When  it  is  inconvenient  or  impossible  to  drive  the  machine
as  a  generator,  a  "motor  saturation"  may  be  made.  In
this  case,  the  machine  is  operated  as  a  free  running  motor.  The
driving  power  must  be  furnished  from  a  variable  voltage  circuit.
A  certain  voltage  is  impressed  upon  the  armature  and  the
motor  field  weakened  or  increased  in  the  case  of  direct  current
machines  to  give  normal  speed,  and  a  record  made  of  the  volts
armature,  amperes  armature,  amperes  field,  and  speed.  The
starting  voltage  should  be  at  least  50  per  cent,  lower  than  the
normal  voltage  of  the  apparatus.  The  applied  voltage  at
the  armature  should  be  increased  step  by  step  to  a  point  25  per
cent,  above  normal  value,  and  the  field  increased  corre-
spondingly to  keep  the  speed  constant,  the  same  readings  being
recorded  at  the  various  steps  as  before.  Readings  should  also
be  taken  at  three  or  four  points,  as  the  impressed  voltage
and  field  current  are  lowered  to  approximately  the  value  at  the
beginning  of  the  test.
Care  should  be  taken  when  testing  direct  current  apparatus,
as  unstable  electrical  conditions  may  develop,  and  excessive

15


226


D.  C.  MOTOR  AND  GENERATOR  TROUBLES


Voli-meter


speeds  result.  The  circuit  breaker  in  the  armature  circuit
of  the  motor  driving  the  machine  must,  therefore,  be  accessible
to  the  tester  reading  the  speed.
