FIELD-EXCITATION OF ROTARY CONVERTERS 197 



Let represent the ratio of transformation, i.e., the ratio of the 

 primary E.M.F. to the secondary E.M.F. Since the armature-resistance 

 is already, by hypothesis, counted once in that of the supply-circuit, 

 it need not be counted again here. We can, therefore, write: 



Ampere-turns of shunt excitation, 



Ampere-turns of series-excitation, 1 



w -j )', ..... (18) 



2 



Ampere-turns of reactive currents, produced by the armature, 



The magnetomotive forces are thus expressed directly in terms of the 

 elements of the primary currents. This will enable the excitations for 

 each load-condition to be calculated. 



If we count as positive the lagging reactive currents (which are 

 magnetizing currents) the total number, A, of exciting ampere-turns 

 corresponding to the load-condition sl w will therefore be 



nkn KN' 



(20) 



~ 



" V 2 v 2 



The +sign is placed before the reactive ampere-turns because, under 

 the normal conditions of current-supply previously considered, the 

 reactive current is directed to the right of D, i.e., it is positive and' 

 magnetizing between zero-load and normal load, and it is demagnetiz- 

 ing beyond normal load. The -fsign is placed before the series ampere- 



7 7T& 



1 From the relation found at the beginning of Chapter I, namely, = - =, we 



Iw V 2 



get 7 2 = - =I W - I 11 Eq. (18), for the sake of greater precision, I w is replaced by 



\/2 



I w j . If the armature had two different windings, the ratio would have to 



N 

 be multiplied by that of the numbers of turns of these two windings, . 



