Quantcast CHAPTER7 ELECTRO-HYDRAULIC  GOVERNING  SYSTEM

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CHAPTER7 ELECTRO-HYDRAULIC  GOVERNING  SYSTEM (CLASS I, PRECISE SETS ONLY) 7-1.   General a.   The   electro-hydraulic   governing   system   is   a speed   (frequency)   sensing stem   used   to   maintain prime   mover   speed   constant   and   therefore   generator output   frequency,   during   periods   of   unchanging load  and  when  load  additions  or  deletions  occur. .   The   system   consists   of   a   control   unit,   load measuring   unit   and   hydraulic   throttle   actuating   unit. c.   The   control   unit   inputs   are   the   generator   output voltage,  and  a  dc  voltage  (0-9.  8  Vdc)  proportional  to the   generator   load,   supplied   by   the   load   measuring unit. The  generator  voltage  input  is  applied  to  a frequency   sensing   network   and   reference   voltage network,   The   differ   ential   output   of   these   two   net- works   determines   the   control   current   of   two   mag- netic   amplifiers   whose   outputs   drive   separate   coils of   the   hydraulic   actuators   pilot   valve.   The   actuator pilot  valve  positions  the  actuator  power  piston  which is  connected  to  the  input  arm  of  the  fuel  injection pump.   The   actuators   hydraulic   system   is   comprised of   a   reservoir,   engine   driven   pump   (300-320   psi,   2 gpm),  cooler  and  filter. d.   Any   deviation   of   engine   speed,   reflected   as   a change  in  frequency  at  the  input  of  the  governor,  pro- duces  a  change  in  the  magnitude  and  direction  of  mag- netic   amplifier   control   field   current.   This   change   in ccntrol   field   current   will   increase   the   strength   of   one coil  of  the  pilot  valve  while  decreasing  the  strength  of the   other, The   resultant   difference   repositions   the pilot  valve  in  turn  repositioning  the  power  piston  which changes  the  output  of  the  fuel  injection  pump,  chang- ing   engine   speed   and   consequently   restoring   generator frequency   to   its   nominal   value. e. The  load  measuring  units  (LMU)  input  to  the governor   control   provides   for   automatic   load   sharing when   two   or   more   sets   are   operated   in   parallel. Each  set  is  equipped  with  an  LMU. f.  If  the  load  added  to  the  system  is  not  equally divided,   the   LMU   inputs   to   their   respective   govern- ors   will   differ.   The   resulting   difference   acting through   additional   windings   of   each   sets   governor (which   are   all   connected   in   parallel)   will   reposition each  sets  actuator  power  piston  such  that  fuel  flow in   the   more   heavily   loaded   set   is   decreased   while that   of   the   lightly   loaded   set   is   increased.   Since   the power  input  of  each  prime  mover  has  been  readjusted, equal  division  of  true  power  (Kw)  occurs  with  no deviation  in  frequency  of  any  set. 7-2.   Malfunction. The  following  procedures  are  to  be  performed  in  the generator   set   unless   otherwise   specified. a.  If  the  FL  -  NL  or  NL  -  FL  transient  exceeds 1-1/2  percent  of  rated  speed  and/or  does  not  re  - establish   stable   engine   operating   conditions   within one   second,   realign   the   control   unit   in   accordance with   paragraph   7-8   . Also   follow   this   procedure if  the  engine  speed  hunts.  If  the  set  cannot  be stabilized,  check  sockets  A  and  B  of  plug  P-21 (refer   to   Operator   and   Organizational   Maintenance Manual)  for  24  Vdc  (A  is  positive). b.   If   the   engine   speed   increases   to   above   nominal operating  speed,  check  sockets  A  and  B  of  P-17  for 120  Vat.  If  it  is  missing,  troubleshoot  the  generator set  wiring.  (Refer  to  Operator  and  Organizational Maintenance   Manual).   If   it   is   present,   check   the resistance   of   the   frequency   adjusting   circuit   con- sisting  of  R4  (250  ohms)  rheostat  R1  frequency  adjust (500  ohms)  and  R5  (250  ohms)  fixed  resistor. Refer   to   Operator   and   Organizational   Maintenance Manual. Measure   the   total   circuit   resistance across   N   and   T   of   harness   plug   P17.   The   circuit resistance   should   be   1000   ohms   (5   percent   tolerance). After   testing   for   correct   total   resistance,   test   the operation   of   the   frequency   adjust   rheostat   by connecting  an  ohmmeter  across  pins  M  and  T  and  re- volving   the   frequency   adjust   rheostat   through its   entire   travel.   The   resistance   should   vary   from 750   ohms   to   250   ohms.   Repeat   this   procedure using  the  ohmmeter  across  pins  M  and  N  of  the harness   plug. If   the   problem   persists,   check   sockets A  and  B  of  plug  P-21  for  24  Vdc  (A  is  positive). c, If  the  engine  speed  remains  below  the  nominal operating   speed,   adjust   R1.   If   there   is   no   improve- ment,   check   the   resistance   of   the   frequency   adjust circuit.   Disconnect   P-17   and   check   the   resistance of   the   frequency   adjust   circuit   consisting of   R4   (250   ohms)   fixed   resistor,   R1 frequency   adjust   rheostat   (500   ohms)   and   R5 (250  ohms,  5  percent)  fixed  resistor.  Refer  to Operator   and   Organizational   Maintenance   Manual. Measure   the   total   circuit   resistance   across   N   and   T of  harness  plug  P17. The  circuit  resistance  should be   1000   ohms   (5   percent   tolerance).   After   testing for  correct  total  resistance,  test  the  operation  of  the frequency   adjust   potentiometer   by   connecting   an ohmmeter  across  pins  M  and  T  and  revolving  the frequency   adjust   potentiometer   through   its   entire travel.  The  resistance  should  vary  from  750  ohms to  250  ohms. Repeat   this   procedure   using   the ohmmeter  across  pins  M  and  N  of  the  harness  plug. Disconnect   actuator   electrical   connector   before making  this  measurement.  Push  actuator  piston  all the  way  down  and  adjust  engine  speed  with  manual throttle.   If   this   value   resistance   is   measured,   check sockets  A  and  B  at  plug  P-21  for  24  Vdc  (A  is  positive). d.  If  the  set  is  operating  at  a  constant  load  and voltage  and  during  an  eight-hour  period  the  change in   ambient   temperature   does   not   exceed   60°F,   the set  frequency  should  not  drift  beyond  1/2  of  one percent   of   rated   frequency.   The   above   requirement assumes   that   the   set   temperatures   were   stabilized 7-1


 


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