How is the speed of the propeller shaft directly coupled to an AC synchronous propulsion motor changed when powered by a variable frequency alternator in an AC turboelectric drive system?

Correct Answer: Option D — varying the turbine speed

In turboelectric systems, the synchronous motor speed is locked to the generator frequency. Since the generator is driven by a turbine, varying the turbine speed directly changes the generator's output frequency. Per IEEE 45, the synchronous speed of the motor is directly proportional to this frequency, allowing precise propeller speed control.