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Control of DC generator voltage

Aviation Mechanic Powerplant (AMP) practice question 11 of 13 on Engine Electrical Systems. ACS element AM.III.F.K4 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONWhat is regulated in order to control the voltage output of a DC generator?

  1. AThe strength of the field current.
  2. BThe speed of the generator drive.
  3. CThe resistance of the load circuit.
Show the answer and explanation

Correct answer: A. The strength of the field current.

  • A — correct The handbook states that regulation of field current strength is used to control DC generator voltage; reducing field current reduces the generated voltage and increasing it raises the output.
  • B — incorrect The drive gearing is fixed at roughly 1 1/8 to 1 1/2 times crankshaft speed and follows engine rpm, so it is not the means used to regulate output voltage.
  • C — incorrect Load resistance is set by the equipment connected to the bus; the regulating action works on the generator field, not on the load.

BACKGROUND

Regulation of field current strength is the means used to control DC generator voltage. In a shunt generator this can be demonstrated with a rheostat in series with the field winding: as field resistance increases, field current and generated voltage drop, and as field resistance decreases, field current and output rise. In an installed aircraft the rheostat is replaced by an automatic control device, the voltage regulator. Checking voltage regulator operation is one of the listed items in a generator system inspection.

STUDY TIP

Anchor the idea that the regulator works on the small field current, not on the heavy output current, and the rest of the theory follows.

KEY POINTS

  • DC generator voltage is controlled by field current strength.
  • More field resistance means less field current and lower voltage.
  • A voltage regulator replaces the manual field rheostat in service.

SOURCES

  • amp-powerplant/ch04.txt "Electrical Load Limits"
  • amp-powerplant/ch04.txt "Parallel (Shunt) Wound DC Generators"
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MORE ON ENGINE ELECTRICAL SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.