Figure 9-79.  A hydraulic constant speed drive for an AC alternator.  Modern aircraft that employ AC alternators use several  computerized control units, typically located in the aircraft’s  equipment bay for the regulation of AC power throughout the  aircraft. Figure 9-81  shows a photo of a typical equipment  bay and computerized control units. Since AC alternators are found on large transport category  aircraft designed to carry hundreds of passengers, their  control systems always have redundant computers that  provide safety in the event of a system failure. Unlike DC  systems, AC systems must ensure that the output frequency  of the alternator stays within limits. If the frequency of an  alternator varies from 400 Hz, or if two or more alternators  connected to the same bus are out of phase, damage occurs to  the system. All AC alternator control units contain circuitry  that regulates both voltage and frequency. These control  units also monitor a variety of factors to detect any system AC Alternators Control Systems  Figure 9-80.  Speed control circuit. Constant speed drive Tach generator AC alternator To load Speed adjustment Tach generator rectifier Control circuit Hydraulic valve assembly 9-45