CHAPTEB XI 

 RECTIFIERS : THE SYNCHRONOUS CONVERTER 



136. Methods of Obtaining Direct Current from Alternating 

 Current.- -At the present time, over 90 per cent, of electrical 

 energy is generated and transmitted as alternating current. A 

 very large percentage of this energy is utilized as alternating 

 current; for example, to operate alternating-current motors, 

 electric furnaces, and many other types of electrical appliances, 

 for illumination purposes, etc. However, there are many cases 

 \\hcn the electrical energy must be in the form of direct current, 

 even although the available supply of energy is alternating cur- 

 rent. For example, direct current must be used for charging 

 storage batteries, for electrolytic work, for telephone exchanges, 

 The direct-current series motor is practically the only type 

 of motor that can be .used for street-railway work and it is also 

 commonly used in railway electrification. In the congested 

 city di<trict<. where the consumers loads are large and close 

 together, direct-current power is preferable to alternating-current 

 power, as capacitive effects in the underground cables are not 

 present when direct current is used and inductive effects in the 

 nt with direct current. Furthermore, in 



such lo.nK the importance of continuity of service requires that 

 a large storage-battery reserve be available. This, again, is an 

 additional reason for supplying direct-current service in such 

 distri 



A- the power supply in the above cases is almost always alter- 

 iiatiiii: riirn-nt. this alternating cm-rent must in some manner he 

 'd to direct current. There are several methods of ac- 

 this, the most common l.eini: the following: 



1 \\<-<-l II.T r.unmutatmu 



mir.-il riTtilirr viln-Mlr 



t Tlii- Tuim.-ii rectifier. 



Induction .,r -\ m dronous-motor-gencrator sets. 

 7. l{.,t;iry or synchronous conv 



333 



