Patent Publication Number: US-10780764-B2

Title: On-board fluid machine

Description:
BACKGROUND ART 
     The present invention relates to an on-board fluid machine. 
     Japanese Laid-Open Patent Publication No. 2010-156271 discloses an on-board fluid machine including, for example, an electric motor and a driver that drives the electric motor. The driver converts DC power, which is supplied from a DC power supply mounted on the vehicle, to AC power. 
     Common mode noise and normal mode noise may both be mixed in the DC power supplied to the driver. In such a case, the noises may interfere with the driver that drives the electric motor. This will affect the operation of the on-board fluid machine. 
     In particular, normal mode noise has a frequency that differs in accordance with the model of the vehicle on which the on-board fluid machine is mounted. It is thus preferred that the normal mode noise be decreased over a wide frequency range so that the on-board fluid machine can be applied to many vehicle models. It is also preferred that this be realized without enlarging the on-board fluid machine. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an on-board fluid machine that reduces the common mode noise and normal mode noise included in the DC power supplied to the driver. 
     To achieve the above object, one aspect of the present invention is an on-board fluid machine including a housing, an electric motor, and a driver. The housing is configured to allow fluid to flow into the housing. The electric motor is accommodated in the housing. The driver is supplied with DC power and drives the electric motor. The driver includes a low-pass filter circuit and an inverter circuit. The low-pass filter circuit is configured to reduce common mode noise and normal mode noise that are included in the DC power. The inverter circuit is configured to convert the DC power, from which the common mode noise and the normal mode noise have been reduced, to AC power. The low-pass filter circuit includes a common mode choke coil and a capacitor. The common mode choke coil includes a ring core and a first coil and a second coil that are wound around the ring core. The capacitor is electrically connected to the common mode choke coil. The driver further includes a damping unit located at a position where magnetic field lines produced by the common mode choke coil generate eddy current. The damping unit is configured to change a frequency characteristic of a phase difference of the common node choke coil. The low-pass filter circuit has a resonant frequency that is set to a value in a frequency range in which the phase difference of the common mode choke coil has been decreased by the damping unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of an on-board motor-driven compressor; 
         FIG. 2  is an exploded perspective view of a driver; 
         FIG. 3  is an exploded perspective view of a common mode choke coil and two parts; 
         FIG. 4  is a front view of a ring core around which two coils are wound; 
         FIG. 5  is an exploded perspective view of the driver; 
         FIG. 6  is a front view of the common mode choke coil accommodated in a damping unit; 
         FIG. 7  is a cross-sectional view of the driver corresponding to line  7 - 7  in  FIG. 6 ; 
         FIG. 8  is a circuit diagram of the driver and an electric motor; 
         FIG. 9  is a graph illustrating the frequency characteristics of the gain of a low-pass filter circuit; 
         FIG. 10  is a graph illustrating the frequency characteristics of the phase difference of the common mode choke coil; and 
         FIG. 11  is a cross-sectional view of a damping unit in a modified example. 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     One embodiment of an on-board fluid machine will now be described. The on-board fluid machine of the present embodiment is an on-board motor-driven compressor including a compression unit that compresses fluid. The on-board motor-driven compressor is used with an on-board air-conditioner. Thus, the compression subject of the on-board motor-driven compressor in the present embodiment is a refrigerant. 
     As shown in  FIG. 1 , an on-board air-conditioner  200  includes an on-board motor-driven compressor  10  and an external refrigerant circuit  201  that supplies the on-board motor-driven compressor  10  with a refrigerant serving as a fluid. The external refrigerant circuit  201  includes, for example, a heat exchanger, an expansion valve, and the like. The on-board air-conditioner  200  cools or warms the passenger compartment using the on-board motor-driven compressor  10  to compress the refrigerant and the external refrigerant circuit  201  to exchange heat with the refrigerant and expand the refrigerant. 
     The on-board air-conditioner  200  includes an air-conditioning ECU  202  that controls the entire on-board air-conditioner  200 . The air-conditioning ECU  202  is configured to acknowledge the set temperature or the like of the on-board air-conditioner  200 . Based on such parameters, the air-conditioning ECU  202  sends various commands, such as ON/OFF commands, to the on-board motor-driven compressor  10 . 
     The on-board motor-driven compressor  10  includes a housing  11 . The housing  11  includes a suction port  11   a . Refrigerant is drawn from the external refrigerant circuit  201  through the suction port  11   a.    
     The housing  11  is formed from a thermally conductive material (e.g., metal such as aluminum). The housing  11  is connected to ground by the body of the vehicle. 
     The housing  11  includes a suction housing portion  12  and a discharge housing portion  13  that are coupled to each other. The suction housing portion  12  is tubular and includes a flat end wall  12   a  and a side wall  12   b , which extends from the circumferential portion of the end wall  12   a  toward the discharge housing portion  13 . Further, the suction housing portion  12  has an opening faced toward the discharge housing portion  13 . The end wall  12   a  is, for example, flat, and the side wall  12   b  is, for example, generally tubular. The discharge housing portion  13  is coupled to the suction housing portion  12  and closes the opening of the suction housing portion  12 . This defines a cavity in the housing  11 . 
     The suction port  11   a  is formed in the side wall  12   b  of the suction housing portion  12 . In detail, the suction port  11   a  is located in the side wall  12   b  of the suction housing portion  12  closer to the end wall  12   a  than the discharge housing portion  13 . 
     The housing  11  includes a discharge port  11   b  from which the refrigerant is discharged. More specifically, the discharge port  11   b  is formed in the discharge housing portion  13  at a location facing toward the end wall  12   a  of the discharge housing portion  13 . 
     The on-board motor-driven compressor  10  includes a rotation shaft  21 , a compression unit  22 , and an electric motor  23  that are accommodated in the housing  11 . 
     The rotation shaft  21  is rotationally supported by the housing  11 . The axial direction of the rotation shaft  21  coincides with the thickness-wise direction of the end wall  12   a  (i.e., axial direction of tubular side wall  12   b ). The rotation shaft  21  is coupled to the compression unit  22 . 
     The compression unit  22  is located in the housing  11  closer to the discharge port  11   b  than the suction port  11   a  (i.e., end wall  12   a ). The compression unit  22  rotates the rotation shaft  21  to compress the refrigerant drawn into the housing  11  from the suction port  11   a  and discharge the compressed refrigerant from the discharge port  11   b . The compression unit  22  may be of any construction such as that of a scroll type, a piston type, or a vane type. 
     The electric motor  23  is located in the housing  11  between the compression unit  22  and the end wall  12   a . The electric motor  23  rotates the rotation shaft  21  in the housing  11  to drive the compression unit  22 . The electric motor  23  includes, for example, a cylindrical rotor  24  that is fixed to the rotation shaft  21  and a stator  25  that is fixed to the housing  11 . The stator  25  includes a tubular stator core  26  and coils  27  that are wound around the teeth of the stator core  26 . The rotor  24  is opposed to the stator  25  in the radial direction of the rotation shaft  21 . The coils  27  are energized to rotate the rotor  24  and the rotation shaft  21  and compress refrigerant with the compression unit  22 . 
     As shown in  FIG. 1 , the on-board motor-driven compressor  10  includes a driver  30  and a cover member  31 . The driver  30  is supplied with DC power and drives the electric motor  23 . The cover member  31  defines an accommodation compartment S 0  that accommodates the driver  30 . 
     The cover member  31  is formed from a thermally and electrically conductive, non-magnetic material (e.g., metal such as aluminum). 
     The cover member  31  is tubular and includes a closed end and an open end. The opening of the open end is faced toward the housing  11 , more specifically, the end wall  12   a  of the suction housing portion  12 . The open end of the cover member  31  is joined with the end wall  12   a  of the housing  11  and fastened to the end wall  12   a  by bolts  32 . The end wall  12   a  closes the opening of the cover member  31 . The accommodation compartment S 0  is defined by the cover member  31  and the end wall  12   a.    
     The accommodation compartment S 0  is located outside the housing  11  at the side of the end wall  12   a  opposite to the electric motor  23 . The compression unit  22 , the electric motor  23 , and the driver  30  are lined in the axial direction of the rotation shaft  21 . 
     A connector  33  is arranged on the cover member  31 , and the driver  30  is electrically connected to the connector  33 . The connector  33  electrically connects the air-conditioning ECU  202  and the driver  30 . Further, the driver  30  is supplied with DC power from an on-board electric storage device  203  installed in the vehicle. The on-board electric storage device  203  is a DC power supply, such as a rechargeable battery or a capacitor, installed in the vehicle. 
     As shown in  FIG. 1 , the driver  30  includes a circuit board  40 , an inverter circuit  41  laid out on the circuit board  40 , two connection lines EL 1  and EL 2  electrically connected to the connector  33  and the inverter circuit  41 , and a low-pass filter circuit  42  arranged on the connection lines EL 1  and EL 2 . 
     The circuit board  40  is flat and spaced apart from the end wall  12   a  by a predetermined distance in the axial direction of the rotation shaft  21 . The circuit board  40  includes a board surface  40   a  faced toward the end wall  12   a.    
     As shown in  FIG. 2 , the circuit board  40  includes terminal holes  40   b  and wires  40   c  that are connected to terminals inserted through the terminal holes  40   b . The wires  40   c  each form at least a portion of the two connection lines EL 1  and EL 2 . In detail, the wires  40   c  are used to electrically connect the connector  33  to the low-pass filter circuit  42  and to electrically connect the low-pass filter circuit  42  to the inverter circuit  41 . 
