Functions of Generator Control Systems Voltage Regulation Overvoltage Protection Parallel Generator Operations Generator Controls for High Output Generators Overexcitation Protection Differential Voltage Reverse Current Sensing Most generator control systems perform a number of  functions related to the regulation, sensing, and protection  of the DC generation system. Light aircraft typically  require a less complex generator control system than larger  multiengine aircraft. Some of the functions listed below are  not found on light aircraft.  The most basic of the GCU functions is that of voltage  regulation. Regulation of any kind requires the regulation  unit to take a sample of a generator output and compare  that sample to a known reference. If the generator’s output  voltage falls outside of the set limits, then the regulation unit  must provide an adjustment to the generator field current.  Adjusting field current controls generator output.  The overvoltage protection system compares the sampled  voltage to a reference voltage. The overvoltage protection  circuit is used to open the relay that controls the field  excitation current. It is typically found on more complex  generator control systems. On multiengine aircraft, a paralleling feature must be  employed to ensure all generators operate within limits. In  general, paralleling systems compare the voltages between  two or more generators and adjust the voltage regulation  circuit accordingly.  When one generator in a paralleled system fails, one of the  generators can become overexcited and tends to carry more  than its share of the load, if not all of the loads. Basically,  this condition causes the generator to produce too much  current. If this condition is sensed, the overexcited generator  must be brought back within limits, or damage occurs. The  overexcitation circuit often works in conjunction with the  overvoltage circuit to control the generator.  This function of a control system is designed to ensure all  generator voltage values are within a close tolerance before  being connected to the load bus. If the output is not within  the specified tolerance, then the generator contactor is not  allowed to connect the generator to the load bus.  If the generator cannot maintain the required voltage level,  it eventually begins to draw current instead of providing it.  This situation occurs, for example, if a generator fails. When  a generator fails, it becomes a load to the other operating  generators or the battery. The defective generator must be  removed from the bus. The reverse current sensing function  monitors the system for a reverse current. Reverse current  indicates that current is flowing to the generator not from  the generator. If this occurs, the system opens the generator  relay and disconnects the generator from the bus.  Most modern high output generators are found on turbine- powered corporate-type aircraft. These small business  jets and turboprop aircraft employ a generator and starter  combined into one unit. This unit is referred to as a starter- generator. A starter-generator has the advantage of combining  two units into one housing, saving space and weight. Since  the starter-generator performs two tasks, engine starting and  generation of electrical power, the control system for this unit  is relatively complex.  A simple explanation of a starter-generator shows that the  unit contains two sets of field windings. One field is used to  start the engine and one used for the generation of electrical  power.  [Figure 9-55] During the start function, the GCU must energize the series  field and the armature causes the  unit to act like a motor.  During the generating mode, the  GCU must disconnect  the series field, energize the  parallel field, and control the  current produced by the armature.  At this time, the starter- generator acts like a typical  generator. Of course, the GCU  must perform all the functions  described earlier to control  voltage and protect the system. These functions include  voltage regulation, reverse current sensing, differential  voltage, overexcitation protection, overvoltage protection,  and parallel generator operations. A typical GCU is shown  in  Figure 9-56 . Figure 9-55.  Starter-generator. LOAD Armature Series field Parallel  (shunt) field 9-35