Patent Publication Number: US-2016245292-A1

Title: Motor-driven compressor

Description:
TECHNICAL FIELD 
     The present invention relates to a motor-driven compressor including an electric motor that drives a compression mechanism. 
     BACKGROUND ART 
     Japanese Laid-Open Patent Publication No. 2011-247215 discloses an example of a motor-driven compressor. The motor-driven compressor includes a compression mechanism, which draws in refrigerant and compresses the refrigerant, an electric motor, which drives the compression mechanism, a housing, which accommodates the compression mechanism and the motor, and a drive circuit unit, which drives the motor. The drive circuit unit includes a circuit board formed by a multi-layer board. Electronic components such as power elements are arranged on the circuit board. 
     In the above motor-driven compressor, when current fluctuation occurs in an electronic component of the drive circuit unit, the electronic component generates noise that leaks from the drive circuit unit. For example, when a vehicle air conditioner is activated, noise radiated from an electronic component of the motor-driven compressor interferes with the signals received by an on-vehicle radio and affects the output of the radio. In particular, electronic components such as a transformer and a capacitor that have coils generate stronger noise than other electronic components and greatly affect peripheral devices. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a motor-driven compressor in which the noise that leaks to the surroundings of electronic components of a drive circuit unit is reduced. 
     One aspect of the present invention is a motor-driven compressor including a compression mechanism, an electric motor, a housing, a drive circuit unit, a cover body, and a shield. The compression mechanism has a compression chamber and compresses refrigerant. The electric motor drives the compression mechanism. The housing accommodates the compression mechanism and the electric motor. The drive circuit unit drives the electric motor. The drive circuit unit includes a multi-layer board and electronic components mounted on the multi-layer board. The multi-layer board includes a ground layer. The cover body covers the drive circuit unit. The cover body is arranged at an outer side of the housing. The shield encompasses at least some of the electronic components. The shield is electrically connected to the ground layer of the multi-layer board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a motor-driven compressor according to a first embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of a main section of the motor-driven compressor shown in  FIG. 1 . 
         FIG. 2A  is an enlarged view of the encircled portion shown in  FIG. 2 . 
         FIG. 3  is a cross-sectional view showing a main section of a motor-driven compressor according to a second embodiment of the present invention. 
         FIG. 3A  is an enlarged view of the encircled portion shown in  FIG. 3 . 
         FIG. 4  is a cross-sectional view showing a main section of a motor-driven compressor according to a third embodiment of the present invention. 
         FIG. 4A  is an enlarged view of the encircled portion shown in  FIG. 4 . 
         FIG. 5  is a cross-sectional view showing a main section of a motor-driven compressor according to a fourth embodiment of the present invention. 
         FIG. 6  is a cross-sectional view showing a main section of a motor-driven compressor according to a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A first embodiment of a motor-driven compressor will now be described with reference to the drawings. The first embodiment of the motor-driven compressor is an on-vehicle motor-driven compressor installed in a hybrid vehicle that travels by drive sources including an electric motor and an internal combustion engine. The motor-driven compressor forms part of a refrigerant circuit in a vehicle air conditioner. The vehicle air conditioner includes an electric motor, a condenser (not shown), a receiver (not shown), a cooling unit (not shown) including an expansion valve and an evaporator, and pipes (not shown) that connect these devices. 
       FIG. 1  shows a motor-driven compressor that includes a compression mechanism  11 , which compresses refrigerant that serves as fluid, and an electric motor  12 , which is used by the compressor to drive the compression mechanism  11 . The compression mechanism  11  and the electric motor  12  are integrated in the motor-driven compressor. The motor-driven compressor includes a metal housing  13 , which is formed from an aluminum metal material in the first embodiment. 
     The housing  13  includes a first housing body  14  and a second housing body  15 . An end surface of the first housing body  14  is joined with an end surface of the second housing body  15 . Bolts  16  integrally fix the first housing body  14  and the second housing body  15 . The first housing body  14  includes a cylindrical portion  17  and an end wall  18 , which is formed integrally with the cylindrical portion  17  to close one end of the cylindrical portion  17 . In other words, the first housing body  14  is cylindrical and has a closed end. The motor-driven compressor of the first embodiment is transversely mounted in an engine compartment. 
     The compression mechanism  11  and the electric motor  12  are accommodated in the first housing body  14 . In the cylindrical portion  17 , a suction port  19  is located near the end wall  18 . The suction port  19  is connected to an external refrigerant circuit (not shown), which is in communication with the inner side of the first housing body  14 . During a compression operation of the motor-driven compressor, low-pressure refrigerant from the external refrigerant circuit flows through the suction port  19  into the first housing body  14 . 
