Abstract:
An electric motor has a stator, a rotor rotatable with respect to the stator, motor windings, a connection plate electrically connected to the motor windings, and a power lead for connecting the motor to a power source. The power lead is connected to the connection plate by an electrical connection and by a separate mechanical connection. The mechanical connection protects the electrical connection from vibration or movement which may otherwise break the electrical connection.

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. 201410127039.X filed in The People&#39;s Republic of China on Mar. 31, 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 an arrangement for attaching power leads to an electric motor. 
       BACKGROUND OF THE INVENTION 
       [0003]    Conventionally, electric motors with power leads have the power leads attached by connection point which serves to make both the electrical connection and the mechanical connection. This is particularly the case for small electric motors. However, in applications where such motors are subject to high vibration or repeated moving where there are forces applied to the power leads, the connection is known to be broken either electrically or mechanically, leading to failure of the motor to operate correctly. Such known connections include solder connections, weld connections, crimp connections and terminal connections. 
         [0004]    Weld connections are preferred as they are fast and cheap to make with good mechanical strength and electrical properties. However, vibrations can cause the weld connection to break. 
         [0005]    Mechanical connections, such as those using wire connectors, give good mechanical connection but the electrical connection is subject to deterioration as the vibrations cause micro-arcing between the connector and the terminal leading to electrical erosion and high resistance joints, affecting the performance of the motor. In extreme cases the electrical connection can be completely lost. 
       SUMMARY OF THE INVENTION 
       [0006]    Thus there is a desire for an improved way of connecting the lead wires to terminals of the motor which can withstand vibrations or frequent movement. 
         [0007]    This is achieved in the present invention by using a separate connection to mechanically fix the power lead to the terminal to stabilize the electrical connection. 
         [0008]    Accordingly, in one aspect thereof, the present invention provides an electric motor comprising: a stator, a rotor rotatable with respect to the stator, motor windings, a connection plate electrically connected to the motor windings, and a power lead for connecting the motor to a power source, wherein the power lead is connected to the connection plate by an electrical connection and by a separate mechanical connection. 
         [0009]    Preferably, the rotor is a wound rotor having a rotor core and a commutator mounted on a shaft, and the motor windings are formed by coils wound about teeth of the rotor core and connected to segments of the commutator, and wherein the connection plate is electrically connected to the windings by brushes making sliding contact with the commutator. 
         [0010]    Preferably, the stator has a housing and permanent magnets fixed to the housing. 
         [0011]    Preferably, an end cap is fitted to the housing and supports brush gear, including a plurality of the bnishes, the connection plate and a brush mounting plate to which the brushes and the connection plate are mounted. 
         [0012]    Preferably, each brush is part of a separate brush assembly comprising the brush, a brush cage in which the brush is slidably mounted and a brush spring for urging the brush into contact with the commutator. 
         [0013]    Preferably, each brush has a shunt and at least one of the shunts is electrically connected to the connection plate. 
         [0014]    Preferably, there are two connection plates, two power leads respectively connected to the connection plates and two groups of brush assemblies with the brushes of each group of brush assemblies being connected to a corresponding connection plate. 
         [0015]    Preferably, there are two brush assemblies in each group of brush assemblies and the brushes of each group are arranged alternately about the commutator. 
         [0016]    Preferably, the power lead is mechanically connected to the connection plate by a wire connector. 
         [0017]    Preferably, the connection plate has an extension in the form of a male spade terminal and the wire connector is resiliently mated with the male spade terminal. 
         [0018]    Preferably, the power lead is electrically connected to the connection plate by weld connection. 
         [0019]    Preferably, the electrical connection is between the mechanical connection and the adjacent end of the power lead. 
         [0020]    In view of the forgoing, the stability of the connections between the power leads and the motor terminals are greatly improved in a simple manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    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. 
