D.  C.  COMMUTATING  MACHINES  175 

sarily  be  lap  or  coil  windings.  In  Fig.  90  is  shown  a  series  drum 
winding  with  35  coils  and  commutator  segments,  and  a  single 
turn  per  coil  arranged  as  wave  winding.  This  winding  may  be 
compared  with  the  35-coil  series  drum  winding  in  Fig.  83. 

40.  Drum  winding  can  be  divided  into  full-pitch  and  frac- 
tional-pitch windings.  In  the  full-pitch  winding  the  spread  of 
the  coil  covers  the  pitch  of  one  pole;  that  is,  each  coil  covers 


FIG.  90. — Series  drum  wave  winding. 

one-sixth  of  the  armature  circumference  in  a  six-pole  machine, 
etc.     In  a  fractional-pitch  winding  it  covers  less  or  more. 

Series  drum  windings  without  cross-connected  commutator  in 
which  thus  the  number  of  coils  is  not  divisible  by  the  number  of 
poles  are  necessarily  always  slightly  fractional  pitch;  but  gen- 
erally the  expression  " fractional-pitch  winding"  is  used  only  for 
windings  in  which  the  coil  covers  one  or  several  teeth  less  than 
correspond  to  the  pole  pitch.  Thus  the  multiple  drum  winding 
in  Fig.  81  would  be  a  fractional-pitch  winding  if  the  coils  spread 


176      ELEMENTS  OF  ELECTRICAL  ENGINEERING 

over  only  four  or  five  teeth  instead  of  over  six.     As  five-sixths 
fractional-pitch  multiple  drum  winding  it  is  shown  in  Fig.  91. 

Fractional-pitch  windings  have  the  advantage  of  shorter 
end  connections  and  less  self-inductance  in  commutation,  since 
commutation  of  corresponding  coils  under  different  poles  does 
not  take  place  in  the  same,  but  in  different,  slots,  and  the  flux 
of  self-inductance  in  commutation  is  thus  more  subdivided. 
Fig.  91  shows  the  multiple  drum  winding  of  Fig.  81  as  a  frac- 


FIG.  91. — Multiple  drum  five-sixth  fractional  pitch  winding. 

tional-pitch  winding  with  five  teeth  spread,  or  five-sixths  pitch. 
During  commutation  the  coils  a  b  c  d  e  f  commutate  simultane- 
ously. In  Fig.  81  these  coils  lie  by  twos  in  the  same  slots,  in 
Fig.  91  they  lie  in  separate  slots.  Thus,  in  the  former  case  the 
flux  of  self-inductance  interlinked  with  the  commutated  coil  is 
due  to  two  coils;  that  is,  twice  that  in  the  latter  case.  Frac- 
tional-pitch windings,  however,  have  the  disadvantage  of  reduc- 
ing the  width  of  the  neutral  zone,  or  zone  without  generated 
e.m.f.  between  the  poles,  in  which  commutation  takes  place,