By Steve Bush 8th March 2022
There is a novel sort of bicycle dynamo that holds a strong magnet loosely in a little box next to the wheel rim.
Once the wheel starts to rotate, the magnet’s field starts to interact with the passing aluminium (it only works with aluminium wheel rims) and the magnet starts to spin around in its box.
A coil around the box then picks off a little of the moving magnet’s energy, and that electricity powers a small led – we are talking a ‘be-seen’ marker light rather than a searchlight here. Update: Realising the search term ‘eddy current generator’ might help, uncovered patent EP2703264A1 on this very technique.
I saw this idea years ago, and it immediately occurred to me that the moving magnet is an unnecessary intermediary, and that there must be a two-coil all-electric way of achieving the same effect – which admittedly might need a little electrical push to get it going.
In my imagination, this is an ac generator, as a homopolar-like dc generator action seems a bit too much to ask for.
A one-jfet oscillator might do it – of the sort that starts up from 20mV, but with its feedback transformer somehow incorporating the passing bicycle rim.
The circuit on the right is such an oscillator, in this case cut down from the circuit of the LTC3018, designed to get useful power from low-voltage high-current power sources like thermo-electric generators.
In it, the jfet starts oscillating spontaneously when more than 20mV is present at Vin, and the boosting action of the transformer eventually establishes enough voltage on its output for the following circuitry (the synchronous rectifier, for example) to start working and create a reliable power rail at the (unseen in this case) far end.
My though is that a similar oscillator could have no Vin, and moving aluminium near the two coils rather than a magnetic core.
Heaven knows what the coils would look like, how many turns they would have, or how the phase would be optimised for oscillation, but you get the idea. Horseshoe cores spring to mind.
It might even start from cold once the rim started moving due to noise in the system, or it might need an initial electrical prod to get it to spring into life.
Update: Just to add something to this, when I was at university, two of the lecturers had a company that designed non-contact equipment to measure the speed of hot steel in a rolling mill.
As I remember, it used two coils slightly separated along one face of the hot metal. One was a ‘record head’ (like a tape recorder) recording a temporary magnetic field into the surface of the steel, and slightly down stream was the read head.
By controlling for a fixed phase relationship between the two waveforms, if I remember correctly, the operational frequency became proportional to speed of the steel sheet – very neat.
Tagged with: EinW Engineer in Wonderland generator inductance power
Hi Steve, and hi Steve – The first alternators I played with were fitted to motor-cycles and the central rotor was of the permanent magnet type – no wiring, it just bolted onto the end of the crankshaft. And no residual magnetism. Polarity of the whole electrical system was done by reversing the two DC wires on the rectifier. That was not the end of the story as quite often the positive terminal was the bolt that mounted the rectifier to the frame. I think ‘flashing’ used to be necessary when early electronic ignition systems began to appear, but I could be wrong. The alternatos had 3 pairs of coils, two of which were connected in parallel. The third was switched in when a few more amps were needed to power the headlamp. On most machines you could test the output of the alternator by connecting it to an AVO set to 10 amps AC and running the engine at about 3,000 RPM. The high voltage (‘HT’) for the spark plugs was usually obtained using a coil and contact-breakers. The even older bikes were fitted with dynamos and a fairly complicated thing in a box called the regulator. From what I remember, voltage- and current-sensitive relays performed the regulating by varying the dynamo field current. These devices were kicked into action by the residual magnetic field that remained within the armature after the engine was stopped. Sometimes they needed ‘flashing’ after a long period of inaction during which the residual magnetism dribbled away somewhere. The HT for the spark plugs was provided by a magneto. There was no connection between this and the rest of the electrical system. Oh, yes, nearly forgot – The batteries needed reversing on both systems… Nowadays the alternators are 3-phase, and the rotor current is controlled by electronics. This current is supplied via one slip-ring and one ‘button’ in the centre of the exposed face of the rotor. Gone are the days of 25 watt headlamp bulbs, variable-frequency horns, and flickery brake lights!
Morning Luke Hear I have re-wired a few motorcycles ? My proudest moment was realising that the 6V system on a Moto Morini Canguro 500 was powered by a strangled three-phase 12V generator, and managing to isolate and re-wire the coils to get full output – those were the days ? Boyer Bransden Power Boxes became my default solution – rugged and did the job – looks like the company is still around: http://boyerbransden.com/
the s/w isn’t showing a “reply” button on Mr. Bush’s last message, so let me quote the text. “Permanent magnets are perfectly acceptable in my thought experiment – just not moving ones.” my reply is: that’s a much tougher problem! Eddy current brakes are relatively common, needed only for a stationary coil to create a magnetic field in an adjacent spinning conductive disc. To be able to use some field in the disc to then induce current in a stationary coil is not trivial. It seems that the first problem is that the field in the spinning disc must be AC. My first thought is that perhaps radial slots can be cut in the disc. Stationary permanent magnets would induce eddy currents in the spinning disc, and there should be at least some variation in the resulting eddy currents. This should produce a varying magnetic field that a nearby stationary coil might pick up. It seems like the efficiency would be pretty low, though. I do like the idea of getting rid of moving parts, but having to move energy via magnetic fields twice seems to imply that the efficiency will be low.