     The wires  40   c  may be formed on the board surface  40   a  or on the opposite surface of the board surface  40   a . Alternatively, the wires  40   c  may be formed in multiple layers. The wires  40   c  may be of any structure. For example, the wires  40   c  may be wire patterns formed on or embedded in the board. Alternatively, the wires  40   c  may be bars, like bus bars, or be flat. 
     The first connection line EL 1  is electrically connected by the connector  33  to a positive terminal of the on-board electric storage device  203  and to the inverter circuit  41 . The second connection line EL 2  is electrically connected by the connector  33  to a negative terminal of the on-board electric storage device  203  and to the inverter circuit  41 . The DC power supplied from the on-board electric storage device  203  to the connector  33  is transmitted over the two connection lines EL 1  and EL 2 . 
     The low-pass filter circuit  42 , which is arranged on the two connection lines EL 1  and EL 2 , is located at the input side of the inverter circuit  41 . The low-pass filter circuit  42  is configured to receive DC power from the connector  33 . The low-pass filter circuit  42  reduces (attenuates) normal mode noise and common mode noise that are included in the DC power supplied to the driver  30 . 
     Common mode noise is the noise that flows through the two connection lines EL 1  and EL 2  in the same direction. Common mode noise may be produced when, for example, the driver  30  (i.e., on-board motor-driven compressor  10 ) and the on-board electric storage device  203  are electrically connected through a path (e.g., body of vehicle) other than the two connection lines EL 1  and EL 2 . 
     Normal mode noise is noise that has a predetermined frequency and is superposed on DC current. Further, normal mode noise is noise in which current momentarily flows through the two connection lines EL 1  and EL 2  in opposite directions. Thus, normal mode noise can be referred to as an inflow ripple component included in the DC power supplied to the driver  30 . The low-pass filter circuit  42  will be described in detail later. 
     The inverter circuit  41  is connected by the wires  40   c  to the output side of the low-pass filter circuit  42 . The inverter circuit  41  is supplied with the DC power output from the low-pass filter circuit  42 , that is, the DC power of which the normal mode noise and common mode noise have been reduced by the low-pass filter circuit  42 . 
     The inverter circuit  41  converts the DC power to AC power. In detail, the inverter circuit  41  is a three-phase inverter including switching elements Qu 1 , Qu 2 , Qv 1 , Qv 2 , Qw 1 , and Qw 2  (hereafter simply referred to as the switching elements Qu 1  to Qw 2 ). The switching elements Qu 1  to Qw 2  are cyclically activated and deactivated to convert DC power to AC power. 
     The inverter circuit  41  is electrically connected by some of the wires  40   c  and hermetic terminals (not shown) formed in the end wall  12   a  to the coils  27  of the electric motor  23 . The AC power converted from DC power by the inverter circuit  41  is supplied to the coils  27  to drive the electric motor  23 . 
     In the present embodiment, the inverter circuit  41  is located between the board surface  40   a  and the end wall  12   a . Instead, the inverter circuit  41  may be located at the opposite side of the board surface  40   a  or beside the circuit board  40 . 
     The configuration of the low-pass filter circuit  42  will now be described in detail with reference to  FIGS. 1, 2 and 3 to 7 . To facilitate illustration, the low-pass filter circuit  42  is shown without an insulator  111  in  FIG. 3 . 
     Referring to  FIG. 3 , the low-pass filter circuit  42  includes a common mode choke coil  50 . The common mode choke coil  50  includes a looped ring core  51 . The ring core  51  of the present embodiment is rectangular and includes rounded corners so as to be looped (ring-shaped) in an axial view of the ring core  51 . Further, the ring core  51  includes two long sides  61  and  71  (extensions), extending straight in a longitudinal direction in the axial view of the ring core  51 , and two short sides  62  and  72 , extending straight in a lateral direction in the axial view of the ring core  51 . 
     The two long sides  61  and  71  are opposed to each other, and the two short sides  62  and  72  are opposed to each other. The opposing direction of the two long sides  61  and  71  is orthogonal to the opposing direction of the two short sides  62  and  72 . 
     To aid understanding, the opposing direction of the long sides  61  and  71  will be referred to as the X-axis direction, the opposing direction of the two short sides  62  and  72  will be referred to as the Y-axis direction, and the axial direction of the ring core  51  will be referred to as the Z-axis direction. The X-axis direction may also be referred to as the lateral direction of the ring core  51  or the extending direction of the two short sides  62  and  72 . The Y-axis direction may be referred to as the longitudinal direction of the ring core  51  or the extending direction of the two long sides  61  and  71 . The X-axis direction, the Y-axis direction, and the Z-axis direction are orthogonal to one another. 
     In the present embodiment, as shown in  FIGS. 1 and 2 , the Z-axis direction, which is the axial direction of the ring core  51 , coincides with the axial direction of the rotation shaft  21 . However, the Z-axis direction does not have to coincide with the axial direction of the rotation shaft  21 , and the common mode choke coil  50  may be directed in any direction. For example, the X-axis direction or the Y-axis direction may coincide with the axial direction of the rotation shaft  21 . 
     The ring core  51  includes two first corners  63 , located at the two ends of the first long side  61  in the Y-axis direction, and two second corners  73  located at the two ends of the second long side  71  in the Y-axis direction. The two first corners  63  connect the first long side  61  to the two short sides  62  and  72 . The two second corners  73  connect the second long side  71  to the two short sides  62  and  72 . The corners  63  and  73  are each curved and shaped to be sectoral as viewed in the Z-axis direction. 
     As shown in  FIGS. 3 and 4 , the common mode choke coil  50  includes a first coil  64  and a second coil  74  that are wound around the ring core  51 . 
     The first coil  64  is wound around the entire first long side  61 , which includes a central portion  61   a  of the first long side  61  in the Y-axis direction, and the first corners  63 . The first long side  61  and the first corners  63  form a first winding portion around which the first coil  64  is wound. 
     The first coil  64  includes a first high-density portion  64   a  and first low-density portions  64   b . The first high-density portion  64   a  has a winding density that differs from that of the first low-density portions  64   b . The winding density is the number of windings per unit length in the winding axis direction. The winding density of the first high-density portion  64   a  is higher than the winding density of the first low-density portions  64   b.    
     The first high-density portion  64   a  is arranged on and around the central portion  61   a  of the first long side  61 . The first low-density portions  64   b  are arranged at the two opposite sides of the first high-density portion  64   a . More specifically, the first low-density portions  64   b  are arranged on the two ends of the first long side  61  in the Y-axis direction and on the first corners  63 . 
     Since the first corners  63  are curved, the winding density of the first coil  64  wound around the first corners  63  has a tendency to become lower than that of the first coil  64  wound around the first long side  61 . 
     The second coil  74  is wound around the entire second long side  71 , which includes a central portion  71   a  of the second long side  71  in the Y-axis direction, and the second corners  73 . The second long side  71  and the second corners  73  form a second winding portion around which the second coil  74  is wound. 
     The second coil  74  includes a second high-density portion  74   a  and second low-density portions  74   b . The second high-density portion  74   a  has a winding density that differs from that of the second low-density portions  74   b . The winding density of the second high-density portion  74   a  is higher than the winding density of the second low-density portions  74   b.    
     The second high-density portion  74   a  is arranged on and around the central portion  71   a  of the second long side  71 . The second low-density portions  74   b  are arranged at the two opposite sides of the second high-density portion  74   a . More specifically, the second low-density portions  74   b  are arranged on the two ends of the second long side  71  in the Y-axis direction and on the second corners  73 . 
     Since the second corners  73  are curved, the winding density of the second coil  74  wound around the second corners  73  has a tendency to become lower than that of the second coil  74  wound around the second long side  71 . 
     As shown in  FIG. 4 , the two coils  64  and  74  are not wound around the two short sides  62  and  72 . The two short sides  62  and  72  may be referred to as non-winding portions around which the two coils  64  and  74  are not wound. Thus, the short sides  62  and  72  include side surfaces  62   a  and  72   a  around which the two coils  64  and  74  are not wound, respectively. The side surfaces  62   a  and  72   a  of the short sides  62  and  72  define the two outer end surfaces of the ring core  51  in the Y-axis direction. Hereinafter, the side surface  62   a  will be referred to as the first non-winding side surface  62   a  of the first short side  62 , and the side surface  72   a  of the second short side  72  will be referred to as the second non-winding side surface  72   a . The two non-winding side surfaces  62   a  and  72   a  intersect (more specifically, are orthogonal) the Y-axis direction, which is the extending direction of the two long sides  61  and  71 . In the present embodiment, the two non-winding side surfaces  62   a  and  72   a  extend in the X-axis direction and the Z-axis direction. The two non-winding side surfaces  62   a  and  72   a  are opposed to each other in the Y-axis direction. 
     In the present embodiment, the first non-winding side surface  62   a  corresponds to “the first side surface,” and the second non-winding side surface  72   a  corresponds to “the second side surface.” Further, in the present embodiment, the first long side  61  corresponds to “the first extension,” and the second long side  71  corresponds to “the second extension.” The winding axis direction of the first high-density portion  64   a  is the Y-axis direction and coincides with that of the second high-density portion  74   a.    
     The two coils  64  and  74  are opposed to each other in the X-axis direction, which is orthogonal to the Z-axis direction that is the axial direction of the ring core  51 . The extending direction of the two long sides  61  and  71  intersects (preferably, is orthogonal to) the opposing direction of the two coils  64  and  74  and the axial direction of the ring core  51 . 
