regulator  (fig.  12-6)  controls  the  voltage  output of the generator portion of the MG. This voltage may be either 450 volts or 120 volts, depending on  the  application.   The  frequency  regulator controls  the  speed  of  the  MG  motor,  and  therefore the  output  frequency  of  the  MG.  Voltage-  and frequency-sensing  circuits  continuously  sample  the output of the MG and provide feedback to their respective regulators. By doing this, they maintain the output voltage and frequency at the required level  (usually  to  plus  or  minus  one-half  percent of   the   rated   value).   A   magnetic   controller   is provided  to  start  and  stop  the  unit.  It  also protects  the  MG  from  operating  at  continuous overload and removes power to the MG if there is  an  undervoltage  condition. STATIC  FREQUENCY  CHANGERS Frequency  changers  step  up  and  refine  the frequency of the ship’s 60-Hz electrical power to 400 Hz. Most of the frequency changers installed on  board  combat  ships  are  static  frequency changers.   Static   frequency   changers   have   no rotating  parts—they  are  all  solid  state.  Static frequency  changers  are  reliable  and  efficient;  they are  the  only  ones  that  provide  the  high-quality power  demanded  by  modern  weapon  systems. A static frequency changer usually consists of a  three-phase  rectifier  and  a  three-phase  inverter. The   rectifier   changes   the   60-Hz   ac   incoming power.    The   inverter   converts   the   dc   power delivered by the rectifier into 400-Hz output power through   the   use   of   many   input   filters   and transformers. SHIPBOARD  POWER DISTRIBUTION Most ac power distribution systems in naval ships  are  450-volt,  three-phase,  60-Hz,  three-wire systems.  The  ship’s  service  generator  and distribution  switchboards  are  interconnected  by bus ties. This arrangement makes it possible to connect  any  switchboard  to  feed  power  from  its generators  to  one  or  more  of  the  other  switch- boards.  The  bus  ties  also  connect  two  or  more switchboards so that the generator plants can be operated  in  parallel.  In  large  installations  (fig. 12-7), distribution to loads is from the generator and  distribution  switchboards  or  switchgear groups  to  load  centers,  distribution  panels,  and the  loads,  or  directly  from  the  load  centers  to some  loads. On some ships, such as large aircraft carriers, zone control of the ship’s service and emergency distribution  is  provided.  A  load  center  switch- board  supplies  power  to  the  electrical  loads  within the  electrical  zone  in  which  it  is  located.  Thus, zone control is provided for all power within the electrical   zone.   Emergency   switchboards   may supply  more  than  one  zone. GENERATOR   AND DISTRIBUTION  SWITCHBOARDS Ship’s  service  450-volt,  ac  switchboards  are generally  of  the  dead-front  type  (no  live  con- nections  exposed).  These  switchboards  are  built to  provide  efficient  and  safe  operation  of  the electrical  system.  A  typical  power  distribution system in a destroyer consists of four generators (two  forward  and  two  aft)  and  two  distribution switchboard.  The  distribution  switchboards  are set up so that each one controls two generators. All the necessary apparatus for generator control and   power   distribution   is   incorporated   in   its associated  switchboard  (fig.  12-8). The  ship’s  forward  distribution  switchboard is  also  used  as  the  control  switchboard.  This switchboard has instruments and controls for the aft generators. These instruments and controls are necessary to parallel the generators to equalize the load. An automatic voltage regulator is mounted on each switchboard to control the generator field excitation and to maintain a constant ac generator voltage  during  normal  changes  in  load. Two emergency diesel generator sets provide electric  power  for  limited  lighting  and  for vital   auxiliaries   if   the   ship’s   service   power should  fail.  These  units  are  located  in  the  forward and aft emergency generator rooms. The forward emergency   switchboard   is   normally   energized from the forward ship’s service switchboard. The aft emergency switchboard is normally energized from  the  aft  ship’s  service  switchboard. Dc power distribution systems are in use on some older ships that have large deck machinery loads. These systems, which consist of the ship’s service generator and distribution switchboards, are similar to the ac systems. On newer ships, dc power is provided at the load with rectifiers that change the ac power to dc power, when required. COMPONENTS  OF  A  SWITCHBOARD Each switchboard includes one or more units, such as a bus tie unit, a power distribution unit, lighting  distribution  units  or  transformers,  and 12-7