THE ROTARY CONVERTER. 327 



Effect of Varying Excitation of a Rotary Converter. The effect 

 of a variation in exciting current upon the behaviour of a 

 rotary converter is different according as to the side from 

 which the converter is driven. 



When driven from the direct-current side a lessening 

 of the exciting current will cause the machine to run more 

 rapidly, since the driving side behaves similarly to any 

 direct-current motor. The increase of speed will not, 

 however, produce a corresponding increase in voltage on 

 the alternating side, since the increase of speed is not 

 more than enough to make up for the decrease in the 

 strength of the field in which the armature rotates. The 

 conductors will therefore rotate more rapidly, but in a 

 weakened field, and, if the losses remained the same as 

 before, it would appear that the voltage generated should 

 be unchanged by change in excitation. It will be found 

 in the experiment which follows that the voltage is affected 

 to a considerable extent by changes in excitation. 



EXPERIMENT XL VII. DETERMINATION OF EFFECT OF VARIA- 

 TION OF EXCITATION UPON RATIO OF TRANSFORMATION OF A 

 ROTARY CONVERTER. . (1) CONVERTER DRIVEN FROM DIRECT- 

 CURRENT SIDE. 



DIAGRAM OF CONNECTIONS. 

 Same as for Experiment XLVI., Fig. 160. 



Instructions. Make connections as described in the case 

 of Experiment XL VI., above. 



With the alternating-current circuit open, vary the 

 excitation, and for each value of the exciting current take 

 readings of the direct and alternating voltages, the current 

 spent in driving the machine, and the speed. The applied 

 voltage should be kept constant during the test. 



The alternating circuit should then be closed and a 

 similar series of readings taken with one or two values of 

 the load current. For each set of readings the resistance in 

 the load circuit should be varied so as to maintain the 

 current at a constant value. 



The readings should be entered in columns as shown 

 for Experiment XLVI. 



Curves showing the variation in secondary voltage 

 should be plotted on a base of exciting current, each 



