness direction). The power module  20  has an upper surface  32  on the upper side. The power module  20  has a first end surface  33  on one side in the first width direction W 1 . The power module  20  has a second end surface  34  on an opposite side in the first width direction W 1 . 
     A negative-electrode-side input terminal  35  is connected to a lower side of the first end surface  33  and on one side in the second width direction W 2 . A positive-electrode-side input terminal  36  is connected to an upper side of the first end surface  33  and on an opposite side in the second width direction W 2 . The negative-electrode-side input terminal  35  and the positive-electrode-side input terminal  36  are disposed apart from each other in an up-down direction (thickness direction). An output terminal  37  is connected to a central portion of the second end surface  34 . 
     The lower arm element  21  and the upper arm element  22  are accommodated inside a package (box body) of the power module  20 . The negative-electrode-side input terminal  35  is connected to the lower arm element  21 . The positive-electrode-side input terminal  36  is connected to the upper arm element  22 . The output terminal  37  is connected between the lower arm element  21  and the upper arm element  22 . 
       FIG.  7    is a cross sectional view of the inverter  15  when seen from a side opposite to the motor  3 .  FIG.  8    is a vertical sectional view of the inverter  15  along line VIII-VIII. As shown in  FIG.  7   , an axial through hole  40  through which the rotation shaft  3   a  of the motor  3  passes is provided in the center of the inverter  15 . A cylindrical boss portion  41  is formed around the axial through hole  40 . The smoothing capacitor  19  is disposed at a center 0 of the inverter  15 . The smoothing capacitor  19  is disposed along the boss portion  41 . The smoothing capacitor  19  is formed with a hollow polygonal shape having an axial through hole through which the rotation shaft  3   a  passes. 
     Each power module  20  (the first U-phase power module  20   u   1 , the first V-phase power module  20   v   1 , the first W-phase power module  20   w   1 , the second U-phase power module  20   u   2 , the second V-phase power module  20   v   2 , and the second W-phase power module  20   w   2 ) are disposed on an outer peripheral side with respect to the smoothing capacitor  19 . On the outer peripheral side with respect to the smoothing capacitor  19 , each power module  20  is disposed side by side in the peripheral direction of the motor  3 . The input terminals  35  and  36  (the first end surface  33 ) and the output terminal  37  (the second end surface  34 ) of each power module  20  face the peripheral direction of the motor  3  (the inverter  15 ). Each power module  20  is disposed so that the thickness direction t coincides with the axial direction of the motor  3 . The smoothing capacitor  19  and each power module  20  are disposed in a space defined by an outer peripheral wall portion  42  and the boss portion  41  of the inverter  15 . 
     Each of the phase power modules  20   u   1 ,  20   v   1 , and  20   w   1  of the first power module group P 1  is disposed on one side in the radial direction of the motor  3  (the inverter  15 ). Each of the phase power modules  20   u   2 ,  20   v   2 , and  20   w   2  of the second power module group P 2  is disposed on an opposite side in the radial direction of the motor  3 . 
     The first U-phase power module  20   u   1  of the first power module group P 1  and the second U-phase power module  20   u   2  of the second power module group P 2  oppose each other in the radial direction of the motor  3  (the inverter  15 ). The first V-phase power module  20   v   1  of the first power module group P 1  and the second V-phase power module  20   v   2  of the second power module group P 2  oppose each other in the radial direction of the motor  3 . The first W-phase power module  20   w   1  of the first power module group P 1   and the second W-phase power module  20   w   2  of the second power module group P 2  oppose each other in the radial direction of the motor  3 . 
     As shown in  FIGS.  7  and  8   , a heat sink  60  is provided on a motor  3  side of the inverter  15 . The heat sink  60  is primarily used for cooling each power module  20 . The heat sink  60  is disposed between the outer peripheral wall portion  42  and the boss portion  41  of the inverter  15 . The heat sink  60  has an upper wall portion  60   a , an outer peripheral wall portion  60   b , a lower wall portion  60   c , and an inner peripheral wall portion  60   d . 
     An upper surface  65  of the upper wall portion  60   a  of the heat sink  60  constitutes a placement surface orthogonal to the axial direction of the motor  3  (may hereunder be referred to as “placement surface  65 ”) . The lower surface  31  of each power module  20  faces the motor  3 . Specifically, the lower surface  31  of each power module  20  is placed side by side on the same placement surface  65 . 
     As shown in  FIGS.  7  and  8   , the smoothing capacitor  19  is one in which an assembly of a plurality of columnar capacitors  45  is covered by a flat plate from both sides in the thickness direction. The smoothing capacitor  19  has a lower surface  19   a  on a lower side (one surface side, the one side in the thickness direction). The smoothing capacitor  19  has an upper surface  19   b  on a side opposite to the lower surface  19   a . The lower surface  19   a  and the upper surface  19   b  of the smoothing capacitor  19  are constituted by flat plates. The lower surface  19   a  of the smoothing capacitor  19  faces the motor  3 . Specifically, the lower surface  19   a  of the smoothing capacitor  19  is placed on the placement surface  65 . 
