Patent Publication Number: US-2023138904-A1

Title: Electric drive unit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosed technology relates to an onboard electric drive unit including a motor and an inverter. 
     2. Description of the Related Art 
     In recent years, vehicles configured to travel by using electricity, such as hybrid vehicles and electric vehicles, have become remarkably popular. A vehicle of this type is equipped with a drive motor and a battery. Direct-current power supplied from the battery is converted into alternating-current power by the inverter. The alternating-current power is controlled and supplied to the drive motor. Rotational power that is thereby generated is used by the vehicle to travel. 
     An inverter of this type handles high electric power. Thus, a large voltage is applied thereto, and large current flows therein. Accordingly, a large amount of heat is generated during operation, and cooling is required. A large surge voltage is also generated. Therefore, the sizes of individual electronic components constituting the inverter are large, and the weights thereof tend to be heavy. Accordingly, inverters known in the art are a factor that hinders an improvement in fuel consumption and electricity consumption. 
     An inverter is usually disposed near a drive motor to shorten a power transmission distance. In a vehicle, however, a large number of devices are installed and a space for disposing an inverter is limited. It is also necessary to consider the balance of the vehicle body. It is thus difficult to dispose a large and heavy inverter in a vehicle appropriately. 
     To address such an issue, a technology that integrates an inverter with a motor to reduce the size and the weight of the inverter has been proposed (for example, Japanese Unexamined Patent Application Publication No. 2004-274992). 
     In Japanese Unexamined Patent Application Publication No. 2004-274992, coolability of a switching element by air-cooling is improved by devising the structure of an inverter case. 
     SUMMARY OF THE INVENTION 
     An inverter includes a smoothing capacitor, a power module including a switching element, and the like. As described above, when the inverter is compatible with high-voltage power sources, these electronic components are generally large and heavy. 
     Large current also flows through a metal part (busbar), which is an electronic component connecting these components. The metal part is thus also large and heavy. When the wiring length (corresponding to a distance over which current flows) of the busbar increases, electric resistance increases accordingly, and a copper loss is generated during energization. The busbar also generates a large amount of heat. Moreover, since large current is turned on and off at a high speed by switching control in the inverter, a large change in magnetic field is generated in the busbar accordingly. 
     Consequently, when the inverter is operated, noise, vibration, electromagnetic interference, and the like may be generated in the busbar due to the changing magnetic field. These effects may lead to an energy loss and may adversely and variously affect the performance of the vehicle. Accordingly, countermeasures are required. When the busbar has a complex shape, the effect thereof is more noticeable. 
     In the inverter disclosed in Japanese Unexamined Patent Application Publication No. 2004-274992, although it may be possible to achieve a size reduction by integrating the inverter with the motor, the electronic components are the same as those known in the art. Accordingly, there is room for improvement in, for example, the structures and arrangement of the electronic components. 
     Thus, the disclosed technology achieves an electric drive unit in which the size and the weight can be reduced while performance can be improved by devising the structures, arrangement, and the like of main electronic components that constitute an inverter. 
     The disclosed technology relates to an electric drive unit including a motor and an inverter disposed adjacent to one end of the motor in a rotational axis direction. 
     The inverter includes a plurality of power modules each of which includes at least one switching element, the plurality of power modules constituting an inverter circuit configured to convert direct-current power into alternating-current power; a smoothing capacitor that constitutes, together with the plurality of power modules, the inverter circuit; a busbar that connects each of the power modules to the smoothing capacitor; and a thin case that has a thickness of less than 50 mm in the rotational axis direction and that accommodates the power modules, the smoothing capacitor, and the busbar. 
     The plurality of power modules are disposed at the periphery of the smoothing capacitor. The smoothing capacitor and each of the power modules are disposed in an inner portion of the thin case and arranged on the same plane orthogonal to the rotational axis direction. 
     The busbar is formed to extend in a circumferential direction. An inner edge portion of the busbar connected to a terminal of the smoothing capacitor has an arc shape or a circular shape extending in the circumferential direction. 
     According to this electric drive unit, the inverter includes the thin case having a thickness of less than 50 mm in the rotational axis direction and is disposed adjacent to one end of the motor in the rotational axis direction. It is thus possible to achieve size and weight reductions of the electric drive unit. 
     It is necessary to connect the motor and the inverter to each other by a plurality of busbars (output-side busbars) while it is possible to shorten the wiring lengths thereof and reduce the inductances of the output-side busbars by disposing the inverter to be adjacent to one end of the motor in the rotational axis direction. In addition, since the motor and the inverter are disposed coaxially, it is possible to make the wiring lengths of the output-side busbars more uniform by disposing the output-side busbars in accordance with the motor. It is thus possible to equalize the inductances of the output-side busbars and possible to improve the controllability of the motor. 
     In this electric drive unit, the shapes and the arrangement of the power modules, the smoothing capacitor, and the busbar, which are main electronic components constituting the inverter circuit, are devised to enable these electronic components to be accommodated in the above-described thin case, which has difficulty in accommodating those with technology known in the art. 
     That is, the plurality of power modules are disposed at the periphery of the smoothing capacitor. It is thus possible to efficiently dispose these electronic components and possible to reduce the installation space. 
     Since the busbar connecting these electronic components is formed to extend in the circumferential direction in accordance with the space between these electronic components, the busbar has a shape having a width larger than a length required for connecting these electronic components. Therefore, it is possible to reduce the wiring length of the busbar and increase the width of the busbar orthogonal to the wiring length. Consequently, it is possible to reduce the inductance of the busbar effectively. Since the busbar has a large surface area and improved heat dissipation properties, it is possible to improve the performance of the electric drive unit. 
     Since the inner edge portion of the busbar connected to the terminal of the smoothing capacitor has the arc shape or the circular shape extending in the circumferential direction, the shortest length from a terminal of each of the power modules to the inner edge portion can be substantially the same at any part. Therefore, it is possible to make the wiring lengths of the power modules more uniform by connecting the inner edge portion to the smoothing capacitor. Consequently, it is possible to equalize the inductance between each power module and the smoothing capacitor and thus possible to improve the controllability of the motor. 
     Further, the smoothing capacitor and each of the power modules are disposed in the inner portion of the thin case so as to be arranged on the same plane orthogonal to the rotational axis direction. Therefore, it is possible to dispose these electronic components, even when the number thereof is large, in the direction orthogonal to the rotational axis direction, that is, in a region in the thin case having a large area. Since all of these electronic components are arranged on the same plane, it is possible to minimize the height from the plane. Accordingly, even the thin case can accommodate a large number of these electronic components. 
     The electric drive unit may be configured such that the thin case has an outer shape formed in a disk shape corresponding to the motor. 
     In this case, it is possible to achieve the electric drive unit that has a small size and that is lightweight. Flexibility in vehicle design is greatly increased, and it is possible to achieve a high-performance vehicle. The electric drive unit is also suitable for an in-wheel motor. 
     The electric drive unit may be configured such that the smoothing capacitor and the power modules each have a flat shape having an installation surface at one side and are placed on a common coolable support surface via the installation surface. 
     When the smoothing capacitor and the power modules each have a flat shape and are placed on a common support surface (corresponding to the same plane) via respective installation surfaces provided at one side thereof, the thickness of the inverter can be further reduced. 
     Since the installation surfaces of these electronic components having large areas are placed on the coolable support surface, it is possible to cool these electronic components effectively. Due to the flat shape, the front surfaces of these electronic components present opposite to the installation surfaces also have excellent heat dissipation properties. Therefore, it is possible to effectively cool these electronic components, which generate a large amount of heat during operation, and thus possible to improve the performance of the electric drive unit. 
