The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 19 Old machines may often be met with where the method of connection is by screws. This method has, indeed, certain peculiar advantages ; for example, that a spare commutator can be put in by an unpractised hånd, and quickly too. This advantage is, however, entirely overweighed by the danger of some of the screws of the lugs getting loose after the machine has been running for a long time. Then all the coils of the armature have no longer the same resistance, since under certain conditions, a very considerable resistance may be produced through the loosening of the screws. This results in sparking occurring at particular seg- ments of the commutator, by examination of which it can easily be seen whereabouts the faulty connection exists. With connections that have been well sweated in with solder the fault is of extremely rare occurence, and, as a matter of faet, can only occur when many wires are soldered into one segment, for then it may happen that some of the wires have not been suffi- ciently covered with the solder. Break in a Winding.—A breakdown due to the complete break of an armature turn by fracture of the wire appears oftener than the preceding. It often happens, especially with thin wires (i.e. with machines of low output, high voltage, or low speed), that a bad spot in the wire, which is not noticed during winding, breaks after the machine has been running a short time. This naturally occurs more often at those places on the armature where the wires are in tension, c 2