II.  Armature  Winding 

37.  Fig.  80  shows  a  six-pole  multiple  ring  winding,  and  Fig. 
81  a  six-polar  multiple  drum  winding.  As  seen,  the  armature 
coils  are  connected  progressively  all  around  the  armature  in 
closed  circuit,  and  the  connections  between  adjacent  armature 
coils  lead  to  the  commutator.  Such  an  armature  winding  has 
as  many  circuits  in  multiple,  and  requires  as  many  sets  of  com- 
mutator brushes,  as  poles.  Thirty-six  coils  are  shown  in  Figs. 
80  and  81,  connected  to  36  commutator  segments,  and  the 
two  sides  of  each  coil  distinguished  by  drawn  and  dotted  lines. 
In  a  drum-wound  machine,  usually  the  one  side  of  all  coils  forms 
the  upper  and  the  other  side  the  lower  layer  of  the  armature 
winding. 

Fig.  82  shows  a  six-pole  series  drum  winding  with  36  slots 
and  36  commutator  segments.  In  the  series  winding  the  circuit 
passes  from  one  armature  coil,  not  to  the  next  adjacent  armature 
coil  as  in  the  multiple  winding,  but  first  through  all  the  armature 
coils  having  the  same  relative  position  with  regard  to  the  magnet 
poles  of  the  same  polarity,  and  then  to  the  armature  coil  next  ad- 
jacent to  the  first  coil.  That  is,  all  armature  coils  having  the 
same  or  approximately  the  same  relative  position  to  poles  of  equal 
polarity  form  one  set  of  integral  coils.  Thus  the  series  winding 
has  only  two  circuits  in  multiple,  and  requires  two  sets  of  brushes 
only,  but  can  be  operated  also  with  as  many  sets  of  brushes  as 
poles,  or  any  intermediate  number  of  sets  of  brushes.  In  Fig.  82, 
a  series  winding  in  which  the  number  of  armature  coils  is  divisible 
by  the  number  of  poles,  the  commutator  segments  have  to  be 
cross  connected.  Therefore  this  form  of  series  winding  is  hardly 
ever  used.  The  usual  form  of  series  winding  is  the  winding  shown 
by  Fig.  83.  This  figure  shows  a  six-polar  armature  having  35 
coils  and  35  commutator  segments.  In  consequence  thereof  the 
armature  coils  under  corresponding  poles  which  are  connected 
in  series  are  slightly  displaced  from  each  other,  so  that  after  pass- 
ing around  all  corresponding  poles  the  winding  leads  symmetric- 
ally into  the  coil  adjacent  to  the  first  armature  coil.  Hereby  the 
necessity  of  commutator  cross  connections  is  avoided,  and  the 
winding  is  perfectly  symmetrical.  With  this  form  of  series 
winding,  which  is  mostly  used,  the  number  of  armature  coils  must 
be  chosen  to  follow  certain  rules.  Generally  the  number  of 
coils  is  one  less  or  one  more  than  a  multiple  of  the  number  of  poles. 


/).  C.  COMMUTATING  MACHINES 
s 


169 


FIG.  80. — Multiple  ring  armature  winding. 


FIG.  81. — Multiple  drum  full  pitch  winding. 


170     ELEMENTS  OF  ELECTRICAL  ENGINEERING 


FIG.  82. — Series  drum  winding  with  cross-connected  commutator. 


FIG.  83. — Series  drum  winding.