           [0022]      FIG. 1  illustrates an electric motor according to the preferred embodiment of the present invention; 
           [0023]      FIG. 2  illustrates a stator of the motor of  FIG. 1 ; 
           [0024]      FIG. 3  illustrates a rotor assembled to the stator of  FIG. 2 ; 
           [0025]      FIG. 4  illustrates brush assemblies added to the rotor and stator assembly of  FIG. 3 ; 
           [0026]      FIG. 5  illustrates an end cap, being a part of the motor of  FIG. 1 ; 
           [0027]      FIG. 6  shows the end cap of  FIG. 5 , viewed from the other side; 
           [0028]      FIG. 7  illustrates brush gear of the motor, comprising the brush assemblies, power leads and connection plates; 
           [0029]      FIG. 8  shows the brush gear of  FIG. 7 , viewed from the other side; 
           [0030]      FIG. 9  illustrates a wire connector; 
           [0031]      FIG. 10  illustrates one of the connection plates of  FIG. 7 ; and 
           [0032]      FIG. 11  illustrates the connection between a power lead and a connection plate. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]      FIG. 1  illustrates an electric motor according to the preferred embodiment. In this embodiment, the motor is a PMDC (permanent magnet, direct current) motor having a permanent magnet stator  10 , a wound rotor  20 , and an end cap  30 . The rotor  20  has a main body or core disposed within the stator  10  and an output shaft extending through the end cap  30 . Two power leads  40   a ,  40   b  extend through the end cap  30  for connection to a power source (not shown) via connectors  42   a ,  42   b.    
         [0034]    As shown in  FIG. 2 , the stator has a deep drawn cup like housing having magnets fitted to an inner surface thereof. The stator thus has one closed end and one open end. In the illustrated embodiment, four permanent magnets  14  are uniformly mounted to the inner surface of the housing  12  by a magnet mounting bracket  16 . Optionally, steel structures (not shown) may be disposed on inner surfaces of the magnets  14  to help hold the magnets and prevent breakage of the magnets from obstructing rotation of the rotor  20 . 
         [0035]      FIG. 3  illustrates the rotor assembled with the stator. The rotor  20  comprises a shaft  22 . A rotor core  23 , having a plurality of teeth  24 , is fixed to the shaft. A commutator  26  is fixed to the shaft next to the rotor core. The shaft is journalled in bearings supported by the closed end of the housing and the end cap. A rotor winding (not shown) is wound about the teeth of the rotor core in a generally known manner. The winding includes a number of coils, the ends of which are connected to segments of the commutator  26 . 
         [0036]      FIG. 4  illustrates brush assemblies  31  added to the rotor and stator assembly of  FIG. 3 . There are two brush assemblies  31   a  and two brush assemblies  31   b , alternately and evenly arranged about the periphery of the commutator  26  and arranged such that the brushes make sliding contact with the commutator  26 . A brush mounting plate (see  FIG. 6 ,  35 ) for positioning the brush assemblies is not shown in  FIG. 4 . Shunts  32   a ,  32   b  extend out from each of the brushes respectively. During operation, as the rotor  20  rotates, the brushes of the assemblies  31   a  and  31   b  sequentially contact segments of the commutator  26 . 
         [0037]      FIGS. 5 and 6  illustrate the end cap  30  from outside and inside. The end cap  30  includes a cover  34 . An annular brush mounting plate  35  is disposed at an inner side of the cover  30 . The brush assemblies  31   a  and  31   b  are fixedly mounted to the brush mounting plate  35 . Shunts  32   a  and  32   b  are connected with connection plates  33   a  and  33   b , respectively. 
         [0038]      FIG. 7  and  FIG. 8  illustrate, from above and below, the brush gear comprising the four brush assemblies, the two connection plates and the two power leads. Each brush assembly comprises a brush cage, a brush slidably received in the brush cage and a brush spring arranged to urge the brush out of the brush cage. For clearer illustration, the brush mounting plate  35  is not shown. The two brush assemblies  31   a  and two brush assemblies  31   b  are alternately arranged. Two shunts  32   a  extend from the two brush assemblies  31   a , respectively. Two shunts  32   b  extend from the two brush assemblies  31   b , respectively. The free ends of the shunts are connected to the connection plates, preferably by welding. Shunts  32   a  are connected to connection plate  33   a  and shunts  32   b  are connected to connection plate  33   b . Power lead  40   a  is connected to the connection plate  33   a  and power lead  40   b  is connected to connection plate  33   b , in a manner described herein after. As such the two shunts  32   a  are electrically connected with power lead  40   a , and the two shunts  32   b  are electrically connected to power lead  40   b . As such, the two brush assemblies  31   a  are commonly connected to the power lead  40   a , and the two brush assemblies  31   b  are commonly connected to the power lead  40   b.    