     As shown in  FIGS.  7  and  8   , the smoothing capacitor  19  and each power module  20  are connected to each other by a negative-electrode-side busbar  51  and a positive-electrode-side busbar  52 , serving as input busbars  50  (may hereunder be simply referred to as “busbars  50 ”). The negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  each have a plate shape. Specifically, the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  are longer in the width direction W and a length direction L than in the thickness direction t. 
     As shown in  FIG.  7   , the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  are formed with wide widths so as to be extend along the peripheral direction of the motor  3  (the inverter  15 ). In other words, the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  are formed with wide widths so as to extend along a direction in which each power module  20  is disposed side by side. The width direction W of the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  is the peripheral direction of the motor  3  (an arc shape is formed). The negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  each have a fan shape. The length direction L of the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  is the radial direction of the motor  3 . 
     One end portion  51   i  of the negative-electrode-side busbar  51  (input busbar  50 ) is connected to a negative-electrode terminal  19   c  provided on the lower surface  19   a  side of the smoothing capacitor  19 . The other end portion  51   o  of the negative-electrode-side busbar  51  (input busbar  50 ) is connected to a negative-electrode-side input terminal  35  of each power module  20 . One end portion  52   i  of the positive-electrode-side busbar  52  (input busbar  50 ) is connected to a positive-electrode terminal  19   d  provided on the upper surface  19   b  side of the smoothing capacitor  19 . An other end portion  52   o  of the positive-electrode-side busbar  52  (input busbar  50 ) is connected to a positive-electrode-side input terminal  36  of each power module  20 . 
     The negative-electrode-side busbar  51  has a lower surface  51   a  on the lower side (one surface side, the one side in the thickness direction). The lower surface  51   a  of the negative-electrode-side busbar  51  faces the motor  3 . Specifically, the lower surface  51   a  of the negative-electrode-side busbar  51  is placed on the placement surface  65 . 
     As shown in  FIG.  7   , output busbars  54  are each connected to the output terminal  37  of a corresponding one of the power modules  20 . There are a total of six output busbars  54  each corresponding to a corresponding one of the power modules  20   u   1 ,  20   v   1 ,  20   w   1 ,  20   u   2 ,  20   v   2 , and  20   w   2  (the coils  17   u   1 ,  17   v   1 ,  17   w   1 ,  17   u   2 ,  17   v   2 , and  17   w   2 ). The output busbars  54  are each interposed between the corresponding one of the power modules  20  and a corresponding one of the coils  17 . The output busbars  54  each have a plate shape. In addition to the output busbars  54 , a wire harness or the like may be interposed between the corresponding one of the power modules  20  and the corresponding one of the coils  17 . 
     Six inverter-side terminal blocks  46  are provided at an outer peripheral portion of the inverter  15 . The inverter-side terminal blocks  46  correspond to the respective power modules  20 . Specifically, each inverter-side terminal block  46  is disposed at a corresponding peripheral-direction position and at a radial-direction outer side with respect to a corresponding one of the power modules  20 . 
     The output busbars  54  extend up to the respective inverter-side terminal blocks  46 . Electrically conductive members (busbars, wire harnesses, or the like) are each interposed between a corresponding one of the inverter-side terminal blocks  46  and a corresponding one of the motor-side terminal blocks  18 . 
     Cooling Path 
     As shown in  FIGS.  7  and  8   , the cooling path (cooling jacket)  61  is provided inside the heat sink  60 . The cooling path  61  is defined by the upper wall portion  60   a , the outer peripheral wall portion  60   b , the lower wall portion  60   c , and the inner peripheral wall portion  60   d . The cooling path  61  is formed with a doughnut shape (ring shape, cylindrical shape) over the entire periphery when seen in the axial direction of the motor  3  (the inverter  15 ) (see  FIG.  7   ). The rotation shaft  3   a  of the motor  3  passes through an inner side of the inner peripheral wall portion  60   d . As described above, the upper surface of the upper wall portion  60   a  of the heat sink  60  is the placement surface  65 . 
     The cooling path  61  is provided closer to the motor  3  than the placement surface  65 . A cooling medium H flows in the cooling path  61 . The cooling medium H is, for example, cooling water or cooling oil. 
     A plurality of fins  64  are provided inside the heat sink  60  (the cooling path  61 ). The fins  64  extend downward inside the cooling path  61  from the upper wall portion  60   a . That is, the fins  64  are provided closer to the motor  3  than the placement surface  65 . 
