FAA Aviation Mechanic General (AMG)Fundamentals of Electricity and ElectronicsHard
A 4-pole AC generator rotates at 12,000 RPM. Using the formula frequency (Hz) = (number of poles x RPM) / 120, what output frequency does this generator produce?
- A800 Hz
- B100 Hz
- C400 Hz
- D200 Hz
Show answer & explanationAnswer & explanation
Correct answer: C. 400 Hz
Applying the formula: f = (4 poles x 12,000 RPM) / 120 = 48,000 / 120 = 400 Hz, which matches the standard aircraft AC electrical system frequency.
Why the other options are wrong
- A. This is double the correct value, resulting from a math or formula error.
- B. This value results from an incorrect division or using the wrong number of poles.
- D. This is half the correct value, likely from a calculation error.
AC Generator Frequency Formula
The frequency of an AC generator's output depends on the number of magnetic poles and rotational speed: f = (P x RPM)/120.
- f = (Poles x RPM) / 120
- More poles or higher RPM increases frequency
- Aircraft AC systems standardize on 400 Hz
Memory trick: 'Poles times speed, divided by 120' gives the Hertz.