IX.  Saturation  Curves 

57.  As  saturation  curve  or  magnetic  characteristic  of  the  com- 
mutating machine  is  understood  the  curve  giving  the  generated 
voltage,  or  terminal  voltage  at  open  circuit  and  normal  speed, 
as  function  of  the  ampere-turns  per  pole  field  excitation. 

Such  curves  are  of  the  shape  shown  in  Fig.  105  as  A.  Owing 
to  the  remanent  magnetism  or  hysteresis  of  the  iron  part  of  the 
magnetic  circuit,  the  saturation  curve  taken  with  decreasing 
field  excitation  usually  does  not  coincide  with  that  taken  with 
increasing  field  excitation,  but  is  higher,  and  by  gradually  first 
increasing  the  field  excitation  from  zero  to  maximum  and  then 
decreasing  again,  the  looped  curve  in  Fig.  106  is  derived,  giving