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Alternator Output Control

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Figure 2-28.—Typical diode. Since  the  engine  speed  varies  in  a  vehicle,  the frequency   also   varies   with   the   change   of   speed. Likewise, increasing the number of pairs of magnetic north  and  south  poles  will  increase  the  frequency  by the  number  pair  of  poles.  A  four-pole  generator  can generate twice the frequency per revolution of a two- pole rotor. Alternator Output Control A  voltage  regulator  controls  alternator  output  by changing the amount of current flow through the rotor windings.  Any  change  in  rotor  winding  current changes the strength of the magnetic field acting on the stator windings. In this way, the voltage regulator can maintain  a  preset  charging  voltage.  The  three  basic types of voltage regulators are as follows: Contact point voltage regulator, mounted away from the alternator in the engine compartment Electronic   voltage   regulator,   mounted   away from the alternator in the engine compartment Electronic  voltage  regulator,  mounted  on  the back or inside the alternator The contact point voltage regulator uses a coil, set of points, and resistors that limits system voltage. The electronic  or  solid-state  regulators  have  replaced  this older  type.  For  operation,  refer  to  the  "Regulation  of Generator  Output"  section  of  this  chapter. The electronic voltage regulators use an electronic circuit  to  control  rotor  field  strength  and  alternator output.  It  is  a  sealed  unit  and  is  not  repairable.  The electronic  circuit  must  be  sealed  to  prevent  damage from  moisture,  excessive  heat,  and  vibration.  A rubberlike gel surrounds the circuit for protection. An integral voltage regulator is mounted inside or on the rear of the alternator. This is the most common type  used  on  modern  vehicles.  It  is  small,  efficient, dependable,  and  composed  of  integrated  circuits. An electronic voltage regulator performs the same operation  as  a  contact  point  regulator,  except  that  it uses  transistors,  diodes,  resistors,  and  capacitors  to regulate  voltage  in  the  system.  To  increase  alternator output,  the  electronic  voltage  regulator  allows  more current into the rotor windings, thereby strengthen the magnetic  field  around  the  rotor.  More  current  is  then induced   into   the   stator   windings   and   out   of   the alternator. To  reduce  alternator  output,  the  electronic regulator  increases  the  resistance  between  the  battery and  the  rotor  windings.  The  magnetic  field  decreases and less current is induced into the stator windings. Alternator speed and load determines whether the regulator increases or decreases charging output. If the load is high or rotor speed is low (engine at idle), the regulator  senses  a  drop  in  system  voltage.  The regulator  then  increases  the  rotors  magnetic  field current until a preset output voltage is obtained. If the load  drops  or  rotor  speed  increases,  the  opposite occurs. Alternator  Maintenance Alternator  testing  and  service  call  for  special precautions   since   the   alternator   output   terminal   is connected to the battery at all times. Use care to avoid reversing  polarity  when  performing  battery  service  of any kind. A surge of current in the opposite direction could bum the alternator diodes. Do  not  purposely  or  accidentally  "short"  or "ground"   the   system   when   disconnecting   wires   or connecting  test  leads  to  terminals  of  the  alternator  or regulator. For example, grounding of the field terminal at   either   alternator   or   regulator   will   damage   the regulator. Grounding of the alternator output terminal will damage the alternator and possibly other portions of the charging system. 2-20



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