FAA Aviation Mechanic General (AMG)Fundamentals of Electricity and ElectronicsMedium
A technician is testing a three-phase AC generator. If the generator has 4 poles and is rotating at 1,800 revolutions per minute (RPM), what is the frequency of the AC output in Hertz?
- A400 Hz
- B60 Hz
- C30 Hz
- D120 Hz
Show answer & explanationAnswer & explanation
Correct answer: B. 60 Hz
The frequency (f) of an AC generator is calculated by the formula f = (P * N) / 120, where P is the number of poles and N is the RPM. So, f = (4 * 1800) / 120 = 7200 / 120 = 60 Hz.
Why the other options are wrong
- A. 400 Hz is common in aircraft, but not the result of this specific calculation.
- C. Incorrect calculation; perhaps divided by 240 instead of 120.
- D. Incorrect calculation; possibly multiplied RPM by poles but not divided correctly.
AC Generator Frequency
The frequency of the alternating current produced by a generator is determined by the number of magnetic poles and the speed of rotation, calculated as f = (P × N) / 120.
- f = frequency in Hertz (Hz)
- P = number of poles
- N = speed in revolutions per minute (RPM)
Memory trick: Poles and RPM, divide by 120, that's the Hz we've known.