href="@public@vhost@g@gutenberg@html@files@48846@[email protected]#AUXILIARY_APPARATUS" class="pginternal" tag="{http://www.w3.org/1999/xhtml}a">AUXILIARY APPARATUS

Switches—switch classification and construction—difficulty encountered in opening the circuit—various switches: knife, snap, and quick break types—fuses—circuit breakers: maximum, minimum, reverse current, maximum and reverse current, no voltage breaker—discriminating cut out—time limit attachments—rheostats—starting boxes—switchboards.

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.

Fig. 247.—Ring armature of four pole dynamo: diagram of winding and connections, showing direction of the induced currents. The currents in the windings under the upper N and S poles are opposed to each other and flow to the external circuit by the positive brush 1, and back to this half of the armature by the negative brushes 3 and 4. At the same instant the opposed currents in the lower windings flow to the external circuit by positive brush 2 and return to the armature through negative brushes 3 and 4. The armature is thus divided into four circuits and four brushes are required which must be placed between the poles so as to short circuit the coils as they pass through the neutral space. In this form of winding there is no difference of potential between the + brushes, so that they are connected in parallel, as are also the negative brushes, and then to the external circuit. In multipolar machines there are as many brushes as pole pieces. Since opposite commutator bars are of the same potential on this four pole dynamo they may be joined by a cross connecting wire and two brushes, as 2 and 4, dispensed with. This can only be done when there is an even number of coils. The armature is said to be "cross connected."

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.

Fig. 248.—Early form of Gramme ring armature, the core being shown cut through, and some of the coils displaced to make it clearer. The core, F, consists of a quantity of iron wire wound continuously to form a ring of the shape shown by the section. Over this is wound about thirty coils of insulated copper wire, B C D, etc., the direction of the winding of each being the same, and their adjacent ends connected together. The commutator segments consist of a corresponding number of brass angle pieces, m, n, which are fixed against the wooden boss, o, carried on the driving shaft. The junction of every two adjacent coils is connected to one of the commutator segments, as shown at n.

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.

Fig. 249.—Modern form of Gramme ring armature. The core consists of a number of thin flat rings of well annealed charcoal iron, the outer diameter of each ring or disc being 11½ inches, and its inner diameter 9¼ inches. Sheets of thin paper insulate each disc from its neighbors to prevent the flow of eddy currents. The armature is mounted on a steel shaft to which is keyed a four armed metal "spider," the extremities of whose arms fit into notches cut in the inner edges of the soft iron core rings, so that a good mechanical connection is obtained between the core and the shaft. The spider is made of a non-magnetic metal, to reduce the tendency to leakage of lines of force across the interior of the armature. The armature inductors consist of cotton covered copper wire of No. 9 standard wire gauge, wound around the core in one layer, and offering a resistance, from brush to brush, of 0.048 ohm. There are two convolutions in each section, the adjacent ends of neighboring sections being soldered to radial lugs projecting from the commutator bars.

Ques. Describe a ring armature.

Ans. It consists essentially of an iron ring, around which is wound a number of coils. These various