THE REVOLUTIONARY DUALMODE TRANSPORTATION SYSTEM
Chapter 9
Magnetic Levitation and Propulsion
LEVITATION ISN’T JUST A MAGIC TRICKIf it were magic, magnetic levitation would be a good trick because the vehicles it levitates are a lot heavier than is the magician’s beautiful assistant. Repulsion maglev was first proposed by US rocket scientist Robert Goddard, and described in the November 1909 issue of SCIENTIFIC AMERICAN. It took us a half-century to use Goddard’s rocket inventions, and it will be over a full century before we will use his magnetic-levitation inventions significantly.
Magnetic levitation (maglev) is a well-developed and tested modern technology. It is practical, efficient, safe, quiet, and economically sound. It has already carried a total of over two and two-thirds million paying train passengers in Japan, Canada, and Germany, without an accident (a safety record railroad tracks could never meet.) In October 1993 the Transrapid 07 maglev train ran 279mph at Emsland, Germany. Another type of maglev train, the ML-500 ran at 343 miles per hour in Japan on April 14, 1999. The American Maglev Star train, which is being proposed for installation between the Kennedy Space Center and Cape Canaveral, is planned for speeds up to 350 mph. Another maglev train recently went into operation at Shanghai, China.
But all is not rosy in the railroad business. In recent years a number of maglev trains have been planned then cancelled. As we saw in the previous chapter, coupled trains have major disadvantages that make that concept largely obsolete whether the cars are supported by steel wheels on rails or by maglev. For that reason conventional railroads are dying and I predict that maglev trains will also die.
Currently the association of maglev with the disadvantages of trains is giving maglev technology an undeserved bad name. But divorced from the old concept of coupled trains, magnetic levitation per se is a wonderful technology, and the coming dualmode guideways will be a perfect application for it. Dualmode maglev cars will be physically independent, separately owned, separately powered, and will go their separate ways, just as vehicles on our highways do.
In addition to speed, some of the other advantages of magnetic levitation are obvious: If the cars aren’t touching the guideway the wheels or tires won’t be wearing out or going flat. And the guideways won’t wear either, so we won’t have the equivalent of broken rails or potholes. The noise and vibration generated by steel wheels on rails, or tires on highways, will also be eliminated. Floating on magnetic force is like floating on air, or flying. At this time (April 2006) Google shows over a million items under the word “maglev.”
HOW MAGLEV WORKS
I think it was Robert Fulghum who wrote, “All I need to know is what I learned in kindergarten.” Not quite: To understand maglev we need to go several grades higher; to where we learned, “Like poles of magnets repel and unlike poles attract.” There are two basic types of maglev system, and a number of variations in each of these. One type supports the cars by magnetic repulsion, and the other type supports them by magnetic attraction.In some attraction-maglev trains the sides of the cars extend down outside and below the edges of raised guideways. In the levitated position the magnets in the car sides are still slightly below magnets in the sides of the guideway. As you will visualize, in this configuration, magnetic attraction lifts the car up above the guideway rather than pulling it down to the guideway. Because of this wrap-around requirement, attraction maglev is probably unusable in the guideways since it makes the necessary full-speed switching from one guideway to another difficult if not impossible. Therefore repulsion maglev is assumed throughout this book.
ENERGY REQUIRED FOR SUPPORT
Lifting the cars off the guideways initially will require a little power and energy. As an example: It would take just less than a half horsepower to lift a one-and-a-half-ton car to a three-inch operating height in three seconds. But the requirement for initial lifting power would last for only several seconds, no further output power or energy will be required to hold the cars at operating height.
If that statement sounds like getting something for nothing, think about the studs in the walls of a house. They require no power or energy in order to hold the roof up. Permanent magnets and superconducting magnets are likewise 100% efficient in this respect. I have a novelty gadget that floats a ballpoint pen in the air by means of permanent magnets. It is ten years old, and the pen floats just as high now as it ever did; yet it has no battery or electric power cord. This isn’t violating any laws of physics. Work (energy) equals Force times Distance. In magnetic levitation there is a “force,” the weight of the object being levitated, but there is no further change in “distance” (levitation height) once the car is lifted, so no further energy is expended.
