D.  C.  COMMUTATING  MACHINES 


195 


as  average  saturation  curve  the  curve  shown  in  Fig.  105  as  A 
and  as  central  curve  in  Fig.  106. 

Direct-current  generators  are  usually  operated  at  a  point  of 
the  saturation  curve  above  the  bend,  that  is,  at  a  point  where  the 
terminal  voltage  increases  considerably  less  than  proportionally 
to  the  field  excitation.  This  is  necessary  in  self-exciting  direct- 
current  generators  to  secure  stability. 

The  ratio 


increase  of  field  excitation 
total  field  excitation 

that  is, 


corresponding  increase  of  voltage 

total  voltage 
F*      de 


FIG.  105. — Saturation  characteristics. 

is  called  saturation  factor  s,  and  is  plotted  in  Fig.  105.  It  is  the 
ratio  of  a  small  percentage  increase  in  field  excitation  to  a  corre- 
sponding percentage  increase  in  voltage  thereby  produced.  The 

quantity  1 is  called  the  percentage  saturation  of  the  ma- 

s 

chine,  as  it  shows  the  approach  of  the  machine  field  to  mag- 
netic saturation. 

58.  Of  considerable  importance  also  are  curves  giving  the 
terminal  voltage  as  function  of  the  field  excitation  at  load. 
Such  curves  are  called  load  saturation  curves,  and  can  be  constant 


196     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

current  load  saturation  curve,  that  is,  terminal  voltage  as  func- 
tion of  field  ampere-turns  at  constant  full-load  current  through 
the  armature,  and  constant  resistance  load  saturation  curve,  that 
is,  terminal  voltage  as  function  of  field  ampere-turns  if  the 
machine  circuit  is  closed  through  a  constant  resistance  giving 
full-load  current  at  full-load  terminal  voltage. 

A  constant  current  load  saturation  curve  is  shown  as  B,  and 
a  constant  resistance  load  saturation  curve  as  C  in  Fig.  105. 


FIG.  106. — Saturation  curves.