RECTIFYING DEVICES 



389 



is required. For this purpose a motor-generator set is 

 used instead of a synchronous converter. An induction, 

 or synchronous, motor takes power from the a-c. line and 

 drives a continuous-current generator, the field excitation 

 of which may be varied. Of, course, a motor-generator set 

 costs more to install than a synchronous converter of equal 

 capacity and in addition it is not as efficient in operation 

 as the converter, but in some cases the advantages gained 



10 



20 30 40 



Continuous Current load in Amperes. 



FIG. 255. Curves Showing Compounding Possible on a Small Auxiliary 

 Pole Synchronous Converter. 



by an adjustable, continuous voltage offset these dis- 

 advantages. 



122. Vibrating Rectifiers Rotating Commutator Rec- 

 tifiers. Many times a small amount of c-c. power is 

 desired where only a-c. power is at hand; the charging of 

 automobile storage batteries in the principal instance of 

 this kind. Generally the amount of power desired is not 

 sufficient to warrant the installation of a motor-generator 