     A discharge chamber  20 , which is in communication with the compression mechanism  11 , is defined in the second housing body  15 . The upper side of the second housing body  15  includes a discharge port  21 , which is in communication with the external refrigerant circuit. The second housing body  15  includes a communication passage  22 , which allows communication between the discharge chamber  20  and the discharge port  21 . During a compression operation of the motor-driven compressor, high-pressure refrigerant is discharged out of the compression mechanism  11  into the discharge chamber  20 . Then, the high-pressure refrigerant flows through the communication passage  22 , reaches the discharge port  21 , and enters the external refrigerant circuit. 
     The compression mechanism  11  includes a fixed scroll  23 , which is fixed to the inner side of the first housing body  14 , and a movable scroll  24 , which orbits with respect to the fixed scroll  23 . Compression chambers  25  are defined between the fixed scroll  23  and the movable scroll  24 . The refrigerant drawn from the suction port  19  into the first housing body  14  enters the compression chambers  25 . The electric motor  12  is driven so that the movable scroll  24  orbits with respect to the fixed scroll  23 . This changes the volumes of the compression chambers  25 . 
     A support  26  is arranged between the electric motor  12  and the fixed scroll  23  in the first housing body  14 . The support  26  forms part of the compression mechanism  11  and supports one end of a rotation shaft  27  of the electric motor  12 . The other end of the rotation shaft  27  is supported by a bearing with respect to the end wall  18 . The end wall  18  includes a flat outer surface  28 , which extends in a direction orthogonal to the axis of the rotation shaft  27 . 
     The electric motor  12  is driven by three-phase alternating-current power. The electric motor  12  includes a stator  29  and a rotor  30 , which is fitted into the stator  29  and fixed to the rotation shaft  27 . The stator  29  includes a stator core  31 , which is fixed to an inner wall of the first housing body  14 , and U-phase, V-phase, and W-phase stator coils  32 , which are wound around the stator core  31 . One end of a wire that forms the stator coil  32  of each phase is drawn from the stator coil  32  as a lead wire  33 , which receives power that is supplied from a drive circuit unit  36  (described below). 
     A base  34  and a cover body  35  are arranged on the outer side of the housing  13 . The base  34  is arranged on the outer surface  28  (surface opposite to the surface facing electric motor  12 ) of the end wall  18  of the first housing body  14 . The cover body  35  is arranged on the base  34 . The base  34  and the cover body  35  define an accommodation cavity  37 , which accommodates the drive circuit unit  36  that drives the electric motor  12 . The base  34  and the cover body  35  are fixed to the first housing body  14  by a bolt  38 . The cover body  35 , which covers the drive circuit unit  36 , is formed from an aluminum metal material in the same manner as the first housing body  14 . The drive circuit unit  36  is arranged on the outer surface  28  of the end wall  18  of the first housing body  14  and accommodated in the accommodation cavity  37 . The drive circuit unit  36  includes a power module  39 , which includes switching elements, other electronic components (only transformer  40  is shown in  FIGS. 1 and 2 ), and a multi-layer board  41 , on which electronic components are mounted. The power module  39  is supplied with power for driving the motor-driven compressor from the outside and converts the supplied direct-current power into alternating-current power. 
     The base  34  of the first embodiment is formed from an aluminum metal material having superior thermal conductance. The base  34  has the form of a plate and includes a first flat surface  42 , which abuts against the outer surface  28  of the end wall  18 , and a second flat surface  43 , which is located at the side opposite to the first flat surface  42 . Since the first flat surface  42  abuts against the end wall  18  of the first housing body  14 , the base  34  is electrically connected to the first housing body  14 , which is electrically grounded. The power module  39  is arranged on (in contact with) the second flat surface  43 . This allows heat to be released from the base  34  to the end wall  18 , the heat being generated during activation of power module  39 . Legs  44  extend toward the multi-layer board  41  from the second flat surface  43  of the base  34 . A threaded hole  46  is formed at the center of each leg  44 . A bolt  45  is fastened to each threaded hole  46  to fix the multi-layer board  41  to the leg  44 . 
