operating position; the points are closed and current is  flowing from the generator to the aircraft electrical loads.  As current flows to the loads, the current coil is energized  and the points remain closed. If there is no generator output  due to a system failure, the contact points open because  magnetism in the relay is lost. With the contact points open,  the generator is automatically disconnected from the aircraft  electrical system, which prevents reverse flow from the load  bus to the generator. A typical three-unit regulator for aircraft  generators is shown in  Figure 9-63 .  As seen in  Figure 9-63 , all three units of the regulator work  together to control generator output. The regulator monitors  generator output and controls power to the aircraft loads as  needed for flight variables. Note that the vibrating regulator  just described was simplified for explanation purposes.  A typical vibrating regulator found on an aircraft would  probably be more complex. DC alternators (like generators) change mechanical energy  into electrical energy by the process of electromagnetic  induction. In general, DC alternators are lighter and more  efficient than DC generators. DC alternators and their related  controls are found on modern, light, piston-engine aircraft.  The alternator is mounted in the engine compartment driven  by a v-belt, or drive gear mechanism, which receives power  from the aircraft engine.  [Figure 9-64]  The control system  of a DC alternator is used to automatically regulate alternator  output power and ensure the correct system voltage for  various flight parameters. The voltage coil is wired in parallel to the generator output  and is energized any time the generator output reaches its  operational voltage. As the voltage coil is energized, the  contact points close and the current is then allowed to flow  to the aircraft electrical loads, as shown by the dotted lines.  The diagram shows the reverse current relay in its normal  The third unit of a three-unit regulator is used to prevent  current from leaving the battery and feeding the generator.  This type of current flow would discharge the battery and is  opposite of normal operation. It can be thought of as a reverse  current situation and is known as reverse-current relay. The  simple reverse-current relay shown in  Figure 9-62  contains  both a voltage coil and a current coil. DC Alternators and Controls Reverse-Current Relay Figure 9-62.  Reverse-current relay. Figure 9-63.  Three-unit regulator for variable speed generators. 9-38 DC generator Three-unit regulator Current limiter Voltage regulator + − Armatu re A Reverse current relay DC generator Reverse-current relay Armatu re Voltage regulator  and current  limiter To electrical loads Current coil Voltage coil Contact points (N.O.)