=Ques. What may be said with respect to the coil ends?=

Ans. Several methods of bringing out the ends of coils are shown in figs.

238 to 241. In fig. 239 copper strip, laid in behind an end sheet of insulating material, makes connection to the inner end, as shown in the right side of the figure, while another strip, shown on the left side similarly inlaid, serves as a mechanical and electrical attachment for the outer end of the winding.

[Ill.u.s.tration: FIG. 239.--Core and edge strip winding for shunt field coils of large multi-polar dynamo. The winding consists of a copper strap S carefully insulated and placed edgewise on the core C in a single layer of winding. With this arrangement, the s.p.a.ce occupied by insulation is reduced to a minimum, and, although the cooling surface is small, each turn of the winding has one edge on the outer surface, being ample for adequate cooling.]

Two other methods are shown in figs. 240 and 241. A simple device for securing the outer end is to fashion a terminal piece so that it can be laid upon the winding, the last three or four turns of which are wound over its base, and after winding, are bared at the place and securely soldered.

=Ques. How are the coils insulated?=

Ans. The spools upon which the coils are wound are usually insulated with several layers of paper preparations; a thickness of one-tenth of an inch made up of several superposed layers is generally sufficient. Varnished canvas is useful as an underlay, and vulcanized fibre for lining the f.l.a.n.g.es. It is important to protect the joint between the cylindrical part and the f.l.a.n.g.es. A core paper may be laid upon every four layers of winding. Between series and shunt coils, in compound wound machines there should be an insulation as efficient as that on the cores. When the winding is completed, two layers of pressed board or equivalent are laid over and bound with an external winding of hard rope or tape. This protective external lagging covering the outer surface of the completed coils is not altogether a benefit for it tends to prevent dissipation of heat.

[Ill.u.s.tration: FIG. 240.--One mode of bringing out the coil ends, in which copper strip is laid in behind an end sheet of insulating material.]

[Ill.u.s.tration: FIG. 241.--Another mode of bringing out the coil ends. A narrow insulated strip of thin copper G, leading to terminal H, is connected with the end _e_ of the coil before winding.]

[Ill.u.s.tration: FIGS. 242 and 243.--Square and hexagonal order of "bedding." The term bedding is an expression used to indicate the relation between the cross sectional area of the winding when wound square, as in fig. 242, and where wound in some other way, as in fig. 243. In the square order of bedding, the degree of bedding equals zero.]

[Ill.u.s.tration: FIG. 244.--Method of securing coils in position when the pole pieces are simply extensions of the core without enlargement.]

=Ques. How are the coils attached?=

Ans. Where the pole pieces are simply extensions of the cores without enlargement, the coils can be slipped over the ends, but some kind of clamping device is necessary to hold them in place, as for instance, the method shown in fig. 244.

In case the pole piece be made larger than the core and separate therefrom, it is put into position after the coils are in place, thus serving the double purpose of pole piece and clamp.

=Ques. Describe the coil connections.=

Ans. Coils are generally united in series so that the same magnetizing current may flow through all of them. The coils should be so connected that they produce alternate north and south poles.

If all the coils be similarly wound with respect to the terminals, and similarly placed; that is, so placed that the winding, considered from the coil terminal nearest the pole face, starts in all the coils in the same direction, then the connections will come at the north end and at the south end of the spools.

[Ill.u.s.tration: FIG. 245.--Western Electric set of former wound field coils for four pole dynamo. These coils are wound around a former or template, and are then slipped over the cores before the latter are bolted to the yokes or frame.]

=Heating.=--The heat generated in the magnetizing coils is dissipated in three ways; by:

1. Induction; 2. Radiation; 3. Convection.

In the first instance, it pa.s.ses through the copper and the insulation, either to the external surface, whence it pa.s.ses off by radiation and convection into the air, or to the magnet core and yoke, which in turn conduct it away. In large multi-polar machines the ma.s.ses of metal in the pole cores and frame are more efficient in dissipating heat than the external surface of the coil.

[Ill.u.s.tration: FIG. 246.--Fort Wayne compound wound rectangular ventilated spool field coil. The series and shunt coils are wound side by side, ventilating pa.s.sages being provided lengthwise through each coil and between the shunt and series coils as shown.]

=Ventilation.=--Sometimes provision is made for ventilation of the field magnet coils as shown in fig. 246. Here the series and shunt coils are wound side by side, ample ventilation being provided lengthwise through and between the coils.

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