Carbon brushes are critical to the operation of the paper machine with direct current (dc) motors.Worn or failed brushes cost thousands of dollars per hour.Proper application of carbon brushes, for varying conditions, will provide optimal brush life and minimal commutator wear electrographitic and graphite(resin-bonded) are the two carbon brush families used extensively.
Electrographitic brushes are made from lampblack or carbon black with pitch and tar binders. The carbon material may be kiln fired twice at 1100 degrees Centigrade to burn off the volatiles. Then,it is placed in electric induction or resistive furnaces to graphitization and the temperatures is about 2600 to 3000 degrees Centigrade.
Electrographitic grades can also impregnated with resins to enhance their brush life and improve friction characteristics.
properties:
1、 High Carbon Brush to Commutator Contact Drop
2、 Good Commutatio
3、 Low Friction
4、 Low Abrasiveness
5、Current Rating of Approximately 80 APSI.
Graphite grades, also called resin-bonded grades, are processed at lower temperatures than electrographitic grades. To manufacture resin-bonded grades, natural graphite or artificial graphite is mixed and bonded with pitch and resins.
The graphite materials are baked at temperatures ranging from 200 to 800 degrees Centigrade.
Graphite grades can also be impregnated with resins to enhance brush performance.
properties:
1、 Reduce Threading
2、 Good Filming
3、 Polishing Action
4、 Wider Resistivity Range
5、 Current Range of 10 to 65 APSl
6、 Laminated Structure
Sometimes,various grades operate satisfactory when mixed together in the same motor, but we don’t advocate this. When different brush grades are installed in a dc machine,the electrical and magnetic balance can be affected.
The brush holder spring is used to properly hold the carbon brush in contact with the commutator to allow electrical current to flow. It also has an affect on the brush life.The recommended spring pressure for commutator brushes is 3 to 6 PSI.If the brush pressure is below 3 PSI, arcing will occur between the brush and commutator. If the pressure is above 6 PSI,the brush friction will increase, affecting commutator and brush wear.
