104 ALTERNATING CURRENTS 



as shown in Fig. 105 (c), without changing the electrical charac- 

 teristics of the winding. This gives a full-pitch, half-coil, 

 barrel winding. Therefore, the spiral winding for which this 

 can be substituted is considered as a full-pitch winding. The 

 differential action of the coil sides of Fig. 105, due to their not 

 having a full pitch, is taken into consideration by the belt- 

 factor constant. (See page 121, Par. 58.) 



The inside coil shown dotted at a may be added to the winding, 

 but it contributes so little to the generated electromotive force, 

 because of its small pitch, that to use it is wasteful. This wind- 

 ing has but one coil-side per slot, so that it is also a single-layer 

 winding. As the ends of the coils may be bent so that they all 

 lie in a single vertical plane, as shown at (6), Fig. 105, it is called 

 a single-range winding. Two- and three-range windings are 

 also used in practice. 



At the present time, single-phase machines are somewhat 

 limited in their field of application. They are used more or 

 less extensively for single-phase railway electrification and for 

 some electric furnace work. Instead of building a single-phase 

 machine for these purposes, however, Y-connected, three-phase 

 machines are commonly used, as such machines are standard. 

 Two phases of the Y are used in series. A spare phase is also 

 available. 



53. Two -phase Windings. Two-phase windings are merely two 

 single-phase windings displaced 90 electrical space-degrees from 

 each other on the armature. If another winding be added to 

 Fig. 103, the coil sides of this new winding being midway be- 

 tween those shown in Fig. 103, a two-phase winding results, as 

 shown in Fig. 106. These two windings are 90 electrical space- 

 degrees apart, so that their voltages differ in time-phase by 90. 



Figure 107 shows a two-phase spiral or chain winding. This 

 is merely adapting the winding of Fig. 105 to two phases. There 

 are now eight slots per pole rather than six. As the coil ends 

 in this winding must necessarily lie in two different vertical 

 planes, in order to pass one another, the winding is called a two- 

 range winding. 



The chief advantage of a chain winding is the considerable 

 space between the coil ends, so that there is little opportunity 

 for electrical breakdown at these points. Therefore, they are 



