I.  General 

35.  Commutating  machines  are  characterized  by  the  combina- 
tion of  a  continuously  excited  magnet  field  with  a  closed-circuit 
armature  connected  to  a  segmental  commutator.  According  to 
their  use,  they  can  be  divided  into  direct-current  generators 
which  transform  mechanical  power  into  electric  power,  direct- 
current  motors  which  transform  electric  power  into  mechanical 

power,  and  direct-current  con- 
verters which  transform  electric 
power  into  a  different  form  of 
electric  power.  Since  the  most 
important  class  of  the  latter 
are  the  synchronous  converters, 
which  combine  features  of  the 
synchronous  machines  with  those 
of  the  commutating  machines, 
they  shall  be  treated  in  a  sepa- 
rate chapter. 

By  the  excitation  of  their  mag- 
net fields,  commutating  machines 

are  divided  into  magneto  machines,  in  which  the  field  consists  of 
permanent  magnets;  separately  excited  machines;  shunt  machines, 
in  which  the  field  is  excited  by  an  electric  circuit  shunted  across 
the  machine  terminals,  and  thus  receives  a  small  branch  current 
at  full  machine  voltage,  as  shown  diagrammatically  in  Fig.  77; 
series  machines,  in  which  the  electric  field  circuit  is  connected  in 
series  with  the  armature,  and  thus  receives  the  full  machine  cur- 
rent at  low  voltage  (Fig.  78) ;  and  compound  machines,  excited  by 
a  combination  of  shunt  and  series  field  (Fig.  79).  .  In  compound 
machines  the  two  windings  can  magnetize  either  in  the  same  direc- 
tion (cumulative  compounding)  or  in  opposite  directions  (dif- 
ferential compounding).  Differential  compounding  has  been 
used  for  constant-speed  motors.  Magneto  machines  are  used 
only  for  very  small  sizes. 

166 


FIG.  77. — Shunt  machine.