Question 26 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is D **Explanation for Option D (Changing the generator engine speed):** In a standard DC diesel-electric drive system (common in older locomotives and some marine applications), the diesel engine drives the main DC generator. The speed of the DC propulsion motor is primarily determined by the voltage supplied to it, as the motor is typically run with a nearly constant field strength (Option A is used for fine-tuning or high-speed operation, but not primary control). The primary way to control the output voltage of the main DC generator is by controlling the speed of the engine driving it. Increasing the diesel engine speed increases the rotational speed of the generator armature, which in turn increases the generated electromotive force (EMF) and thus the output voltage supplied to the DC propulsion motor. Higher voltage results in higher motor speed. This method provides the broadest and most fundamental control over the motor speed and tractive effort. **Why the other options are incorrect:** * **A) changing the motor field excitation current:** While weakening the motor field (reducing excitation current) can increase motor speed (used often at high speeds to achieve the maximum possible speed, known as "field weakening"), it simultaneously reduces the available torque. It is usually a secondary control mechanism used only after maximum generator voltage is reached, and is not the *primary* method for controlling the entire speed range from zero to maximum. * **B) changing the generator field excitation current:** Changing the generator field excitation current is a highly effective way to control the generator output voltage, and thus the motor speed. However, in most large diesel-electric systems (especially locomotives), the engine speed (D) is tied to the generator voltage control (B) through complex power management systems. If the system is designed specifically for **variable engine speed control (D)**, then D is the superior primary control method, as it also optimizes fuel consumption by matching engine output (HP) directly to the power demand. If the system used a constant engine speed, then B would be the correct answer, but historically, older and simpler systems heavily relied on variable engine speed (notably the classic locomotive throttle notched system). Given the general context of large DC diesel-electric drives, varying the diesel engine speed (D) is the defining characteristic of primary power and speed control. * **C) changing the polarity of the generator field:** Changing the generator field polarity reverses the polarity of the voltage supplied to the motor, which is used specifically to reverse the direction of rotation (forward/reverse), not to modulate the speed within a given direction.