340 THE ROTARY CONVERTER. 



By adding a series winding on the magnets, arranged 

 to carry the direct current of the load circuit, an increase 

 in excitation is produced proportional to the load. If 

 this is correctly proportioned, the required increase of 

 excitation to produce the necessary change in phase of 

 the current may be obtained. The general action is then 

 as follows: The field windings are so proportioned that 

 at no load the converter is under-excited, and a lagging 

 current is consequently produced in the supply circuit. 

 This has the effect of lowering the output voltage, as can 

 be seen from the curves in Fig. 168. It is arranged that 

 the voltage thus obtained should be that required for all 

 loads. If the converter is now loaded the excitation will 

 increase on account of the series turns, and the current 

 and voltage will come more nearly into phase, the ratio of 



conversion -n- - being thus increased. It is 



alternating voltage 



thus possible to make the increase in the conversion ratio 

 just counteract the armature drop due to increase of load, 

 and thus to maintain a constant voltage, or, if desired, to 

 produce a rise in voltage approximately proportional to 

 the load, in order to make up for losses in the trans- 

 mission line. 



Sometimes the regulating action is assisted by the 

 insertion of an inductance in the supply circuit, so as to 

 produce a greater lag in the current, which can be counter- 

 acted by over-excitation of the converter fields at full 

 load. 



The power-factor corresponding to any required ratio 

 of conversion for the rotary converter can be determined 

 from the curves taken in the last experiment. From 

 curves taken at full load and no load the required increase 

 in excitation in order to maintain a constant secondary 

 voltage can be at once obtained. 



To maintain a constant voltage at all loads the excita- 

 tion is usually adjusted to give unit power factor at full 

 load. To give 10 per cent, over-compounding the fields 

 are made to give unit power factor at about three-quarters 

 of full load, and consequently a leading current at 

 full load. 



