Magnet levetation and a negative resistor - Page 1
Forum section: electronics / projects

author: John Heath
author_rank: Frequent Contributor
timestamp: June 02, 2016, 04:22:29 am
content: There was a clever engineer that found a way to design the impossible theoretically perfect electric motor. Perfect in the sense that constant voltage equals constant RPM regardless of load on the motor. The way he did this was to simulate a negative resistor in series with the DC motor to counter act DC resistance of copper winding and magnetic domains. It worked so well that it replaced spinning weights used to regulate speeds for cassette capstan motors for the last ten years they were made. A raised beer for this elegant solution of negative resistance for a perfect electric motor.With this in mind it should be possible to simulate a perfect degassing coil by putting a negative resistor in series with the winding as a closed loop circuit. If the spool of wire is 20 ohms then a 20 ohms negative resistor should return it to being a perfect coil of wire with 100 percent counter EMF when in a closed loop. In short a super conductor. If a magnet were placed on top of such a coil it should levitate , I think. Has anyone here made a negative resistor or have thoughts on this.

author: Zeranin
author_rank: Regular Contributor
timestamp: June 02, 2016, 05:00:02 am
content: Quote from: John Heath on June 02, 2016, 04:22:29 amMany years ago, I independently ‘invented’ the same method of adding ‘negative resistance’ to a DC motor driver, to improve speed regulation, and I’m sure it has been done independently many times both before and after. My motor drivers had a trimpot to set the negative resistance value, usually set to cancel the motor winding resistance, and thus provide RPM that was load independent, without the need for a tacho-based speed regulation circuit. If desired, one could set the negative resistance higher than the physical resistance, for a net negative resistance, with the cute result that the motor RPM wouldincreaseunder load.Inprinciple, you may be right, and a magnetic coil with active resistance cancellation would be able to provide magnetic levitation via the Meissner effect. In practice, I suspect it won’t work for at least 3 reasons. The first is that while a superconducting coil can provide up to infinite currents to dynamically cancel the PM fields and provide levitation, the actively controlled copper winding cannot. Secondly, the response time of the superconducting loop is ‘infinitely’ fast, whereas the actively controlled winding is not. Finally, the active cancellation of coil resistance is by it’s nature open loop, that is, you set a trimpot (or equivalent) as accurately as you can so as to match the negative and positive resistances, but such matching can never be perfect. Furthermore, the winding resistance will change with temperature, further exposing the limitations of trying to match the negative resistance to the winding resistance.How close you might get to achieving levitation with a ‘negative resistance’ circuit I don’t know. The idea is cute.

author: Zeranin
author_rank: Regular Contributor
timestamp: June 02, 2016, 05:29:32 am
content: Thinking further about your idea, I think it flawed right from the outset. A small permanenet magnet will levitate above aslab or discof a superconducting material, not above a superconducting magnetic coil, so your idea was never going to work, even if wecouldperfectly cancel the resistance of a current-carrying coil. Am I missing anything?Superconducting maget coils are fairly common, such as in MRI machines, and have the interesting property of 'sucking' in steel objects, such as a large oxygen gas cyclinder that was once left unrestrained in a hospital MRI room, which jumped into the machine at great speed, seriously smashing up both the patient and the MRI machine, but not producing levitation AFAIK.

author: zlymex
author_rank: Frequent Contributor
timestamp: June 02, 2016, 06:30:46 am
content: Magnet levetation is very common by applying active feedback such as magnetic levitation trains.I bought one of the magnetic levitation globe many years ago and all the electronic parts are in the base unit. The globe is just permanent magnet.

author: Zeranin
author_rank: Regular Contributor
timestamp: June 02, 2016, 06:59:51 am
content: Quote from: zlymex on June 02, 2016, 06:30:46 amYes, that is true, also magnetic bearings, but AFAIK these active feedback systems rely on some sort of position measurement? But I could be wrong, so please describe how your levitation globe works. Whereas levitation above a superconducting slab of material does not require position measurement at all. One can also levitate aspinningmagnetic top above a permanenet magnet base, without active feedback, but the top needs to be spinning for this to work.

author: zlymex
author_rank: Frequent Contributor
timestamp: June 02, 2016, 01:01:19 pm
content: Quote from: Zeranin on June 02, 2016, 06:59:51 amThat's right. The levitation does need positional sensors, and the levitation only take place in a small area which requires very skillful initial positioning of the globe. I didn't open the base to examine the principle but reading from some article about it.There is another sort of passive(no electricity, no cooling, no spinning or other sort of energy required) top level(the floating object is above the base, no other object on top of the floating object), that is, diamagnetic. I took this photo several years ago.

