Fig. 236 shows a one piece frame of a six pole dynamo having cast welded into it, combined cores and pole pieces.
Ques. What is the disadvantage of laminating a core?
Ans. It necessitates a nearly square or rectangular section, which requires more copper for the winding than the cylindrical form.
The Magnetizing Coils.—The object of the magnetizing coils, is to provide, under the various conditions of operation, the number of ampere turns of excitation required to give the proper flux through the armature to produce the desired electromotive force.
With respect to the manner in which magnetizing coils are wound they are said to be:
1. Spool wound; 2. Former wound.
Ques. Describe the methods of constructing spool wound coils.
Ans. The spool is made in various ways, sometimes entirely of brass, or of sheet iron with brass flanges, or of very thin cast iron. Some builders use sheet metal with a flange of hardwood, such as teak. If a spool be simply put upon a lathe to be wound, the inner end of the wire, which must be properly secured, should be brought out in such a way that it cannot possibly make a short circuit with any of the wires in the upper layers. To avoid this difficulty, the wire is sometimes wound on the spool in two separate halves, the two inner ends of which are united, so that both the working ends of the coil come to the outside as shown in fig. 238.