The Diseases Of Electrical Machinery 1904
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14 THE DISEASES OF
mutators may be caused by the shrinking of the in-
sulation between the commutator segments and the
damping rings. If vulcanized fibre has been used for
insulation, in the course of time this will always
happen. On the other hånd, the damping rings which
hold the segments in place may be responsible. In
any case, it will hardly be the duty of the engineer
who erects the machinery to correct this. Commu-
tators which have become loose on their shaft naturally
cause sparking, at any rate with carbon brushes, since
they become untrue, and cause the brushes to jog up
and down. With metal brushes, owing to their greater
flexibility, sparking from this or other of the above
causes is much less to be feared.
Commutator out of Truth.—A commutator may
also become untrue in consequence of the varying
hardness of the segments. This fault is indeed less
to be feared when using hard-drawn or drop-forged
copper ; but it often appears when the segments used
are of cast metal, such as simple brass, or (better)
bronze, such as silicium-bronze or phosphor-bronze.
The use of such bronzes for commutators is therefore
not quite satisfactory, although it is not to be con-
demned absolutely.
Brushes and Brush-Holders.—The brushes have
an important influence upon sparking. Errors may
arise, of a disastrous kind, both in form, number and
material. In modern machines, having two or more
rows of collecting brushes, it is the invariable practice