I have done a lot of transformer and inductor design over the years, always
said "you can't focus or direct a magnetic field", apart from ordinary
magnetic materials of course. On day I found something that someone had put
together on the web: He pointed out that if you have a resonant coil in a
magnetic field then the field it creates cancels the field around the
outside, and enhances it in the center on the coil. The effect is just like
"sucking" in the field into the coil!
I can always learn.....
-----Original Message-----
From: EV [mailto:ev-boun...@lists.evdl.org] On Behalf Of Martin WINLOW via
EV
Sent: Wednesday, 23 July 2014 8:19 p.m.
To: Lee Hart; Electric Vehicle Discussion List
Subject: Re: [EVDL] EVLN: BMW&Daimler developing 3-Hour Wireless Inductive
EVSE for i3 EV
"Calling Nikola Tesla! Calling Nikola Tesla! Come back! All is forgiven!"
I bet *he'd* have something to say on this subject! MW
On 21 Jul 2014, at 20:24, Lee Hart via EV wrote:
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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