FAULTS IN ARMATURES. of this current the moisture will be gradually, dispersed. When thoroughly dry, and whilst still warm, a coat of shellac or rubber varnish should be applied to the whole of the windings. (2) Grounds in Armatures. (a) Armature Coils Grounded or Connected to Core or Frame of Armature.-When this fault is confined to a single coil, it is not in itself liable to do any damage. If, however, a similar fault develops in some other part of the dynamo, or in the external circuit, the coil is liable to be burned out. Mr. F. Bain, in the Western Electrician, has described a simple method of locating a coil grounded to the frame of an armature, which is reprinted below. Fig. I clearly shows the arrangements of the details. B is a battery or dynamo circuit, giving a current of a few amperes through the armature by its own brushes (1 and 2). At î, a roughly-made galvanometer, to carry some 25 amperes or so, is placed, one terminal being in connection with the shaft of the armature, and the other attached to a movable brush (3). Since the function of the particular galvanometer is simply to show a deflection when a current is passing, and to mark zero when there is none, a coil of thick wire, with a pocket-compass in the centre, will do all that is required, but care must be taken to remove it sufficiently far away from the disturbing effects of the armature magnetism The manner of testing is as follows: Assume a steady current to be flowing from battery (B) through the armature, touch the commutator with brush (3), FAULTS IN ARMATURES. and a current will flow through (G). Slowly rotate the armature or the brush (3), until the galvanometer (G) shows no deflection. The coil in contact with 3 will be found to be grounded or connected to the frame. A hand regulator or rheostat (R) may be inserted in series with the battery or dynamo circuit to regulate the strength of the current passing. The main advantage of this test is that the damaged coil can be located without unsoldering the coils from the commutator, which is sometimes a difficult operation without proper tools; and further, the fault can frequently be repaired without disconnecting any of the wires if its exact position be determined. FAULTS IN ARMATURES. (3.) Disconnections in Armature Circuit.—A partial or complete disconnection in the armature circuit is always characterized by heavy sparking at the commutator, but not, as a rule, by an excessive heating of the armature or slipping of the belt, and this enables this fault to be distinguished from a short circuit. The faulty part can always be readily located by the "flat" which it produces upon the surface of the commutator. The armature circuit being open at the faulty part, heavy sparking results at every half revolution as the brushes pass over it, and as a consequence the corresponding segments are "pitted" or "flattened" with respect to the others, and may easily be discovered on examination. The fault may occur in either the commutator or in the coils of the armature. To ascertain whether it is in the latter, carefully examine the winding of the faulty coil. The defect may be sought for more particularly at the commutator end of the armature, as breaks in the wire are most frequent where the connections are made with the commutator segments. If no break can be discerned, try passing a heavy current through the faulty coil by means of the brushes. If a disconnection exists in sufficient contact to pass a current, the coil will be heated in the neighborhood of the disconnection, and may be discovered by running the fingers over the coil. When located, the fault may be repaired by rewinding the coil, or carefully cleaning the broken ends and jointing. The fault may also be temporarily repaired by soldering the adjacent commutator segments together without disconnecting the coil. FAULTS IN ARMATURES. (4.) Flats on Commulator.-This is the name given to a peculiar fault which developes on one or more bars of the commutator. It is not confined to dynamos of bad design or construction, but frequently appears on those of the highest class, and may be recognized as a “pitting” or flattening" of one or more segments. It is always accompanied by sparking at the brushes, and may be due to a periodical jumping of the brushes, caused by a bad state of the commutator, or a bad joint in the driving belt, or to a flaw, or a difference in the composition of the metal of the particular bar upon which it appears. But more frequently it may be traced to a more or less developed fault, such as a disconnection, either partial or complete, in the armature coil. The disconnection may occur either in the coil itself, or at the point where its ends make connection with the lug of the commutator, or at the point where the lug is soldered to the segment of the com mutator. To remedy the fault, the brushes should be examined to see if any periodical vibration takes place. If such is the case, the cause should be removed, and the flat carefully filed or turned out, and the brushes readjusted. If it is due to a difference in the composition of the metal of which the segment is made, the flat will exist as long as the particular segment is in use, and will need to be periodically turned out if a new segment is not fitted into the commutator. Now that the construction of commutators has been improved, however, by the use of hard-drawn copper or phosphor-bronze segments, this fault is rarely due to this last-mentioned |