The Diseases Of Electrical Machinery 1904
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ELECTRICAL MACHINERY.
il
struction may both lead to sparking. In the former
case this shows itself only after some weeks’ working,
so that it is not found out in the test room.
Too Hard Mica Insulation.—The varying de-
grees of hardness of mica must be here noticed. Mica
which, as is well known, is an exceedingly good in-
sulator, is very variable in regard to its hardness. If
we are to use mica in building commutators, we must
see that it is about as hard as the copper, in order
that the brushes should wear away the copper and
the mica at the same rate. As it is certainly not
easy always to fulfil this condition, we have become
accustomed to using mica which is softer than the
copper. Since it is impossible to examine each
mica slip in the workshop, it may happen that some
piece of mica is used which is harder than the copper.
It is then clear that the copper will be worn away
sooner than the mica, so that, after some weeks’ work-
ing, the mica stands out a little above the copper.
This cannot be satisfactorily determined either by the
eye or by touch, since a piece that may project less
than the hundredth of an inch is sufficient to start the
brushes chattering, and thus may cause sparking.
This sparking is very dangerous, since it damages the
copper in a remarkably short time. Grinding with
glass-paper is no use, since such hard mica is not
touched by glass-paper. The application of glass-
paper only increases the fault, since the copper alone
is rubbed away. In such cases a light cut must be