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 result?
- A. The motor stalls because torque is proportional to field flux
- B. The motor runs at reduced speed to compensate for lower counter-EMF
- ✓ The motor accelerates toward a destructive overspeedCorrect
- D. The motor draws reduced armature current and slows gradually to a stop
Why C is correct
Counter-EMF depends on field flux. When field flux is lost, counter-EMF collapses, armature current surges, and the motor accelerates uncontrollably toward destructive overspeed. Field-loss relays are provided to guard against this hazard. (NEETS Mod. 5 §2-1)
Cited:NEETS Mod. 5 §2-1