     As shown in  FIGS.  7  and  8   , when seen in the axial direction of the motor  3  (the inverter  15 ) , the cooling path  61  faces the lower surface  31  of each power module  20 , the lower surface  51   a  of the negative-electrode-side busbar  51 , and the lower surface  19   a  of the smoothing capacitor  19 . 
     Similarly, when seen in the axial direction of the motor  3 , the fins  64  face the lower surface  31  of each power module  20 , the lower surface  51   a  of the negative-electrode-side busbar  51 , and the lower surface  19   a  of the smoothing capacitor  19 . 
     As shown in  FIG.  2   , the inlet pipe  62  and the outlet pipe  63  are connected to an upper portion of the outer peripheral portion of the inverter  15 . The inlet pipe  62  and the outlet pipe  63  communicate with the cooling path  61 . The cooling medium H introduced into the cooling path  61  via the inlet pipe  62  is discharged to the outside via the outlet pipe  63  after having flowed inside the cooling path  61 . 
     Busbar Inductance Sensitivity 
       FIG.  9    is a perspective view of a busbar  50 .  FIG.  10    shows graphs showing the relationship between size and inductance sensitivity of busbar  50 . As a result of assiduous studies, the inventor of the present application and others have found the following with regard to the relationship between the size and inductance sensitivity of busbar  50 . 
     As shown in  FIGS.  9  and  10   , the larger a width W (mm) of busbar  50 , the smaller an inductance sensitivity (nH) of busbar  50 . 
     Basically, the larger a length L (mm) of busbar  50 , the larger the inductance sensitivity (nH) of busbar  50 . However, as shown in the middle graph in  FIG.  10   , a minimum value (inflection point) M exists in the relationship between the length L (mm) and the inductance sensitivity (nH) of busbar  50 . Therefore, even if lengths L differ from each other, the inductance sensitivity (nH) may become the same. Specifically, the inductance sensitivity (nH) of busbar  50  (51,  52 ) is a function of the length L (mm) extending from the one end portion  51   i  or  52   i  of busbar  50  (51,  52 ) (the terminal  19   c  or  19   d  of the smoothing capacitor  19 ) to the other end portion  51   o  or  52   o  of busbar  50  (51,  52 ) (the input terminal  35  or  36  of each power module  20 ). The function has the minimum value M so that the inductance sensitivity K (nH) becomes the same at a first length L 1  (mm) and a second length L 2  (mm) that differ from each other. The second length L 2  (mm) is larger than the first length L 1  (mm). 
     Even if a thickness t (mm) of busbar  50  changes, the inductance sensitivity (nH) of busbar  50  does not change. 
     As shown in  FIG.  7   , the width of the negative-electrode-side busbar  51  and the width of the positive-electrode-side busbar  52  are substantially the same. As shown in  FIG.  8   , a length L- of the negative-electrode-side busbar  51  and a length L+ of the positive-electrode-side busbar  52  differ from each other. The length L- of the negative-electrode-side busbar  51  corresponds to the first length L 1 . The length L+ of the positive-electrode-side busbar  52  corresponds to the second length L 2 . The length L+ of the positive-electrode-side busbar  52  (the second length L 2 ) is larger than the length L- of the negative-electrode-side busbar  51  (the first length L 1 ). However, due to the existence of the minimum value M above, the inductance sensitivity of the negative-electrode-side busbar  51  and the inductance sensitivity of the positive-electrode-side busbar  52  are equal to each other. 
     Positional Relationship between Power Module and Coil 
     As shown in  FIG.  7   , when seen in the axial direction of the motor  3  (the inverter  15 ), the first U-phase power module  20   u   1  of the first power module group P 1  is disposed at a position overlapping the first U-phase coil  17   u   1  of the first coil group C 1 . When seen in the axial direction of the motor  3 , the first V-phase power module  20   v   1  of the first power module group P 1  is disposed at a position overlapping the first V-phase coil  17   v   1  of the first coil group C 1 . When seen in the axial direction of the motor  3 , the first W-phase power module  20   w   1  of the first power module group P 1  is disposed at a position overlapping the first W-phase coil  17   w   1  of the first coil group C 1 . 
     As shown in  FIG.  7   , when seen in the axial direction of the motor  3  (the inverter  15 ), the second U-phase power module  20   u   2  of the second power module group P 2  is disposed at a position overlapping the second U-phase coil  17   u   2  of the second coil group C 2 . When seen in the axial direction of the motor  3 , the second V-phase power module  20   v   2  of the second power module group P 2  is disposed at a position overlapping the second V-phase coil  17   v   2  of the second coil group C 2 . When seen in the axial direction of the motor  3 , the second W-phase power module  20   w   2  of the second power module group P 2  is disposed at a position overlapping the second W-phase coil  17   w   2  of the second coil group C 2 . 
