Re: Wireless power transfer (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 08 Jun 2007 13:12:28 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Wireless power transfer (fwd)

Hi Steve,

Non-contact electric toothbrush and shaver chargers use simple 
transformer action at line frequency, and because of closer proximity, 
significantly higher coupling coefficients - it's basically a 
transformer with relatively loose coupling (compared to a normal 
iron-core transformer), so it has significantly high leakage inductance.

The approach used by the MIT folks uses a pair of inductively coupled 
tuned circuits operating at the common resonant frequency of the tuned 
circuits, driven from a CW RF source. By using high Q transmitter and 
receiver coils and relatively high coupling coefficient (k), energy 
coupling can approach 100%. With lower coupling, it will take longer for 
the receiver to "ring up" to full power.

Using looser coupling simply means that it take longer to completely 
transfer energy from one system to the other, and various system losses 
become more important in limiting overall efficiency. As one would 
expect, the total energy that can be continuously extracted from the 
secondary system rapidly declines as coupling is reduced.

Bert
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Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Fri, 8 Jun 2007 11:21:05 -0600
> From: S&JY <youngs@xxxxxxxxx>
> To: Tesla List <tesla@xxxxxxxxxx>
> Subject: Wireless power transfer
> 
> FYI - an article in the June 8, 2007 Wall Street Journal.  If any of you
> have more details on the "magnetically coupled resonators", please post it
> on this list.  For example, how is it different from magnetically coupled
> electric toothbrush chargers?
> 
> --Steve Y.
> 
>  
> 
>  
> 
> MIT Scientists Pave the Way
> 
> For Wireless Battery Charging
> 
>  
> 
> By WILLIAM M. BULKELEY
> 
> June 8, 2007; Page B6
> 
>  
> 
> CAMBRIDGE, Mass. -- A 60-watt light bulb that glows even though it isn't
> wired to a power source could pave the way for wireless recharging of
> cellphone and laptop batteries and the operation of mobile robots without
> batteries, scientists reported.
> 
>  
> 
> A team of researchers at Massachusetts Institute of Technology reported in
> the online publication of the journal Science, that they had demonstrated
> wireless transmission of electric power by magnetically coupled resonators.
> They dubbed their technology "WiTricity" for wireless electricity.
> 
>  
> 
>  
> 
> Marin Soljacic, the physics professor who led the team, said the technology
> is sufficiently proved and that "now is a good time to start thinking about
> commercializing it." He said that while further development is needed to
> improve efficiency, he thinks commercial products could be on sale in "a few
> years, if you started working very seriously." He said MIT would handle
> licensing of the technology.
> 
>  
> 
> Prof. Soljacic said that over short distances, wireless transmission appears
> to be about 80% as efficient as wired transmission. The resonant devices
> interact with each other without interfering with either biological
> processes or other electrical devices. Prof. Soljacic also noted that
> electricity pioneer Nicola Tesla worked on similar methods for wireless
> transmission in the early 20th century, but dropped the idea because it
> didn't work over long distances.
> 
>  
> 
> While wireless transmission of electricity over radio waves is well known,
> it's very inefficient because most of the power is dissipated in directions
> other than the device that needs the power. Prof. Soljacic's team theorized
> that two copper coils tuned to resonate in identical magnetic fields would
> assure the power would go only where it was needed.
> 
>  
> 
> Resonant objects interact only with objects of the same resonance. For
> example, if an opera singer holds a high note in a room of wine glasses
> filled to different levels, only one glass, which resonates with that note,
> is likely to shatter.
> 
>  
> 
> The researchers' technology uses a copper coil attached to a power outlet to
> transmit electromagnetic waves at set frequencies. A receiving coil attached
> to the base of the light bulb, can receive the power for a distance up to
> seven feet, making it appropriate for rooms.
> 
>  
> 
> Prof. Soljacic, a 33-year-old who joined MIT from Croatia, said he thinks
> manufacturers could include such coils in battery-powered devices such as
> laptop computers to make automated, wireless recharging possible.
> 
>  
> 
> He said he started considering the problem several years ago because he was
> often awakened at night by the sound of his cellphone beeping because its
> battery was getting low and he had forgotten to put it on its charger.
> 
>  
> 
> The technology doesn't work over long distances, but it functions well
> within average sized rooms in homes, he said. That might make it possible to
> build wireless devices that don't require batteries. He noted that many
> devices such as robot vacuum cleaners and laptop computers run on less
> energy than the light bulb his team lit up.
> 
>  
> 
> 
> 
>