Chapter 8 Electrical Propulsion
Marine electric propulsion systems utilize generators to provide power for electric motors, which in turn drive the ship’s propeller. The electric generators may be driven by any suitable prime mover, most commonly, diesel engines. While it may initially seem counterintuitive to interpose complex and costly equipment between the prime mover and the propeller when a simple line shaft and gearbox could transmit the power instead, there are valid justifications for the added complexity and expense.
The advantages of electric propulsion include:
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Layout Flexibility.The ability to install the prime mover in any convenient location, without being restricted to a direct line with the propeller. Power is transmitted through flexible cable runs, as opposed to rigid line shafts. Furthermore, instead of relying on a single large main engine, multiple smaller engines can be used, allowing for placement in smaller spaces and more convenient locations.
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Efficient Operation.Eliminating the need for reduction gears. Typically, diesel engines and steam turbines operate most efficiently at high or medium speeds, while propellors are most efficient at low speed, below about 100 RPM. When propulsion power is supplied electrically, speed reduction is accomplished by selecting the appropriate number of poles in the generator and varying the electrical frequency supplied to the motor. This setup allows each component to operate within its most efficient range, maximizing overall energy efficiency and reducing fuel consumption.
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Redundancy and Reliability.Inherent redundancy and enhanced reliability compared to traditional propulsion systems. With multiple smaller generator sets, the system can continue to function even if one or more generators or motors fail or require maintenance. This redundancy reduces the risk of complete propulsion failure and ensures that the propulsion system remains operational.
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Economical Part-Load Operation.Improved efficiency at low loads. Ships can secure one or more engine/generator sets if they are not needed. The remaining engines can then operate at nearly full load, resulting in improved efficiency compared to having more engines operating at part-load. Load management systems continuously monitor the electrical demand and promptly initiate the startup of an additional generator when necessary.
