Question 28 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is D **Explanation for Option D (Correct Answer):** In a DC diesel-electric drive propulsion system, the direction of rotation of the main propulsion motor (and thus the propulsion shaft) is primarily controlled by reversing the polarity of the power supplied from the main generators to the main motors. This reversal is achieved by controlling the excitation (field current) of the **generators**. The generator field current is typically supplied and controlled by a **controlled rectifier**. If the **controlled rectifier fails to reverse the polarity** supplied to the **generator field windings**, the polarity of the voltage output by the main generators cannot be reversed. Consequently, the main propulsion motors will only receive current in one direction (causing only ahead rotation), regardless of control lever input requesting astern movement. **Explanation of Incorrect Options:** * **A) A failure of the main propulsion diesel-generators engines to reverse direction of rotation.** * This is incorrect because the diesel engines driving the generators in a diesel-electric system operate continuously in one direction (usually constant speed) and **do not** reverse direction. The electrical generators produce power whose polarity is reversed electrically, not mechanically by the prime mover. * **B) A failure of the field rheostats to reverse polarity to the motor field windings.** * While field rheostats control the strength (speed) of the motor, the **motor field polarity** itself is generally *not* reversed to change direction in this type of system (especially one using generator voltage reversal for direction control). Furthermore, if the motor field failed to reverse, the motor might run erratically or weakly, but the primary direction control comes from the main power source (the generators). * **C) A failure of the non-controlled rectifier to reverse polarity to the motor field windings.** * A non-controlled (diode) rectifier is designed to provide DC power from an AC source without the ability to regulate or reverse the voltage polarity. Non-controlled rectifiers are often used to provide stable excitation to the motors. Since they cannot reverse polarity, their failure to do so is expected and would not be the cause of the inability to reverse the propulsion shaft, which is controlled upstream at the generator level.