But unlike superconductors and permanent magnets, ordinary electromagnets have electrical resistance and are therefore not a hundred percent efficient. It they are used our maglev system would require some continuing input electric power in order to maintain levitation.
INDUCTIVE MAGLEV
This type of maglev system uses permanent magnets in the cars and passive shorted conductor loops in the guideways, or just a solid electrically conductive but non-magnetic plate in the guideways. It requires no electrical input for levitation, but levitates only when the cars are moving. The magnets in the moving cars induce electric currents and associated magnetic fields in the guideway conductors. These guideway fields repel the fields of the moving magnets. In this “inductive maglev” the electrical resistance of the guideway conductors indirectly produces some magnetic drag force that drains a little mechanical energy from the moving cars. A small additional propulsive force from the linear motors is required to counter this drag force. See SCIENTIFIC AMERICAN, January 2000. It should be noted however, that at high speeds the aerodynamic drag on the vehicles is many times the small magnetic drags that are inherent in some maglev systems. One way of looking at the efficiency of such dynamic support is by “Lift-to-Drag ratio,” or (L/D), a parameter widely used in aeronautics. The L/D of the Bechtel maglev system is said to be 100, and that of the Foster-Miller maglev system is 170. The L/D of modern jet transport airplanes averages only 18 to 20.
SUPERCONDUCTING MAGLEV
In theory the efficiency of superconducting magnets is 100%. One example of superconducting maglev is the Miyazaki test track in Japan, which propelled a vehicle at 268 mph back in the mid 1990s. Gordon Danby and James Powell of the United States invented superconducting maglev (Ref: US Patent #3,470,828). Its major advantage is much higher magnetic field strengths than are possible with even the best room-temperature permanent magnets. For instance, In MRI machines without superconductivity a flux density of 0.4 tesla is achievable only with a permanent magnet weighing many tons; but with superconductive magnets, 2.0 tesla is readily achievable.
But superconducting maglev also has many disadvantages, including high initial costs, developmental problems, and essential thermal requirements that are difficult to meet and sustain. With the present state of the art, superconductors are usually special alloys of niobium such as NbTi and NbSn, operating at extremely low “cryogenic” temperatures.
The superconducting electromagnet systems in maglev trains must be cooled down to a few degrees above absolute zero, and then energized with circulating currents of “several hundred thousand amperes.” In order to keep the conductivity “perfect,” to keep this very high circulating current and resulting magnetic field “permanent,” the ultra-low temperature must be continuously maintained by on-board liquefied gas such as helium. Even with the best-insulated “Dewar vessels” (thermos bottles) the onboard supply of cryogenic liquid will boil away in a few hours whether the vehicle is in use or not. Any comparisons between the efficiencies of ordinary and superconductive systems must include the energy expended to liquefy the cryogenic gas.
Superconducting maglev would not be a satisfactory design choice for the private dualmode cars, since the tank of cryogenic liquid in the cars would have to be refilled regularly, or an onboard cryorefrigerator would have to be operated constantly, even if the cars were not used regularly. However, dualmode taxis, buses, trucks, and guidetainers, which would have regular maintenance and much higher use factors, might use superconducting maglev to advantage.
PERMANENT-MAGNET MAGLEV
Modern ceramic and rare-earth-containing magnets are extremely powerful compared to earlier permanent magnets such as Alnico. The latest magnets are over an order of magnitude stronger than the best magnets of a decade ago. And they show virtually no degradation in strength in a decade. But we don’t want to use anything that is “rare,” because we will be building millions of dualmode vehicles and want them to be as inexpensive as possible. However the “rare earth elements” were given that collective title long before we knew much about them or had uses for them. Fortunately most of them are not really rare. Neodymium, which is used in one of the best permanent magnets, is over twice as plentiful in the earth’s crust as lead.
Maglev development is progressing rapidly in the United States, with dozens of technical papers on maglev being presented and dozen of maglev patents being granted every year. There have been many improvements in all of the different types of maglev, such as by the use of “null flux,” and “Halbach Arrays” to optimize the magnetic fields. Like most other technical equipment, maglev is getting better all of the time. It would fill the bill for REV very nicely today, and will be still better by the time we start designing the system in earnest.
