The Diseases Of Electrical Machinery 1904
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22
THE DIS EAS ES OF
S
the poles of the magnets one above the other, it is
customary to decentre the armature by making the
air-gap between the upper pole and the armature
smaller than that between the armature and the under
pole, in order, by the attraction of the upper pole, to
relieve the bearings of a part of the weight of the
armature. This must not be done with multipolar
machines having more than two paths through the
armature winding. It is done in some railway
motors.
The heating of the commutators of low-voltage
machines is often caused by there being too heavy
masses of copper, too massive segments, in which,
owing to the heavy currents, eddy-currents ensue.
The heating roughens the commutator surface, and so
leads to sparking.
Having now dealt sufficiently with those faults
which show themselves by heavy sparking, we pass
on to other phenomena.
The Different Kinds of Armature Short-
Circuits.—One often hears the cause of a break-
down referred to as a short-circuit or, as workmen
call it, a “short” in the armature. Very few of those
who use these words picture to themselves the
various and differing faults which are included in
this name. First, the insulation, between.the armature
winding and the iron core of the armature, may have
broken down at some point. Secondly, the insulation
of one coil may have given way in two places, so that