Abstract:
A commutator has a conductive layer, a segment layer and an insulatim layer separating the conductive layer and the segment layer. The segment layer includes multiple commutator segments. A mounting hole is defined along an axis of the commutator passing through the conductive layer. The three-layer structure of the commutator forms a capacitor having an increased confronting area and reduced inter-plate distance. The capacitor thus has a greater capacitance and hence greater EMI absorbing capability, making it possible to reduce EMI emissions without additional EMI reduction components outside the commutator. A rotor and a motor employing the commutator are also disclosed.

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. 201410768577.7 filed in The People&#39;s Republic of China on Dec. 12, 2014, the entire contents of which are hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an electric motor, and in particular, to a commutator for an electric motor, especially a micro or sub-fractional horsepower DC motor. 
       BACKGROUND OF THE INVENTION 
       [0003]    As shown in  FIG. 5 , during operation of a DC commutator motor, sparks may be generated between a commutator and a carbon brush, which may cause electromagnetic interference (EMI). To suppress or eliminate EMI, a typical measure is to dispose a structure made of an EMI-absorbing material in the motor and surrounding the commutator. However, this increases the weight of the motor, which is not desirable. In addition, this EMI-suppressing structure and its associated mounting structure make the motor structure more complicated, which leads to a more complex manufacturing process and higher manufacturing costs. 
       SUMMARY OF THE INVENTION 
       [0004]    Hence there is a desire for a more simple solution for absorbing EMI in a DC commutator motor. 
         [0005]    Accordingly, in one aspect thereof, the present invention provides a commutator comprising: a conductive layer, a segment layer including a plurality of commutator segments, and an insulating layer fixed between the conductive layer and the segment layer, the insulating layer electrically isolating the segment layer from the conductive layer, wherein the conductive layer, the insulating layer and the segment layer form a capacitor connecting the commutator segments to ground. 
         [0006]    Preferably, the insulating layer has a thickness between the conductive layer and the segment layer in the range of 0.8 mm to 2.0 mm, and most preferably, the thickness is 1.0 mm. 
         [0007]    Preferably, a concave-convex engagement structure is formed at contact surfaces between the conductive layer and the insulating layer, and/or, between the insulating layer and the segment layer to increase the surface area of the respective contact surfaces. 
         [0008]    Preferably, the conductive layer is electrically grounded. 
         [0009]    Preferably, the commutator is a face plate commutator and the conductive layer and segment layer are essentially planar. 
         [0010]    Preferably, the commutator is a cylindrical commutator and the conductive layer forms an inner ring radially surrounded by the insulating layer. 
         [0011]    According to another aspect, the present invention provides a rotor comprising a motor shaft and a commutator as described above, fixed to the motor shaft, wherein the motor shaft passes through a mounting hole formed in the commutator. 
         [0012]    Preferably, the motor shaft is made of a conductive material, and the conductive layer is indirectly electrically grounded through the motor shaft. 
         [0013]    Alternatively, the motor shaft is made of an insulating material and the motor shaft defines a through hole, a conductor extends from the conductive layer, passes through the through hole in the motor shaft and is then electrically grounded. 
         [0014]    According to a further aspect, the present invention provides a motor comprising: a stator, and a rotor as described above. 
         [0015]    Preferably, the stator includes a housing and the conductive layer of the commutator is grounded through the housing. 
         [0016]    By adopting this solution of the present invention, the three-layer structure of the commutator forms a capacitor having an increased confronting area and reduced inter-plate distance. The capacitor thus has a greater capacitance and hence greater EMI-absorbing capability, which makes it possible to absorb EMI without additional EMI-absorbing components outside the commutator. EMI can be suppressed by directly grounding the conductive layer, by electrically grounding the motor shaft, or indirectly grounding the conductive layer through the motor shaft and the housing. EMI suppression can be successfully achieved using the remaining original parts and structure without any modification or with only a minor modification to the structure of the motor. As such, the extra EMI-absorbing structure and/or components added in the conventional motor are no longer necessary. Removing the original EMI-absorbing structure can reduce the weight of the motor as well as simplify the structure and fabrication process of the motor. It is to be understood that, in order to achieve more thorough EMI-suppression, the entire or part of the extra EMI-absorbing structure/components that are conventionally added may also remain in the motor adopting the structure of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    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. 
