D.  C.  COMMUTATING  MACHINES  221 

economy  of  high  voltage  alternating-current  transmission  and 
distribution.  For  railroading  generally  the  series  motor  type  is 
used,  either  the  plain  compensated  series  motor,  or  inductive 
modifications  thereof,  as  the  repulsion  motor  etc.  In  the  repul- 
sion motor  the  armature,  instead  of  being  connected  in  series 
with  field  and  compensating  winding,  is  closed  on  itself  and  thus 
traversed  by  a  secondary  current  induced  by  the  compensating 
winding  as  primary  that  is,  the  armature  is  connected  inductively 
in  series. 

2.  As    constant-speed    motor    where     considerable    starting 
torque  is  required,  as  for  elevators,  hoists,  etc.,  and  in  general 
as  self-starting  single-phase  motors.     For  this  purpose,  com- 
binations of  repulsion  and  induction  type  or  of  series  and  in- 
duction type  are  used. 

3.  As  adjustable  speed,  alternating-current  motor  of  single- 
phase  and  of  polyphase  type.     The  synchronous  motor  and 
the  induction  motor  both  are  constant  and  fixed  speed,  the 
former  synchronous,  the  latter  near  synchronous.     Operating 
the  induction  motor  materially  below  synchronism,  by  arma- 
ture resistance,  is  inefficient  and  gives  a  speed  which  varies  with 
the  load.     By  changing  the  number  of  poles,  or  by  concatena- 
tion,   multi-speed  induction    motors    can    be    produced.     The 
gradual  speed  adjustment,  as  given  by  field  control  of  direct- 
current  motors,   requires,  however,  a  commutator  on  the  al- 
ternating-current motor.     If  into  the  secondary  of  the  induction 
motor  an  e.m.f.  is  introduced,  the  speed  of  the  motor  can  be 
varied  by  varying  the  introduced  e.m.f.;  and  lowered,  if  this 
e.m.f.  is  in  opposition;  raised  beyond  synchronism,  if  this  e.m.f. 
is  in  the  same  direction  as  the  e.m.f.  induced  in  the  motor 
secondary.     As,  however,  the   e.m.f.  induced  in  the  induction 
motor  secondary  is  of  the  frequency  of  slip,  the  speed  controlling 
e.m.f.  must  either  be  supplied  through  the  commutator  or  de- 
rived from  a  low  frequency  commutating  machine  as  source. 

4.  For  power-factor  compensation.     In  an  inductive  circuit, 
the  current  lags  behind  the  voltage  or,  what  is  the  same,  the 
voltage  leads  the  current,  and   the  power-factor  thus  can  be 
raised  by  compensation  either  by  introducing  a  leading  current, 
as  from  condenser  or  overexcited  synchronous  motor,  or  by  in- 
troducing a  lagging  voltage.     In  the  commutating  machines, 
the  voltage  induced  in  the  armature  by  its  rotation  is  in  phase 
with  the  field  magnetism,  and  by  lagging  the  field  exciting  current, 


222     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

the  commutating  machines  thus  can  be  made  to  give  a  lagging 
voltage,  that  is,  to  compensate  for  low  power-factor  due  to  lagging 
current.  Thus,  by  inserting  such  a  commutating  machine  into 
the  secondary  of  an  induction  machine,  the  latter  can  be  made 
to  give  unity  power-factor  or  even  leading  current. 

Such  phase  compensation  is  frequently  used  in  alternating- 
current  commutator  motors  to  get  good  power-factor.  Thus  in 
the  series  motor,  by  shunting  the  field  by  a  non-inductive  re- 
sistance, and  thereby  lagging  the  field  exciting  component  of 
the  current  and  with  it  the  field  flux  and  the  voltage  induced 
in  the  armature  by  its  rotation,  behind  the  main  current,  the 
series  motor  can  at  higher  speeds  be  made  to  give  unity  power- 
factor.  At  low  speeds,  such  complete  compensation  is  not 
possible,  as  the  compensating  voltage  is  proportional  to  the 
speed. 


C.  SYNCHRONOUS  CONVERTERS