The Diseases Of Electrical Machinery 1904
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ELECTRICAL MA CH1NER Y.
15
to have at least two, and frequently more than two,
brushes in each row. A row of narrow brushes will
make better contact than one wide brush. In the case
of carbon brushes, as many narrow brushes as possible
should be used side by side in each row. A single
wide brush would be far too stiff and immovable, and
the brush holder would be too heavy to follow small
inequalities in the commutator. In general, a brush-
width * of 1 inch should not be exceeded ; carbon
brushes of above i|th inch in width, should be al-
lowed in exceptional cases only. It is not only the
width, but also the breadth f of the brushes which is
of importance. In this respect the breadth, as used
originally by the builder, should be followed. He
alone is in a position to prescribe the most suitable
breadth for the brushes, since this dimension is of
great influence upon the commutation. It is therefore
a mistake to oi'der carbon brushes later from another
maker, who might supply brushes of a wrong breadth.
Carbon Brushes.—The quality of the brushes
also plays an important part. The resistance of the
different qualities is very different. It is not desirable
for every continuous-current machine that the brushes
should have an especially high or low resistance.
* The term “width” means here the width in a direction parallel
to the axis of the machine.
f The term “ breadth ” is here used for the breadth in the direction
around the commutator. The breadth of a carbon brush is usually at
least twice the breadth of one of the segments of the commutator, and
frequently as great as that of three segments.