eddy current bike light?

[daldesign]

A couple of guys in Holland or Germany created the Magnic Bike Light.

http://urbanvelo.org/magnic-light/

I understand eddy currents, but am skeptical that the amount of power available is enough to drive the LED. They claim no moving parts other than the rim of the bike wheel.

 

[ScottyUK]

I don't doubt it at all. Google 'homopolar generator' and see how much current can be produced. An interesting device, and innovative - thanks for the link. I wish them well.

 

[LiteYear]

It's almost a homopolar generator but there's no complete circuit including the rotating member. I wonder if their reference to Eddy currents instead of Lorentz force (the principle behind homopolar generators) means that the method is different.

To me it looks more like an eddy current type absorber used in dynamometers, except backwards so it works as a generator. Ergo, permanent magnets in the light impart a magnetic field into the conductive rim. When the rim passes through the field, eddy currents are generated to oppose the change in magnetic field (Lenz's law), creating an attractive force and slowing the rim. In a dynamometer you energise the magnet with a coil to change that attractive force, so I guess in this design the force drives the coil instead, giving you electrical energy.

 

[btrueblood]

A guess on my part: permanent magnets, perhaps 3 or 5, arrayed on a small rotor with their poles perpendicular to the rotor axis. Mount rotor so that the magnet faces approach the rim as close as possible - the eddy current "drag" induced by each magnet in the bike rim should impose an equal/opposite torque on the rotor - now you have a rotor with magnets attached spinning around. Left as an excersize for the sparkies is the best method to extract current from said spinning rotor. Of secondary interest - this could also provide tach/speed info., if you could keep the slip loss fairly constant or predictable or measureable...

 

[itsmoked]

From that movie it looked like the light was piddly and wholly inadequate for the task. But maybe that was just a camera issue.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[daldesign]

btrueblood, I think you might be right. Yours is the best explanation I have heard. At first I thought they were capturing the tail of the eddy current time constant (as used for crack detection etc) but a simple experiment shot that down. I think the Reelight is a better design, which has PMs mounted on the spokes.

 

[btrueblood]

"I think the Reelight is a better design, which has PMs mounted on the spokes"

Until you had to change a tire, and popped 7 or 8 of the stupid little magnets off the rim whilst prying on the tire bead. Or had to wipe/scrape the iron filings and Fe(2) oxides off the magnets once a week...

 

[jraef]

I think it's cool, in fact I made one sorta like it for my 6th grade science fair! My father was using Eddy Current Brakes on some machines he was building and I had asked how they worked to stop a spinning disc without touching it. He explained it to me and I got the idea to use that in my science fair project. Mind you I did not have neodymium magnets or LEDs, so mine was considerably more bulk, ugly and less effective. But watch the video closely. They NEVER show the light without the wheel turning. That's because with no batteries, the light will go dark every time you stop. Not so great in urban settings and I think their statement about "never" having to worry about lights on your kid's bikes is a little over zealous.

And although they do highlight that it is "frictionless", that does not mean there is no energy transferred, there is. The magnets acting on the rims to capture energy from eddy currents is indeed going to be imparting a retarding force on the rim, just like the Eddy Current Brake system. I could feel it on mine. What makes it viable now is the high energy to lumen ratio of the LEDs because it would keep the effort from the peddler low. I would consider also coupling it with a slow discharge cap so that at least it could stay illuminated for a minute of so when at a stop light.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[davidbeach]

high energy to lumen ratio

Um, perhaps, did you mean high lumen to energy ratio?

 

[jraef]

Um, yes.
Low energy, high lumens.

Another brain fart...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[endurozw]

I wonder how difficult this would be to build at home, I love mountainbiking but I really can't visualize how this works. The mental picture I'm getting involves the eddy currents turning magnets inside the device / generating electricity but in reality building it seems like a daunting task for someone that has never built a generator.