The test fixture was used to test a novel superconducting Maglev suspension based on a new "flux-canceling" Maglev topology. The basic principles of EDS Maglev are shown in the figure below. The N-S-N-S arrangement of superconducting magnets creates a magnetic field; this magnetic field travels with the train. Magnetic induction due to the interaction of this moving magnetic field and the guideway loops creates an induced current in the guideway loops.
If the guideway coils are centered around the magnets as the train moves down the track, there is no net flux in the coil, and no induced current. If the train moves up or down, there is a net flux in the guideway loops which creates an induced current, which in-turn creates a restoring force.
Our magnet is based on an iron-core arrangement. This results in less stray flux in the far field (i.e. in the passenger cabin) and allows us to tailor the magnetic field in the vicinity of the superconductor, so that a higher current density can be run in the coils.
"Flux canceling" Maglev geometry
At zero velocity, there is zero lift force since there is no induced current in the guideway loops. At intermediate train speed, the drag force reaches a finite peak, and at high speed the drag force decreases while the lift force reaches a maximum. This effect is easy to understand if you consider simple L-R models of the guideway loops.
EDS lift and drag force vs. train velocity for ideal Maglev system
A 1/5-scale model of the "flux-canceling" EDS Maglev suspension was built, and testing done using a rotating wheel test facility in the Charles Stark Draper Laboratory. The test wheel is approximately 2 meters in diameter and is composed of an aluminum hub and a non-conductive composite fiberglass/epoxy rim where the guideway conductors are embedded. The test wheel has a maximum rotation speed of 1000 RPM and a peripheral velocity of up to 300 km/hour under computer control. For initial testing, the suspension magnet was mounted to a 6-axis force sensor, and forces and moments at various test wheel speeds were measured. We were able to operate at speeds significantly higher than the EDS drag peak.
Rotating test wheel facility
The suspension magnet is a dual-row octapole with a pole pitch of p = 0.126 meters, constructed with laminated sheets of M19 steel. For initial testing, the magnet was run with copper coils.
Octapole suspension magnet
The guideway conductors were built from 0.093" thick copper sheets. The conductor pattern was cut with a water jet, and brazed together before being embedded in the periphery of the fiberglass test wheel. This method of manufacture is a low-cost alternative to standard guideway geometries. Below is shown a small section of the guideway coils, showing the end sections.
Shown next is a prototype high temperature superconducting (HTSC) coil. The coil was made from Bi-2223 tape and operated in liquid nitrogen at 77K.
Prototype HTSC coil
An electrodynamic system model was developed where lift and drag forces were predicted with good results. Shown next are test results for measurement of lift and drag forces using the test fixture. The models were used to scale the results for a full-scale Maglev system.
Lift and drag force measurements. Top traces are mode shapes of guideway currents.
For vertical suspension tests, the magnet was mounted to a one-DOF air bearing which allowed vertical motion, with little damping. The resultant suspension and control system was tested using a magnetic vertical control system. This eliminates the need for costly and heavy mechanical damping. The air bearing is shown below.
With the air bearing energized and the control system deactivated, the vertical magnet position was perturbed approximately +1 centimeter from the equilibrium position and the resultant transient decay of magnet position was observed. The oscillation frequency is at 1.15 Hz with a damping ratio of approximately 1%, corresponding to the expected underdamped EDS response.
Vertical motion of suspension magnet, active control deactivated
With the control system activated, the damping is significantly higher (approximately 40%). This is a secondary magnetic suspension, and no passive damping or mechanical shock-absorbers are needed. Further improvement can be made in the transient response by adjusting the loop parameters.
Results with control system activated
Further tests were run to determine if significant lift is possible at zero train velocity by exciting the levitation coils with AC currents. It is desirable to remove the requirements for a mechanical suspension (for instance, wheels or air bearings) for low speed operation of the EDS train. The fact that high temperature superconductors are robust with regard to AC losses is a further motivation, and such control is difficult using low temperature superconductors due to quenching. Using the test facility, this "AC lift" was demonstrated. Test results were used to predict performance of a full-scale system using HTSC coils. Detailed results and scaling laws are given in my thesis.
T. Thompson, Ph.D.
Thompson Consulting, Inc.