D.  C.  COMMUTATING  MACHINES  181 

With  the  brushes  set  midway  between  adjacent  field  poles, 
the  armature  m.m.f.  is  additive  on  one  side  and  subtractive  on 
the  other  side  of  the  center  of  the  field  pole.  Thus  the  magnetic 
intensity  is  increased  on  one  side  and  decreased  on  the  other. 
The  total  m.m.f.,  however,  and  thus,  neglecting  saturation,  the 
total  flux  entering  the  armature,  are  not  changed.  Thus,  arma- 
ture reaction,  with  the  brushes  midway  between  adjacent  field 
poles,  acts  distorting  upon  the  field,  but  neither  magnetizes  nor 
demagnetizes,  if  the  field  is  below  saturation. 

The  distortion  of  the  magnetic  field  takes  place  by  the  arma- 
ture ampere-turns  beneath  the  pole,  or  from  B  to  C.  Thus,  if 
T  =  pole  arc,  that  is,  the  angle  covered  by  pole  face  (two  poles 
or  one  complete  period  being  denoted  by  360  degrees),  the  dis- 

rFa 
torting  ampere-turns  of  the  armature  reaction  are 


As  seen,  in  the  assumed  instance,  Fig.  94,  where  F 


the  m.m.f.  at  the  two  opposite  pole  corners,  and  thus  the  mag- 
netic densities,  stand  in  the  proportion  1  to  3.  As  seen,  the 
generated  e.m.f.  is  not  changed  by  the  armature  reaction,  with 
the  brushes  set  midway  between  the  field  poles,  except  by  the 
sftiall  amount  corresponding  to  the  flux  entering  beyond  D  and 
G,  that  is,  shifted  beyond  the  position  of  brushes.  At  D,  how- 
ever, the  flux  still  enters  the  armature,  depending  in  intensity 
upon  the  armature  reaction;  and  thus  with  considerable  arma- 
ture reaction  the  brushes  when  set  at  this  point  are  liable  to 
spark  by  short-circuiting  an  active  e.m.f.  Therefore,  under  load, 
the  brushes  are  shifted  toward  the  following  pole,  that  is,  toward 
the  direction  in  which  the  zero  point  of  magnetic  flux  has  been 
shifted  by  the  armature  reaction. 

45.  In  Fig.  95,  the  brushes  are  assumed  as  shifted  to  the  cor- 
ner of  the  next  pole,  E  respectively  B.  In  consequence  thereof, 
the  subtractive  range  of  the  armature  m.m.f.  is  larger  than  the 
additive,  and  the  resultant  m.m.f.  F  =  F0  +  Fa  is  decreased; 
that  is,  with  shifted  brushes  the  armature  reaction  demagnet- 
izes the  field.  The  demagnetizing  armature  ampere-turns  are 

f~ir> 

PM  =  7^TfFa.    That  is,  if  TI  =  angle  of  shift  of  brushes  or  angle 

of  lead  (  =  GB  in  Fig.  95),  assuming  the  pitch  of  two  poles  =  360 
degrees,  the  demagnetizing  component  of  armature  reaction  is 
2  T  V  V 

*";  the  distorting  component  is  ^-^.,  where  T  =  pole  arc. 

loU  loU 

12 


182     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

Thus,  with  shifted  brushes  the  field  excitation  has  to  be  in- 
creased under  load  to  maintain  the  same  total  resultant  m.m.f., 

that  is,  the  same  total  flux  and  generated  e.m.f.     Hence,  in 

*?    jf        ff 

Fig.  95  the  field  excitation  F0  has  been  assumed  by     **a  =  - 

loU          o 

larger  than  in  the  previous  figures,  and  the  magnetic  distribution 
BI  plotted  for  these  values. 


J 


FIG.  95. — Distribution  of  flux  with  current  in  the  armature  and  brushes 
shifted  from  the  magnetic  neutral.