This article explains features of a ship propulsion motor with superconducting coils that hushes machinery noise and hollows out confined engine rooms. American Superconductor Corp. of Westborough, MA, announced plans to build a high-capacity manufacturing plant for its superconducting wire. This will move the company’s high temperature superconductor (HTS), wire from a developmental phase into large-scale production. IPS is exploring more than just propulsion motor technology. Engineers are evaluating the entire shipboard electrical system, from ship’s service power, to dc distribution, to power electronics—as well as the propulsion motor itself. The Navy, having decided upon electric drive for its next warship, has left the door open for superconducting motors. 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. 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.
The promise of superconducting motors inched closer to reality last month after American Superconductor Corp. of Westborough, Mass., announced plans to build a high-capacity manufacturing plant for its superconducting wire. This will move the company’s high- temperature superconductor, or HTS, wire from a developmental phase into large-scale production, said company spokesman Kevin Coates.
The announcement follows a U.S. Navy statement in January that it would use electric drive for the main propulsion in its next class of combatant ships, the DD-21 land attack destroyer. The news improved the chances for superconducting motors being aboard the first of the new vessels when it is christened toward the end of the decade.
Even if a superconducting motor does not propel the lead ship, the decision will put enough necessary systems and equipment on the ships so they could easily be back-fitted eventually with superconductor motors.
“It almost doesn’t matter what the first motor is,” said Maribel Soto, a program manager in the materials division at the Office of Naval Research. Whether the first vessel carries an induction motor, a permanent magnetic motor, or a superconducting motor, the decision to use an electric final drive vindicates much of Soto’s work in heading up what she called the “basic science” of ONR’s superconductor motor development. The Navy, she said, now recognizes the importance of electric drive. The superconductor motor program has moved into an applications phase, she added.
Just the decision to go with electric drive is a “radical change” for the Navy, Soto said. There will be a cost associated with it coming on board. The transition may be under way for some time before the Navy begins to realize any economic advantage, she added.
Jon Walman, a spokesman for the Navsea Program Executive Office for Surface Strike, said that for the first time the Navy is leaving much of the ship design up to the two teams preparing proposals. The Navy is becoming more of an overseer and evaluator of proposals. The days of shipyards building to Navy blueprints is drawing to a close, he said.
Both contractors bidding the DD-21 class—General Dynamics’ Bath Iron Works in Maine and Litton Ingalls Shipbuilding of Pascagoula, Miss.—recommended electric drive to the Navy as propulsion for its next warship.
Actually, electric drive is only a part of the larger recommendation to use an integrated power system in the next class of ships, Walman explained. Navy ships have carried electric propulsion before, he said. An integrated power system, however, ties the production of the electricity consumed by propulsion motors and the rest of a ship’s electrical equipment to shared generators.
As for which electric drive technology would move the first of the new ships, Walman said a decision would surface next year when the Navy selects a DD-21 shipbuilder.
Tim Zavertnik, an electrical engineer working in the Navsea Integrated Power Systems program, said, “IPS is exploring more than just propulsion motor technology.” Engineers are evaluating the entire shipboard electrical system, from ship’s service power, to dc distribution, to power electronics—as well as the propulsion motor itself.
At a test site in Philadelphia, Zavertnik said, IPS has installed a full-scale 25,000-horsepower induction motor, along with adjustable frequency converters and ship’s service distribution. The induction motor represents the “lower-tech, more mature” approach to electric ship drive, he said. Permanent magnet motors and superconducting motors are further away.
So, the Navy’s decision does not automatically qualify superconductor motors for shipboard duty. Still, the ONR’s 1999 award to American Superconductor of a $1.5 million preliminary design contract for a 33,000-hp ship propulsion motor is a sure sign of Navy interest.
Why Electric Anyway
Before extolling the virtues of superconductor motors, it might help to take a walk through a modern naval engine room. This should spell out the benefits of electric ship propulsion in general. Clifford Whitcomb, a Navy lieutenant commander and MIT professor of naval architecture, leads the way.
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 high-torque propellers at 100 to 120 rpm.
In contrast, the Navy emphasizes speed, in addition to fuel efficiency, Whitcomb said. Many of its surface ships are capable of cruising above 30 knots. 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.
When most of the shipping industry was looking at diesels to replace an aging steam fleet, the Navy opted for gas turbines. The Navy has been using gas turbines almost exclusively since the 1980s, Whitcomb said. Exceptions, of course, are the nuclear carriers and submarines that rely on pressurized water reactor cycles and steam turbines.
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. Yet Navy ships still continued to carry a great deal of propulsion machinery in the form of a prime mover (or two), massive bull gear and pinion sets, plus the shafts needed to deliver all that horsepower back through stern tubes to the propeller blades. In the switch to gas turbines, the Navy also gave up something; gas turbines don’t run in reverse.
To circumvent this last hitch, the Navy installed controllable pitch propellers, which can direct their thrust backward. Controllable pitch propellers also help to maintain steady turbine speed even as ship speed changes during maneuvers. But to make the controllable pitch system work requires a complex network of hydraulics running through the propeller shaft.
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. The same generator can provide power for propulsion, combat systems, auxiliary equipment, and ship’s services. According to Whitcomb, a typical naval combatant might need 50 to 100 megawatts for propulsion, and another 5 to 15 MW for hotel loads and weaponry. Gas turbines tied mechanically to the ship’s propulsion are unavailable to supply the electrical power needed by other systems. Thus, additional smaller-capacity gas turbine or diesel generators must be carried on board and dedicated to producing electrical power.