OFFICIAL EVALUATION OF MAGLEV
A September 1993 U.S. Army Corps of Engineers and Department of Transportation document, FINAL REPORT ON NATIONAL MAGLEV INITIATIVE (Doc No. PB 94-100237), makes the following statements: “The NMI study concluded that maglev technology has been demonstrated as a technically feasible transportation system. A United States-developed maglev would yield several design improvements that could result in significant performance and economic benefits compared to other high-speed ground alternatives. Most important, by developing an advanced maglev system, the U.S. could compete in both the nontechnical and technical aspects of the global maglev market.
“The NMI study recommends that the Federal Government proceed with a U.S. maglev prototype development program because of the significant public benefits. The recommended program is a three-phase development plan leading to a technical demonstration at a test site.
“The GMSA team found that any maglev system, foreign or U.S. developed, would offer many benefits, including high speed, high capacity, low wear and maintenance, modest land requirements, low energy consumption, low operating costs, alternative fuel choices, and low noise levels. The U.S. concepts, however, offer even better performance potential than foreign maglev systems in the areas of energy efficiency, guideway design, motor design, power transfer, refrigeration demand [for superconducting maglev], and materials and techniques.”
That was thirteen years ago. The thing the NMI study missed entirely at that early date, was that the coming Dualmode Transportation System will use this wonderful maglev technology for cars, buses and trucks to great national and international advantage; while maglev trains, which they had in mind, will largely die out.
POWERING THE CARS ON THE GUIDEWAYS
Maglev vehicles are almost always propelled by “linear motors.” These special electric motors are integrated with the magnetic levitation system and run on power from the guideways. Linear motors are also used in a number of applications that do not use maglev: They are now powering roller coasters, aircraft catapults on carriers, automatic material-handling systems, and are even being studied for satellite launching.Linear motors are different from regular electric motors in that they are not round and don’t have a rotating shaft, they run along a line, they are linear. Compared to ordinary motors they are “unwrapped” or straightened out and stretched. Their stationary parts are built into the guideway and their moving (not rotating) parts are rigidly attached to the moving cars. In conventional-motor terms, the “armature” can be either in the guideway or in the car, and the “field” in either opposite location.
Either wheels or maglev could be used to support linear-motor-powered cars, but such cars would be propelled by their linear motors, not by wheel traction.
LINEAR SYNCHRONOUS MOTORS
There are two basically different types of alternating-current electric motors: “induction motors”, and “synchronous motors.” Induction motors slow down and provide more torque (turning force) as we load them more heavily, while synchronous motors will only run at the speed dictated by the frequency of the AC power supplied to it. If the load gets too heavy for a synchronous motor it immediately stops: It cannot run at any speed out of synchronism with its power. Two synchronous motors of the same type that are connected to the same power source will run at exactly the same speed. Good examples are plugged-in electric clocks. These clocks have synchronous motors and run from the same precisely controlled alternating current, therefore they all keep exactly the same time.Both induction motors and synchronous motors are built and used in the linear form as well as in the conventional rotary form. The Linear Synchronous Motor (LSM), is the only logical propulsion for The National Dualmode Transportation System guideways. The synchronism of LSM is the vital feature that makes it perfect for maintaining the spacing of the cars on the guideways, and allowing very close spacing. The fact that the efficiency of synchronous motors is considerably higher than that of induction motors is another advantage.
A simple way to visualize synchronous-linear-motor propulsion is to compare it with surfboarding. The electric coils in the LSMs will produce traveling magnetic waves comparable in many ways to ocean waves. A surfer positions him/herself on the front side of a wave and “rides it” (gets pushed along). Cars on LSM guideways will be pushed along by magnetic waves in a comparable manner. Each car will ride its own wave, and all of the magnetic waves will travel at exactly the same speed. The distance between the magnetic waves and therefore the distance between the cars on the guideway won’t change; therefore the cars can never crash into each other. They will be like boxes traveling on a conveyor belt.