     The two coils  64  and  74  are set to have the same number of windings. The two coils  64  and  74  are wound so that the magnetic flux generated by the coil  64  and the magnetic flux generated by the coil  74  strengthen each other when common mode currents, which are currents that flow in the same direction, flow through the two coils  64  and  74  and so that the magnetic flux generated by the coil  64  and the magnetic flux generated by the coil  74  cancel each other when normal mode currents, which are currents that flow in opposite directions, flow through the two coils  64  and  74 . 
     As shown by the single-dashed lines in  FIG. 4 , some of the magnetic flux leaks even when normal mode currents flow through the two coils  64  and  74 . This produces leakage flux Bx (i.e., magnetic field lines) in the common mode choke coil  50 . Thus, the common mode choke coil  50  has a predetermined inductance with respect to normal mode current. In other words, the common mode choke coil  50  has a relatively large impedance (in detail, inductance) with respect to common mode currents and a relatively small impedance with respect to normal mode currents. 
     The two coils  64  and  74  are not wound around the two short sides  62  and  72 . Thus, magnetic flux has a tendency to leak from the ring core  51 . As a result, the leakage flux Bx has a tendency to be large compared with a structure in which a coil is wound entirely around the ring core  51 . Further, the coils  64  and  74  include the low-density portions  64   b  and  74   b . Thus, the leakage flux Bx has a tendency to be large compared with a structure in which the coils  64  and  74  are formed by only the high-density portions  64   a  and  74   a.    
     As described above, the ring core  51  does not have the form of a circular ring that is free from straight portions. Rather, the ring core  51  is non-circular and includes the long sides  61  and  71 , the short sides  62  and  72 , and the curved corners  63  and  73 . The winding density of the coils  64  and  74  wound around the corners  63  and  73  has a tendency to be lower than locations that are straightly formed. In this regard, the shape of the ring core  51  including the long sides  61  and  71 , the short sides  62  and  72 , and the curved corners  63  and  73  forms the high-density portions  64   a  and  74   a  and the low-density portions  64   b  and  74   b.    
     As shown in  FIG. 4 , the leakage flux Bx is produced at each of the two coils  64  and  74  and has the form of a loop extending from one of the two non-winding side surfaces  62   a  and  72   a  to the other one of the two non-winding side surfaces  62   a  and  72   a  in the Y-axis direction. The leakage flux Bx has a tendency to concentrate more at the two non-winding side surfaces  62   a  and  72   a , which intersect (in detail, extends orthogonal to) the winding axis direction of the high-density portions  64   a  and  74   a , than the side surfaces of the long sides  61  and  71 . In the present embodiment, the coils  64  and  74  are wound around the corners  63  and  73  in addition to the long sides  61  and  71 . Thus, the leakage of magnetic flux from the side surfaces of the corners  63  and  73  is limited, and the leakage flux Bx has a tendency to concentrate at the two non-winding side surfaces  62   a  and  72   a.    
     As shown in  FIGS. 3 and 5 , the common mode choke coil  50  includes a first input terminal  65 , a first output terminal  66 , a second input terminal  75 , and a second output terminal  76 . The first input terminal  65  and the first output terminal  66  extend from the first coil  64 . The second input terminal  75  and the second output terminal  76  extend from the second coil  74 . The terminals  65 ,  66 ,  75 , and  76  are located at the inner side of the ring core  51  and extend in the Z-axis direction. In the present embodiment, the two input terminals  65  and  75  are located closer to the central part of the common mode choke coil  50  than the first non-winding side surface  62   a , and the two output terminals  66  and  76  are located closer to the central part of the common mode choke coil  50  than the second non-winding side surface  72   a . As shown in  FIG. 5 , the terminals  65 ,  66 ,  75 , and  76  are inserted through the terminal holes  40   b  of the circuit board  40  and electrically connected to the wires  40   c . This couples the common mode choke coil  50  to the circuit board  40 . 
     The two input terminals  65  and  75  are electrically connected by the wires  40   c  to the connector  33 , and the two input terminals  65  and  75  are supplied with DC current from the on-board electric storage device  203 . The two output terminals  66  and  76  are electrically connected to the inverter circuit  41  by the wires  40   c.    
     As shown in  FIG. 1 , the low-pass filter circuit  42  includes an X capacitor  80  that is electrically connected to the common mode choke coil  50 . In the present embodiment, the driver  30  includes two Y capacitors  81  and  82  in addition to the X capacitor  80 . 
     In the present embodiment, the common mode choke coil  50  and the capacitors  80  to  82  are located between the board surface  40   a  and the end wall  12   a . Instead, at least one of the common mode choke coil  50  and the capacitors  80  to  82  may be located on the surface of the circuit board  40  opposite to the board surface  40   a  or beside the circuit board  40 . 
     The capacitors  80  to  82  each include a terminal inserted through the corresponding terminal hole  40   b  and fixed to the circuit board  40 . This couples the capacitors  80  to  82  to the circuit board  40  in a state electrically connected to the common mode choke coil  50  and the inverter circuit  41 . The electrical connection to the capacitors  80  to  82  will be described later in detail. 
     The driver  30  includes a damping unit  90  located at a position where the magnetic field lines (leakage flux Bx), which are produced by the common mode choke coil  50 , generate eddy current Ie. The location of the damping unit  90  is set so that the magnetic field lines (leakage flux Bx), which are produced by the common mode choke coil  50 , generate eddy current Ie at the damping unit  90 . 
     As shown in  FIGS. 2 and 3 , the damping unit  90  includes a first part  91  and a second part  101 . The parts  91  and  101  are box-shaped and respectively include openings  92  and  102 , each opening in one direction, and end walls  93  and  103  (bottom walls). The two parts  91  and  101  are arranged with their openings  92  and  102  opposed to each other. In detail, the two openings  92  and  102  are opposed to each other in the Y-axis direction, which is the direction orthogonal to the Z-axis direction and which intersects (preferably, extends orthogonal to) the X-axis direction. The two parts  91  and  101  cooperate to accommodate the common mode choke coil  50 . In this case, the two parts  91  and  101  cover most of the common mode choke coil  50 . 
     As shown in  FIG. 6 , the damping unit  90  (i.e., parts  91  and  101 ) is located at a position penetrated by the leakage flux Bx, which is produced at the common mode choke coil  50 , that is, a position intersecting the leakage flux Bx. The leakage flux Bx penetrates the damping unit  90  so that the eddy current Ie flows through the damping unit  90  and generates magnetic flux By in a direction that cancels the leakage flux Bx. The damping unit  90  (i.e., parts  91  and  101 ) is formed from a non-magnetic conductive material, such as aluminum or brass, and has a relative permeability set to 0.9 to 3. 
     As shown in  FIGS. 6 and 7 , the first part  91  includes the first end wall  93  (bottom wall) that covers the first non-winding side surface  62   a  and a first peripheral wall  94  (side wall) that extends from the first end wall  93  toward the second part  101 . 
     The first end wall  93  is flat and slightly larger than the first non-winding side surface  62   a  as viewed in the Y-axis direction. The first end wall  93  is opposed to the first non-winding side surface  62   a  in the Y-axis direction. In the present embodiment, the first end wall  93  corresponds to “the first opposing portion.” 
     The first peripheral wall  94  is frame-shaped and surrounds the common mode choke coil  50  as viewed in the Y-axis direction. The first peripheral wall  94  surrounds both of the two long sides  61  and  71 . The first peripheral wall  94  covers substantially one half of the common mode choke coil  50 , that is, the side corresponding to the first non-winding side surface  62   a . The first peripheral wall  94  covers the side of each of the two coils  64  and  74  that corresponds to the first non-winding side surface  62   a . The first peripheral wall  94  includes a first distal end  95  that defines the first opening  92 . 
     In the present embodiment, the first end wall  93  is rectangular. Further, the first peripheral wall  94  extends from the edges of the first end wall  93  and has the form of a rectangular frame as viewed in the Y-axis direction. However, the first end wall  93  and the first peripheral wall  94  may have any form. For example, the first end wall  93  may be oval and have the form of an ellipsoid. 
     As shown in  FIG. 5 , the first peripheral wall  94  includes a first recess  96  that extends from the first distal end  95  toward the first end wall  93 . The first recess  96  is formed in the first peripheral wall  94  at a portion corresponding to the circuit board  40  and extends from the first distal end  95  to an intermediate position in the first peripheral wall  94  in the Y-axis direction. 
     The two input terminals  65  and  75  are extended through the first recess  96  and inserted through the terminal holes  40   b . In detail, the two input terminals  65  and  75  are extended through the first recess  96  toward the circuit board  40  and inserted through the terminal holes  40   b . This avoids interference of the two input terminals  65  and  75  with the first part  91 . 
     As shown in  FIGS. 6 and 7 , the second part  101  includes the second end wall  103  (bottom wall) that covers the second non-winding side surface  72   a  and a second peripheral wall  104  (side wall) that extends from the second end wall  103  toward the first part  91 . 
     The second end wall  103  and the first end wall  93  are identical in shape. The second end wall  103  is flat and slightly larger than the second non-winding side surface  72   a  as viewed in the Y-axis direction. The second end wall  103  is opposed to the second non-winding side surface  72   a  in the Y-axis direction. The first end wall  93  and the second end wall  103  are opposed to each other in the Y-axis direction. In the present embodiment, the second end wall  103  corresponds to “the second opposing portion.” 
