Abstract:
An electric motor includes a stator and a rotor. The stator includes a motor housing with an open end and an end cover mounted to the open end. The rotor includes a rotor shaft that extends through the end cover. The end cover includes a bearing that supports the rotor shaft. The rotor shaft and motor housing are made of an electrically conductive material and are electrically connected with each other. The motor housing, end cover, bearing and rotor shaft cooperatively form an electromagnetic shield suppressing electromagnetic interference generated by the motor. Optionally, the rotor shaft drives a coaxial worm shaft of a gearbox by an insulating member, which further suppresses electromagnetic interference.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 201410252289.6 filed in The People&#39;s Republic of China on Jun. 9, 2014, the entire contents of which are hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to an electric motor and in particular, to a motor and an actuator having the motor. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electric motors having a commutator and brushes generate electromagnetic radiation by the switching of current that occurs as the brushes slide over the commutator. This electromagnetic radiation may cause electromagnetic interference with sensitive devices located nearby. Actuators incorporating electric motors are frequently used in vehicles to control the position of movable elements such as air flaps in an air conditioning system. Electromagnetic radiation from the motors of these actuators is required to be kept below desired levels to reduce the effects of electromagnetic interference on nearby computer systems. Currently, a filtering circuit is used to suppress the electromagnetic radiation but these components are expensive and as the desired level of allowable electromagnetic interference continues to decrease the cost of the filtering circuit increases to ensure electromagnetic compatibility. 
       SUMMARY OF THE INVENTION 
       [0004]    Hence there is a desire for electric motors and actuators with increased electromagnetic compatibility. 
         [0005]    Accordingly, in one aspect thereof, the present invention provides a motor comprising a stator and a rotor, the stator comprising a motor housing with an open end and an end cover mounted to the open end of the motor housing, the rotor comprising a rotor shaft through which the rotor is rotatably mounted to the stator, and a bearing mounted to the end cover for supporting the rotor shaft, wherein the rotor shaft and the motor housing are made of an electrically conductive material, and the rotor shaft and the motor housing are electrically connected with each other. 
         [0006]    Preferably, the bearing is made of an electrically conductive material, and the rotor shaft is electrically connected to the motor housing by the bearing and a conductor connected between the bearing and the motor housing. 
         [0007]    Preferably, the rotor shaft, bearing, end cover and motor housing cooperatively define a closed chamber. 
         [0008]    Preferably, the rotor comprises a rotor core and a commutator mounted to the rotor shaft, the stator comprises a brush assembly, the brush assembly is mounted adjacent the open end and located within the closed chamber. 
         [0009]    Preferably, the brush assembly comprises a filtering element located in the closed chamber. 
         [0010]    Preferably, the end cover is made of an insulating material, and the bearing is electrically connected to the motor housing by an electrically conductive layer disposed over the end cover or an electrically conductive member mounted to the end cover. 
         [0011]    Preferably, the end cover is made of an electrically conductive material, and the rotor shaft is electrically connected to the motor housing by the bearing and the end cover. 
         [0012]    Preferably, one of the end cover and the motor housing comprises a protruding connecting tab, and the other of the end cover and the motor housing comprises a receiving portion for receiving the connecting tab to achieve electrical connection therebetween. 
         [0013]    Preferably, the bearing is one of an electrically conductive sleeve bearing and an electrically conductive rolling bearing. 
         [0014]    According to a second aspect, the present invention provides an actuator comprising a motor and a gearbox connected to the motor, the gearbox comprising a worm wheel and a worm shaft with a worm, the worm wheel meshed with the worm, the motor comprising a stator and a rotor, the stator comprising a motor housing with an open end and an end cover mounted to the open end of the motor housing, the rotor comprising a rotor shaft connected with the worm shaft, and a bearing mounted to the end cover for supporting the rotor shaft, wherein the rotor shaft and the motor housing are made of an electrically conductive material, and the rotor shaft and the motor housing are electrically connected with each other. 
         [0015]    Preferably, the worm shaft is made of an electrically conductive material, and the worm shaft is electrically insulated from and coaxially connected with the rotor shaft by an insulating member. 
         [0016]    Preferably, the insulating member is disposed on one side of the end cover adjacent the worm shaft. 
