The Diseases Of Electrical Machinery

34 THE DISEASES OF Short-Circuit in a Coil.—This fault is better recognised by an increase of the shunt current, and by a decrease in the resistance of the defective coil, than by the heating, which is probably always present to a certain extent. The resistance is decreased by the bridging over of several of the layers, and there- fore the number of turns in the coil which are effec- tively at work is less than the actual number in the coil. Both these causes tend to increase the shunt current (at the same voltage), and therefore raise the heating of the coils that are at work. A direct short- circuit in a coil may produce so great a bridging-over of the turns that the whole coil no longer has any magnetizing effect; as, for examplQ, if the end and beginning of the winding have become electrically con- ..nected together. With modern machines this will not often occur; but formerly, where the bobbins were made of zinc, which is not a mechanically satisfactory material, this fault has often happened. Such a com- pletely short-circuited coil will naturally not become heated in working, since no current flows through it. With a bipolar machine this cutting out of one coil will have no further influence on the working of the machine ; since, as is well known, when there are two opposite poles, one only need be wound, while the other will be a resultant pole without any winding. In spite of this, the strength of both poles (not taking into account the magnetic dispersion) will be the same as if both coils were at work ; and since by the short-