The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 33 all, both the beginning part and the end part of every coil ought to be completely free from any drag or strain. This is practically carried out in many different ways, as may be found described in treatises on the construction of dynamos. A break in the winding of the field in the case of a continuous-current dynamo is not in itself dangerous, since the machine (even with separate excitation) will not work. But in the case of motors this fault is an exceedingly dangerous one, since the motor, if running without load, will immediately race. The fault is also a dangerous one in the case of dynamos that are being operated in parallel on the ’bus-bars of a station ; for if one machine thus loses its excitation, the other machines in parallel with it will at once pour their currents into its armature. The presence of such a fault can easily be ascertained by the absence of a spark when opening or closing the field regulating switch. Also the ampere- meter in the exciting circuit will show no current. The Different Kinds of Short-Circuits.—Short- circuits can be caused in very different ways. First, in the case of a shunt machine, a coil may be short- circuited in itself in consequence of rough treatment during winding, the insulation being broken through at several places. The current will then pass directly across between different layers, and a number of turns are completely short-circuited and thereby bridged over, so that they are excluded from having any in- fluence upon the production of the magnetic flux. D