 Why Electric Anyway?
Most commercial ships today rely on diesels as their source of motive power. Whether they are slow-speed engines directly coupled to the propeller shaft or medium- and high-speed machines operating through massive reduction gearing, diesels have essentially supplanted steam turbines as the primary way to spin hgh-torque propellers at 100 to 120 rpm.
Diesel engines, whose efficiency makes them popular with commercial shippers mindful of fuel costs, simply cannot deliver the high-end power needed to turn a screw upwards of 200 rpm without becoming prohibitively heavy.
Through the switch from steam to gas turbines, one part of the propulsion system, the reduction gears, stayed. The gas turbine helped reduce a ship's weight by eliminating the heavy steam turbine casings and boilers.
Electric drive severs the connection between prime mover and propeller. Electrical generators can be located anywhere on the ship, their power transmitted through cables to the propulsion motors.
There is another advantage to electric drive. Its input voltage and current can be manipulated to cancel machinery and propeller noise; a ship can operate with greater stealth.
Those are the arguments in favor of electric drive. Superconducting motors enhance the case for electric drive in a number of ways.
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Superconducting Motor
With their high power densities, practically zero electrical resistance, and low noise, superconducting motors could bring forth nearly silent propulsion. (Magnetic flux concentrations typical of conventional motors are main sources of noise and do not develop in dc machines.) The superconducting motors also could produce full torque rating at low speeds, promising a direct coupling with propellers while remaining small enough to fit in a narrow hull. These advanced motors could bring about the use of larger, slower, and consequently quieter propellers.
The brushes remain a weak point today in the dc homopolar motor program, although advances in solid copper fiber and foil brush systems may prove these technologies better suited to a life at sea than their liquid metal counterparts.
Superconducting motors can develop the same torque and horsepower within a motor frame that is nearly a third the size of a comparably rated conventional motor. Losses can be half those of a conventional motor of equal power. The main factor leading to an HTS motor's smaller size for a given horsepower output is the magnetic-field strength that superconducting magnets create. Iron teeth, used to enhance magnetic flux in conventional rotors and stators, are not needed by superconducting motors.
As for speed control, superconducting ac machines could be driven by pulse-width modulated converters using the latest insulated gate bipolar transistor, or IGBT, technology, just like ordinary motors. Thus, the complexity associated with changing speed and direction using controllable pitch propellers would vanish. |
Scale model of a 33,000-hp superconducting ac propulsion motor. The design marries a conventional ac stator and a superconducting dc rotor.
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One 33,000-hp ship propulsion motor in two forms: an induction machine dwarfs a superconductor motor. The smaller machine, approximately 90 x 65 inches full scale, owes its svelte figure to iron tooth loss and higher flux density.
Article compiled by Mr. Srinivas K Patnaik, Fleet Manager
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