The GCU is the main computer that controls alternator  functions. The BPCU is the computer that controls the  distribution of AC power to the power distribution buses  located throughout the aircraft. There is typically one GCU  used to monitor and control each AC alternator, and there  can be one or more BPCUs on the aircraft. BPCUs are  described later in this chapter; however, please note that the  BPCU works in conjunction with the GCUs to control AC  on modern aircraft.  A typical GCU ensures the AC alternator maintains a  constant voltage, typically between 115 to 120 volts. The  GCU ensures the maximum power output of the alternator  is never exceeded. The GCU provides fault detection and  circuit protection in the event of an alternator failure. The  GCU monitors AC frequency and ensures the output if the  alternator remains 400 Hz. The basic method of voltage  regulation is similar to that found in all alternator systems; the  output of the alternator is controlled by changing the strength  of a magnetic field. As shown in  Figure 9-82,  the GCU  controls the exciter field magnetism within the brushless  alternator to control alternator output voltage. The frequency  is controlled by the CDS hydraulic unit in conjunction with  signals monitored by the GCU.  failures and take protective measures to ensure the integrity  of the electrical system. The two most common units used to  control AC alternators are the bus power control unit (BPCU)  and the GCU. In this case, the term “generator” is used, and  not alternator, although the meaning is the same.  Figure 9-81.  Line replaceable units in an equipment rack.  Main AC alternator armature winding Exciter  electromagnet  field Pilot exciter armature 3 phase  bridge rectifier N S Main AC alternator field Exciter armature Exciter  permanent  magnet field GCU Figure 9-82.  Schematic GCU control of the exciter field magnetism. 9-46