     Specifically, when seen in the axial direction of the motor  3  (the inverter  15 ), at least a part of each power module  20  is disposed so as to overlap at least a part of the corresponding one of the coils  17 . 
     That is, as shown in  FIG.  7   , when seen in the axial direction of the motor  3  (the inverter  15 ), a distance  du   1  between the first U-phase power module  20   u   1  of the first power module group P 1  and the first U-phase coil  17   u   1  of the first coil group C 1 , a distance  dv   1  between the first V-phase power module  20   v   1  of the first power module group P 1  and the first V-phase coil  17   v   1  of the first coil group C 1 , and a distance  dw   1  between the first W-phase power module  20   w   1  of the first power module group P 1  and the first W-phase coil  17   w   1  of the first coil group C 1  are equal to each other. 
     Similarly, as shown in  FIG.  7   , when seen in the axial direction of the motor  3  (the inverter  15 ), a distance  du   2  between the second U-phase power module  20   u   2  of the second power module group P 2  and the second U-phase coil  17   u   2  of the second coil group C 2 , a distance  dv   2  between the second V-phase power module  20   v   2  of the second power module group P 2  and the second V-phase coil  17   v   2  of the second coil group C 2 , and a distance  dw   2  between the second W-phase power module  20   w   2  of the second power module group P 2  and the second W-phase coil  17   w   2  of the second coil group C 2  are equal to each other. 
     Further, as shown in  FIG.  7   , when seen in the axial direction of the motor  3  (the inverter  15 ), the distance  du   1  between the first U-phase power module  20   u   1  of the first power module group P 1  and the first U-phase coil  17   u   1  of the first coil group C 1  and the distance  du   2  between the second U-phase power module  20   u   2  of the second power module group P 2  and the second U-phase coil  17   u   2  of the second coil group C 2  are equal to each other. When seen in the axial direction of the motor  3 , the distance  dv   1  between the first V-phase power module  20   v   1  of the first power module group P 1  and the first V-phase coil  17   v   1  of the first coil group C 1  and the distance  dv   2  between the second V-phase power module  20   v   2  of the second power module group P 2  and the second V-phase coil  17   v   2  of the second coil group C 2  are equal to each other. When seen in the axial direction of the motor  3 , the distance  dw   1  between the first W-phase power module  20   w   1  of the first power module group P 1  and the first W-phase coil  17   w   1  of the first coil group C 1  and the distance  dw   2  between the second W-phase power module  20   w   2  of the second power module group P 2  and the second W-phase coil  17   w   2  of the second coil group C 2  are equal to each other. 
     The distances  du   1 ,  dv   1 ,  dw   1 ,  du   2 ,  dv   2 , and  dw   2  are all zero. Any standard may be set for the distances  du   1 ,  dv   1 ,  dw   1 ,  du   2 ,  dv   2 , and  dw   2 . 
     Operational Effects of First Embodiment 
     According to the present embodiment, since the first power module group P 1  and the second power module group P 2  of the inverter  15  are connected in parallel, the output of the motor  3  can be increased. 
     Since each power module  20  is placed side by side on the same placement surface  65  orthogonal to the axial direction of the motor  3 , the axial-direction length of the inverter  15  can be suppressed from being increased even though the number of power modules  20  has been increased. 
     Since the motor  3  and the inverter  15  are disposed adjacent to each other in the axial direction of the motor  3 , the distance between the phase power modules  20  and the respective phase coils  17  can be reduced. Therefore, inductances of electrical paths (including the output busbars  54 ) extending from the phase power modules  20  to the respective phase coils  17  can be reduced. 
     Further, in at least the first power module group P 1  and the first coil group C 1 , when seen in the axial direction of the motor  3 , the distances  du   1 ,  dv   1 , and  dw   1  between a corresponding one of the phase power modules  20   u   1 ,  20   v   1 , and  20   w   1  and a corresponding one of the phase coils  17   u   1 ,  17   v   1 , and  17   w   1  are equal to each other between the U phase, the V phase, and the W phase. Therefore, in at least the first power module group P 1  and the first coil group C 1 , the inductances of the electrical paths (including the output busbars  54 ) extending from the phase power modules  20   u   1 ,  20   v   1 , and  20   w   1  to the respective phase coils  17   u   1 ,  17   v   1 , and  17   w   1  can be equalized between the U phase, the V phase, and the W phase. 
     Accordingly, while suppressing the drive unit A including the motor  3  and the inverter  15  from increasing in size, the output of the motor  3  is increased, and the inductances of the electrical paths extending from the phase power modules  20  to the respective phase coils  17  can be reduced and equalized. 
