8-B] 



CIRCLE DIAGRAM. 



287 



30. Efficiency. Extend the line AP" back to L and draw LL' 

 parallel to OE. The scale //', of 100 equal parts, is drawn parallel to 

 AP'" at any convenient distance. The point / on LL' is marked 100; 

 the point /' on AP" is marked zero. 



To determine the efficiency corresponding to any primary current 



E 



60 50\ 40 30 2O 10 



O L 



FIG. 3. Graphical method of obtaining power factor, slip and efficiency. 



OP, locate the point e where the line LP cuts the efficiency scale. 

 The per cent, efficiency* is I'e. 



* (3oa). Proof of Efficiency. for any point P, the efficiency is 

 PK -*- PM. 



The triangles LKM and I' LI are similar; hence 



d) 



(2) 



(3) 



But 



PM/PM=n/l'l. (4) 



Subtracting (3) from (4), we have 



(PM KM} *- PM = (VI el} ~ 77 ; 

 whence 



which is the efficiency. 



The triangles LPM and eLl are similar; hence 



PM :LM = Ll \el 

 Dividing (i) by (2), we have 



