D.  C.  COMMUTATING  MACHINES 


171 


All  these  windings  are  closed-circuit  windings;  that  is,  starting 
at  any  point,  and  following  the  armature  conductor,  the  circuit 
returns  into  itself  after  passing  all  e.m.fs.  twice  in  opposite  direc- 
tion (thereby  avoiding  short  circuit).  An  instance  of  an  open- 
coil  winding  is  shown  in  Fig.  84,  a  series-connected  three-phase 
star  winding  similar  to  that  used  in  the  Thomson-Houston  arc 
machine.  Such  open-coil  windings,  however,  cannot  be  used  in 
commutating  machines.  They  are  generally  preferred  in  syn- 
chronous and  in  induction  machines. 


FIG.  84. — Open-circuit  three-phase  series  drum  winding. 

38.  By  leaving  space  between  adjacent  coils  of  these  windings 
a  second  winding  can  be  laid  in  between.  The  second  winding 
can  either  be  entirely  independent  from  the  first  winding,  that 
is,  each  of  the  two  windings  closed  upon  itself,  or  after  passing 
through  the  first  winding  the  circuit  enters  the  second  winding, 
and  after  passing  through  the  second  winding  it  reenters  the  first 
winding.  In  the  first  case  the  winding  is  called  a  double  spiral 
winding  (or  multiple  spiral  winding  if  more  than  two  windings 
are  used),  in  the  latter  case  a  double  reentrant  winding  (or 


172     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

multiple  reentrant  winding).  In  the  double  spiral  winding  the 
number  of  coils  must  be  even;  in  the  double  reentrant  winding, 
odd. 

Multiple  spiral  and  multiple  reentrant  windings  can  be  either 
multiple  or  series  wound;  that  is,  each  spiral  can  consist  either 
of  a  multiple  or  of  a  series  winding.  Fig.  85  shows  a  double 
spiral  multiple  ring  winding,  Fig.  86  a  double  spiral  multiple 
drum  winding,  Fig.  87  a  double  reentrant  multiple  drum  winding. 
As  seen  in  the  double  spiral  or  double  reentrant  multiple  wind- 
ing, twice  as  many  circuits  as  poles  are  in  multiple.  Thus  such 


FIG.  85. — Multiple  double  spiral  ring  winding. 

windings  are  mostly  used  for  large  low-voltage  machines,  but  as 
very  few  large  direct-current  generators  are  built  nowadays,  and 
alternating-current  generation  with  synchronous  converters  usu- 
ally preferred,  and  as  multiple  spiral  or  reentrant  windings  are 
inconvenient  in  synchronous  converters,  their  use  has  greatly 
decreased. 

39.  A  distinction  is  frequently  made  between  lap  winding  and 
wave  winding.  These  are,  however,  not  different  types;  but 
the  wave  winding  is  merely  a  constructive  modification  of  the 
series  drum  winding  with  single-turn  coil,  as  seen  by  comparing