Magnic Light: contactless bicycle dynamo light

[Martin]

Last week the Eurobike show was on in Friedrichshafen / Germany.
I visited the Reelite booth and learned about a new light system that doesn't need spoke magnets but works just from being close to the rotating rim. And the prinicple is similar to the Magnic Light, I was told. It's the first time that I heard of Magnic Light and I took a note to check this out later.

Now the thing I've seen at the Reelite booth was a little plastic case, roughly 1" by 3" and rather slim, with a red LED in it. When held near a rotating rim, the LED slowly lit up. It wasn't very bright and the slow startup suggested a capacitor got charged in there.
The box was partially translucent for the LED to shine through, and I spotted a copper winding inside. Sure I was curious to learn how exactly it has been made inside but staff wasn't prepared to explain this in detail.

Today I googled Magnic Light and was surprised how much discussion it has generated - and no detailed explanation of how it works, yet. It got me hooked and I did some experiments of my own, trying to confirm my idea how it might work.

I have a motor-driven bicycle front wheel in a test stand near my electronic workbench, perfekt for this test. I brought an inductor (a salvaged mains transformer primary) close to the rim, the oscilloscope showing the open-circuit voltage at the inductor terminals.
I then played with strong rare-earth magnets near the inductor and around the rim.
I tried various positions for the magnets and the inductor, different inductor cores, magnets inside the inductor. I settled on an arrangement with an I-core in the inductor, a disc magnet at the end of the core, the magnet close to the rim. And yes, I measured an AC waveform at the inductor terminals, its frequency related to the wheel speed, it's amplitude higher with the assembly closer to the rim. Looks like this concept works !

I observed that magnets were more difficult to hold by hand, the closer they got to the rim: At one point of the rim, the joining point, it pulled so strongly on the magnets, that it was hard to prevent touching the rim. Well, this joint appeared to have a piece of steel inside (confirmed on stationary wheel). The rest of the rim made the magnets vibrate slightly.
The inductor waveform was close to sinusodial and my optimum arrangement yielded around 300mVpp. Once per rotation, a rather large pulse (1.5Vpp) occured.

Next I took a look at the frequency of the large pulse and the steady sinusodial signal. The large pulse was obviously one per rotation of the wheel. But what could determine the sinusodial signal's frequency ?
I set the wheel to 96 rpm (=1.6 Hz and period T=0.625s), the signal has a period of 17.5ms, that is 35.7 full signal periods per rotation. Familiar number ? Well, the wheel has 36 spokes ! (Allow for some inaccurracy of my analog scope).

Conclusion: The spoke nipples are slightly disturbing the magnetic field, creating the AC signal that I'm seeing. Without spoke nipples and rim joint, no output. So this works like a hammond organ, the rim with nipples is the tonewheel !

Now I believe that the Magnic Light (and the new Reelight thing I have seen) is much more refined than what I built from bits and pieces that I had around. But will it really extract enough energy to power an LED ? Isn't the strong force on the magnet a problem for the mounting ? And does it rely on the rim joint or on the nipples or on both to be there ?
Could it be the Magnic Light is working in a different way ?
I hope we soon see the first samples being sold !

 

[2_i]

Thanks, Martin, for this long write-up. Your observations are generally consistent with mine. I was thinking about the spokes, but did not do accurate enough measurements to observe their potential effect. In fact, I am not sure I even used a wheel where a significant spoke effect could be present. Is your test wheel equipped with eyelets? Are your spokes out of regular or stainless steel? I am asking because the typical stainless steel lacks ferromagnetic properties. Are your nipples out of regular steel? Quite often they are out of brass. I ran around the house with a magnet and found different answers for different wheels lying around.

 

[Martin]

My test wheel has stainless steel spokes, nipples that are non-magnetic, but eyelets that are ferromagnetic. So it must be the eyelets that save me. By the way, the tire on that wheel contains steel cables (clincher) and so a magnet slightly attracts the Al wheel all along the rim, though more at the eyelets and much more at the joint.
In a final experiment, I managed to make an LED flash with a modifed setup, exploiting the steel joint of my rim: A strong magnet on a hinge, magnet close to the rim and pulled away from the rim by a rubber band, I got one powerful movement of the hinge on every rotation. Also attached to the hinge was a smaller magnet that fit into my inductor. At first a little tricky to align, but then, on every movement of the hinge, the small magnet dived into the inductor and the LED flashed.
With the Magnic Light videos suggesting a more constant supply and uncritical alignment, this is probably not what they do.

