206 ELEMENTS OF ELECTRICAL ENGINEERING. 



The load current /' in the primary coil being opposite to /" 

 in phase is behind E' , as shown in Fig. 173. 



Case 3. Receiving circuit containing a condenser. In this 

 case, which is shown in Fig. 174, I" is ahead of E" in phase 

 and /' is ahead of E' in phase, as indicated in the figure. 



100. Equivalent resistance and reactance of an ideal transformer 

 feeding a given receiving circuit. -^ The primary of a transformer 

 takes from the supply mains a definite current at a definite phase 

 lag when the secondary of the transformer is delivering current 

 to a given circuit. Consider a simple circuit, of resistance r and 

 reactance x, which, if connected directly to the supply mains, 

 would take the same current that is delivered to 'the primary of the 

 transformer as above and at the same phase lag. This simple 

 circuit is equivalent to the transformer and the secondary receiv- 

 ing circuit, and r and x are called the equivalent primary re- 

 sistance and the equivalent primary reactance, respectively, of the 

 secondary receiving circuit. 



In using the ideas of equivalent primary resistance and react- 

 ance, magnetizing current is always ignored. 



Let R be the actual resistance of the secondary receiving 

 circuit, X its actual reactance, N f the number of primary 

 turns, and N" the number of secondary turns ; then 



N f z 



and 



f * 



(24) 



To derive these two equation sj resolve the primary electro- 

 motive force E' , Fig. 175, into components parallel to and per- 

 pendicular to I 1 as shown. The component parallel to I 1 is 

 rl 1 and the component perpendicular to I 1 is xl' . The 

 triangle whose sides are E 1 , rl 1 and xl' is similar to the 

 triangle whose sides are E" , Rl" and XI" , therefore 



xT_ E^ 

 XI" ~~ E" 



