USCG Exam PracticeElectrical fundamentals — DC/AC, motors, distribution
An AC induction motor draws 5–7 times its full-load current at the moment of starting. What physical condition of the motor at standstill causes this high inrush?
- A. The rotor is spinning at synchronous speed, creating maximum back-EMF.
- B. The stator windings are open-circuited, forcing all current through the rotor.
- ✓ At standstill the motor behaves like a short-circuited transformer, presenting very low impedance.Correct
- D. Single-phasing at start-up forces the remaining two phases to carry excess current.
Why C is correct
NEETS Mod. 5 §4-1 states that at standstill a squirrel-cage motor behaves like a short-circuited transformer, which is why starting inrush is five to seven times full-load current at a poor power factor. This is why larger motors use reduced-voltage or soft starters to limit the surge and the resulting bus-voltage dip.
Cited:NEETS Mod. 5 §4-1