Technology for Magnetically Levitated Trains

The term “maglev” describes magnetically levitated trains, trains that are suspended on a magnetic cushion above a magnetized track, rather than on wheels.  Because they are levitated, they offer the possibility of operating more smoothly, quietly, quickly, and efficiently than conventional passenger trains.  But an additional feature they bring to the transportation arena is a lower cost system for people movers; the vehicle without undercarriage is lighter, so the elevated guideway costs are less.

The Center for Electromechanics (CEM) at the University of Texas at Austin is working on such a system.  The primary focus is on the design of electric motors that will be used to power the maglev train.  The initial system will use two 600 kW linear electric motors.  To put this in perspective, each 600 kW motor will produce the equivalent peak power of  eight Toyota Prius hybrid cars which have a peak rated power of slightly more than 80 kW each.

The new electric motors are called “linear induction motors” (LIM).  “Linear” indicates they move in a straight line rather than spin like most motors, and “induction” indicates they induce a current in the reaction rail that creates a secondary magnetic field against which the primary field from the vehicle can push.  Various reaction rails are being investigated, including rails with window shaped conductors, to determine one that will minimize cost and maximize performance.

WHY MAGLEV?

High-speed conventional steel-on-steel passenger trains

  • Require 30-40 miles to get up to speed
  • Weigh close to 1,000,000 lbs.
  • Require a great deal of maintenance
  • Require overpasses in place of grade crossings which are too dangerous
  • Are expensive to build – there is a significant difference in cost to build a high speed conventional rail vs. a low speed steel-on-steel system

Maglev

  • Economics, specifically how fast and how much petroleum fuel costs rise, along with population density, dictates this technology’s penetration rate into society.
  • Cost significantly less than their conventional counterparts
  • Set the world speed record for trains (361mph)
  • Low Speed Maglev (Electromagnetic systems (EMS)) as people movers makes sense in high population areas where space and cost are critical
  • Road congestion is already a problem
  • The infrastructure for conventional people movers cannot compete with Maglev
  • High Speed Maglev (Electrodynamic systems (EDS)) as an alternative to planes makes sense over highly traveled corridors less than 500 miles

REQUIRED TECHNICAL COMPONENTS

All maglev systems require three fundamental components:

  • Lift, the use of magnetic forces to suspend the vehicle - thse forces may be induced by movement or imposed by injecting current
  • Guidence, to keep the vehicle from sliding off the track laterally
  • Propulsion, to move the vehicle without contact

Lift

Electrodynamic System (EDS)

The lift in an EDS system results from currents that are induced by the movement of magnets past shorted coils.  This type of lift is inherently stable above a threshold speed.  The currents exert a force to move the magnets so they link less magnetic flux with the coil.

 

The Japanese Rail (JR) System uses figure “8” null flux coils to create this lift, but the mechanism for inducing the current is the same.

 

 

Electromagnetic System (EMS)

In an EMS system, the magnets wrap around a steel U shaped track and are controlled actively using eddy current sensors that sense the gap.  CEM is working on this type of system and has optimized the track to guarantee a minimum of 5,000 lbs of lift per 20” magnet, while minimizing the material in the track.

In EMS systems, special attention should be given to the shape of the track in order to minimize costs.  A rounded track is most economical.  The dimensions, x, y, a, v, and u, are treated as unknowns in a field theory based optimization to minimize track volume while achieving the targeted lift.

Guidance

In EDS systems, guidance is accomplished using cross-connections or repulsive magnets such as those sketched below.

Balanced - no induced current
When unbalanced a current is induced in the rail to re-center the magnet


By contrast, EMS systems, such as the one below,offer inherent guidance due to the fact that the magnets want to align laterally with the steel track.

Close-up of magnet wrapping under track
Propulsion

Linear Induction Motor

Conceptually, a linear induction motor is created by unrolling a conventional round motor.  The electromagnetic forces running their rotary counterparts can be used to create linear motion.  A traveling magnetic field forms its essence.  When the cylinder of a conventional motor rotates, its field also rotates and can interact with magnets or lumps of steel on a surrounding annulus to rotate them as well.  The figure shows a rotary motor with two linear variants.  The field can be rolled into a tube to make a linear tube or “pipe” motor or it can be laid flat, as in the case of the linear electric motors.  The moving field is realized by a winding carrying current.  If the current in adjacent positions is timed properly, the magnetic field will simulate the various movements of the field indicated.

 

Rotary, tubular, and linear motors.

Adapted from Eric Laithwaite’s book, Linear Electric Motors,

Mills and Boon, London, 1971.

The most prevalent electric motor is the induction motor because the secondary member has no excitation and the device is quite robust.  The figure shows the unraveling of that motor to form a linear induction motor; the armature winding is excited with phased current to simulate the field pattern in the previous figure.

The primary difference between linear electric motors and conventional electric motors is the length of either the primary or the secondary.  If the portion annotated “armature winding” is the long member, the motor is called a long stator motor.  Magnetically levitated transportation is perhaps the preeminent application for linear electric motors, one in which both linear induction and linear synchronous motors have been applied successfully.  If the windings form the long member on the ground, the track becomes expensive, but the challenge of getting propulsion power to the vehicle is circumvented.

 

Unwinding an induction motor

 

The linear induction motor developed by CEM has

  • 5,000 lbs of thrust
  • 750 V
  • 1500 A
  • 1800 lbs

It is an induction motor optimized to maximize the product of the thrust/weight and the power factor.  This index represents a compromise between size/weight and electrical performance.  Parametric finite element codes are employed to perform the optimization numerically.  This motor uses a very unique inside knuckle connection which allows the use of form wound coils even though the slots are wider than the teeth.

PLANNED USE AND FUTURE RESEARCH

When the motor designs are completed and evaluated, it is likely that additional research will focus on optimizing the track itself: characterizing the lift magnets that provide the levitation forces and determining the best shape for the fixed reaction rail against which the LIMs will push. 

The program is targeted to produce a prototype train for demonstration within 2006.  CEM will also help with the control and operation of the LIM.  The deployed system is planned for Asia.

“Much of the development and demonstration of maglev trains has been in Europe and Asia,” said Dr. Robert Hebner, Director of the Center for Electromechanics.  “Through this project, we hope that some of the important technology used for maglev systems worldwide will be designed and manufactured in the U.S.”

For additional information, please contact:

Kent Davey

Program Manager

(512) 232-1603

k.davey@mail.utexas.edu