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Inductrack questions

  1. Sep 23, 2009 #1
    I'm an undergraduate EE student about to make a project proposal on building a miniature vactrain, that is a maglev track encassed in a vacuum, and what kind of efficiency and speeds it can achieve while it's not powered. However, I'm still trying to wrap my mind around the Inductrack concept. Most the resources I've found didn't went into detail since they were directed to the layman, and those that did went into detail still didn't give me a good understanding.

    What I do understand is the concept of Halbach arrays and how they can double the magnetic field on a single side, why there needs to be a minimum velocity as the current induced in the coils cause a repulsing magnetic field due to Lenz's law, and if I'm not mistaken, the propulsion would be a separate linear motor that can push the train while in both non-levitating and levitating stages. What puzzles me are the magnetic field arrangements when the magnets interact with the coils and vice versa. What kind of magnetic drag would inductrack trains be subjected to at various speeds? Are there any clear diagrams or animations showing these arrangements?
  2. jcsd
  3. Sep 24, 2009 #2
    The Halbach array gives (essentially) a sinusoidal component to the field strength along the length.

    From the point of view of the coil, it's the *change* of the field causes an EMF in the closed loop (by Faraday's law). But the change of the field *leads* the field by 90 degrees. So the EMF is 90 degrees ahead of the vehicle at all speeds.

    At low speeds the coil is almost a simple resistance, so the EMF creates a current that creates a field and repels the magnets, and causes perhaps a bit of levitation, but mostly drag- it mostly pushes the vehicle backwards, in accordance with lenz's law, because the generated field is ahead of the vehicle's magnets.

    At high speeds the rate of change of the field is higher, giving higher voltages, but the coil sees a higher frequency field (w) and the impedance of the coil at higher frequency is mostly inductive (r+iwL). This inductance pushes the phase of the *current* backwards so that it generates a field that almost lines up with the magnets. If you think about it, this causes mostly lift and hardly any drag.

    The other critical thing is that because the impedance goes up with speed (becomes almost purely inductive wL), so this limits the current. In fact if you think about it at high speed, the current is independent of speed, since the voltage and the inductive impedance are both proportional to speed.

    The net upshot is that the power needed to push the vehicle along is constant, and the drag force is inversely proportional to speed (force * distance/time = power)
    Last edited: Sep 24, 2009
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