D.  C.  COMMUTATING  MACHINES  177 

since  the  one  side  of  the  coil  enters  or  leaves  the  field  before  the 
other.  Therefore,  in  commutating  machines  it  is  seldom  that 
a  pitch  is  used  that  falls  short  of  full  pitch  by  more  than  one 
or  two  teeth,  while  in  induction  and  synchronous  machines 
occasionally  as  low  a  pitch  as  50  per  cent,  is  used,  and  two-thirds 
pitch  is  frequently  employed. 

For  special  purposes,  as  in  single-phase  commutator  motors 
fractional-pitch  windings  are  sometimes  used. 

41.  Series  windings  find  their  foremost  application  in  machines 
with  small  currents,  or  small  machines  in  which  it  is  desirable 
to  have  as  few  circuits  as  possible  in  multiple,  and  in  machines 
in  which  it  is  desirable  to  use  only  two  sets  of  brushes,  as  in 
smaller  railway  motors.     In  multipolar  machines  with  many  sets 
of  brushes  a  series  winding  is  liable  to  give  selective  commutation; 
that  is,  the  current  does  not  divide  evenly  between  the  sets  of 
brushes  of  equal  polarity. 

Multiple  windings  are  used  for  machines  of  large  currents,  thus 
generally  for  large  machines,  and  in  large  low-voltage  machines 
the  still  greater  subdivision  of  circuits  afforded  by  the  multiple- 
spiral  and  the  multiple-reentrant  winding  is  resorted  to. 

To  resume,  then,  armature  windings  can  be  subdivided  into 

(a)  Ring  and  drum  windings. 

(6)  Closed-circuit  and  open-circuit  windings.  Only  the  former 
can  be  used  for  commutating  machines. 

(c)  Multiple  and  series  windings. 

(d)  Single-spiral,  multiple-spiral,  and  multiple-reentrant  wind- 

ings.    Either  of  these  can  be  multiple  or  series  windings. 

(e)  Full-pitch  and  fractional-pitch  windings.