The Diseases Of Electrical Machinery

 ELECTRIC AL MACHINERY. 29 tact with the shell or spider), we see clearly from Figs. 6 and 7 that the short-circuited path for the current extends over all the coils lying between the two faults. The machine, therefore, will work no longer, and in the majority of cases will burn out. With drum-armatures, in such an event, the complete rewinding of the armature is usually necessary; while with ring-armatures, the renewal of the two coils only is necessary, unless the other short-circuited coils have been carbonised by the heavy current passing for a considerable time. From Figs. 4-7 (which have been put together in one diagram) one can further see what has been pre- viously stated, viz. that a single contact is in itself of no account, but that it only becomes dangerous when a second point of contact comes into existence. When repairing continuous-current machines, it may happen that the commutator is removed and in- correctly replaced, so that the wires leading to the segments are wrongly connected. The replacing of a commutator, and the connection of it with the winding, must not therefore be left to an inexperienced hånd. Fig. 8. If, for example, the correct method of connection is that represented in Fig. 8, it would be quite possible, even for a trained man, unless he had noticed the correct method of connection before removing the I II 1 I