1 86 ELEMENTS OF ELECTRICAL ENGINEERING. 



exactly opposite in phase to the alternating electromotive force 

 B that is induced in that portion of the armature winding. 



This second condition means that the synchronous converter 

 is assumed to operate at unity power factor, and that the point P 



of the clock diagram, Fig. 

 150, is located as shown in 

 Fig. 1 53, in which B and 7 

 are opposite in phase, so 

 that, the angle (7?7) being 

 equal to 180, the power 

 intake of the machine per 

 phase is equal to BL This 

 second condition can in 

 general be realized, for any 

 given load on the converter, 

 by adjusting the field ex- 

 citation of the machine until 



Fig. 153. 



the alternating current in- 

 take is a minimum as explained in section 2, of Art. 78. 



Current in each section of the converter armature. Let E Q be 

 the electromotive force between direct-current brushes and 7 

 the direct-current output of the machine. Then the power out- 

 put is 7? 7 . Let E n be the effective value of the induced 

 electromotive force in one section of the armature between adja- 

 cent collector rings and 7 n the effective value of the alternating 

 current in the same section of the armature winding. Then, 

 since E n and 7 n are assumed to be exactly opposite to each 

 other in phase, the intake of power per phase is EJ n and the 

 total intake of power is nEJ n . Therefore, assuming 100 per 

 cent, efficiency, we have 



whence, substituting the value of E n in terms of E Q from equa- 

 tion (ii) of Art. 90 and solving for 7 n , we have 



/.-- * 



;/ sn 

 n 



