What are the reasons for initial inrush curent at start of an AC motor?
The coils of an electric motor have inductive reactance but very little resistance. The coils initially act similar to a short circuit when the switch is closed and the motor shaft is stationary. Once the motor begins to rotate, the motor develops back EMF (voltage in reverse polarity to source voltage) which counters the source voltage, reducing the effective voltage across the motor coils, and reduces current flow. Back EMF is highest in a motor that is at full operating speed and under no mechanical load. When the motor is under mechanical load, back EMF is reduced until the motor is under full load, increasing the current to full load rating.