         [0017]    According to a third aspect, the present invention provides an actuator comprising a motor and a gearbox connected to the motor, the gearbox comprising a worm wheel and a worm shaft with a worm, the worm wheel being in mesh with the worm, the motor comprising a rotor shaft connected with the worm shaft, wherein the worm shaft and the rotor shaft are made of an electrically conductive material and are electrically insulated from each other, the worm shaft is coaxially connected with the rotor shaft by an insulating member to rotate with the rotor shaft. 
         [0018]    Preferably, one end of the rotor shaft adjacent the worm shaft comprises a connecting head connected with the insulating member, and one end of the worm shaft adjacent the rotor shaft comprises a connecting head connected with the insulating member. 
         [0019]    Preferably, both connecting heads are non-circular. 
         [0020]    Preferably, the one end of the rotor shaft adjacent the worm shaft and the one end of the worm shaft adjacent the rotor shaft are isolated from each other by an air gap or a portion of the insulating member. 
         [0021]    Preferably, one end of the rotor shaft adjacent the worm shaft comprises a flat portion, a reinforcement member is mounted to the flat portion, and the reinforcement member is fixedly connected to the insulating member. 
         [0022]    Preferably, the reinforcement member has a through hole in a central portion thereof for receiving the flat portion, at least two projections extend outwardly from an outer periphery of the reinforcement member, the insulating member comprises at least two position-limiting blocks alternately arranged with the projections. 
         [0023]    Preferably, the insulating member has a connecting hole at a central portion thereof, one end of the worm shaft comprises a connecting head, multiple axial ribs are formed on an outer periphery of the connecting head, and the multiple axial ribs are fixedly connected in the connecting hole. 
         [0024]    In view of the foregoing, the motor housing, end cover, bearing and rotor shaft of the motor cooperatively form an electromagnetic shield which effectively suppresses the electromagnetic interference of the motor thus increasing the electromagnetic compatibility of the motor. The rotor shaft of the motor and the worm shaft of the gearbox may be coaxially connected by an insulating member, which further increases the electromagnetic compatibility of the motor by reducing electromagnetic interference radiated by the rotor shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    A preferred embodiment of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same reference numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
           [0026]      FIG. 1  illustrates an actuator according to a first embodiment of the present invention; 
           [0027]      FIG. 2  is a sectional view of the actuator of  FIG. 1 ; 
           [0028]      FIG. 3  illustrates a rotor and worm shaft of the actuator of  FIG. 1 ; 
           [0029]      FIG. 4  illustrates a bearing seat for the rotor of  FIG. 3 ; 
           [0030]      FIG. 5  illustrates a rotor shaft and worm shaft of the actuator; 
           [0031]      FIG. 6  is a sectional view of the rotor shaft and worm shaft of  FIG. 5 ; 
           [0032]      FIG. 7  illustrates the worm shaft; 
           [0033]      FIG. 8  illustrates an insulating member for connecting the worm shaft to the rotor shaft; 
           [0034]      FIG. 9  illustrates the rotor shaft; 
           [0035]      FIG. 10  illustrates a reinforcement member mounted to the end of the rotor shaft adjacent to the worm shaft; 
           [0036]      FIG. 11  is a partial, enlarged view of  FIG. 6 : and 
           [0037]      FIG. 12  is a partial, sectional view of the rotor shaft and worm shaft according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    Referring to  FIG. 1  to  FIG. 4 , a actuator  20  in accordance with a first embodiment of the present invention includes a gearbox  30  and a motor  40 . The gearbox  30  includes a gear housing  31 , a worm wheel  33  and a worm shaft  81  disposed in the gear housing  31 , and an output gear  34 . The output gear  34  rotates coaxially along with the worm wheel  33 . A worm is mounted on or formed with the worm shaft  81  and meshes with the worm wheel  33 . A bearing  35  is mounted in the gear housing  31  for supporting the worm shaft  81 . 
