The three-unit regulator used with DC generator systems is  made of three distinct units. Each of these units performs  a specific function vital to correct electrical system  operation. A typical three-unit regulator consists of three  relays mounted in a single housing. Each of the three relays  monitors generator outputs and opens or closes the relay  contact points according to system needs. A typical three- unit regulator is shown in  Figure 9-59 . The voltage regulator section of the three-unit regulator  is used to control generator output voltage. The voltage  regulator monitors generator output and controls the  generator field current as needed. If the regulator senses  that system voltage is too high, the relay points open and the  current in the field circuit must travel through a resistor. This  resistor lowers field current and therefore lowers generator  output. Remember, generator output goes down whenever  generator field current goes down.  As seen in  Figure 9-60 , the voltage coil is connected in parallel  with the generator output, and it therefore measures the  voltage of the system. If voltage gets beyond a predetermined  limit, the voltage coil becomes a strong magnet and opens the  contact points. If the contact points are open, field current  must travel through a resistor and therefore field current goes  down. The dotted arrow shows the current flow through the  voltage regulator when the relay points are open. Since this voltage regulator has only two positions (points  open and points closed), the unit must constantly be in  adjustment to maintain accurate voltage control. During  normal system operation, the points are opening and closing  at regular intervals. The points are in effect vibrating. This  type of regulator is sometimes referred to as a vibrating- type regulator. As the points vibrate, the field current raises  and lowers and the field magnetism averages to a level that maintains the correct generator output voltage. If the system  requires more generator output, the points remain closed  longer and vice versa.  The current limiter section of the three-unit regulator is  designed to limit generator output current. This unit contains  a relay with a coil wired in series with respect to the generator  output. As seen in  Figure 9-61,  all the generator output  current must travel through the current coil of the relay. This  creates a relay that is sensitive to the current output of the  generator. That is, if generator output current increases, the  relay points open and vice versa. The dotted line shows the  current flow to the generator field when the current limiter  points are open. It should be noted that, unlike the voltage  regulator relay, the current limiter is typically closed during  normal flight. Only during extreme current loads must the  current limiter points open; at that time, field current is  lowered and generator output is kept within limits.  Current Limiter Voltage Regulator Three-Unit Regulators  Figure 9-60.  Voltage regulator.  DC generator Voltage regulator Armatu re Resistance To electrical loads Contact points Voltage coil Field current Field circuit DC generator Current limiter Armatu re To electrical loads Contact points (NC) Current coil Field circuit Figure 9-61.  Current limiter. Figure 9-59.  The three relays found on this regulator are used to  regulate voltage, limit current, and prevent reverse current flow. 9-37