350 



ALTERNATING CURRENTS 



At 100 per cent, efficiency and unity power-factor 

 VI = 2(3Fs7 6 ) 



but 



= ST. 7 



V 141 4 



-~^ = 2.828 (Par. 141) 



- 0.4717 



(82) 



(a) (6) 



FIG. 321. Currents and voltages in a double-Y, 6-phase system. 



If the efficiency be 77 and the power-factor P.F., 



0.471 7 



7 6 



77 X "P.F. 



(83) 



Summarizing the above, (for unity power-factor and 100 per 

 cent, efficiency) : 



No. OF 

 SLIP-RINGS 



2 

 3 

 4 

 6 



No. of 

 PHASES 



1 



3 



4 

 6 



RATIO 

 I AO/ I D 



1.414 

 0.943 

 0.707 

 0.471 



Example. A 500 kw. converter, Fig. 322, has an efficiency of 92 per cent. 

 at full load and operates at a power-factor of 0.94. The direct-current 



ROPERTY OF ELECTRICAL 



FACULTY OF APPLIED SCIcNCE. 



