Has anyone heard of Magnic Lights?

[[XAP]Bob]

The rim does cross the field, think about the rim as a series of slices, not as a ring.

I did one of my physics A Level main practical projects on the effect - it is real.

[Mick F]

Ok. Big magnet in the box.
Rim moving last it, conductor in a magnetic field therefore eddy currents.
These currents create their own magnetic field, which passes by the coil in the box (because the rim is moving).
this coil then produces an output voltage.

Rim moving PAST it, I suppose what you mean.

The rim isn't moving past it. Slices or otherwise. As far as the magnet is concerned, the rim is almost stationary.
It's the spokes and nipples "as a whole wheel thing" that induces the eddy currents and the EMF.

How lorry braking works, I don't know.

[[XAP]Bob]

The rim is moving past it. Unless you're still.

On a phone, so yes, last should have read past!

The rim of a bike moves at quite some speed, the eddy currents are set up and stay with the rim, moving forwards, not alot, but enough.

[Mick F]

Hi Bob,
What's your number?

The idea that an alu rim "not moving" across a conductor can create an EMF in a magnet-and-coil unit, leaves me a bit mystified. I could happily accept that a "wheel" could do it, but not a rim.

[[XAP]Bob]

http://en.m.wikipedia.org/wiki/Eddy_current_brake

Look at the linear brake, then look at a wheel(rim) as a slightly curved straight line.

Does that help?

[robc02]

This video explains eddy currents and magnetic braking well. The effect is more pronounced with a continuous conductor, such as a bicycle rim, than with a segmented one.

The same principle is at work in the induction motor, where a rotating magnetic field is created and it induces eddy currents in a stationary (to begin with) conducting core. - The reverse of the situation in the lighting system being discussed, but with the same relative movement between magnetic field and conductor. The eddy currents can be very large and create correspondingly large magnetic fields that produce a strong interaction with the rotating magnetic field - and therefore torque in the conductor.

There is no doubt that eddy currents will be produced in the rotating rim. Its just a question of how effectively they can be harnessed to produce an emf in the stationary coil assembly.

[Brucey]

mmm, nothing to do with the spokes....

BTW the eddy current effect is marked. I have just spent a few minutes messing about with magnets and wheels, and a magnet held near the aluminium rim does exert some retarding force on the wheel presumably because of the eddy currents generated.

This effect will be present whether the light is on or not, so although it should be small, there will be a small drag from this system unless it is moved away from the rim when the light is off.

cheers

[robc02]

BTW the eddy current effect is marked.

Yes, the rim is near enough a short circuit so the eddy currents can circulate freely and produce the effect you note.
The video linked above shows the effect very clearly.

[Geoff.D]

I'm with Brucey, Bob and Rob on this one. Here's how I've figured it for myself.

The rim does indeed cut through a magnetic field, because the filed is 3 dimensional and the rim is passing in a 2 dimensional arc through a portion of it. Any point in the rim will move a cross the field as it traverses the arc. This results in and induced current. It isn't the linear current induced in a straight wire moving across a filed, or the more sophicitcated (but essentially the same) coil in a generator. It's an eddy cuurent. Followed by another and another, and so on. In this way there'll be a series of eddies induced as the rim approaches the maget from below, geting stronger as they close in on the magnet, and fading as they pass away on the top side. But, it'll be a never ending stream going past the pickup coil.

The job of the coil is to convert these small fields, associated with the eddies, into an induced current. Because they'll all be travelling unformly in the the same direction and orientation, the coil will produce a linear current. It's acting as a generator, but with the moving magnets not on the same axis.

The trick is that the rim has two functions. First it is a conductor, responding to the magnet field on the magnet in the housing (by hosting eddy currents). Secondly it provides a moving magnetic field (the fields of the eddies) to induce a current back to the coil in the housing. Neat idea. Two inductions. No mechanical interactions.

However, it remains to be seen as to how efficient it will be. All systems have losses. Lack of touching, moving parts isn't the be-all-and-end-all.

[Mick F]

mmm, nothing to do with the spokes....

BTW the eddy current effect is marked. I have just spent a few minutes messing about with magnets and wheels, and a magnet held near the aluminium rim does exert some retarding force on the wheel presumably because of the eddy currents generated.

I think your experiment has a great deal to do with the spokes. Your experiment was to do with magnetism, not the production of EMF. Alu isn't magnetic - or if it is, it's a tiny amount.

