April 2002 Excerpt
Old is New Again
Another Option in Electric Drive
By Leslie Spaulding
following article is based on a paper presented at the Third Naval Symposium
on Electric Machines in December. The paper titled A Study of the Magnetic
Field Effects Upon Metal Fiber Current Collectors in a High Critical Temperature
Superconducting Homopolar Motor, was authored by Lynn Petersen, Damian
Urciuoli, Thomas Fikse, Laura Stubbs, and Neal Sondergaard (all Code 812).
It was co-authored by Doris Kuhlman Wilsdorf, Matthew Bednar, Richard Johnson,
Jon Moore (all University of Virginia), Richard Martin, Wallace Elger (both
Noesis, Inc.) and Michael Heiberger (General Atomics Corp).
Testing is currently underway in support of another option for electric propulsion for the U.S. Navy Fleeta superconducting homopolar motor.
Past studies have shown that the use of electric drive for both combatant and large commercial ships would be advantageous. Recent efforts in Europe and the United States have focused on using alternating current (AC), such as the Integrated Power System, which was tested by the Division. However, engineers in the Electrical Systems Department of the Machinery R&D Directorate (80) recently wrote a paper describing the homopolar motor, in which they stated, Homopolar electric motors have many advantages over their AC counterparts including higher efficiency, smaller size for the same power level, simplicity of control, and lower acoustic noise. The motors produce smooth torque to a drive shaft through the interaction of direct current with the stators magnetic fields. The intensity of the stator field can be efficiently increased through the use of superconducting magnets. A homopolar motor lacks the inherent time varying forces that are a source of noise in alternating current electric machine concepts. Recent system studies have shown that a DDG 51-like surface ship with superconducting homopolar (SCHP) motor drive would possess range benefits up to three times greater than conventional drive.
The current testing is the result of what Neal Sondergaard (812) called the ebb and flow of technology. The superconducting homopolar motor is the relatively new application of recently-developed technology to what is essentially an old idea. A homopolar motor is a simple device which operates on direct current. It was discovered in the early 19th Century by Michael Faraday. The stationary part of the motor is the stator. The rotor is electrically connected to the stator by brushes. The current goes through the motor and interacts with the magnetic field to turn the rotor around. With the SCHP, a superconducting magnet is used to efficiently generate a very intense magnetic field. This strong magnetic field allows the motor to produce higher torque at smaller sizes.
The Division has been involved in SCHP technology development for approximately 30 years. The country faced an energy crunch in the early 1970s. Homopolar machines are very efficient (99%), so the Navy was interested in propelling its Fleet with this technology. The 963 Class destroyers were being implemented with gas turbines, which were less efficient when not operating at full power. The superconducting homopolar motors, as the principle component of an electric drive system, offered among many other benefits, a highly efficient way to couple the turbines with the propellers electrically to achieve efficiency through all ranges of speed. The Annapolis Lab worked on this project throughout the 1970s and early 1980s.
Engineers at the former Annapolis Laboratory built a 400-horsepower system, which went to sea in 1980. Simultaneously, they worked with contractors General Electric and Garret Air Research to build a 3,000-horsepower system, which went to sea in 1983. The systems were demonstrated on the Jupiter II Test Craft. The technology at this time required liquid helium systems to keep the magnets cold and superconducting. Liquid metal brushes were also required. Although these demonstrations proved the feasibility of the propulsion concept, issues associated with long-term reliability of the advanced technologies remained.
Since the early 1980s, developments in the advanced technologies continued. Under the ALISS Minesweeping Program in the 1990s, great strides were made in the engineering development of both magnet and cryogenic cooling technology. The cryocooler work eliminated the 1980s logistics requirements of liquid helium production and distribution and reduced the cost, size, and complexity of the cooling system. In 1987, high temperature superconductors were discovered and with their discovery came the possibility of greatly simplified applications of superconductivity. In 1995, ONR funded R&D in HiTc materials, which resulted in a replacement of the low-temperature magnet system with a high-temperature magnet system in a NSWC 400-hp machine.
In 1997, the liquid cryogen-based cooling system for the high-temperature superconducting magnet was replaced by a conductively cooled, cryocooler-based system. In 1999, a structureborne and airborne acoustic assessment was conducted on the cryogenic support equipment for both the minesweeping magnet system and the 400-hp homopolar generator by the Division.
Prior to the current study, the SCHP motors developed by the Navy all employed liquid metal brushes. Supported by DARPA and NAVSEA, liquid metal wetted fiber current collectors were demonstrated at full-scale in a test rig at NSWCCD in 1995. Liquid metals, however, require high quality cover gas systems to protect the liquid metal from oxidation or hydrolysis.
In the late 1970s and early 1980s, researchers at the University of Virginia, Westinghouse, and University of Paris showed that fiber brush technology was better than conventional brushes and offered an alternative to liquid metals for homopolar motors. Relatively high current densities and good wear rates at reasonable efficiency were possible with these brushes. However, initial attempts at implementing the technology in these high-speed machines were disappointing.
Metal fiber brush research continued at the University of Virginia and NSWCCD. Under SBIR sponsorship, Noesis, Inc. teamed with the University of Virginia to continue the evolution of this technology.
The current work being done at the Division involves exploring the use of these brushes for both a forward fit into the superconducting homopolar motors for tomorrow, as well as a retrofit for current Fleet motors that use carbon brushes.
In Code 81s Small Motors Lab in Bldg. 87, metal fiber brushes were put into one of the 400-hp, in-house built superconducting motors. This motor has been used in the lab since the earlier 1980 demonstration on the Jupiter II. Its been modified to have newer high temperature superconducting magnets, as well as the fiber brushes. Because of its small size, this motor offers a good compromise of realistic brush parameters for brushes at a moderate cost. The data obtained will help validate and develop design models for the brushes and future large-scale motors.
The brushes are operating as though they are in a full-scale machine. Were not making a lot of power with these things, but we're running them under the same conditions that they would see if they were running in a big propulsion motor, said Sondergaard. Were developing models of how they ought to behave and obtaining data to further retire any risk associated with using these brushes. Code 81 engineers plan to run this motor for an extended period to validate the long-term performance of the metal fiber brushes.
General Atomics, through a Congressional Plus-Up, has been funded to build a 5,000-hp SCHP motor. They are using brush data collected by Code 81 engineers to help in the design of their machine. Division personnel will continue to provide technical data and general support to the GA design effort.
This gives us a chance to demonstrate integration of these advanced technologies at a higher power scale, said Sondergaard. The General Atomics motor is in the beginning design state now and will be built and tested in the next three years. If everything proves out on the 5,000-hp, a full-scale motor will be built and tested, providing an efficient, quiet option for electric propulsion in the Fleet.
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