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. Whitcomb, himself a submariner, said that though the need for quiet running is not as obvious for surface ships as it is for submarines, it is nearly as important. Surface ships want to avoid broadcasting acoustic signatures to eavesdropping submarines (or other warships), which might use such information to sift the identities of friends and foes from an ocean full of radiated noise.
Those are the arguments in favor of electric drive. Superconducting motors enhance the case for electric drive in a number of ways.
Dc Superconducting Motor
The Navy recognized the benefits of superconductivity for ship propulsion early on, demonstrating in 1980 a 300-kilowatt superconducting de homopolar generator and a 400-hp motor aboard the test craft Jupiter II. In 1983, the Navy tested a 3,000-hp superconducting dc motor on the same vessel.
The Navy saw that its submarine and surface fleets could benefit enormously from superconducting motors. 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.
These two machines completed their sea trials well before the 1986 discoveries of high-temperature superconductors. The ONR’s Soto remembered the trials being “implementation nightmares.” The motors, she said, were soaked in liquid helium at 4.2 Kelvin to keep them superconductive. To chill the motors, the Navy relied on cumbersome helium liquefaction and recovery systems that worked only sporadically. Also, liquid metal brushes for collecting current in the dc motors used fussy, complex motor seals.
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.
Still, with the coming of better cryogenic refrigeration such as that developed in 1992 for magnetic resonance imaging systems, the Navy continues its research into dc superconducting motors. “The Navy is moving forward to demonstrate a large-scale de homopolar motor,” Soto said. It will be operational at 4.2 K, she added, and should cost less to demonstrate than an ac synchronous superconducting motor using HTS wire.
Ac Superconducting Motor
ONR awarded American Superconductor a contract in June last year to design a 33,000- hp ac ship propulsion motor based on HTS technology. American Superconductor’s development of bismuth-2223 wire, a shortened name for the superconducting cupric compound (Bi,Pb)2Sr2Ca2Cu3O10, helped bring ac polyphase superconducting motors a step closer to practicality.
HTS ceramics are brittle, which makes forming wires from them difficult. Yet, these materials, according to American Superconductor’s director of engineering, Bruce Gamble, reach a critical current density, Jc, of 24,000 amps per square centimeter at 77 K. When the Bi-2333 materials are cooled to 30 K, Jc climbs to 45,000 A/cm2 and sometimes as high as 75,000 A/cm2. Most important for motor coils, though, the material retains 90 percent of its critical current density under stresses reaching 11,500 psi.
HTS motors use cryocoolers, which have proven themselves in semiconductor manufacturing over the past decade. These coolers operate on a modified Stirling refrigeration cycle. Due to less stringent temperature demands, truly practical, reliable, efficient cooling systems for superconducting coils seem closer at hand for HTS motors than for low-temperature superconductor motors. This is one advantage that could push the Navy toward HTS rather than LTS machines.
The very popularity of ac motors in industry may be another factor that ultimately draws the Navy into an HTS design. Motor manufacturers are inclined to build something for which they can sell many copies, Whitcomb said, and ac superconducting motors would be popular indeed for their low cost and energy savings. That kind of plum could accelerate the pace of ac machine development.
With ships and subs, Whitcomb said, it is efficiency, size, reliability, and silence that matter. Both HTS and LTS motors need less engine-room space than a conventional electric drive. They would add less weight to a finished ship, especially for motors located aft, where the longitudinal moment must be considered. They would give hull designers flexibility in their search for more efficient forms. They would free naval architects from the “tyranny of the drivetrain.”
Indeed, the greater power density of superconducting motors will probably rate higher at sea than on land. On shore, where space is plentiful, efficiency is more the rule.
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, Whitcomb said. 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, he said.
“This means that the current densities in the active regions are not limited by iron saturation. The stator only requires the use of back iron acting primarily as a shield to keep magnetic flux inside the machine. A resulting HTS air core configuration, with higher flux density, is significantly smaller and lighter than conventional ac synchronous motors,” Whitcomb said.
Gamble, at American Superconductor, added, “Our estimates are that the active length of a superconducting motor will be on the order of one-quarter of that for a permanent magnet alternative. This will reduce the overall hull length by up to three times the length savings in the drivetrain, depending on hull balance and overall trimming of the mass of the boat.”
No More Winding Slots
Removing iron teeth dispenses with the need for winding slots. That yields several advantages. It eliminates “cogging torque,” which Whitcomb called a significant source of radiated noise. Less iron equates to less synchronous and subtransient reactance as well, enhancing a machine’s transient stability. Power density improves after the iron comes out since more space can be set aside for windings.
How quiet could the ac machines be? Probably not as noiseless as a homopolar machine with its constant current and magnetic field that remains uniform throughout the air gap. It is possible, however, that ac machines could make use of the so-called quiet technology that has been developed for permanent magnet motors, Gamble said. And air core machines could use helically wound, monolithic armatures with their fewer parts and bonded components to reduce noise further, he said.
Whitcomb said that superconductor motors can be configured for across-the-line starting. “The ac synchronous motors being designed today by American Superconductor will be capable of self-start by switching on full voltage at zero speed,” he said.
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, Gamble said. Thus, the complexity associated with changing speed and direction using controllable pitch propellers would vanish.
The Navy, having decided upon electric drive for its next warship, has left the door open for superconducting motors. Gamble, reinforcing Soto’s comment, said, “The present generation permanent magnet machines may be designed to allow retrofit of HTS components. This allows the Navy the option of retrofitability with moderate additional investment in its permanent magnet machines.”