Ques. Describe the construction of former wound coils.
Ans. Former wound coils are wound upon a block of wood having temporary flanges to hold the wire together during the winding. Such coils have pieces of strong tape inserted between the layers and lapped at intervals over the windings to bind them together. Coils are usually soaked with insulating varnish and stove dried.
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.
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 flanges. It is important to protect the joint between the cylindrical part and the flanges. 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.
