USCG Exam PracticeElectrical fundamentals — DC/AC, motors, distribution
Single-phasing of a three-phase AC motor is considered especially destructive primarily because:
- A. The motor immediately stalls and draws no current, causing the thermal overload to reset
- ✓ The motor continues to run on two phases at greatly increased current, threatening insulationCorrect
- C. Loss of one phase causes the rotor to reverse direction, producing mechanical shock
- D. The rotating magnetic field collapses entirely, inducing zero rotor current and zero torque
Why B is correct
Single-phasing causes the motor to keep running on two phases at greatly increased current. This sustained overcurrent can cook the insulation before a standard overload relay trips, which is why phase-loss protection is provided on important machines. (DOE-HDBK-1011 Vol.4 §12-1)