     The electric drive unit may be configured such that the busbar includes a third busbar that has a plate shape extending in a fan shape along an upper surface of each of the power modules, and that the third busbar is connected to one of a positive electrode terminal and a negative electrode terminal of each of the power modules and to a corresponding terminal of the smoothing capacitor. 
     Due to having the plate shape extending in the fan shape in accordance with the arrangement of the plurality of power modules, the third busbar has a large surface area and excellent heat dissipation properties. Moreover, since the third busbar extends along the upper surface of each of the power modules, it is possible to efficiently cool the power modules. In addition, the third busbar has a large lateral width and thus has a low inductance. Due to the plate shape, it is possible to accelerate size and weight reductions of the inverter. 
     The electric drive unit may be configured such that the busbar includes a first busbar that has a plate shape extending in a band shape between and along each of the power modules and the smoothing capacitor and that is connected to another of the positive electrode terminal and the negative electrode terminal of each of the power modules and to a corresponding terminal of the smoothing capacitor. 
     That is, in the electric drive unit, in addition to the third busbar, the first busbar connected to the positive electrode terminal or the negative electrode terminal that is not connected to the third busbar has the plate shape extending in the band shape between and along each of the power modules and the smoothing capacitor. 
     Therefore, the first busbar has a short wiring length and a large busbar width orthogonal to the wiring length. Consequently, it is possible to effectively reduce the inductance of the first busbar. Due to the plate shape, it is possible to accelerate size and weight reductions of the inverter. 
     The electric drive unit may be configured such that lengths and widths of the first busbar and the third busbar are set such that inductance is substantially the same between the first busbar and the third busbar. 
     Originally, inductance is different between the first busbar and the third busbar having different sizes. In contrast, in this electric drive unit, the lengths and the widths of these busbars are set on the basis of the findings by the present inventors such that inductance is substantially the same between the two busbars having different sizes. Accordingly, inductance is equalized between the busbars of both the positive and negative electrodes, which can improve the controllability of the motor. 
     The electric drive unit may be configured such that each of the power modules includes the positive electrode terminal and the negative electrode terminal, a half bridge circuit, and an output terminal, the at least one switching element being two switching elements, the half bridge circuit being connected between the positive electrode terminal and the negative electrode terminal in a state in which the two switching elements are connected in series, the output terminal being connected between the two switching elements. 
     In this case, a plurality of circuits provided in accordance with the phases of the motor in the inverter circuit can be configured by incorporating each of these power modules. As a result, layout design of the electronic components in the inner portion of the inverter is facilitated. The size reduction of the inverter can be accelerated. 
     The electric drive unit may be configured such that the switching element is constituted by a silicon carbide metal oxide semiconductor field effect transistor. 
     The SiC MOSFET has low electric resistance and excellent heat resistance compared with an insulated gate bipolar transistor (IGBT) and a power metal oxide semiconductor field effect transistor (MOSFET), which are generally used as switching elements of this type. Consequently, the chip size of the SiC MOSFET can be reduced more than the IGBT and the power MOSFET having the same performance. 
     Accordingly, it is possible to reduce the size of the electronic component incorporating the switching element. In particular, since the two switching elements are incorporated in series in each power module described above, the size can be more effectively reduced. 
     The electric drive unit may be configured such that the smoothing capacitor is constituted by a plurality of element capacitors connected in parallel, and that the plurality of element capacitors that are arranged to face the plurality of power modules are disposed along the arrangement of the plurality of power modules. 
     To be compatible with high voltages, the smoothing capacitor is required to have a large capacity. Meanwhile, in this electric drive unit, the number of the element capacitors is selectable in accordance with a desired capacity, which increases flexibility in design. Moreover, the outer shape of the smoothing capacitor can be freely set by changing the arrangement of these element capacitors. When these element capacitors are disposed laterally, a thickness reduction of the inverter can be accelerated. 
     Further, when the plurality of element capacitors arranged to face the plurality of power modules are disposed along the arrangement of the plurality of power modules, the distance between these element capacitors and each of the power modules can be shortened, which can accelerate reduction and equalization of the inductance. 
     According to the electric drive unit to which the disclosed technology is applied, size and weight reductions can be achieved while performance is improved. Therefore, when the electric drive unit is mounted on a vehicle, fuel consumption and electricity consumption can be improved, and flexibility in vehicle design is increased. Noise and the like can be also suppressed, and thus, a high-performance vehicle can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is one example of a vehicle to which a disclosed electric drive unit is applied. 
         FIG.  2    is a view roughly illustrating a first electric drive unit mounted on the vehicle. 
         FIG.  3    is a schematic view for describing a structure of a motor constituting the electric drive unit. 
         FIG.  4    is a basic circuit diagram of an inverter constituting the electric drive unit. 
         FIG.  5    is a view for comparison between chips of an IGBT and a SiC MOSFET. 
         FIG.  6    is a view for describing a power module. 
         FIG.  7    is a view roughly illustrating a principal portion of an inverter of the first electric drive unit. 
         FIG.  8    is a schematic sectional view of a part indicated by arrow Y 7  in  FIG.  7   . 
         FIG.  9    is a view roughly illustrating a busbar for description. 
         FIG.  10    is a graph showing the relationship between the shape (width, length, and thickness) of the busbar and inductance. 
         FIG.  11    is a view corresponding to  FIG.  7    and illustrating an electric drive unit according to a second embodiment. 
         FIG.  12    is a schematic sectional view of a part indicated by arrow Y 11  in  FIG.  11   . 
         FIG.  13    is a view corresponding to  FIG.  7    and illustrating an electric drive unit according to a third embodiment. 
         FIG.  14    is a schematic sectional view of a part indicated by arrow Y 13  in  FIG.  13   . 
         FIG.  15    is a view for describing a principal portion of an inverter and a structure of a power module. 
         FIG.  16    is a view for describing an electric drive unit according to a fourth embodiment. 
         FIG.  17    is a view corresponding to  FIG.  7    and illustrating an electric drive unit according to a fifth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the disclosed technology will be described using a plurality of preferable embodiments. Fundamentally, the following description is merely exemplary. The matters described in each embodiment are not limited to the embodiment and are applicable also to the other embodiments. 
     An electric drive unit in which the disclosed technology is used is suitable for power sources of vehicles such as hybrid vehicles and electric vehicles that travel using electricity. Accordingly, an onboard electric drive unit is presented as an example in the following embodiments. 
       FIG.  1    is a schematic view in which a vehicle  1  according to the present embodiment is viewed from below. The vehicle  1  is a so-called hybrid vehicle. As main driving sources, an engine  2  and a drive motor  20 A (one example of the disclosed motor) are mounted. The disclosed technology is applicable not only to a hybrid vehicle but also to an electric vehicle that travels with only a motor. 
     In the vehicle  1 , an engine room  1   a  is disposed at a front side of the vehicle interior. The engine  2  is disposed in the engine room  1   a . Rear wheels  3  are driving wheels. That is, the vehicle  1  is a so-called front-engine rear-drive (FR) vehicle. 
     The vehicle  1  is also equipped with, in addition to the engine  2  and the drive motor  20 A described above, first and second inverters  30 , an automatic transmission  4 , a high-voltage battery  5 , a propeller shaft  6 , a differential  7 , and the like, as drive system devices. The drive motor  20 A is integrated (constitutes the disclosed electric drive unit) with the first inverter  30 . The drive motor  20 A and the first inverter  30  (first electric drive unit  10 A) integrated with each other are disposed adjacent to the rear side of the engine  2 . 
