Coil gun drives high-speed-rail technology

DN Staff

September 25, 1995

8 Min Read
Coil gun drives high-speed-rail technology

Albuquerque, NM--At first glance, Sandia National Laboratories' linear induction motor appears an unlikely candidate to solve the country's transportation woes. Surrounded by a heavy steel truss, it looks more like a shortened particle accelerator.

But when physicists demonstrate it, all skepticism disappears. A wheeled aluminum plate weighing 35 pounds streaks through the middle of the truss as if shot from a cannon. In a distance of just 13 feet, the plate's velocity reaches 33 mph.

And that's merely a fraction of the technology's ultimate potential. The offshoot of a Sandia coil-gun program, the linear induction motor has moved steel plates and cylinders at a speed of a kilometer per second. Physicists estimate that the technology could launch satellites at the incredible speed of six kilometers per second. The National Aeronautics and Space Administration has even considered it as a means to launch the Space Shuttle.

Now, however, Sandia scientists propose an unexpected new role for the technology: high-speed rail. A project called SERAPHIM (for SEgmented RAil PHased Induction Motor) would employ linear induction motors to propel trains at speeds of 200 mph or more. Using the new technology, high-speed trains could operate on existing rights-of-way, create less rail wear, reduce the need for track maintenance, offer improved grade-climbing capabilities, and provide a smoother, quieter ride than is now available.

What's more, the scientists say their system offers advantages over much ballyhooed magnetic levitation technology. "It requires neither powered guideways nor superconducting magnets," says Barry Marder, distinguished member of Sandia's technical staff. "As a result, this system offers a low-risk, cost-effective route to very-high-speed ground transportation."

Faster than a bullet. Unlike conventional high-speed-train technologies--such as those used on the French TGV (Design News, 7-24-95) and the Japanese Bullet Trains--the Seraphim concept employs passive steel wheels. No traction is needed--either for acceleration or braking.

Instead, the SERAPHIM concept propels the train through magnetism. Electrified coils on the train react against a segmented reaction rail mounted on the roadway. The reaction rails can be one of two types: single- or double-sided. In either case, the coils for the linear induction motor mount on the train.

SERAPHIM's linear induction motor provides motive force by magnetic interaction with the reaction rail. Unlike linear induction motors of the past, however, it operates not by embedding flux in a conductor, but by excluding it. In the double-sided version, pairs of closely spaced coils on the vehicle straddle a segmented aluminum rail. As the train moves forward, on-board sensors "watch" for the approach of an aluminum reaction plate. When the coils overtake the plate, the system's modulator pulses current to the coil, creating a magnetic field, which induces surface currents on the plate. Those surface currents repel the coil.

In essence, Sandia scientists say, the pulsed coils push off the edges of the plate. How? At high frequency operation, the flux has insufficient time to penetrate.

Operating in this fashion, each coil can produce 3.5 kN of thrust. With 30 coil pairs mounted on each powered railroad car, the system can provide 6 MW (8,000 hp).

During operation, each powered rail car would employ two gas turbine power units. Power modulators would supply the pulsed voltage to the coils, with each modulator powering six coil pairs.

A sense-and-fire circuit would control the pulsing of the power modulators. Each modulator, in turn, would provide 1.2 MW average power in pulses 2 ms half-width at frequencies ranging from 100-230 Hz.

Sandia scientists say that the system's maximum cruise speed is limited primarily by available power, aerodynamic drag, and grade. Cruise speeds of 200 mph on straight and level track could be achieved with a supplied power of 6.3 MW (8,500 hp).

In their New Mexico labs, Sandia scientists have constructed working models of the SERAPHIM concept. Using a three-stage motor, they accelerated a 14.4 kg aluminum plate along a 4m track to speeds of 15 m/sec. In the test, they achieved peak thrusts of 18 kN per coil, using coils with two windings of 51 turns each. The windings employed copper straps insulated with DuPont's Kapton(R) film and Dacron(R) polyester.

Air travel alternative. Sandia's concept isn't the first of its kind. During the 1970s, engineers working for the Federal Railroad Administration conducted tests on a similar concept at the Transportation Test Center in Pueblo, CO. Using demonstrator vehicles with linear induction motors and passive steel wheels, trains reached speeds of 200 mph.

