HTS Motors Promise Savings

After 17 years of development work aimed at so-called “high-temperature superconducting (HTS) motors”—which promise notable improvements in electric motor size, weight and operating efficiency—Rockwell Automation Inc.(www.rockwellautomation.com) believes the payoff may now finally be in sight.

The Rockwell effort is targeting large industrial HTS motors at 1,000 horsepower and above, for which the technology is expected to be cost-effective. “We’re hoping to have [an HTS] motor into a pilot industrial application within the next three or four years,” says Rick Schiferl, director of advanced technology for Reliance Electric motors, at Rockwell’s Advanced Development Laboratory, in Euclid, Ohio. A commercial version could be on the market within a year or two after that, Schiferl says. “So it’s not next week, but it is relatively soon.”

One reason for the optimism is the Rockwell lab’s recent successful demonstration of a 7.5 horsepower HTS motor with coils made using second-generation, or 2G, superconducting wire provided by SuperPower Inc. (www.igc.com/superpower), a Schenectady, N.Y.-based subsidiary of Intermagnetics General Corp. Rockwell is working with SuperPower on the HTS motor project, in cooperation with the U.S. Department of Energy’s Superconductivity Partnerships with Industry (SPI) program.

Under the federally funded initiative, Rockwell has built and demonstrated motors up to 1,600 horsepower using earlier, first-generation, or 1G, HTS wire. But the 1G wire is now deemed to be too expensive for application in motors. The 2G wire, made of a ceramic material, can be manufactured using a thick-film deposition process that is less labor intensive and less costly than the powder-based production method used for the 1G wire, Schiferl says. In high-volume

production, the 2G wire is ultimately expected to be three to four times less expensive than the 1G wire. That should be sufficient to produce large HTS motors that are cost-competitive with conventional electric motors, according to Schiferl.

All things are relative, and that’s certainly the case when it comes to the term, “high-temperature,” as it relates to superconducting electric motors.

Superconductivity—or the ability of certain materials to conduct electric current with nearly zero resistance—in fact occurs at very low temperatures. The phenomenon was first observed in 1911 by Dutch physicist H. K. Onnes, using wire made of elemental mercury cooled to 4 Kelvin, or about -268 Celsius, the temperature of liquid helium.

But the term “high-temperature superconductivity” was born in 1987, after the discovery of a class of ceramic materials that could superconduct at temperatures up to 90 K. This meant that liquid nitrogen, at about 77 K, or –196 C, could be used as a coolant, making the use of superconductivity more practical for a range of proposed commercial applications, including motors. “As soon as the new materials were discovered, we had a program started in the 1988 timeframe,” Schiferl relates.

Keep it cool

In the 7.5-horsepower motor recently demonstrated by Rockwell, liquid nitrogen is introduced into the center of the rotor to cool the superconducting coils, then exhausted as nitrogen gas through the motor frame. But in high-horsepower commercial HTS motors, the coolant will be recaptured, re-chilled and recirculated in a closed-loop system.

Schiferl expects rapid progress toward higher-output HTS motors as SuperPower ramps up its ability to produce the 2G wire in lengths longer than the current 100 meters. At that length, too many wire splices would be required for a high-horsepower motor, impeding performance. “But when the wire is produced at lengths of about 1,000 meters, we’ll be able to build a sizeable motor, and that’s when energy savings will really add up,” Schiferl says.

Indeed, energy losses in HTS motors are expected to be about half those seen in conventional motors. That means that the 97 percent efficiency range typical of today’s high-horsepower motors could be improved to about 98.5 percent with an HTS motor, says Schiferl. For a 5,000-horsepower HTS motor running continuously, the savings would be about $50,000 per year based on California energy prices, he notes. HTS motor size and weight will also be about half that of conventional motors, he adds.

Other companies, including Germany-based Siemens AG (www.siemens.com), are also working on high-horsepower HTS industrial motors. Initial applications are expected in utilities, petrochemicals and other industries that rely on large motors running continuously.

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