FAULTS IN ARMATURES.

The armature and commutator are the most vulnerable parts of a dynamo, and being subjected whilst rotating to various detrimental influences, are a prolific source of faults, the chiefest of which may be enumerated as follows:

(1) Short circuits in armature.

(2) Grounds in armature.

(3) Disconnections in armature circuit.

(4) Flats on commutator.

(5) Short circuits in commutator.

(6) Rough and uneven surface of commutator.

(7) Segments loose or knocked in.

(1.) Short Circuits in Armatures.-The classes of short circuits which occur in the armature are- (a) Short circuits in individual sections or coils; (b) Short circuits between adjacent coils; (c) Short circuits between sections through frame or core of armature; (d) Short circuits between sections through binding wires; (e) Partial short circuits in arma

ture.

FAULTS IN ARMATURES.

(a) Short Circuits in Individual Sections or Coils.-This is an exceedingly common fault, which makes its presence known by a violent heating of the armature, flashing at the commutator, flickering of the light, and by a smell of burning varnish or overheated insulation. When these indications are present, the machine should be stopped at once, otherwise the armature is liable to be burnt out. The fault is due either to metallic dust lodging in the insulation between adjacent bars of the commutator, or to one or more convolutions of the coils coming into contact with each other, either through a metallic filing becoming embedded in the insulation or damage to the insulation. When the machine is stopped, the faulty coil, if not burnt out, can generally be located by running the fingers over the armature, by its excessive temperature over the rest of the coils, and by the baked appearance of the varnish or insulation. If the machine will not build, and it is suspected that the fault arises from short circuited armature coils, the field magnets should be excited by the current from a storage battery or another dynamo, and having raised the brushes from contact with the commutator, the armature should be run for a short time. In stopping, the faulty coil or coils may be located by the heat generated by the short circuit. When the dynamo is started to localize a short circuit, great precautions should be taken, and the machine only run for a few minutes at a time until the faulty coil is detected. A short circuited coil may also be located by the fall or potential method. Fig. 107 illustrates diagramatically this method of testing. Discon

FAULTS IN ARMATURES.

nect the external and field circuits from the armature, and pass a large current-say from 20 to 100 amperes-from a

C

C

FIG. 107.

battery (B) or another dynamo through the whole armature by means of the brushes. Then, having previously well

FAULTS IN ARMATURES.

cleaned the commutator, measure the difference of potential between adjacent segments all round the commutator (c), by means of a voltmeter or galvanometer (G), the terminals of which are connected to adjacent segments, as shown.

The short circuited coil or coils will be located by the difference of potential between the corresponding segments being little or nothing. It may be remarked, however, that this is not always a decisive test. In some cases the short circuit may be intermittent, or may disappear as soon as the armature ceases to rotate. I. such cases, the short circuit is caused by the wire coming into contact through the action of the centrifugal forces developed by the rotation of the armature.

The former test is therefore the more reliable in all cases. When the faulty coil has been located, the insulation between the segments of the commutator to which its ends are connected should be carefully examined for anything that may bridge across from plate to plate, and scraped clean from any adherent metallic particles or burrs. If the commutator is apparently all right, the fault probably lies in the winding. The insulation of this should be carefully examined, and any metallic filing or other particle discovered therein carefully removed, and a little shellac varnish applied to the faulty part.

It will sometimes happen that a small portion of the insulation has been abraded from two adjacent conductors, thus causing them to come into electrical contact with each other. In such circumstances a small boxwood or other