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. BPC is 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, the GCU controls the exciter field
magnetism within the brushless alternator to control the alternator output voltage.
The frequency is controlled by the CDS hydraulic unit in conjunction with signals
monitored by the GCU.
