248 ELECTRICAL ENGINEERING 



either lengthened or shortened by - th of the pole pitch. Thus 



Yl 



the wave form of the e.m.f. generated in a short-pitch winding is 

 not the same as the wave of the flux distribution in the air gap. 



Similarly the wave form of any distributed winding differs 

 from the wave of flux distribution, since the terminal e.m.f. is 

 the sum of a number of waves displaced from one another. A 

 fully distributed winding gives an e.m.f. wave of approximately 

 sine form at no load regardless of the flux distribution. 



FIG. 226. Flux wave with fifth FIG. 227. Elimination of fifth harmonic, 

 harmonic. 



146. Effects of Distributing the Winding. (1) The core is used 

 to better advantage since a number of small slots evenly spaced 

 are used instead of a few large ones. (2) The copper is evenly 

 distributed over the armature surface and thus the copper loss is 

 also distributed and the heat developed by it can more easily be 

 dissipated. A higher current density in the copper can, therefore, 

 be used. (3) The self-inductive reactance is very largely de- 

 creased by distributing the winding in a large number of slots, 

 since the coefficient of inductance of a coil is proportional to the 

 square of the number of turns. (4) The terminal e.m.f. is de- 

 creased as shown in Art. 144 but the wave form is made more 

 nearly sinusoidal. 



147. Multiple-circuit Windings. The windings already dis- 

 cussed are all single circuit, that is, all the turns of one phase are 

 connected in series. In low-voltage machines with a large current 

 output it is necessary to connect the coils forming each phase in 

 multiple circuit. When connected two-circuit the terminal e.m.f. is 

 reduced to one half and the current output is doubled; the power 

 output, therefore, remains the same. 



The e.m.f.'s generated in the sections of the windings which are 

 connected in multiple must be of the same value and must be in 

 phase or circulating currents will flow. It is also necessary that 



