ELECTRICAL/MECHANICAL CHARACTERISTICS TESTING AND ADJUSTING phase  rotating  armature  is  mounted  on  an  extension  of the rotor shaft.  Six rotating diodes mounted on the rear fan   convert   this   three-phase   AC   current   to   DC   for excitation of the main revolving field. Voltage Regulation The static voltage regulator is compact.  It has only one moving   part   -   a   hermetically   sealed   voltage   build-up relay which operates once each time the generator starts up and shuts down.  The relay is integral with the solid- state epoxy encapsulated module. The   regulating   system   continually   monitors   generator output   voltage   (as   well   as   line   current   and   operating power    factor)    and    corrects    differences    between    the preset    voltage    level    and    the    actual    voltage    level. Differences are sensed by the  regulator  which  raises  or lowers  the  field  excitation  of  the  brushless  exciter.    In turn,   the   brushless   exciter   raises   or   lowers   generator field   excitation   to   maintain   the   desired   preset   voltage level.    This  regulating  and  control  system  functions  at low   voltage   and   current   levels   within   the   exciter   field circuit.    Direct  current  output  from  the  brushless  exciter is    continuous.        The    resulting    voltage    and    current variations  in  the  generator  field  are  smooth.    The  result is voltage regulation within ± 2% from no load to full load with     3%     speed     droop.          At     isochronous     speeds, regulation  is  ±  1%  from  no  load  to  full  load.    At  steady state   conditions   (no   change   in   load   on   generator), voltage   level   is   maintained   within   ±   0.2%   of   preset voltage level. All SR 4 Generators use "volts-per-hertz" type regulation, which provides the best possible response by matching   generator   output   to   engine   capability.      This type  of  regulation  prevents  engine  stalling  under  heavy load application (See engine data sheet 71.3).  Over the normal  load  range,  the  voltage  gain  control  maintains  a nearly   constant   voltage   even   if   the   governed   engine speed droops as much as 5% on an 0.8 P.F. load. Radio Noise Suppression Although   the   voltage   regulating   and   exciting   system generate  a  very  minimum  of  radio  frequency  noise,  the SR   4   Generator   does   include   noise   suppression   as standard equipment.  Radio frequency noise level of SR 4   Generators   is   well   within   commercially   acceptable limits. Telephone Influence Factor (TIF) Caterpillar  SR  4  Generator  performance  falls  well  within accepted standards for minimum telephone interference. A  factor  of  250  to  350  is  commercially  acceptable,  in accordance with NEMA.  All SR 4 Generators have a TIF of less than 50. Thermal Protection A thermal protective circuit breaker is used in the exciter field  circuit  in  addition  to  a  protective  fuse.    These  two devices  provide  protection  against  overheating  due  to prolonged   operation   at   low   power   factor   loads.      The circuit   breaker   is   resettable,   and   the   fuse   must   be replaced in event it opens. These protection devices do not replace the normal load line circuit breaker or line fuses and disconnect switch. Manual Adjustments There are three controls on the exciter panel: Voltage Level  -  provides  a  minimum  adjustment of  generator  output  voltage  5%  above  or  below the standard nameplate voltage on all generators,   except   the   50   Hertz   240-480   volt generators    which    have    an    adjustment    10% above and 5% below nameplate voltage. Voltage Gain - an adjustment to compensate for voltage   drop   caused   by   generator   load   and governor  speed  droop.    It  is  used  to  optimize voltage  regulation  with  either  an  isochronous  or speed droop governor. Voltage     Droop     -     Standard     on     all     SR     4 Generators.    This  adjustment  provides  for  equal division   of   reactive   KVA   when   two   or   more generators  are  operated  in  parallel.    It  is  set  at zero for single unit operation. SERVICE INFORMATION Complete    service    information    on    SR    4    Generators, including  detail  of  automatic  start-stop  systems  in  the Package Generator Sets, is found in Service Department Form   No.      REG01106-02,   "SR   4   Generator   Service Manual." 82