     As illustrated in  FIG.  2   , the drive motor  20 A has a front shaft  21  projecting from the front end thereof, and a rear shaft  22  projecting from the rear end thereof. Although not illustrated, the front shaft  21  is coupled to the output shaft (crankshaft) of the engine  2  via a clutch and the like. The rear shaft  22  is coupled to the automatic transmission  4 . 
     As illustrated in  FIG.  1   , a central part in the vehicle width direction of a floor panel  1   b  constituting the floor surface of the vehicle interior is provided with a floor tunnel  1   c  projecting toward the vehicle interior side and extending in the front-rear direction. In a state of being accommodated in the floor tunnel  1   c , the automatic transmission  4  is disposed adjacent to the rear side of the first electric drive unit  10 A. A front end portion of the propeller shaft  6  is coupled to the rear side of the automatic transmission  4 . 
     A rear end portion of the propeller shaft  6  is coupled to the differential  7 . It is configured such that rotational power output from the engine  2  and the like is transmitted to the rear wheels  3  through a wheel shaft  8  extending leftward and rightward from the differential  7 . An exhaust pipe extends rightward from the engine  2 , and a catalytic device  2   a  is installed on the upstream side of the exhaust pipe. Although not illustrated, the exhaust pipe extends rearward, and an end portion thereof on the downstream side is connected to a silencer installed at a rear portion of the vehicle  1 . 
     To enable driving of front wheels  9  in the vehicle  1  in the present embodiment, an in-wheel motor  20 B (one example of the disclosed motor) having a columnar outer shape is incorporated in each of the left and right front wheels  9 . Each in-wheel motor  20 B rotatably drives a corresponding one of the front wheels  9  independently. The in-wheel motors  20 B function as, for example, assist motors that each generate power and transmit the power to the front wheels  9  corresponding thereto at a time of starting the vehicle  1 . Note that when the drive motor  20 A and the in-wheel motors  20 B are not particularly distinguished from each other, these motors may be collectively referred to as the motor  20 . 
     The second inverter  30  to which the disclosed technology is applied is integrated (constitutes the disclosed electric drive unit) with one end of the in-wheel motor  20 B of the vehicle  1  in the rotational axis direction. The in-wheel motor  20 B and the second inverter  30  (second electric drive unit  10 B) integrated with each other are assembled to the vehicle body of the vehicle  1  in a state in which the side of the second inverter  30  is directed inward in the vehicle width direction. 
     The high-voltage battery  5  for driving is installed on each of left and right sides of the floor tunnel  1   c  on the lower side of the floor panel  1   b . The high-voltage battery  5  is constituted by a plurality of mutually coupled battery modules each constituted by a plurality of battery cells (lithium-ion batteries or the like). The high-voltage battery  5  has a large capacity and thus has a flat shape extending widely along the floor panel  1   b.    
     The high-voltage battery  5  supplies direct-current power to each of the first and second inverters  30  (also simply referred as the inverter  30  when these inverters  30  are not particularly distinguished from each other). The inverter  30  converts supplied direct-current power into alternating-current power. The inverter  30  supplies the alternating-current power to the drive motor  20 A to rotate the rear wheels  3  and supplies the alternating-current power to the in-wheel motors  20 B to rotate the front wheels  9  (so-called power running). 
     The vehicle  1  also performs regeneration processing. In other words, the drive motor  20 A and the in-wheel motors  20 B are also utilized as power generators. When power is generated by the drive motor  20 A or the in-wheel motors  20 B at a time of deceleration of the vehicle, the alternating-current power is converted into direct-current power by the inverter  30 . The direct-current power is supplied to the high-voltage battery  5  to charge the high-voltage battery  5 . 
     As illustrated in  FIG.  2   , the disclosed inverter  30  is thin and lightweight differently from inverters known in the art. 
     Specifically, the first inverter  30  is disposed adjacent to one end (rear end) of the drive motor  20 A in the rotational axis direction (direction in which a rotational axis J extends in  FIG.  2   ) to be integrated therewith. The drive motor  20 A has a columnar outer shape centered at the rotational axis J. Meanwhile, the outer shell of the first inverter  30  is constituted by a case (thin case  31 ) having a thin thickness and has an outer shape in a disk shape having an outer diameter substantially the same as the outer diameter of the drive motor  20 A. The thin case  31  is assembled to the rear end of the drive motor  20 A in a state in which the thickness direction thereof coincides with the rotational axis direction of the drive motor  20 A. 
     In the present embodiment, a thickness (the size in the rotational axis direction) Wiv of the thin case  31  is designed to be less than or equal to 50 mm. Preferably, the thickness Wiv of the thin case  31  is less than or equal to 30 mm. Similarly, the second inverter  30  integrated with the in-wheel motor  20 B also has a thin thickness. 
     With technology known in the art, incorporating electronic components that constitute an inverter in a case having such a thickness is extremely difficult and is not achieved actually. In contrast, in the disclosed inverter  30 , the shapes and the layout of main electronic components of the inverter are devised as described later. Consequently, it is possible to accommodate these electronic components in a space having a small thickness and, as a result, possible to achieve such a thin and lightweight inverter  30 . 
     With such reductions in the thickness and the weight of the inverter, the vehicle  1  in which the inverter is mounted can obtain various merits. For example, a large and heavy inverter known in the art is generally disposed, as indicated by an imaginary line α 1  in  FIG.  1   , below the floor panel  1   b  adjacent to a side of the drive motor  20 A. 
     In such a case, the inverter is required to have a flat shape due to height limitation. The inverter occupies a specific large region below the floor panel  1   b . Thus, the high-voltage battery  5  with known technology can be installed only in a range indicated by an imaginary line α 2 . As a result, layout design of devices below the floor panel  1   b  is significantly limited. 
     It may be possible to dispose the inverter on the upper side of the drive motor  20 A or the automatic transmission  4 . In such a case, however, the size of the floor tunnel  1   c  is increased and generate a problem that the vehicle interior is narrowed. In contrast, the inverter  30  according to the disclosed technology can be installed with little restriction on the side of the vehicle  1 . 
     Accordingly, flexibility in layout design of onboard devices is increased. Consequently, as shown in the vehicle  1 , it becomes possible to, for example, increase the capacity of the high-voltage battery  5  and possible to improve the performance of the vehicle  1 . In the disclosed inverter  30 , a weight reduction is also accelerated, and thus, fuel consumption and electricity consumption are also improved. Further, combining with the in-wheel motors  20 B is facilitated, as in the vehicle  1 , and thus, the vehicle  1  having higher performance can be achieved. 
     Motor 
       FIG.  3    simply illustrates a structure of the motor  20 . The motor  20  is a so-called permanent magnet-type synchronous motor. A shaft  24  (constituting the front shaft  21  and the rear shaft  22 ) is rotatably supported at the center of a cylindrical motor case  25 . To the shaft  24 , a columnar rotor  26  is fixed. At the outer peripheral part of the rotor  26 , a permanent magnet  26   a  is installed. Consequently, a plurality of N-poles and S-poles are alternately disposed to be arranged at equal intervals in the circumferential direction. 
     At the periphery of the rotor  26 , a stator  27  is coaxially disposed. The stator  27  has a plurality (six in the illustrated example) of teeth  27   a  projecting radially at equal intervals from an annular core toward the rotor  26 . The tip of each of the teeth  27   a  faces the rotor  26  with a gap therebetween. An electric wire is wound around each of these teeth  27   a  to form a plurality of coils  27   b.    
