D.  C.  COMMUTATING  MACHINES 


207 


and  compound  machines.  Magneto  machines  and  separately 
excited  machines  are  very  similar  in  their  characteristics.  In 
either,  the  field  excitation  is  of  constant,  or  approximately 
constant,  impressed  m.m.f.  Magneto  machines,  however,  are 
little  used,  except  for  very  small  sizes. 

By  the  direction  of  energy  transformation,  commutating  ma- 
chines are  subdivided  into  generators  and  motors. 

Of  foremost  importance  in  discussing  the  different  types  of 
machines  is  the  saturation  curve  or  magnetic  characteristic; 
that  is,  a  curve  relating  terminal  voltage  at  constant  speed  to 
ampere-turns  per  pole  field  excitation,  at  open  circuit.  Such 
a  curve  is  shown  as  A  in  Figs.  109  and  110.  It  has  the  same 


1      8      9      4      5      6       78      9     19    U     12    13     H    15    16    I?     J«     W    20     81 

FIG.  109. — Generator  saturation  curves. 


general  shape  as  the  magnetic  flux  density  curve,  except  that  the 
knee  or  bend  is  less  sharp,  due  to  the  different  parts  of  the 
magnetic  circuit  saturation  successively. 

Thus,  in  order  to  generate  voltage  ac  the  field  excitation  oc 
is  required.  Subtracting  from  ac  in  a  generator,  Fig.  109,  or 
adding  in  a  motor,  Fig.  110,  the  value  ab  =  ir,  the  voltage  con- 
sumed by  the  resistance  of  the  armature,  commutator,  etc., 
gives  the  terminal  voltage  be  at  current  i,  and  adding  to  oc  the 
value  ce  =  bd  =  iq  =  armature  reaction,  or  rather  field  excita- 
tion required  to  overcome  the  armature  reaction,  gives  the  field 
excitation  oe  required  to  produce  the  terminal  voltage  de  at 


208     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

current  i.  The  armature  reaction  iq,  corresponding  to  current  i, 
is  calculated  as  discussed  before,  and  q  may  be  called  the  coef- 
ficient of  armature  reaction. 

69.  Such  a  curve,  D,  shown  in  Fig.  109  for  a  generator,  and 
in  Fig.  110  for  a  motor,  and  giving  the  terminal  voltage  de  at 
current  i,  corresponding  to  the  field  excitation  oe}  is  called  a 
load  saturation  curve.  Its  points  are  respectively  distant  from 
the  corresponding  points  of  the  no-load  saturation  curve  A  a 
constant  distance  equal  to  ad,  measured  parallel  thereto. 

Curves  D  are  plotted  under  the  assumption  that  the  armature 
reaction  is  constant.  Frequently,  however,  at  lower  voltage  the 


1231567 

FIG.  110. 


9   10  11  12  13  14  .15  16  17  18  19  20  21 

-Motor  saturation  curves. 


armature  reaction,  or  rather  the  increase  of  excitation  required 
to  overcome  the  armature  reaction  iq,  increases,  since  with 
voltage  commutation  at  lower  voltage,  and  thus  weaker  field 
strength,  the  brushes  have  to  be  shifted  more  to  secure  spark- 
less  commutation,  and  thus  the  demagnetizing  effect  of  the 
angle  of  lead  increases.  At  higher  voltage  iq  usually  increases 
also,  due  to  increase  of  magnetic  saturation  under  load,  caused 
by  the  increased  stray  field.  Thus,  the  load  saturation  curve 
of  the  continuous-current  generator  more  or  less  deviates  from 
the  theoretical  shape  D  toward  a  shape  shown  as  G.