FAULTS IN ARMATURES.

hardwood wedge, coated with shellac varnish, and driven in tightly between the wire, will generally effect a cure. If the faulty coil cannot be expeditiously repaired, and the dynamo is urgently wanted, the coil may be cut out of circuit altogether, and the corresponding commutator segments connected together with a piece of wire, of a size proportionate to the amount of current to be carried, soldered to each. It will not be necessary to cut out and remove the entire coil.

If the active portions only are separated from one another, so that they do not form a closed circuit, it will answer the purpose-e. g., if the wires are cut with a chisel at the point where they pass over the ends of the core, and the ends separated, it will be quite as effective as removing the entire coil. It is wise, of course, to rewind the coil at the first opportunity.

(b) Short Circuits between Adjacent Coils.—Whilst in the ring armature the presence of this fault does not necessarily imply that the machine will not build, in the drum armature wound into a single layer of conductors it entirely prevents this occurring. Reference to Fig. 128 will show that adjacent coils are during a certain period of the revolution at the full difference of potential generated by the machine. Hence, if any two adjacent coils are connected together or short circuited, the whole of the armature will be practically closed on itself, any current generated flowing within the armature only, without passing to the external circuit.

Large drum armatures wound with compressed and stranded bars and connectors are particularly susceptible to this fault, a

FAULTS IN ARMATURES.

slight blow generally forcing one or more of the strands into contact with the adjacent bars, thus short circuiting the armature, and rendering it practically useless so far as the generation of current is concerned. In this class of short circuit in drum armatures, the method of locating the faulty coils by exciting the field, and running the armatures on open circuit, is not applicable, for the reason that the whole armature will be heated equally. Mr. Loomis, in the Electrical Engineer of New York, has described a method of locating faults of this description, and, as it appears reliable, it is given below:

"In following this plan, no arrangement for belting need be made. It is only necessary to fasten a monkey wrench to the rim of the pulley, or, better still, a crank to the shaft. Now, excite the fields, and, to make the effects more marked, connect the coils in parallel, as the excessive current will only be used for a moment. When this has been done the strongest man will scarcely be able to rotate the armature, and then only with extreme slowness, except at one position. When this position has been found, mark the armature at points in the centre of the pole pieces (A B), as shown in the accompanying diagram (Fig. 109), and at both ends of the armature. The explanation is that both halves of the armature oppose one another at this position; but when not at these points a continuous circuit is formed, and the resultant magnetic effect is enormous. As the 'cross' or 'short' circuit will be found at one of these four marked points, it becomes desirable to know in which one it is most likely to be found.

FAULTS IN ARMATURES.

Experience has shown that it is nearly always on the commutator end in the last half of the winding, where the wires pass down through the first half terminals. This applies to an unequal winding. In armatures where the windings are equal, it is as liable to occur at one point as at another. With this method a defect can be found and remedied in a few moments, for it has always been a simple matter to repair it

when discovered. These results can be observed in a perfect armature by connecting the opposite sections of the commutator.

The above will be understood to apply to armatures having Siemens winding." Faults of this description can frequently be discovered by a careful inspection of the windings of the armature without recourse to testing. When located, the fault can usually be repaired with a hardwood

FAULTS IN ARMATURES.

wedge, as explained above, or a piece of mica or vulcanized fibre cemented in place with shellac varnish.

(c) Short Circuits between Sections through Frame or Core of Armature.-The localization of this fault can be effected by the methods described above, and by disconnecting the whole of the armature coils from the commutator and from each other, and testing each separately with a battery and galvanometer coupled up as in Fig. 137, one wire being connected to the shaft and the other to the end of the coil under test. As a rule, there is no way of remedying this fault other than unwinding the defective coils, reinsulating the core, and rewinding new coils.

(d) Short Circuits between Sections through Binding Wires.-This fault is the result of a loose winding, and is caused by the insulation upon which the binding wires are wound giving way, thus bringing coils at different potentials together. As a consequence to the heavy current which flows, the binding wires are as a rule unsoldered or burned. The location of the fault can therefore be effected by simple inspection. To remedy, it will be necessary to unwind and rewind on new binding wires, on bands of mica or vulcanized fibre, soldering at intervals to obviate flying asunder.

(e) Partial Short Circuits in Armatures.-This is, as a rule, due to the presence of moisture in the windings. To remedy, the armature should be taken out and exposed to a moderate dry heat, or subjected to a current equal to that ordinarily given by the dynamo. Under the action of heat or