     The second peripheral wall  104  and the first peripheral wall  94  are identical in shape. The second peripheral wall  104  is frame-shaped and surrounds the common mode choke coil  50  as viewed in the Y-axis direction. The second peripheral wall  104  surrounds both of the two long sides  61  and  71 . The second peripheral wall  104  covers substantially one half of the common mode choke coil  50 , that is, the side corresponding to the second non-winding side surface  72   a . The second peripheral wall  104  covers the side of each of the two coils  64  and  74  that corresponds to the second non-winding side surface  72   a . The second peripheral wall  104  includes a second distal end  105  that defines the second opening  102 . 
     As shown in  FIG. 5 , the second peripheral wall  104  includes a second recess  106  that extends from the second distal end  105  toward the second end wall  103 . The second recess  106  is formed in the second peripheral wall  104  at a portion corresponding to the circuit board  40  and extends from the second distal end  105  to an intermediate position in the second peripheral wall  104  in the Y-axis direction. 
     The two output terminals  66  and  76  are extended through the second recess  106  and inserted through the terminal holes  40   b . In detail, the two output terminals  66  and  76  extend through the second recess  106  toward the circuit board  40  and are inserted through the terminal holes  40   b . This avoids interference of the two output terminals  66  and  76  with the second part  101 . Thus, the terminals  65 ,  66 ,  75 , and  76  of the two coils  64  and  74  are extended through one of the two recesses  96  and  106  and inserted through the terminal holes  40   b  to extend through the circuit board  40 . 
     As shown in  FIG. 7 , the driver  30  includes the insulator  111  that insulates the common mode choke coil  50  and the damping unit  90 . The insulator  111  is, for example, an insulation coating, which is applied to the surface of the common mode choke coil  50 , or an insulation film. The insulator  111  functions to prevent short-circuiting between the common mode choke coil  50  and the damping unit  90 . 
     The terminals  65 ,  66 ,  75 , and  76  extend through the insulator  111 . An insulation coating is applied to the basal ends of the terminals  65 ,  66 ,  75 , and  76  to prevent short-circuiting of the terminals  65 ,  66 ,  75 , and  76  with the two parts  91  and  101 . 
     The insulator  111  is also arranged between the end walls  93  and  103  and the corresponding non-winding side surfaces  62   a  and  72   a . The end walls  93  and  103  and the non-winding side surfaces  62   a  and  72   a  are in contact with the insulator  111 . In this case, an opposing distance Y 1  between the end walls  93  and  103  and the corresponding non-winding side surfaces  62   a  and  72   a  is the same as the thickness of the insulator  111 . 
     In the present embodiment, the first peripheral wall  94  and the second peripheral wall  104  are slightly larger than the common mode choke coil  50  as viewed in the Y-axis direction so that the insulator  111  is spaced apart from the two peripheral walls  94  and  104 . 
     However, the insulator  111  may be in contact with the two peripheral walls  94  and  104 . This will allow the heat of the common mode choke coil  50  to be transmitted in a preferred manner to the two parts  91  and  101  and improve the heat dissipation of the common mode choke coil  50 . 
     The insulator  111  may have any structure. For example, the insulator  111  may be an insulation coating applied to the inner surfaces of the two parts  91  and  101 . In  FIG. 7 , the insulator  111  is illustrated thicker than actual. 
     The two parts  91  and  101  are coupled to the common mode choke coil  50  from the Y-axis direction and positioned relative to the common mode choke coil  50  in a state in which the end walls  93  and  103  and the non-winding side surfaces  62   a  and  72   a  are in contact with the insulator  111 . Consequently, the opposing distance Y 1  (thickness of insulator  111 ) is constant. In other words, the two parts  91  and  101  are positioned relative to the common mode choke coil  50  so that the opposing distance Y 1  is constant regardless of dimensional errors of the two parts  91  and  101  and the common mode choke coil  50 . 
     The two parts  91  and  101  may be positioned relative to the common mode choke coil  50  by any structure. For example, the structure may involve engagement, fitting, or adhering. Further, the driver  30  may include a clamp that clamps the two parts  91  and  101  in the Y-axis direction. In this case, the two parts  91  and  101  are also coupled to the common mode choke coil  50  in a state in which displacement of the two parts  91  and  101  is restricted in the Y-axis direction. The clamp may include, for example, two urging members that urge the two parts  91  and  101  toward each other. 
     As shown in  FIG. 7 , the two parts  91  and  101  are spaced apart from each other in a state in which the two distal ends  95  and  105  are opposed to each other in the Y-axis direction. Thus, a gap  112  extends between the first distal end  95  and the second distal end  105 . The gap  112  extends over a distance that is greater than the opposing distance Y 1  from the end walls  93  and  103  to the corresponding non-winding side surfaces  62   a  and  72   a . The damping unit  90  does not cover portions of the two coils  64  and  74  corresponding to the gap  112  extending between the two distal ends  95  and  105 . 
     The gap  112  and the central portions  61   a  and  71   a  of the two long sides  61  and  71  are located at corresponding positions in the Y-axis direction. Thus, the damping unit  90  does not cover the sections of the high-density portions  64   a  and  74   a  in the two coils  64  and  74  corresponding to the gap  112 , that is, the sections of the two coils  64  and  74  wound around the central portions  61   a  and  71   a  of the two long sides  61  and  71 . Further, the damping unit  90  does not cover the section of the two coils  64  and  74  corresponding to the two recesses  96  and  106 . 
     The first end wall  93  and the first peripheral wall  94  define a first part accommodation compartment S 1  in the first part  91 . Further, the second end wall  103  and the second peripheral wall  104  define a second part accommodation compartment S 2  in the second part  101 . The first part accommodation compartment S 1  and the second part accommodation compartment S 2  are opposed to each other in the Y-axis direction. In this case, the common mode choke coil  50  is accommodated in the first part accommodation compartment S 1  and the second part accommodation compartment S 2 . That is, the two parts  91  and  101  cooperate with each other in a state in which the openings  92  and  102  are opposed to each other in order to accommodate the common mode choke coil  50 . In detail, the two parts  91  and  101  accommodate the common mode choke coil  50  from the Y-axis direction that extends orthogonal to the Z-axis direction and intersects (in present embodiment, extends orthogonal to) the X-axis direction, which is the opposing direction of the two coils  64  and  74 . 
     The two parts  91  and  101  of the damping unit  90  are in contact with the housing  11  (i.e., end wall  12   a ) to allow for heat exchange between the damping unit  90  and the housing  11 . This cools the two parts  91  and  101  with the housing  11 . 
     The electrical configuration of the electric motor  23  and the driver  30  will now be described. 
     As shown in  FIG. 8 , the coils  27  of the electric motor  23  form, for example, a three-phase construction including a u-phase coil  27   u , a v-phase coil  27   v , and a w-phase coil  27   w . The u-phase coil  27   u , the v-phase coil  27   v , and the w-phase coil  27   w  are connected to one another in, for example, a Y connection. 
     The inverter circuit  41  includes u-phase switching elements Qu 1  and Qu 2  that correspond to the u-phase coil  27   u, v -phase switching elements Qv 1  and Qv 2  that correspond to the v-phase coil  27   v , and w-phase switching elements Qw 1  and Qw 2  that correspond to the w-phase coil  27   w . The switching elements Qu 1  to Qw 2  are, for example, power switching elements such as insulated gate bipolar transistors (IGBTs). The switching elements Qu 1  to Qw 2  include flywheel diodes Du 1  to Dw 2  (body diodes). 
     The u-phase switching elements Qu 1  and Qu 2  are connected to each other in series by a connection wire, which is connected to the u-phase coil  27   u . The series-connected body of the u-phase switching elements Qu 1  and Qu 2  is electrically connected to the two connection lines EL 1  and EL 2 , and the series-connected body is supplied with DC power from the on-board electric storage device  203 . 
     The other switching elements Qv 1 , Qv 2 , Qw 1 , and Qw 2  are connected in the same manner as the u-phase switching elements Qu 1  and Qu 2  except in that the corresponding coil is different. In this case, the switching elements Qu 1  to Qw 2  are connected to the two connection lines EL 1  and EL 2 . 
     The driver  30  includes a controller  113  that controls the switching elements Qu 1  to Qw 2 . The controller  113  may be, for example, one or more dedicated hardware circuits and/or a circuitry realized by one or more processors running on a computer program (software). Each processor includes a CPU and a memory such as a RAM and a ROM. The memory stores, for example, program codes or commands configured to have the processor execute various types of processing. The memory, or computer-readable medium, is any medium that is accessible and usable by a versatile or dedicated computer. 
     The controller  113  is electrically connected by the connector  33  to the air-conditioning ECU  202  and cyclically activates and deactivates the switching elements Qu 1  to Qw 2  based on commands from the air-conditioning ECU  202 . In detail, the controller  113  executes pulse-width modulation (PWM) control on the switching elements Qu 1  to Qw 2  based on commands from the air-conditioning ECU  202 . More specifically, the controller  113  uses a carrier signal (carrier wave signal) and a command voltage signal (comparison subject signal) to generate a control signal. Then, the controller  113  uses the generated control signal to control activation and deactivation of the switching elements Qu 1  to Qw 2  and convert DC power to AC power. 
     As shown in the circuit diagram of  FIG. 8 , the low-pass filter circuit  42  is located between the connector  33  and the inverter circuit  41 . 
