. 



FIELD-EXCITATION OF ROTARY CONVERTERS 195 



(volts) induced, and the abscissas represent the corresponding ampere- 

 turns of excitation. 



2. The equivalent magneto-motive force of the armature, i.e., the 

 number of ampere-turns which, if acting upon the field-cores (same as 

 the ampere-turns of the regular field-winding) would produce in the 

 magnetic circuit the same magnetomotive force as is produced by the 

 reactive current /</ circulating through the armature. 



These equivalent average ampere-turns can be represented, as in 

 an alternator, by the expression, 



in which N'= the number of peripheral wires of the armature- winding 

 per double field (i.e., per magnetic circuit); K being a coefficient of 

 reduction which depends upon the number of phases and of armature 

 slots, and on the width of the poles; and Id being the effective value 

 of the reactive current. 



K is determined by calculation, by taking /^=i ampere, with 

 zero-lag, and calculating the mean magnetic potential produced under 

 one pole. 



The value of K can be obtained by experimental measurement. 

 This can be done with an approximation which is generally sufficient 

 for all purposes (since the leakage-reactance, ws, is practically negligible), 

 by running the converter at normal speed by power applied mechanically, 

 and then making it supply (on the A.-C. side) with normal voltage, a 

 purely inductive external circuit (composed of a reactance giving a 

 power-factor cos equal, at most, to 0.20); the difference between 



