Questions About Highly Coupled Magnetic Resonance

[G Cooke]

This is one of very few in-depth sources of information I can find online about Highly Coupled Magnetic Resonance: https://www.google.com/url?sa=t&sou...FjACegQIBRAB&usg=AOvVaw2h6obU5MmoUtjYup9I5zxA

The last sentence of the third-to-last paragraph says that research is underway that may someday lead to vehicle charging on the move. I'm guessing that the reason that this requires extra research is that relative motion between the source and device resonators affects power transfer. So I'm curious, how would you begin to conceptualize the effect of motion (both linear and rotational) on Highly Coupled Magnetic Resonance?

On a related note, I'm curious what the equation for the magnetic coupling coefficient k mentioned in equation 9 would be? The second paragraph after equation 9 defines k as "a dimensionless parameter representing the fraction of magnetic flux that is coupled between the source and device resonators" and describes it as "a function of the relative sizes of the resonators, the distance between them and their relative orientation". Based on the description, I imagine it contains a cos(theta)/r^3, where theta is the relative angle of the resonators and r is the distance between them, but I'm not sure where the relative sizes of the resonators fit in.

I know that I could replace theta with 2*pi*t/T + phi, where T is the period of rotation, phi is the initial relative angle, and t is time, to see the effect of rotation on power transfer over time; likewise, I could replace r with |v*t + x|, where v is relative velocity and x is the initial distance, to see the effect of linear motion on power transfer over time; however, I don't think WiTricity would need to do extra research into charging on the move if those were the only effects. I think there must be a specific effect on the magnetic coupling itself as a result of relative motion.

 

[sophiecentaur]

I have four children, in different parts of the world, none of whom can run an electric vehicle because of the ' electrical connection' problem on public roads where they park. Contactless power transmission would be really handy for them but the necessary infrastructure would / will be very expensive. Energy metering would be 'trivial' so you could park anywhere as long as people used the designated spots. (That would involve some education for some drivers.)

For doing this on the move, the infrastructure would need to be many thousands of times bigger if every car can use it over most of its journey each time. Efficient power transmission on the move, as the vehicle is bouncing up and down and moving from pad to pad, sounds like too much additional trouble in control and capital cost.

PS I wonder why people have to use the name of Tesla to justify their proposals. He never actually built such equipment. It was one of his pipe dreams.

 

[G Cooke]

Thanks for your reply, Sophie.

I have a few questions.

1) Why would the infrastructure for contactless power transmission be expensive? I'm imagining the resonator circuit shown in the document being duplicated as needed, along with meters. I don't immediately see anything there I would think of as expensive, though admittedly I don't have enough experience to have an intuitive sense of the costs involved.

2) Why would the infrastructure need to be many thousands of times bigger for on-the-move charging? It seems like it could actually be smaller since instead of dotting the roads and cities with enough charging pads for the expected number of cars, you could just have central source resonators feeding multiple cars each. (Q can be very large to compensate for low k, k being low because the distance is large.)

3) How does bouncing up and down affect power efficiency? I don't see how any non-relativistic-scale movement would affect power transmission in any significant way since the resonators are coupled via frequency matching of a field which is propagating at the speed of light, which means the doppler effect from such non-relativistic relative motion is negligible. (This is also where I'm stuck in trying to conceptualize the effect of motion on coupling as per my original question.)

4) This isn't a question about your reply, but I found seemingly conflicting information about the effect of solenoid layer count on Q. This Wikipedia page says that using single-layer solenoids gives "improved Q" (under last section "Mechanism details", under second-to-last subsection "Transmitter coils and circuitry"), while this electronics magazine says that they add multiple layers because "it increases the Q of the coil" (last paragraph). Could you help me understand this apparent discrepancy? The electronics magazine wasn't speaking specifically about magnetic resonance, but I'd still think the same definition of Q would apply.

I take it from your answer that there is in fact no specific effect on coupling due to motion (other than at relativistic speeds, which are irrelevant to the motion we're considering).

The Tesla thing is curious. I think a lot of people idolize Tesla to some degree (myself included), so mentioning his name probably adds a lot to one's own feeling of accomplishment when furthering his goals.