           [0018]      FIG. 1  is an end view of a commutator according to one embodiment of the present invention. 
           [0019]      FIG. 2  is a perspective view of a rotor according to one embodiment of the present invention. 
           [0020]      FIG. 3  is a perspective view of a motor according to one embodiment of the present invention. 
           [0021]      FIG. 4  is a perspective view of part of the rotor of the motor of  FIG. 3 . 
           [0022]      FIG. 5  illustrates a cylindrical commutator according to the present invention. 
           [0023]      FIG. 6  is a sectional view of the commutator of  FIG. 5 . 
           [0024]      FIG. 7  illustrates a planar commutator according to the present invention. 
           [0025]      FIG. 8  is a sectional view of the commutator of  FIG. 7 . 
           [0026]      FIG. 9  shows another planar commutator according to the present invention. 
           [0027]      FIG. 10  is a sectional view of the commutator of  FIG. 9 . 
           [0028]      FIG. 11  is a perspective view of a known commutator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]      FIG. 1  is an end view of a commutator  1  in accordance with a first embodiment of the present invention. The commutator is of the cylindrical type and has a conductive layer  11  in the form of an inner ring, an insulating layer  12  radially surrounding the conductive layer  11 , and a segment layer  13  radially surrounding the insulating layer  12 . A mounting hole  10 , used to mount the commutator on a motor shaft, is formed along an axis of the commutator. The conductive layer  11  and the segment layer  13  are isolated from each other by the insulating layer  12 . The segment layer  13  includes multiple commutator segments  16 . The segment layer  13  may be made of copper, the insulating layer  12  may be made of a resin material, and the conductive layer  11  may be made of copper, aluminum or another electrically conductive material. The conductive layer  11  and the segment layer  13  are electrically isolated by the insulating layer  12 , thus forming a capacitor, with the conductive layer and the segment layer forming the pole plates of the capacitor. In addition to being directly proportional to the dielectric constant of the insulating material, the capacitance of the capacitor is also directly proportional to the confronting surface areas of the conductive layer  11  and the segment layer  13 , on opposite sides of the insulating layer  12 , and inversely proportional to an inter-plate distance between the conductive layer  11  and the segment layer  13 . The addition of the conductive layer  11  not only increases the confronting surface area of the conductors on opposite sides of the insulating layer  12 , but it also decreases the inter-plate distance between the conductors, thereby providing a capacitor with a greater capacitance. 
         [0030]    After the commutator  1  of the present embodiment is mounted on the motor shaft  2  as shown in  FIG. 2  and  FIG. 4 , the conductive layer  11  or segment layer  13  of the commutator  1  is directly or indirectly grounded. EMI generated during operation of the motor is absorbed by the capacitor with greater capacitance and released through the ground connection. As a result, the adverse effect caused by the EMI is significantly reduced or eliminated. 
         [0031]    Theoretically, from the capacitor perspective, the insulating layer  12  has a uniform radial thickness (i.e. thickness of the layer) and a uniform radial distance between an inner surface of the insulating layer  12  and the axis, thus forming the same capacitance over the entire insulating layer  12  to ensure the same capability of absorbing EMI. However, considering the process complexity and the need of assembly and fixing of the insulating layer  12  to the conductive layer  11  and the segment layer  13 , respectively, the layer thickness of the insulating layer  12  may not be uniform over the entire insulating layer  12 . 
         [0032]    Preferably, the thickness of the insulating layer  12  is in the range from 0.8 mm to 2 mm. If the thickness is too large, the conductive layer  11  and the segment layer  13  form a capacitance that is too small to absorb the EMI. If the thickness is too small, the thickness can not be guaranteed by the current fabrication processes. More preferably, the thickness is 1 mm, in which case, a large capacitance (the capacitance was found to be as large as 5 pf to 40 nf by testing a commutator of the same model) between the plates can be achieved without imposing a too-high requirement on the fabrication process. 
