USCG Exam PracticeMotor controllers and remote control
A squirrel-cage induction motor is started across the line. Which of the following best describes the starting inrush current and its effect on the ship's bus?
- A. Starting inrush is approximately equal to full-load current at unity power factor, causing negligible bus voltage dip
- ✓ Starting inrush is five to seven times full-load current at poor power factor, causing a bus voltage dipCorrect
- C. Starting inrush is two to three times full-load current at leading power factor, improving bus voltage momentarily
- D. Starting inrush equals locked-rotor current only if the motor exceeds 50 horsepower; smaller motors start without surge
Why B is correct
NEETS Mod. 5 §4-1 states that because a squirrel-cage motor at standstill behaves like a short-circuited transformer, its starting inrush is five to seven times full-load current at a poor power factor, producing a bus-voltage dip. This is why reduced-voltage or soft starting is used on large motors.
Cited:NEETS Mod. 5 §4-1