Why is RUF not a Maglev System?
Palle R Jensen
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 the 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 engineers 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.
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.
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.
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.
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 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 doesnt suffer from this limitation and in RUF all passengers are seated.
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).
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).
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.
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.
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.
My conclusion is that even if the maglev technology is very fascinating, there are good reasons not to use it in a dualmode system.
Last modified: October 04, 2000