     As shown in  FIGS. 2 and 2A , the multi-layer board  41  of the first embodiment includes four-layer wiring layers  51  to  54 . In the following description, the four-layer wiring layers  51  to  54  will be described as, in sequence from the one which is most distant from the base  34  to the one which is closest to the base  34 : the first wiring layer  51 , the second wiring layer  52 , the third wiring layer  53 , and the fourth wiring layer  54 . Insulating layers  55  are arranged between the first wiring layer  51  and the second wiring layer  52 , between the second wiring layer  52  and the third wiring layer  53 , and between the third wiring layer  53  and the fourth wiring layer  54 , respectively. The insulating layers  55  are formed by an insulator. The wiring layers  51  to  54  have conductive wiring patterns that are formed from copper foil. In the first embodiment, the first and fourth wiring layers  51  and  54  are used for communication, the second wiring layer  52  is used for power supply, and the third wiring layer  53  is used for ground. The second wiring layer  52  is electrically connected to the power module  39 . The third wiring layer  53  of the first embodiment corresponds to a ground layer of the multi-layer board  41  and is electrically connected to the electrically-grounded first housing body  14  through the bolts  45  and the base  34 . 
     As shown in  FIG. 2 , the transformer  40 , which serves as an electronic component, is coupled to a surface of the fourth wiring layer  54  that is located at the side opposite to the third wiring layer  53 . The transformer  40  of the first embodiment has the form of a cylindrical column. Electronic components other than the transformer  40  (not shown) are mounted on the surface of the first wiring layer  51  that is located at the side opposite to the second wiring layer  52  and on the surface of the fourth wiring layer  54  that is located at the side opposite to the third wiring layer  53 . As shown in  FIG. 2 , the multi-layer board  41  includes through holes  56 , which are formed at locations corresponding to the threaded holes  46  of the legs  44 . The through holes  56  are aligned with the legs  44 , and the bolts  45  are inserted through the through holes  56  and fastened to the threaded holes  46  of the legs  44  to fasten and fix the multi-layer board  41  to the legs  44 . Each of the through holes  56  functions as a via (a via hole). The legs  44  and the third wiring layer  53  of the multi-layer board  41  are electrically connected through the bolts  45  that are inserted through the through holes  56 . 
     A shield  57  extends from the second flat surface  43  toward the multi-layer board  41 . As shown in  FIGS. 1 and 2 , the shield  57  has the form of a cylinder and substantially encompasses the entire transformer  40  except for the surface of the transformer  40  that is opposed to the multi-layer board  41 . The shield  57  is electrically connected to the third wiring layer  53  by electrically connecting the legs  44  to the third wiring layer  53  of the multi-layer board  41 . The shield  57 , which encompasses the transformer  40 , reduces leakage of noise radiated from the transformer  40  to the surroundings of the transformer  40 . That is, by arranging the shield  57  on the base  34 , the base  34  not only functions as a heat-releasing member that releases heat from the power module  39  but also functions to reduce leakage of noise radiated from the transformer  40 . 
     An insulating gap G 1  extends between the shield  57  and the circumferential surface of the transformer  40 . An insulating gap G 2  extends between the top of the transformer  40  and the base  34 . In the first embodiment, a gap G 3  extends between the end  58  of the shield  57  that is opposed to the multi-layer board  41  and the surface of the multi-layer board  41  (fourth wiring layer  54 ). This separates the shield  57  from the multi-layer board  41 . The gap G 3  allows for relative movement of the shield  57  and the multi-layer board  41 . Thus, even when the motor-driven compressor vibrates, physical contact is avoided between the shield  57  and the multi-layer board  41 . 
     The operation of the motor-driven compressor of the first embodiment will now be described. When power is supplied from the drive circuit unit  36  to the electric motor  12 , the electric motor  12  is driven to activate the compression mechanism  11 . As a result, the refrigerant drawn through the suction port  19  into the first housing body  14  enters the compression chambers  25 . As the compression chambers  25  decrease in volume, the refrigerant in the compression chambers  25  is compressed and discharged to the discharge chamber  20 . 
     When the motor-driven compressor is driven, the power module  39  performs a switching operation that generates heat. The heat generated by the power module  39  is released through the base  34  to the end wall  18  of the first housing body  14 . In this manner, heat is released from the power module  39 . 
     When the motor-driven compressor is driven, current fluctuation occurs in the drive circuit unit  36 . This fluctuates the magnetic field intensity of the transformer  40  and generates noise. By transmitting the noise radiated from the transformer  40  to the first housing body  14 , the shield  57  reduces leakage of the radiated noise to the surroundings of the transformer  40 . The noise radiated from the transformer  40  toward the multi-layer board  41  is transmitted to the first housing body  14  through the third wiring layer  53  and the legs  44 . 
