1.  Field of the Invention
The present invention relates generally to an electro-machine system, and more particularly to a brush holder of a slip ring. 
2.  Description of the Related Art
When an electro-mechanical system, such as electric machine or power generator, is running, a slip ring serves the function of electrical signal transmission between a running device and a static device to input/output signals and to change the direction.  The slip ring has several advantages including raising the system's performance, simplifying the structure, and avoiding the wire from being breaking when the system is running. 
Typically, the slip ring is provided with carbon brushes for signal transmission.  FIG.  1 shows a conventional brush holder of the slip ring on which the carbon brushes are provided.  The brush holder includes a substrate 60 and a plurality of brush frames 70.  The substrate 60 has a conductor pattern 62 thereon, and each brush frame 70 is provided with two carbon brushes 72 on opposite ends.  The brush frames 70 are fixed on a bottom side of the substrate 60 by welding and are electrically connected to the conductor pattern 62 that the carbon brushes 72 are electrically connected to wires 75 through the conductor pattern 62.  User may control the connection between the carbon brushes 72 and the wires 75 through the conductor pattern 62.  However, the conductor pattern 62 of the substrate 60 is very complex that it costs much to design and manufacture the substrate 60. 
To improve the above drawback, an improved brush holder of a slip ring was provided.  As shown in FIG.  2, the brush holder includes a substrate 80 and a plurality of brush frames 90.  The substrate 80 is provided with a plurality of bores 82.  Each brush frame 90 is provided with a bore 92 at middle that a bolt 85 is inserted into the bores 92, 82, and then engages a nut 86 to fix the brush frames 90 on a bottom side of the substrate 80.  Wires 95 are connected to the bolts 85 respectively that user may control the signal transmission through the bolts 85 and the wires 95.  This design has the advantages of low cost and fast manufacture.  However, the bolt 85 and the nut 86 will be loose or get rust after a time of use that the electrical signal transmission will become unstable. 
Besides, aforesaid slip rings have a fixed size that the specific brush holder has a fixed number of brush frames, and only may be mounted in a specific slip ring.  For a slip ring with different numbers of brush frames, the substrate and the conductor pattern thereon have to be redesigned.  Therefore, it still has some places that need to be improved in the conventional slip ring.