AC Generation and Distribution

36.1 Advantages of AC Power

• Large modern transport aircraft use AC as their primary power source due to several
• advantages:
•  AC generators (alternators) are simpler, more robust, and have a better power-to-weight ratio
• than DC generators.
•  Voltage can be easily stepped up or down using transformers.
•  AC is easily converted to DC where needed using Transformer Rectifier Units (TRUs).
•  Three-phase AC motors are simpler and more efficient than DC motors.

36.2 AC Terminology

•  Cycle: One complete series of alternating values.
•  Frequency: The number of cycles per second, measured in Hertz (Hz). The standard
• frequency for aircraft AC systems is 400 Hz.
•  Three-Phase AC: Most large aircraft use a three-phase system, where three separate AC
• utputs are generated 120° out of phase with each other. This provides a smoother, more
• continuous supply of power.

36.3 Constant Frequency Systems

• To maintain a constant 400 Hz frequency, the alternator must be driven at a constant speed,
• even as the engine RPM varies. This is achieved by a Constant Speed Drive Unit (CSDU).
•  CSDU: A complex hydromechanical gearbox that takes a variable speed input from the
• engine and produces a constant speed output to drive the alternator.
•  Integrated Drive Generator (IDG): A modern unit where the CSDU and the generator are
• combined into a single housing.
•  Paralleling Alternators: To connect multiple AC generators to a common bus bar, their
• voltage, frequency, and phase sequence must be synchronized. The system then automatically
• manages load sharing to ensure each generator takes an equal share of the Real Load (kW)
• and Reactive Load (kVAR).

36.4 AC System Components and Backup Power

•  Transformer Rectifier Unit (TRU): A device that uses a transformer to step down the 115V
• AC to a lower voltage and a rectifier to convert it to 28V DC for powering DC components.
•  Inverter: A device that converts DC power (from a battery) into AC power (typically 115V /
• 400 Hz). This is used to power essential AC instruments in the event of a total AC generator
• failure.
•  Auxiliary Power Unit (APU): The APU drives its own AC generator, which can power the
• entire aircraft electrical system on the ground and serve as a backup generator in flight.

•  Ram Air Turbine (RAT): A final emergency power source. The RAT is a small turbine that
• deploys into the airstream and spins a small generator or hydraulic pump to power only the
• most essential flight controls and instruments.

36.5 Bonding and Screening

•  Bonding: The process of electrically connecting all metal parts of the aircraft with flexible
• wire straps. This ensures that all parts are at the same electrical potential, preventing static
• electricity from building up and creating a spark (fire) hazard.
•  Screening: Enclosing electrical wires in a metal sheath to prevent the electromagnetic fields
• they create from causing interference with radio and navigation equipment.
•  Static Wicks: Small, pointed devices on the trailing edges of wings and control surfaces.
• They are designed to safely discharge static electricity that builds up on the airframe during
• flight back into the atmosphere, preventing it from interfering with communications.