         [0039]    The motor  40  includes a stator  45  and a rotor  70 . The stator  45  includes a motor housing  51 , an end cover  53  mounted to one end of the motor housing  51  to enclose an opening of the motor housing  51 , permanent magnets (not shown) mounted to an inner surface of the motor housing  51 , a brush assembly and an end cover  61  mounted to another open end of the motor housing  51 . The rotor includes a rotor shaft  71 , a rotor core  73  and a commutator  75  fixed to the rotor shaft  71 , and rotor windings (not shown) wound around teeth of the rotor core  73 . The brush assembly includes a brush card or brush holder  56  and at least two brushes  57  supported by the brush holder  56  and arranged to make sliding contact the commutator  75 . Bearings  55 ,  59  are respectively mounted in the end covers  53 ,  61 , for supporting the rotor shaft  71 . The rotor is rotatably mounted to the stator  45  through the bearings. Rotating the rotor causes the worm shaft  81  to rotate coaxially with the rotor shaft  71  to achieve an output with reduced speed by means of the worm wheel  33  and the output gear  34 . 
         [0040]    In this embodiment, the motor housing  51  and the end covers  53 ,  61  are made of an electrically conductive material. The end cover  61  forms a bearing seat or bearing holder for mounting of the bearing  59 . The bearing  59  is made of an electrically conductive material (e.g. copper). Therefore, the rotor shaft  71  is electrically connected with the motor housing  51  through the bearing  59  and end cover  61 . As such, the motor housing  51 , end cover  53 , end cover  61  and rotor shaft  71  cooperatively define a closed chamber. The closed chamber acts as an electromagnetic shield which effectively prevents electromagnetic radiation produced by the brush assembly, rotor windings and commutator  75  inside the motor interfering with external devices. Preferably, during use, the electromagnetic compatibility (EMC) of the actuator  20  may be further increased by grounding of the motor  40 . 
         [0041]    In this embodiment, the brush assembly is located within the electromagnetic shield, such that the electromagnetic interference of the brush assembly with external devices during current reversal can be reduced or avoided. In this case, filtering elements such as capacitors or inductors that are mounted to the brush assembly are also located within the electromagnetic shield. 
         [0042]    Referring to  FIG. 4 , in this embodiment, the end cover  61  includes a cover body  62 . The cover body  62  is recessed at a central portion to form an axially-extending cylindrical flange  64 . The cylindrical flange  64  forms the bearing seat. Connecting tabs  66  extend outwardly from an outer edge of the cover body  62 . Correspondingly, the motor housing  51  of the motor includes receiving portions  52  (e.g. through holes, slots or notches) that receive the connecting tabs  66  to achieve electrical connection between the motor housing  51  and the end cover  61 . The outer edge of the cover body  62  may also form a cutout  67  for engaging with the brush holder  56  of the brush assembly in a recess-protrusion engagement manner for facilitating positioning and mounting of the end cover  61 . 
         [0043]    In this embodiment, the bearing  59  is implemented as a sleeve bearing (also referred to as oil-impregnated bearing). While the rotor shaft  71  is not inference-fit within the inner hole of the sleeve bearing and there is a small clearance between the sleeve bearing and the rotor shaft  71 , it is noted, however, that the rotor shaft  71  is supported by and thus establishes electrical connection with the sleeve bearing  59  at any time. Therefore, the implementation of the bearing  59  as the sleeve bearing does not affect the electrical connection between the rotor shaft  71  and the motor housing  51 . 
         [0044]    It is also to be understood that the bearing  59  may also be implemented as a rolling bearing such as a ball bearing. In this case, the rotor shaft  71  is interference-fit with an inner ring of the rolling bearing to establish the electrical connection therebetween. An electrical connection between the inner ring and an outer ring of the rolling bearing is established through rolling elements such as steel balls. The outer ring of the rolling bearing is interference-fit with the bearing seat of the end cover to establish an electrical connection therebetween. 
         [0045]    In this embodiment, the end cover  61  is made of electrically conductive material, which acts as a conductor electrically connecting the bearing  59  and the motor housing  51 . Alternatively, the end cover  61  may be made of an insulating material (e.g. plastic), and the bearing  59  and motor housing  51  are electrically connected by an electrically conductive member mounted to the end cover  61  or an electrically conductive connecting layer (e.g. a copper foil layer) disposed over the end cover  61 . 
