CHAPTER XVII

THE ARMATURE

The armature of a dynamo consists of coils of insulated wire wound around an iron core, and so arranged that electric currents are induced in the wire when the armature is rotated in a magnetic field or the field magnets rotated and armature held stationary.

The commutator is in fact a part of the armature, but is of sufficient importance to be considered in a separate chapter.

=Ques. What are the practical objections to the elementary armature, described in fig. 165?=

Ans. It induces a very feeble current, which is not of constant pressure, but pulsating; that is, it consists of two pronounced impulses in each revolution as shown in fig. 168.

=Ques. Why does the elementary armature produce a pulsating current?=

Ans. The pulsations are due to the coil moving alternately into, and out of, the positions of best and least action in the magnetic field.

=Ques. How is a continuous current, or one of uniform pressure obtained?=

Ans. If an additional coil be added to the elementary armature, at right angles to the existing coil, and its ends suitably connected to a four part commutator, as in fig. 185, so that one coil is in the position of best action, while the other is in the position of least action, the pulsations of the resulting current will be of less magnitude. By increasing the coils and suitably altering the construction of the commutator to accommodate the ends of these coils, the resultant current may be represented by practically a straight line, indicating the so called _continuous current_, instead of the wavy resultant curve No. 6, as illustrated in fig. 187.

An armature for practical use has a large number of coils, suitably arranged upon an iron core, so that a large proportion of them are always actively cutting the lines of force, or moving into the positions of best action in the magnetic field.

=Types of Armature.=--Although there are many forms of armature, all may be divided into three classes, according to the arrangement of the coils or winding on the core, as:

1. Ring armatures; 2. Drum armatures; 3. Disc armatures.

Each of these forms of armature has its own special advantages for particular purposes, the disc type being least in favor and not having had any extensive application in this country.

=Ques. What is the comparison between ring and drum armatures?=

Ans. The drum armature is electrically and mechanically the more efficient, possessing, as it does, possibilities in the way of better mechanical construction of the core, and in the arrangement and fixing of the inductors thereon not to be found in the ring form. Less wire and magnetizing current are required for the field magnets for a given output than with the ring armature. Drum winding is not so simple as ring winding, and it is more difficult to ventilate a drum than a ring armature, it being necessary to provide special ventilating ducts.

=Ques. Describe a ring armature.=

Ans. It consists essentially of an iron ring, around which is wound a number of coils. These various coils are wound on separately, the wire being carried over the outside of the ring, then through the center opening and again around the outside, this operation being repeated until the winding for that individual section is completed. The adjacent coil is then wound in the same way, the ends of each being brought out to the commutator side of the armature, the arrangement of the coils on the ring and connections with the commutator being shown in fig. 247, examples of actual construction being shown in figs. 248 and 249.

=Ques. For what conditions of operation is the ring armature specially adapted, and why?=

Ans. It is well suited to the generation of small currents at high voltage, as for series arc lighting, because the numerous coils can be very well insulated.

=Ques. Why does a ring armature require more copper in the winding than a drum armature?=

Ans. For the reason that those inductors which lie on the inner side of the iron ring, being screened from practically all the lines of force, as shown in fig. 250, do not generate any current.

Numerous attempts have been made to utilize this part of the winding by making the pole pieces extend around the ring in such a manner that lines of force will pass to the inside of the ring, also by arranging an additional pole piece on the inside of the armature, but mechanical considerations have shown these methods to be impractical.

=Ques. Is any portion of the winding of a drum armature inactive?=

Ans. Yes; the end connectors do not generate any current.

=Ques. What is the chief advantage of the drum armature?=

Ans. It reduces considerably the large amount of dead wire necessary with the ring type.

=Ques. How is this accomplished?=

Ans. By winding the wire entirely on the outer surface of a cylinder or _drum_, as it is called, as shown in fig. 251, thus none of the wire is screened by the metal of the core.

Fig. 252 shows an elementary four coil drum armature. Starting from the point _a_ and following the winding around without reference at first to the commutator, it will be found that the rectangular turns of the wire form a closed circuit, and are electrically in series with one another in the order of the numbers marked on them.

With respect to the connections to the four segments _w_, _x_, _y_, _z_, of the commutator it will be found that at two of these, _x_ and _y_, the pressures in the windings are both directed _from_, or both directed _toward_ the junction with the connecting wire. At the other two segments, _z_ and _w_, one pressure is toward the junction and the other directed from it. If, therefore, the brushes be placed on _x_ and _y_ they will supply current to an external circuit, _z_ and _w_, for the moment being idle segments.

=Disc Armatures.=--The inductors of a disc armature move in a plane, perpendicular to the direction of the lines of force, about an axis parallel to them as shown in fig. 253. The main difficulty with this type has been in constructing it so that it will be strong and capable of resisting wear and tear. It was introduced in an effort to avoid the losses due to eddy currents and hysteresis present in the other types of armature.

On account of the nature of the construction of a disc armature, it is necessary that the coils subject to induction occupy as small a space as possible in the direction of their axes. This requirement, as well as the connection of the inductors with each other and with the commutator, prevented the general adoption of this form of armature, and subsequent experience failed to justify the existence of the type.