Figs. 1,510 and 1,511.—Multi-wire inductors. When the cross section of inductor necessary to carry the current is large, the use of a single wire would present difficulties in winding on account of its stiffness. Accordingly two or more smaller wires are used in parallel to secure the required cross section. Bare wire is used and the several sections encased in insulation as shown, the combination being more flexible than an equivalent single wire.
Fig. 1,512.—Two coil slot for whole coil winding. The slot has two recesses A and B for the reception of separate coils. In assembling the winding, the inner wedge is first placed in position and then the slot line with the insulating material. This usually consists of alternate layers of mica and pressboard. The coils composed of several turns of wire or copper strip are wound in place, and after covering with a layer of insulation, the outer wedge is pushed in place to retain the inductors in position.
Ques. What precaution is taken in insulating a wire wound coil containing a large number of turns?
Ans. On account of the considerable difference of pressure between layers, it is necessary to insulate each layer of turns as well as the outside of the coil, as shown in fig. 1,513.
Fig. 1,513.—Method of winding a coil containing a large number of turns, when there is considerable difference of pressure between the layers. In such cases to guard against short circuits or breakdown of the insulation, each layer of turns is insulated from the next layer by the insulating strips A, B, C, in addition to the regular insulation around each wire. After the coil is made up it is wound with insulating tape, varnished and baked.
Ques. Do distributed coils require insulation between the separate layers?
Ans. Since they are subdivided into several coils insulation between layers is usually not necessary.