         [0046]    Referring to  FIG. 3 ,  FIG. 5  and  FIG. 11 , in this embodiment, the worm shaft  81  and rotor shaft  71  are made of a metal material. The worm shaft  81  and the rotor shaft  71  are coaxially connected through an insulating member  90 , such that the worm shaft  81  rotates along with the rotor shaft  71 . Because the worm shaft  81  is electrically insulated from the rotor shaft  71 , conduction of the electromagnetic interference from the rotor shaft  71  to the worm shaft  81  is reduced or eliminated, thus further increasing the EMC of the actuator. 
         [0047]    Preferably, the location where the rotor shaft  71  is insulated from the worm shaft  81  is located outside the electromagnetic shield and close to the end cover  61 . As such, the electromagnetic shield is substantially closed, and most parts of the rotor shaft  71  is located within the electromagnetic shield. 
         [0048]    In this embodiment, one end of the rotor shaft  71  adjacent the worm shaft includes a flat portion for connecting with a reinforcement member  87 . The reinforcement member  87  has a through hole  88  at a central portion thereof for receiving the flat portion  72 . Three projections  89  extend outwardly from a circumferential periphery of the reinforcement member  87 . One end of the worm shaft  81  close to the rotor shaft forms a connecting head  82  with a reduced outer diameter. Multiple axial ribs  83  are formed on an outer periphery of the connecting head  82 . The insulating member  90  preferably is made of Bakelite and is connected with the reinforcement member  87  and the connecting head  82  of the worm shaft to coaxially fix the rotor shaft  71  and the worm shaft  81  to each other. In this embodiment, the insulating member  90  includes a cylindrical main body  91 , three position-limiting blocks  95  at one side of the main body  91 , and an annular body  97  surrounding the three position-limiting blocks  95 . A connecting hole  93  is formed in a central portion of the main body  91 . The connecting head  82  of the worm shaft  81  is interference-fit in the connecting hole  93 . The ribs  83  on the outer periphery of the connecting head  82  reinforce the connection between the connecting head  82  and the main body  91 . The reinforcement member  87  is mounted in the annular body  97  of the insulating member  90 , with the three position-limiting blocks  95  of the insulating member  90  alternately arranged with the three projections  89  of the reinforcement member  87  for the transmission of the driving force. An outer circumferential surface of the annular body  97  is flush with an outer circumferential surface of the main body  91 , such that the insulating member  90  has a continuous smooth surface which reduces rotational resistance. 
         [0049]    In this embodiment, an axial length of the connecting head  82  of the worm shaft  81  is less than a depth of the connecting hole  93  of the main body  91 , such that a gap  98  is formed between the rotor shaft  71  and the worm shaft  81  ( FIG. 11 ). This gap  98  ensures that the worm shaft  81  and the rotor shaft  71  are electrically insulated from each other. 
         [0050]    Alternatively, as shown in  FIG. 12 , the main body  91  may have a bottom portion  99  at one end of the central connecting hole  93  adjacent the rotor shaft  71 . This bottom portion  99  is integrally formed with the insulating member  90  for isolating the rotor shaft  71  from the worm shaft  81 , thereby ensuring electrical isolation between the rotor shaft  71  and the worm shaft  81 . 
         [0051]    It is noted that the connecting head  82  of the worm shaft  81  should not be limited to the shape as described above. Rather, any shape of the connecting head  82  is possible as long as it enables the coaxial rotation of the insulating member  90  and the worm shaft  81 . Similarly, the end of the rotor shaft  71  close to the worm shaft is not intended to be limited to the structure having the flat portion  72 . 
         [0052]    It is also noted that the reinforcement member  87  is used mainly to reinforce the connection between the rotor shaft  71  and the insulating member  90 . Therefore, the reinforcement member  87  may be omitted in another embodiment. In addition, the flat portion  72  of the rotor shaft  71  is used mainly to achieve torque transfer to prevent relative movement. Therefore, the flat portion of the rotor shaft  71  may be replaced with another non-circular head such as a square head or a hexagonal head. 
         [0053]    It is also noted that the design of the insulating connection between the rotor shaft and worm shaft as shown in  FIG. 5  to  FIG. 7  may be also applied in another type of actuator. 
         [0054]    In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item or feature but do not preclude the presence of additional items or features. 
         [0055]    It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
         [0056]    The embodiments described above are provided by way of example only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined by the appended claims.