GENERAL PRINCIPLES OF SYNCHRONOUS MOTORS 29 







Jow, as Fig. 20 shows, the mean strength of the commutated current 

 is proportional to the algebraical sum of two areas, one of which (Si) 

 is positive, and the other (2) is negative. The former diminishes and 

 the latter increases, when the lag increases, the effect being a rapid 

 decrease in the excitation. If we let /o = time at which the commuta- 



tion occurs; if T= the period, and 27;^= the lag in question, the 

 exciting ampere-turns will be given by the following equation: 



_ 2 N f 

 -LVtav. -=r 



1 Jto 



io sin I 271 .B )dt = Nio cos 271 , 



1/71 1 



i.e., they are reduced in the proportion indicated by the factor cos j^ 

 as compared with their value when the commutation occurs at the 



FIG 20. 



proper moment of passing through zero (at which time the factor 

 cos =^- equals unity) . It is necessary, therefore, to vary the position 



of the brushes with the load. This cannot be done, however, during 

 the fluctuations of load, and it is conceivable that this decrease of 

 excitation under the influence of the lag may then greatly reduce the 

 stability of operation. It appears, therefore, that the method of exci- 

 tation by commutated currents is a process open to criticism. 



Advantage might be taken of the use of the transformer in Fig. 

 21, as the author showed in 1892 (Lumi&re Electrique, loc. cit.), to 

 compound the excitation in such a way as to cause the inducing 

 flux to increase with the load, and thus produce an increase in 

 stability. It would be sufficient, to this end, to wind on the same 

 transformer a second primary coil with a large conductor through 

 which the armature or rotor current passes. 



(3) Another method of commutating the current, which is pref- 

 erable to the preceding one, consists in combining with the armature- 