     These coils  27   b  constitute a coil group of these phases including a U-phase, a V-phase, and a W-phase. The coils  27   b  of respective phases are disposed to be arranged alternately in the circumferential direction. The motor  20  is provided with relay terminals  28  of each of the U-phase, the V-phase, and the W-phase. To these relay terminals  28 , the electric wires constituting the coils  27   b  of the respective phases are connected. Alternating-current power controlled by the inverter  30  is supplied to the coils  27   b  of the respective phases, and a magnetic field change is thereby periodically generated between the rotor  26  and the stator  27 . The magnetic field change acts on the permanent magnet  26   a , and the rotor  26  and the shaft  24  are thereby rotated around the rotational axis. 
       FIG.  3    schematically illustrates a basic structure of the motor  20 . The number of the magnetic poles, the structure, and the like of the rotor  26  or the stator  27  are selected in accordance with the specifications of the motor  20 . For example, a motor of a type (so-called concentrated winding) in which the coil  27   b  is formed for each of the teeth  27   a  is illustrated in  FIG.  3   . However, a motor of a type (so-called distributed winding) in which an electric wire is wound around the plurality of teeth  27   a  may be employed. Alternatively, the motor  20  may be of an outer rotor type in which the rotor  26  is positioned on the outer side of the stator  27 . 
     Inverter 
       FIG.  4    illustrates a basic circuit diagram of the inverter  30 . In accordance with the motor  20  of a three-phase type, the inverter  30  is provided with an inverter circuit  40  that converts direct-current power into alternating-current power of three phases (the U-phase, the V-phase, and the W-phase) and outputs the alternating-current power. The inverter circuit  40  is publicly known. 
     The inverter circuit  40  is provided with a positive wire  42  having a positive-side direct-current terminal  41  at one end thereof, and a negative wire  44  having a negative-side direct-current terminal  43  at one end thereof. The positive-side direct-current terminal  41  is connected to the positive electrode of the high-voltage battery  5 . The negative-side direct-current terminal  43  is connected to the negative electrode of the high-voltage battery  5 . Between the positive wire  42  and the negative wire  44 , three circuits (half bridge circuits  45 ) for energizing the coils  27   b  of the respective phases are connected in parallel. 
     Each of the half bridge circuits  45  has two switching elements  46  that are connected in series. A free wheel diode  46   a  is connected anti-parallel to each of the switching elements  46 . Between these two switching elements  46 , output wires  47  are connected. Each of the output wires  47  is connected via the relay terminals  28  to a corresponding one of the coils  27   b  of the respective phases of the motor  20 . 
     During the operation of the inverter  30 , each of the switching elements  46  is turned on and off at a high speed (switching control). Consequently, current having a pseudo alternating-current waveform is formed, and the current flows to the motor  20  via the positive wire  42  and the output wire  47  of any one of the phases. Via the output wires  47  of the remaining phases, the current flows from the motor  20  to the negative wire  44 . The switching elements  46  for turning on and off large current at high speed under a high voltage is required to have high durability both electrically and mechanically. 
     As the switching elements  46  capable of fulfilling such requirements, an insulated gate bipolar transistor (IGBT) or a power metal oxide semiconductor field effect transistor (MOSFET) is generally used. The IGBT is mainly used in many cases. 
     In contrast, in the disclosed inverter  30 , a silicon carbide metal oxide semiconductor field effect transistor (SiC MOSFET) is used as the switching elements  46 . 
     The SiC MOSFET is publicly known. The SiC MOSFET has low electric resistance and excellent heat resistance compared with the IGBT and the power MOSFET since SiC, in which physical properties are more stable than Si, is used as a material in the SiC MOSFET. Consequently, the chip size of the SiC MOSFET can be reduced more than the IGBT and the power MOSFET having the same performance. 
       FIG.  5    schematically illustrates a chip  50  of the IGBT and a chip  50  of the SiC MOSFET. Each of the chips  50  of the SiC MOSFET and the IGBT has a thin plate shape, and the lower surface thereof is joined to a heat dissipation plate  51  for cooling by solder  52 . A peripheral part of the upper surface of each of the chips  50  is provided with a gate and the like (not illustrated). In a state of being embedded in an insulating resin, each chip  50  is incorporated, together with a wire, in an electronic component. 
     Accordingly, a reduction in the size of the chip can reduce the size of the electronic component in which the chip is incorporated. In particular, the size of a later-described power module  70  can be more effectively reduced since the power module  70  includes two switching elements  46  connected in series. 
     Meanwhile, in the case of the IGBT, a relatively large coolable range (the range indicated by a two-dot chain line α 3 ) can be obtained also at the upper surface since the surface area of the IGBT is large. Accordingly, it is possible to perform cooling effectively by joining the heat dissipation plate  51  also to the upper surface similarly to the lower surface. Meanwhile, in the SiC MOSFET having a small surface area, it is not possible to reduce the size of a section provided with the gate and the like, and a coolable range is thus very small. 
     Therefore, in the case of the SiC MOSFET, cooling the upper surface thereof is inefficient. Accordingly, it is effective in the SiC MOSFET to cool the lower surface thereof actively without cooling the upper surface thereof actively. Thus, the disclosed inverter  30  is devised to be capable of efficiently cooling the lower surface of the SiC MOSFET. As a result, a cooling structure for cooling the upper surface of the SiC MOSFET is omitted or simplified, and the SiC MOSFET can be thinner than the IGBT. 
     Smoothing Capacitor 
     As illustrated in  FIG.  4   , a smoothing capacitor  60  is connected via a bridging wire  48  to a part of the positive wire  42  and the negative wire  44  that are positioned between the positive-side and negative-side direct-current terminals  41  and  43  and the three half bridge circuits  45 . The smoothing capacitor  60  constitutes the inverter circuit  40  and smooths a voltage applied between the positive wire  42  and the negative wire  44 . 
     To be compatible with high voltages, the smoothing capacitor  60  is required to have a large capacity. Therefore, the disclosed inverter  30  is constituted by a plurality of element capacitors  60   a  connected in parallel, as described later. In other words, small-capacity capacitors (in detail, film capacitors) are connected in parallel to each other to constitute the smoothing capacitor  60  having a desired capacity. That is, the smoothing capacitor  60  of the disclosed inverter  30  is not a single capacitor but is a unitized capacitor (constituting a smoothing capacitor unit). 
     With the unitized smoothing capacitor  60 , flexibility in design is increased since the number of the element capacitors  60   a  is selectable in accordance with a desired capacity. Further, in the disclosed inverter  30 , the form of the smoothing capacitor  60  is also devised (details will be described later) to achieve a thickness reduction and an efficiency increase. 
     Busbar 
     A large current flows through the electric wires including the positive wire  42 , the negative wire  44 , the bridging wire  48 , and the like illustrated in  FIG.  4   . Therefore, generally, a metal part (busbar) constituted by a copper plate or the like is used to constitute each of these electric wires. When the busbar is long, electric resistance is high, and a copper loss increases. The amount of generated heat also increase. 
     Moreover, since the large current is turned on and off at a high speed by switching control, a large magnetic change is generated in the busbar accordingly. As a result, noise, vibration, electromagnetic interference, and the like are generated in the busbar in response to the inverter being operated. Those may lead to an energy loss and may adversely and variously affect the performance of the vehicle  1 . 
     When the busbar has a complex shape by, for example, being bent, the effect thereof is more noticeable. A difference in the shapes of the busbars required to have uniformity, such as positive and negative electrodes, also deteriorates controllability of the motor. 
     To suppress these inconveniences, reducing and equalizing the inductances of the busbars are effective. Thus, in the disclosed inverter  30 , the shapes and the arrangement of the busbars are devised (details will be described later) so that the inductances of the busbars can be reduced and equalized to reduce heat generation, noise, and the like generated due to the busbars while addressing thickness and weight reductions and an efficiency improvement of the inverter. 
