Navy Scientists Demonstrate World's Highest Power Superconductive Motor

12/19/1997 - 77-97r
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Scientists and engineers at the Naval Research Laboratory (NRL) and the Naval Surface Warfare Center (NSWC) - Carderock Division have set a new world record in superconductive motor performance using high temperature (Tc) superconducting field magnets. This record-setting performance was demonstrated in a laboratory at NSWC/Annapolis, where the motor produced an output of 104 horsepower (hp) with its high Tc field windings cooled to 77° Kelvin (K) or -320.8° F using liquid nitrogen, according to Mr. Don Waltman, NSWC/Annapolis. The previous known record for a motor having a high Tc field winding and operating at a temperature of 77 K was 5 hp. At a field winding temperature of 28 K (-438 degrees F) using liquid neon, the motor produced 230 hp of shaft power and at a temperature of 4.2 K, cooled with liquid helium, the machine produced 320 hp.

The motor demonstration may have direct impact on ongoing Navy programs in minesweeping, electric drive and superconductive magnet energy storage. There are also a host of other applications which will benefit including: transportation, electric power transmission and distribution, magnetic resonance imaging and other medical devices. NRL has been responsible for the development and testing of HTS conductors used in the magnets.

The field magnet for the motor consists of six, discrete coils wound with high Tc superconducting wire made with bismuth strontium calcium copper oxide (2223) material. The superconducting wire and the coils were made by American Superconductor Corp., an industrial partner in this research, to specifications developed by NRL and NSWC, explain Drs. Donald Gubser and Louis Toth, of NRL's Materials Science and Technology Division.

The performances of these new superconducting coils represent significant progress and improvement over high Tc superconducting coils that had been previously installed and tested in the same motor in the fall of 1995 when motor outputs of 167 hp and 122 hp were measured at field winding temperatures of 4.2 K and 28 K, respectively.

The NSWC motor being used to demonstrate the performance capabilities of high Tc superconductors and coils was designed and built at Annapolis and is a reduced-scale demonstration model for an electric propulsion machine for Naval ships. The motor is a dc homopolar or acyclic machine whose original design and construction used field magnets fabricated with niobium titanium superconducting wire, which is the most widely used superconducting material for magnet applications. Niobium titanium superconducting wire belongs to the class of superconducting materials commonly called low temperature (low Tc) superconductors which are superconducting only in the temperature range of 20K or less. In most applications, such as the magnets for magnetic resonance imaging systems used by many hospitals, the niobium titanium superconducting magnets are cooled using liquid helium. In contrast, the high Tc superconducting materials offer the advantage of being cooled with liquid nitrogen, which is more plentiful and less costly than liquid helium.

The NSWC homopolar motor was selected to demonstrate the performance of the high Tc materials, because it is a fundamental property of homopolar machines that there are no forces developed on the field winding of the motor in reaction to the electromagnetically induced torque in its armature (rotor). This property, therefore, reduces the design complexity of the superconducting magnets (both low Tc and high Tc) and the suspension components to structurally support the magnets in the machine.

In addition to the use of superconducting magnets, the NSWC homopolar motor employs advanced technology electrical current collectors or brushes. These current collectors which were designed and developed at NSWC employ sodium potassium liquid metal to transfer electrical current from the stator of the machine to its rotating armature. Liquid metal current collectors have the advantage of being able to operate very efficiently at high levels of electrical current. For the results reported here, the current collectors of the homopolar machine performed reliably at values of electrical current in excess of 40,000 Amperes.

Plans for the future are to continue to use the NSWC homopolar motor as a test bed for high Tc superconducting magnets. Currently the high Tc field winding is being redesigned and modified to be cooled using a cryocooler refrigerator. A cryocooler is a commercially available, low temperature, mechanical refrigerator that can cool magnet systems to a temperature range of 4 to 77 Kelvin without the use of liquid cryogens such helium, neon, or nitrogen. When completed, this facility, in addition to being the world's first demonstration of a superconductive motor having a field winding cooled with a cryocooler refrigerator, will provide the capability to measure the performance of the high Tc field winding over a broad range of temperatures.

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