By developing the new linear induction technology, Sandia scientists have resurrected the idea, but have added one important twist. Up to now, linear motors have had inherent velocity limitations related to their lengths. That is, the longer they were, the greater their speed. But by employing rapidly-pulsed magnetic fields and segmented reaction rail--instead of low-frequency fields and continuous rails--the scientists eliminated the inherent speed limitation.

"There's really no practical limit to the speed you can achieve with this technology," Marder says. "If you had the power to do it, you could launch the train at any speed. All it requires is faster switching of the coils."

The technology, Marder says, also offers other important advantages:

It requires no new rail technology. Its construction would cost about one-fourth as much as a comparable maglev system.

  • Its track would be subjected to less wear, because SERAPHIM doesn't rely on wheel adhesion.

  • It would offer greater grade climbing capabilities, because it doesn't suffer from wheel slippage.

  • Its locomotives would be lighter, because no extra weight is needed to improve traction.

Sandia scientists hope to test the concept at the Pueblo, CO, Test Center. Plans are to build a "ladder-like" reaction rail with aluminum plates that lie horizontally within the track bed. This configuration would provide single-sided linear motor technology, allowing downward-facing coils to "push off" the aluminum plates. Though it's not as efficient as the double-sided configuration, it should easily allow test trains to run at 200 mph.

Ultimately, Sandia scientists believe that the technology could offer a viable alternative to air travel. Because airline costs are highest for planes as they ascend and descend, short flights might one day be less cost effective than rail travel. Concludes Marder: "A train travelling at these speeds could certainly compete."

A smokeless Space-Shuttle launch?

Since the glory days of the Redstone rockets, NASA has launched its space missions amid great plumes of smoke and fire. Now comes a completely different idea.

Instead of a great belch of smoke, some NASA scientists are looking at a huge pop of electrical current to launch the Space Shuttle. Using Sandia's coil gun technology, they could replace the first stage of the Shuttle with a re-usable "MagLifter," they say.

If it were ever deemed feasible, the MagLifter would propel the Space Shuttle up a mountainside using the magnetic coil-gun technology. Employing a technology similar to SERAPHIM's, electrical coils on the ground would react magnetically with specially-designed plates on the Shuttle's first stage. This magnetic propulsion scheme could ultimately push the Shuttle up a track to a speed of 600 mph, giving it sufficient speed for a mountainside launch. "Then you could bring the launch vehicle back down for the next launch," says Barry Marder, distinguished scientist for Sandia National Laboratories.

According to the researchers, the MagLifter could supply sufficient thrust for higher-speed launches, were it not for excessive G-force build-up on the crew. Using the coil gun for satellite launches, they estimate a build-up of more than 2,000 Gs on the satellite--a figure that's obviously excessive for passengers.

How likely is the MagLifter concept to see action? All Marder will say: "We've proposed this propulsion scheme to NASA, and they're considering it."

Sandia's linear induction motor isn't the only electromotive train technology to employ DuPont's Kapton(R) polyimide film as an insulation medium.

The material also plays a vital role in the most powerful universal four-axle locomotive in the world, the Re 465. Designed by ABB Transportation Systems Ltd., Zurich, Switzerland, and Swiss Locomotive and Machine Works, the 82-ton engine has a top speed of 140 mph and can pull a 650-ton train at 60 mph up a 2.7% gradient. The new locomotive runs on a Swiss trans-Alpine rail link between northern and southern Europe. For the Re 465's traction motor, DuPont engineers developed a special, corona-resistant version of the material, known as Kapton CR. Kapton CR helps insulate the copper conductors in the motor's stator from steep voltage peaks that can lead to partial electrical discharges, or corona. These discharges can break down organic insulation materials.

To develop Kapton CR, DuPont engineers undertook a joint program with traction motor manufacturers ABB and Siemens AG. The result: Corona resistance is said to be orders of magnitude better than that of standard Kapton, and thermal conductivity is twice as high.

Corona resistance is now said to be 100,000 hours at 20 kV/mm at 50 Hz. The propulsion system employs power thyristors and three-phase ac motors.

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