It is not only "how it could be done" but also how it already *is* being
done. One commercial bus line in Utrecht, the Netherlands is being
charged wirelessly at the end station of the bus line. The driver
received some training how to position the bus such that it is aligned
with the charging pad, but since bus drivers are already pretty good at
positioning a bus at a stop, this was no burden - just a matter of
knowing where to place it to make sure that the bus will be charged by
the time it is making its next trip. Indeed the bus stop has an embedded
coil in the bus bay and the bus has the (resonant) pickup.
Buses in the Netherlands also are equipped with ride-height control that
allows them to "kneel" at every stop, giving disabled passengers an
easier way than to climb in/out the bus - typically the front of the bus
is *level* with the curb while the bus is stopped. This no doubt also
helps to reduce
the distance between charging pad and pickup coil on the bus.

Cor van de Water
Chief Scientist
Proxim Wireless Corporation
Email: Private:
Skype: cor_van_de_water Tel: +1 408 383 7626

-----Original Message-----
From: EV [] On Behalf Of Peter C.
Thompson via EV
Sent: Wednesday, July 23, 2014 7:40 AM
Subject: Re: [EVDL] EVLN: BMW&Daimler developing 3-Hour Wireless
Inductive EVSE for i3 EV

Took some digging, but using the phrase "resonant magnetic induction" 
produce this paper:

For those that aren't into heavy equations, it shows how you can design 
coils to absorb resonating magnetic fields over air gaps as large as 
200mm with efficiency over 90%.

Note that larger air gaps can be acheived with different antenna designs

- this will be very useful for larger vehicles (e.g. heavy trucks or


On 7/23/14, 2:53 AM, George Tyler via EV wrote:
> I have done a lot of transformer and inductor design over the years,
> 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
> "sucking" in the field into the coil!
>       I can always learn.....
> -----Original Message-----
> From: EV [] On Behalf Of Martin WINLOW
> 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
> EVSE for i3 EV
> "Calling Nikola Tesla! Calling Nikola Tesla! Come back! All is
> 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
> difference in their conductivity, so we can tightly control where the
> current flows.
>> With magnetics, we have no good conductors, and no good insulators.
> 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
> shorts everything to everything else!
>> (Superconductors can give us good magnetic insulators; but they don't
> except at cryogenic temperatures).
>>> I don't know wave theory but I believe directional radio
>>> work by having two or more antennas. Can something similar be done
> inductive coils?
>> Yes; sort of. Every electric field inevitably has a magnetic field,
> vice versa. That's why we call it "electromagnetics". However, for
> fields to act like waves, which we can focus and direct like light,
> 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
> too hard to make antennas with multiple elements in parallel to focus
> direct these frequencies (like the traditional TV antennas that look
> giant metal combs).
>> At 1 MHz (the AM radio broadcast band) the wavelength is about 300
> thus the tremendously high towers needed to effectively transmit it
> whole tower is the antenna). It's hopeless to make receiving antennas
> big. We have to use far smaller antennas, that are far less efficient
> 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
> 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
> But to do so, they require *very* tight coupling between the primary
> secondary -- minimal gap between them. This is the opposite of the
> requirement to have some separation between primary and secondary as
> 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
> secondary coil or the charging station's primary coil so they touch.
>> --
>> Excellence does not require perfection. -- Henry James
>> --
>> Lee A. Hart now includes
>> GE EV-1
> _______________________________________________
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> _______________________________________________
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