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Wireless Power Transfer via Strongly Coupled Magnetic Resonances

  1. Mar 27, 2012 #1
    Hi eveyone. I really need your help for this project that I will be undertaking soon. But there is much I still don't understand and my lecturers aren't around to help.

    The setup that I'm envisioning is like this-- Source(230V 50hz) connects to signal generator, signal generator to 1st RLC circuit and hence, magnetic induction of EMF in the 2nd RLC circuit. The 2 RLC circuits have the same resonant frequency and so they exchange power efficiently.

    So the questions I have are basically:

    1.) What voltage should I use for the output of the signal generator? Does it depend on the ratings of the capacitors and inductors?

    2.) There will always be a resistance in the LC circuits due to the resistance of the inductor. But on top of that, I need to add a separate resistor because during resonance, the reactances of the L and C cancel each other. So there would be very low impedance in the circuit , resulting in high current thus causing the power to trip. So how do i know what value of resistance to use? And is a resistor necessary for the receiving coil?

    3.) Resonant frequency= 1/(2*∏*√(L*C)). So by changing my values of L and C, I will change the resonant frequencies of the RLC circuits which I am trying to tune both circuits to. Am I supposed to adjust the L and C values until I get a frequency in the ISM band?

    4.) I don't understand the significance of 'Q' in power transfer. What is its implication on efficiency? Also is it true that silver plating the inductor coils can increase Q?

    5.)How would I reduce the size of the receiving coil to fit into a portable device?

    Sorry if these questions are very basic but I would really appreciate any help. Thanks a lot.
  2. jcsd
  3. Mar 27, 2012 #2

    If you are going to use an RLC circuit, additional calculations must be done accounting for the resistance. I think you'll want a resistor that will make your circuit critically dampened (but I really don't know for sure). From what I understand, a high Q means high efficiency. I have done something similar in the past using two identical LC circuits and a crude astable multivibrator as a power source, yet only got reasonable power transfer a few millimeters. How far are you looking to transmit? How large is your current setup? And how small do you want to make the receiver?
    Last edited: Mar 27, 2012
  4. Mar 28, 2012 #3
    "How far are you looking to transmit?"

    About 2 metres with an efficiency of about 40% should be fine

    "How large is your current setup?"

    Sorry but I don't have a setup yet. I need to understand how to do the setup first before I start. And which part of the setup are you asking about?

    "And how small do you want to make the receiver?"

    Around 2-3cm radius coil to fit into an ordinary cellphone.
  5. Mar 28, 2012 #4


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    Yes, this is relevant point because the load resistor is a significant damping factor.

    If you are after an efficiency of 40% then I think you should bear in mind that a properly designed iron-cored transformer still manages to lose 2%. This is when 'all' the flux is linked between the two windings. A separation of 2m will involve virtually all the primary flux leaking elsewhere (why should it go from primary to secondary, rather than anywhere else?) - giving an efficiency value of more like the 2% that the conventional transformer loses.
    There is a very long and turgid thread here which discusses some experiments on 'wireless power', based on ideas of the dreaded Nicola Tesla, who is a God for some people.
  6. Mar 28, 2012 #5
    " A separation of 2m will involve virtually all the primary flux leaking elsewhere (why should it go from primary to secondary, rather than anywhere else?) - giving an efficiency value of more like the 2% that the conventional transformer loses."

    I'm sure you've heard of the MIT team that managed to transfer over 40% of the input power over a distance in excess of 2m. They did this using strongly coupled magnetic resonances. So it is possible but what I'm asking is how to do it.

    To everyone: Please help me answer the 5 questions in my first post. Your help is much appreciated.
  7. Mar 28, 2012 #6


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    But you are discussing a 50Hz system?
  8. Mar 28, 2012 #7
    I initially considered adjusting the L and C values to get a resonant frequency of 50hz. But it seems to be impractical as the values of L get too large. So I will use a signal generator to get a higher frequency voltage.
  9. Mar 28, 2012 #8


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    The actual choice of frequency is extremely relevant and I suggest that you read the details of any other work you can find out about. You should also make sure that any frequency you choose to work at will not generate interference to radio services - or you will have the 'heavies' from the regulatory services knocking on your door.
  10. Mar 28, 2012 #9
    Don't you need an antenna to focus the energy? I think you will lose a large amount of energy from spreading losses and attenuation.

    Also, in their experiment they used a 9MHz frequency, not 50 Hz.

    Your Q factor has to do with the resistance in your RLC circuit and how damped the oscillations will be. A perfect harmonic resonanting circuit will not dissipate any energy and will be tuned to a single frequency and will have the maximum Q factor, but this is impossible physically because of losses. This means that the higher the Q factor, the more energy you will capture at your resonant frequency.

    Q factor has different interpretations and applications, but it has to do with how much fraction of gain you have at your resonant frequency to how much the rest of your gain is spread out across a bandwidth of other frequencies. You will want a very sharp peak at your resonant frequency, basically a bandpass filter with very high attenuation at all frequencies other than the resonant frequency.
  11. Mar 28, 2012 #10


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    This matter of Q is a bit hard to nail. If your transformer has a load which is dissipating power then surely the resistance is relevant and will produce damping. I think the point of using a resonance is to achieve matching so that the coupling between coils is at a suitable impedance such that the damping of the load is minimised. You need to get the mutual impedance to be comparable with (higher than?) the self impedance.

