STABILITY OF OPERATION OF ROTARY CONVERTERS 215 



the converter had to decrease, in order to maintain the same induced 

 E.M.F. The converter therefore varied in speed in order to maintain 

 the induced E.M.F. constant. 



In the second place, when the load was heavier than the critical 

 load of the machine, an increase of reactive current was produced 

 by a speed increment. Hence, there was a decrease in the total excita- 

 tion flux. The speed of the motor therefore tended to increase until 

 the current supplied from the battery was limited by the resistance of 

 the circuits. As the armature of the converter was then in a state of 



A'AL.L I 



K 



i Axis or Reactive Currents B 



FIG. 27. 



unstable dynamic equilibrium its speed soon began to decrease; and 

 thus speed-oscillations were produced. 



In practice, therefore, it is desirable that the load put on the rotary 

 converter should not exceed the critical load above mentioned. 



Let us see, now, what can be done to raise this load-limit. We 

 will use the same diagram as before (Fig. 27). 



We see readily that what is necessary is to increase the vector 

 ON. This may be accomplished in different ways: 



First, if the frequency is decreased, then, for a given voltage, other 

 things being equal, the magnetizing current will have to be increased. 



Therefore the diameter of the circle corresponding to^l i) will 



\<7 / 



be increased, and, consequently, the point of contact I/, of the line 



