FIELD-EXCITATION OF ROTARY CONVERTERS 



201 



power, and "with the secondary voltage constant at the brushes. It is 

 the same problem as the preceding one except that it is not the difference 

 of potential at the terminals, but the induced E.M.F. e, which must 

 be supposed constant, in this case. 



The locus of the point C is again the right line CD itself. But, 

 as we have added the condition that the segment OB remains constant, 

 the point O will, therefore, remain fixed (Fig. 17*1). 



M 



B ^"P 



FIG. 170. 



For each position of the point C the corresponding segment CB 

 indicates the current, and OC indicates the E.M.F. required at the 

 generator. Taking the E.M.F. values as abscissas and the current 

 values as ordinates, we obtain V-shaped curves (Fig. 18), which, in 



(. E.m.f. of Current-Supply 



FIG. 18. 



consequence of making s constant, are independent of the field- 

 winding and of the law of saturation. 



It is more interesting, however, to take the excitation ampere-turns 

 as abscissas, as in the case of synchronous motors, without distinguish- 

 ing those due to the series and the shunt windings. This amounts 

 to the same thing as assuming the machine to be separately excited. 



