I7 8 TRANSFORMERS. [Exp. 



the advantage that all connections with supply lines and instruments 

 are at low potential; see Electric Journal, p. 64, Vol. VI., and Fig. 

 322, Karapetoff's Exp. Elect. Engineering. 



A modified form of opposition test can be applied to a single 

 transformer; see Foster's Handbook. 



57. Note on Efficiency. If the rated secondary voltage is 

 E 2 =ioo, the customary and most simple procedure is to take the 

 core loss for this voltage from Fig. 5 (thus, W = 4i.6) and to 

 compute the full load efficiency as in 31. To be accurate, however, 

 the secondary core voltage or flux voltage, s , should be taken as 

 E 2 plus the secondary RI drop. Taking this drop as 1.28 (=%r in 

 35)> we nave ES = 101.28 and the corresponding core loss, 

 W 42.5; this gives the correct efficiency of 95.51 instead of 95.56. 

 The difference between these values is so little that the method of 

 31 is usually sufficiently correct. 



Approached in another way, we might consider E B = 100 and 

 W = 4i.6. Then 2 =ioo 1.28 = 98.72. To get the rated out- 

 put of 2 K. W., since E 2 is decreased, the current must be increased 

 by the factor I -f- 98.72. The copper loss must then be increased by 

 the factor (i -=~98.72) 2 , giving an efficiency of about 95.5. 