author: John Heath
author_rank: Frequent Contributor
timestamp: June 02, 2016, 11:40:37 pm
content: Quote from: zlymex on June 02, 2016, 06:30:46 amI was thinking of the magnet inducing electron movement in the coil. The coil of wire has the benefit of 0 resistance by a op-amp compensation for negative resistance so the current should continue holding the magnet up. For my 2 cents I think your first assessment was closer to the mark. The delay in the coil to react to a magnet that could be rotating. Also one grand spool of wire could not counter a magnetic field that is not uniform. This would require many small 0 ohms coils and they would have to be fast to keep up with a magnet that is rotating.

author: rs20
author_rank: Super Contributor
timestamp: June 03, 2016, 12:10:54 am
content: Quote from: Zeranin on June 02, 2016, 05:29:32 amA shorted-out superconducting coil (shorted out, of course, with more superconductor)willcause levitation. After all, a slab/disc is just a coil with a single turn.Quote from: Zeranin on June 02, 2016, 05:29:32 amAn MRI coil contains superconductor to reduce losses, but there's still a CC power supply driving the thing. This gives the entire coil ahighimpedance to AC, hence why it behaves much like a permanent magnet itself. Once again, if the two terminals were hooked up to a superconducting short (with initial current = 0) instead of a power supply, it'd act more like a traditional slab of superconductor, although exactly what sort of behaviour would be manifested is beyond me.(Note I'm only 90% sure an MRI machine has a power supply. Maybe they do have a superconducting short. Nevertheless, the coil is initialized in this case with a high current, which makes it look like a permanent magnet and overwhelms any counter-currents that an incoming permanent magnet may attempt to induce.)

author: Zeranin
author_rank: Regular Contributor
timestamp: June 03, 2016, 01:48:24 am
content: Quote from: zlymex on June 02, 2016, 01:01:19 pmYes, I was aware of that. Earnshaw's theorom says that you can't (without active feedback) levitate a non-spinning PM magnet above another magnet. However, just as you say, diamagnetic materials such as bismuth or pyrolytic carbon can be levitated above a powerful permanenet magnet. Very cute!

author: Zeranin
author_rank: Regular Contributor
timestamp: June 03, 2016, 02:09:39 am
content: Quote from: rs20 on June 03, 2016, 12:10:54 amAre you sure about that? One reason levitation of a small magnet above a large magnet fails is because the small magnet flips over. If you mechanically prevent the flipping such as a magnetic bead on a thin nylon thread, then levitation happens perfectly, because you already have the right sort of repulsion-vs-distance characteristic, whereby the closer the small PM gets to the big one, the larger the repulsive force. So what makes you think that a superconducting coil, as opposed to a slab/disc of superconducting material, will achieve levitation of a small permanent magnet? A quick net search revealed no example of such levitation.It is not at all clear that a slab/disc of superconducting material is ‘just a coil with a single turn’. I would argue strongly that it is not.

author: John Heath
author_rank: Frequent Contributor
timestamp: June 03, 2016, 09:23:37 am
content: Quote from: Zeranin on June 03, 2016, 02:09:39 amOne way around the magnet wanting to flip is quantum flux pinning. Would like to speak it this but can not as I do not understand flux pinning , ha.

