https://www.kickstarter.com/projects/14 ... hoverboard
I could imagine some metal skate parks being quite interesting.
Some discussion I came across on this:
OK, does someone actually fluent in electromagnetics (i.e., not me) want to read their patent and determine whether it’s B.S.?
Credentials: Bachelor’s degree in engineering physics (mechatronics specialization); Master’s thesis on motor design, currently do magnetomechanics as a career.
It is workable in principle. However, their claims that it will eventually run over non-conducting surfaces *is* absurd. This technology will likely only run on copper or aluminum surfaces.
Rather than suggesting that the hoverboard will run over non-conductive surfaces, Hendo’s patent suggests that a variety of surfaces may in the future be rendered electrically conductive by suitable additives, e.g. electrically conductive concrete. That’s… a plausible future technology, but comes across to me as wishful thinking.
They use a rotating disc of magnets, which is a pretty smart idea. The alternating N-S poles sweep across the copper surface, which induces currents that attempt to counter the field of permanent magnets. The copper plane behaver as a ‘mirror': the induced electric currents in the copper create the same magnetic field as would be created by a mirror-image of the spinning disc below the surface. That means the N poles on the hoverboard will be facing the N poles on the mirror image, and likewise with the S poles. Like poles repel, and the lift is provided this way.
If the copper were instead a superconductor, the magnets would not have to rotate at all. Even a stationary magnet lowered down onto a superconducting surface would induce a perfect mirror image current that would repel it. However, in a non-superconductor, the currents will decay with time due to the finite resistance of the metal, and the magnets would settle down to rest on the copper surface as the currents disappear. Spinning the magnets keeps the current going, and thereby sustains the lift, making up for the fact that copper is not a superconductor.
There are some practical concerns, so I’ll quickly run some numbers. The important ones are:
* Magnetic field needed to lift a practical load
* Power needed to sustain the lift
The hoverboard discs look about 20 cm in diameter, and there’s four of them, and they have magnets just along the perimeter. To support someone who weighs 80 kg, 800 N of thrust is needed. So the force per unit area is something like 20 kN per sq. metre.
The pressure between two repelling magnets can be approximated roughly as 400,000*(B^2) N/m^2. A reasonable magnetic field density obtained by neodymium magnets is between B=0.1 Tesla to 0.5 Tesla, depending on the ratio of the magnet thickness to hover height. It looks like about 0.2 T would be enough to develop the necesary lift, which is definitely practical. It will take a lot of magnet though, which makes it pricey.
The spacing of the N-S poles of the magnets should be not smaller than the hovering clearance, or else the magnetic field won’t be forced to go through the copper. It also shouldn’t be to much bigger or else it wastes a lot of magnet material. Assuming it hovers 1 cm off the ground, there could be about 20 north/south pole pairs.
It’s very hard to calculate the resistance for current flowing through a solid copper surface, but I’ll do a first-order approximation by assuming that it flows in a uniform rectangular pattern along the top side and back along the bottom side. If the copper is 6 mm thick, and the magnets are spaced at a pitch of 25 mm, then about 10,000 A of current flows in the copper and the equivalent resistance is about 0.8 micro-ohms.
So that amounts to 80 watts of power draw to keep the magnets spinning, or 320 watts for the whole hoverboard. (Again, this is a very rough calculation; I don’t know the size and specs of the actual Hendo design nor its surface, so I’m just choosing the numbers that I would use if I built one). 320 watts is quite high, but for a lithium-battery system it’s not too impractical, since the battery life doesn’t need to be that long. A 1.5 kg battery pack will keep it going for an hour.
The power loss will appear as drag torque on the spinning magnet discs. The discs will need to be arranged in counter-rotating pairs to keep the whole hoverboard from spinning (just like the rotors on a Chinook helicopter).
Directional thrust could probably be achieved by tilting the plane of the rotating magnets, although I’m not going to work out the details of this.
My conclusion: It’s not a hoax. But it will only hover on metal surfaces for the forseeable future.
Hendo technology probably won’t make maglev trains cheaper; actually, it’s based on inductrack maglev train technology. The rotating permanent magnets are a smart way of saving energy, rather than a more traditional approach of using electromagnets in an alternating-current configuration. But in enormous vehicles like trains, it becomes cheap to have a liquid-nitrogen cryogenic system and use superconducting magnets (Japan’s approach, currently with a working prototype). Meanwhile inductrack uses the train’s forward motion to generate the lift, rather than needing the magnets to spin.