A propulsion alternative is to transfer power to the vehicle and then have a magnetic structure on the vehicle that can create a moving field that, in turn, interacts with a conducting guide way to provide propulsion. This is called a short stator linear induction motor (or linear asynchronous motor), generally referred to as a Linear Induction Motor. It is possible to put the powered winding on the guide way and induce currents in a conductor on the vehicle, in which case it would be a long stator design, but this turns out to be an expensive and inefficient approach to maglev propulsion. The Linear Induction Motorcreates forces via induced current much as an Electrodynamic Suspension design creates suspension via induced currents. The rotary induction motor was invented by Nikola Tesla in 1886 and the linear version for transportation was pioneered by Eric Laithwaite in the 1950s.
The advantage of the Linear Induction Motor is the reduced cost of the guideway and the ability to operate closely spaced vehicles without the block limitation of the linear synchronous motor. The Linear Induction Motor has been used for propelling both wheel based vehicles and magnetically suspended vehicles over a very wide range of speeds.
The principal disadvantage of the short stator design is the need to transfer power to the vehicle. If this is done with sliding contacts, as with conventional electric trains, then some of the advantages of maglev technology are lost. If it is done via inductive transfer, then the guideway cost is increased.
A second disadvantage is the need to have substantial amounts of power conditioning on the vehicle. This equipment must be sized to match the highest force and speed for the entire system, even if it rarely needs such high peak power. If the air gap is large this power conditioning equipment can be heavy and expensive.
Note that the popular press often misuses terms like “induction” and “synchronous” when applied to linear motors.
In 1966 Danby and Powell at Brookhaven National Laboratory conceived of using vehicle-mounted superconducting magnets that induced current into their novel “null-flux” winding on the guide way. This concept helped precipitate maglev research efforts in the early 1970s, but these were soon terminated by political action. In 1987 Senator Patrick Moynihan created the U.S. National Maglev Initiative that was managed by the Federal Railway Administration. It led to four preliminary designs, but the effort was terminated by political action. In the late 1990s a maglev effort was reinitiated, but the combination of political and technical dispute makes the future uncertain. There is an effort by the U.S. Federal Transit Administration to develop an Urban Maglev system that can be an alternative to urban rail-based transit systems. The objective is to achieve speeds up to 161 km/h (100 mph) with lower noise, lower energy consumption and lower operating cost that any other fixed guide way system. Based on the success of the Birmingham, England system it is likely that a viable urban system can be built. If the lower speed design was well conceived it could evolve into a design suit- able for operation at higher speeds, just as the railroad system evolved.
