Michael Richardson, American Superconductor’s program manager/commercial motors, and David Madura, product manager/HTS synchronous condensers, examine a prototype 5,000-hp HTS motor. Developed in Westborough, Mass., the motor passed a full-load test in January.
Superconductivity is the basis for various research programs now bringing prototype motors to the field-test stage. The result could be major changes in the designs of propulsion motors resulting in powerful, smaller and lighter electric marine propulsion systems.
In February 2002, the Office of Naval Research in Alexandria, Va., awarded American Superconductor Corp. in Westborough, Mass., an $8 million contract to build and deliver a 6,500-hp, variable-speed, 230-maximum-rpm high-temperature superconductor (HTS) motor designed specifically for ship propulsion. The HTS motor is expected to be approximately 5 feet long, 5 feet in diameter and weigh about 26 tons.
Gary Jebsen, submarine program technology manager at the ONR, explained, “We are not trying to build this motor as small as it ultimately can be built. It is designed as a technology demonstrator for a future larger machine. We also have an eye on future podded HTS propulsion applications where the size of a motor is particularly critical.”
Electric-propulsion systems give marine architects the freedom to locate the electric generators and their diesel or turbine power sources remotely from the propulsion motors. This arrangement eliminates the need for long propeller shafts. Another benefit is the ability to use podded propulsors.
Along with the growing commercial applications, mainly in cruise ships, electric propulsion got a major boost from the U.S. Navy in January 2000. The Navy said the decision to design the proposed DD21 land attack destroyer around electric drives and integrated power systems for reliability would free up “large amounts of internal space,” thereby “opening immense opportunities for redesigning ship architecture.”
“We always felt there were a lot of payoffs for electric ships in general, and the Navy was ultimately going to go that way,” Jebsen noted.
The long road to practical use
Superconductors are generally referred to as high-temperature (HTS) and low-temperature (LTS). David Paratore, general manager-electric motors and generators at American Superconductor, explained the critical temperatures for HTS are roughly between 30Â° Kelvin (K) and 80Â° K, while LTS critical temperatures are less than 10Â° K. When materials are chilled to or below their critical temperature, “they display the properties [of superconductors], while above those temperatures, they are just regular materials,” Paratore said.
Absolute zero, or 0Â° K, represents the point at which all molecular motion ceases. The scientists of 1911 predicted that, at absolute zero, metals with the proper interatomic structure would offer zero electrical resistance; that is, they would exhibit superconductivity. The original discovery was made after helium was liquefied for the first time at 4.2Â° K (-452Â° F) at atmospheric pressure. That made it possible to achieve zero electrical resistance in mercury wire and later in other metals at somewhat higher temperatures, such as lead at 7.2Â° K. Superconductivity applications became more practical over time as scientists discovered materials that became superconductive at higher temperatures.
In 1960 the first organic material was synthesized that exhibited stable conducting characteristics, and in 1979 a superconducting organic material was discovered. In 1986 researchers discovered an oxide that was superconductive at 30Â° K. A year later, they discovered yttrium barium copper oxide (YBCO), which becomes superconductive at 90Â° K. Other compounds were found that worked at even higher temperatures, including BSSCO (bismuth, strontium, calcium, copper and oxygen), which is superconductive at up to 110Â° K, and thallium compounds that performed at up to 127Â° K.
The steady stream of suitable materials for different superconductor applications continues. American Superconductor, which has targeted marine propulsion and the power industry, currently uses BSSCO, a material it has found suitable for use in power cables, ship motors or generators. However, a number of possible technological difficulties associated with superconductor motors are still being researched, such as possible brittleness of superconductor wiring subjected to high magnetic fields, combined with long periods of continuous operation.
Development programs exploring improved electric motor performance technologies for shipboard use have been underway at the ONR since the 1980s. The designs include the HTS synchronous motors, LTS DC homopolar motors and advanced permanent magnet motors. Jebsen listed some of the anticipated benefits of the HTS and LTS technologies: “It looks like they have the potential to be significantly smaller and lighter than conventional motors in large ship propulsion motor sizes. They will also have an advantage in efficiency, particularly at low speeds and low loads. And they offer a large improvement in power density over conventional AC synchronous motors.”
The ONR’s research and development has also been pursuing the DC homopolar motor technology, “which is more power-dense than a conventional DC motor because the magnetic fields are much higher with superconducting magnets,” he said. That current work uses LTS technology. Disadvantages of DC systems include the need for brushes and that the DC homopolar motors need high current at low voltage. Such systems would be difficult to integrate with the primary AC ship wiring and controls. But this technology has the potential to help the Navy make its ships quieter and harder to detect.
