XII.  Conclusion 

103.  Of  the  types  of  machines,  converter,  inverted  converter, 
and  double-current  generator,  sundry  combinations  can  be  de- 
devised  with  each  other  and  with  synchronous  motors,  alternators, 
direct-current  motors  and  generators.     Thus,   for  instance,   a 
converter  can  be  used  to  supply  a  certain  amount  of  mechanical 
power   as   synchronous   motor.     In   this   case   the   alternating 
current  is  increased  beyond  the  value  corresponding  to  the  direct 
current  by  the  amount  of  current  giving  the  mechanical  power, 
and  the  armature  reactions  do  not  neutralize  each  other,  but 
the  reaction  of  the  alternating  current  exceeds  that  of  the  direct 
current  by  the  amount  corresponding  to  the  mechanical  load. 
In  the  same  way  the  current  heating  of  the  armature  is  in- 
creased.    An  inverted  converter  can  also  be  used  to  supply 
some  mechanical  power.     Either  arrangement,  however,  while 
quite  feasible,  has  the  disadvantage  of  interfering  with  auto- 
matic control  of  voltage  by  compounding. 

Double-current  generators  can  be  used  to  supply  more  power 
into  the  alternating  circuit  than  is  given  by  their  prime  mover, 
by  receiving  power  from  the  direct-current  side.  In  this  case  a 
part  of  the  alternating  power  is  generated  from  mechanical  power, 
and  the  other  converted  from  direct-current  power,  and  the 
machine  combines  the  features  of  an  alternator  with  those  of  an 
inverted  converter.  Conversely,  when  supplying  direct-current 
power  and  receiving  mechanical  power  from  the  prime  mover  and 
electric  power  from  the  alternating  system,  the  double-current 
generator  combines  the  features  of  a  direct-current  generator  and 
a  converter.  In  either  case  the  armature  reaction,  etc.,  are 
the  sum  of  those  corresponding  to  the  two  types  of  machines 
combined. 

104.  A  combination  of  the  converter  with  the  direct-current 
generator  is  represented  by  the  so-called  "motor  converter,"  which 
consists  of  the  concatenation  of  a  commutating  machine  with  an 
induction  machine. 

If  the  secondary  of  an  induction  machine  is  connected  to  a 
second  induction  or  synchronous  machine  on  the  same  shaft,  and 
of  the  same  number  of  poles,  the  combination  runs  at  half 
synchronous  speed,  and  the  first  induction  machine  as  frequency 
converter  supplies  half  of  its  power  as  electric  power  of  half 
frequency  to  the  second  machine,  and  changes  the  other  half 


262     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

as  motor  into  mechanical  power,  driving  the  second  machine  as 
generator.  (Or,  if  the  two  machines  have  different  number  of 
poles,  or  are  connected  to  run  at  different  speeds,  the  division  of 
power  is  at  a  different  but  constant  ratio) .  Using  thus  a  double- 
current  generator  as  second  machine,  it  receives  half  of  its 
power  mechanically,  by  the  induction  machine  as  motor,  and  the 
other  half  electrically,  by  the  induction  machine  as  frequency 
converter.  Such  a  machine,  then,  is  intermediate  between  a 
converter  and  a  direct-current  generator,  having  an  armature 
reaction  equal  to  half  that  of  a  direct-current  generator. 

Such  motor  converters  have  been  recommended  for  high-fre- 
quency systems,  as  their  commutating  component  is  of  half 
frequency,  and  thus  affords  a  better  commutator  design  than  a 
high-frequency  converter.  They  are  necessarily  much  larger 
than  standard  converters,  but  are  smaller  than  motor  generator 
sets,  as  half  the  power  is  converted  in  either  machine.  One 
advantage  of  this  type  of  machine  for  phase  control  is  that  it 
requires  no  additional  reactive  coils,  as  the  induction  machine 
affords  sufficient  reactance. 

The  use  of  the  converter  to  change  from  alternating  to  alter- 
nating of  a  different  phase,  as,  for  instance,  when  using  a  quarter- 
phase  converter  to  receive  power  by  one  pair  of  its  collector 
rings  from  a  single-phase  circuit  and  supplying  from  its  other 
pair  of  collector  rings  the  other  phase  of  a  quarter-phase  system, 
or  a  three-phase  converter  on  a  single-phase  system  supplying 
the  third  wire  of  a  three-phase  system  from  its  third  collector 
ring, .  is  outside  the  scope  of  this  treatise,  and  is,  moreover,  of 
very  little  importance^  since  induction  or  synchronous  motors 
are  superior  in  this  respect. 

APPENDIX