Question 24 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is C ### Why Option C (Shunt Wound) is Correct: In DC propulsion drive systems, the desired operational characteristic is **speed control that is independent of the load (torque)**, and the ability to maintain relatively constant speed under varying conditions. A **shunt-wound DC motor** (where the field winding is connected in parallel with the armature) provides these characteristics. Its key advantages for propulsion are: 1. **Excellent Speed Regulation:** The motor speed drops only slightly as the load increases. 2. **Easy Speed Control:** Speed can be efficiently and smoothly controlled over a wide range by adjusting the field current (field weakening) or by adjusting the armature voltage (voltage control), which is crucial for maneuvering ships or vehicles. Historically, the shunt-wound DC motor was the standard choice for electrically driven propellers and traction systems due to its robust and controllable speed characteristics. ### Why the Other Options are Incorrect: * **A) Permanent Magnet (PM) Motor:** While PM DC motors are used in smaller, modern applications (especially brushless types), they are generally unsuitable for large, high-power DC propulsion drives. Their maximum torque is limited by the fixed magnetic field, and speed control often requires complex electronic commutation, which was not the typical historical approach for large DC propulsion systems. * **B) Differentially Compounded Motor:** This motor type has a shunt field and a series field wired to oppose each other. As the load increases, the net flux decreases rapidly, causing the motor speed to rise dangerously high (runaway). This unstable speed characteristic makes it completely unsuitable for propulsion or any application requiring steady speed under varying load. * **D) Series Wound Motor:** In a series-wound motor, the field winding is in series with the armature. This configuration results in very high starting torque, making it suitable for traction applications where massive starting torque is needed (like trains or hoists). However, a series motor has **poor speed regulation**—its speed drops significantly as the load increases, and critically, if the load is removed or reduced substantially, the motor speed can become dangerously high (runaway). This instability makes it generally unsuitable for direct use in marine propulsion or applications where precise, constant speed is required under light load.