d. Between sections of armature;

e. Partial short circuits.

2. Grounds;

3. Breaks in armature circuit.

Short Circuit in Individual Coils.--This is a common fault, which makes its presence known by a violent heating of the armature, flashing at the commutator, flickering of the light on lighting circuits, 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.

Ques. How is the faulty coil located?

Ans. When the machine is stopped, the faulty coil, if not burnt out, can generally be located by the baked appearance of the varnish or insulation, and by its excessive temperature over the rest of the coils, being detected also by the baked appearance of the varnish or insulation.

Ques. What should be done if the machine do not build, and it be suspected that the fault is due to short circuited armature coils?

Ans. 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 for the purpose of localizing a short circuit, precautions should be taken, and the machine only run for a few minutes at a time until the faulty coil is detected.

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 segment to segment, and scraped clean. If the commutator be apparently all right, the fault probably lies in the winding. The insulation of the winding should be carefully examined, and any metallic filings or other particles discovered therein carefully removed, and a little shellac varnish applied to the faulty part.

Ques. If the insulation on adjacent conductors has been abraded, how should it be repaired?

Ans. A small boxwood or other hardwood wedge, coated with shellac varnish should be driven in tightly between the wire; this will generally be sufficient.

Ques. If a faulty coil cannot be quickly repaired and the dynamo be needed, what should be done?

Ans. The coil may be cut out of circuit, 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 be separated so that they do not form a closed circuit, it will answer the purpose. If the wires be 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.

Short Circuits between Adjacent Coils.--In ring armatures the presence of this fault does not necessarily imply that the machine will not build; in drum armatures, wound into a single layer of conductors, it entirely prevents this occurring.

Reference to a winding diagram will show that adjacent coils are during a certain period of the revolution at the full difference of pressure generated by the machine. Hence, if any two adjacent coils be connected together or short circuited, the whole of the armature will be practically closed on itself, any current generated flowing within the armature only.

Large drum armatures wound with compressed and stranded bars and connectors are particularly susceptible to this fault, a 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, does not apply, for the reason that the whole armature will be heated equally.

A method of locating such fault is illustrated in fig. 704. This applies to drum wound armatures. 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 wedge, as explained above, or a piece of mica or vulcanized fibre cemented in place with shellac varnish.

Short Circuits between Sections through Frame or Core of Armature.--Detection 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. 705, 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.

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 pressures together. As a consequence of 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.

Partial Short Circuits in Armatures.--This is usually due to the presence of moisture in the windings. To remedy the fault, the armature should be taken out and exposed to a moderate heat, or subjected to a current equal to that ordinarily given by the dynamo. Under the action of heat or of this current the moisture will be gradually dispersed. When thoroughly dry, and while still warm, a coat of shellac should be applied to the whole of the windings.

Burning of Armature Coils.--The reason for the burning of an armature coil may be explained as follows: The coil, segments, and the short circuit between the segments form a closed circuit of low resistance so that it is only necessary to have a low pressure set up in the active portion of the coil to force a very large current through the coil and the short circuited commutator bars. The heating effect of this current is sufficient to burn out the coil.

Cutting Out Damaged Armature Coils.--To cut out a damaged coil from an armature, first, disconnect the coil from the commutator, and after cutting off the leads, insulate the exposed parts with tape. Then connect the commutator bars (which were connected with the leads) with a wire of the same size as the wire winding.

To remove the coil entirely, cut the band wire or remove the wedges, and lift up a sufficient number of leads and coils to permit of the removal of the damaged coil.

Grounds in Armatures.--These faults occur when the armature coils become connected to the frame or core of the armature. When this grounding is confined to a single coil, it is not in itself liable to do damage. A simple method of locating a grounded coil is illustrated in fig. 708.

Ques. What is the advantage of this test?

Ans. The damaged coil can be located without unsoldering the coils from the commutator, which is sometimes a difficult operation without proper tools; further, the fault can frequently be repaired without disconnecting any of the wires if its exact position be determined.

Magneto Test for Grounded Armatures.--A magneto test for grounded armatures is not to be recommended, as armatures often possess sufficient static capacity to cause a magneto to ring even though there be no leak. This is due to the alternating current given by the magneto for when the circuit has capacity it acts as a condenser and at each revolution of the armature of the magneto a rush of current goes out and returns, charging the surfaces of the conductor alternately in opposite directions, and ringing the bell during the process.

Breaks in Armature Circuit.--A partial or complete break in the armature circuit is always accompanied 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 the 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 become "pitted" or "flattened" with respect to the others; they may easily be discovered on examination.

Breaks in the armature circuit may occur in either the commutator or in the coils of the armature. To ascertain whether it be 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 discovered, try passing a heavy current through the faulty coil by means of the brushes.

If a partial break exist with sufficient contact to pass a current, the coil will be heated at that point 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.