HBCT1F1BRS: TIIK SYNCHRONOUS CONVERTER 367 



Tin- transformer primaries are connected in delta, and the second- 

 aries can be connected either diametrical, star, or double-Y. 

 (If this were a split-pole type of converter, the primaries could not 

 be connected in delta, but they must be connected in Y without 

 neutral return to main generator. ) The advantage of the star 

 and the inter-connected double-Y connection is the fact that a 

 neutral is accessible. The voltages and currents at each part 

 of the system are shown. Unity power-factor, 98 per cent. 

 efficiency for the transformers and 95 per cent, efficiency for the 

 converter are a-sumed. 



The double-delta connection of secondaries may also be used. 

 Such a connection for a converter is shown in Fig. 334. The 

 arrows point in the relative directions in which the voltages act. 

 Nn neutral is available if this method of connecting the trans- 

 formers is used. 



150. The Inverted Synchronous Converter. When a converter 



operates from a direct-current source and delivers alternating 



current, it is known as an inverted synchronous converter. The 



direct -cum -nt side has characteristics very similar to those of a 



shunt or compound motor. The alternating-current side has 



characteristics very similar to those of an alternator. A con- 



verter when operating inverted has the same rating as when 



operating direct. When operating from the alternating-current 



supply, the speed of the converter must be in synchronism with 



ipply. and hence constant. When operating from the direct - 



current supply, the speed is determined by the back electromotive 



and the flux, just as in any direct-current motor, and the 



: may vary. In fact, at times there i> n tendency for the 



inverted cnnverter to race, so that inverted com hould 



-|x>ed-limit ing devices. An inductive load on the alternat- 



mvnt <ide weakens the held through armature re.-iction. in 



the same manner that the field of an alternator is weakened under 



similar conditions. The w-ak-ninL r of the field increases the 



ol the converter. This increased speed causes the current 



to lag still more^tan 9 = fT) l)(ir;ll i s e of the increased 



A is cumulative. ;md may cause the 



armature to reach dangerous speeds, t he necessity for using a 

 'l-limitii:. -us. 