     The common mode choke coil  50  is arranged on the two connection lines EL 1  and EL 2 . As described above, the common mode choke coil  50  produces the leakage flux Bx when normal mode current flows. In this regard, the common mode choke coil  50  includes the hypothetical normal mode coils L 1  and L 2  in addition to the two coils  64  and  74 . More specifically, in an equivalent circuit, the common mode choke coil  50  of the present embodiment includes the two coils  64  and  74  and the hypothetical normal mode coils L 1  and L 2 . The hypothetical normal mode coils L 1  and L 2  are respectively connected in series to the coils  64  and  74 . 
     The X capacitor  80  is arranged in a stage subsequent to the common mode choke coil  50 , or at the side corresponding to the inverter circuit  41 , and electrically connected to the two connection lines EL 1  and EL 2 . The common mode choke coil  50  and the X capacitor  80  form an LC resonant circuit. That is, the low-pass filter circuit  42  of the present embodiment is an LC resonant circuit that includes the common mode choke coil  50 . 
     The low-pass filter circuit  42  has a cutoff frequency fc set to be lower than a carrier frequency fp that is the frequency of the carrier signal. The carrier frequency fp may also be referred to as the switching frequency of each of the switching elements Qu 1  to Qw 2 . 
     The vehicle includes, for example, a PCU (power control unit)  204 , which serves as an on-board device, in addition to the driver  30 . The PCU  204  uses the DC power supplied from the on-board electric storage device  203  to drive a travel motor or the like installed in the vehicle. In the present embodiment, the PCU  204  and the driver  30  are connected in parallel to the on-board electric storage device  203 , and the on-board electric storage device  203  is shared by the PCU  204  and the driver  30 . 
     The PCU  204  includes, for example, a boost converter  205  and a power capacitor  206 . The boost converter  205  includes a boost switching element and cyclically activates and deactivates a boost switching element to step-up the DC power of the on-board electric storage device  203 . The power capacitor  206  is connected in parallel to the on-board electric storage device  203 . Although not illustrated in the drawings, the PCU  204  includes a travel inverter that converts the DC power stepped-up by the boost converter  205  to drive force that can drive the travel motor. 
     Noise generated by the switching of the boost switching element may enter the driver  30  as normal mode noise. In this case, the normal mode noise includes noise components corresponding to the switching frequency of the boost switching element. The switching frequency of the boost switching element differs between vehicle models. Thus, the frequency of the normal mode noise differs between vehicle models. Noise components corresponding to the switching frequency of the boost switching element may include harmonic components in addition to noise components having the same frequency as the switching frequency. 
     The two Y capacitors  81  and  82  are connected to each other in series. In detail, the driver  30  includes a bypass line EL 3  that connects one end of the first Y capacitor  81  and one end of the second Y capacitor  82 . The bypass line EL 3  is connected to ground by the vehicle body. 
     A series-connected body of the two Y capacitors  81  and  82  is located between the common mode choke coil  50  and the X capacitor  80  and electrically connected to the common mode choke coil  50 . The other end of the first Y capacitor  81  is connected to the first connection line EL 1 , that is, the section of the first connection line EL 1  connecting the first coil  64  (first output terminal  66 ) and the inverter circuit  41 . The other end of the second Y capacitor  82  is connected to the section of the second connection line EL 2  connecting the second coil  74  (second output terminal  76 ) and the inverter circuit  41 . 
     The frequency characteristics of the low-pass filter circuit  42  will now be described with reference to  FIG. 9 .  FIG. 9  is a graph illustrating the frequency characteristics of the gain G (attenuation amount) of the low-pass filter circuit  42  relative to the entering normal mode noise. The solid line in  FIG. 9  illustrates the frequency characteristics when the damping unit  90  exists, and the double-dashed line illustrates the frequency characteristics when the damping unit  90  does not exist. In  FIG. 9 , the horizontal axis represents the frequency as a logarithm. The gain G is a parameter indicating the amount that the normal mode noise can be reduced. 
     As shown by the double-dashed line in  FIG. 9 , when the damping unit  90  does not exist, the Q factor of the low-pass filter circuit  42  (i.e., LC resonant circuit including common mode choke coil  50  and X capacitor  80 ) is relatively high. Thus, it is difficult to reduce the normal mode noise at frequencies that are close to the resonant frequency f 0  of the low-pass filter circuit  42 . In other words, the normal mode noise has a tendency to increase at frequencies close to the resonant frequency f 0  of the low-pass filter circuit  42 . 
     In this respect, the present embodiment includes the damping unit  90  that is located at a position where the eddy current Ie is generated by the magnetic field lines (the leakage flux Bx) produced at the common mode choke coil  50 . The damping unit  90  is located at a position penetrated by the leakage flux Bx. The penetration of the leakage flux Bx generates the eddy current Ie that produces the magnetic flux By in a direction canceling the leakage flux Bx. Thus, the damping unit  90  functions to lower the Q factor of the low-pass filter circuit  42 . Accordingly, as shown by the solid line in  FIG. 9 , the Q factor of the low-pass filter circuit  42  is low. Thus, the low-pass filter circuit  42  also decreases normal mode noise at frequencies close to the resonant frequency f 0  of the low-pass filter circuit  42 . 
     The existence of the damping unit  90  lowers the inductance of the hypothetical normal mode coils L 1  and L 2 . Thus, the resonant frequency f 0  of the low-pass filter circuit  42  in the present embodiment is slightly higher than that when there is no damping unit  90 . 
     As shown in  FIG. 9 , the tolerable value of the gain G required in accordance with the specification of the vehicle is referred to as the gain Gth. The Q factor of the gain G of the low-pass filter circuit  42  that becomes equal to the tolerable gain Gth when the frequency of the normal mode noise is the same as the resonant frequency f 0  is referred to as a specific Q factor. In the present embodiment, the Q factor of the low-pass filter circuit  42  is lower than the specific Q factor because of the damping unit  90 . Thus, the gain G of the low-pass filter circuit  42  when the frequency of the normal mode noise is the same as the resonant frequency f 0  is smaller than the tolerable gain Gth (larger in absolute value). In other words, the damping unit  90  is configured to lower the Q factor of the low-pass filter circuit  42  from the specific Q factor. 
     One speculation of why the Q factor of the low-pass filter circuit  42  becomes low because of the damping unit  90  will now be described. The description hereafter is a speculation and does not negate the validity of the damping unit  90 . 
     The magnetic flux By in the direction that cancels the leakage flux Bx functions as magnetic resistance to the leakage flux Bx of the common mode choke coil  50 . Thus, the magnetic flux By, which cancels the leakage flux Bx, impedes the flow of normal mode current through the common mode choke coil  50 , which causes the leakage flux Bx. In this manner, the magnetic flux By in the direction that cancels the leakage flux Bx functions as a resistance component to the normal mode current. 
     The leakage flux Bx in the direction that cancels the magnetic flux By has a tendency to increase more easily as the eddy current Ie generated at the damping unit  90  increases. The eddy current Ie generated at the damping unit  90  has a tendency to increase more easily as the normal mode noise increases. The normal mode noise (normal mode current) has a tendency to increase at frequencies close to the resonant frequency f 0  of the low-pass filter circuit  42 . Thus, the magnetic flux By in the direction that cancels the leakage flux Bx has a tendency to increase at frequencies close to the resonant frequency f 0  of the low-pass filter circuit  42 . Accordingly, the resistance components of the damping unit  90  have a tendency to increase at frequencies close to the resonant frequency f 0  of the low-pass filter circuit  42 . This decreases the Q factor of the low-pass filter circuit  42 . 
     The frequency characteristics of a phase difference θ of the common mode choke coil  50  will now be described with reference to  FIG. 10 . The phase difference θ of the common mode choke coil  50  is the difference between the phase of the voltage applied to the common mode choke coil  50  and the phase of the current flowing through the common mode choke coil  50 .  FIG. 10  is a graph showing changes in the phase difference θ of the common mode choke coil  50  relative to changes in the frequency of normal mode noise (normal mode current). In  FIG. 10 , the horizontal axis represents the frequency as a logarithm. 
     As shown in  FIG. 10 , the phase difference θ of the common mode choke coil  50  varies in accordance with the frequency of the normal mode noise. The arrangement of the damping unit  90  on the common mode choke coil  50  changes the frequency characteristics of the phase difference θ of the common mode choke coil  50 . That is, the damping unit  90  (two parts  91  and  101 ) changes the frequency characteristics of the phase difference θ of the common mode choke coil  50 . 
     In detail, when the phase difference θ of the common mode choke coil  50  in a case in which there is no damping unit  90  is defined as the first phase difference θx, as shown in the double-dashed line in  FIG. 10 , the first phase difference θx gradually increases as the frequency increases in a relatively low frequency range. However, the first phase difference θx is substantially constant and remains high in a relatively high frequency range. 
     When the phase difference θ of the common mode choke coil  50  in a case in which the damping unit  90  is used is defined as the second phase difference θy, the frequency characteristics of the second phase difference θy is as shown by the solid line in  FIG. 10  and plotted along a curve including a maximum value θm and a minimum value θn. The second phase difference θy is the same as the first phase difference θx in a relatively low frequency range but smaller than the first phase difference θx in a relatively high frequency range. 
     The frequency range in which the second phase difference θy is smaller than the first phase difference θx is defined as a specific frequency range fb. The specific frequency range fb is the frequency range in which the phase difference θ of the common mode choke coil  50  is decreased by the damping unit  90 . When the upper limit value of the frequency range in which the first and second phase differences θx and θy are both the same is defined as the lower limit frequency fb 0 , the specific frequency range fb is a frequency range that is greater than the lower limit frequency fb 0 . 
