XI.  Unbalancing  of  Polyphase  Synchronous  Machines 

23.  The  preceding  discussion  applies  to  polyphase  as  well  as 
single-phase  machines.  In  polyphase  machines  the  nominal 
generated  e.m.fs.  or  nominal  counter-generated  e.m.fs.  are  neces- 
sarily the  same  in  all  phases  (or  bear  a  constant  relation  to  each 
other).  Thus  in  a  polyphase  generator,  if  the  current  or  the 


SYNCHRONOUS  MACHINES  151 

phase  relation  of  the  current  is  different  in  the  different  branches, 
the  terminal  voltage  must  become  different  also,  more  or  less. 
This  is  called  the  unbalancing  of  the  polyphase  generator.  It  is 
due  to  different  load  or  load  of  different  inductance  factor  in  the 
different  branches. 

Inversely,  in  a  polyphase  synchronous  motor,  if  the  terminal 
voltages  of  the  different  branches  are  unequal,  due  to  an  unbal- 
ancing of  the  polyphase  circuit,  the  synchronous  motor  takes 
more  current  or  lagging  current  from  the  branch  of  higher  vol- 
tage, and  thereby  reduces  its  voltage,  and  takes  less  current  or 
leading  current1  from  the  branch  of  lower  voltage,  or  even 
returns  current  into  this  branch,  and  thus  raises  its  voltage. 
Hence  a  synchronous  motor  tends  to  restore  the  balance  of  an 
unbalanced  polyphase  system ;  that  is,  it  reduces  the  unbalancing 
of  a  polyphase  circuit  caused  by  an  unequal  distribution  or 
unequal  phase  relation  of  the  load  on  the  different  branches. 
To  a  less  degree  the  induction  motor  possesses  the  same  property.