Having completed the pole pieces, the fields themselves and the spools for supporting them can now be completed. Plate J covers coils and spools for the main fields. These main fields consist of two parts - a series field consisting of a few turns of heavy copper (in this case one turn), which carries the main armature current; and a shunt field consisting of a large number of turns of small wire connected across the armature and carrying a small current. All the electrical characteristics of these coils will be given by the designer - the number of turns for each, . the size of copper, and such other things as are fixed by the results to be obtained.
For the shunt field coils instructions will be given to the winder by specification, since a drawing is not suitable for giving such information. As to the general arrangement of the coils and spool, however, a drawing must be made. Notice the plans and elevation of two complete adjacent poles, Figs. 2 and 3. These show the directions of the windings and the general locations of the terminals. The arrangement of the shunt field terminals is shown in the two sections through AA and BB.
The series field, as stated above, must carry the full armature current. The coil, therefore, consists of a number of leaves of copper laid together in multiple and wound around the spool, shown in side and end views in Fig. 1. In order to keep the heating of this coil to a minimum without Using too much material, proper provision must be made for ventilation. In this case, this is accomplished by placing wooden space blocks (indicated by small figures 6 and 7) in such a manner that the coil is divided into two parts and so that there is space for air circulation between the collars and the coil. The two parts of the coil are riveted together through the space blocks. Dowel pins are passed through one of the collars, through the space blocks, and into recesses in the other collar, so that the coil is held rigidly in place. Since the coil occupies only three sides of the spool, a long wood space block, shown in Section AA, is provided to fill the fourth side. This block is held in place in the spool by wood pins the same as the dowels which hold the coil.
Connections must be made to this coil by copper bars. The ends of the laminations are therefore carried past the side of the spool, Figs. 1, 2, and 3, and divided so as to form slots for taking quarter-inch copper bars. The drawing gives the number of laminations in each division and the thickness of each lamination, as well as the dimension of the spaces for the connection bars.
The spool and flanges are next drawn. The spool proper, Fig. 14, is of sheet steel. The ends are turned over one-half inch to form supports for the flanges; the sides are lapped and riveted. These spools must be made to fit the pole pieces and must have proper dimensions to take the windings with adequate allowance for the flanges.
The flanges are of veneered maple, to give stiffness and prevent warping. Note that the two flanges enclosing the series coil, Figs. 15 and 16, are identical except in thickness and in the character of the holes for the dowel pins. One plan and two sections are therefore sufficient to show both of these flanges.
The third flange, Figs. 17 and 18, must be different on adjacent poles because of the different location of the terminals. The difference is indicated on the plan and section, however, by showing the location of one notch by full lines and the other notch by dotted lines. This makes the specifications just as clear and saves time.
An assembly of two adjacent poles is drawn to show the location of the flanges, Figs. 19 and 20. This is for the benefit of the assembler and is as necessary as the other views of drawings for the man who builds the parts.