         [0039]    Connection plate  33   a  includes an extension which is bent 90 degrees to form a terminal plate  44   a . The connection plate  33   b  likewise includes an extension which is bent 90 degrees to form terminal plate  44   b . The terminal plates  44   a ,  44   b  are generally rectangular, forming male spade terminals. The power leads  40   a ,  40   b  are respectively connected to the terminal plates through wire connectors  50 . A middle part of each of the power leads  40   a  and  40   b  between opposite ends thereof is wrapped with an insulating layer  41  to ensure that the power leads are safely connected to the power source without being electrically connected to another component. 
         [0040]      FIG. 9  illustrates a wire connector  50 . Preferably, the wire connector  50  is formed from copper and has a base plate  51  with top and bottom ends. A first bent portion  52  is disposed at the bottom end of the base plate  51 , and a second bent portion  54  is disposed at the top end of the base plate  51 . The second bent portion  54  has two generally U-shaped sides. The distance between the two U-shaped sides is slightly greater than the width of the terminal plate  44   a ,  44   b , to receive the terminal plate there between. The free ends of the sides curl back towards the base plate and are spaced therefrom by a distance less than the thickness of the terminal plate such that the sides are resiliently deformed when the terminal plate is pressed into the wire connector to tightly grip the terminal plate. The first bent portion  52  is arranged to be crimped to the power lead to mechanically fix the power lead to the wire connector and thus to the terminal plate. The wire connector  50  is arranged to be crimped to the power lead such that the free end of the power lead extends passed the wire connector. 
         [0041]      FIG. 10  illustrates connection plate  33   a , to more clearing show the structure of terminal plate  44   a . Terminal plate  44   a  has a generally rectangular shape. A detent  45  is formed at a middle area of a side of the pin plate  44   a  for limiting downward movement of the wire connector  50  relative to the terminal plate  44   a . The detent is in the form of a step in the side of the terminal plate which forms a stop which bears against the axial edge of the second bent portion  54 . The detent  45  may be formed at another location, as long as it limits the downward movement of the wire connector  50  with respect to the terminal plate  44   a . In this embodiment, the terminal plate  44   a  is integrally formed with the conductive plate  33   a  by stamping and is bent 90 degrees to form the pin plate  44   a  perpendicular to the conductive plate  33   a . In another embodiment, the connection plate and terminal plate may extend in the same direction or at an angle other than 90 degrees. 
         [0042]      FIG. 11  illustrates the connection between the power lead  40   a  and the connection plate  33   a . Power lead  40   b  and connection plate  33   b  are similarly connected. The end of the power lead  40   a  is stripped of insulation. The bared end is crimped to the wire connector  50   a  and the wire connector is fitted to the terminal plate  44   a  to establish a mechanical connection between the power lead and the connection plate. The wire connector  50   a  is crimped to the power lead  40   a  in such a manner that the bared end  46  of the power lead extends beyond the wire connector. The terminal plate  44   a  is inserted into the second bent end  54 , resiliently deforming the ends of the c-shaped sides which then press the terminal plate  44   a  to the base plate  51  of the wire connector. The terminal plate is inserted through the wire connector until detents  45  contact the c-shaped sides  54  of the wire connector, thus establishing the correct position of the wire connector with respect to the terminal plate. At this position the free end  46  of the power lead extending beyond the wire connector lays against the distal end of the terminal plate  44   a . With the power lead physically connected to the terminal plate in a stable manner, the end of the power lead  40   a  is electrically connected to the terminal plate  44   a  by a separate electrical connection. This electrical connection is preferably a weld connection, such as made for example using a spot welding technique. Other types of electrical connections, such as those made by soldering, fusing or crimping, may be used depending on the application and manufacturing process needs. Indeed, other types of mechanical fixation methods may be used to physically fix the power lead to the terminal plate. 
         [0043]    Hence, the power leads are connected to the motor terminals through a mechanical connection and a separate electrical connection. Thus the electrical connection is isolated from strain and vibrations applied to the power leads ensuring a good and stable electrically connection over the life of the motor. 
         [0044]    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 but not to exclude the presence of additional items. 
         [0045]    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. 
         [0046]    Although the invention is described with reference to a preferred embodiment, it should be appreciated by those skilled in the art that various modifications are possible. For example, the mechanical connection may not be a wire connector or the wire connector may be directly welded to the terminal plate. Also the terminal plate may have a shape different to a male spade connector. Therefore, the scope of the invention is to be determined by reference to the claims that follow.