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Big Superconducting Motor Passes Its First Tests

08 July 2009

After decades of planning and designing, real motors that use superconducting elements have been built and are now being tested by the U.S. Navy. American Superconductor Corporation and its partner Northrop Grumman say they have completed successful full-power tests of a 36.5 MW (49,000 hp) high-temperature superconductor (HTS) ship propulsion motor at a naval site in Philadelphia.

HTS ship generator
HTS ship generator

The motor uses coils of HTS wire that carry 150 times the power of similar-sized copper wire, yet is less than half the size of conventional motors designed for the new series of U.S. Navy DSG-1000 guided missile destroyers. The motor reduces ship weight by approximately 200 metric tons; thus newer ships built with superconducting motors will be more fuel-efficient and can free up space for other uses.

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All photos courtesy of American Superconductor

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High Temperature Superconductor (HTS) material was first discovered in 1986. The resistance-free conductors are made of ceramic materials that exhibit superconducting properties at “high” temperatures between -250° and -150° C (In contrast to low temperature superconductors, which operate below -262° C. Both high and low temperature superconducting wire need to be cooled to cryogenic temperatures in order to exhibit the property of superconductivity, but HTS systems require less expensive cooling systems than low temperature superconductors. Liquid nitrogen at -200° C can be used for some HTS applications.

Details of the cooling system for the 36.5 MW motor were not revealed, but it is believed to be similar to a prototype 5 MW motor built and tested five years ago. The smaller motor is cooled with a refrigeration system that circulates helium gas in a closed loop to maintain the HTS field winding at cryogenic temperature. The performance of off-the-shelf cryocoolers has improved significantly in the past decade and have made HTS motors more feasible.

Standard motor (rear) and HTS motor
Standard motor (rear) and HTS motor

Successful tests

“The successful load test of our HTS motor marks the beginning of a new era in ship propulsion technology,” said Dan McGahn, senior vice president and general manager of AMSC Superconductors.

“This motor provides the U.S. Navy with a truly transformational capability relative to size, stealth, endurance and survivability, providing our Navy with a clear performance advantage for years to come.”

A typical naval destroyer will need two 36.5 MW engines for propulsion. Building such a motor with conventional technology results in an engine that is four to five times heaver than an HTS motor. The HTS motor operates at 120 rpm and at a temperature of approximately 32K, which is -241°C. The cryogenic cooling system power consumption is less than a few percent of the total losses in the machine.

Degaussing coils

Cruise Ship
Cruise Ship

Earlier last year, the Navy successfully installed an HTS degaussing coil onboard the USS Higgins. Powered by American Superconductor HTS wire and magnet cable technology, the coil system will undergo sea trials during the next two years.

Degaussing coils, a vital part of today's military ships, are used to make military ships magnetically “invisible.” They are used primarily a countermeasure to the threat of magnetic influenced mines. Since ships are mainly constructed of steel, they disturb the earth's natural magnetic field, which can be detected by sensors and weapons such as naval mines. Degaussing coils are composed of a network of electrical cables installed around the circumference of a ship's hull, running from bow to stern on both sides. Electrical current is passed through these cables to counteract the ship's inherent magnetic field, controlled in such a way so that the ship is magnetically invisible. HTS technology can reduce the weight of the ship's degaussing system by up to 10 times that of a copper cable solution.

The U.S. Navy says it has invested more than $100 million in the development of HTS technology, paving the way not only for use in Navy ships but also in commercial vessels, such as cruise liners and liquefied natural gas (LNG) tankers.

HTS rotating machine technology is also being applied to the renewable energy industry. Wind generator systems utilising HTS wire instead of copper wire are expected to be much smaller, lighter and more efficient than current systems. This will lower the cost of wind-generated electricity, particularly for offshore wind farms.

To visit the American Superconductor website and watch video clips of the ship propulsion system and superconductor motor testing, CLICK HERE


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