XIV.  Three-wire   Generator  and  Converter 

107.  A  machine  based  upon  the  principle  of  the  direct-current 
converter  is  frequently  used  to  supply  a  three-  wire  direct-current 
distribution  system  (Edison  system).  This  machine  may  be  a 
single  generator  or  synchronous  converter,  which  is  designed  for 
the  voltage  between  the  outside  conductors  of  the  circuit  (the 
positive  and  the  negative  conductor),  220  to  280  volts,  while  the 
middle  conductor  of  the  system,  or  neutral  conductor,  is  con- 


SYNCHRONOUS  CONVERTERS 


271 


nected  to  the  generator  by  autotransformer  and  collector  rings, 
or,  in  the  case  of  a  synchronous  converter,  is  connected  to  the 
neutral  of  the  step-up  transformers,  and  the  latter  thus  used 
as  autotransformers. 


-*; 

to     

v        2  "* 

n 

C, 

2                                0         1        , 

o 

)T               ' 

k    g 

to       t.<  

2 

FIG.  146. — Three-wire  machine  with  single  autotransformer. 

A  three-wire  generator  thus  is  a  combination  of  a  direct- 
current  generator  and  a  direct-current  converter,  and  a  three- 
wire  converter  is  a  combination  of  a  synchronous  converter  and 
a  direct-current  converter.  Such  a  three-wire  machine  has  the 
advantage  over  two  separate  machines,  connected  to  the  two 


2C 


i-l.   < 

Fro.  147. — Three-wire  system  with  two  machines. 

sides  of  the  three- wire  direct-current  system,  of  combining  two 
smaller  machines  into  one  of  twice  the  size,  and  thus  higher 
space-  and  operation-economy  and  lower  cost,  and  has  the  further 
advantage  that  only  half  as  large  current  is  commutated  as  by 


272     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

the  use  of  two  separate  machines;  that  is,  the  positive  brush 
of  the  machine  on  the 'negative  and  the  negative  brush  of  the 
machine  on  the  positive  side  of  the  system  are  saved,  as  seen  by 
the  diagrammatic  sketch  of  the  machine  in  Fig.  146  and  the  two 
separate  two-wire  machines  in  Fig.  147.  The  use  of  three-wire 
220-volt  machines  on  three-wire  direct-current  systems  thus  has 
practically  displaced  that  of  two  separate  110- volt  machines.