256 



PRINCIPLES OF ELECTRICAL DESIGN 



to fractional pitch lap windings, in which the two sides of a coil 

 are not similarly placed relatively to the center lines of the poles, 

 with the result that tooth harmonics in the e.m.f. wave may be 

 almost eliminated. 



(b) Single-layer Winding. With' this winding there is only 

 one coil-side in a slot, and the number of conductors per slot 

 may, therefore, be either odd or even. Several shapes of coil are 



/v 



FIG. 95. Three-phase, single-layer winding; three slots per pole per phase. 



necessary in order that the end connections may clear each other, 

 and this involves a larger number of special tools or formers and 

 a larger number of spare coils than for a double-layer winding. 

 These disadvantages are, however, sometimes outweighed by the 

 fact that the total number of coils in the machine is smaller. 

 Good insulation is easily obtained because the end connections 

 may be separated by large air spaces, and the single-layer winding 

 is, therefore, suitable for high voltages. 



FIG. 96. Three-phase, single layer winding; four slots per pole per phase 



Considering each phase winding separately, the full number of 

 turns per pole may encircle one pole only as shown in Fig. 95, or 

 they may be divided between a pair of poles, in two equal parts, 

 as shown in Fig. 96. Both diagrams show one phase only of a 

 three-phase generator. In Fig. 95 there are three slots, while in 

 Fig. 96 there are four slots, per pole per phase. The coils of the 

 other phase windings would be similarly arranged in the re- 

 maining slots, the ends projecting beyond the slots being shaped 



