B.  THBEE-WIRE  CONVERTER 

109.  In  a  converter  feeding  a  three-wire  direct-current  system 
the  neutral  can  be  derived  by  connection  to  the  transformer 
neutral.  Even  in  this  case,  however,  frequently  a  separate  auto- 
transformer is  used,  connected  across  a  pair  of  collector  rings  of 
the  converter,  since,  as  seen  above,  with  the  moderate  unbalanc- 
ing usually  existing,  such  a  compensator  is  very  small. 

When  connecting  the  direct-current  neutral  to  the  transformer 
neutral  it  is  necessary  to  use  such  a  connection  that  the  trans- 
former can  operate  as  autotransformer,  that  is,  that  the  direct 
current  in  each  transformer  divides  into  two  branches  of  equal 
m.m.f.,  otherwise  the  direct-current  produces  a  unidirectional 
magnetization  in  the  transformer,  which  superimposed  upon 
the  magnetic  cycle  raises  the  magnetic  induction  beyond  satura- 
tion, and  thus  causes  excessive  exciting  current  and  heating, 
except  when  very  small. 


SYNCHRONOUS  CONVERTERS 


275 


k       *T.     \ 


FIG.  149. — Neutral  of  Y-connected  transformers  connected  to  neutral  of 
three-wire  system  supplied  from  a  three-phase  converter. 


FIG.  150. — Quarter-phase  converter  with  transformer  neutral  connected  to 
direct-current  neutral. 


FIG.  151. — Three-phase  converter  with  neutral  of  the  T-connected  trans- 
formers as  direct-current  neutral. 


FIG.  152. — Three-phase  converter  with  transformer  neutral  connected  to 
direct-current  neutral. 


276     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

For  instance,  with  Y  connection  of  the  transformers  supplying 
a  three-phase  converter,  Fig.  149,  each  transformer  secondary 
receives  one-third  of  the  neutral  current,  and  if  this  current  is  not 
very  small  and  comparable  with  the  exciting  current  of  the 
transformer — which  can  rarely  be — the  magnetic  density  in 
the  transformer  rises  beyond  saturation  by  this  unidirectional 
m.m.f.  This  connection  thus  is  in  general  not  permissible  for 
deriving  the  neutral. 

In  a  quarter-phase  converter,  as  shown  in  Fig.  150,  the  trans- 
former neutral  can  be  used  as  direct-current  neutral,  since 
in  each  transformer  the  direct  current  divides  into  two  equal 
branches,  which  magnetize  in  opposite  direction,  and  so  neu- 
tralize. 

The  T  connection,  Fig.  151,  can  be  used  for  three-phase  con- 
verters with  the  neutral  derived  from  a  point  at  one-third  the 
height  of  the  teaser  transformer,  as  seen  in  Fig.  151. 

Delta  connection  on  three-phase  and  double  delta  on  six-phase 
converters  cannot  be  used,  as  it  has  no  neutral,  but  in  this  case  a 
separate  compensator  is  required. 

The  diagrammatical  connections  of  transformers  can,  however, 
be  used  on  six-phase  converters,  and  the  connection  shown  in 
Fig.  152,  which  has  two  coils  on  each  transformer,  connected  to 
different  phases,  on  three-phase  converters. 


D.  ALTERNATING-CURRENT  TRANSFORMER