A.  GENERATORS 


209 


Separately  Excited  and  Magneto  Generator 

70.  In  a  separately  excited  or  magneto  machine,  that  is,  a 
machine  with   constant  field  excitation  FQ)   a  demagnetization 


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FIG.  111. — Separately  excited  or  magneto-generator  demagnetization  curve 
and  load  characteristic  with  constant  shift  of  brushes. 


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FIG.  112. — Separately  excited  or  magneto-generator  demagnetization  curve 
and  load  characteristic  with  variable  shift  of  brushes. 

curve  can  be  plotted  from  the  magnetization  or  saturation  curve 
A  in  Fig.  109.  At  current  i,  the  resultant  m.m.f .  of  the  machine 
is  FQ  —  iq,  and  the  generated  voltage  corresponds  thereto  by 
the  saturation  curve  A  in  Fig.  110.  Thus,  in  Fig.  Ill  a  de- 
magnetization curve  A  is  plotted  with  the  current  ob  =  i  as 


210     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

abscissas  and  the  generated  e.m.f.  ab  as  ordinates,  under  the 
assumption  of  constant  coefficient  of  armature  reaction  q,  that 
is,  corresponding  to  curve  D  in  Fig.  109.  This  curve  becomes 
zero  at  the  current  ?o,  which  makes  i$q  =  FQ.  Subtracting  from 
curve  A  in  Fig.  Ill  the  drop  of  voltage  in  the  armature  and 
commutator  resistance,  ac  =  ir,  gives  the  external  characteristic 
B  of  the  machine  as  generator,  or  the  curve  relating  the  terminal 
voltage  to  the  current. 

In  Fig.  112  the  same  curves  are  shown  under  the  assumption 
that  the  armature  reaction  varies  with  the  voltage  in  the  way 
as  represented  by  curve  G  in  Fig.  109. 

In  a  separately  excited  or  magneto  motor  at  constant  speed 
the  external  characteristic  would  lie  as  much  above  the  demag- 
netization curve  A  as  it  lies  below  in  a  generator  in  Fig.  Ill, 
and  at  constant  voltage  the  speed  would  vary  inversely  pro- 
portional hereto. 

Shunt  Generator 

71.  The  external  or  load  characteristic  of  the  shunt  generator 
is  plotted  in  Fig.  113  with  the  current  as  abscissas  and  the 
terminal  voltage  as  ordinates,  as  A  for  constant  coefficient  of 
armature  reaction,  and  as  B  for  a  coefficient  of  armature  reac- 
tion varying  with  the  voltage  in  the  way  as  shown  in  G,  Fig.  109. 
The  construction  of  these  curves  is  as  follows: 

In  Fig.  109,  og  is  the  straight  line  giving  the  field  excitation 
oh  as  function  of  the  terminal  voltage  hg  (the  former  obviously 
being  proportional  to  the  latter  in  the  shunt  machine).  The 
open-circuit  or  no-load  voltage  of  the  machine  is  then  kq. 

Drawing  gl  parallel  to  da  (assuming  constant  coefficient  of 
armature  reaction,  or  parallel  to  the  hypothenuse  of  the  triangle 
iq,  ir  at  voltage  og,  when  assuming  variable  armature  reaction), 
then  the  current  which  gives  voltage  gh  is  proportional  to  gl, 
that  is,  i  :  iQ  =  gl  :  da,  where  iQ  is  the  current  at  the  voltage  de. 

As  seen  from  Fig.  113,  a  maximum  value  of  current  exists 
which  is  less  if  the  brushes  are  shifted  than  at  constant  position 
of  brushes. 

From  the  load  characteristic  of  the  shunt  generator  the 
resistance  characteristic  is  plotted  in  Fig.  114;  that  is,  the  de- 
pendence of  the  terminal  voltage  upon  the  external  resistance 

„       terminal  voltage      ~  .      ^.  , 

R  =  — • — *-•     Curve  A   in   Fig.    114   corresponds   to 

current