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DC selsyn transmitter winding taps

Aviation Mechanic Airframe (AMA) practice question 15 of 20 on Aircraft Instrument Systems. ACS element AM.II.H.K5 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONIn a DC selsyn position indicating system, the transmitter's circular resistance winding is tapped off in three fixed places. How are those taps normally spaced?

  1. A120° apart around the winding
  2. B90° apart around the winding
  3. C60° apart around the resistance coil
Show the answer and explanation

Correct answer: A. 120° apart around the winding

  • A — correct The handbook states the resistance winding is tapped off in three fixed places, usually 120° apart, and these taps feed the three toroidal windings of the indicator motor.
  • B — incorrect Ninety degrees is the spacing used between gimbal bearing points in a gyroscope, not the tap spacing of a selsyn transmitter winding.
  • C — incorrect Three taps at 60° would not distribute current evenly around a circular winding; the stated spacing is 120°.

BACKGROUND

A synchro system is a two-unit electrical system that measures a parameter at one point and indicates it at another. Most position-indicating instruments, such as the flap position indicator, are built around a synchro system, and synchros are also used as remote position indicators for landing gear, autopilot systems, and radar. On aircraft with direct current, or DC, electrical systems, the DC selsyn is widely used. Its transmitter has a circular resistance winding and a rotatable contact arm whose shaft is linked to the moving unit. The winding is tapped in three fixed places, usually 120° apart, and those taps carry varying current to the three toroidal windings in the indicator, whose resultant magnetic field turns a permanent magnet and the attached pointer to match the transmitter arm position.

STUDY TIP

Trace the current path from the transmitter contact arm through the three taps to the indicator coils; the 120° spacing follows naturally from three equal divisions of a circle.

KEY POINTS

  • Synchro systems transmit position data electrically from a remote unit to a flight deck indicator.
  • The DC selsyn transmitter winding is tapped in three places, usually 120° apart.
  • A permanent magnet in the indicator aligns with the resultant field and carries the pointer.

SOURCES

  • sources/ama-airframe/ch10.txt "DC Selsyn Systems"
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MORE ON AIRCRAFT INSTRUMENT 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.