Question 20 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is B ### Explanation of Correct Option (B) **B) varying the turbine speed** The speed ($N$) of an AC synchronous motor is rigidly linked to the frequency ($f$) of the power supply and the number of motor poles ($P$) by the formula: $$N = (120 \times f) / P$$ In an AC turboelectric system utilizing a variable frequency alternator, the alternator (generator) is driven by the prime mover, which is the gas or steam turbine. 1. **Turbine Speed = Alternator Speed:** Adjusting the throttle of the turbine changes its rotational speed, which directly changes the speed of the coupled alternator. 2. **Alternator Speed = Frequency:** The frequency ($f$) generated by the alternator is directly proportional to its rotational speed. 3. **Frequency = Motor Speed:** Since the propeller shaft is coupled to the synchronous motor, controlling the input frequency ($f$) by varying the turbine speed provides smooth, continuous control over the motor speed and thus the propeller shaft speed. This method allows the continuous and precise speed variation required for ship maneuvering, making varying the turbine speed the primary control mechanism. *** ### Explanation of Incorrect Options **A) varying the number of motor poles** While changing the number of poles ($P$) would change the synchronous speed ($N$), this requires complex, switchable motor windings (pole-changing motors) and only provides discrete, stepped changes in speed (e.g., high and low speed settings). It does not provide the smooth, continuous speed regulation needed for precise ship control, especially during acceleration and deceleration. **C) varying the field strength of the generator** Varying the field strength of the generator controls the magnitude of the output voltage ($V$). It does not change the generator’s rotational speed, which is dictated by the turbine throttle. Since synchronous motor speed is determined by frequency, controlling the voltage alone does not change the speed. **D) varying the field strength of the motor** Varying the field strength (excitation) of a synchronous propulsion motor is used to control the reactive power flow (kVAr) and maintain a desired power factor. It does not affect the synchronous speed of the motor, which is fixed by the supply frequency ($f$) and the number of poles ($P$).