 

[2_i]

Their cast suggested a magnet rotating in the horizontal plane, with the magnetization axis horizontal and the rotation axis vertical. Funny to think of it, but for the ferromagnetic operation optimal would be a diametric magnetization for a disk magnet and for Eddy - a radial magnetization for a ring. That latter magnetization would switch off the ferromagnetic operation completely.

 

[Steamdonkey]

I know because it would violate physics and repeatable experiments. Eddy breaking is employed in practice and you do not need to prove it. Efficiency of the breaking depends roughly on the square of the speed. Before you mentioned a Dremel and for a typical Dremel the speed would have been roughly 340 km/h. At this speed even the air drag would matter. I quit this discussion because it is pretty pointless. I tried to address the issues you raised rationally but the target is moving and I do not see any benefit from this discussion.

340 km/h??? Not even close. For the power measurement I did not use a Dremel, I spun the aluminum disk with a hobby motor running at approx. 2.5V, see previous post. It wasn't spinning anywhere near 30,000 RPM. Also, the disk didn't vaporize, melt or even get very warm, isn't that supposed to happen with eddy currents of this magnitude?

Where did 10 km/h come from? That's really slow for a bike. Is that what Magnic claims? I could see getting a watt or so from a wheel spinning at a cruising speed of 18 mph if a better magnet & coil setup were used.

 

[AnAppleSnail]

I took 28k rpm (off my decades old Dremel) * 60 *pi *.024*2.5 = 335 km/h. Did you mean where pi came from?

Nah, probably I forgot minutes vs hours. Thanks kindly!

 

[vinnieman]

If the source of power for the spinning magnet were eddy currents on the surface of the rim, wouldn't any ferromagnetic material near the dynamo unit be a huge source of inefficiency?

 

[Steve K]

If the source of power for the spinning magnet were eddy currents on the surface of the rim, wouldn't any ferromagnetic material near the dynamo unit be a huge source of inefficiency?

seems like a reasonable assumption. From the info that the Magnic folks have provided, I don't see any other metallic objects nearby.

I should add that ferrous material will provide alternate paths for the magnetic flux, potentially reducing the flux that couples into the rim, which would reduce the eddy currents, etc., reducing power to the light. If the Magnic works by having a magnet spin in the light, then any nearby conductive material would also have eddy currents that would produce losses. The brake (or bracket) that the light attaches to would appear to be the closest conductive material. As such, this design might not work as well if there is an aluminum fender/mudguard in close proximity.

Steve K.

 

[2_i]

If the source of power for the spinning magnet were eddy currents on the surface of the rim, wouldn't any ferromagnetic material near the dynamo unit be a huge source of inefficiency?

If the rim were superconducting than the magnetic field of induced currents would be about as strong as the ferromagnetically induced fields. However, due to the losses in a conductor, the magnetic field of induced current is weaker at low speeds, at first linearly increasing with the rim speed. The gain that the Eddy current field has, though, over the ferromagnetically iduced field of the rim joint, is that it appears everywhere along the rim. The fields induced in the fork etc. that Steve mentions are generally weaker, than in the rim, because they are induced by the motion of the magnet and those in the rim stem from the relative motion of the rim and the magnet. Now at sufficiently high rim speed, the Eddy current field will eventually become as strong as the ferromagnetic, but the issue is when that will happen. The coefficient in the linear rise of Eddy current should depend e.g. on thickness of the rim etc. In railway practice Eddy-current breaks are only used or designed for super-fast trains, because at low speeds such breaks cannot be effective. Also there will be heat-loss cost to the Eddy currents.

Sorry that it is not smoothed up, but I have to run.

 

[vinnieman]

So from what I gather, if we remove all ferromagnetic materials from nearby the dynamo, this light system would work just via eddy currents rotating a diametrically charged cylindrical magnet near the alloy rim.