 

[sophiecentaur]

@G Cooke the cost of rebuilding all roads with buried coils over their whole length would be immense. Bad enough if enough static spots were available for all vehicles. It’s a bit like underfloor heating only built much more robustly. And what about repairs?
The spacing(and lateral position) between vehicle and supply coils would affect the mutual inductance and, if you need resonance, I think there would need to be automatic continuous tuning to compensate.
I know it won’t be long before portable devices are charged cordlessly but that’s easy to arrange on a table top with very small spacing. Efficiency for that doesn’t matter either.
Tesla will live for ever, since David Bowie played his character in that film.

 

[G Cooke]

@G Cooke the cost of rebuilding all roads with buried coils over their whole length would be immense.

This is true; however, if Q is designed very large, and assuming that there is in fact no specific effect on coupling due to motion, then it follows from equation 9 that:

instead of dotting the roads and cities with enough charging pads for the expected number of cars, you could just have central source resonators feeding multiple cars each.

In the case of central source resonators, there would be no need to excavate roads. Thus, the size of the infrastructure required would be much smaller, and the cost would be significantly reduced.

The spacing(and lateral position) between vehicle and supply coils would affect the mutual inductance and, if you need resonance, I think there would need to be automatic continuous tuning to compensate.

This is true, but if Q is again designed very large, then according to equation 9, which shows how mutual inductance is included within k and how what matters is the product of k and Q, high Q can compensate for low mutual inductance. As for resonance, the resonators consist of RLC circuits, which are tuned according to equation 1, which depends solely on the chosen inductance and capacitance values. If designed correctly, then as long as the source resonator is powered at the resonant frequency (or a harmonic thereof), the resonators will resonate. Automatic continuous tuning is necessary in applications where a variable inductance or capacitance is manipulated via a feedback loop to control a voltage gain, for example.

Lastly, I'd like to point out that I'm only using WiTricity and its vehicle charging potential as a reference point to make it easier to ask questions about the topic -- essentially, I'm taking advantage of the fact that Highly Resonant Magnetic Coupling is already "out there" to some degree; however, my purpose for asking my original two questions as well as questions 1-4 of my previous post is to help me complete a project of my own which is related to, but very different from, WiTricity and its vehicle charging aspirations. If it would be possible, I would appreciate your and any other experts' answers to those questions.

 

[DaveE]

if Q is again designed very large

Yes, but how. It sounds easy when you're doing maths, but the real world is lossy in many ways. One of those ways is when you are extracting energy from the resonators to charge your car.

 

[sophiecentaur]

This is true, but if Q is again designed very large,

There is a huge problem with the idea and it's a fundamental one. If you want to 'deliver' 1kW of power to the car and you have a "very large" Q - say 1000, the mean Power that's oscillating around the resonator will be 1000kW, (that's what the Q factor means) with massive Currents and Voltages. In reality, to charge a car in a reasonable would require something around 10KW. So the structure (under every car) would need to handle several times those volts and currents. That would involve heavy gauge copper loops to limit dissipative losses etc. etc.

This problem is not very serious for low power charging because inefficiency is not relevant when you're talking about a Watt or two. I think you imagine the car charging station would consist of a few turns of 1mm wire and a capacitor off the shelf. In all Engineering projects, the actual Numbers are what counts. Things don't just scale.

Edit: Also, the sensitivity of the natural frequency of a resonator to things like spacing and position will be dependent on the Q. Furthermore, the mythical idea of 'broadcasting' power from a central source just doesn't hold water.

 

[alan123hk]

It seems like it could actually be smaller since instead of dotting the roads and cities with enough charging pads for the expected number of cars, you could just have central source resonators feeding multiple cars each

Could you briefly explain how this "central source resonators feeding multiple cars" works ?
For example, I think each resonator must have its own automatic tuning impedance matching network, and this impedance matching network is complicated and not low cost.

Q can be very large to compensate for low k, k being low because the distance is large

This is theoretically correct, but, as described in #7, there are many limitations, so the Q value of the inductor cannot be increased indefinitely. Moreover, when the Q value increases, the technical requirements of the impedance matching network also increase

https://www.electronics-notes.com/articles/basic_concepts/q-quality-factor/inductor-q-factor.php
https://economictimes.indiatimes.co...on-the-move/articleshow/59118553.cms?from=mdr
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5579727/

 

[Frodo]

Off topic but electric trams with overhead wires charge batteries on the move - see https://en.wikipedia.org/wiki/Nice_tramway.

Most of the Nice routes are overhead wires but the tram batteries are used across scenic places like Place Massina and Place Garibaldi where overhead wires would be an eyesore.