USCG Exam PracticeElectrical fundamentals — DC/AC, motors, distribution
A large three-phase squirrel-cage induction motor is started across the line. The starting inrush current and its effect on the distribution bus are best described as:
- A. One to two times full-load current at unity power factor, causing negligible bus-voltage disturbance
- ✓ Five to seven times full-load current at poor power factor, potentially causing a significant bus-voltage dipCorrect
- C. Ten times full-load current at unity power factor, requiring the emergency generator to pick up the load
- D. Four times full-load current at leading power factor, which actually supports bus voltage during starting
Why B is correct
At standstill a squirrel-cage motor behaves like a short-circuited transformer, drawing 5–7 times full-load current at poor (lagging) power factor. This surge can cause a significant bus-voltage dip, which is why reduced-voltage or soft starters are used on larger motors. (NEETS Mod. 5 §4-1)
Cited:NEETS Mod. 5 §4-1