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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