How to Select Good carbon brushes:
Brush grade selection involves considerable tests both on the test bed and under
actual service conditions. It is sometimes found that brushes which are considered
satisfactory on the test bed do not operate satisfactorily in service. In view of this, the proper grade can only be selected after suitable service trials and evaluation.
Indian Standard Specification IS-3003 covers dimensions, requirements and test
procedures for carbon brushes. Divergence in the physical properties and dimensions of carbon brushes can cause considerable trouble in service. Verification of the properties involves exhaustive testing, and since the carbon brushes are required to be procured rather frequently, it is not practicable to carry out such a large amount of tests on each lot purchased. It is extremely important, therefore, to restrict the brush procurement from established and well proven sources only, even if the prices may be higher. Also, whenever a new supply source or a new brush grade is considered, detailed tests/service trials should be carried out before approving the same for bulk use.
Some of the defects usually noticed on the carbon brushes are:
Dimensions not confirming to the drawing
Bowed/Curved and chipped carbons
Poor quality of pig tails, which results in their getting frayed/broken in service
Bad joints between pigtail and carbons, resulting in high unequal voltage drops
across the same
Hair line, invisible cracks at pigtail to carbon joints
Physical properties not conforming to the grade