Abstract:
A multi-brush cage assembly for a subfractional horsepower electric motor has two brushes  22, 23  received in separate chambers  24, 25  of the cage and urged by springs  26, 27  towards a commutator  15  of the motor. An L-shaped changeover lever  30  is located on a pivot  33  between the chambers and keeps one brush  23  retracted until the other brush  22  has worn out.

Description:
[0001]    This invention relates to subfractional horsepower electric motors of the universal type and in particular, to a cage brush assembly for such motors.  
           [0002]    Universal motors have many uses and are commonly used in household appliances. One drawback of using universal motors is that they have a wound rotor and use brushes rubbing on a commutator to transfer electrical energy to the rotor windings. This introduces a life limitation. The higher the power of the motor, the higher the current drawn and the faster the brushes wear. For many small electric motors, the cost of replacing the brushes is prohibitive and most appliances come to the end of their useful life when the motor wears out. However, many appliance manufacturers would like to guarantee their appliances to run for so many hours and this has become an important marketing advantage. As other parts of the motor have improved, the life limiting factor has become the life of the brushes. While the life can be increased by brush composition and the power of the motor, there are other factors which must be taken into account in these choices and as so often is the case, compromises have to be made. However, there is still a need for a longer life motor.  
           [0003]    Another option is to make the brushes longer. However, in many applications, the brushes are already as long as practical or there simply is not the room in the appliance to allow for longer brushes to be used.  
           [0004]    Thus, the desire to provide longer life from the brush assembly has gone unfulfilled until now.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention fulfills this desire by providing a double brush assembly in which, when a first brush reaches its wear limit, a second brush is brought into operation thereby extending the life of the brush assembly.  
           [0006]    Accordingly, in one aspect thereof, the present invention provides a brush assembly for subfractional horsepower electric motor, comprising: first and second brushes; guide means for guiding the brushes; first and second springs for respectively urging the brushes in one direction under the guidance of the guide means from a first position towards a second position, and a changeover mechanism for switching operation from the first brush to the second brush when the first brush reaches the second position, wherein the changeover mechanism includes a lever pivotably mounted on a pin located between the brushes whereby the second brush is held in the first position against the urgings of the second spring until the first brush reaches the second position.  
           [0007]    Preferably, the guide means comprises a brush cage having two compartments in which the brushes are slidably received under the urgings of the springs and the pin is mounted in the cage and extends transversely of the cage between the compartments.  
           [0008]    Preferably, the first brush has a recess near a non-contact end in a side facing the lever, said recess being arranged to receive one arm of the lever when the first brush is in the second position, to allow the lever to pivot and release the second brush.  
           [0009]    Preferably, the lever upon releasing the second brush retracts the first brush from the second position.  
           [0010]    Preferably, the lever is L-shaped with a first arm arranged to retract the first brush when the first brush reaches the second position and a second arm arranged to release the second brush when the first brush is retracted.  
           [0011]    In a second aspect, the invention also provides a subfractional horsepower universal motor incorporating two brush assemblies as described above.  
           [0012]    By use of the inventive brush assembly, the life of the motor can be extended with the expectant brush life increased up to double the previous life without increasing the radial width of the motor, although the axial length of the motor may need to be increased slightly to accommodate the extra brush. If longer brush life is required, the brush assembly can be adapted to receive any number of additional brushes. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0014]    [0014]FIG. 1 illustrates a universal motor incorporating a brush assembly according to the preferred embodiment;  
         [0015]    [0015]FIG. 2 is a schematic illustration of the brush assembly;  
         [0016]    [0016]FIG. 3 is a schematic illustration similar to FIG. 2 at the moment of change over; and  
         [0017]    [0017]FIG. 4 is a schematic illustration similar to FIG. 2 just after the change over. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    [0018]FIG. 1 illustrates a universal motor  10  which apart from the brush gear is of conventional construction. As such, it has a wound stator core  11  supporting stator windings  12  and end brackets  13 ,  14  which hold bearings. A wound rotor having a rotor core, rotor windings and a commutator  15  mounted on a shaft  16 . The shaft  16  is journalled in the bearings. One end bracket  13  also supports the brush gear which comprises two brush assemblies  20 . A lead  17  connects the stator windings to the brush assemblies and power leads  18  connect the stator windings to the electric supply.  
         [0019]    As the brush assemblies are identical, only one will be described. FIGS. 2-4 schematically illustrate the construction and operation of a brush assembly.  
         [0020]    The brush assembly  20  has a cage  21  which guides first and second carbon brushes  22 ,  23  into contact with the commutator  15 . Each brush  22 ,  23  is slidably received in a separate compartment  24 ,  25  and urged into contact with the commutator  15  by a spring  26 ,  27 . Shunts  28  electrically connect the brushes to the cage and to terminals  29  (FIG. 1). An L-shaped changeover lever  30  has a first arm  31  and a second arm  22  and is located between the brush compartments  24 ,  25  at the commutator end of the cage  21 . The changeover lever  30  is pivotably mounted on a pin  33  and is able to pivot between an initial position shown in FIG. 3 to a release position shown in FIG. 4. Brush  22 , which is to be used first, has a cutout portion  34  in the side facing the changeover lever  30  at the shunt end. The purpose of the cutout portion  34  will be described hereinafter.  
         [0021]    During assembly, the first brush  22  is slid into its compartment  24 , the spring  26  is compressed on top of the brush  22 , the shunt  28  is connected to the cage  21  and a flap  35  or two at the end of the compartment  24  is bent over to retain the spring  26 . Brush  22  pushes the first arm  31  of the changeover lever  30  out of compartment  24  by pivoting the lever about the pin  33 . The first arm  31  of the lever  30  is now laying alongside the first brush  22  while the second arm is projecting across the second compartment  25 . The second brush  23  is now slid into compartment  25  but is prevented from reaching the commutator  15  by the changeover lever  30 . A second spring  27  is pressed on top of brush  23 , the shunt  28  is connected and one or two flaps  35  are bent to retain the spring  27  within compartment  25 . The assembly is shown in FIG. 2.  
         [0022]    As the motor is used, the first brush  22  is worn away until, approaching end of useful life, the situation shown in FIG. 3 is reached. At this point, the cutout portion  34  in brush  22  is aligned with the first arm  31  of the changeover lever  30  allowing the lever  30  to pivot clockwise releasing the second brush  23  which has been held up by arm  32 .  
         [0023]    As the second brush  23 , under urgings of spring  27 , pivots the lever  30  clockwise in its rush toward the commutator  15 , it causes the lever  30  to retract the first brush  22  from the commutator  15  slightly, preventing it from wearing further on the commutator and reduces sparking. This occurs by the first arm  31  contacting the upper surface of the cutout portion  34  before arm  31  has reached end of travel when it extends perpendicularly across the compartment  24 . Arm  32  is now parallel to the brush direction and the lever  30  is prevented from pivoting anticlockwise to release the first brush  22  by the second brush  23  which is now blocking the return of the second arm  32 , as shown in FIG. 4.  
         [0024]    The cutout portion  34  may be any convenient shape but the shape shown, namely a quarter circle cutout with a rounded upper outer corner has an advantage of minimal material removal to keep the increase in brush resistance and loss of mechanical strength to a minimum. The rounded upper outer corner facilitates the retraction of the brush and allows the lever to start pivoting with little resistance from the first brush  22 , allowing the arm  31  to gently roll along the upper surface of the cutout  34  as it retracts the brush  22 .  
         [0025]    The embodiment described above is given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention as defined in the appended claims.