FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsHard
An AC alternator on an aircraft has 4 poles and is driven at a constant speed of 1,800 RPM. What output frequency does the alternator produce?
- A60 Hz
- B120 Hz
- C30 Hz
- D400 Hz
Show answer & explanationAnswer & explanation
Correct answer: A. 60 Hz
AC generator frequency is calculated with the formula f = (P × N) / 120, where P is the number of poles and N is the RPM. Substituting the values gives f = (4 × 1,800) / 120 = 7,200 / 120 = 60 Hz.
Why the other options are wrong
- B. This doubles the correct answer, likely from a formula error.
- C. This would result from an incorrect division, not the correct formula application.
- D. 400 Hz is the typical aircraft AC frequency but requires a much higher RPM or more poles, not this combination.
AC Generator Frequency Formula
The output frequency of an AC generator is determined by the number of poles and rotational speed using f = (P × N)/120.
- P = number of poles, N = RPM
- Standard aircraft AC frequency is typically 400 Hz
- Higher pole count or higher RPM increases frequency
Memory trick: Poles times speed, divided by one-twenty, gives frequency plenty