FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A starter motor on a reciprocating engine uses a series-wound DC motor design rather than a shunt-wound motor. A technician explains this choice to a trainee. What operating characteristic makes the series motor best suited for this application?
- AIt maintains constant speed regardless of load changes
- BIt requires no external power source to operate
- CIt develops maximum torque at low speed under heavy load
- DIt produces a constant voltage output at all engine speeds
Show answer & explanationAnswer & explanation
Correct answer: C. It develops maximum torque at low speed under heavy load
Series-wound DC motors develop their highest torque at low or zero speed because armature and field current are the same and both are high during initial cranking, exactly the condition needed to overcome engine compression and inertia at startup.
Why the other options are wrong
- A. Series motors are known for widely varying speed with load, not constant speed.
- B. All DC motors, including series motors, require an external power source (the battery).
- D. A motor does not produce voltage output; this describes a generator characteristic, not a motor.
Series-Wound Starter Motor
A DC motor with field and armature windings in series, producing very high starting torque at low speed, ideal for engine cranking.
- Field and armature current are equal
- Highest torque at zero/low speed
- Speed increases greatly if load is removed (risk of overspeed)
Memory trick: Series motor = 'muscle at zero speed' for cranking a stubborn engine.