     Even in the second power module group P 2  and the second coil group C 2 , as in the first power module group P 1  and the first coil group C 1 , when seen in the axial direction of the motor  3 , the distances  du   2 ,  dv   2 , and  dw   2  between a corresponding one of the phase power modules  20   u   2 ,  20   v   2 , and  20   w   2  and a corresponding one of the phase coils  17   u   2 ,  17   v   2 , and  17   w   2  are equal to each other between the U phase, the V phase, and the W phase. Therefore, even in the second power module group P 2  and the second coil group C 2 , as in the first power module group P 1  and the first coil group C 1 , inductances of electrical paths (including the output busbars  54 ) extending from the phase power modules  20   u   2 ,  20   v   2 , and  20   w   2  to the respective phase coils  17   u   2 ,  17   v   2 , and  17   w   2  can be equalized between the U phase, the V phase, and the W phase. 
     Further, between the first power module group P 1  and the first coil group C 1  and the second power module group P 2  and the second coil group C 2 , the inductances of the electrical paths (including the output busbars  54 ) extending from the phase power modules  20   u   1 ,  20   v   1 ,  20   w   1 ,  20   u   2 ,  20   v   2 , and  20   w   2  to the respective phase coils  17   u   1 ,  17   v   1 ,  17   w   1 ,  17   u   2 ,  17   v   2 , and  17   w   2  can be equalized. 
     When seen in the axial direction of the motor  3 , the phase power modules  20   u   1 ,  20   v   1 , and  20   w   1  of the first power module group P 1  are disposed at positions overlapping the respective phase coils  17   u   1 ,  17   v   1 , and  17   w   1  of the first coil group C 1 . Therefore, in the first power module group P 1  and the first coil group C 1 , the distances  du   1 ,  dv   1 , and  dw   1  between the corresponding one of the phase power modules  20   u   1 ,  20   v   1 , and  20   w   1  and the corresponding one of the phase coils  17   u   1 ,  17   v   1 , and  17   w   1  can be reduced. Consequently, in the first power module group P 1  and the first coil group C 1 , the inductances of the electrical paths (including the output busbars  54 ) extending from the phase power modules  20   u   1 ,  20   v   1 , and  20   w   1  to the respective phase coils  17   u   1 ,  17   v   1 , and  17   w   1  can be reduced. 
     When seen in the axial direction of the motor  3 , the phase power modules  20   u   2 ,  20   v   2 , and  20   w   2  of the second power module group P 2  are disposed at positions overlapping the respective phase coils  17   u   2 ,  17   v   2 ,  17   w   2  of the second coil group C 2 . Therefore, even in the second power module group P 2  and the second coil group C 2 , as in the first power module group P 1  and the first coil group C 1 , the distances  du   2 ,  dv   2 , and  dw   2  between the corresponding one of the phase power modules  20   u   2 ,  20   v   2 , and  20   w   2  and the corresponding one of the phase coils  17   u   2 ,  17   v   2 , and  17   w   2  can be reduced. Consequently, even in the second power module group P 2  and the second coil group C 2 , as in the first power module group P 1  and the first coil group C 1 , the inductances of the electrical paths (including the output busbars  54 ) extending from the phase power modules  20   u   2 ,  20   v   2 , and  20   w   2  to the respective phase coils  17   u   2 ,  17   v   2 , and  17   w   2  can be reduced. 
     In the motor  3  (in particular, a concentrated winding type in which the number of coils  17  is even), coils  17  of the same phase are often disposed so as to oppose each other in the radial direction of the motor  3  (see  FIG.  3   ). According to the present embodiment, as with the coils  17  of the same phase, power modules  20  of the same phase are disposed so as to oppose each other in the radial direction of the motor  3 . Therefore, the phase power modules  20  are easily positioned with respect to the respective phase coils  17 . 
     Since the smoothing capacitor  19  and each power module  20  are placed on the same placement surface  65  orthogonal to the axial direction of the motor  3 , such a structure is advantageous in terms of suppressing an increase in the axial-direction length of the inverter  15 . Further, since the distance between the smoothing capacitor  19  and each power module  20  can be reduced, inductances of the input busbars  50  (the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52 ) that connect the smoothing capacitor  19  and each power module  20  to each other can be reduced. 
     Since each power module  20  is disposed side by side in the peripheral direction of the motor  3  on the outer peripheral side with respect to the smoothing capacitor  19 , the distances between the smoothing capacitor  19  and each power module  20  can be made equal to each other. Further, since the input busbars  50  (the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52 ) are formed with wide widths so as to be extend along the peripheral direction of the motor  3 , in the input busbars  50 , inductances of electrical paths between the smoothing capacitor  19  and each power module  20  can be equalized. 