     Power Module 
       FIG.  6    illustrates the power module  70  according to the present embodiment. The power module  70  is a small flat electronic component having a small thickness. Each of the upper and lower surfaces of the power module  70  has a rectangular shape. One (first end portion) of short end portions thereof is provided with one output terminal  71 . The other (second end portion positioned opposite the first end portion) of the short end portions thereof is provided with two terminals (a positive electrode terminal  72  and a negative electrode terminal  73 ). 
     In the power module  70 , the positive electrode terminal  72  and the negative electrode terminal  73  are positioned away from each other in the up-down direction and positioned away from each other also in the width direction to avoid contact therebetween. The positive electrode terminal  72  is displaced toward the upper surface, and the negative electrode terminal  73  is displaced toward the lower surface. The lower surface of the power module  70  is constituted by a flat surface (installation surface  70   a ). 
     As simply illustrated in  FIG.  6   , the half bridge circuit  45  including the two switching elements  46  (SiC MOSFET) is configured in the inner portion of the power module  70 . The two switching elements  46  connected in series are disposed along the long sides of the power module  70 . As described above, the chip  50  of the SiC MOSFET is used in each switching element  46 . Thus, the size and the thickness of the power module  70  are also reduced. 
     The positive electrode side of the half bridge circuit  45  is connected to the positive electrode terminal  72 . The negative electrode side of the half bridge circuit  45  is connected to the negative electrode terminal  73 . The output terminal  71  is connected between the two switching elements  46 . In the inverter  30 , three power modules  70  are used to constitute the half bridge circuits  45  of the U-phase, the V-phase, and the W-phase. Specific Configuration of Power Module, Smoothing Capacitor, Busbar, and the like, and Arrangement thereof 
       FIG.  7    is a view in which the inner portion (the inner portion of the thin case  31 ) of the first inverter  30  is viewed from the side opposite to the drive motor  20 A.  FIG.  8    is a schematic sectional view of the part indicated by arrow Y 7  in  FIG.  7   . In description, the near side in  FIG.  7    is referred to as the upper side. 
     In the thin case  31 , the three power modules  70 , the smoothing capacitor  60 , a negative-electrode-side busbar  80  (first busbar), a positive-electrode-side busbar  81  (second busbar), output-side busbars  82 , and the like are accommodated in a predetermined arrangement. Although, in addition to those, members such as a control substrate for executing switching control and the like are accommodated in the thin case  31 , illustration thereof is omitted. The inner portion of the thin case  31  is filled with an insulating resin. Illustration thereof is also omitted. 
     Cooling Plate 
     As described above, the thin case  31  has a disk shape. A center portion thereof is provided with a cylindrical shaft cylinder portion  31   a  for inserting the rear shaft  22 . In the present embodiment, a semi-circular cooling plate  32  is disposed in a substantially half region of the thin case  31 . As illustrated in  FIG.  8   , the cooling plate  32  is a hollow metal member excellent in thermal conductivity. The cooling plate  32  is disposed on the lower side (the side of the drive motor  20 A) of the thin case  31 . 
     The upper surface of the cooling plate  32  constitutes a flat support surface  32   a  that extends in a direction orthogonal to the rotational axis direction. The lower surfaces of the smoothing capacitor  60  and each of the power modules  70  constitute flat installation surfaces  60   b  and  70   a , respectively. The smoothing capacitor  60  and each of the power modules  70  are placed on the support surface  32   a  via the installation surfaces  60   b  and  70   a , respectively. 
     In other words, the smoothing capacitor  60  and each of the power modules  70  are disposed in the inner portion of the thin case  31  so as to be arranged on the same plane (on the support surface  32   a ) orthogonal to the rotational axis direction. 
     As illustrated in  FIG.  2   , the thin case  31  is provided with a fluid inflow pipe  33  and a fluid outflow pipe  34 . It is configured such that a cooling fluid is supplied to and circulated in the inner portion of the cooling plate  32  through the fluid inflow pipe  33  and the fluid outflow pipe  34 . A plurality of projections  32   b  are provided on the back side of the support surface  32   a  facing the inner portion of the cooling plate  32 . 
     Due to these projections  32   b , the area of the back side of the support surface  32   a  is increased. Consequently, heat exchange with the cooling fluid is accelerated, and the coolability of the support surface  32   a  is improved. As a result, the smoothing capacitor  60  and each of the power modules  70  placed on the support surface  32   a  can be effectively cooled. 
     Smoothing Capacitor 
     In a state of being adjacent to the shaft cylinder portion  31   a , the smoothing capacitor  60  is disposed at the center side of the thin case  31 . The three power modules  70  are disposed at the outer peripheral side of the thin case  31  to be arranged with intervals therebetween in the circumferential direction (direction around the rotational axis J) and are disposed to be arranged along the periphery of the smoothing capacitor  60 . 
     In the present embodiment, each of the power modules  70  is disposed such that respective terminals are directed in the circumferential direction. Specifically, the positive electrode terminal  72  and the negative electrode terminal  73  of each of the power module  70  are positioned at one (first circumferential end portion  75 ) of end portions (circumferential end portions  75 ) mutually facing in the circumferential direction orthogonal to an inner-side end portion (inner end portion  74 ) facing the smoothing capacitor  60 . These terminals  72  and  73  mutually face in the clockwise direction in  FIG.  7   . Each output terminal  71  is positioned at the other (the second circumferential end portion  75  positioned opposite the first circumferential end portion  75 ) of the circumferential end portions  75 . The output terminals  71  mutually face in the counterclockwise direction in  FIG.  7   . 
     In the present embodiment, a side surface portion (alternating-current-side side surface portion  61 ) of the smoothing capacitor  60  facing the three power modules  70  has a polygonal shape constituted by three opposing surfaces opposite a corresponding one of the power modules  70 . As illustrated in  FIG.  7    and  FIG.  8   , a pair of alternating-current-side terminals  62  (the lower side is the negative side and the upper side is the positive side) extending along the edge of the alternating-current-side side surface portion  61  are provided at positions on the alternating-current-side side surface portion  61  vertically away from each other. 
     Each of the power modules  70  and the smoothing capacitor  60  are disposed such that the sizes of gaps between each of the power modules  70  and the smoothing capacitor  60  are substantially the same. 
     These electronic components  60  and  70  are connected to each other by the negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  each having a plate shape. Accordingly, current flows through the busbars  80  and  81 . Thus, the gap between these electronic components  60  and  70  corresponds to the wiring length. By making the wiring lengths substantially the same, it is possible to equalize the inductance between each power module  70  and the smoothing capacitor  60 , as described later. Equalizing the inductance can suppress deterioration in the controllability of the motor  20 . 
     As described above, the smoothing capacitor  60  is constituted by the plurality of element capacitors  60   a  connected in parallel. Accordingly, the outer shape of the smoothing capacitor  60  can be freely set by changing the arrangement of these element capacitors  60   a . The smoothing capacitor  60  can be formed in a flat shape by selecting low-height element capacitors  60   a  and laterally disposing the element capacitors  60   a . In this case, even when the capacity is increased, the thin thickness of the inverter  30  can be maintained since the capacity increase can be performed by only expanding the size of the smoothing capacitor  60  in the lateral direction. 
     Preferably, the element capacitors  60   a  arranged to face the three power modules  70  are disposed along the alternating-current-side side surface portion  61  so as to be along the arrangement of the power modules  70 . In this case, the distance between the element capacitors  60   a  and each of the power modules  70  can be shortened, and reducing and equalizing the inductance can be accelerated. 
