The Diseases Of Electrical Machinery 1904
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ELECTRIC AL MACHINERY.
23
the armature coil is short-circuited on itself. The
same effect may be caused by copper dust or dirt
between two segments of the commutator. Or, thirdly
and finally, a coil may have formed a direct short-
circuit with another coil, as may easily occur with
those drum armatures in which many wires cross one
another at the front and back, either by reason of an
unsuitable method of winding, or in consequence of a
knock which the armature has received. In addition
to these three quite different kinds of faults, which all
take the name of short-circuits in the armature, there
exists a series of defects of a similar character.
Contact between the Winding and the Frame.
A chance connection of any point of the armature
winding with the iron of the armature is of no import-
ance so long as no other live part of the machine is
likewise connected with the iron of the machine. A
precisely similar state of things exists continually
in the case of all those machines of which one pole is
put to earth. In the case of such machines, for the
sake of security, the frame is also always earthed.
The result of this is that any small fault in the winding
immediately develops into a short-circuit. In most
cases this will result in the burning out of the arma-
ture winding. This same fault will also occur (i) with
railway motors ; (ii) with motors which are connected
to a tramway Circuit; (iii) with motors with earthed
frame working on a three-wire system, having the
middle wire earthed. Often, too, in ordinary plants,