Question 19 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is D **Explanation for why Option D is correct:** In modern AC propulsion drive systems (which typically use synchronous or induction motors), the motor's speed and direction are controlled by the power electronics, specifically the variable frequency drive (VFD) or power converter (inverter). The rotation direction of an AC motor is determined by the phase sequence (the order in which the voltage peaks occur on the three phases: R-S-T or A-B-C) of the three-phase AC power supplied to the stator windings. To reverse the motor's direction, the power converter electronically switches the firing sequence of its semiconductor devices (like IGBTs or Thyristors) to swap any two of the three output phases (e.g., changing the sequence from A-B-C to A-C-B). This is a fast, efficient, and solid-state method of reversal, requiring no mechanical contactors or high-current switching components in the main power path, making it the standard method for modern AC drives. **Explanation for why other options are incorrect:** * **A) power directional relays:** Power directional relays are used primarily for protection (detecting power flow in the wrong direction) in large systems, not for actively controlling the direction of a motor. Directional control is handled by the motor drive's control unit. * **B) reversing the direction of current flow in the armature:** This method (reversing armature polarity while keeping the field polarity constant) is the traditional and standard way to reverse a **DC motor**. It is not applicable to three-phase AC motors where the rotation is dictated by the rotating magnetic field established by the three-phase stator windings. * **C) changing the direction of current flow in the motor's field winding:** Changing the field current direction in a **DC motor** would reverse the motor (similar effect to reversing the armature). However, AC induction motors (the most common type for modern propulsion) do not have a separate DC field winding that controls the primary direction of rotation in this manner. Even in large AC synchronous motors that do have field windings, the reversal is still achieved by changing the stator phase sequence (Option D).