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DC motor type for high starting torque

Aviation Mechanic General (AMG) practice question 38 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K27 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONAn aircraft application such as an engine starter requires high starting torque under a heavy load. Which type of DC motor is used?

  1. ASeries motor
  2. BShunt motor, for its constant field current
  3. CCompound motor, for the highest torque
Show the answer and explanation

Correct answer: A. Series motor

  • A — correct Because of its low winding resistance the series motor draws a large starting current through both field and armature, producing the high starting torque that is its principal advantage; series motors are used as engine starters.
  • B — incorrect The shunt motor's starting torque is less than that of a series motor of equal size; it is chosen when constant speed is wanted and high starting torque is not needed.
  • C — incorrect A compound motor's starting torque is higher than a shunt motor's but lower than a series motor's.

BACKGROUND

There are three basic types of DC motors, differing in how the field and armature coils are connected. In the series motor the field windings, a few turns of heavy wire, are in series with the armature, so the same current flows through both; the low resistance lets it draw a large starting current and produce high starting torque, but its speed varies greatly with load and it can overspeed if the load is removed. Series motors are used in aircraft as engine starters and for raising and lowering landing gear, cowl flaps, and wing flaps. The shunt motor has its field in parallel with the armature, varies little in speed with load, and has lower starting torque. The compound motor combines both windings, with starting torque between the two.

STUDY TIP

Match the motor to the job: series for starting torque, shunt for constant speed, compound for a mix.

KEY POINTS

  • Series motor: high starting torque, speed depends heavily on load.
  • Shunt motor: nearly constant speed, less starting torque.
  • Compound motor: characteristics between series and shunt.

SOURCES

  • FAA-H-8083-30B ch12 "Types of DC Motors" - "Series DC Motor", "Shunt DC Motor", "Compound DC Motor"
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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.