Why Doesn't RUF  Use Linear Motors?


Palle R Jensen

The concept of linear motors is fascinating. We are used to rotary electric motors where magnetic poles are pulled by magnetic fields in order to make the rotor rotate. Suddenly somebody gets the idea of straighten out the rotor and let it glide along an array of magnetic poles.

This concept can be used to make linear movements and is widely used within the industry. It seems natural to use the principle in a system where vehicles are moving along a line. What is the problem?


The efficiency of an electric motor is very much dependent on the air gap between rotor and stator. The smaller the air gap the better. With good bearings it is relatively easy to obtain a very small air gap in a rotary electric motor.

In a LIM/LSM motor, the air gap needs to be larger for different reasons:

There is some kind of wheel involved in order to maintain the air gap between stator (guideway) and "rotor" (electromagnetic array). This wheel has bearings with some tolerance, but more important, it rolls on a surface which is exposed to the weather. This surface cannot be kept completely clean, so some extra distance is needed.

The guideway needs to be able accommodate a small radius of curvature (26 m in RUF). This means that the length of the electromagnetic array cannot be very long. If it is 1 m long, the air gap in the middle will be increased by 4.8 mm. This is a very large air gap! The efficiency will become very poor.


LIM uses eddy currents in a metal surface to create the force for propulsion. This can be done in a simple way where nothing needs to be added to the guideway. The surface of the guideway can be used as "reaction rail" for the LIM. If the efficiency was OK, LIMs might be OK for a single mode system which only drives on a guideway.

LSM has some interesting advantages. It allows for very precise longitudinal control of the vehicle. This is attractive, especially in a system with vehicles which are put together into trains.

But LSM has this advantage at a high price because the guideway needs magnets everywhere.

The beauty of rotary synchronous AC motors is that they use the same magnets over and over again. It is much more cost effective and just as easy to control precisely.

LSM proponents argue that wheels will slip due to limited friction and different wheel diameters. This may be true in some systems, but in RUF we will probably use metal wheels (for reliability) on a rubber surface. This means that the wheel diameter will remain almost the same during the lifetime of the vehicle. A special advantage in RUF is its ability to adjust the pressure so that the needed friction can be obtained. I admit that RUF cannot obtain as precise a level of synchronization as a LSM system, but its precision is good enough. I think that any system will need a method for making precise separation-distance measurements to ensure a smooth functioning. With feedback from such a system, it is no problem to synchronize the vehicles.

Number of propulsion units.

Whereas LIM/LSM may be relevant in a single mode system, it seems to be a waste of money to have two propulsion systems.

The problem is that a Dual Mode vehicle cannot be propelled by a LIM/LSM on conventional roads. It will need an electric motor for this purpose. This motor would be idle when the vehicle is using LIM/LSM on the guideway and the LIM/LSM would be idle when using the roads.

In the RUF concept the same motors are used in both cases. This will ensure a smooth transition and save weight and money.


I see no compelling reasons for using LIM/LSM in a Dual Mode system.


Last modified: August 13, 2002