As for the other points, I remain to be convinced that this system will produce much power at all. Up thread, the comment was made about the rim needing to be VERY true. I'm not surprised by that, as the unit would need to be very close to the wheel to be of any use.

I note the explanation of these lights says that they won't work on "full carbon" wheels. What do they mean by full carbon? Would this system work with a carbon rim and steel spokes and brass nipples? Or would it work with an alu rim with CF spokes and nipples?

Personally, I'd rather have battery lights or a dynamo.

[Mick F]

Arthur C Clark's third law:
Any sufficiently advanced technology is indistinguishable from magic.

[robc02]

mmm, nothing to do with the spokes....

BTW the eddy current effect is marked. I have just spent a few minutes messing about with magnets and wheels, and a magnet held near the aluminium rim does exert some retarding force on the wheel presumably because of the eddy currents generated.

I think your experiment has a great deal to do with the spokes. Your experiment was to do with magnetism, not the production of EMF. Alu isn't magnetic - or if it is, it's a tiny amount.

As for the other points, I remain to be convinced that this system will produce much power at all. Up thread, the comment was made about the rim needing to be VERY true. I'm not surprised by that, as the unit would need to be very close to the wheel to be of any use.

I note the explanation of these lights says that they won't work on "full carbon" wheels. What do they mean by full carbon? Would this system work with a carbon rim and steel spokes and brass nipples? Or would it work with an alu rim with CF spokes and nipples?

Personally, I'd rather have battery lights or a dynamo.

As you say, Mick, the retarding force in Brucey's experiment could not be directly due to magnetism between the magnet and rim as aluminium is not (for all intents and purposes) magnetic. However the movement of the rim across the magnetic field induced an EMF in the rim, this created eddy currents which in turn created magnetic flux. It was this flux that interacted with that of Brucey's magnet and caused the retarding effect. Brucey will also have experienced a small (maybe too small to feel but readily measurable) force trying to drag the magnet around with the rim. I have seen similar experiments done with plain non-magnetic electrically conductive discs - i.e. no discrete spokes as in a bicycle wheel. These are the principles harnessed in the induction motor - one of the most common and elegant electrical machines made.

As stated elsewhere, these effects could be used to induce and emf in a stationary coil, the question is - how efficiently?

As regards air gaps - the smaller the better. Commercial electrical machines have very small gaps, just enough to allow rotation without interference. But Reelights manage OK with much larger gaps than optimal - they are just less efficient than they could be. The point for most users is "are they bright enough?" and "will they noticeably slow me down?" The latest LEDs produce a lot of light for very little current, so less powerful energy sources are becoming more viable for some applications.

[Audax67]

How it works is less important than how well it works. I wouldn't expect the current version to replace a dynamo light. In the video it doesn't appear to cast a light all that far and the rider was on what looked like a nice smooth road, unlike some of the pothole-riddled horrors we find going through forests here, where if you can't see well you're picking up bits. But I like to have bright light close to the bike as well as out ahead, and for that it would be fine. You'd need one each side, though, since the wheel blocks off ~50% of the beam.

Can't quite imagine how the rearlight would work with panniers, though. Or rather I can: it wouldn't.

[Mick F]

I have a HUGE magnet - four inches across and weighs nearly three pounds. It came from an I Band radar magnetron.

I offered it to my rear rim whilst the bike was on the stand. I spun the back wheel and held the magnet close to the rim - and away from the frame!

What happened?
Well, the wheel was slowed down ok, but only once per revolution - as in a "pulse". This is because that in Rigida Chrina rims, there's two steel pins that hold the extrusion together. The magnet tried to "grab" these pins.

There was no other effect.

[Geoff.D]

The video highlighted by Rob gives a good explanation of Brucey's results. The magnetic fields associated with the induced eddy currents will be acting in opposition to the (magnetic) forces that created them. Hence the retardation. This will be one of the inefficience in the system (analogous to friction in mechanically driven generators).

It's possible, Mick, that the effect of the "grab" on the steel pins in your experiment was so big that you didn't (couldn't) notice the much smaller effect reported by Brucey. That small background retardation could have got lost outside the boundaries of your detection system.

Hoever, Audax67's point is, actually, the bottom line. What will be the best use for this set of lights? It seems to me that they're more likely to be a warning of your presence to other road users than an illumination of the way ahead for the rider.

Innovation is exciting, essential and interesting. But, I guess I'll continue to be happy with my exisiting lighting set up, which serves me well for my (low key) needs at the moment. However,come my next huge win on the lottery.............