WINDINGS OF ARMATURES.

Figs. 125 and 126 are simple diagrams showing the way in which wire is wound on drum and ring armatures, respectively. In both figures the coils are shown separated to more

FIG. 125.

readily illustrate the prin

ciple. A little examina

DRUM WINDING.

tion of the figures will show that each section of the coil is connected to the next in order to it; the whole of the windings constituting, therefore, a single closed coil.

Also the end of each section and the beginning of the next are both connected with a segment of the commutator.

WINDING OF ARMATURES.

Let it be remembered that in closed-coil armatures, whether of the "ring" or the "drum" type, there are usually as many segments to the commutator as there are sections or groups of coils in the circuit of the armature. In the case of open-coil armatures, the separate coils are not connected up together in series, and a special form of commutator is used instead of the usual arrangement of a large number of parallel bars.

The principle of the ring or gramme armature is shown in Fig. 70. An iron ring capable of revolving upon an axis is arranged in the magnetic field between the poles N and S of an electro magnet. Upon this ring is wound a number of coils or loops of insulated copper wire, so as to cover the whole of the surface of the iron ring. The ends of each of the coils are connected to the ends of the adjacent coils, so that a continuous closed spiral is formed all around the ring; and, at the points where connection is made between the coils, connection is also made to strips of copper, which are insulated from each other and arranged around the axis of rotation into a circular commutator, as shown in the figure, against the two strips situated at opposite ends of a diameter press, two metallic brushes, B1, B2, which, remaining stationary, serve to convey the current generated in the coils of the armature to the external circuit E.

The arrangement of the lines of force in the magnetic field between the two poles N and S, when the ring is inserted therein is shown in the dotted lines in the figure.

WINDING OF ARMATURES.

From this it will be seen that the lines of force issuing from the north pole pass by way of the armature core to the south pole of the magnet, one-half of the lines passing through the upper portion of the core and the other half passing through the lower portion of the core. Owing to this peculiar arrangement, a very intense magnetic field is created between the outer surface of the armature core and the polar faces, whilst the interior space within the core remains almost entirely free from lines of force.

The drum or Siemen's type of armature differs essentially from the ring armature only in the manner in which the conductors are arranged upon the iron core. In the ring armature the core consists of a ring, and is overwound with conductors passing along the outer surface and through the interior; in the drum armature, the core is in most cases a ring also, or may be regarded as a ring, and is overwound with conductors passing along the outer surface, but in place of passing through the interior the conductors are carried completely around it axially, in the manner represented in Fig. 127.

This shows a drum armature in perspective, upon which only two adjacent conductors or coils have been wound. Since the windings of the drum armature pass over the ends of the core, it is impossible to represent the whole of them in perspective, and, therefore, they are exhibited diagramatically in Fig. 128, which illustrates what is known as a right-handed winding, with eight-part commutator (see in cut a, b, c, d, etc.). In the figure shown, the windings are supposed to be

WINDING OF ARMATURES.

viewed from the commutator or front end of the armature. The windings passing along the length of the drum are represented by the small circles upon which are marked

WINDING OF ARMATURES.

The manner in which the individual loops or coils are arranged will be rendered clear by following the course of a single loop or coil upon the armature.

Starting from the commutator segment (a) upon which the positive brush is resting at the upper portion of the armature, the conductor proceeds up the face of the drum to b, thence along the top and across the back end to the lowest part of the drum, from whence it proceeds across the bottom of the drum to 15. From 15 the conductor is brought around the face of the drum and connected to the commutator section (h), next to the one from which it started.

Another coil starts from the segment (h) and follows a similar course upon the surface of the armature, and is connected to the segment (g), from which another coil starts, and so on all around the armature. A continuous closed spiral is thus formed all around the armature, in a somewhat similar manner to the ring armature. The arrangement of the line of force in the magnetic field, when the armature is inserted therein, is similar to that of the ring armature, and with the armature rotating in the direction indicated by the arrow, it will be seen that the E. M. F.s and currents induced in the conductors, on either side of the armature, have the same relative directions as those induced in the conductors arranged upon the outer surface of the ring armature, the only difference being that the current flows across each end of the drum in place of flowing through the interior; the conductors at the ends being thus the only idle portions of the winding in the drum armature.