D.  C.  COMMUTATING  MACHINES  183 

In  Figs.  96,  97,  98,  99,  curves  are  plotted  corresponding  to 
those  in  Figs.  92,  93,  94,  and  95.  As  seen,  the  spread  of  mag- 
netic flux  at  the  pole  corners  is  greatly  increased,  but  farther 
away  from  the  field  poles  the  magnetic  distribution  is  not 
changed. 

47.  The  magnetizing,  or  rather  demagnetizing,  effect  of  the 
load  with  shifted  brushes  is  not  changed.  The  distorting  effect 


FIG.  96. — Flux  distribution  under  a  single  pole. 

of  the  load  is,  however,  very  greatly  decreased,  to  a  small  per- 
centage of  its  previous  value,  and  the  magnetic  field  under  the 
field  pole  is  very  nearly  uniform  under  load. 

The  reason  is:  Even  a  very  large  increase  of  m.m.f.  does  not 
much  increase  the  density,  the  ampere-turns  being  consumed  by 
saturation  of  the  iron,  and  even  with  a  large  decrease  of  m.m.f. 
the  density  is  not  decreased  much,  since  with  a  small  decrease 


'I/       •    \\      L 


FIG.  97. — Distribution  of  flux  and  m.m.f.   at  no  load. 

of  density  the  ampere-turns  consumed  by  the  saturation  of  the 
iron  become  available  for  the  air  gap. 

Thus,  while  in  Fig.  95  the  densities  at  the  center  and  the  two 
pole  corners  of  the  field  pole  are  8000,  12,000,  and  4000,  with  the 
saturated  structure  in  Fig.  99  they  are  8000,  9040,  and  6550. 

At  or  near  the  theoretical  neutral,  however,  the  saturation  has 
no  effect. 

That  is,  saturation  of  the  armature  teeth  affords  a  means  of 


184     ELEMENTS  OF  ELECTRICAL  ENGINEERING 


reducing  the  distortion  of  the  magnetic  field,  or  the  shifting  of 
flux  at  the  pole  corners,  and  is  thus  advantageous  for  machines 
which  shall  operate  over  a  wide  range  of  load  with  fixed  position 
of  brushes,  if  the  brushes  are  shifted  near  to  the  next  following 
pole  corner. 

Fo 


J 


FIG.  98. — Distribution  of  flux  and  m.m.f.  at  load,  with  Brushes  at  neutral. 

It  offers  no  direct  advantage,  however,  for  machines  corn- 
mutating  with  the  brushes  midway  between  the  field  poles,  as 
converters. 

An  effect  similar  to  saturation  in  the  armature  teeth  is  produced 


J 


L 


FIG.  99. — Distribution  of  flux  and  m.m.f.  at  load,  with  brushes  shifted  to 

next  pole  corner. 

by  saturation  of  the  field  pole  face,  or  more  particularly,  satura- 
tion of  the  pole  corners  of  the  field.