     In the input busbars  50  (the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52 ) that connect the smoothing capacitor  19  and each power module  20  to each other, by using the minimum value (the inflection point) M (see  FIG.  10   ), even though the length L- of the negative-electrode-side busbar  51  (the first length L 1 ) and the length L+ of the positive-electrode-side busbar  52  (the second length L 2 ) differ from each other, the inductance of the negative-electrode-side busbar  51  and the inductance of the positive-electrode-side busbar  52  can be equalized with respect to each other. 
     As shown by an alternate long and short dash lines in  FIG.  1   , in the related art, an inverter  15 ′ is often disposed near a second battery unit  12   b ′ of a battery  12 ′. According to the present embodiment, since the inverter  15  can be disposed adjacent to the motor  3  in the axial direction, the inverter  15  no longer needs to be disposed near the second battery unit  12   b . Therefore, the layout of the second battery unit  12   b  can be provided with greater freedom, and the second battery unit  12   b  can be increased in size. 
     When seen in the axial direction of the motor  3 , the phase power modules  20   u   1 ,  20   v   1 , and  20   w   1  of the first power module group P 1  are disposed at positions overlapping the respective phase coils  17   u   1 ,  17   v   1 , and  17   w   1  of the first coil group C 1 . Therefore, in the first power module group P 1  and the first coil group C 1 , this is advantageous in terms of equalizing the inductances of the electrical paths extending from the phase power modules  20   u   1 ,  20   v   1 , and  20   w   1  to the respective phase coils  17   u   1 ,  17   v   1 , and  17   w   1  between the U phase, the V phase, and the W phase. 
     When seen in the axial direction of the motor  3 , the phase power modules  20   u   2 ,  20   v   2 , and  20   w   2  of the second power module group P 2  are disposed at positions overlapping the respective phase coils  17   u   2 ,  17   v   2 , and  17   w   2  of the second coil group C 2 . Therefore, even in the second power module group P 2  and the second coil group C 2 , as in the first power module group P 1  and the first coil group C 1 , this is advantageous in terms of equalizing the inductances of the electrical paths extending from the phase power modules  20   u   2 ,  20   v   2 , and  20   w   2  to the respective phase coils  17   u   2 ,  17   v   2 , and  17   w   2  between the U phase, the V phase, and the W phase. 
     Modification 1 of First Embodiment 
       FIG.  11    is a view, corresponding to  FIG.  7   , of Modification  1  of the first embodiment. According to the present modification, in the first power module group P 1  and the first coil group C 1  and in the second power module group P 2  and the second coil group C 2 , when seen in the axial direction of the motor  3 , the phase power modules  20   u   1 ,  20   v   1 ,  20   w   1 ,  20   u   2 ,  20   v   2 , and  20   w   2  are disposed at locations that do not overlap the respective phase coils  17   ul ,  17   v   1 ,  17   w   1 ,  17   u   2 ,  17   v   2 , and  17   w   2 . Specifically, when seen in the axial direction of the motor  3 , the phase power modules  20   u   1 ,  20   v   1 ,  20   w   1 ,  20   u   2 ,  20   v   2 , and  20   w   2  are displaced from the respective phase coils  17   ul ,  17   v   1 ,  17   wl ,  17   u   2 ,  17   v   2 , and  17   w   2  in the peripheral direction of the motor  3 . 
     When seen in the axial direction of the motor  3 , the distances  du   1 ,  dv   1 ,  dw   1 ,  du   2 ,  dv   2 , and  dw   2  between the corresponding one of the phase power modules  20   u   1 ,  20   v   1 ,  20   w   1 ,  20   u   2 ,  20   v   2 , and  20   w   2  and the corresponding one of the phase coils  17   ul ,  17   v   1 ,  17   w   1 ,  17   u   2 ,  17   v   2 , and  17   w   2  are equal to each other, and are d (&gt; 0). 
     Modification 2 of First Embodiment 
       FIG.  12    is a view, corresponding to  FIG.  7   , of Modification  2  of the first embodiment. According to the present modification, the stator of the motor  3  is further provided with a third U-phase coil  17   u   3 , serving as a U-phase coil  17   u , a third V-phase coil  17   v   3 , serving as a V-phase coil  17   v , and a third W-phase coil  17   w   3 , serving as a W-phase coil  17   w . The third U-phase coil  17   u   3 , the third V-phase coil  17   v   3 , and the third W-phase coil  17   w   3  constitute a third coil group C 3 . 
     The inverter  15  further includes a third U-phase power module  20   u   3 , serving as a U-phase power module  20   u , a third V-phase power module  20   v   3 , serving as a V-phase power module  20   v , and a third W-phase power module  20   w   3 , serving as a W-phase power module  20   w . The third U-phase power module  20   u   3 , the third V-phase power module  20   v   3 , and the third W-phase power module  20   w   3  constitute a third power module group P 3 . 
