The Diseases Of Electrical Machinery

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,