July 2006 Mechanical Engineering Departments: Tech Focus, Pt. 2

This section was written by
Associate Editor Alan S. Brown.

Power Transmission and Motion Control

Small, but Powerful

Imagine a 4- to 10-megawatt generator with only 10 to 20 percent of the size and weight of today's portable systems. That's exactly the type of innovative power module that high-temperature superconductors make possible, according to Larry Long of Long Electromagnetics Inc. in Pittsburgh.

Long's entire 5 MW module—generator, turbine, power conditioner, and cooling system—measures only 14 x 7 x 7 feet (roughly 4 x 2 x 2 meters), small enough to containerize or put on a small truck. At just under 6,000 pounds (or about 2,700 kilograms), the entire module weighs 40 percent less than a conventional generator alone. It's small enough to truck in to provide instant power for natural disasters, spot power shortages, remote worksites, or military deployments.

The high current density of superconducting wires powers this 5-megawatt generator. It is small and portable enough to truck to emergencies.

The module features several innovative technologies. This starts with a high-speed generator based on high-temperature superconducting wires. Because the wires conduct far more current than copper, Long can make the generator much smaller than conventional units of similar generating capacity. The wires are commercially available, and the equipment needed to chill them to superconducting temperature
(-240°C) has grown cheaper and highly reliable over the past decade.

One advantage of a smaller generator with lighter windings is that it can rotate at extremely high speeds. Long's unit typically runs at 12,000 to 15,000 rpm, four to five times faster than most conventional generators. This is fast enough to link it directly to a small, high-speed commercial turbine engine without using a gearbox.

The module runs on kerosene, jet fuel, natural gas, or other regionally available fuels. It includes its own air cooling system and an electronic power conditioning and fault management unit.

Long has built his first prototype and expects testing to begin within the next few months. He has good reason to be optimistic about the results. Long has designed several high-speed multimegawatt superconducting military and commercial generators. He is currently working on two projects for the U.S. Air Force and NASA, including a 5 MW, 15,000-rpm generator powered by a Rolls-Royce AE1107 turbine that resembles the prototype he just completed.

No Gears or Batteries

Electrical drives use absolute encoders to determine the number of completed turns and the position within the last turn. This feature comes in handy for positioning, and also to eliminate the need for a reference run in case of a power interruption.

Most encoders rely on either a multistage gearbox or electronic devices, but both have their limitations. Gearboxes are subject to wear, especially when used with high-speed motors that alter speeds quickly. Nor does their geometry permit large through-hole hollow shafts (the type used in gantries, oil rigs, and steel production). Moreover, precision is limited to 12 bits (4,096 positions).

Electronic absolute encoders resolve many of these problems by using rotating magnets instead of gears. This allows 16-bit precision (65,536 positions). Unfortunately, their electronics require battery backup. Not only do batteries wear quickly at high industrial operating temperatures, but their use is restricted in areas where there is a risk of explosion. Such systems may also send erroneous signals when shocked by braked motors.

The answer, according to a German manufacturer, Hübner Electromaschinen GmbH of Berlin, is a new type of microgenerator-based encoder that eliminates gearboxes and batteries. Others have tried this approach previously, but Hübner claims that its unit generates more energy than earlier versions, making it easier to install and use in nasty industrial environments.

The system consists of two parts. One comprises a magnetizable leaf spring within a coil embedded in a plastic housing. The second is a cobalt-samarium magnet pair of opposing polarity on a disk that rotates with the encoder shaft. As the magnets pass the leaf spring, they pull it until spring tension causes it to snap back. This generates a very strong voltage pulse in the coil that surrounds the spring.

The unit has resolution as good as the best electronic systems, uses no wear parts, and resists shock. It handles high continuous speeds, even in stop-and-start operation, and can be operated in explosion-hazard areas.

Deep Freeze on Friction

Two German companies, Adelwitz Technologiezentrum GmbH (ATZ) of Adelwitz and Siemens AG, have recently demonstrated bearings that could bring the dream of contactless bearings closer to reality. Both use ceramic-based, high-temperature superconductors, which can be chilled by commercially available cryocoolers.

The manufacturers are looking at applications in energy equipment, including flywheels.

Superconductors, materials that show zero resistance to electrical current, are typically used for their electrical properties. Yet their magnetic properties—including the ability to lock and suspend a conventional magnet in place above them—promise to usher in a new generation of extremely efficient contactless bearings.

Siemens and Nexans have developed an industrial bearing based on high-temp superconductors.

The bearings produce no friction and support high rotational speeds. Unlike conventional magnetic bearings, suspended rotors automatically center themselves among the surrounding superconductors of the cylindrical stator. This eliminates the need for active stabilization controls.

ATZ's bearing has a 200-millimeter outside diameter. The company's founder, Frank Werfel, said it can support a 10,000-newton axial load (equivalent to levitating a 1 metric ton rotor) and a 4,700-newton radial load.

For the superconductor to work, ATZ runs liquid nitrogen coolant at 72 kelvin through the walls of the cylinder that holds its 28 superconductors in place. The bearing weighs 55 kilograms.

Werfel said he is collaborating with Magnet-Motor GmbH to build a 5-kilowatt-hour, 250-kilowatt energy flywheel storage system. The company is also working with Boeing Co. and German utility E.ON AG on flywheel development.

Meanwhile, Siemens has teamed with Nexans Superconductors GmbH to introduce a superconducting bearing for use with a 4-megavolt-amp generator.

In some ways, the Siemens bearing is a larger and more robust cousin of ATZ's device. It handles higher radial loads and uses a reliable industrial cryocooler for full-time operation.

According to a Siemens researcher, Wolfgang Nick, the vacuum housing provides so much insulation that the bearing could spin for two hours without active cooling before its capacity began to degrade.

The bearing handles speeds of 3,600 revolutions per minute, so it can generate a 60-hertz alternating current. An electrodynamic damping unit attenuates shaft vibrations induced by resonant frequencies. This limits vertical and horizontal shaft oscillations to less than 0.01 millimeter.

Nexans plans to leverage this design to build a flywheel-ready bearing before the end of the year. Meanwhile, Siemens researchers continue to test superconductors in new applications. The company recently began running its first high-temperature superconductor generator at its testing facility in Nuremberg. The unit produces 4,000 kilovolt-amps at 3,600 revolutions per minute.

In addition to flywheels, developers of superconducting bearings envision applications ranging from motors and generators to turbines and compressors.