The Diseases Of Electrical Machinery 1904
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k6 THE DISEASES OF
Nor does it follow that a machine which runs well
with copper brushes will run well with carbon brushes,
or vice versa. It depends upon the build of the
machine, upon the dimensions of the pole shoes, and
upon the number of segments, whether high or low
resistance brushes are needed. It must also be pointed
out that although a brush may have very good me-
chanical properties, yet it may not work satisfactorily.
From the question of resistance we now pass
to the mechanical properties. Many carbon brushes
are too hard to be used ; they break off at the cor-
ners, and thereby lead to sparking. Other brushes,
again, are too soft ; they dirty the commutator, and
wear away very rapidly. The various kinds of carbon
brushes which are on the market very rarely fulfil all
the requirements which must be expected. Here,
again, one firm—the company “ Le Carbone ”—has
made itself prominent by the production of really good
carbon brushes. Brushes of the finest quality are in-
deed fully six times as dear as those of other makes.
They are, however, extremely satisfactory in working,
and will stand a high current density, so that from a
brush of i square inch of working surface one may
take, in continued working, a current of 30-40 amperes;
and, at times of momentary overload, easily twice as
much.
The commutator material for use with carbon
brushes must be without exception either hard-drawn
or drop-forged copper. The use of bronze or cast-