USCG Exam PracticeMotors and generators
On a three-phase induction motor, what is the effect of single-phasing — the loss of one supply phase — while the motor remains under load?
- A. The motor immediately stalls because the rotating magnetic field collapses entirely
- ✓ The motor continues to run on two phases at greatly increased current, risking insulation damageCorrect
- C. The motor speed increases to synchronous speed because slip is eliminated
- D. The motor reverses direction because the rotating field direction changes
Why B is correct
DOE-HDBK-1011 Vol.4 §12-1 explains that single-phasing is especially destructive because the motor keeps running on two phases at greatly increased current. This is why phase-loss protection is provided on important machines.