Types of Diesel Electric Propulsion
The two systems dominating the market today are Frequency controlled A.C Motors and SCR controlled D.C Motors.
Frequency controlled A.C Motor drive system were generally more cost effective below 500 H.P and SCR controlled D.C motors systems at the higher end. The offshore drilling industries favor SCR controlled DC drives.
Modern SCR and frequency controlled systems have efficiencies approaching 97% in power conversion. The selection of one over the other is an application issue. The deep draft cruise ship industry, due to the high hotel-like power requirements, is adopting high-power diesel electrical propulsion systems in most of its new builds.
Both technologies have a proven record of efficiencies and reliability. For a direct current propulsion motor, the electrical power may be from one or more DC generators or may be form an alternator and then delivered through a rectifier as a DC supply. The power for direct current motors is limited to about 8 MW, and so AC machines are used for high outputs unless an effort is made to install DC motors in tandem. The rectification scheme can incorporate speed control and a means of reversing.
Power for AC propulsion motor is supplied obviously by an alternator; the prime movers may be a diesel engine, a gas turbine, or a boiler and steam turbine installation.
The choice of diesel electrical system as the power source for a propulsion system of a vessel has nothing to do with hydrodynamic efficiency. The propulsion system of a vessel provides thrust to move the vessel and is still chosen by the designer based on merits for the vessel’s application. Conventional propellers, controllable pitch propellers, azimuthing Z drives, transverse tunnel thrusters, and low speed water jet systems can be driven with equal effectiveness by a diesel-electrical system.
Diesel-electrical propulsion becomes viable when the installed KW for propulsion approaches or is exceeded by the KW installed for other purposes. The convenience of electric power distribution makes it possible to optimally locate the primary power source, i.e. diesel generators, exclusive of consideration as to whether it is for propulsion, thrusters, or cargo handling purposes. A large variation in propulsion power requirements, such as long periods of low speed operation or the necessity to shift power from main propulsion to thrusters for dynamic positioning purposes, can also justify diesel electric systems.
Modern turbo-charged diesel engines are efficient over a relatively narrow operating load and RPM range. They are not suitable for long period of low speed, low load, low RPM, high torque requirements for reversing large propellers. Modern generator systems with load sharing, auto-start, and load shedding features make it possible to efficiently utilize the installed horsepower of a diesel electrical system.