Ans. Wire larger than about number 8 B and S gauge (.1285 inch diameter) is not easily handled, hence for large inductors, two or more wires may be wound together in parallel.
According to the mechanical features and manner of assembling on the core, drum windings may be divided into several classes, as follows:
1. Hand winding;
2. Evolute or butterfly winding;
3. Barrel winding;
4. Bastard winding;
5. Former winding.
Hand Winding.—The first windings were put on by hand and proved objectionable on account of the clumsy overlapping of the wires at the ends of the armature, which stops ventilation and hinders repairs, while the outer layers overlying those first wound, bring into close proximity inductors of widely varying voltage. The method is still used in special cases and for small machines. Such a winding has rarely, if ever, been made with one continuous wire.
Figs. 369 and 370.—Evolute and "straight out" connectors. In small machines the connectors must be curved as in fig. 369, but in large machines, especially where the teeth are wide, they may be straight as in fig. 370. These connectors may take either of the following forms: 1. involute or evolute connectors—An involute is the curve drawn by the extremity of a piece of string which is unwound from a cylinder; 2. spiral connectors—These consist of double spirals, the commutator being usually connected to the junction of the two spirals. These connectors are also known as "butterfly" connectors.
Evolute or Butterfly Winding.—This mode of winding, was introduced by Siemens for electroplating dynamos to overcome the objections to hand winding. It takes its name from the method of uniting the inductors by means of spiral end connectors as shown in [fig. 374], also in figs. 369 and 370, which show more modern forms.
Fig. 371.—Holzer Cabot armature; rear view showing back head and coil guard. The construction of core and winding is described in [fig. 367]. The shaft is of crucible steel ground to gauge. The commutator segments are of drop forged copper in the smaller and hard-drawn copper in the larger sizes. The insulating material between the segments is mica. On the larger sizes, the commutator shell is fitted with a thread and mounted on a spider. This construction provides openings between the commutator and shaft for ventilation.
Ques. What are evolute connectors?