     A side surface portion (direct-current-side side surface portion  63 ) of the smoothing capacitor  60  that is present opposite to the alternating-current-side side surface portion  61  is provided with a pair of direct-current-side terminals  64 . The direct-current-side terminals  64  each constitute a corresponding one of a positive-side direct-current terminal  41  and a negative-side direct-current terminal  43  of the inverter circuit  40  and are each connected indirectly to a corresponding electrode of the high-voltage battery  5 . 
     Negative-Electrode-Side Busbar and Positive-Electrode-Side Busbar 
     The negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  connect the smoothing capacitor  60  to the three power modules  70  constituting the half bridge circuits  45 . In other words, in the inverter circuit  40 , these busbars  80  and  81  constitute parts that correspond to the bridging wire  48 , the positive wire  42 , and the negative wire  44  that are positioned between the smoothing capacitor  60  and the half bridge circuits  45 . 
     Therefore, the large current flows through these busbars  80  and  81 . Moreover, the large current fluctuates due to switching control. Consequently, a large amount of heat and a large magnetic change are generated in these busbars  80  and  81 . When the negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  have complex shapes, the effect thereof is more noticeable. The magnetic change in these busbars  80  and  81  causes noise, vibration, electromagnetic interference, and the like. Those may lead to an energy loss and may adversely and variously affect the performance of the vehicle  1 . 
     In contrast, in the disclosed inverter  30 , these busbars  80  and  81  each have a plate shape extending in the circumferential direction and having a large lateral width (although the name of these busbars is preferably “bus plate” in consideration of the shape thereof, the name “busbar,” which is a common name, is used here). An outer edge portion (edge portion positioned on the outer side in the radial direction) of one of the busbars is connected to the positive electrode terminal  72  or the negative electrode terminal  73  of each of the power modules  70 , and an inner edge portion (edge portion positioned on the inner side in the radial direction) of the other of the busbars is connected to a corresponding terminal of the smoothing capacitor  60 . 
     Specifically, an outer edge portion  80   b  of the negative-electrode-side busbar  80  is connected to the negative electrode terminal  73  of each of the power modules  70 , and an inner edge portion  80   a  of the negative-electrode-side busbar  80  is connected to the alternating-current-side terminal  62  of the negative side of the smoothing capacitor  60 . An outer edge portion  81   b  of the positive-electrode-side busbar  81  is connected to the positive electrode terminal  72  of each of the power modules  70 , and an inner edge portion  81   a  of the positive-electrode-side busbar  81  is connected to the alternating-current-side terminal  62  of the positive side of the smoothing capacitor  60 . 
     The negative-electrode-side busbar  80  is positioned on the lower side of the positive-electrode-side busbar  81  and, as clearly illustrated by dots in  FIG.  7   , is formed and disposed to be fitted into a gap between each of the power modules  70  and the smoothing capacitor  60 . Specifically, the inner edge portion  80   a  of the negative-electrode-side busbar  80  has a polygonal shape corresponding to the polygonal shape of the alternating-current-side surface portion  61 . The outer edge portion  80   b  of the negative-electrode-side busbar  80  has a polygonal shape similar to the shape of the inner edge portion  80   a.    
     Parts that overlap a corresponding one of the power modules  70  are notched, and three recessed portions  83  are thereby formed on the side of the outer edge portion  80   b  of the negative-electrode-side busbar  80 . The power modules  70  are fitted into the recessed portions  83  corresponding thereto. Consequently, in a state of being fitted into the gap between each of the power modules  70  and the smoothing capacitor  60 , the negative-electrode-side busbar  80  extends along each of the power modules  70  and the smoothing capacitor  60  in the circumferential direction. 
     As illustrated in  FIG.  8   , in a state of being placed on the support surface  32   a  of the cooling plate  32 , the negative-electrode-side busbar  80  is joined to the alternating-current-side terminal  62  of the negative side of the smoothing capacitor  60  and the negative electrode terminal  73  of each of the power modules  70 . 
     The positive-electrode-side busbar  81  is disposed above the negative-electrode-side busbar  80  and, as illustrated in  FIG.  7   , has a plate shape extending along the surface of each of the power modules  70 . Specifically, as with the negative-electrode-side busbar  80 , the inner edge portion  81   a  of the positive-electrode-side busbar  81  has a polygonal shape corresponding to the polygonal shape of the alternating-current-side side surface portion  61 . The outer edge portion  81   b  of the positive-electrode-side busbar  81  has a polygonal shape similar to the shape of the inner edge portion  81   a  thereof. 
     Consequently, in a state of extending along the surface of each of the power modules  70 , the positive-electrode-side busbar  81  extends along each of the power modules  70  and the smoothing capacitor  60  in the circumferential direction. 
     As illustrated in  FIG.  8   , in a state of vertically facing the negative-electrode-side busbar  80  with an interval therebetween, the positive-electrode-side busbar  81  is joined to the alternating-current-side terminal  62  of the positive side of the smoothing capacitor  60  and the positive electrode terminal  72  of each of the power modules  70 . 
     The negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  are each designed to have a flat plate shape having a small thickness and have a short wiring length and a large lateral width (the length in a direction orthogonal to the wiring length). In the present embodiment, due to the shape and the arrangement of the power modules  70 , the positive-electrode-side busbar  81  has a longer wiring length than the negative-electrode-side busbar  80 . 
     Relationship Between Shape of Busbar and Inductance 
     It is common technical knowledge that inductance decreases as the lateral width of a busbar increases and that inductance increases as the wiring length of the busbar increases. However, as a result of examining the relationship between the shape of a busbar and inductance thereof, the present inventors have found that there is a case in which the same inductance is obtained even when wiring lengths are different. 
       FIG.  9    illustrates a busbar as an example. This busbar represents a busbar having a width W larger than a length L and imitating the negative-electrode-side busbar  80  and the positive-electrode-side busbar  81 . The character t denotes thickness.  FIG.  10    is a graph summarizing the relationship between a busbar having such a shape and inductance thereof (in detail, inductance sensitivity). 
     The upper graph shows the relationship between the width W of the busbar and the inductance. The middle graph shows the relationship between the length L of the busbar and the inductance. The lower graph shows the relationship between the thickness t of the busbar and the inductance. 
     In the busbar in  FIG.  9   , when the width W increases, the inductance gradually decreases accordingly. It had been considered that, when the length L increases, the inductance similarly increases accordingly. However, it was confirmed that, in the case of the length L, an inflection point is present in a region in which the length L is short. 
     Thus, as illustrated in  FIG.  10   , a same inductance H 1  can be obtained in both a busbar having a length L 1  and a busbar having a length L 2 . That is, it is possible to design even busbars having different lengths L to have the same inductance as long as the lengths L are in a predetermined range. 
     It was also confirmed that the thickness t of a busbar hardly affects the inductance. That is, the thickness t of a busbar is practically sufficient to have a required minimum size. 
     From such findings, the present inventors focused on the fact that reducing the length L, increasing the width W, and a minimum required thickness t are sufficient to reduce and equalize inductances of busbars and that busbars can have the same inductance even when having different lengths. On the basis of these findings, the shapes and the arrangement of the negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  are devised. 
     Specifically, in the present embodiment, the negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  have different lengths. Thus, the lengths and the widths of the negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  are adjusted such that inductance is substantially the same therebetween. In other words, as with the busbar having the length L 1  and the busbar having the length L 2  described above, the length L (−) of the negative-electrode-side busbar  80  and the length L (+) of the positive-electrode-side busbar  81  are adjusted together with the widths thereof such that inductance is the same therebetween. Consequently, the inductances of these busbars are equalized. As a result, it is possible to suppress deterioration in the controllability of the motor  20 . 
