Aircraft Electrical Power Generation Systems

AC Generation

Large aircraft typically require High Voltage AC power (115/200V, 400Hz, 3-phase).

• Brushless AC Generators: Use a set of permanent magnets for initial excitation.
  • Exciter Generator: Mounted on the same shaft.
  • Permanent Magnet Generator (PMG): Specifically used for initial excitation current and powering the generator control.
• Voltage Regulation: The Voltage Regulator controls the output voltage (keeps it constant at varying loads/speeds) by adjusting the excitation current to the field coil.
  • Wired in series with the field coil.
  • If electrical load increases -> Voltage drops -> Regulator increases excitation current -> Voltage returns to normal.

Constant Speed Drive (CSD) & IDG

AC generators must spin at a constant speed to maintain a constant frequency (400Hz), despite the engine's varying RPM.

• CSD (Constant Speed Drive): A hydromechanical unit that converts variable engine speed to constant generator speed.
  • Monitors/Adjusts: Output Frequency.
  • Has its own independent oil system for lubrication and cooling.
  • Faults: High oil temperature, low oil pressure.
• IDG (Integrated Drive Generator): Combines the CSD and the Generator into one single unit.
  • Disconnect: In case of a fault (e.g., high oil temp, low pressure), the pilot operates the IDG disconnect switch.
  • Consequence: The drive shaft is mechanically disconnected. The generator is unusable for the rest of the flight.
  • Reset: Can ONLY be reconnected on the ground by maintenance with the engine stopped.

Frequency Wild

• Frequency Wild Generators: Generators where the frequency varies with engine speed (no CSD/IDG).
• Application: Only suitable for resistive loads (e.g., anti-icing mats/heaters) because inductance/reactance are frequency-dependent.

DC Generation & Conversion

• TRU (Transformer Rectifier Unit):
  • Combines a Transformer and a Rectifier.
  • Function: Converts AC (115V) to DC (28V).
  • Used to power DC buses and charge batteries from the main AC supply.
  • Often includes a smoothing/filter circuit to minimize output fluctuation.
• Inverter:
  • Function: Converts DC (28V) to AC (115V, 400Hz).
  • Static Inverter: Solid-state device (no moving parts) used for essential AC loads if main AC fails.
• Starter-Generator:
  • Combines starter motor and generator functions (common on turbine engines).
  • Acts as a starter motor to spool the engine up to self-sustaining speed.
  • Switches to become a DC generator once the engine is running.

Protection & Control

• GCU (Generator Control Unit):
  • Checks for faults (over-voltage, over-excitation).
  • Controls the Generator Circuit Breaker (GCB) and Exciter Control Relay.
• Split Bus System: If a generator fails, GCBs/BTBs (Bus Tie Breakers) reconfigure to ensure essential loads are powered.