Abstract:
A multi-pole PMDC motor has brush gear for transferring electrical power from motor terminations to its windings via a commutator. The brush gear comprises a brush card supporting at least four brush assemblies. The brush assemblies are separated into two brush groups with the brush assemblies of each group being electrically connected together by links of flexible wire.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 200810141964.2 filed in The People&#39;s Republic of China on Aug. 22, 2008. 
     FIELD OF THE INVENTION 
     This invention relates to an electric motor and in particular, to a multi-pole PMDC motor having a brush card assembly. 
     BACKGROUND OF THE INVENTION 
     For electric motors, especially PMDC motors, with 4 or more stator poles, it is usual to have the same number of brushes as stator poles, cooperating with a commutator to energize the rotor windings in the correct sequence. The brushes are divided into two groups with all the brushes within each group being electrically connected together. Some recent improvements in motor design use equalizers or electrical connections to join together various segments of the commutator, such as diametrically opposed segments for a four pole motor, to reduce the number of brush sets. However, this means that the remaining brushes must carry greater current and this is not always possible or desirable. To avoid using equalizers, the brushes are electrically connected by using lead wires which are connected together outside of the motor. This is considered unsightly and clumsy. One solution has been to insert mold strips of metal into the card with supports the brush assemblies and connect the strips to the motor terminals or even form the motor terminals from the strips, with the brush assemblies being connected, usually by the brush shunts, to the appropriate metal strip. This is a good solution but requires a relatively large brush card and an expensive insert molding process. 
     Hence there is a desire for a simple arrangement for connecting associated brush assemblies in a multi-pole motor, especially a four pole PMDC motor. 
     SUMMARY OF THE INVENTION 
     Accordingly, in one aspect thereof, the present invention provides a PMDC motor comprising: motor terminations; a stator having at least four stator poles; a wound rotor; brush gear for transferring electrical power from the motor terminations to the rotor, the brush gear comprising a brush card supporting two groups of brushes, each brush group having at least two brushes; and links electrically connecting brushes of each group together, wherein the links are flexible wires. 
     Preferably, the flexible wires are single core wires with enamel insulation. 
     Alternatively, the flexible wires are multi-strand wires with PVC insulation. 
     Preferably, the flexible wires are copper wires. 
     Preferably, connectors connect the links to shunts of the brushes, and the connectors have a press-in terminal for receiving the links. 
     Preferably, the motor terminations are two lead wires and each lead wire is connected to a respective group of brushes by being crimped to the connector with one of the shunts. 
     Preferably, the press-in terminal comprises a hole and at least two curved fingers extend from the edge of the hole, radially across the hole and axially in the direction of insertion, the fingers being adapted to resiliently grip the wire there between to prevent withdrawal of the wire from the terminal. 
     Preferably, the links have soldered tips at each end. 
     Preferably, the fingers are adapted to scrape solder from the soldered tips as the tips are pressed passed the fingers, forming a buildup of solder under the fingers to increase the area of electrical contact between the wire and the connector. 
     Preferably, the brush card has a plurality of channels and the links are disposed within respective channels. 
     Preferably, the channels are formed on one side of the brush card and the connector is disposed on the other side of the brush card and the links pass through holes in the brush card and communicating with the channels to engage the connectors. 
     Preferably, the links are captured within the channels. 
     Preferably, the links are a press fit within the channels. 
     Preferably, each brush assembly further comprises a brush guidance mechanism for guiding the brush towards the rotor; each guiding mechanism comprises a pair of substantially parallel rails slidingly received in grooves extending along opposite sides of the brush and a spring for resiliently urging the brush along the rails. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 labelled 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. 
         FIG. 1  is an isometric view of an electric motor in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is an isometric view of a brush card, being a part of the motor of  FIG. 1 ; 
         FIG. 3  is an isometric view from below of the brush card of  FIG. 2 ; 
         FIG. 4  is a schematic view similar to  FIG. 3  with the brush card and various other components removed to show brushes, motor terminations and their interconnections; 
         FIG. 5  is a schematic view of wires forming a part of the interconnections of  FIG. 4 ; 
         FIG. 6  is an isometric view of the bare brush card of  FIG. 2 ; 
         FIG. 7  is an isometric view of a connector, being a part of the brush card of  FIG. 2 ; and 
         FIG. 8  is a schematic sectional view of a part of the connector of  FIG. 7  illustrating the connection with a wire of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates a multi-pole PMDC motor  10  in accordance with the present invention. PMDC stands for permanent magnet direct current and the motor has a permanent magnet stator comprising an magnetic housing accommodating either separate permanent magnet pieces or a single magnet formed into a ring and magnetized to form a number of magnetic poles. Multi-pole refers to a motor having four or more stator poles. The illustrated motor has four stator poles for simplicity of description but the invention is applicable to motors with more than four stator poles. 
