EXCLUSIVE: Superconducting motors: 2nd generation HTS wire boosts development
The lofty goal remains unchanged: develop high-temperature superconducting (HTS) technology practical for use in large industrial motors.
Ben Hothem, Rockwell Automation laboratory assistant, cools down the rotor of the 2-hp HTS motor (inside enclosure) in preparation for testing. Torque transducer and generator for the dynamometer are shown at right.
The lofty goal remains unchanged: develop high-temperature superconducting (HTS) technology practical for use in large industrial motors. However, a new direction is at hand. Second-generation (2G) HTS wires and coils promise to make these super-efficient machines cost-effective and available in the not-too-distant future. HTS technology’s premise is to virtually eliminate resistance to current flow in motor windings, allowing an HTS motor to cut power losses 50% compared to best traditional motors available today. In early June 2005, Rockwell Automation —in cooperation with the U.S. Department of Energy’s (DOE) “Superconductivity Partnerships with Industry” (SPI) program—successfully demonstrated first use of 2G HTS wire in a 2-hp Reliance Electric HTS motor. HTS wire remains the key to making this technology practical.
Rockwell Automation has a history of experience in the HTS arena. For more than 10 years, it has worked with the DOE’s SPI program and partner supplier companies in a series of HTS motor demonstrations that showed impressive progress, reaching 1,600-hp output in 2001—all using first-generation (1G) HTS wire. (Even higher output was demoed in a 5,000-hp HTS motor by another developer, American Superconductor Corp ., in July 2001, also using 1G wires.) However, cost per motor of first-generation wire was projected to exceed total cost of an existing motor; something new was needed, according to Dr. Rich Schiferl, Rockwell Automation’s director of advanced technology for Reliance Electric motors. “First-generation HTS wire technology was just too expensive to be successful for motors,” he told Control Engineering .
2-hp motor, temperature perspective
Dr. Rich Schiferl compares the size of conventional 400-amp copper wire (right) and a piece of second-generation (2G), 400-amp superconducting wire.
The HTS motor used for the 2-hp demonstration was a synchronous machine with superconducting field windings (rotor) and a conventional stator. Cooling for the HTS coils came from liquid nitrogen introduced into the center of the rotor and exhausted through the motor frame (for demo purpose only, see photo). It provided a temperature of about–321 °F (-196 °C) inside the rotor. In an actual motor, coolant would be recirculated. “In large industrial HTS Motors, the superconducting-coil coolant will be recaptured from the rotor and rechilled in a closed-loop cooling system so that no cold gas or liquid will escape from the motor,” says Schiferl.
High-temperature superconductivity is a very relative term. “High” refers to ability of materials to show superconductive properties at temperatures higher than near absolute zero (0 K). For 2G HTS wire, it refers to a still-mighty-cold region around–321 °F (77 K), but this “warmer” temperature also permits use of more commonly available (and less costly) liquid nitrogen as the cooling medium. Prior developments with 1G HTS wire mentioned above required further cooling to around 30 K to produce superconductivity. This also made motors with 1G wire more costly, since more exotic cooling was needed, such as helium gas or liquid neon, explains Schiferl. “Cost of cooling represents lost energy for HTS motors,” he says.
Toward kilometers of wire
2G HTS wire coil used in the 2-hp motor (left) is dramatically smaller and has higher performance than an older vintage 1G HTS coil of equivalent power rating.
The 2-hp demo motor used 14 meters (46 ft) of ceramic-based, coated conductor 2G HTS wire—wound into two rotating field coils—supplied by SuperPower Inc ., a subsidiary of Intermagnetics General Corp. Second-generation HTS wire provides performance superior to the previous generation and is expected to be cost-effective.
SuperPower currently produces 2G wires in continuous lengths up to 100 m, and this capability is reportedly growing rapidly. Actually, about 10 times the present continuous wire length will be needed for motors around 5,000 hp, where the large hoped-for energy savings can be achieved. Splicing will add to total wire length. “Kilometers of wire will be required for large HTS motors,” explains Schiferl.
Manufacturing the wire is a significant achievement. It relies on SuperPower’s proprietary MOCVD (metal organic chemical vapor deposition) process that is said to yield much higher production rates (throughput) and uniformity of performance than earlier pulsed laser deposition (PLD) processing. MOCVD technology has already been used in semiconductor wafer production, but the company believes it’s the first to adapt the process “to this extent” in fabrication of 2G wire.
When can we expect to see HTS motors at work? Schiferl replied that he believes in about five years HTS motors will be applied in industry in small numbers. Ramp up to wider use of such motors will come later. “Smallest unit sizes expected will be around 1,000 hp, with most at 5,000 hp and above. Also, 2G HTS wire available in the next five years will likely meet the necessary cost and length requirements,” he says. Electricity production and ship propulsion are likely first applications.
Five years doesn’t seem all that far away. While crucial tasks of HTS development need to be completed, we look forward to witnessing major progress milestones along the way.
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—Frank J. Bartos, executive editor, Control Engineering, firstname.lastname@example.org