author: Zeranin
author_rank: Regular Contributor
timestamp: June 03, 2016, 10:14:19 am
content: Quote from: John Heath on June 02, 2016, 11:40:37 pmI may regret telling you this JH, but I can easily model superconducting materials with the FEA magnetic modelling program. Superconductors reject all magnetic flux, which is another way of saying their magnetic permeability is zero, or magnetic 'resistance' is infinite, which is easily handled with FEA modelling. Likewise, any diamagnetic material (relative permeability less than1) is easily FEA modelled. So if you want to see the magnetic field lines when a small permanent magnet (PM) is placed above a slab/disc of superconducting material, then easily done. I guarantee that the distorted shape of the magnetic field of the PM willnotbe the same as you would get if you substituted a current winding in place of the slab of superconducting material, reinforcing my point that the two are not equivalent. 'Flux pinning' as per you very cool video is another step further that I can't model, and in this case the flux does pass through the superconductor, but this is not necessary to produce levitation.I agree with you that moving the position of a small PM above a zero resistance winding will induce a permanent current in the winding, but maintain that you cannot achieve stable magnetic levitation of a small PM above a current-carrying coil, regardless of how the current in the coil is controlled, or varies as a function of the PM position.Here's the problem. To achieve levitation, you need a stable equilibrium in every one of the 6 degrees of freedom, 3 translational and 3 rotational. When a small PM is placed on-axis above a large PM or current-carrying coil, then it's easy to obtain an equilibrium in all 6 degrees of freedom, without the need for any feedback or zero resistance of the coil. The PM will move downward until the downward force of gravity is exactly balanced by the upward magnetic repulsive force. This force balance in the vertical direction is stable, because if the PM moves below the balance point then it will be pushed upward, and if it moves above the balance point then it will be pushed downward. The forces in the other 5 degrees of freedom will also be balanced, provided the PM is exactly on-axis, and exactly vertical, but in these degrees of freedom it's anunstableequilibrium. For example, if the magnet should tilt even the tiniest amount away from vertical, then the resultant unbalanced torque acts to tilt the PM still further off vertical, and so on, with the result that the PM 'flips over', and then the repulsion turns to attraction, and all levitation is lost as the PM gets pulled down onto the large PM below.Blah, blah, you know all that. But here is the crunch. It does not matter how or how well you control the coil current, including current in a superconducting coil, you can't alter the fact that when using a current carrying coil to 'levitate' a small PM, you only have a stable equilibrium in the vertical direction. It can't work and it won't work. As a corollary to that, no matter what coil current is chosen as a function of the position of the small PM above, you can't exclude flux from passing through the coil (as happens with a superconducting disc/slab) because the SHAPE of the field produced by your coil is fixed, and therefore cannot cancel the different shape of field produced my the small PM. I can easily demonstrate all of this with FEA modelling, but I don't need to. I'm sure you can see what I'm saying anyway.

author: rs20
author_rank: Super Contributor
timestamp: June 03, 2016, 11:09:25 am
content: Quote from: Zeranin on June 03, 2016, 02:09:39 amConsider the argument of gradual change. The change from a slab of superconductor, to a slab of superconductor with a hole in it, to a slab of superconductor with holes and grooves in the side, to a closed loop/helix of superconductor, is a perfectly gradual change. If you propose that the solid slab levitates, and the shorted coil doesn't, I think the onus is on you to explain where in this gradual morph the effect suddenly breaks down and the magnet "flips over" instead of levitating. Perhaps an infinitesimally thin superconducting coil can't levitate a magnet, but that's not related to the claim I'm making about a superconducting coil where the superconducting wire has considerable thickness, and therefore can not be simplified to a single-dimensional device.IIUC, a superconductor effectively provides a mirror image to the magnet floating above it. If the magnet starts to turn, the mirror image turns as well, and thus no divergent behaviour is possible. Thus, the behaviour of a superconductor is totally different to a permanent magnet.Finally, don't confuse Type I and Type II superconductors. Type I superconductors cannot be penetrated by magnetic fields. Type II can.Addendum: It's a joke to think your FEA program can handle superconductors, unless it is some specialized progam specially designed for the purpose. Simulation programs are amplifiers of knowledge, not sources. Just readthis, and try to tell me that your FEA program can handle this wacky s***:Quote

author: Zeranin
author_rank: Regular Contributor
timestamp: June 03, 2016, 11:22:33 pm
content: Quote from: rs20 on June 03, 2016, 11:09:25 amWe are at cross purposes. Yes, a slab of superconducting material with a hole in the centre is (or at least may be) capable of levitation, which is really what you are saying. However, that misses the point of the OP, which was whether you can produce levitation with a coil of wire that is made to behave as if it had zero resistance, as per the topic name of the thread, and the answer is that you can't, for all the reasons I gave. In the same context, a slab/disc of superconducting material is clearly not 'just a coil with a single turn'.What I said about magnetic modelling FEA is correct. I made it clear that standard FEA (quite obviously) can't handle quantum flux pinning. However, Type2 superconductors can produce levitation, and may be regarded as a perfect diamagnetic material, that can be modelled satisfactorily by standard FEA, that will accurately predict the shape and direction of the Bfield when a small magnet is placed above the superconducting material. I'll happily provide such a simulation if you like, and if you can fault it, then I doff my hat to you.Thus, the behaviour of a superconductor is totally different to a permanent magnet.Of course it is. We both agree on that, and it is also completely different to the field behaviour that can be produced by a current-carrying non-superconducting coil, regardless of how the coil current is controlled, which was the original question. I'm hoping that with a few minor misunderstandings out of the way we now agree on everything, as scientists and engineers almost always do, in the end.