     Moreover, the negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  are shared among the power modules and are each formed to have a relatively short length and a large lateral width. Therefore, the inductances of the negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  themselves are also reduced. Further, these busbars  80  and  81  each have a large surface area and thus are excellent in heat dissipation properties. The negative-electrode-side busbar  80  having a relatively small surface area is in surface contact with the cooling plate  32  and can be cooled effectively. 
     The positive-electrode-side busbar  81  not in contact with the cooling plate  32  has a relatively large surface area. Therefore, the positive-electrode-side busbar  81  itself has excellent heat dissipation properties. In addition, due to being in contact with the upper surface of each of the power modules  70 , the positive-electrode-side busbar  81  can accelerate heat dissipation of each of the power modules  70 . 
     These busbars  80  and  81  each have a flat plate shape, and the shape is simple. Processing thereof is easily, and a magnetic change can be suppressed. The busbars  80  and  81  have an advantage for a thickness reduction and can be accommodated even in the thin case  31  having a small thickness. Since the positive and negative terminals of each of the three power modules  70  are connected by one busbar, it is also advantageous in terms of the number of components and the number of processes. 
     Output-Side Busbar 
     The output-side busbars  82  constitute parts that correspond to the output wires  47  of the inverter circuit  40 . That is, the output-side busbars  82  are connected to a corresponding one of the output terminals  71  of each of the power modules  70  and to the relay terminals  28  of a corresponding one of the phases of the drive motor  20 A. 
     In the present embodiment, the output terminals  71  of two of the three power modules  70  are positioned on the lower side of the positive-electrode-side busbar  81 . Therefore, an opening portion  84  is formed at each of parts of the upper surface of the positive-electrode-side busbar  81  overlapping the output terminals  71  of these two power modules  70 . 
     One end portion of one of the output-side busbars  82  is joined in a state as it is to the output terminal  71  corresponding thereto, and one end portion of each of the other two of the output-side busbars  82  is joined to the output terminals  71  corresponding thereto through respective opening portions  84 . The relay terminals  28  of the respective phases are disposed to correspond to the arrangement of each of the power modules  70 . That is, the arrangement of the relay terminals  28  of the respective phases is designed such that the wiring lengths of the output-side busbars  82  are the same. 
     Therefore, the three output-side busbars  82  are the same metal parts and are the same in length, shape, and the like. Consequently, respective inductances of these output-side busbars  82  are also the same, and the inductances of the output-side busbars  82  are also equalized. 
     As described above, according to the first electric drive unit  10 A in the present embodiment, the size and the weight can be reduced while performance thereof is improved compared with an electric drive unit in which an inverter known in the art is used. Accordingly, when the first electric drive unit  10 A is mounted on the vehicle  1 , fuel consumption and electricity consumption can be improved while flexibility in design of the vehicle  1  is increased. In addition, noise and the like can be suppressed, and thus, the vehicle  1  that has high performance can be achieved. Although the first electric drive unit  10 A has been described in the present embodiment, the contents thereof are also applicable to the second electric drive unit  10 B. 
     Second Embodiment 
       FIG.  11    and  FIG.  12    illustrate a second embodiment of the disclosed electric drive unit.  FIG.  12    is a schematic sectional view of a part indicated by arrow Y 11  in  FIG.  11   . 
     Basic configurations of the motor  20 , the inverter  30 , and the like in the present embodiment are the same as those in the above-described embodiment. Therefore, description of the same configurations as those in the above-described embodiment is omitted by using the same reference signs. Then, configurations that differ from those in the above-described embodiment will be described specifically (the same applies to the other embodiments). 
     In the present embodiment, the directions of the power modules  70  are different. That is, each power module  70  in the present embodiment is disposed in a state of being rotated by 90 degrees from the state of each power module  70  in the above-described embodiment. 
     Consequently, each of the power modules  70  in the present embodiment is disposed such that respective terminals are directed in the radial direction (radial direction around the rotational axis J). Specifically, each of the power modules  70  is disposed radially and has both the positive electrode terminal  72  and the negative electrode terminal  73  at the inner end portion  74  positioned at the center side of the thin case  31 . Each of the power modules  70  has the output terminal  71  at an end portion (outer end portion  76 ) that is positioned at the outer peripheral side of the thin case  31  and that does not face the smoothing capacitor  60 . 
     Consequently, as illustrated in  FIG.  12   , the positive electrode terminal  72  and the negative electrode terminal  73  of each of the power modules  70  are in a state of each facing a corresponding one of the pair of alternating-current-side terminals  62  of the smoothing capacitor  60 . Accordingly, the positive-electrode-side busbar  81  and the negative-electrode-side busbar  80  can have the same shape. 
     Specifically, the same metal part is used for the positive-electrode-side busbar  81  and the negative-electrode-side busbar  80  in the present embodiment. Therefore, these busbars  80  and  81  are the same in terms of length, lateral width, and thickness. In addition, the gap between each of the power modules  70  and the smoothing capacitor  60  is substantially the same, and these busbars  80  and  81  (corresponding to the first busbar) are fitted into the gap. 
     Each of the positive-electrode-side busbar  81  and the negative-electrode-side busbar  80  in the present embodiment has a plate shape extending in a band shape and has a large lateral width while having a short length. Each of the positive-electrode-side busbar  81  and the negative-electrode-side busbar  80  has a bent shape so as to extend along each of the power modules  70  and the smoothing capacitor  60 . The length L (+) of the positive-electrode-side busbar  81  and the length L (−) of the negative-electrode-side busbar  80  are the same. Accordingly, the inductances of the positive-electrode-side busbar  81  and the negative-electrode-side busbar  80  are the same. 
     The output terminal  71  of each of the power modules  70  is positioned on the radially outer side. Therefore, the output-side busbars  82  connected to these output terminals  71  extend radially as they are and are each connected to a corresponding one of the relay terminals  28  by the shortest distance. The length of each of the output-side busbars  82  is also designed such that wiring lengths are the same. Therefore, the inductances of the output-side busbars  82  are also the same. 
     Even when the shapes, the arrangement, and the like of the power modules  70  and the busbars  80 ,  81 , and  82  are configured as in the present embodiment, the same effects as those in the above-described embodiment can be obtained. Accordingly, when employing the electric drive unit of the present embodiment, it is possible to achieve size and weight reductions while improving performance compared with an electric drive unit using an inverter known in the art. When mounting the electric drive unit of the present embodiment on a vehicle, it is possible to improve fuel consumption and electricity consumption while increasing flexibility in design of the vehicle. It is also possible to suppress noise and the like and thus possible to achieve a high-performance vehicle. 
     Third Embodiment 
       FIG.  13   ,  FIG.  14   , and  FIG.  15    illustrate a third embodiment of the disclosed electric drive unit.  FIG.  14    is a schematic sectional view of a part indicated by arrow Y 13  in  FIG.  13   .  FIG.  15    is an explanatory view of a principal portion of the inverter  30  and a structure of the power module  70 . 
     In the present embodiment, the alternating-current-side side surface portion  61  of the smoothing capacitor  60  has an arc shape. Along the edge of the arc shape, the pair of alternating-current-side terminals  62  provided at the alternating-current-side side surface portion  61  also extend in an arc shape. 
     In the present embodiment, the arrangement of the positive electrode terminal  72 , the negative electrode terminal  73 , and the output terminal  71  of each power module  70  differs from those in the above-described embodiments. Specifically, as illustrated in  FIG.  15   , the positive electrode terminal  72  and the negative electrode terminal  73  are disposed in mutually opposite directions in accordance with the array direction of the half bridge circuits  45 . The output terminal  71  is also disposed at the upper surface (in detail, an intermediate part thereof in the longitudinal direction) of the power module  70  in accordance with the array direction of the half bridge circuits  45 . 