“The DC homopolar motor is probably the quietest motor you can envision,” Jebsen said. “In an application interested in absolute quiet, where you have fewer opportunities to silence a motor in other ways, you may tend to favor the DC homopolar.”
However, HTS motor designs offer other benefits, according to American Superconductor. “Because HTS wire can carry significantly larger currents than copper wire, these windings are capable of generating much more powerful magnetic fields in a given volume of space. Advances in coil design make it possible for a superconducting machine to match the power output of an equally rated conventional motor with as little as one-third the size and weight,” Jebsen said.
In addition to reductions in size and weight, ship designers are also looking for greater efficiency, Paratore said.
American Superconductor believes that the combination of those characteristics will be the key to a new series of ship propulsion motors. “On a 100 percent load full-throttle cruising, we are one-half to two percentage points more efficient than a traditional synchronous machine, and at really low loads, it can be as much as 20 percentage points,” Paratore explained. According to the American Society of Mechanical Engineers, projections of the efficiency of HTS motors are supported by research conducted several years ago at Rockwell Automation’s Reliance Electric subsidiary. The researchers concluded that “the efficiency losses of an HTS motor will be about half of those on an energy-efficient AC induction motor.”
Commercializing HTS motors
Over the last 15 years, various R&D programs have tested a variety of superconductor motor designs, including stationary HTS field windings, rotating HTS field windings, HTS rotor assemblies, and cooling components using liquefied nitrogen, liquefied helium or gaseous helium. Last year, American Superconductor built a 5,000-hp 1,800-rpm HTS motor for in-house testing that represented the largest HTS motor built to date. The tests were successful under full load at rated voltage, current and power. According to American Superconductor, the “system was peak-load tested to 7,000-hp at rated speed.”
“The larger the motor gets, the more advantage we have,” Paratore said. Ultimately, he expects the new 6,500-hp HTS motor project to be about half the weight of a conventional motor. The weight should keep coming down. “As the hp rating goes up, an HTS motor could be one-third to one-quarter as light,” he said. The size of an HTS motor compared with a conventional synchronous motor will be similarly reduced.
Stuart Karon, director of business development at ACS, noted that “chilling equipment is not needed on regular motors, so I guess that could be a negative for HTS motors. But that is more than offset by the rotor itself staying cool. Rotors are often a major issue for motor maintenance. In terms of less rebuilding of rotors, you are saving a ton of money with HTS.”
The American Superconductor HTS synchronous motor makes the rotor the superconducting element by using HTS wire in place of copper wire and employs a stator of conventional construction. That makes the rotor the only element that requires cryogenic chilling.
To operate as a superconductor device, the HTS elements must be maintained at the proper cold temperatures. The HTS cooling systems have proven to be reliable and efficient, Paratore said. American Superconductor currently uses off-the-shelf gaseous helium cryogenic cooling units because of their reliability and efficiency.
Karon noted, “That’s an important distinction. Working with gaseous helium is very safe. Working in liquid helium is less safe and much more expensive. HTS allows working in the gaseous state, and that makes a big difference in the cost structure.” Liquid helium has been used for many LTS applications at temperatures of 4.2Â° K.
One of the development challenges has been “getting the coldness into the motor,” Jebsen noted. It may take a week or so to get a motor from normal ambient temperatures down to operating temperature. Once the system is operational, multiple cooling units would be needed for reliability to keep the motor at the critical temperature, even during port layovers.
Total cooling-system failure would not immediately reduce superconductor motor output. “It’s a degradation of performance similar to power going off on a refrigerator,” Paratore said. The current thinking at American Superconductor is that if the cooling system failed, the HTS propulsion motor would probably provide power for half-speed operation for about five days, according to Paratore.
Jebsen concurred: “You would be able to run the HTS motor as a superconducting motor for a while, because it will take a long time to warm up. Once it passes the critical point, it is no longer superconducting, but it would still run [under reduced power] so you can limp home.”
The bottom line is if the current R&D program’s motors prove the commercial viability of superconducting motors, operational savings could generate a substantial market for the new equipment. A 1999 study found that conventional electric motors larger than 1,000 hp consumed about $64 billion worth of energy per year in the United States. Superconductor motors could cut energy costs considerably. Ultimately, market demand for superconductor motors will be driven by their initial cost and their actual efficiency in service.
For the maritime industry, commercial acceptance of superconducting motors for propulsion may be at hand. Said Paratore, “I think there is general acceptance that electric propulsion is the right way to go for many people. We don’t have to convince them to use electric motors. All we have to do is help them understand the value HTS brings.”