Thanks for your layman explanations. I follow.
------ Original Message ------
From: "Lee Hart" <leeah...@earthlink.net>
To: "Peri Hartman" <pe...@kotatko.com>; "Electric Vehicle Discussion
List" <ev@lists.evdl.org>
Sent: 21-Jul-14 12:24:27 PM
Subject: Re: [EVDL] EVLN: BMW&Daimler developing 3-Hour Wireless
Inductive EVSE for i3 EV
From: Peri Hartman
Is it possible to use multiple coils to focus the "beam"?
Magnetic fields are devilishly difficult to direct or focus.
With electricity, we have great conductors (copper, silver, etc.) and
great insulators (air, plastics, etc.) There are *many* orders of
magnitude difference in their conductivity, so we can tightly control
where the current flows.
With magnetics, we have no good conductors, and no good insulators.
It's as if our best electrical conductor was carbon (which we make
resistors out of), and our best insulator was water (which conducts
pretty well, especially if dirty). Imagine trying to make a circuit
work where the conductors are all carbon, and it's submerged in water,
which partially shorts everything to everything else!
(Superconductors can give us good magnetic insulators; but they don't
work except at cryogenic temperatures).
I don't know wave theory but I believe directional radio transmitters
work
by having two or more antennas. Can something similar be done with
inductive coils?
Yes; sort of. Every electric field inevitably has a magnetic field, and
vice versa. That's why we call it "electromagnetics". However, for
these fields to act like waves, which we can focus and direct like
light, the frequencies need to be very high. The elements of a
directional antenna need to have dimensions on the order of 1/4
wavelength or more.
Wavelength (in meters) = 300 / Frequency (in MHz). At 100 MHz (the
frequency of FM radio and the old VHF television), the wavelength is
about 3 meters -- so a 1/4 wave antenna is about 0.75 meters or 30"
long. It's not too hard to make antennas with multiple elements in
parallel to focus and direct these frequencies (like the traditional TV
antennas that look like giant metal combs).
At 1 MHz (the AM radio broadcast band) the wavelength is about 300
meters; thus the tremendously high towers needed to effectively
transmit it (the whole tower is the antenna). It's hopeless to make
receiving antennas this big. We have to use far smaller antennas, that
are far less efficient and require substantial amplification to work.
The inductive chargers mentioned here are using 85 KHz. The wavelength
is on the order of 3500 meters! It's impossible to direct such
frequencies with the techniques used for radio antennas.
Vicor makes switchmode converters that operate just over 1 MHz; about
the highest practical frequency for state of the art switchmode
converters. They had to go to heroic lengths to get their transformers
to operate with reasonable efficiency (90%). Such frequencies are not
yet practical for high power converters.
Lower frequency transformers are more efficient. Conventional 60 Hz
transformers can be over 99% efficient, if you use enough copper and
iron. But to do so, they require *very* tight coupling between the
primary and secondary -- minimal gap between them. This is the opposite
of the requirement to have some separation between primary and
secondary as imposed by the wireless charging proponents.
I think the only way to make a practical "wireless" charger will be to
use more or less ordinary frequencies, and mechanically position the
primary and secondary coils as close as possible. This means either
moving the car's secondary coil or the charging station's primary coil
so they touch.
--
Excellence does not require perfection. -- Henry James
--
Lee A. Hart http://www.sunrise-ev.com/controllers.htm now includes the
GE EV-1
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