    Btw, this is not an 'antenna' system. It is a near field system which is based on coupling, rather than the launching of a far field wave. The wavelength at 9MHz is around 30m, so the two halves of the 'transformer' are well within a fraction of a wavelength.
  12. Mar 28, 2012 #11
    I'm not sure I understand what you mean..

    How do you get 30m? Using v=fλ? If yes then what is the value of v?
  13. Mar 28, 2012 #12
    Its the propgation of the speed of light in air.
  14. Mar 28, 2012 #13


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    Mutual impedance:
    When you have two elements, there is an Impedance Matrix which tells you the Voltage in any either element in terms of the currents in the two elements. It's like the matrix that describes how a transformer performs. You have self and mutual inductances in that case. If elements are wide apart, the mutual impedance will be low (low induced voltage for a given current in the other element) . Look up antenna impedance matrix.
  15. Mar 28, 2012 #14
    What will be the approximate physical sizes of your coils?
    How will they be oriented with respect to each other?
    Last edited: Mar 28, 2012
  16. Mar 28, 2012 #15
    For the transmitting coil, the radius used depends on the inductance I want to create for the resonant frequency I want to use since fres=1/(2*π*√(L*C)) . But I don't know what fres to use. Can it be anything within the ISM band? Like i was wondering why the MIT team chose 10 Mhz. Was it random or is a higher frequency better for efficiency?

    Regarding the orientation, the receiving coil will be inside a cellphone. And if I'm not mistake, the receiving and transmitting coils cannot be 90 deg to each other or there will be no cutting of flux right? Which means the handphone (assuming the coil is laid flat inside the case) must be held parallel to the source coil.
    Last edited: Mar 28, 2012
  17. Mar 28, 2012 #16
    From: http://web.mit.edu/newsoffice/2007/wireless-0607.html
    The above suggests that the coils were large, for a distance of 2 meters, the coils may have had a diameter of 0.2 meters or larger. It seems to me the larger diameter your coils the better your coupling will be. Likewise I would guess that the higher the Q the better the coupling.

    I don't know why they chose the frequency they did. The lower your frequency the farther the near field or non-radiative field will reach. You might want to wind many turns on a ferrite rod for antennas. The rods may be oriented either parallel or inline for good coupling.

    There is a band from 160 kHz to 190 kHz that will allow you to transmit 1 watt. If you don't want to get a license, check out part 15 of the FCC regulations to find out what you can do.

    http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=a0c46034a7c291b54e709279ac262483&tpl=/ecfrbrowse/Title47/47cfr15_main_02.tpl [Broken]
    Last edited by a moderator: May 5, 2017
  18. Mar 9, 2013 #17
    Wireless Transfer by Magnetic Resonance Coupling

    I need some help for designing Wireless energy transfer by designing transmitting and receiving modules via Magnetic Resonance coupling through coils. I have some basic questions if anybody can help me:
    1- What RLC values should I use to design transmitting and receiving modules and producing high Q factor?
    2- What output should use from function generator?
    3- How can I reduce the size of receiving coil?
    Please help me if any one have already worked on such project.
  19. Mar 9, 2013 #18


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    What have you found out already about this?
    What frequency did you plan to use and how much wireless power did you want to transport. Also, how far did you want the wireless link to operate?
    There are a number of You tube videos showing bench-top projects that have shown the principle and I have seen constructional details on some (I don't remember where, but they are out there). You could search as well as I could and you would find a lot more stuff on the way, which could be more useful for you than some link I could find for you.
    It would be a fun project btw.
  20. Mar 10, 2013 #19
    Thanks for your Reply
    As output from receiving module I want to gain power to charge a mobile phone or operate simple 9 V motor. I have tried with many coils but still not getting good Result so trying to get information before designing complete modules.
    Resonance Frequency I need to produce definitely high but exact value I am not sure , If I am not wrong that also depend upon the selection of RL and C values that I really need to know…
    Distance should me about 1to 1.5 m . Yes You tube is good source to search , if you also get good video link that exactly shows values of RL & C please share with me.
  21. Mar 10, 2013 #20


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    This is definitely not something you can achieve by trial and error - there are too many important variables involved. It's Engineering, with a very big capital EEE, in which the actual numbers count. You need to be able to answer the questions in my last post if you want to make any headway. It's really not a trivial thing. If it were, all our phones and laptops would already be getting their power this way. Systems may be available before too long at a reasonable price but it's not in all the shops, is it?
    The way forward for you, imo, is to find a link which describes someone's successful design and start with that. As I wrote before, there are Videos with most of what you need. They will help you avoid endless disappointments and frustration.
    Can you light an LED or run your motor with the direct output from your signal generator? Then can you do the same thing with a 10kOhm resistor in series? That could represent the amount of power loss in any system you might design / choose randomly.
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