     When seen in the axial direction of the motor  3 , the phase power modules  20   u   1 ,  20   v   1 ,  20   w   1 ,  20   u   2 ,  20   v   2 ,  20   w   2 ,  20   u   3 ,  20   v   3 , and  20   w   3  are disposed at positions overlapping the respective phase coils  17   u   1 ,  17   v   1 ,  17   w   1 ,  17   u   2 ,  17   v   2 ,  17   w   2 ,  17   u   3 ,  17   v   3 , and  17   w   3 . When seen in the axial direction of the motor  3 , the distances  du   1 ,  dv   1 ,  dw   1 ,  du   2 ,  dv   2 ,  dw   2 ,  du   3 ,  dv   3 , and  dw   3  between a corresponding one of the phase power modules  20   u   1 ,  20   v   1 ,  20   w   1 ,  20   u   2 ,  20   v   2 ,  20   w   2 ,  20   u   3 ,  20   v   3 , and  20   w   3  and a corresponding one of the phase coils  17   u   1 ,  17   v   1 ,  17   w   1 ,  17   u   2 ,  17   v   2 ,  17   w   2 ,  17   u   3 ,  17   v   3 , and  17   w   3  are equal to each other, and are zero. 
     According to the present modification, the output of the motor  3  can be further increased by connecting the first power module group P 1 , the second power module group P 2 , and the third power module group P 3  by three parallel connection. 
     Modification 3 of First Embodiment 
       FIG.  13 A  is a view, corresponding to  FIG.  7   , of Modification 3 of the first embodiment. The inverters  16  according to the present modification are disposed adjacent to the respective in-wheel motors  14  in the axial direction of the in-wheel motors  14  (see  FIG.  1   ). The inverters  16  do not have an axial through hole  40  and a boss portion  41 . The smoothing capacitor  19  has a columnar shape. The smoothing capacitor  19  does not have an axial through hole. 
     Modification 4 of First Embodiment 
       FIG.  13 B  shows an output busbar  54  according to Modification  4  of the first embodiment. In the present modification, each output busbar  54  is formed with a wide width so as to extend along the peripheral direction of the motor  3 . In other words, the width direction of each output busbar  54  is the peripheral direction (an arc shape is formed). Each output busbar  54  has a fan shape. Therefore, since each output busbar  54  is easily formed with a wide width, the inductance of each output busbar  54  is easily reduced. 
     Other Modifications of First Embodiment 
     The number of coil groups of the motor  3  and the number of power module groups of the inverter  15  may differ from each other. For example, when the number of coil groups of the motor  3  is three (the number of coils is nine), the number of power module groups of the inverter  15  may be two (the number of power modules may be six). In contrast, when the number of coil groups of the motor  3  is two (the number of coils is six), the number of power module groups of the inverter  15  may be three (the number of power modules  20  may be nine). 
     Although not shown, in the motor  3 , the U-phase coils  17   u , the V-phase coils  17   v , and the W-phase coils  17   w  may be subjected to distributed winding. 
     For the input busbars  50 , the length L- of the negative-electrode-side busbar  51  may be the second length L 2 , and the length L+ of the positive-electrode-side busbar  52  may be the first length L 1 . 
     The placement surface  65  may include a plurality of surfaces that are positioned in the same plane orthogonal to the axial direction of the motor  3 . 
     Second Embodiment 
       FIG.  14    is a view, corresponding to  FIG.  7   , of a second embodiment, and is a cross sectional view of an inverter  215  when seen from a side opposite to the motor  3 .  FIG.  15    is a view, corresponding to  FIG.  8   , of the second embodiment, and is a vertical sectional view of the inverter  215 . A detailed description of structures that are the same as those of the embodiment above may not be given. 
     In the present embodiment, each power module  20  is disposed on an outer peripheral side with respect to a smoothing capacitor  19 . On the outer peripheral side with respect to the smoothing capacitor  19 , the power modules  20  are disposed side by side in a peripheral direction of the motor  3 . 
     The input terminals  35  and  36  (first end surface  33 ) and the output terminal  37  (second end surface  34 ) of each power module  20  face a radial direction of the motor  3  (the inverter  215 ). Specifically, the input terminals  35  and  36  (the first end surface  33 ) of each power module  20  face an inner peripheral side. The output terminal  37  (the second end surface  34 ) of each power module  20  faces the outer peripheral side. Each power module  20  is disposed radially from the center 0, which is the origin, of the inverter  215  (the motor  3 ). 
     As shown in  FIG.  14   , the width of the negative-electrode-side busbar  51  and the width of the positive-electrode-side busbar  52  are the same. As shown in  FIG.  15   , the length L- of the negative-electrode-side busbar  51  and the length L+ of the positive-electrode-side busbar  52  are the same. Therefore, the inductance of the negative-electrode-side busbar  51  and the inductance of the positive-electrode-side busbar  52  are equal to each other. 
