Chapter 36: 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.