DISTORTION OF MAGNETIC FIELD IN THE DYNAMO.

As soon as a dynamo commences to supply current to a circuit, a number of reactions between the armature and magnetic field immediately takes place. The chiefest of these

FIG. 119.

reactions is the distortion of the magnetic field, and consequent shifting of the neutral points. This distortion of the magnetic field is directly due to the fact that the armature, when

DISTORTION OF MAGNETIC FIELD.

working, becomes itself an electro magnet, the poles of which exert an attracting or repelling force upon the magnetic field. The nature of this armature magnetism will be understood by reference to Fig. 119. It will be seen that the currents generated in the windings on each side of the armature, by its motion in the magnetic field, are flowing from the top to the bottom of the ring. On applying the rule given on page 81, it will be found that these currents tend to produce north and south poles on each half of the core at the points where the current enters and leaves the armature. There will thus be two north poles at the top of the ring, and two south poles at the bottom; but as these poles are adjacent to one another, the external effect will be equivalent to a single north and south pole situated at the top and bottom of the ring. The resulant effect of these two armature poles upon the magnetic field is to twist the line of force round into an oblique direction as shown in Fig. 120. Since in order to secure sparkless collection of the current and to obtain the greatest difference of potential between the brushes, it is necessary to place these latter at points situated at right angles to the direction of the lines of force in the armature, the twisting of the lines necessarily involves the shifting of the brushes round the commutator in the direction of rotation, otherwise the armature coils will be short circuited by the brushes while actively cutting the lines of force, and considerable sparking at the brushes will result.

EDDY CURRENTS.

Another very important action which occurs when the dynamo is working is the production of eddy currents in different parts of the machine.

FIG. 120,

This term is applied to the currents which are always pro duced when a solid metallic mass is rotated in a magnetic field, or is subjected to the action of a magnetic field which is undergoing change in its intensity or strength, for the reasor that the currents generated always tend to flow in more or less circular paths. When produced in large solid metallic masses, the strength to which these eddy currents attain is frequently very considerable, owing to the low electrical resistance of the masses in which they flow; and, in addition to consuming a large amount of energy, they frequently occasion a large and dangerous rise in the temperature. The

EDDY CURRENTS,

cores of armatures being made of iron, are, unless suitably constructed, subject to the detrimental influence of these eddy currents, as is also the case with the conductors wound upon the core if these are of large cross-sectional area. To entirely prevent the generation of these eddy currents is impossible; they can, however, be prevented attaining any considerable strength, by suitably interposing resistance in their path. With this object, the cores and conductors of armatures are laminated, or subdivided into a number of small parts, each of which is electrically insulated from the other by some insulating material.

COUPLING UP FIELD MAGNET COILS.

In coupling up the exciting coils of dynamo field magnets, the primary essential is to so arrange the connections of the coils that the magnetizing current flowing through them produces the requisite polarity in the respective pole pieces. In those field magnets provided with a single coil only (as in Fig. 76), no mistake can obviously be made; when two or more coils are used, however, it is possible to so arrange the connections that poles are produced in the yokes or other portions of salient pole field magnets; or in case of consequent pole field magnets, the connections may be so arranged that the coils neutralize each other, and no external field whatever is produced in the manner in which the coils of the various types of salient and consequent pole field magnets are coupled.

DISEASES OF DYNAMOS.

Taken as a whole, an electric machine is remarkably durable, and the cost of maintenance very small. There are only three parts that can really wear away-these are the two journals and the commutator. The shafts in nearly all machines are made of steel, and in some are even hardened and ground. Therefore the wear in the journals takes place mostly on the boxes, and these, being interchangeable, can be replaced in a few minutes and at small expense when worn out. The commutators, if they run without sparking, should last several years.

As there are virtually only three parts that really wear out, it might be supposed that the renewal of these parts would be all that would be required to keep the machine in perfect running order. But experience shows that such is not the

case.

At least four-fifths of the mishaps and break-downs that occur with dynamos arise from causes more strictly within the province of the engineer than in that of the electrician. On the other hand, many of the mechanical faults that develop themselves in the machine might have been avoided had the engineer been possessed of a better knowledge of the electric and magnetic conditions which obtain in the running of the machine.