D.  C.  COMMUTATING  MACHINES  187 

ampere-turns  per  pole.  Choosing  then  8000  ampere-turns  per 
commutating  pole  F',  leaves  2000  ampere-turns  as  resultant  com- 
mutating  m.m.f .  at  full  load,  half  as  much  at  half  load,  etc.  The 
resultant  m.m.f.  of  the  main  field  FQ,  the  armature  Fa,  and  the 
commutating  pole  Ff  is  represented  in  Fig.  100  by  Fz,  and  the 
flux  produced  by  it  is  shown  in  Fig.  101.  As  seen,  with  the  com- 
mutator brushes  midway  between  the  field  poles,  that  is,  in  the 
center  of  the  commutating  pole,  a  commutating  flux  proportional 
to  the  armature  current  enters  the  armature  at  the  brush  B 
and  5',  and  is  cut  by  the  revolving  armature  during  commutation. 
The  use  of  the  commutating  pole  or  interpole  thus  permits 
controlling  the  commutation,  with  fixed  brush  position  midway 
between  the  field  poles,  and  commutating  poles  therefore  are 


FIG.  101. — Magnetic  flux  distribution  with  commutating  pole. 

extensively  used  in  larger  machines,  especially  of  the  high-speed 
type. 

The  commutating  pole  makes  the  commutation  independent 
of  the  main  field  strength,  and  therefore  permits  the  machines 
to  operate  with  equally  good  commutation  over  a  wide  voltage 
range,  and  at  low  voltage,  that  is,  low  field  strength,  as  required 
for  instance  in  boosters,  etc. 

50.  With  multiple-wound  armatures,  at  least  one  commutat- 
ing pole  for  every  pair  of  main  poles  is  required,  while  with  a 
series-wound  armature  a  single  commutating  pole  would  be 
sufficient  for  all  the  sets  of  armature  brushes,  if  of  sufficient 
strength.  In  general,  however,  as  many  commutating  poles  as 
main  poles  are  used. 

With  the  position  of  the  brushes  at  the  neutral,  as  is  the  case 
when  using  commutating  poles,  the  armature  reaction  has  no 


188     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

demagnetizing  component,  and  the  only  drop  of  voltage  at  load 
is  that  due  to  the  armature  resistance  drop  and  the  distortion  of 
the  main  field,  which  at  saturation  produces  a  decrease  of  the 
total  flux,  as  shown  in  Fig.  98. 

As  is  seen  in  Fig.  101,  the  magnetic  flux  of  the  commutating 
pole  is  not  symmetrical,  but  the  spread  of  flux  is  greater  at  the 
side  of  the  main  pole  of  the  same  polarity.  As  result  thereof,  the 
total  magnetic  flux  is  slightly  increased  by  the  commutating 
poles;  that  is,  the  two  halves  of  the  commutating  flux  on  the 
two  sides  of  the  brush  do  not  quite  neutralize,  and  the  com- 
mutating flux  thus  exerts  a  slight  compounding  action,  that  is, 
tends  to  raise  the  voltage.  This  can  be  still  further  increased 
by  shifting  the  brushes  slightly  back  and  thus  giving  a  magnet- 
izing component  of  armature  reaction.  This  can  be  done  with- 
out affecting  commutation  as  long  as  the  brushes  still  remain 
under  the  commutating  pole.  In  this  manner  a  compounding 
or  even  a  slight  over-compounding  can  be  produced  without  a 
series  winding  on  the  main  field  poles,  or,  inversely,  by  shifting 
the  brushes  slightly  forward,  a  demagnetizing  component  of 
armature  reaction  can  be  introduced.  Furthermore,  the  current 
induced  in  the  short-circuited  armature  coil  by  the  commutating 
field  is  magnetizing,  that  induced  by  the  magnetic  field  of  arma- 
ture reaction,  demagnetizing. 

In  operating  machines  with  commutating  poles  in  multiple, 
care  therefore  must  be  taken  not  to  have  the  compounding  action 
of  the  commutating  poles  interfere  with  the  distribution  of  load ; 
for  this  purpose  an  equalizer  connection  may  be  used  between 
the  commutating  pole  windings  of  the  different  machines,  and 
the  commutating  windings  treated  in  the  same  way  as  series 
coils  on  the  main  poles,  that  is,  equalized  between  the  different 
machines  to  insure  division  of  load. 

51.  The  advantage  of  the  commutating  pole  over  the  shift 
of  brushes  to  the  edge  of  the  next  field  pole,  in  constant  poten- 
tial machines — shunt  or  compound  wound — thus  is  that  the 
commutating  flux  of  the  former  has  the  right  intensity  at  all 
loads,  while  that  of  the  latter  is  right  only  at  one  particular  load, 
too  high  below,  too  low  above  that  load.  In  series-wound 
machines,  that  is,  machines  in  which  the  main  field  is  excited  in 
series  with  the  armature,  and  thus  varies  in  strength  with  the 
armature  current,  armature  reaction  and  field  excitation  are 
Always  proportional  to  each  other,  and  the  distribution  of  mag-