Resonant Coupling for Optimal Power Transfer - Page 1
Forum section: electronics / projects

author: electricfan
author_rank: Newbie
timestamp: October 16, 2023, 04:56:01 pm
content: I am trying to read the following paper:https://www.cs.cmu.edu/~claytronics/papers/cannon-tranpe09.pdfand understand their design for a four coil power transfer.The proposed system utilizes resonant coupling to wirelessly transfer power from a single source coil to multiple receiver coils. The experimental setup, as depicted in Fig. 1, involves a signal generator that supplies power to two LEDs through resonant coupling between large green coils and smaller black coils. Fig2 shows the schematic of their design.In essence, my question is why can we just not use the first coil as both receiving power, and having resonance(make a two coil system, and not four coil system).The paper in section VII briefly talks about why this might be the case, but unfortunately I do not understand their explanation. They refer to thisAnd here is their explanation:"The circuit model provides a simple way to understand what is accomplished by the resonant coupling mechanism and how it differs from simple inductive coupling. Fig. 4 shows three circuits providing inductive coupling to the load. Fig. 4(a) shows simple nonresonant coupling, while (b) and (c) shows variations of resonant coupling. In Fig. 4(b), the source and load resistances are approximately in parallel with parallel LC resonant circuits if the ohmic resistances R2 , R3 are small. Here the circuit Q is determined by the loading provided by RS , RL. In contrast, in Fig. 4(c), which corresponds to the system described in this paper, the source and load impedances are inductively coupled to the resonant circuits. In this case, the turns ratios can be chosen so that RS , RL are transformed into much larger effective resistances in parallel with the LC resonant circuits, resulting in a larger Q. Fig. 5 shows |(VL /VS )(ω)| as computed using these circuit models and component values appropriate for our experiment, as given in the Appendix. For the case of simple inductive coupling [Fig. 4(a)], the transfer function is almost constant at −58 dB for frequencies between 1 and 10 MHz, with low and high frequency 20 dB/decade roll-offs. For the values chosen, the Q of the circuit shown in Fig. 4(b) is relatively small, with only a weak resonant peak at a frequency that corresponds to ω0. In contrast, the impedance transformation provided by the coupling coils L1 , L4 for the circuit shown in Fig. 4(c) results in a much higher Q and coupling that is stronger by about 40 dB than the circuit shown in Fig. 4(b). Thus, the low Q and purely inductive systems have efficiencies many orders of magnitude lower than the high Q system."It seems to me like the first coil is optimizing for "power transfer", and the second and third resonant coils are optimizing for "resonant frequency". Wouldn't the most optimal power transfer be at the resonant frequency, and using two coils instead of two would introduce unnecessary losses? I dont really get which formulas necessarily would prove that the four coil setup is better compared to the two coil one. I know that Q = (w_0*L)/R which leads me to think that more coils = more resistance and lower Q factor.Thanks!

author: moffy
author_rank: Super Contributor
timestamp: October 16, 2023, 09:05:42 pm
content: Without looking too deeply into it I would expect you are correct for your reasons given. Had a similar situation, though not air coupling, some years ago where our lab was using two transformers in series, one driving the other, to generate a 1000V AC output. Another lab said it would be more efficient to use just a single transformer, so to prove my boss correct, whose design it was, I did a series of calculations and ended up proving that a single core would be more efficient. The other lab was surprised when I shared the result.

author: jonpaul
author_rank: Super Contributor
timestamp: October 16, 2023, 09:09:36 pm
content: See Nikola TESL A (NOT THE CAR!) Colorado Springs papers and notes 1880s..1900"Extra coil"Old newsJon

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 17, 2023, 03:12:30 am
content: QuoteI think you can, if you wanted to.Often the research setups explore more than simple coupling. That's been done alreadyOne simple test you can do here is to just take the resonant coil away, and see what happens to energy in : energy out.This looks like a test for a more general coupling use, and where the coils are not tightly coupled or placed.- I can imagine the top 3 are on a vehicle, for example.The resonant coil matches the driver coil much better than the small pickups, so it can extract energy and set up a larger resonant field.The smaller coils now have a higher field to extract energy from.Losses will still be significant IN:OUT, but placement may have less effect with the resonant coil used.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 17, 2023, 07:13:06 am
content: FYI, also posted here:https://electronics.stackexchange.com/questions/685489/resonant-coupling-for-optimal-power-transferin case that leads to any cross-pollination.Tim

author: electricfan
author_rank: Newbie
timestamp: October 17, 2023, 02:10:42 pm
content: I did come across a summary of his notes, but was not able to find anything related to "Extra coil" unfortunately.

author: electricfan
author_rank: Newbie
timestamp: October 17, 2023, 02:22:28 pm
content: They also provide resonance values for the coil: k12 = 0.17 k13=0.0010, k14 = 0.021, k23 = 0.0047, k24 = 0.021 k34 = 0.76. You are indeed correct that 2 and 3 are not coupled. So it seems to me like we want resonance for coils 2 and 3 due to that lower coupling so we are able to be as efficient as possible with transmission. However, why can we not just also have resonance in the first coil which would allow us to effectively get rid of the second one?