106 



ALTERNATING CURRENTS 



54. Two-phase Lap Winding. The lap winding is the most 

 common type of alternator winding. With it, there are very few 

 limitations in the choice of number of slots, pitch, etc. The 

 coils are all alike, requiring the minimum number of spares, and 

 the winding is very flexible in the matter of connections. For 

 example, with a lap winding it is a simple matter to change a 

 440-volt winding to one of 220 volts by paralleling. 



To obtain a lap winding, more coils are added to the winding 

 shown in Fig. 103. The connections of the coils of any one phase 

 are almost identical with those in the direct-current windings 

 described in Vol. I, Chap. X. Direct-current lap windings may 

 be used for single-phase and for polyphase voltages by taps at 

 suitable points, with connections to slip rings, as is done in the 

 synchronous converter. (See page 342, Par. 138.) 



FIG. 108. Two-phase, full-pitch, lap winding, four slots per pole per phase. 



Figure 108 shows a two-phase lap winding, in which there are 

 eight slots per pole, making four slots per pole per phase. This is 

 a full-pitch winding, the coil pitch being 8 slots, which is the 

 number of armature slots per pole. The connections of phase B 

 are omitted for the sake of clearness as they are identical with 

 those of phase A. It will be observed that the coil sides in any 

 one slot are both of the same phase. This is not the case with 

 fractional-pitch windings. 



55. Three-phase Windings. The difference between two- 

 phase and three-phase windings is merely in the number of 

 phase-belts per pole. Figure 109 shows the simple winding of 

 Fig. 103 adapted to three-phase. For clearness the end con- 

 nections of phase A alone are shown, It is necessary merely to 



