t 

 TRANSFORMERS 375 



posed of the exciting current OMi, which is practically constant 

 for all loads and the load current M\A\. The impressed primary 

 e.m.f. ODi is a little greater than the primary counter e.m.f. 

 OBi on account of the resistance drop B\C\ and the reactance 

 drop CiDi, the values being the product of the primary current 

 OA i times the resistance and leakage reactance, respectively, of 

 the primary winding. The former is in phase with the current, 

 the latter in quadrature. 



Induced e.m.f. The relation between the counter e.m.f. of 

 a transformer and the various factors, such as flux density, number 

 of turns, frequency, etc., are determined by the following formula: 



# = 4.44X/XnX4>XHT 8 ; 



in which # = mean effective e.m.f.; 



/= frequency in cylces per second; 



n= total number of turns of the primary winding; 



= total magnetic flux in maxwells. 



This equation is based on the assumption that the e.m.f. is a 

 true sine wave. 



Ratio. The A. I. E. E. Standardization Rules state that 

 " The voltage ratio of a transformer is the ratio of the r.m.s. 

 primary terminal voltage to the r.m.s. secondary terminal voltage 

 under specified conditions of load." It also defines " the ratio of 

 a transformer, unless otherwise specified, as the ratio of the 

 number in turns in the high-voltage winding to that in the low- 

 voltage winding, i.e., the turn-ratio." 



The two ratios are equal when one of the windings is open 

 and the transformer does not carry any load. When loaded, the 

 resistance and inductance of the windings cause a drop in the 

 voltage, thus modifying the ratio of transformation slightly. 



The ratio of a transformer refers, of course, to the turns 

 which are connected in series, high-voltage as well as low-voltage. 

 In many instances it is desirable for the sake of interchangeability 

 and standardization to split up the windings in groups of sections 

 which may be connected either in series, parallel, or series-parallel. 

 This is almost always the case with distributing transformers, 



