The Diseases Of Electrical Machinery

26 THE DIS EAS ES OF P Short-Circuit in the Winding.—While the above mentioned single armature short-circuit only causes breakdowns when a second contact has occurred or is already present, a short-circuit in a winding, that is to say a contact between two points of an armature coil, is at once destructive and causes a serious breakdown. In a dynamo the usual result, unless the fault has short-circuited only a very small part of the winding, is that the machine refuses to excite itself. If the field-magnet is now separately excited, and the machine is run, it still does not reach its full voltage ; and one can also clearly observe that the machine uses up a great deal of power, even when the external circuit is open. In quite a short time, | to a minute, a smell of burning will be noticed. If the machine is at once stopped, and one then touches the coils to ascertain their heating, one can find, by the unequal heating of the coils, that one coil in which the short-circuit exists—it will be much hotter than the others. This heating results from the heavy current which is produced in the short-circuited turn of the armature. To this also is due the great absorption of power, in spite of the circumstances that the external circuit is opened and that no current is being generated for the purpose of excitation. If this kind of fault occurs in a continuous-current motor the effect is that the motor runs very slowly backwards. In certain circumstances the motor may not run at all. One can then, however, if the field-magnet is excited,