     The resonant frequency f 0  of the low-pass filter circuit  42  is set to a value in the specific frequency range fb. In detail, the resonant frequency f 0  of the low-pass filter circuit  42  is set to be higher than the lower limit frequency fb 0 . In the present embodiment, the resonant frequency f 0  of the low-pass filter circuit  42  is set to a value that is closer to a minimum frequency fn corresponding to the minimum value θn than a maximum frequency corresponding to the maximum value θm. 
     The present embodiment has the advantages described below. 
     (1) The on-board motor-driven compressor  10 , which serves as the on-board fluid machine, includes the housing  11  that allows refrigerant serving as fluid to flow therein, the electric motor  23  that is accommodated in the housing  11 , and the driver  30  that drives the electric motor  23  and is supplied with DC power. The driver  30  includes the low-pass filter circuit  42  and the inverter circuit  41 . The low-pass filter circuit  42  reduces (attenuates) the common mode noise and the normal mode noise included in the DC power. The inverter circuit  41  converts the DC power from which the two noises have been reduced by the low-pass filter circuit  42  to AC power. The low-pass filter circuit  42  includes the looped ring core  51 , the common mode choke coil  50  that includes the two coils  64  and  74  wound around the ring core  51 , and the X capacitor  80  electrically connected to the common mode choke coil  50 . 
     The driver  30  further includes the damping unit  90  set at a position where the leakage flux Bx (magnetic field lines) produced at the common mode choke coil  50  generates the eddy current Ie at the damping unit  90 . The damping unit  90  changes the frequency characteristics of the phase difference θ of the common mode choke coil  50 . The resonant frequency f 0  of the low-pass filter circuit  42  is set to a value in the specific frequency range fb that is a frequency range in which the damping unit  90  decreases the phase difference θ. 
     The common mode choke coil  50  reduces the common mode noise included in the DC power supplied to the driver  30 . Further, normal mode current flows through the common mode choke coil  50  and produces the leakage flux Bx. This reduces the normal mode noise with the common mode choke coil  50  and the low-pass filter circuit  42  including the X capacitor  80  that are electrically connected to each other. Accordingly, there is no need to use a dedicated coil that reduces normal mode noise. Further, the inverter circuit  41  can be supplied with DC power from which common mode noise and normal mode noise have both been reduced. Thus, enlargement of the driver  30  can be avoided. This limits enlargement of the on-board motor-driven compressor  10 . 
     Further, the damping unit  90  lowers the Q factor of the low-pass filter circuit  42 . In detail, the magnetic field lines (leakage flux) at the common mode choke coil  50  generate eddy current at the damping unit  90  and lower the Q factor of the low-pass filter circuit  42 . This reduces the normal mode noise at frequencies close to the resonant frequency f 0  of the low-pass filter circuit  42 . Thus, the versatility is improved while limiting enlargement of the on-board motor-driven compressor  10 . 
     As described above, when the low-pass filter circuit  42  has a high Q factor, it will be difficult to reduce the normal mode noise at frequencies close to the resonant frequency f 0  of the low-pass filter circuit  42 . Thus, the low-pass filter circuit  42  that has a high Q factor will not effectively function on normal mode noise having frequencies close to the resonant frequency f 0 . This may result in erroneous operation of the driver  30  or shorten the life of the low-pass filter circuit  42 . Thus, when the Q factor of the low-pass filter circuit  42  is high, the low-pass filter circuit  42  cannot be applied to a vehicle model that generates normal mode noise having frequencies close to the resonant frequency f 0 . In the present embodiment, the damping unit  90  lowers the Q factor. This decreases the normal mode noise at frequencies close to the resonant frequency f 0 . More specifically, the resonant frequency f 0  of the low-pass filter circuit  42  can be included in the frequency range in which the low-pass filter circuit  42  is able to reduce the normal mode noise, that is, the frequency range to which the driver  30  is applicable. This widens the frequency range of the normal mode noise that can be reduced by the low-pass filter circuit  42  so that the on-board motor-driven compressor  10  can be applied to a wide variety of vehicle models. 
     To decrease the Q factor, for example, a damping resistor may be connected in series to the common mode choke coil  50 . However, a damping resistor needs to correspond to relatively large currents and is thus relatively large. This increases the power loss and generated heat. Thus, heat dissipation and the like need to be taken into consideration when connecting the damping resistor to the common mode choke coil  50 . This may result in enlargement of the on-board motor-driven compressor  10 . 
     In the present embodiment, the eddy current Ie is generated at the damping unit  90 . However, the eddy current Ie is smaller than the current that flows through a damping resistor. Thus, the damping unit  90  generates a smaller amount of heat. This limits enlargement of the on-board motor-driven compressor  10  and reduces the two types of noise while improving the versatility. 
     The inventors of the present invention have found that that the change in the frequency characteristics of the phase difference θ of the common mode choke coil  50 , which results from the damping unit  90 , and the decrease in the phase difference θ contribute to the effect for lowering the Q factor of the low-pass filter circuit  42  (hereafter referred to as “the damping effect”). By setting the value of the resonant frequency f 0  of the low-pass filter circuit  42  in the specific frequency range based on this observation, the phase difference θ of the common mode choke coil  50  can be decreased at frequencies close to the resonant frequency f 0 . 
     The resistance component of the damping unit  90  with respect to the leakage flux Bx varies in accordance with the phase difference θ. More specifically, the resistance component of the damping unit  90  increases as the phase difference θ decreases. Thus, the Q factor of the low-pass filter circuit  42  may be further decreased by setting the resonant frequency f 0  to a frequency at which the phase difference θ is small. In this regard, the damping unit  90  of the present embodiment sets the resonant frequency f 0  to a value in the specific frequency range fb in which the phase difference θ is small. This allows the Q factor of the low-pass filter circuit  42  to be further reduced. 
     The damping unit  90  has a first characteristic that generates the eddy current Ie to lower the Q factor and a second characteristic that changes the frequency characteristics of the phase difference θ of the common mode choke coil  50 . With regard to the second characteristic, the value of the resonant frequency f 0  can be set in the specific frequency range fb to further reduce the Q factor. 
     The Q factor of the low-pass filter circuit  42  is lowered to reduce the normal mode noise at frequencies close to the resonant frequency f 0 . Thus, the damping effect may be referred to as an effect for reducing normal mode noise at frequencies close to the resonant frequency f 0 . 
     (2) The damping unit  90  is located at a position that the leakage flux Bx produced by the common mode choke coil  50  penetrates. The damping unit  90  is configured so that the penetration of the leakage flux Bx results in the flow of the eddy current Ie generated by the magnetic flux By in the direction canceling the leakage flux Bx. This obtains advantage (1). 
     (3) The two coils  64  and  74  are opposed to each other in the X-axis direction that is orthogonal to the axial direction of the ring core  51  (Z-axis direction). The ring core  51  includes the first non-winding side surface  62   a  and the second non-winding side surface  72   a  that interest the Y-axis direction, which is orthogonal to both of the Z-axis direction and the X-axis direction. The damping unit  90  includes the first end wall  93 , which serves as the first opposing portion opposing the first non-winding side surface  62   a , and the second end wall  103 , which serves as the second opposing portion opposing the second non-winding side surface  72   a . In the common mode choke coil  50  in which the two coils  64  and  74  are opposed to each other in the X-axis direction, flux easily leaks from the two non-winding side surfaces  62   a  and  72   a , which intersect the Y-axis direction. Thus, the leakage flux Bx has a tendency to concentrate at the two non-winding side surfaces  62   a  and  72   a . In this regard, the end walls  93  and  103  are opposed to the non-winding side surfaces  62   a  and  72   a  in the present embodiment. Thus, the leakage flux Bx easily penetrates the two end walls  93  and  103 . In other words, the amount of the leakage flux Bx that does not penetrate the damping unit  90  can be reduced. Accordingly, the damping effect can be improved. 
     (4) The damping unit  90  includes the box-shaped parts  91  and  101 . The parts  91  and  101  include the end walls  93  and  103  and the peripheral walls  94  and  104 , respectively. The peripheral walls  94  and  104  extend from the end walls  93  and  103  and are frame-shaped so as to surround the common mode choke coil  50  as viewed in the Y-axis direction, which is the opposing direction of the end walls  93  and  103 . The distal ends  95  and  105  of the peripheral walls  94  and  104  define the openings  92  and  102 , respectively. In a state in which the openings  92  and  102  are opposed to each other, the two parts  91  and  101  cooperate to accommodate the common mode choke coil  50 . 
     The end walls  93  and  103  cover the non-winding side surfaces  62   a  and  72   a  where the leakage flux Bx has a tendency to concentrate. Thus, the leakage flux Bx easily penetrates the two end walls  93  and  103 . Further, the leakage flux Bx that penetrates the two end walls  93  and  103  generates the eddy current Ie at the peripheral walls  94  and  104 . The eddy current Ie flows in the circumferential direction of the frame-shaped peripheral walls  94  and  104 . That is, the eddy current Ie forms a closed loop as viewed in the Y-axis direction. This easily forms the magnetic flux By in a direction that cancels the leakage flux Bx flowing in the Y-axis direction (i.e., extending direction of first and second long sides  61  and  71 ). Thus, the damping effect can be further improved. 
