The Diseases Of Electrical Machinery 1904
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i o THE DIS EAS ES OF
mistake (which the author has met with several times)
of inserting carbon brushes in the holder without any
grinding of the surface whatever. One cannot suffi-
ciently strongly point out that a badly ground brush
has been the destruction of many a commutator. This
work, therefore, must not be carelessly done; but a
fair amount of time should be devoted to it, and the
machine only set again in full working when one is
convinced that there is a good and perfect contact
at the sliding surface. It is for this reason advisable
to run a machine with new brushes for several hours
without load, so that the brushes may become
thoroughly bedded to the outline of the commutator.
Brushes are ground to shape by bending a piece
of glass-paper (rough side uppermost) round the com-
mutator, placing the brushes in the brush-holder, and
screwing them up tight against the glass-paper. The
brush rocker is then moved backwards and forwards
until the brushes, by rubbing upon the glass-paper,
have taken the same curvature as the commutator
surface, and bed properly against it.
Unsuitable Commutator Material.—We now
come to the actual faults which may lead to spark-
ing. We must in the first place mention the purely
mechanical cause of unsuitable material. For moclern
commutators with carbon brushes, the segments are
made of hard-drawn or drop-forged copper, with layers
of mica insulation between the segments. With these
commutators unsuitable material and improper con-