THE ARMATURE REACTIONS OF ALTERNATORS 259 



TABLE VII 



EMPIRICAL COEFFICIENTS 



Three-phase winding with 6 short distributed coils per double field. 



tions are markedly reduced by the employment of short coils. But 

 it may be readily shown that with a sinsuoidal field-magnet flux of a 

 breadth equal to the pitch, the E.M.F. induced in the winding is 

 reduced approximately in the same ratio. In fact, the mean breadth 

 of the short bobbins, Fig. 16, is \ of the pitch, while the mean breadth 



FIG. 16. 



of the long bobbins, Fig. 10, is % the pitch. The straddling arrange- 

 ment of coils in Fig. 10 involves only the coefficient of reduction 



2 cos 



(- 



\I2 



= 0.966, while the arrangement of Fig. 16 reduces the 



E.M.F. in the same ratio as the flux linkage, that is to say, by 



the coefficient =sin = . The two E.M.F. 's are then in the ratio 



4 2 



E short bobbins 0.707 



= ^=0.73. 



E long bobbins 0.966 



