Re: [Phys-L] maglev

[John Denker <jsd@av8n.com>]

On 03/31/2014 10:20 AM, Bill Norwood wrote:
> I cannot for the life of me see how a moving vehicle supported in a
> magnetic field would not create eddy currents which would dissipate a
> substantial portion of the mechanical energy of the train.

Setting aside various nitpicks, the interesting physics 
works like this:
  a) Any non-conducting items in the vicinity contribute
   nothing to the dissipation.
  b) Any very-good-conducting items in the vicinity
   contribute little (if anything) to the dissipation.
   The field just doesn't have time to penetrate.
  c) You get a lot of dissipation if the resistivity
   is baby-bear just right.

Overall, at any given frequency, the situation is loosely 
analogous to the termination at the end of a coax cable.
A short is non-dissipative, an open is non-dissipative, 
and an impedance-matched termination absorbs 100% of the 
energy.

Less loosely speaking, the situation is verrry frequency
dependent.  There is a characteristic L/R time for the
resistive metal, and if the applied field is changing 
rapidly on that timescale, you see mostly the L and not
the R, which I suspect is the essential physics answer
to the question that was asked.

When building maglev trains, they do worry about stray pieces
of resistive metal, such as rebar in the concrete.
  http://www.rle.mit.edu/cehv/documents/59-IEEETransactionsonMagnetics.pdf

Now for various nitpicks and tangents:

Tangential remark:  So-called asynchronous motors aka
induction motors are based on essentially the same physics
as eddy currents.  Again the situation is verrry frequency
dependent.  If you want a farm tractor to win the stump-
pulling contest, or if you want a locomotive with lots of
torque at zero speed, you do *not* want an induction motor.
Real locomotives use DC aka "universal" motors.  Clock
motors are synchronous, but for another reason.  OTOH for
something like a fan, where the load is zero at low speed,
induction motors work fine, with very little dissipation.

If you are interested in trains strictly speaking, most of 
the trains I know of rely primarily on plain old magnets, 
using eddy currents only for stability and damping.  This
has obvious advantages when the train needs to slow down 
or stop.

  I have no idea how they keep the permanent magnets clean.

OTOH there are things such as motors and gyroscopes that do 
rely on eddy currents, so eddy-current levitation is still 
worth thinking about.  Undesirable parasitic eddy currents
show up all over the place.

The dissipation of a levitation system doesn't have to be 
zero; it just has to be reasonable in comparison to the
dissipation of ordinary wheels and bearings, which is far 
from zero.  One train project famously states that the 
levitation uses less power than the air conditioning.
  http://www.transrapid.de/cgi-tdb/en/basics.prg?a_no=41