Why RUF Should Use Maglev
by
Palle wrote:
"In this interesting debate about dualmode systems, the development has been towards the following characteristics:
1) It has to be true dualmode.
Pallet solutions may be an option (as it is in the RUF system), but the goal is that the final system uses vehicles created for using the guideway directly.
2) The vehicles have to look almost like normal cars (or busses).
It is important to realize how much in society is based upon boxes on 4 wheels having reasonable well-defined dimensions. In order to gain accept of a new system, it is not a good idea to ask people to buy something which looks very different from today's mainstream cars. The car has found its form because it is extremely convenient for a normal family.
3) The vehicles must be supported by the guideway, not suspended.
Suspended vehicles cannot easily be made so that they look "normal". The roof-mounting device will create problems both inside and outside the vehicle.
In a new system, it is very important that the users feel safe. It is not enough that it is safe from an engineer’s viewpoint, if people don't feel it. I think that "riding" on top of a RUF guideway is a much more safe feeling than hanging under a guideway. There is also problems with visual impact and side wind forces."
I agree with all of the above, Palle.
"In the RUF system, the lateral guidance is perfect and the users can understand that when the RUF is "riding" on top of the triangular guideway, it cannot possible derail. Our tests have shown that the channel in the middle of the vehicle creates only minor problems.
The channel under the cars would be tolerable if it was necessary, but with maglev it would not be necessary. Such a channel somewhat violates your stated requirement for the cars to look "normal," and it reduces the passenger/cargo space, the space for vehicle systems, or both.
No problems are better than "minor problems."
One remaining difference between dualmode system designers seems to be: Maglev or wheels?
I have looked at maglev several times, because many people from the start assumes that RUF is a maglev system. My arguments for choosing wheels for the guideway are:
Wheels are needed anyway for emergency landing.
If something happens to the electromagnets levitating the vehicles (permanent magnets in the vehicle), it has to be able to continue on wheels. Even in the Inductrack system (Halbach array) I think wheels are needed to get started if the vehicle is brought to an unpredicted stop.
Four wheels for emergency landing and for exiting the guideways in the event of a power failure are already there. Use the street-mode wheels and the street-mode motor in emergencies on the guideways. HiLoMag proposes to use the inductive maglev (Inductrack) concept only laterally for contact-free guidance and switching.
The cost of a maglev guideway is much higher than for a simple guideway like the RUF triangular monorail. The article about maglev in Scientific American (Jan. 2000) states that the cost of guideway may be 80% higher than for an ordinary track. Since the track is the most expensive part of a dualmode system, this is very critical.
Maglev guideways will cost more, but we get what we pay for. Highways and railroads also cost more than paths through the woods.
Fixed rubber wheels are almost as smooth as a maglev solution.
Rubber is a beautiful material and very well-developed. It can be made in almost any quality, so it will be possible to carry the vehicles smoothly on a smooth guideway (no potholes) with very low rolling resistance. The trick in the RUF concept is that the wheels carrying the vehicle are not the same as the one driving the vehicle. The driving wheels are rolling on the top of the guideway and the friction can be adjusted to be high during traction or braking and low during free rolling.
Maglev will depend upon magnetic forces not upon friction forces, therefore maglev is not sensitive to weather or tire wear.
Maglev braking cannot exceed maglev traction.
In emergency braking situations, it may be a problem that a maglev system (linear induction) may not be able to brake harder than the maximum force used for acceleration. This may be OK in a system with standing passengers, where powerful braking may be dangerous, but it will limit the capacity of a system since safety distances become longer.
The rail brake in the RUF system doesn’t suffer from this limitation and in RUF all passengers are seated.
With maglev and all of the cars synchronously propelled there will be no reason for any braking on the guideways, except for the regenerative braking that will occur automatically on steep downgrades to keep those vehicles from exceeding synchronous velocity. For maximum capacity all acceleration and deceleration should be in offline ramps—the velocity of vehicles on the guideways should be absolutely constant twenty-four hours a day. Braking on the exit ramps will also be regenerative only since the power frequency will gradually decrease. Since no levitated car can "stall" and all the cars will be going at exactly the same speed, any "emergency" braking of one or more cars on the guideways would be disastrous.
Maglev switching is binary.
As I understand the switch in maglev systems, there are only 2 directions to choose among (like in an old fashioned train). If you have a system configured as a network with bi-directional guideways, and you need to be able to get from any direction to any direction, you end up with a huge structure in a maglev system. Exit ramps have to be long if you are going to decelerate from high speed to road speed on the exit ramps. If you turn at high speed, you will occupy a lot of expensive land for a 90 degree turn (side acceleration may not exceed 0.2 G).