     When accommodating the common mode choke coil  50  with the two box-shaped parts, for example, when coupling two parts to the common mode choke coil  50  from the X-axis direction, the peripheral wall will be frame-shaped as viewed in the X-axis direction and not frame-shaped as viewed in the Y-axis direction. Such a structure limits the generation of the eddy current Ie forming a closed loop at the peripheral wall as viewed in the Y-axis direction. This decreases magnetic flux in the direction canceling the leakage flux Bx. 
     In the present embodiment, as described above, the existence of the peripheral walls  94  and  104  that are frame-shaped as viewed in the Y-axis direction will easily generate the eddy current Ie that forms a closed loop at the peripheral walls  94  and  104 . This sufficiently generates the magnetic flux By in a direction that cancels the leakage flux Bx and further improves the damping effect. 
     In the present embodiment, the two parts  91  and  101  cooperate to accommodate the common mode choke coil  50 . Thus, in comparison with a structure that accommodates the common mode choke coil  50  with a single part, the common mode choke coil  50  can be accommodated in a relatively easy manner. 
     When accommodating the common mode choke coil  50  with a single part, the part may include an opening so that the common mode choke coil  50  can be inserted from the opening. In this case, the damping unit will not entirely cover a single surface of the common mode choke coil  50 . This will adversely affect the Q factor lowering effect (hereafter referred to as the damping effect) of the low-pass filter circuit  42 . 
     For example, when the damping unit has a size that allows for the accommodation of the entire common mode choke coil  50  and is formed by a single part including an opening directed in the Y-axis direction and enabling the insertion of the common mode choke coil  50 , one of the two non-winding side surfaces  62   a  and  72   a  will not be covered by the damping unit. Thus, it will be difficult for the leakage flux Bx to penetrate the damping unit. Further, for example, when the damping unit has a size that allows for the accommodation of the entire common mode choke coil  50  and is formed by a single part including an opening directed in the X-axis direction or the Z-axis direction and enabling the insertion of the common mode choke coil  50 , the damping unit will not be looped and closed as viewed in the Y-axis direction and will be U-shaped and open at one side. This will hinder the flow of the eddy current Ie, which forms a closed loop as viewed in the Y-axis direction, through the damping unit. In contrast, the damping unit  90  of the present embodiment is formed by the two parts  91  and  101 . Thus, the above problem does not occur. 
     (5) The peripheral walls  94  and  104  are frame-shaped and do not include gaps or slits as viewed in the Y-axis direction. Thus, the eddy current Ie that flows through the peripheral walls  94  and  104  is not hindered by gaps or slits. This increases the eddy current Ie, which, in turn, increases the damping effect. 
     (6) The gap  112  is formed between the two distal ends  95  and  105 . This limits variations in the opposing distance Y 1  of the non-winding side surfaces  62   a  and  72   a  from the end walls  93  and  103  caused by dimensional errors of the two parts  91  and  101  and the common mode choke coil  50 . Thus, variations are limited in damping effect of the two parts  91  and  101 . 
     More specifically, the damping effect produced by the two parts  91  and  101  varies in accordance with the opposing distance Y 1  of the non-winding side surfaces  62   a  and  72   a  from the end walls  93  and  103 . Thus, there is a need to keep the opposing distance Y 1  constant in order to obtain a stable damping effect. 
     When the two parts  91  and  101  are formed so that the two distal ends  95  and  105  are not spaced apart by the gap  112 , the two parts  91  and  101  can be positioned when the two distal ends  95  and  105  come into contact with each other. In this case, the opposing distance Y 1  may vary because of dimensional errors of the two parts  91  and  101  and the common mode choke coil  50 . 
     In the present embodiment, the gap  112  is formed between the two distal ends  95  and  105 . Thus, the two parts  91  and  101  are not positioned by contact of the two distal ends  95  and  105 . This allows the gap  112  to vary in correspondence with the dimensional errors described above to keep the opposing distance Y 1  constant. Thus, the advantages described above can be obtained. 
     (7) The insulator  111  is located between the end walls  93  and  103  and the non-winding side surfaces  62   a  and  72   a . The two parts  91  and  101  are positioned in a state in which the non-winding side surfaces  62   a  and  72   a  and the end walls  93  and  103  are in contact with the insulator  111 . This allows the opposing distance Y 1  to be decreased and improves the damping effect. 
     (8) The damping unit  90  does not cover the portion of the common mode choke coil  50  corresponding to the gap  112 . This may lower the damping effect. In this regard, in the present embodiment, the gap  112  is located at a position corresponding to the central portions  61   a  and  71   a  in the extending direction of the long sides  61  and  71  of the ring core  51 . The central portions  61   a  and  71   a  of the long sides  61  and  71  is where the coils  64  and  74  (i.e., high-density portions  64   a  and  74   a ) exist. At such portions, the leakage of magnetic flux is limited. This limits decreases in the damping effect even if the gap  112  is formed between the two distal ends  95  and  105 . 
     (9) The driver  30  is provided with the circuit board  40  that includes the inverter circuit  41  and the low-pass filter circuit  42 . The peripheral walls  94  and  104  include the recesses  96  and  106  that extend from the distal ends  95  and  105  toward the end walls  93  and  103  to intermediate positions of the peripheral walls  94  and  104 . The first input terminal  65  and the first output terminal  66  that extend from the first coil  64  and the second input terminal  75  and the second output terminal  76  that extend from the second coil  74  are extended through one of the two recesses  96  and  106  and inserted into the terminal holes  40   b  of the circuit board  40 . This electrically connects the common mode choke coil  50  and the circuit board  40 . 
     When just electrically connecting the common mode choke coil  50  and the circuit board  40 , the peripheral walls  94  and  104  may include slits extending from the distal ends  95  and  105  to the end walls  93  and  103 . However, when the peripheral walls  94  and  104  includes such slits, it will be difficult to form a looped that is closed as viewed in the Y-axis direction in the two parts  91  and  101 . This limits the generation of the eddy current Ie in the two parts  91  and  101 . In this regard, in the present embodiment, the recesses  96  and  106  extend to intermediate positions of the peripheral walls  94  and  104 . Thus, at least the portions of the peripheral walls  94  and  104  corresponding to the side of the end walls  93  and  103  have the form of a closed frame. This forms a closed loop through which the eddy current flows in the peripheral walls  94  and  104  and obtains the damping effect. 
     The portions of the two peripheral walls  94  and  104  at the side corresponding to the distal ends  95  and  105  contributes less to the damping effect than the portions of the two peripheral walls  94  and  104  at the side corresponding to the end walls  93  and  103  and the end walls  93  and  103  of the two parts  91  and  101 . Thus, even when the peripheral walls  94  and  104  include the recesses  96  and  106 , the damping effect does not decrease. Accordingly, the common mode choke coil  50  and the circuit board  40  can be electrically connected while decreasing the damping effect. 
     (10) The terminals  65 ,  66 ,  75 , and  76  are located closer to the central part of the common mode choke coil  50  than the two non-winding side surfaces  62   a  and  72   a . This allows the recesses  96  and  106  to have smaller dimensions. Thus, the cross-sectional area of the eddy current Ie, which flows in the circumferential direction of the peripheral walls  94  and  104 , can be increased. This limits decreases in the damping effect caused by the recesses  96  and  106 . 
     (11) The inverter circuit  41  includes the switching elements Qu 1  to Qw 2 , and the switching elements Qu 1  to Qw 2  are PWM-controlled to convert DC power to AC power. Further, the cutoff frequency fc of the low-pass filter circuit  42  is set to be lower than the carrier frequency fp, which is the frequency of the carrier signal used to PWM-control the switching elements Qu 1  to Qw 2 . This reduces (attenuates) ripple noise, which results from switching of the switching elements Qu 1  to Qw 2 , with the low-pass filter circuit  42  and limits the ripple noise that is released from the on-board motor-driven compressor  10 . More specifically, the low-pass filter circuit  42  functions to reduce the normal mode noise and common mode noise that enters the on-board motor-driven compressor  10  during operation of the PCU  204  and functions to reduce the ripple noise that is released during operation of the on-board motor-driven compressor  10 . 
     When widening the frequency range of the normal mode noise that can be reduced by the low-pass filter circuit  42 , the resonant frequency f 0  may be set to be higher than the expected frequency range of the normal mode noise to avoid the occurrence of resonance. However, this will also increase the cutoff frequency fc of the low-pass filter circuit  42 . Thus, it will be difficult for the cutoff frequency fc to be lower than the carrier frequency fp. Further, a situation in in which the carrier frequency fp increases as the cutoff frequency fc rises is not preferable because this will increase the switching loss of the switching elements Qu 1  to Qw 2 . 
     In the present embodiment, the damping unit  90  reduces the normal mode noise at frequencies close to the resonant frequency f 0 . Thus, there is no need to increase the resonant frequency f 0  in accordance with the expected frequency range of the normal mode noise. Accordingly, the cutoff frequency fc can be lower than the carrier frequency fp without increasing the carrier frequency fp in excess. This limits the release or ripple noise, which results from the switching of the switching elements Qu 1  to Qw 2 , from the on-board motor-driven compressor  10 , without increasing the power loss of the inverter circuit  41 . 
     (12) The frequency characteristics of the phase difference θ of the common mode choke coil  50  changed by the damping unit  90  includes the maximum value θm and the minimum value en. In this configuration, the resonant frequency f 0  of the low-pass filter circuit  42  is set to a value closer to the minimum frequency fn, which corresponds to the minimum value en, than the maximum frequency fm, which corresponds to the maximum value θm. This decreases the phase difference θ of the common mode choke coil  50  at frequencies close to the resonant frequency f 0 . Thus, normal mode noise at frequencies close to the resonant frequency f 0  can be reduced in a further suitable manner. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms. 
