196 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



the voltage (e) at the terminals, with this load, and the voltage (e) 

 obtained with open circuit, with the same excitation, is the measure, 

 in volts, of the armature-reaction corresponding to the current supplied 

 to the external circuit by the converter. 



Taking the values e and e thus obtained and referring them to the 

 excitation-curve (Fig. 15), they will correspond to different points on 

 this curve. The difference between the abscissas, A and a, correspond- 

 ing to these two points will be a measure of the equivalent ampere- 

 turns of the armature (neglecting, as before, the effects due to magnetic 

 leakage). It is only necessary, in order to obtain K, to divide a A 

 by the product of the effective reactive current Id measured in the above- 

 mentione*d experimental test, by half the number of turns of wire, 



N' /- 



, and by the constant v 2. We will have 

 2 



K = ~ ........ (16) 



Compound-Excitation. Different Factors of this Excitation. 



Rotary converters are excited by current taken from their secondary 

 (D.-C.) side. The field-excitation may, obviously, be either shunt, 

 series, or compound. 

 Let 2= secondary voltage at the terminals; 



72= secondary or output current delivered by the rotary converter; 

 = number of turns of the series- winding; 

 r resistance of one turn of the shunt-winding. 

 The shunt ampere-turns and the series ampere-turns will be, 

 respectively, equal to 



E 2 



-j- and n! 2 . 



It is these ampere turns which, conjointly with the equivalent 

 ampere-turns of the armature, determine the total excitation. 



In the most general case, that of a compound-wound converter, 

 the question is, therefore, quite complex, since there are three variable 

 factors, the first of which varies with the voltage, the second with the 

 output (direct) current, and the third with the reactive (alternating) 

 current. 



As already seen, the secondary ampere-tums are proportional to 

 the ampere-turns due to the useful primary current, I W JQ. 