Cool, I want to go make one now!

 

[2_i]

I have magnets on order to try a version where the ferromagnetic effects are practically eliminated. The issue is that the power under normal cycling conditions is then likely to be gone too, but it still can be fun to try that out and particularly to compare the ferromagnetic vs nonferromagnetic versions.

 

[Steve K]

well, the adjacent aluminum and steel isn't really a big concern regardless. As long as it is significantly further away from the dynamo's magnets than the rim is, it's a small concern. The big trick is... well, we really don't know what the big trick is. We've all got our favorite theories why it works, or perhaps doesn't work. Now we're just waiting for someone to get their hands on one and tell us if it really works or not!

 

[2_i]

Steve, both the ferromagnetic and Eddy current interaction go like B^2, where B is the field of the magnet. Eddy current has an extra factor of v/rho, where rho is resistivity. If in experimentation the Eddy current effect is nonexistent compared to the ferromagnetic effect at bicycle v, you cannot beat it - for a stronger magnet still the ferromagnetic effect would be stronger. The way to do it would be either to go down in rho but you have alu there and this is it, or you have to go up in v and this at least an order of magnitude or more. I was even thinking about having an auxiliary motor inside the dynamo to increase the relative speed, but take it easy. Anyway, to make it short what I am waiting for is the reaction of the investors once this thing starts to unfold.

 

[Steve K]

my main concern with the ferrous material is that it might provide an alternate path for the magnetic flux that should be getting into the rim and creating the eddy currents that will make the light work. Generally, there no reason that there should be much metal near the dynamo... other than metal fenders/mudguards... I don't think I've seen any videos of the magnic where fenders were used.

The investors should always be concerned and informed of risks associated with their investment decisions, and should be prepared for the possibility of large losses. I'm not familiar enough with Kickstarter to know if the investors understand that there is risk, or understand the magnitude of that risk. Having been involved with the design of electronics for 25 years or so, I know that there is always risk when designing a product, and that risk exists even after it is in production. I think it would help if the designer would tell people what the schedule is for getting into production. Has there been any info on this matter?

 

[burt68]

I dont know much but i made this http://www.youtube.com/watch?v=-FooPy2OLXk&feature=plcp ,coil is from a 240v fan, magnets are disc shaped and there are 3 of them, led is small old type voltage unknown. I know this is not working off the rim but thought it might be of some interest.the video should be self explanatory,with magnets working like a Farady torch

from burt68

 

[2_i]

I made this http://www.youtube.com/watch?v=-FooPy2OLXk&feature=plcp

Very nice: simple and effective. You could actually also make a device without employing any ferromagnetic effect, just a magnet oscillating inside a coil, on account of the bicycle shaking, accelerating and slowing down. I think someone mentioned such a device in the context of the Magnic discussions. For a smoother operation, one should throw in a capacitor that would be getting charged. After all there are also commercial flashlights that get charged by shaking.

 

[fyrstormer]

...

Burt, PLEASE add some punctuation to your post. It's nearly impossible to read.

 

[jdp298]

Back on topic, is there a view on whether this Magnic would work on a racing carbon rim where the only metal is the spoke tensioners? Clearly it'd be less than a 36-spoke alu rim, but something might come of it?

 

[EnabLED]

[Would the Magnic light] work on a [...] carbon rim where the only metal is the spoke tensioners?

I think there is not yet a general view whether it works or is a scam, so from that point of view, no.

The statement from the Magnic folks is it works from eddy currents in the rim, so it requires a conductive rim http://www.kickstarter.com/projects/dynamodirk/magnic-light-get-new-energy. If that's so, then carbon is an electrical conductor and some carbon is a good conductor, but the carbon used in carbon-fiber rims is a poor conductor. So from that view, no.

Some other folks suggest it is due to e.g., steel nipples or even spokes, in which case yes, it would work with a carbon fibre rim.

 

[burt68]

I am thinking scam ,as we are not just talking about a bike light here ,this magnic light is a generator and claims to be a very good one.If it is as good as it looks then they have invented a better way to make any power generator.