214 ELEMENTS OF ELECTRICAL ENGINEERING. 



The power output is equal to E"I" on non-inductive load. If 

 the receiving circuit is inductive the power output is E" I" multi- 

 plied by the power factor of the receiving circuit. 



The power input is equal to the power output plus the losses, 

 so that, if the losses can be calculated, the power input is known, 

 and then the efficiency may be calculated by dividing output by 

 input. 



The copper loss may be easily found from the equation 



W e = R'I n + R"I"\ 



In order to determine the iron loss, calculate <> from equation 

 (25) Art. 101 and divide by the sectional area of core to find the 

 maximum flux density $>. Then, knowing volume of iron V t 

 thickness of laminations /, frequency f t and the two empirical 

 constants a and b in equations (26) and (27) of Art 103, the 

 values of P h and P e may be calculated. 



105. Transformer regulation. When a transformer is at zero 

 load the ratio of primary to secondary voltage is almost exactly 

 equal to the ratio N f JN", but with increasing load the secondary 

 terminal voltage decreases, the supply voltage being constant, or, 

 in other words, the secondary terminal voltage increases in value 

 as the load on the transformer is decreased. The degree of regu- 

 lation is usually specified by expressing the increase of secondary 

 terminal voltage from full load to zero load in percent of full-load 

 voltage. Thus a given transformer supplied from constant voltage 

 mains gives a secondary terminal voltage of 108.5 volts at full 

 load and 112 volts at zero load and its percentage regulation is 



112 108.5 



x 100 = 3.22 per cent. 



The decrease of secondary terminal voltage of a transformer 

 with increase of load is due to coil resistances and to magnetic 

 leakage. When the receiving circuit is non-inductive the decrease 

 of secondary terminal voltage is due almost entirely to coil resist- 

 ances, when the receiving circuit is very highly inductive the 



