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Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
This thread will be about unicorns.
I remember reading an article a long time ago about how you can put one turn of wire around your house, feed some audio into it and be able to pick that up anywhere in the house with a coil and an amplifier. So I figure the same might apply to inductive power transfer.
Proceeded to make myself some big coils. About 10 inches or so in diameter, coil one is a 2+2 turn one hooked up to what else but a royer. With a 680nF cap the thing resonates at 67kHz, so the inductance is 8.3uH. My harvesting coil is 4 turns of similar diameter, the load being an SSCP-7 LED, attached with no regard for its well being.
Initial tests with no resonant cap seemed decent. From the brightness of the LED and my memory, it seemed like it was running at it's max rating of 19W.
A book's worth of distance decreased the power to 75%, and it all but stopped glowing at about 6 inches.
Bringing some resonance to the party with a 480nF cap across the LED, there was what seemed to be a 2 fold increase in power. The max distance increased to 10 inches and the poor little LED was being beaten with about 5 amps here Interestingly, the entire system is still more efficient than just burning off the extra voltage from that battery with a resistor.
I noticed that there were dead zones in where there was no transfer. Along the horizontal axis, there seems to be a certain area both outside and inside the powermaking coil where nothing happens. Some sharpie marker to clarify, seems to be about 1/6'th the diameter of the coil, 1/6'th of the way in and out.
And a youtube video.
With the coils perpendicular, the dead zone seemed to be in the center, 1/3 the diameter of the coil. Moving it out of here made light.
I plan on making larger and larger coils to see if anything interesting happens. Next coil will probably be 3 feet across...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi grenadier
The phasing of magnetic flux (think of it as static field lines if you wish) passing through your receiving coil is opposite for when the coil is on side of the transmitter, compared to being atop of it
As you move the coils perpendicularly it's natural to expect that at some point the total flux caught by receiving coil will be zero. I also demonstrate the effect in my unicorn video.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
W00t! My hypothesis appears to be correct.
No matter where the harvester coil is located within the solenoid, power transfer remains unchanged. I'm about to make a 4 foot coil now to see if this scales up...
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Alright sorry for post #3 but seeing as this is already #1 on the threadlist...
It did not scale up. Center was dead as can be.
So I decided to play with different coil shapes, and it looks like a long thin coil works ok.
However, most surfaces are not long and thin. :-/ So, I decided to put some 'intruders' into the coil in an effort to increase its physical size. It worked but there were dead zones.
Then I realized, I can just squeeze those dead zones tight , thereby eliminating them!
So, I seem to have stumbled on the ideal coil for a flat surface. Neat. I need to get me some foamboard tomorrow...
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Grenadier wrote ...
Alright sorry for post #3 but seeing as this is already #1 on the threadlist...
It did not scale up. Center was dead as can be.
Were you getting a good amount of current through it at a high frequency? The inductance will be much larger with such a large coil, so the amount of current and frequency might have dropped. Plus with the larger area the magnetic flux density will also have dropped. Couple all of that together and the voltage will be lower. The voltage on your pickup coil is the equal to rate at which the total flux through the coil is changing ( ). For a "dumb" load like an LED you want a high voltage to get highest power, so for the most power you want high current, high frequency, and high field intensity. But these are balanced against things like cost, efficiency, and coverage area...
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059
Have you considered making a "star coil" ?
This is essentially a circular coil but with the points facing inwards so that the coil flux is homogenous.
One way to explore this is to use magnetic viewing film, making a small copy of your proposed coil design and passing DC current through it.
Also, another idea to help keep it in resonance is to use two banks of reverse biased LEDs in inverse parallel with a DC bias current applied 180 degrees out of phase with the tuned circuit frequency.
One of my projects is to make a fully wireless Lifter powered by resonant power transfer, estimated altitude could be as much as 6 feet (!)
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Experiments with inductive power transfer
