USCG Exam PracticeElectrical fundamentals — DC/AC, motors, distribution
A DC shunt motor is running at rated speed when its field circuit opens accidentally. What is the most likely result?
- A. The motor will stall immediately because torque drops to zero.
- B. The motor will continue at rated speed because armature voltage is unchanged.
- ✓ The motor will accelerate to a potentially destructive overspeed.Correct
- D. The motor will slow down gradually as counter-EMF increases.
Why C is correct
Loss of field flux removes the counter-EMF limit on armature current and speed. A shunt motor with an open field circuit can race to destructive overspeed, which is why field-loss relays are fitted. NEETS Mod. 5 §2-1.
Cited:NEETS Mod. 5 §2-1