         [0033]    As shown in  FIG. 1  and  FIG. 2 , the conductive layer  11  and the insulating layer  12 , and the insulating layer  12  and the segment layer  13  can be fastened in various manners including, for example, press-fit, adhesive bonding or snap-fit. In the present embodiment, concave-convex engagement structures  14  are formed on each of the contacting surfaces between the conductive layer  11  and the insulating layer  12 , and the contacting surfaces between the insulating layer  12  and the segment layer  13 . The purpose of the concave-convex engagement structures is to enhance the connection tightness between the connected two layers by increasing the binding force or frictional force between the contacting surfaces of the connected two layers. In an alternative embodiment, one or both of the two pairs of contacting surfaces may be connected in another manner. The concave-convex engagement structures also increases the contact surface areas between the layers. 
         [0034]    Referring to  FIG. 2 , a rotor  9  in accordance with one embodiment of the present invention includes a motor shaft  2  and the commutator  1  of the above embodiment. The commutator  1  is mounted on the motor shaft  2  which passes through the motor shaft mounting hole  10 . The motor shaft  2  and commutator  1  may be fastened together by various methods. In the present embodiment, the motor shaft  2  is preferably an interference-fit on the motor shaft mounting hole  10  to make sure that the commutator  1  rotates with the motor shaft  2 . In addition, the interference fit connection manner avoids a gap between the motor shaft  2  and the mounting hole  10  which would cause wobbling of the commutator  1  on the motor shaft  2 . 
         [0035]    In this embodiment, the segment layer  13  or conductive layer  11  can be directly grounded to eliminate EMI. For example, as shown in  FIG. 2 , if the motor shaft  2  is non-conductive, a through hole  20  may be defined in the motor shaft  2 . One end of a conductor  15  passes through the through hole  20  and is electrically connected with the conductive layer  11  of the commutator  1 , and the other end is electrically grounded. As such, the commutator  1  can be electrically grounded without affecting the fabrication and operation of other motor parts by taking advantage of the rod like structure of the motor shaft  2 . 
         [0036]    If the motor shaft  2  is made of a conductive material and the conductive layer  11  is in electrical contact with the motor shaft  2 , then the conductive layer may be grounded by grounding the motor shaft  2 . As such, the conductive layer  11  can be indirectly electrically grounded through the motor shaft  2 . Therefore, electrical grounding of the conductive layer  11  of the commutator  1  can be realized without modifying the structure of the motor shaft  2 . 
         [0037]    When the motor shaft  2  is made of a conductive material, and is fitted with a conventional commutator  1 ′ ( FIG. 11 ) having an insulating layer  12 ′ and a segment layer  13 ′ but having no conductive layer, the insulating layer  12 ′, the conductive segment layer  13 ′ and the conductive motor shaft  2  also form a capacitor. However, because the motor shaft  2  has a relatively small diameter, it has a small surface area confronting the commutator layer  13 ′ and the commutator segments  13   a ″, and the motor shaft  2  and the segment layer  13 ′ have a large distance there between (i.e. the radial thickness of the insulating layer  12 ′), the capacitor has a small capacitance. By testing a commutator of similar size, it was found that the capacitor is less than 1 pf when the thickness of the insulating layer is 4.5 mm. The EMI signal generated by the commutator  1 ′ and carbon brush  3  has a low frequency, usually lower than 6 GHz, which can hardly be absorbed by the capacitor having such small capacitance. Therefore, no one skilled in the art has recognized that the EMI signal can be absorbed and discharged by a capacitor formed by the commutator alone or in combination with the motor shaft. 
         [0038]      FIG. 3  illustrates a first embodiment of a motor  8  of the present invention. The motor illustrated is an outer rotor motor and has a stator including a housing  4  and a rotor  9 . The housing  4  supports the rotor through one or more bearings and also supports the stator core, stator windings, brushes, etc. The rotor is further illustrated in  FIG. 4 . The rotor includes a motor shaft  2 , supporting the commutator  1  and a rotor core  7 . Magnets, not shown, are fitted to an inner surface of the rotor core. Two brushes  3  and brush springs, being parts of the stator, are schematically illustrated for reference only. The commutator  1  of this embodiment is a special commutator having commutator segments and slip rings. 
