Re: Inductive Power through Floor?

[rcochran@xxxxxxxxxx]

E. Lee Dickinson wrote:
> Had an idea today, while trying to figure out how to power a low voltage
> item built into my coffee table.
>
> I was wondering if any of you thought it would be possible or feasible to
> build or buy some sort of induction plates that would work through a floor
> and preferably a layer of carpet.
>
> I imagine this would be great for floor lamps and other things which need
> power but which aren't always able to be close to an outlet.
>
> Any ideas?

Theoretically possible? yes.  Practical?  I don't think so.  Safe?
maybe not.

For efficient transfer of energy via induction, the driving coils need
to be fairly close to the driven coils, and the alignment needs to
be somewhat well controlled.  Ordinary transformers are examples
of inductive devices that are designed for efficiency where the
engineers can easily control the relative placement of the coils.
But they still put out a lot of heat, indicating they're not terribly
efficient.

My toothbrush is charged by an inductive arrangement.  It gets quite
warm, indicating a low efficiency, but the power needed to charge it
is very low.  The advantage is waterproof sealing with no cords or
sockets to corrode.  But it still requires the toothbrush be
nestled in the right position in its base for charging.  It's no
more convenient than the metal contact arrangement used
for charging a cordless phone, and it's less efficient, but it's
less prone to corrosion.

If you put an ordinary piece of metal on top of the driving coils,
it will introduce eddy currents in the metal, producing lots of
heat.  That's how inductive stoves work, by inducing currents
in the pans.  You don't really want strong coils all over your
floor sending power out to all nearby metal objects.  A kid's
metal toy could get hot enough to cause a burn.  So for safely
driving things as power-hungry as a 50 watt lamp, you'd want
to have local power outlet coils that are only energized
when a desired load is "plugged in".

There still might be concerns about the EMF fields from
this kind of arrangement.  There are enough health concerns
already about 60 Hz EMF without introducing something
that's explicitly designed to create more EMF into your
home.  Although I think many of the EMF concerns are
unfounded  and not based on sound science, I imagine it
would be difficult to convince investors to develop products
with such a huge lawsuit potential.  I'd be hesitant myself
to invest in something like this, because even though I
don't believe 60Hz EMF is a huge health concern, I
DO believe it's possible for a jury to find otherwise, so I
think it's a huge lawsuit concern.

For small low power devices, if you can align the driven coils
precisely over the driving coils in the floor, you might be able
to make the technical side of this work.  But I can't see it
being nearly as efficient or practical as just putting ordinary
outlet boxes in the floor.