D.  C.  COMMUTATING  MACHINES 


167 


36.  By  the  number  of  poles  commutating  machines  are  divided 
into  bipolar  and  multipolar  machines.  Bipolar  machines  are 
mainly  used  in  small  sizes.  By  the  construction  of  the  armature, 
commutating  machines  are  divided  into  smooth-core  machines 
and  iron-clad  or  "toothed"  armature  machines.  In  the  smooth- 
core  machine  the  armature  winding  is  arranged  on  the  surface 
of  a  laminated  iron  core.  In  the  iron-clad  machine  the  arma- 
ture winding  is  sunk  into  slots.  The  iron-clad  type  has  the  ad- 
vantage of  greater  mechanical  strength,  but  the  disadvantage  of 
higher  self-inductance  in  commutation,  and  thus  requires  high- 
resistance,  carbon  or  graphite,  commutator  brushes.  The  iron- 
clad type  has  the  advantage  of  lesser  magnetic  stray  field,  due 


FIG.  78. — Series  machine. 


FIG.  79. — Compound  machine. 


to  the  shorter  gap  between  field  pole  and  armature  iron,  and  of 
lesser  magnet  distortion  under  load,  and  thus  can  with  carbon 
brushes  be  operated  with  constant  position  of  brushes  at  all  loads. 
In  consequence  thereof,  for  large  multipolar  machines  the  iron- 
clad type  of  armature  is  best  adapted;  the  smooth-core  type  is 
hardly  ever  used  nowadays. 

Either  of  these  types  can  be  drum  wound  or  ring  wound. 
The  drum  winding  has  the  advantage  of  lesser  self-inductance 
and  lesser  distortion  of  the  magnetic  field,  and  is  generally  less 
difficult  to  construct  and  thus  mostly  preferred.  By  the  arma- 
ture winding,  commutating  machines  are  divided  into  multiple- 
wound  and  series-wound  machines.  The  difference  between 
multiple  and  series  armature  winding,  and  their  modifications,  can 
best  be  shown  by  diagram. 


168     ELEMENTS  OF  ELECTRICAL  ENGINEERING