     As illustrated in  FIG.  13   , each of the power modules  70  is disposed such that the negative electrode terminal  73  (corresponding to the inner terminal) is positioned at the inner end portion  74  and that the positive electrode terminal  72  (corresponding to the outer terminal) is positioned at the outer end portion  76 . The negative electrode terminal  73  is in a state of facing the alternating-current-side terminal  62  of the negative-side. The gap between each of the power modules  70  and the smoothing capacitor  60  is substantially the same. 
     The negative-electrode-side busbar  80  in the present embodiment has a band plate shape extending in a curved manner. Consequently, similarly to the busbars  80  and  81  in the second embodiment, the negative-electrode-side busbar  80  (corresponding to the first busbar) is fitted into the gap. 
     The positive-electrode-side busbar  81  (corresponding to the third busbar) in the present embodiment has a plate shape extending in a curved manner along the surface of each of the power modules  70 . Specifically, as illustrated in  FIG.  14    and  FIG.  15   , the positive-electrode-side busbar  81  in the present embodiment has a main wall portion  90  and an outer peripheral wall portion  91 . 
     The main wall portion  90  is formed to spread in a fan shape in the circumferential direction along the upper surface of each power module  70 . A through hole  92  is formed at a part of the main wall portion  90  corresponding to the output terminal  71  of each power module  70 . The outer peripheral wall portion  91  is continuous with the outer peripheral edge of the arc shape of the main wall portion  90  to be bent in a direction orthogonal thereto and extends along the outer end portion  76  of each power module  70 . 
     Similarly to the inner edge portion  80   a  of the negative-electrode-side busbar  80 , the inner edge portion  81   a  of the positive-electrode-side busbar  81  in the present embodiment has an arc shape extending in the circumferential direction. Accordingly, the inner edge portion  81   a  of the positive-electrode-side busbar  81  in the present embodiment is joined, in a state of being fitted to the alternating-current-side side surface portion  61  of the smoothing capacitor  60 , to the alternating-current-side terminal  62  of the positive side. The outer edge portion  81   b  (the terminal end part of the outer peripheral wall portion  91 ) of the positive-electrode-side busbar  81  in the present embodiment is joined, in a state of being fitted to the outer end portion  76  of each of the power modules  70 , to the positive electrode terminal  72 . 
     The negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  in the present embodiment differ from each other in length. Therefore, the lengths and the widths of the negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  are adjusted such that the inductances thereof are substantially the same. That is, the length L (−) of the negative-electrode-side busbar  80  and the length (the total value of La (+) and Lb (+)) of the positive-electrode-side busbar  81  are adjusted together with these widths. Consequently, the inductances of these busbars  80  and  81  are equalized. As a result, it is possible to suppress deterioration in the controllability of the motor  20 . 
     In a state of being joined to the output terminals  71  of respective power modules  70 , each of the output-side busbars  82  is drawn upward to above the positive-electrode-side busbar  81  through the through hole  92 . The output-side busbars  82  and the positive-electrode-side busbar  81  are insulated from each other by a resin sheet  93 . The output-side busbars  82  and the positive-electrode-side busbar  81  are each formed in a shape that is bent to have an L-shaped cross-section so as to extend along the upper surface of each power module  70 . 
     After extending radially outward as it is, each of the output-side busbars  82  extends in the rotational axis direction toward the motor  20  and is thereby connected to the relay terminal  28  corresponding thereto. The length of each of the output-side busbars  82  is also designed such that the wiring lengths are the same. Therefore, the inductances of the output-side busbars  82  are also the same. 
     Even when the shapes, the arrangement, and the like of the power modules  70  and the busbars  80 ,  81 , and  82  are configured as in the present embodiment, the same effects as those in the above-described embodiments can be obtained. 
     Fourth Embodiment 
       FIG.  16    illustrates a fourth embodiment of the disclosed electric drive unit and includes a schematic sectional view taken along an arrow Y 16 . The basic configuration of the present embodiment is the same as that of the third embodiment. As simply illustrated in  FIG.  16   , each of the power modules  70  is constituted by one switching element  46  in the present embodiment while, in the third embodiment, the half bridge circuits  45  of the respective phases are constituted by the power modules  70  each including two switching elements  46 . 
     These two power modules  70  are connected in series by using a relay busbar  95 . Consequently, the half bridge circuit  45  is constituted. The output-side busbar  82  is joined to the relay busbar  95 . Similarly to the third embodiment, the lengths and the widths of the negative-electrode-side busbar  80  and the positive-electrode-side busbar  81  are adjusted also in the present embodiment such that inductance is substantially the same therebetween. The length of each output-side busbar  82  is also designed such that inductance is the same. 
     Even when the shapes, the arrangement, and the like of the power modules  70  and the busbars  80 ,  81 ,  82 , and  95  are configured as in the present embodiment, the same effects as those in the above-described embodiments can be obtained. 
     Fifth Embodiment 
       FIG.  17    illustrates a fifth embodiment of the disclosed electric drive unit. The present embodiment is an embodiment corresponding to the second electric drive unit  10 B. 
     That is, in the second electric drive unit  10 B, the second inverter  30  is integrated with the in-wheel motor  20 B, and it is thus not essential to insert the shaft  24  into a center portion of the thin case  31 . Accordingly, in the thin case  31  of the present embodiment, the shaft cylinder portion  31   a  is not provided at a center portion as in each embodiment described above. Consequently, the space at the center portion of the thin case  31  is expanded. 
     Thus, by utilizing the space, the smoothing capacitor  60  in the present embodiment is formed in a disk shape coaxial with the thin case  31 . Accordingly, the capacity of the smoothing capacitor  60  can be increased. When the capacity is the same, the thickness can be reduced. In addition, since the length from the center of the thin case  31  to the alternating-current-side side surface portion  61  is the same, it is sufficient to dispose the power modules  70  in point-symmetrically, and the layout design of electronic components is easy. 
     The shapes and the arrangement of each power module  70 , the positive-electrode-side busbar  81 , the negative-electrode-side busbar  80 , and the output-side busbars  82  presented in the present embodiment are the same as those in the third embodiment illustrated in  FIG.  13   . Alternatively, the shapes and the arrangement such as those in the other embodiments may be employed. 
     Even when the shapes, the arrangement, and the like of the power modules  70  and the busbars  80 ,  81 , and  82  are configured as in the present embodiment, the same effects as those in the above-described embodiments can be obtained. 
     OTHER EMBODIMENTS 
     The disclosed technology is not limited to the embodiments described above. The disclosed technology includes also various configurations other than those. 
     For example, in each embodiment described above, the inverter  30  in which each of the power modules  70  and the busbars  80 ,  81 , and  82  is disposed at a portion in the circumferential direction at the outer peripheral side of the thin case  31  is presented as an example. 
     However, the example is a non-limiting example. It may be configured such that, as indicated by an imaginary line α 4  in the fifth embodiment illustrated in  FIG.  17   , the power modules  70  are disposed over any part at the whole circumference of the thin case  31 , the smoothing capacitor  60  is extended accordingly in the circumferential direction, and the busbars  80 ,  81 , and  82  are formed over the whole circumference, for example, in an annular form. In this case, a larger number of the power modules  70  can be installed. It is thus possible to achieve an inverter compatible with a motor of multiple phases or a high-output inverter. 
     The inverter  30  reduced in the thickness and the weight by the disclosed technology is preferably disposed adjacent to one end of the motor  20  in the rotational axis direction. Such an arrangement is, however, not essential. Depending on vehicle specifications, the inverter  30  may be disposed in the vicinity of the motor  20 , for example, at a side of or above the motor  20 .