FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsHard
An engine-driven AC generator has 4 poles and is coupled through a constant-speed drive (CSD) unit that maintains a fixed generator shaft speed regardless of engine RPM fluctuations, in order to produce a steady 400 Hz output. Using the formula frequency = (poles × RPM) / 120, what generator shaft speed must the CSD maintain?
- A6,000 RPM
- B12,000 RPM
- C24,000 RPM
- D9,600 RPM
Show answer & explanationAnswer & explanation
Correct answer: B. 12,000 RPM
Rearranging the formula to solve for RPM gives RPM = (120 × frequency) / poles = (120 × 400) / 4 = 48,000 / 4 = 12,000 RPM. The CSD must hold the generator at this constant shaft speed to maintain stable 400 Hz output.
Why the other options are wrong
- A. This value results from an incorrect rearrangement of the frequency formula.
- C. This doubles the correct RPM value, an arithmetic error.
- D. This is not consistent with the correct calculation using 4 poles and 400 Hz.
Constant-Speed Drive (CSD)
A mechanical device between an engine and AC generator that maintains a constant generator shaft speed despite engine RPM variations, ensuring stable output frequency.
- Frequency formula: f = (P × RPM) / 120
- Keeps generator speed constant for stable 400 Hz output
- Failure of the CSD causes frequency to vary with engine RPM
Memory trick: CSD = 'Constant Spin, Dependable frequency' no matter the engine's mood.