USCG Exam PracticeMotor controllers and remote control
A DC series motor driving a cargo hoist is uncoupled from its load while still energized. Which of the following consequences is most likely, and what characteristic of the series motor causes it?
- A. The motor stalls immediately because series motors require a minimum load to develop counter-EMF sufficient to sustain rotation
- B. The motor runs at nearly constant speed because the shunt field maintains stable flux independent of armature current
- ✓ The motor accelerates uncontrollably because with no load current the series field collapses, reducing flux and allowing speed to rise without limitCorrect
- D. The motor reverses direction because removing the mechanical load reverses the torque angle of the series field relative to the armature
Why C is correct
NEETS Mod. 5 §2-2 states that a series DC motor must never be run without a mechanical load because unloaded it accelerates uncontrollably; the series field flux is proportional to armature current, and with no load the current drops, flux collapses, and speed rises without limit. This is why series motors are directly coupled or geared to their loads rather than connected through a clutch that could release.
Cited:NEETS Mod. 5 §2-2