     The shield  57  is separated from the multi-layer board  41 . Thus, even if the motor-driven compressor vibrates when driven, the shield  57  and the multi-layer board  41  relatively move within the range of the gap G 3  and thus do not physically contact each other. Further, since the shield  57  encompasses the transformer  40 , the transformer  40  is protected from impacts applied from the outside. 
     The motor-driven compressor of the first embodiment has the advantages described below. 
     (1) The shield  57  projecting from the base  34  encompasses the transformer  40 . Thus, most of the noise radiated from the transformer  40  is blocked by the shield  57  and transmitted to the grounded first housing body  14  through the base  34  that has the shield  57 . This reduces leakage of the radiated noise to the surroundings of the transformer  40 . Since leakage of the noise radiated from the transformer  40  is reduced, other wires and electronic components are less likely to receive the radiated noise. 
     (2) The legs  44  that fix the multi-layer board  41  to the first housing body  14  are electrically connected to the third wiring layer  53 , which serves as a ground layer of the multi-layer board  41 . Thus, the noise radiated from the transformer  40  toward the multi-layer board  41  is transmitted to the first housing body  14  through the third wiring layer  53  and the base  34 , which has the legs  44 . 
     (3) The shield  57  is separated from the multi-layer board  41 . Thus, even when the motor-driven compressor vibrates, the shield  57  and the multi-layer board  41  relatively move within the range of the distance between the shield  57  and the multi-layer board  41  (gap G 3 ). This avoids damage of the shield  57  and the multi-layer board  41  that would be caused by the vibration. Further, as compared to when the shield  57  is joined with the multi-layer board  41 , the motor-driven compressor of the first embodiment does not require a means or operation for joining the shield  57  with the multi-layer board  41 . This facilitates assembling of the motor-driven compressor. 
     (4) The shield  57  is located close to the transformer  40  within a range that ensures insulation of the shield  57  from the transformer  40 . This allows heat to be released from the transformer  40  through the shield  57 . As a result, the transformer  40  is easily cooled. This eases heat-withstanding conditions of the transformer  40  and allows the transformer  40  to be smaller than conventional transformers. Employing a compact transformer  40  reduces the size of the motor-driven compressor. 
     (5) The shield  57  encompasses the transformer  40 . Thus, the transformer  40  is physically protected by the shield  57  and is less likely to receive physical damage. For example, even if the motor-driven compressor receives impact when hit, the shield  57  is first damaged. This reduces damage to the transformer  40 . 
     Second Embodiment 
     A motor-driven compressor of a second embodiment will now be described. In the second embodiment, a shield is fixed to the multi-layer board. In the second embodiment, the same reference numerals are given to those components that are the same as the corresponding components of the first embodiment. Such components will not be described in detail. 
     As shown in  FIGS. 3 and 3A , a base  64  and a shield  65  differ in form from the corresponding components of the first embodiment. More specifically, the shield  65  includes an upright portion  66  and a flange  61 . The upright portion  66  projects from the second flat surface  43  of the base  64  toward the multi-layer board  41 . The flange  61  extends from the end  67  of the upright portion  66 , which is opposed to the multi-layer board  41 , along the surface of the multi-layer board  41  (fourth wiring layer  54 ). The flange  61  includes threaded holes  62 , and the multi-layer board  41  includes the through holes  56 , which are arranged at locations corresponding to the threaded holes  62  of the flanges  61 . Bolts  63  are inserted through the through holes  56  and fastened to the threaded holes  62  to join the shield  65  and the multi-layer board  41 . Each of the through holes  56  functions as a via (a via hole) in the same manner as the first embodiment, and the bolts  63  electrically connect the shield  65  and the third wiring layer  53 . 
     The second embodiment has advantages (1), (2), (4), and (5) of the first embodiment. Further, the flange  61  of the shield  65  increases the joining areas of the shield  65  and the multi-layer board  41 . This reinforces the joining of the shield  65  and the multi-layer board  41  with the bolts  63 . As a result, even when the motor-driven compressor vibrates, damage caused by the vibration is avoided in the shield  65  and the multi-layer board  41 . The bolts  63  electrically connect the shield  65  and the third wiring layer  53 , which serves as a ground layer of the multi-layer board  41 . Thus, the noise radiated from the transformer  40  toward the multi-layer board  41  is easily transmitted to the electrically-grounded first housing body  14  through the third wiring layer  53  and the base  64 , which has the shield  65 . 
