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 immediate consequence?
- A. The motor stops immediately because torque drops to zero.
- B. The motor slows down as counter-EMF collapses and armature current increases.
- ✓ The motor accelerates to a potentially destructive overspeed.Correct
- D. The motor continues at rated speed because armature voltage is unchanged.
Why C is correct
NEETS Mod. 5 §2-1 explains that reducing field flux causes a shunt motor to speed up in order to restore counter-EMF; an accidental open in the field circuit removes the limit entirely and can allow the motor to race to a destructive overspeed, which is why field-loss relays are fitted.
Cited:NEETS Mod. 5 §2-1