FIELD-EXCITATION OF ROTARY CONVERTERS 203 



excitation vanish (a negative shunt-excitation being out of the question). 

 This upper limit value 



....... (24) 



must be greater than the minimum limit, unless the number of turns 

 on the armature is excessive. There is no actual need of an upper 

 limit to the reactive current, because the aim is to reduce it as much 

 as possible, on account of the undesirable effects produced by it on 

 the distribution-system, namely, its increasing the ohmic losses, and 

 its necessitating a higher E.M.F. E. What should determine the 

 value which is to be selected for IQ is the condition that, between zero 

 load and normal load, the normal variation of voltage (which deter- 

 mines the range of effect produced by the field-excitation windings), 

 should be that which is suitable for the practical purpose for which 

 the machine is intended, and that which is represented by the current 

 supply-characteristic which has already been denned. 



The investigation of this point is facilitated by a new diagram, 

 which will now be considered, and which indicates the lag characteristics 

 of the converter at constant potential. 



Lag-Characteristics of Rotary Converters at Constant Potential. 

 Let us suppose that, in some way, the potential difference at the brushes 

 of the converter is maintained constant. The shunt-excitation, which 

 is proportional to this voltage, will also remain constant; and the total 

 ampere -turns which produce this voltage will also remain constant.. 

 We will therefore have 



A = constant ; Ad = constant . 

 Hence A s +A a = constant (A AJ) ..... (28) 



xkn KN' 



or =(I w -i ) + -=I d =(A-A d ) ..... (29) 



V 2 V 2 



This formula simply states that, with constant potential, the converter 

 can furnish any secondary current whatever (proportional to /,) 

 which passes through its series-winding, owing to the automatic decrease 

 of the reactive current, Id, as the latter, of itself, takes the value neces- 

 sary to make up the same total magnetomotive force A. 



The relation between I W JQ and Id is simply linear. If, therefore, 

 in Fig. 20, the load-values be represented, as previously, by distances 



