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AC alternator output phases

Aviation Mechanic Airframe (AMA) practice question 1 of 24 on Aircraft Electrical Systems. ACS element AM.II.K.K1 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONWhat type of output does an aircraft AC alternator produce?

  1. AThree-phase AC with phases 120 degrees apart
  2. BSingle-phase AC that is rectified to DC internally
  3. CTwo-phase AC with the phases 90 degrees apart
Show the answer and explanation

Correct answer: A. Three-phase AC with phases 120 degrees apart

  • A — correct For each revolution the AC alternator produces three separate voltages whose sine waves are separated by 120 degrees.
  • B — incorrect Rectifying the output internally describes a DC alternator; an AC alternator does not rectify its output, and it is not single-phase.
  • C — incorrect The output is three-phase, and the separation between the sine waves is 120 degrees, not 90.

BACKGROUND

AC alternators are found on aircraft that use large amounts of electrical power, such as transport category airplanes, where one alternator is normally driven by each engine. For each revolution of the alternator the unit produces three separate voltages, and the sine waves of those voltages are separated by 120 degrees. This wave pattern is similar to the one a DC alternator produces internally, except that the DC alternator rectifies the voltage with diodes while the AC alternator delivers its output as AC. Modern AC alternators are brushless, and energy is moved between stator and rotor by magnetic flux and electromagnetic induction. That brushless design is extremely reliable and needs very little maintenance.

STUDY TIP

Tie the number 120 degrees to three-phase output, and keep in mind that the difference between a DC alternator and an AC alternator is rectification, not the wave pattern produced inside.

KEY POINTS

  • An AC alternator produces three separate voltages per revolution.
  • The three sine waves are separated by 120 degrees.
  • A DC alternator produces a similar pattern but rectifies it to DC.

SOURCES

  • sources/ama-airframe/ch09.txt "AC Alternators"
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MORE ON AIRCRAFT ELECTRICAL SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.