SECTION 1 -INTRODUCTION TO BRUSHES AND CARBON GRADES
Question 1:4 What Additives Are Used?
All manufacture carbon is porous, to some degree, making later treatments possible. Moisture and other substances absorbed and held in the pores can play an important part in the chemical reaction which takes place at the rubbing surfaces and can thus influence the formation of the patina or skin on the collector surface.
Special operating conditions sometimes necessitate the artificial introduction of chemical additives into the brush material to conuteract any adverse effects of such conditions and to help in the control of commutator patina or skin formation.
Additives like paraffin, polyester resin, phenolic resin and special non-mineral oils can improve the performance and brush life under certain circumstances.
As a general rule, these additives reduce the current collecting characteristics and restrict the operating temperature of the brush material. The presence, however, by improving mechanical strength, modifying friction characteristics or by controlling patina formation, often results in a definite net gain in performance under difficult conditions, such as traction motors or machines used on aircraft.
Aircraft application is a particularly good example of how additions can extend the range of satisfactory brush operation.
Electrical machines on aircraft have to operate in extremely low humidity existing at high altitudes. In the absence of adequate atmospheric moisture, brush friction can be very high and before the use of special additives, brush wear on high altitude aircraft was often disastrously high.
Similarly, high friction wear can occur in atmospheres of hydrogen, nitrogen and helium if the water vapour density is unfavourably low. Furthermore, the absence of oxygen can inhibit development of a satisfactory patina or surface skin on the collector, in which case corrective additives in the brush material would be required.
Question 1:5 Why Do Some Brushes Look Like Copper?
The information has been drawn from experience of Morganite Taiwan Limited Application Specialists combined with a wide range of published information from major carbon supplier and motor manufacture. However, the material can not be downloaded, copied or used without written permission.
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