Specifications

170
(i) Incorrect grade of brush and improper brush tension
(ii) Improper bedding of brushes.
(iii) Carbon brush chattering due to:
(a) Excessive clearance between Fit up correct size brush. It should be
carbon brush and its holder. good slide fit in its holder. Clearance
should not be more than 2 mils.
(b) Excessive overhang of brush Normally, the clearance between the
bottom edge of brush holder and the
commutator should only be 1.5mm (1/
16 In. ) or less. The brush holders should
be properly reset and secured in position
so that the clearance is correct.
(iv) Carbon brush too tight in holder This is generally due to accumulation of
carbon dust. All carbon brushes should
be removed entirely out of the holders
once a month at least. Accumulated
carbon dust should be blown off by
compressed air and both the brush and
the holder cleaned thoroughly with dry
cloth. Carbon brushes must slide freely
inside the holder.
(v) Brush tail connections They should be secured properly. If the
tail connection improperly secured
strands are damaged, new brushes
should be fitted.
(vi) Excessive wear of brush Normally, carbon brushes should last for
several months. Excessive wear is
definite sign of poor commutation, it
requires detailed investigation. If the
worn-out brush is not replaced quickly,
the metallic pig tail connection imbedded
inside the carbon brush may damage the
commutator badly.
Care in fitting a new carbon brush
The surface of each carbon brush should be carefully ground to correspond to the curvature
of the commutator so that the brush makes contact over its full area, so as to be able to carry
the full load current. The correct curvature is ensured by drawing a strip of very smooth sand
paper under the brush and over the commutator or the slip ring. It should be moved to and fro in
close contact with the commutator surface. Simultaneously, pressure should be applied on the
brushes. The sand paper should preferably cut the carbons only in the forward direction. Sharp
edges of the carbons should then be slightly beveled and all carbon dust blown off. Emery
paper must never be used for grinding carbon brushes as the emery particles may get
embedded in the carbon. If this happens, the commutator surface would obviously be ruined.
15.12 The usual defects in brushes and brush gear areas are as follows: