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Contactless Electrical Current Transfer?
ferralis (Not an EE) asks: "Recently I've come up with a design for a very fun toy (to be unveiled later if I'm successful). What's missing is a means to send electrical power over a distance of five to ten centimeters (2-4 inches). I've done some research (mostly online) and have found extremely limited information. Even my beloved Google has forsaken me, and even my pleadings to eldritch information deities such as AltaVista have gone unrewarded. Can anyone help?"
"The way I see it, to do this a person needs merely set up a high-frequency electrical field using a larger coil (primary) and a similar but smaller coil (secondary) can be placed within it, creating an air-core transformer. Unfortunately I can't find the math or even anecdotes about what happens when the secondary is off-center, or there is more than one secondary introduced... and I am not looking to build a Tesla coil here. I can imagine that many toys could be built using such a system, and one would think the knowledge would be well known and readily available, but apparently it is not. For this application, efficiency is -not- an issue."
I have an electric toothbrush that charges wirelessly, I assume by using coils. Try dismantling one of those?
Does the person using this toy have to live past the first use? :->
A wireless power cord.
I mean, we've got wireless everything already, and all those radio waves and GHz signals are basically a form of energy streaming through the air, so why couldn't we do it with electricity?
Think of the bliss a wireless power bar would bring.
Patent pending.
Does this help?
I believe what you are trying to do is induction. A current in one coil should produce an opposite current in another coil. For best effect, put an iron core inside, which should make the magnetic field stronger. Or something like that. Can't remember much else from physics 2.
Wouldn't this be akin to minature lightning? With children?
use lightning. i've seen it travel much further than you require.
You could mix up some chemicals, use your electric power source to put some current into them to cause a complex electrochemical reaction. Call this wireless energy reaction "charging", for example. Then when you want to power your toy, just put the chemicals into a sealed metal container in the toy (such as a "cell"). If you need more power, you could even use a group or "battery" of these "cells" to increase the voltage available to your toy. Then when you need more power, simply remove the chemical containers from the toy, "charge" them again, and put them back in.
--- You shall know the truth, and the truth shall make you mad- Neal (not Cowboy) Boortz
or this?
IAAEE
E = -L(di/dt)
Isn't the standard practice for wireless power transfer to use a horde of small, highly skilled mice working smoothly together, each transporting a few columbs of electrons?
What the hell are you people learning in schools these days? Are your teachers clowns?
puts ("Python r0cks\n");
A piece of copper wire of about 10 cm should do the trick.
Let me guess... it's some variation on the "Jump To Conclusions" mat that shocks the user in midair if he is en-route to a dangerous conclusion.
-- If you try to fail and succeed, which have you done? - Uli's moose
Wouldn't it be better if the application was more thoroughly describe or at least the purpose for sending power over that distance? It doesn't take a genius to see that's pretty much what a solar power cell and a flashlight does.
Unfortunately this is one of those questions like you get on sci.electronics.* which doesn't include the basics like how much power is required; at what voltage and current levels, AC or DC, what frequency, can "it" run unconnected (e.g. on battery) and if so for how long (application-wise, not technology-wise), etc. etc.
Barring nice engineer-friendly technical specs, at least outline more specifically what you're trying to do, at least in vague terms, would be more helpful. Starting out by saying it's a product idea (rather than just some hobby thing) was probably mistake if you're paranoid about competitors.
Without some minimal specifications of this sort, absolutely any answer you get will either be hopelessly vague, utterly useless or simply a troll.
google
If you happen to be in Spain this summer, stop by the Wireless Power Transmission Conference.
although its obviouslly a much smaller current, wacom tablets wirelessly power their tablet pens, look into that--also the name passes me now but theres a charger pad that charges devices just by putting them on the pad with a thing on the battery, if i remember ill add a response.
There are few resources for contactless power supplies because they are so darn inefficent that there's almost always a better solution.
Since you didn't give us any information on how much power you actually need (a few milliwatts I hope...) then all I can tell you is to avoid air coils. You'll get almost no practical energy transfer. Make the smaller coil fit inside the larger coil, and put a suitable core inside the smaller coil.
Alternately, if you want two flat faces facing each other, get two large cheap speakers. Remove the cone and coils from the magnet assembly. You may need to remove the magnets themselves and replace them with another ferromagnetic material. Place new coils where the old speaker coils were (wrapped around the core inside the assembly). Face them to each other and put low voltage AC on one side.
There are transformer books which will give you the information you need to accomplish this. It's hard to give you better information than that, though, without knowing the power requirements of your device.
If this PDF treatment on the subject doesn't help, then you probably don't have enough knowledge to correctly design one and you ought to simply start toying around with different designs until you find a suitable match. If/when you mass manufacture the device you'll want to pass it by a real engineer who can redesign it for you. Pay attention to the references in the paper for more information.
-Adam
Man if you can't find a way to do it on Google, it probably can't be done.
As I regularly tell one of my friends : you have used too many pronouns / variables in that sentence in order to come to a conclusion / make any sense. That said, describe in tight detail what exactly you want to do and maybe we can help you figure out a way to do it.
I want to make something something and have it use some power to do something something and it will be cool if I can figure this part out. Surprisingly, given all those facts Google was not much use - how about you guys, how do I transmit an unknown amount of unknown voltage for unknown purpose and unknown duration over 2 to 4 inches?
What do you want to do?
Glonoinha the MebiByte Slayer
Beware that for pretty nearly any method of transferring power you can think of, you will be unable to achieve the FCC's Class B certification for consumer electronics. The device is going to wind up emitting a significant amount of energy in the radio spectrum, and even if you were able to focus them almost entirely in the vertical direction (I'm assuming you're trying to feed power to something you're levitating), the Feds will frown on it.
Use a small solar cell in the receiver, and a diffused semiconductor laser (you may need to use orange or yellow to get enough energy into the solar cell) to beam power over to it.
go to a library and get any decent high school or college text book.
/. questions that could be posted.
Or if you are too lazy, pay some college physics student a couple bucks and they'll get you the info you need in no time.
I hate to make the same point that many have made before, but aren't there any better ask
This sounds simply like an electric transformer. A slightly more efficient way to do it over large (and even larger) distances is using a Tesla coil. These are simply just transformers where the primary is run at some frequency in the order of kilohertz and the secondary doesn't necessarily have to be wound tightly around the primary.
True wireless electrical current transfer occurs as a spark (think lightning). That is current flowing through whatever meduim between highly positively charged and negatively charged areas.
If it's done magnetically (with coils of any description) it's not really current flow, because current involves the physical flow of electrons. If it's a coil of any descriptioon, including a transformer or Tesla coil then it's mostly magnetic. That makes it wireless transfer of energy, which is no great achievement; we've known about that for ever!
I drink to make other people interesting!
You can throw them a lot further than 10 centimeters (about 4 inches for you metrically challenged).
a horrible place
There is a transformer called a lighting transformer which is used to get 60 Hz power onto a 'hot' tower for lighting AM broadcast towers. It is air core and the separation between the primary and secondary is measured in inches.
I have no clue how these transformers work! Everything I know about magnetics tells me that they shouldn't work. I would love to hear the explanation.
(If you want to see one, from a safe distance, they are commonly installed at the bottom of NDB towers. NDBs are the non-directional beacons usually in the vicinity of airports. The towers are about fifty feet high. Ask a pilot where to find one. The lighting transformer carries electricity across the insulator at the bottom of the tower. The transformer looks like two interlocked rings mounted at ninety degrees to each other.)
I am not entirely certain how it is done but using magnetisim it is possible, it is commonly use in medical places where power needs to be transferred to somthng inside the body. Asking a doctor you know that does surgury might reveal some answers.
see: The Sraight Dope
Anyway.
Short-range inductive power transmission works reasonably well. It's commonly used to recharge electric shavers and toothbrushes. Considerable power can be transferred this way. The GM EV1 electric car used an inductive charger, where a flat "paddle" containing a coil was inserted into a rectangular slot in the car.
Efficiency improves with frequency. The EV1 charger ran at 400KHz or so. But you have to take precautions not to become an RF emitter, and get FCC type approval. If you stay with 60Hz, that's usually not a problem.
Coil area helps. If you can use large diameter coils, bigger than the air gap between them, it will probably work.
If you don't need much power but want directionality, one interesting option might be to have a bright light aimed at a solar cell. You'll be lucky to get 1% efficiency. If that's enough, you're done. It's safe.
If you need very little power but have room for a physically large antenna, you might be able to build something that runs off ambient RF fields. Just make a big flat coil, wire it to a diode, and see what comes out. The output will vary enormously depending on how close you are to a transmitter. If you're lucky, you might be able to power a clock.
Here
You can accomplish anything you set your mind to. The impossible just takes a little longer.
Squirt a stream of water across the four inch gap, aimed at a small Pelton wheel spinning a generator on the receiving end.
If you can perhaps have some sealed or mostly sealed air chamber, you could transmit power through the air. Place a speaker at one end of an air chamber, and excite it with AC. You will experiment till you find the most efficient and useful frequency, it would most likely be the resonant frequency of the system though. Now, a speaker at the other end will definitly viabrate, with the same frequency, but a bit lower power level. Just an idea I dreamed up when playing with a couple subwoofers and holding a voltmeter on an unhooked one in a room with alot of bass.
I have an electric toothbrush that charges wirelessly, I assume by using coils. Try dismantling one of those?
I'd imagine so. This technique is also used to recharge the batteries in some pacemakers. It's just a non-conventional transformer.
But the problem he has is the distance. The charger for the pacemaker, toothbrush, electric shaver, etc. can be brought very close to the device being recharged: he wishes to charge something at a distance. The holder will also be aligning the coils very closely.
