Question 27 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is C. ### Why Option C is Correct: Modern SCR (Silicon Controlled Rectifier) rectified DC propulsion drive systems operate by first generating AC power and then converting that AC power into controlled DC power specifically for the propulsion motor. * **Propulsion Generators Produce AC Power:** Virtually all modern marine propulsion systems utilize synchronous alternators (generators) that inherently produce AC power (typically 3-phase AC). It is more efficient and easier to generate power this way than to generate high-power DC directly. * **Conversion to DC Power:** The key component mentioned in the question is the **SCR Rectifier**. The SCR rectifier takes the generated AC power and converts it into variable DC power. The SCRs allow precise control over the voltage and current supplied to the motor. * **DC Propulsion Motor:** The rectified DC power is used to drive a DC propulsion motor. While older, large DC drives often used series-wound motors (especially in diesel-electric locomotives), modern marine SCR drives typically use **shunt-wound DC motors** (or separately excited DC motors) because they offer better speed control and stability under variable load conditions, which is essential for propulsion applications. Therefore, the system architecture involves AC generation $\rightarrow$ SCR Rectification $\rightarrow$ Shunt Wound DC Motor. ### Why Other Options are Incorrect: **A) propulsion generators which produce DC power that is converted to AC power for the propulsion motor** * **Incorrect Generation:** Modern high-power generators produce AC, not DC, power. * **Incorrect Motor Type/Conversion Goal:** If the output was DC, converting it back to AC (using an inverter) would define a modern AC Variable Frequency Drive (VFD) system, not an SCR-rectified DC drive system. **B) propulsion generators which produce AC power that is directly delivered to the synchronous AC propulsion motor** * **Missing Rectification:** This describes a basic AC synchronous drive system (sometimes used in larger vessels) where VFDs control the frequency, but it completely omits the critical component mentioned in the question: the **SCR rectifier** which is central to a DC propulsion package. If the power were delivered directly (without VFDs), the speed control would be fixed and linked to the generator speed. **D) propulsion generators which produce DC power that is directly delivered to the series-wound DC propulsion motor** * **Incorrect Generation:** Modern high-power propulsion generators produce AC power. * **Missing Rectification/Control:** Delivering power "directly" means there is no high-power conversion equipment (like SCRs) for speed and torque control, which is necessary for a propulsion drive. While series-wound motors have been used, the core error here is the assumption of direct DC power generation.