     Third Embodiment 
     A motor-driven compressor of a third embodiment will now be described. In the third embodiment, a shield is arranged on the end wall of the first housing body. In the third embodiment, the same reference numerals are given to those components that are the same as the corresponding components of the first embodiment. Such components will not be described in detail. 
     As shown in  FIGS. 4 and 4A , a shield  72  is arranged on the end wall  18  of the first housing body  14  and extends from the outer surface  28  toward the multi-layer board  41 . The shield  72  has the form of a cylinder and substantially encompasses the entire transformer  40  except for the surface of the transformer  40  that is opposed to the multi-layer board  41 . The shield  72 , which encompasses the transformer  40 , blocks noise radiated from the transformer  40 . That is, the function of reducing leakage of noise radiated from the transformer  40  is added to the first housing body  14 . The base  74  does not have a shield and instead includes openings  71 , through which the shield  72  is inserted. The base  74  and the first housing body  14  are electrically connected to each other. Thus, the shield  72  is electrically connected to the third wiring layer  53  through the bolts  45  and the base  74 , which has the legs  44 . 
     An insulating gap G 1  extends between the shield  72  and the circumferential surface of the transformer  40 . An insulating gap G 2  extends between the top of the transformer  40  and the end wall  18 . In the third embodiment, a gap G 3  extends between the end  73  of the shield  72  that is opposed to the multi-layer board  41  and the surface of the multi-layer board  41  located at the side corresponding to the fourth wiring layer  54 . This separates the shield  72  from the multi-layer board  41 . The gap G 3  allows for relative movement of the shield  72  and the multi-layer board  41 . Thus, even when the motor-driven compressor vibrates, physical contact is avoided between the shield  72  and the multi-layer board  41 . 
     In the third embodiment, the shield  72  projecting from the end wall  18  of the first housing body  14  encompasses the transformer  40 . Thus, most of the noise radiated from the transformer  40  is transmitted to the first housing body  14 , which has the shield  72 . This reduces leakage of the radiated noise to the surroundings of the transformer  40 . Further, the third embodiment has advantages (2) to (5) of the first embodiment. 
     Fourth Embodiment 
     A motor-driven compressor of a fourth embodiment will now be described. In the fourth embodiment, a shield is arranged on the cover body. In the fourth embodiment, the same reference numerals are given to those components that are the same as the corresponding components of the first embodiment. Such components will not be described in detail. 
     As shown in  FIG. 5 , the transformer  40  of the fourth embodiment is coupled to the surface of the multi-layer board  41  that is opposed to the cover body  35 . A shield  81  extends from an inner surface of the cover body  35  (surface of cover body  35  opposed to multi-layer board  41 ) toward the multi-layer board  41 . The shield  81  has the form of a cylinder and substantially encompasses the entire transformer  40  except for the surface of the transformer  40  that is opposed to the multi-layer board  41 . The shield  81 , which encompasses the transformer  40 , blocks noise radiated from the transformer  40  and reduces leakage of the radiated noise to the surroundings of the transformer  40 . That is, the function of reducing leakage of noise radiated from the transformer  40  is added to the cover body  35 . The end of the shield  81  of the fourth embodiment is separated from the multi-layer board  41  in the same manner as the first embodiment. Further, since the base  34  and the first housing body  14  are electrically connected, the shield  81  is electrically connected to the third wiring layer  53  through the base  34  and the bolt  45 . 
     In the fourth embodiment, the shield  81  projecting from the cover body  35  encompasses the transformer  40 . Thus, most of the noise radiated from the transformer  40  is blocked by the shield  81  and transmitted to the electrically-grounded first housing body  14  through the base  34  and the cover body  35 , which has the shield  81 . This reduces leakage of the radiated noise to the surroundings of the transformer  40 . Further, the fourth embodiment has advantages (2) to (5) of the first embodiment. 
     Fifth Embodiment 
     A motor-driven compressor of a fifth embodiment will now be described. In the fifth embodiment, a base having a shield is fixed to the cover body. In the fifth embodiment, the same reference numerals are given to those components that are the same as the corresponding components of the first embodiment. Such components will not be described in detail. 