We may need a “closed-loop” or “feedback” system on order to keep the cars operating at an efficient point on the alternating-current waveform. The 60mph guideway system will require AC power of one frequency, and the 200mph (or whatever speed we decide upon) guideway system will require a higher frequency, or more widely spaced guideway coils. But the frequency or two frequencies will be exact and unchanging throughout the system. The cars on a Miami guideway will therefore travel at exactly the same speed as the cars on a Seattle guideway.
LSM propulsion is usually used in combination with maglev. Parts of these two subsystems, such as permanent or superconductive magnets and electromagnetic coils, may be common to both systems, thus reducing the initial cost and the maintenance costs of the entire system.
Because of their safety, high system-capacity capability, speed, efficiency, simplicity, and above all their ability to synchronize the cars, the use of linear-synchronous-motors in the guideways is the most-vital technical feature of our coming dualmode system. Further chapters of this book will be based upon Linear-Synchronous-Motor guideways. But both maglev technologies and LSM technology are currently in a rapid state of development by over a dozen different organizations and companies. Therefore no predictions will be attempted here as to which dualmode system or LSM system will prevail for The National Dualmode Transportation System.
Without LSM each dualmode guideway car would have to have some kind of sensors to constantly measure the distance between itself and its neighbors, and some kind of speed control system to keep that separation distance constant. This is exactly what human drivers have to do. Our eyes are the sensors that judge the distance to the car ahead, and our foot on the accelerator is the velocity control feature. But human drivers can’t perform these functions anywhere nearly as rapidly, precisely, and safely as synchronous guideways will. These differences between human and LSM capabilities account for the very high capacity potential for the guideways.
As of March 2006, Google listed seven hundred and fifty thousand items containing the phrase “linear synchronous motors.” I strongly recommend the highly technical, authoritative, and detailed 2000 book, LINEAR SYNCHRONOUS MOTORS, Transportation and Automation Systems, by Jacek Gieras and Zbigniew Piech, published by CRC Press.
NO BRAKES ON THE GUIDEWAYS
On the streets the dualmode cars will use conventional brakes, but on the guideways these brakes can do nothing because the wheels aren’t touching the guideway. That is good. We must not have any guideway brakes except the “regenerative braking” provided by the LSM. To “brake” normally means to stop or to slow down. But the regenerative braking will be used to stop a car only in the deceleration ramp at the end of its trip. On a guideway with power loss the regenerative function will serve to help keep all cars at exactly the same speed as all of the other cars.On the highways and streets, drivers who unwisely or unexpectedly put on the brakes cause a high percentage of the accidents. That won’t happen and can’t happen on the guideways, since there will be no way for humans or the LSM system to produce differences in speed between cars—no way to desynchronize them. Except in emergencies (to be covered later) the guideway traffic will never speed up, slow down, or stop. On the guideways synchronous speed is the safe speed. Conventional braking on the guideways would cause immediate disaster, so there will be no provision for guideway braking.
POWER SAVING
Even though maglev and LSMs are very efficient, energized guideway coils will consume significant electricity even when there are no cars on the guideway. Therefore, to save electricity, the power to sections or blocks of guideway with no traffic on them may be automatically turned off. Approaching cars would trigger sensors well in advance of a turned-off block, to turn on the power ahead. Power to that block would be automatically turned off again when the cars have passed—unless there are other cars approaching. The failure mode of this power-saving system would be “power-on.”
Alternatively, if we put the permanent magnets in the guideways instead of into the cars, and put the AC coils in the cars, then only the cars would use power, never the guideways.
DISCLAIMER
The author has a degree in mechanical engineering, and has also had a number of courses in electrical engineering and electronics, but he is not an expert in maglev or linear-motor theory and design. He firmly believes that a system essentially as described in this chapter and book is doable, but this is a complex and rapidly growing field, and the experts currently disagree in many areas of it. This book is intended as only a broad-brush preliminary discussion of the great potential for dualmode transportation. Many thousands of engineers and scientists and many hundreds of thousands of man-hours will be required to design the actual system in detail, and test and develop it. The author is long retired, and regrets that he will not be one of the actual do-ers.
Next: CHAPTER 10
Capacity of the Guideways
Back to: CONTENTS
Last modified: August 01, 2006
