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Alternator output frequency from poles and rpm

Aviation Mechanic General (AMG) practice question 28 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K5 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONWhat is the output frequency of a 4-pole AC generator whose armature turns at 1,800 rpm?

  1. A30 cycles per second
  2. B120 cycles per second, one per pole per rev
  3. C60 cycles per second
Show the answer and explanation

Correct answer: C. 60 cycles per second

  • A — incorrect 30 is only the revolutions per second; it must be multiplied by the number of pole pairs, which is 2 for a 4-pole machine.
  • B — incorrect One cycle is produced for each pair of poles, not each pole, so a 4-pole generator produces 2 cycles per revolution, not 4.
  • C — correct Frequency = (P/2) x (rpm/60) = (4/2) x (1,800/60) = 2 x 30 = 60 cycles per second.

BACKGROUND

In a generator, the voltage passes through one complete cycle each time a conductor passes under a North and a South pole. The number of cycles per revolution therefore equals the number of pairs of poles. Frequency equals cycles per revolution multiplied by revolutions per second, which the handbook writes as F = (P/2) x (rpm/60). A 2-pole generator at 3,600 rpm and a 4-pole generator at 1,800 rpm both produce 60 cycles per second. The standard unit of frequency is the hertz (Hz).

STUDY TIP

Divide the pole count by 2 to get pole pairs and divide rpm by 60 to get revolutions per second, then multiply.

KEY POINTS

  • One cycle per revolution for each pair of poles.
  • F = (P/2) x (rpm/60).
  • 4 poles at 1,800 rpm gives 60 cycles per second.

SOURCES

  • FAA-H-8083-30B ch12 "Frequency Defined" (4-pole, 1,800 rpm example)
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.