D.  C.  COMMUTATING  MACHINES 


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adjusted  for  the  same  voltage  at  no  load  and  at  full  load,  under- 
compounds  at  higher  and  over-compounds  at  lower  voltage,  and 
even  at  open  circuit  of  the  shunt  field  gives  still  a  voltage  op  as 
series  generator.  When  shifting  the  brushes  under  load,  at  lower 
voltage  a  second  point  g  is  reached  where  the  machine  compounds 
correctly,  and  below  this  point  the  machine  under-compounds 
and  loses  its  excitation  when  the  shunt  field  decreases  below  a 
certain  value;  that  is,  it  does  not  excite  itself  as  series  generator.