     The motor has an end cap  12  closing one end of the housing  11  and a rotor  13  disposed within the housing  11 . The rotor has a shaft  14  which is journalled in bearings. The rotor is a wound rotor having a rotor core and a commutator fitted to the shaft and rotor windings wound about the rotor core and connected to the commutator. A fan may be fixed to the rotor for cooling the motor. Motor terminations  15 , illustrated in the form of lead wires, are provided for connecting the motor to a source of electrical power. The end cap  12  supports a brush card assembly  20  in that the end cap fixes the brush card with respect to the housing  11 . 
     The brush card assembly  20  is shown in  FIG. 2 . The assembly comprises a brush card  21  having a central opening  22  arranged to receive the commutator. The brush card supports a number of brush assemblies, in this example, four brush assemblies with diametrically opposed brushes forming a brush group. 
     Each brush assembly comprises a brush  30  adapted to make sliding contact with the commutator, a shunt  31  connecting the brush to a connector  32  and a brush guidance mechanism. The brush guidance mechanism includes a spring  33  for resiliently urging the brush into contact with the commutator and means for guiding the brush to the commutator. The guiding means may be a standard brush cage, the spring itself or as shown in the preferred embodiment a pair of rails  34  which are accommodated in grooves  35  in the brush  30  allowing the brush to slide along the rails only. 
     The connector  32  has a first portion  36  and a second portion  37  as more clearly shown in  FIG. 4  and  FIG. 7 . The first portion  36  is arranged to connect to the shunt  31  and if required to a motor lead wire  15 . It is preferably a crimp terminal being of a cylindrical or tubular form in which the shunt and lead wire is disposed and then crimped to electrical and mechanically fixed the shunt and lead wire to the connector. As there are only two lead wires only two of the connectors are required to connect to the lead wires, one for each brush group. The second portion  37  forms a push in terminal having a through hole  38  and a pair of fingers  39  which extend from the edge of the hole radially inward and axially in the direction of insertion. 
     To electrically connect together the brushes  30  of a brush group, links  40  formed by flexible wires extend between the connectors of a brush group. The links  40  have soldered tips  41  or end portions which are pressed into the push in terminals of the connectors. As the soldered tips  41  are pressed into the hole  38  they resiliently deform the fingers so as to part the fingers to allow passage of the tips  41 . The fingers thus resiliently grip the tips and prevent the withdrawal of the tips. Attempts to withdraw the tips causes the fingers to bite into the tips. As the tips are being pushed passed the fingers, the fingers scrape off some of the solder on the fingers. The removed solder builds up under the fingers and creates a greater contact surface between the tips and the fingers reducing the contact resistance of the so formed connection. The flexible wire of the links may be multi-strand PVC insulated wire or preferably single core enamel insulated wire. Copper wire is preferred although other wire types such as aluminum wire may be suitable The soldered tips, especially on the multi-strand wire gives the tips greater integrity and resistance to damage when being inserted in the push in terminal. The single core wire of the links  40  is shown in  FIG. 5 , preformed ready to be fitted to the brush card with the tips bent at right angles to the body of the wire for insertion into the terminals. 
       FIG. 3  illustrates the reverse side of the brush card assembly  20  to that shown in  FIG. 2 . Here the links  40  are shown disposed in channels  23  connecting together link holes  24  (see  FIG. 6 ) through which the tips  41  extend to engage with the connectors  32 . The links may be captured within the channels, preferably by being a press fit within the channels. The use of insulated links means that there is greater freedom in locating the links as they can cross beneath a brush assembly, as shown in  FIG. 3  with concern of causing a short circuit. This can not be done with insert molded links. 
     The brush card  21  is shown bare in  FIG. 6 . The central opening  22  has peripheral slots for the brushes giving the opening  22  a crucifix form. Terminal holes are provided for passage of the motor terminations  15 . Four posts  26  support and hold the springs  33 . Four lands  27 , appearing as peripheral extensions of the brush card are used to position the brush card within the housing  11  and cooperate with the end cap  12  to fix the brush card in position. 
       FIG. 7  is an enlarged view of one of the connectors  32  in which the push in terminal is shown having two fingers  39  which are arcuate so as to better conform to the surface of the tips of the links. The curved shape of the fingers makes better contact with the tips and gives a wider area for the build up of the scraped solder. They also scrape a wider strip of solder from the tips. 
     The build up of scraped solder is schematically shown in the sectional view of  FIG. 8 . The scraped solder  42  is shown filling in an area between the fingers  39  and the tip  41 . 
     In this manner, the present invention provides a simple solution to the problem of how to electrically connect together the brushes of brush groups without the need for insert molding of connector links. 
     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. 
     Although the invention is described with reference to one or more preferred embodiments, it should be appreciated by those skilled in the art that various modifications are possible. Therefore, the scope of the invention is to be determined by reference to the claims that follow. 
     For example, the motor terminations are shown and described as lead wires but could be other types of terminations such as for example male terminals formed integrally as part of two of the connectors. 
     Also, the push in terminal of the connector may have more or less than two fingers for scraping and gripping the tips of the links.