     The two parts  91  and  101  may accommodate the common mode choke coil  50  from a diagonal direction that extends orthogonal to the Z-axis direction and intersects both of the X-axis direction and the Y-axis direction. The two parts  91  and  101  may also accommodate the common mode choke coil  50  from the Z-axis direction or from the X-axis direction. The opposing direction in which the openings  92  and  102  oppose each other is not limited to the Y-axis direction and may be any direction. 
     The first coil  64  does not have to be wound around the first corners  63  and may be wound around only the first long side  61 . It is only necessary that at least a portion of the first coil  64  be wound around the first long side  61 . The same applies to the second coil  74 . 
     The first coil  64  may be wound around the first short side  62  instead of the first long side  61 . In the same manner, the second coil  74  may be wound around the second short side  72  instead of the second long side  71 . In this case, the two parts of the damping unit may be coupled to the common mode choke coil  50  from the X-axis direction. 
     The two coils  64  and  74  may be wound around the entire ring core  51 . More specifically, the non-winding portions around which the coils are not wound may be omitted from the ring core  51 . In other words, the two coils  64  and  74  may be wound around the side surfaces  62   a  and  72   a  of the short sides  62  and  72  that are planes intersecting the Y-axis direction, which is orthogonal to both of the axial direction of the ring core  51  and the opposing direction of the two coils  64  and  74 . In this case, the leakage flux Bx has a tendency to concentrate at the side surfaces  62   a  and  72   a.    
     The ring core  51  may be circular and formed without the corners  63  and  73 . In this case, the winding density of the two coils  64  and  74  may be fixed. That is, the coils  64  and  74  do not necessarily have to include both of the high-density portions  64   a  and  74   a  and the low-density portions  64   b  and  74   b.    
     The end walls  93  and  103  and the peripheral walls  94  and  104  may include gaps, slits, or through holes. Further, the two parts  91  and  101  may be at least partially meshed, recessed, embossed, or holed. In this manner, the peripheral walls  94  and  104  do not need to have the form of a completely closed frame. 
     The two parts  91  and  101  are identical in shape. Instead, for example, the two peripheral walls  94  and  104  may have different dimensions in the Y-axis direction. 
     The two parts  91  and  101  may include overlapping portions. For example, the distal ends  95  and  105  of the two parts  91  and  101  may be overlapped with each other. In this case, the peripheral wall of one of the two parts  91  and  101  may be larger than that of the other one so that the two distal ends  95  and  105  do not abut against each other. Thus, the gap  112  is not necessary. 
     The gap  112  does not have to be located at a position corresponding to where the central portions  61   a  and  71   a  of the long sides  61  and  71  are located in the Y-axis direction and may be located closer to the central portions  61   a  and  71   a  than the two non-winding side surfaces  62   a  and  72   a  or any other position. 
     Instead of the recesses  96  and  106 , the end walls  93  and  103  or the peripheral walls  94  and  104  may include through holes. In this case, the terminals  65 ,  66 ,  75 , and  76  may be extended through the through holes and inserted through the terminal holes  40   b  of the circuit board  40 . Further, the recesses  96  and  106  may be omitted, and the terminals  65 ,  66 ,  75 , and  76  may be extended through the gap  112 . 
     The non-winding side surfaces  62   a  and  72   a  and the end walls  93  and  103  do not have to be in contact with the insulator  111 . For example, the non-winding side surfaces  62   a  and  72   a  may be spaced apart from the insulator  111 . Alternatively, the end walls  93  and  103  may be spaced apart from the insulator  111 . 
     As shown in  FIG. 11 , a damping unit  120  may be defined by two extended portions  121  and  122  extending from the housing  11  (i.e., end wall  12   a ) toward the circuit board  40 . In the same manner as the housing  11 , the two extended portions  121  and  122  are formed from a non-magnetic conductive material (e.g., aluminum) and formed integrally with the housing  11 . 
     The two extended portions  121  and  122  are opposed to each other in the Y-axis direction. The first extended portion  121  is opposed to the first non-winding side surface  62   a . The insulator  111  is located between the first extended portion  121  and the first non-winding side surface  62   a . Further, the first extended portion  121  and the first non-winding side surface  62   a  are in contact with the insulator  111 . The second extended portion  122  is opposed to the second non-winding side surface  72   a . The insulator  111  is located between the second extended portion  122  and the second non-winding side surface  72   a . Further, the second extended portion  122  and the second non-winding side surface  72   a  are in contact with the insulator  111 . The non-winding side surfaces  62   a  and  72   a  are covered by the extended portions  121  and  122 . In this case, the flux leakage Bx penetrates the two extended portions  121  and  122  and generates the eddy current Ie that interferes with the leakage flux Bx at the extended portions  121  and  122 . This lowers the Q factor of the low-pass filter circuit  42  and changes the frequency characteristics of the phase difference θ of the common mode choke coil  50  to obtain the specific frequency range fb. Thus, advantage (1) is obtained. In this manner, the damping unit  90  does not need the peripheral walls  94  and  104 , and the two parts do not have to be box-shaped. 
     The parts  91  and  101  may be tubular and formed without the end walls  93  and  103 . The leakage flux Bx partially penetrates the two parts  91  and  101 . Nevertheless, it is preferred that the parts  91  and  101  include the end walls  93  and  103  in order to increase the damping effect. 
     The damping unit may include a coupling portion that couples the two parts  91  and  101 . In other words, the damping unit does not have to be formed by two parts and may be formed by a single part. In this case, the damping unit preferably includes an opening that allows for the insertion of the common mode choke coil  50 . Nevertheless, it is preferred that the damping unit be formed by at least two parts so that the common mode choke coil  50  can easily be accommodated in the damping unit. 
     The cover member  31  does not need to have a tubular shape. For example, when the suction housing portion  12  includes an annular rib extending from the end wall  12   a  in a direction opposite to the side wall  12   b , the cover member  31  may be coupled to the suction housing portion  12  in a state contacting the rib. In this case, the end wall  12   a , the rib, and the cover member  31  define the accommodation compartment S 0 . In this manner, the accommodation compartment S 0  may be defined by any structure. 
     The ring core  51  may have any shape and be, for example, a UU core, an EE core, or a toroidal core. The ring core  51  does not need to have the form of a completely closed ring and may include a gap. 
     The circuit configuration of the low-pass filter circuit  42  is not limited to that of the above embodiment. For example, the low-pass filter circuit  42  may include two X capacitors  80 . Further, the low-pass filter circuit may be of any type such as a n-type or a T-type. 
     The Y capacitors  81  and  82  may be omitted. That is, the driver  30  does not necessarily have to include Y capacitors. Nevertheless, it is preferable that the Y capacitors be included since common noise can be reduced in a suitable manner. 
     The boost converter  205  may be omitted. In this case, the normal mode noise is, for example, noise resulting from the switching frequency of switching elements of a travel inverter. 
     The on-board device is not limited to the PCU  204  and may be any device including a switching element that is cyclically activated and deactivated. For example, the on-board device may be an inverter or the like that is separate from the driver  30 . 
     The on-board motor-driven compressor  10  is of an inline type but instead may be of, for example, a camelback type in which the driver  30  is arranged on the outer side of the housing  11  in the radial direction of the rotation shaft  21 . In this manner, the driver  30  may be located at any location. 
     The on-board motor-driven compressor  10  is used with the on-board air-conditioner  200 . Instead, for example, when a fuel cell is installed in a vehicle, the on-board motor-driven compressor  10  may be used with an air supply device that supplies air to the fuel cell. In this manner, the compressed subject is not limited to refrigerant and may be any fluid such as air. 
     The on-board fluid machine is not limited to the on-board motor-driven compressor  10  that includes the compression unit  22  and may be any device. For example, when the vehicle provided with the on-board fluid machine is a fuel cell vehicle, the on-board fluid machine may be an on-board electric pump that supplies hydrogen to the fuel cell. 
     The modified examples may be combined with each other or with the above embodiment. 
     One aspect that can be acknowledged from the above embodiment and the modified examples will now be described. 
     (A) An on-board fluid machine including: 
     a housing configured to allow fluid to flow into the housing; 
     an electric motor accommodated in the housing; and 
     a driver that is supplied with DC power and drives the electric motor, wherein the driver includes
         a low-pass filter circuit configured to reduce common mode noise and normal mode noise that are included in the DC power, and   an inverter circuit configured to convert the DC power, from which the common mode noise and the normal mode noise have been reduced, to AC power, wherein   the low-pass filter circuit includes
           a common mode choke coil including a ring core and a first coil and a second coil that are wound around the ring core, and   a capacitor electrically connected to the common mode choke coil,   
               

     wherein the driver further includes a damping unit located at a position penetrated by leakage flux that is produced at the common mode choke coil, the damping unit is configured so that the penetration of the leakage current through the damping unit generates a flow of eddy current, and the damping unit changes a frequency characteristic of a phase difference of the common mode choke coil, and a resonant frequency of the low-pass filter circuit is set in a frequency range in which a phase difference of the common mode choke coil has been decreased by the damping unit. 
     The phrase of “the damping unit is configured so that the penetration of the leakage current through the damping unit generates a flow of eddy current” indicates that, for example, the first part and the second part are formed from a non-magnetic conductive material. 
     The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.