     The other structures are the same as those of the first embodiment. 
     Third Embodiment 
       FIG.  16    is a view, corresponding to  FIG.  7   , of a third embodiment, and is a cross sectional view of an inverter  315  when seen from a side opposite to the motor  3 .  FIG.  17    is a view, corresponding to  FIG.  8   , of the third embodiment, and is a vertical sectional view of the inverter  315 . A detailed description of structures that are the same as those of the embodiments above may not be given. 
     As shown in  FIGS.  16  and  17   , the negative-electrode-side input terminals  35  are each connected to a lower side of the first end surface  33  of a corresponding one of power modules  20 . The positive-electrode-side input terminals  36  are each connected to a lower side of the second end surface  34  of the corresponding one of the power modules  20 . The output terminals  37  are each connected to the center of the upper surface  32  of the corresponding one of the power modules  20 . 
     The negative-electrode-side busbar  51  connects the negative-electrode terminal  19   c  of the smoothing capacitor  19  and the negative-electrode-side input terminal  35  of each power module to each other. The positive-electrode-side busbar  52  connects the positive-electrode terminal  19   d  of the smoothing capacitor  19  and the positive-electrode-side input terminal  36  of each power module  20  to each other. 
     The positive-electrode-side busbar  52  starts out from the positive-electrode terminal  19   d  of the smoothing capacitor  19  (one end portion  52   i ), and extends along the upper surface  32  to the second end surface  34  from the first end surface  33  of each power module  20 . Then, the positive-electrode-side busbar  52  bends downward and extends along the second end surface  34  up to the positive-electrode-side input terminal  36  (other end portion  52   o ) on the lower side of each power module  20 . In other words, the positive-electrode-side busbar  52  extends so as to be wound around each power module  20  from the upper surface  32 . Each output busbar  54  starts out from the output terminal  37  at the upper surface  32  of each power module  20 , and extends upward. The positive-electrode-side busbar  52  has three opening portions for passing three upwardly extending output busbars  54  therethrough. 
     As shown in  FIG.  16   , the width of the negative-electrode-side busbar  51  and the width of the positive-electrode-side busbar  52  are the same. As shown in  FIG.  17   , the length (L-) of the negative-electrode-side busbar  51  and the length (sum of La+ and Lb+) of the positive-electrode-side busbar  52  differ from each other. The length (L-) of the negative-electrode-side busbar  51  corresponds to the first length L 1 . The length (sum of La+ and Lb+) of the positive-electrode-side busbar  52  corresponds to the second length L 2 . The length (sum of La+ and Lb+, the second length L 2 ) of the positive-electrode-side busbar  52  is larger than the length (L-, the first length L 1 ) of the negative-electrode-side busbar  51 . However, due to the existence of the minimum value M (see  FIG.  10   ), the inductance of the negative-electrode-side busbar  51  and the inductance of the positive-electrode-side busbar  52  are equal to each other. 
     Note that the conditions (for example, materials) of the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  according to the present embodiment differ from those of the embodiments above. Therefore, the mode of the minimum value (M above) (see  FIG.  10   ) also differs. Specifically, the difference between the first length L 1  and the second length L 2  is larger than that in the embodiments above. 
     The other structures are the same as those of the second embodiment. 
     Modification 1 of Third Embodiment 
       FIG.  18    is a view, corresponding to  FIG.  8   , of Modification 1 of the third embodiment, and is a vertical sectional view of the inverter  315 . Although in the embodiments above, each power module  20  accommodates both the lower arm element  21  and the upper arm element  22  in one package, the present modification differs. In the present modification, each power module  20  is separated into a first package  20   a  that accommodates the lower arm element  21  and a second package  20   b  that accommodates the upper arm element  22 . 
     Modification 2 of Third Embodiment 
       FIG.  19    is a perspective view of power modules  20  and busbars  50  according to Modification  2  of the third embodiment. In the present modification, each power module  20  is linearly disposed side by side. The width direction of each of the busbars  51  and  52  is a linear direction (direction in which each power module  20  is disposed side by side). Even in this case, in at least the first power module group P 1  and the first coil group C 1 , when seen in the axial direction of the motor  3 , the distances  du   1 ,  dv   1 , and  dw   1  between the corresponding one of the phase power modules  20   u   1 ,  20   v   1 , and  20   w   1  and the corresponding one of the phase coils  17   u   1 ,  17   v   1 , and  17   w   1  are to be equal to each other between the U phase, the V phase, and the W phase. 
     Other Embodiments 
     Although the present disclosure has been described by way of preferred embodiments above, such descriptions are not limiting matters, and, naturally, various modifications are possible. 
     The present disclosure is applicable to a vehicle drive unit, and thus is very effective and has high industrial applicability.