The guideway switching we propose is binary, but so is highway switching binary. On at-grade street intersections a driver usually have three options, turn left, right, or go straight ahead. By adding a third guide rail and using a ternary switching system we could do the same thing on automatic guideways. But this will be completely unnecessary since there will be no guideway intersections comparable to street intersections. Recognize that all guideway junctions will have overpasses and cloverleafs just as all highway junctions have. Entering and exiting a cloverleaf interchange requires only binary switching, as does entering and exiting the guideway system.
The entry and exit ramps for 60-mph guideways will be the same length as the entry and exit ramps we now use on 60-mph highways. With constant guideway speed the radius of the guideway curves can be less than the radius of highway curves for the same speed, since the guideways will be built with exactly the right bank angle for that speed. Highways don't use nearly optimum (balanced) bank angles because part of the time highway traffic may be slow or stopped. With the proper bank (superelevation) built in, guideway travelers will feel no lateral accelerations, and there will be no lateral forces on the car guidance system except those due to cross winds.
In a RUF system, where speed is lowered to 30 km/h in the switch area, these problems are much smaller. A RUF switch can handle multiple directions using the magnetic guidance principle (not maglev).
Reducing the speed of the cars on the guideway as they approach a switch would enormously reduce the capacity of the guideways both by lowering the average speed and by the requirement for huge headways. With a HiLoMag type of system we feel we can reduce the minimum clearance between cars to about one foot (0.3 meters). At this spacing the capacity of one 60-mph guideway lane will be the equivalent of 12 sixty-mph highway lanes, and a 200-mph guideway will be equivalent to 40 highway lanes. With low-speed switching instead, many lanes of guideway would be required in dense-traffic areas.
Linear Induction Motors (LIM) have poor efficiency compared to electric motors.
In a rotary electric motor, the air gap between rotor and stator can be very small, since it is controlled by ball bearings. In a LIM, the "rotor" is the guideway and the bearings are the maglev (or wheels). The tolerances will be larger in this system and consequently, the efficiency will be lower.
You may have misspoken, Palle, but you will recall that most of us who propose maglev (and some who don't) specify Linear Synchronous Motors (LSM). These are reported to have efficiencies in the ninety- percent range, much higher than the efficiency of LIMs. Our other big reason for using LSMs of course is that the cars will then automatically hold their spacing exactly, without proximity sensors or velocity-control systems. I like the thought that they will be like boxes on a conveyor belt.
A dualmode vehicle has to be able to run on the normal streets where maglev cannot be used. This means that a dualmode maglev vehicle need two propulsion systems and it will have to change smoothly between them. In the RUF system, the vehicle uses the same propulsion system (two electric motors) and there is a smooth transition between the modes because all wheels are rolling all the time.
"Two electric motors" sounds like two propulsion systems to me. The magnets that would be added for maglev and LSM (our second motor) might be comparable in cost to your second rotary motor. The transition from wheels to maglev and vice versa will be "smooth" except in the case of an emergency delevitation due to a power failure. In that case, as the levitation disappears (which could happen suddenly or gradually depending upon the type of maglev used) the wheels will contact the guideway and be suddenly spun up to speed just as the wheels suddenly start spinning upon touchdown of an airplane.
It has been argued, that LIM makes it much easier to control the vehicles than using normal electric motors. This may be true with old fashioned DC motors, but modern AC motors behave exactly the same way as LIM. The field is rotating and the rotor is following the field exactly. In the RUF prototype we use AC motors with permanent magnets.
Again we have the LIM/LSM confusion. Rotary induction motors behave the same as LIM, and rotary synchronous motors behave like LSM. When you wrote "The field is rotating and the rotor is following the field exactly" you were describing synchronous motors (both rotary and LSM) not LIMs. Induction motors do not follow the field exactly, they "slip" below synchronous speed, and the amount of slip is roughly proportional to the load on the motor. I don't think induction motors ever use a permanent-magnet field or a superconductor field.
My conclusion is that even if the maglev technology is very fascinating, there are good reasons not to use it in a dualmode system.
Yes, maglev technology is fascinating. Maglev trains will never be used significantly because the whole concept of railroad trains is obsolete, but maglev with LSM has wonderful advantages over wheels and rotary propulsion systems, and they are an ideal choice for our worldwide dualmode transportation system. In this application the advantages of maglev far outweigh its disadvantages.
Last modified: October 16, 2000