342 ALTERNATING CURRENTS 



low-voltage batteries from alternating-current supply. Their 

 advantage lies in their cheapness and simplicity. 



6. Induction- or Synchronous-motor-generator Sets. None of 

 the foregoing devices is capable of converting alternating to 

 direct current on the large scale required in modern power 

 systems. To convert large amounts of power, induction-motor- 

 or synchronous-motor-generator sets may be employed. The 

 capacity of such units is limited only by the size in which it is 

 possible to construct the direct-current generator. ' The dis- 

 advantage of a motor-generator set is that it requires two ma- 

 chines, with corresponding cost and floor space, and the over-all 

 efficiency is not extremely high, being the product of the effici- 

 encies of the individual units of the set. 



7. The Rotary or Synchronous Converter is a single machine 

 which converts alternating to direct current or vice versa, and 

 may be built to convert large amounts of power efficiently and 

 economically. Because it has only one armature and one field, 

 the synchronous converter usually costs less than an equiva- 

 lent motor-generator set. Because the armature current is 

 small, being the difference between the alternating and the direct 

 currents, this type of machine has a high efficiency when oper- 

 ating under favorable conditions. 



138. Principle of the Synchronous Converter. It has already 

 been demonstrated that alternating current is generated in the 

 armature coils of the ordinary direct-current generator. If 

 taps be brought out properly from the armature winding to slip- 

 rings, alternating current may be taken from this same winding 

 and the machine becomes an alternator. Such an alternator 

 can obviously operate as a synchronous motor. 



The synchronous converter is constructed like the ordinary 

 direct-current generator, although the relative dimensions may 

 be different. It has fixed poles, a rotating armature, a commu- 

 tator, a shunt field, and usually a series field. In addition to 

 the commutator, however, leads are taken from the armature to 

 slip-rings, in the manner shown in Fig. 314 (also see Figs. 315 and 

 316). Figure 314 represents a two-pole, single-phase converter. 



In the synchronous converter, as commonly used, alternating 

 current is supplied to the slip-rings and direct current is taken 

 from the commutator and brushes. If, however, the direct- 



