Main benefits

Electric power generation in ships has been traditionally carried out by using either separate auxiliary gensets or by using so called shaft generators connected to the main engine. Auxiliary gensets typically consist of a constant-speed, 4-stroke diesel engine equipped with a standard asynchronous or synchronous generator. The main benefit of this concept is that due to constant-speed operation, the generator feeds constant voltage and frequency into the ship’s electric grid and therefore, no power electronics are needed for frequency conversion. Additionally, since this concept is independent of the propulsion, power generation can continue during manoeuvring and harbouring. The main drawbacks are that auxiliary gensets require more space, more maintenance, and they cannot run with cheaper heavy fuel oil (HFO) without expensive auxiliary equipment, such as pre-heaters to reduce viscosity.

Maximum freedom

Coupling a PM shaft generator with a frequency converter gives the maximum amount of freedom in optimizing the engine and propeller  efficiencies, as both the speed and propeller pitch angle in the case of a CP propeller can be independently varied using the combinatory mode. The optimal combination of propeller speed and pitch angle minimizes the specific fuel oil consumption. The lower the ship’s speed. the bigger is the benefit. Even when sailing on open seas, lowering the engine speed and simultaneously using a smaller propeller pitch in the case of the CP propeller can result in huge savings in fuel consumption. Although there are shaft generator systems that allow variable speed operation by utilizing a frequency converter, so far they all use electrically excited synchronous generators.