SYNCHRONOUS MACHINERY 



247 



If gh is a full-pitch coil the side h will occupy a position under the 

 adjacent pole similar to that of g and the e.m.f.'s generated in the 

 two sides will be of the same value and wave shape but displaced 

 180 degrees in phase; they therefore act in the same direction 

 around the coil and add directly to give the terminal e.m.f. If e 

 is the effective value of the e.m.f. generated in one side of the 

 coil the terminal e.m.f. is E = 2 e. With a full-pitch concentrated 

 winding the wave form of the generated e.m.f. is the same as the 

 wave of flux distribution under the poles. 



If the coil pitch is less than the pole pitch by an angle a, the 

 e.m.f. wave generated in the side h leads the e.m.f. in g by an 



E.M.F. between 

 Terminals 



E=2e Cos J2 



FIG. 225. Short-pitch coil. 



angle a, Fig. 225, and the terminal e.m.f. is the vector sum of two 

 e.m.f.'s of effective value e displaced in phase by an angle a. It is 



# = 2ecos^, (243) 



and is less than the e.m.f. generated in the full-pitch winding in 

 the ratio cos^ : 1- 



Fractional-pitch windings are sometimes used in order to elimi- 

 nate certain harmonics from the e.m.f. wave of the generator. 

 Take the case of a machine with the wave of flux distribution, 

 shown in Fig. 226, consisting of a fundamental and a fifth har- 

 monic. With a full-pitch winding the e.m.f. wave would consist 

 of a fundamental and the prominent fifth harmonic. If, how- 

 ever, the coil pitch is made only 80 per cent of the pole pitch the 

 e.m.f. hi one side of the coil will lead that in the other by 36 

 degrees and the fifth harmonics in the two sides will be in direct 

 opposition and will disappear. (Fig. 227.) The terminal e.m.f. will 

 consist only of the fundamental and it will be decreased in ratio 

 cos 18 : 1. To eliminate an nth harmonic the coil pitch must be 



