SYNCHRONOUS MACHINERY 251 



watts to a non-inductive load at unity power factor or it can supply 

 1000 X 0.80 = 800 kilowatts to an inductive load of 80 per cent 

 power factor. . 



150. Comparative Ratings of an Alternator Wound Single-, 

 Two- and Three-Phase. Take the case of a machine with six 

 slots per pole. Let e be the effective value of the e.m.f. generated 

 in each coil of the winding and I be the current per conductor. 

 The current will be the same in the three cases for the same tem- 

 perature rise. 



When wound single-phase using all the slots the distribution 

 factor is 0.64 and the terminal e.m.f. is 



E = 6 e X 0.64 

 and the output is 



Pi = El cos </> = 3.84 el cos 0, 



where cos <j> is the power factor of the load. 



When wound single-phase using only four slots per pole the 

 terminal e.m.f, is 



E = 6 e X 0.64 X 0.866 (Art. 144) 



and the output is 



Pi' = El cos = 3.32 el cos 0. 



When wound two-phase with three slots per phase per pole the 

 distribution factor is 0.91, the e.m.f. per phase is 



E = 3 e X 0.91 

 and the output is 



P 2 = 2EI cos = 5.46 el cos 0. 



When wound three-phase with two slots per phase per pole the 

 distribution factor is 0.96, the e.m.f. per phase is 



E = 2 e X 0.96 

 and the output is 



PS = 3 El cos = 5.76 el cos 0. 



Taking the three-phase rating as 100 the comparative ratings 

 are as given below. 



Number of Phases. Rating. 



Three-phase 100 



Two-phase 95 



Single-phase using all the slots 67 



Single-phase using only four slots per pole 57.7 



