The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 27 clearly prove the existence of the short-circuit by turning the armature of the motor, when the motor will begin to run backwards. In these cases rewinding of the armature is usually necessary. At the very least a new coil must be inserted in place of the defective one. Short-Circuit between. Two Armature Coils.— The occurrence of a short-circuit between two different coils is still worse, since in such cases a great part of the armature winding will become carbonised, unless the fault is immediately noticed and the machine immediately stopped. Short-Clrcu.it in the Commutator.—All that has been said above regarding the winding, applies simi- larly to the commutator. If everything else on the machine is all right, a direct short-circuit of one seg- ment with the commutator shell or spider is, in itself, of no account so long as no other short-circuit occurs. If two adj acent segments touch one another, or if both are in contact with the shell or spider, the result is the same as if a coil was internally short-circuited. The coil will burn, and the same effects, the large amount of power taken, the heating and (if a motor) the back- ward running of the machine, will be the same. If two segments at a greater distance from one another are in contact, the result is the same as if two points of two different armature coils were in contact. The burn- ing of the whole intervening winding will be the result. In this last case a dynamo, even with separate excita- tion, will give no voltage, and a motor will not start.