Here's the problem: magnetism follows the law of inverse squares. You double the distance, you get 1/4 the power. Roughly:
B = (a*Bo) / (d^2)
where a is alpha, a constant of proportionality;
d is distance;
B is the strength of the magnetic field at the receiver;
Bo is B naught, the strength of the magnetic field at the transmitter
Noting that the distance is a square term in the denominator, ain't much gonna happen.
Now, the magnetic field generated by the transmitter can be roughly approximated by:
B = muo*i*n
where B is the resultant magnetic field;
muo is mu naught, the permeability constant, approximately 1.26x10^-6 H/m;
i is the current in amperes through the coil;
n is the number of turns of wire in the coil
On the receiver side:
E = N * (dPhi/dt)
where E is the output electromotive force in volts;
N is the number of turns in the coil;
dPhi/dt is the rate of change of the magnetic field experienced by the coil, with respect to time.
Note that the magnetic field is inversely proportional to the square of the distance, as given in the very first expression. Note also that this assumes the coils are in perfect alignment so that 100% of the magnetic field (at whatever strength) is coupled into the secondary coil. In his application, good alignment is going to be virtually impossible. This coil-coil coupling will not work for his use.
Now, alternatives. Build a transmitter and transmit as RF (electromagnetic waves) rather than magnetic fields - not practical, since this would require a fairly large transmitter and would be extremely inefficient. The RF energy will not all be coupled to the toy, and the FCC is likely to get angry about your inefficiencies. Note that this is how Tesla was transmitting power wirelessly - I don't think the FCC even existed back then!
Microwave oven magnetrons? Same thing as above, it's just a transmitter, but >500W of microwave energy is not suitable for any toy.
Solar coupling? Hugely inefficient. Shining a bright light onto a solar cell will waste massive amounts of energy, but so far I think it's the most practical solution.
Fire and Meat. Yummy.
ferralis asks about distance, but doesn't mention the size of the objects on either side. This is crucial if you are trying to use magnetic coupling, because the range of the near field is determined by the size of the objects. However, it would not surprise me if you could get good coupling over a distance of 2-4 inches, using frequencies in the 27 MHz ISM band, using tuned coils. As ferralis is not a double-E he is not going to be building this himself, so he can ask the RF engineer to insert a "seek" circuit on the transmitter which pulses briefly but only remains on if it detects the load from the receiver coil in close proximity. (This is how a grid-dip meter works.)
If you can force the relative orientation of the transmitter and receiver, you can probably improve coupling by using ferrite "loopstick" cores for the coils and let the relative permeability work its magic.
Time is Nature's way of keeping everything from happening at once... the bitch.
Tesla earned kudos for the invention of the AC distribution system and the induction motor, which made possible the fractional-horsepower motor (one of which I am enjoying right now, as it is powering the fan keeping me comfortable). His experiments in wireless power transmission do not belong in the same category.
Worse than that: your mention of them in the same posting proves that it was easier for you to learn to post on Slashdot than to learn what you are talking about, and therefore that computers are too easy to use. ;-)
Time is Nature's way of keeping everything from happening at once... the bitch.
Of course lightning transfers current with out a wire in the conventional sense, but it doesn't go thru empty space. Once the cloud-cloud or cloud-ground potential difference is great enough, you get some ions and a nice toasty ribbon of plasma that the current can flow down. Sure, it's in the vapor state, but lighting does have "wires."
I know that this is a case of pedantic nit-picking coming from an physics ignoramus, but maybe it points ot a way to get this guy's job done. If you get the right frequency laser to shine from the power transmitting module on this guy's toy, to the receiver, hitting a copper target, a "wire" of ionized air would be formed. Then you can pump current into said beam, into the target and capacitor/battery assembly and be done with it. There are so many whopping problems with this approach, obviously.
I like lithium watch batteries, myself.
Why do I have this? I don't smoke.
- try searching for "contactless smartcards" .. i don't know much details about how is done, but these devices use contactless transmited power ... also see ISO14443 & ISO15693
- check Mifare & NFC technologies from Philips and Felica from Sony
- as a side note, my electric toothbrush(philips jordan) has a docking/charging station wich does its job without any metalic contact - it's just a plastic-on-plastic touch there, i remember being a bit boggled by the device when i first saw it :).
Again, no ideea how it's done, but at least you can be sure it's possible and very cheap for tiny distances :)
good luck!
"There is nothing more frightful than ignorance in action." Johann Wolfgang von Goethe
http://www.dissolute.com.au/avweb/522.html
'tis truly dangerous!
It sounds like you're using induction to do the power transfer. Calculate from first principles, most likely the Biot-Savart law directly, since your normal solenoid inductance calculation makes assumptions that aren't valid in your case. You will have to prepare and evaluate some integrals based on your off-axis geometry.
This post expresses my opinion, not that of my employer. And yes, IAAL.