         [0039]    In this embodiment, the segment layer  13  or conductive layer  11  may be directly electrically grounded to eliminate EMI. When the housing  4  is made of a conductive material, the segment layer  13  or conductive layer  11  can further be electrically connected with the housing  4 , and the housing  4  is electrically grounded. As such, the segment layer  13  or conductive layer  11  can be indirectly electrically grounded through the housing  4 , and thus the EMI can be discharged via the housing. 
         [0040]    The segment layer  13  or conductive layer  11  may be directly electrically grounded to eliminate EMI. For example, when the motor shaft  2  and the rotor core  7  are made of an insulating material, a through hole  20  may be defined through the motor shaft  2 , one end of a conductor  15  passes through the through hole  20  to be electrically connected with the conductive layer  11  of the commutator  1 , and the other end is electrically grounded. 
         [0041]    Similarly, when the rotor core  7  is made of a conductive material and the motor shaft  2  is made of an insulating material, one end of the conductor  15  passes through the through hole  20  to be electrically connected with the conductive layer  11  of the commutator  1 , and the other end is electrically connected with the rotor core  7 . As such, the commutator  1  can be electrically grounded without affecting the fabrication and operation of other motor parts by taking advantage of the rod like structure of the motor shaft  2 . When both the housing  4  and motor shaft  2  are made of a conductive material, as shown in  FIG. 4 , the motor shaft  2  is electrically connected with the conductive layer  11  and the housing  4 , allowing the conductive layer to be grounded through the housing. Thus, the EMI absorbed by the capacitor can be successfully discharged by using the current conductive motor shaft  2  and housing  4  without making any structural modification to the motor. 
         [0042]    The invention is applicable to other types of commutators.  FIGS. 5 &amp; 6  illustrated a standard cylindrical type commutator and  FIGS. 7 to 10  illustrate two standard planar type commutators, modified according to the present invention. 
         [0043]    The cylindrical commutator of  FIG. 5  has a plurality of commutator segments  16  arranged about an outer surface of a commutator body, in a manner producing a cylindrical brush contact surface. The commutator body comprises the conductive layer  11  in the form of an inner ring and the insulating layer  12  formed radially about the inner ring. The inner ring has a number of radially extending discs extending towards the segment layer and interleaved with radially extending fingers formed on the commutator segments and forming discontinuous discs extending radially towards the inner ring. The interleaved discs significantly increases the contact surface areas with the insulating layer and allows the thickness of the insulating layer between the segment layer and the conductive layer to be controlled to a small dimension. 
         [0044]      FIG. 7  illustrates a thin planar commutator  1 . The segment layer  13  formed of a plurality of commutator segments  16  forms a planar brush contact surface. As shown in the sectional view of  FIG. 8 , the conductive layer is flat disc separated from the segment layer  13  by the insulating layer  12 . In this embodiment the insulating layer  12  forms the body of the commutator and is directly molded to the segment layer and the conductive layer to fix these two layers in spaced relationship. The segment layer is a single piece disc when the insulating layer is over molded and then cut into individual commutator segments thereafter. The conductive layer is exposed in the shaft mounting hole  10  to make direct electrical connection with the motor shaft, should the motor shaft be electrically conductive. 
         [0045]      FIG. 9  illustrates a thick planar commutator  1 . As shown in the cross sectional view of  FIG. 10 , the conductive layer  11  is cup shaped with the base of the cup extending radially and the shaft mounting hole  10  passing through the base. Again the conductive layer  11  may be exposed in the shaft mounting hole for electrical connection with the shaft. The commutator segments of the segment layer  13  form two discontinuous rings which confront the sides of the cylindrical portion of the cup. The insulating layer is disposed between the conductive layer and the segment layer to hold these parts and to form the dielectric of the capacitor while the confronting portions of the segment layer and the conductive layer  11  form the poles of the capacitor. This increases the surface area of the poles and thus the capacitance of the capacitor. 
         [0046]    The phrase “directly grounded” used herein means that it is electrically grounded without using any conductive structure of or associated with the motor. The phrase “indirectly grounded” used herein means that it is electrically grounded through any conductive structure of or associated with the motor. Because the segment layer  13  mainly comprises multiple separate commutator segments  16 , grounding of the segment layer  13  has great difficulty and complexity. Therefore, preferably, it is the conductive layer  11  that is directly or indirectly grounded. 
         [0047]    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. 
         [0048]    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. 
         [0049]    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.