An example of wave drum winding for a four pole machine is shown in [fig. 267]. For simplicity, very few coils are taken, there being only five as shown in the illustration. To make the winding, one strip should be removed from the wooden core and the others spaced equally around the cylindrical surface. This will give ten slots, the number required for the five coils. The winding is indicated in the following table:
| A | — | 1 | — | 4 | — | C |
| B | — | 3 | — | 6 | — | D |
| C | — | 5 | — | 8 | — | E |
| D | — | 7 | — | 10 | — | A |
| E | — | 9 | — | 2 | — | B |
Accordingly the first coil starting at segment A, is carried to the back of the drum through slot 1, thence across the back and returning through slot 4, ending at segment C the starting point of the second coil. Each coil is wound on in similar manner, the last coil ending at segment A, the starting point of the first coil. A developed view of the winding is shown in [fig. 268].
Double Windings.—In the various drum windings thus far considered, each coil had its individual slots, that is, no two occupied the same two slots. This arrangement gave twice the number of slots as commutator segments.
Fig. 268.—Developed view of the five coil wave winding shown in [fig. 267].
In a double winding there are as many segments as slots, each of the latter containing two inductors, comprising part of two coils.
The Siemens Winding.—In winding drum armatures for bipolar dynamos of two horse power or less, and especially for very small machines as used in fan or sewing machine motors, a form of winding, known as the Siemens winding, which is shown in [fig. 271], is largely used. It consists in dividing the surface of the armature core in one equal number of slots, say 16, and using a 16 part commutator.
In the Siemens winding, the end of the wire used at the start is to be connected to the first commutator bar, but must be fastened to the armature core out of the way so as not to interfere with the winding of the coils.
If eight turns of wire be required to fill a slot with one layer, then the wire is carried from front to back and bent aside so as to clear the shaft; after passing across the back or pulley end of the armature, it is wound in the diametrically opposite section and brought to the front, then across the commutator end and up close to the beginning of the coil.
