X.  Compounding 

59.  In  the  direct-current  generator  the  field  excitation  re- 
quired to  maintain  constant  terminal  voltage  has  to  be  increased 
with  the  load.     A  curve  giving  the  field  excitation  in  ampere- 
turns  per  pole,  as  function  of  the  load  in  amperes,  at  constant 
terminal  voltage,  is  called  the  compounding  curve  of  the  machine. 
The  increase  of  field  excitation  required  with  load  is  due  to : 
1.  The  internal  resistance  of  the  machine,  which  consumes 
e.m.f.  proportional  to  the  current,  so  that  the  generated  e.m.f., 
and  thus  the  field  m.m.f.  corresponding  thereto,  has  to  be  greater 
under  load.     If  p  =  resistance  drop  in  the  machine  as  fraction 

ir 
of  terminal  voltage,  =  —  >    the  generated  e.m.f.  at  load  has  to  be 

£ 

e  (1  +  p),  and  if  ^o=  no-load   field   excitation,  and  s  =  satu-