How is the direction of rotation of the main propulsion motor in a modern AC propulsion drive system reversed?

Correct Answer: Option D — electronically changing the phase sequence of the voltages generated by the power converter

Modern AC drives utilize Variable Frequency Drives (VFDs) to control motor speed and direction. By electronically manipulating the firing sequence of the inverter's power semiconductors (IGBTs or thyristors), the phase sequence of the output voltage is reversed, causing the rotating magnetic field in the stator to reverse direction, thereby reversing the motor.