     As shown in  FIG. 6 , the motor-driven compressor of the fifth embodiment includes a base  91 , which serves as a heat-releasing member. The base  91  is arranged on the inner surface of the cover body  35  (surface of cover body  35  opposed to multi-layer board  41 ). The base  91  and the cover body  35  are fixed by bolts (not shown). The base  91  is formed from an aluminum metal material having superior thermal conductance in the same manner as the cover body  35 . The base  91  has the form of a flat plate and includes a first flat surface  92 , which abuts against the inner surface of the cover body  35 , and a second flat surface  93 , which is located at the side opposite to the first flat surface  92 . The second flat surface  93  contacts the power module  39 , which is electrically connected to the multi-layer board  41 . Legs  94  extend toward the multi-layer board  41  from the second flat surface  93  of the base  91 . A threaded hole  95  is arranged at the center of each leg  94 . A bolt  45  is fastened to the threaded hole  95  to fix the multi-layer board  41  to the leg  94 . The legs  94  are electrically connected to the third wiring layer  53  of the multi-layer board  41 . 
     A shield  96  extends from the second flat surface  93  toward the multi-layer board  41 . The shield  96  has the form of a cylinder and substantially encompasses the entire transformer  40  except for the surface of the transformer  40  that is opposed to the multi-layer board  41 . Since the shield  96  is arranged on the base  91 , the base  91  not only functions to release heat but also reduce leakage of noise radiated from the transformer  40 . 
     An insulating gap G 1  extends between the shield  96  and the circumferential surface of the transformer  40 . An insulating gap G 2  extends between the top of the transformer  40  and the base  91  (reference numerals G 1  and G 2  are omitted in  FIG. 6 ). In the fifth embodiment, a gap G 3  extends between the end  97  of the shield  96  that is opposed to the multi-layer board  41  and the surface of the multi-layer board  41  located at the side corresponding to the first wiring layer  51 . This separates the shield  96  from the multi-layer board  41 . The gap G 3  allows for relative movement of the shield  96  and the multi-layer board  41 . Thus, even when the motor-driven compressor vibrates, physical contact is avoided between the shield  96  and the multi-layer board  41 . 
     In the fifth embodiment, the shield  96  projecting from the base  91  encompasses the transformer  40 . Thus, most of the noise radiated from the transformer  40  is transmitted to the electrically-grounded first housing body  14  through the shield  96  and the base  91 . This reduces leakage of the radiated noise to the surroundings of the transformer  40 . Further, the fifth embodiment has advantages (2) to (5) of the first embodiment. 
     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. 
     In the above embodiments, the drive circuit unit  36  is arranged on the outer surface of the end wall  18  of the first housing body  14 . However, the drive circuit unit  36  does not have to be arranged on the outer surface of the end wall  18  of the first housing body  14 . As long as the drive circuit unit is located outside the housing, the drive circuit unit may be arranged anywhere. For example, the drive circuit unit of the motor-driven compressor may be arranged on the outer surface of the cylindrical portion of the first housing body. 
     The motor-driven compressor of the third embodiment includes the base  74 . However, as long as the shield  72  and the third wiring layer  53  are electrically connected, there is no need for the base  74 . For example, the shield  72  may be electrically connected to the third wiring layer  53  by bolts. 
     The motor-driven compressor of the fourth embodiment includes the base  34 . However, as long as the shield  81  and the third wiring layer  53  are electrically connected, the motor-driven compressor does not have to include the base  34 . For example, the shield  81  may be electrically connected to the third wiring layer  53  by bolts. 
     In the above embodiments, the electronic component is a transformer. However, this is exemplary only. The electronic component may be a capacitor. In particular, the present invention is effective for electronic components that generate a large noise. Further, a plurality of electronic components may be shielded by a shield. 
     In the above embodiments, the shield is formed to be cylindrical in accordance with the column-shaped electronic component (transformer). However, the shield may have any shape as long as the shield covers the electronic component. For example, the shield may be polygonal. 
     In the above embodiments, a gap extends between the shield and the electronic component (transformer). However, this is exemplary only. For example, an insulating resin having superior thermal conductance may be molded around the electronic component so that the resin abuts against the shield. This allows heat to be easily transmitted from the electronic component to the shield through the resin. Thus, the release of heat from the electronic component is facilitated. 
     In the second embodiment, the shield is joined with the multi-layer board by bolts. However, this is exemplary only. The shield may be joined with the multi-layer board by, for example, an adhesive or solder. Alternatively, conductive grease may be filled between the shield and the multi-layer board. 
     In the third to fifth embodiments, the shield is separated from the multi-layer board. However, the shield does not have to be separated from the multi-layer board. For example, the shield may include a flange that is joined with the multi-layer board in the same manner as the second embodiment. 
     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.