360 



ALTERNATING CURRENTS 



converter, as shown in Fig. 328. By raising the field of the alter- 

 nator or booster, the alternating voltage of the converter is raised. 

 The converter voltage may be lowered, not only by decreasing 

 the booster field, but by reversing it as well. 



When the booster voltage is assisting the converter voltage, 

 the booster acts as an alternator and takes mechanical power from 

 the converter armature. This increases the energy component 

 of the resultant armature current in the converter and hence 

 changes the cross-magnetizing effect of the armature. When the 

 booster voltage bucks the converter voltage, the booster receives 

 electrical energy and delivers mechanical energy to the converter 



Booster 

 Field Rheostat 



Conve 



Converter 

 Field Rheostat 



FIG. 328. Synchronous converter with series booster. 



shaft. That is, it operates as a synchronous motor and tends to 

 drive the converter mechanically. Therefore, the energy current 

 in the converter armature is decreased and may even be reversed. 

 This causes a variation of the cross magnetization which in turn 

 requires that the strength of the commutating poles be changed 

 accordingly. This is accomplished by separate windings on the 

 commutating poles, the current in these windings being controlled 

 by the booster field-rheostat. (For further information, see 

 Standard Handbook, Sec. 9.) The distinct advantage of this 

 method of control is that the voltage may be varied independ- 

 ently of power-factor. The objection to this type of voltage 

 control is the additional machine. Figures 329 and 330 show 

 converters having booster-generators. 



Transformer Taps. The converter voltage may be adjusted 

 approximately to the desired value by taps on the transformers. 