Good afternoon Mr Kurt Indeed it is a tough problem – at least for me. Only bicycle rims allowed in my thought experiment, so I fear no slots in the rotor. My (poorly educated) instinct is that permanent magnets will not help. If I stare at the diagram of how an eddy current brake works, I feel on the edge of understanding how to crack the problem – almost certainly using a self-oscillating circuit – but the method is refusing to (ie, I have not got the imagination to make it ? fall into place. For one thing, the movement is at right angles to any possible coil – Mr Faraday, Mr Lenz and Mr Tesla, could you pop in here for a moment…. However, two coils (possibly with a lot of turns), a phase shift (or capacitive resonator) and some gain feels like the necessary block diagram, plus this magic magnetic circuit. In principle, if something can be made to work (so long as it does not have to oscillate too fast), I feel it should be at least as efficient as the thing with the spinning magnet. (BTW my original hypothetical aim was only to be able to light a single 5mm red led at the back and a white one at the front for safety) I only wish I had a bigger brain, that had more electromagnetic theory installed ?
There was an eddy current sort of dynamo for bikes about .. 10?… years ago. Similar to a bottle dynamo, it used a magnetic wheel that was rotated by the eddy currents in the nearby rim. There was a winding around the magnetic wheel which produced the output power. I think the output power was about 2 watts. The designers did put it into production and it was modestly successful. It doesn’t seem to be the same as what you are describe, though. I do like the general idea, primarily for the lack of drag from a rubber roller on the rim. There must be eddy current losses in the rim, which reduces efficiency. Also, the lack of of direct contact with the rim seems to be what limits the output power.
Good morning Mr Kurt That is very similar to the the thing I was trying to describe – maybe I mis-remembered the same thing. My mental quest is for a solid-state version of either of the things we remember. I suspect efficiency would be low, but my thought is even something that could drive a 5mm led at 20mA would be a benefit for children’s bikes – a single self-contained block that includes the led and can be mounted near the rim to give light at all times – maybe one for the front and one for the back. Over the last couple of days I have been puzzling over the electromagnetic circuit, and have frankly got nowhere -I need to re-study magnetic drag brakess and induction motors – what is Nicola Tesla’s email address???
I doubt that the wheels will be perfectly circular and nor will they be perfectly ‘true’ (i.e. there will be some side-to-side movement of the rim as they rotate). Could even a tiny amount of imperfection start the generating process? Or, would this mysterious effect be enhanced by putting the pickup device near the spokes instead of the rim?
Morning Luke Hear Good bicycle wheels are pretty true – much better than 1mm – maybe 0.3mm. The rotating magnet generator certainly works on the rim – Steve Kurt (see other comment) knows more about it that I do. I am still trying to think of a solid-state mechanism that will work* – maybe varying distance would if there was a small permanent magnet to get things going – sort of, the generator equivalent of a parametric amplifier? *but brain not big enough at this end ?
I wish I could recall the name of the eddy current based bike dynamo light. It did mount to the brake caliper, and did have integrated LEDs and optics. A nice compact arrangement that reminds me of the old block dynamos (of which I still have a couple). An interesting side effect of mounting on the brake caliper was that actuating the brake moved the light’s spinning disc of magnets closer to the rim, improving the coupling of magnetic flux, and making the light brighter. This had the effect of making it like a brake light! As far as Mr. Tesla.. I’m not sure where he resides. I’m pretty sure he’s not responding to emails, though. A side note on Mr. Tesla… I recently became aware of the fact that his AC motor designs were 2 phase, not the 3 phase designs that we see more often now.
Good morning Mr Kurt The spinning magnet idea is clever, but can you think of a way to make it a solid-state system? It must be possible. BTW – Mr Tesla has certainly not responded to any of my emails. He was unbelievably clever to think up the induction motor – I didn’t realise his was two phase.
solid state? ..as in “no vacuum tubes”? ? I’ll read between the lines and assume that your intent is to avoid the use of permanent magnets? Don’t car alternators do this, mostly? I have the vague impression that they use residual magnetism in a core to get the field current started. That’s probably not true, but it seems that a small magnet could be used in a bike dynamo to initiate current to a field winding. If it would be similar to the design with permanent magnets,then I think this coil would have to also be a rotor, so you might have to have a circuit board that rotates with the rotor. A stator winding would then be needed to capture energy from the rotor, as in most/all bike dynamos. Seems like a lot of work just to get rid of permanent magnets. Or is the goal similar to car alternators, where the field current is controlled in order to get the desired output voltage?
Morning Mr Kurt. Definitely no electron beams, unless travelling in a semiconductor. Permanent magnets are perfectly acceptable in my thought experiment – just not moving ones. The aim is to generate electricity with only fixed parts adjacent to the rotating rim – so like the cunning dynamo design whose rotating magnet is dragged by the aluminium rim, bit with no rotating magnet, although you can have as many static magnets as you wish – it must be possible, and I think it needs two coils, spaced some how and feedback between them. On the subject of alternators – the ones that I know about use rotating electromagnets to create their fields, but will sometimes start-up from residual fields without electrical excitation if spun fast enough. In ancient times, when you switched a positive earth car (automobile) with a dynamo (dc generator) to negative earth, you used to have to ‘flash’ the electromagnet to reverse that residual field – I might have remembered that wrongly, it was a long long time ago….
Your email address will not be published. Required fields are marked *
Get our news, blogs and comments straight to your inbox! Sign up for the Electronics Weekly newsletters: Mannerisms, Gadget Master and the Daily and Weekly roundups.
Read our special supplement celebrating 60 years of Electronics Weekly and looking ahead to the future of the industry.
Read the Electronics Weekly @ 60 supplement »
Read the first ever Electronics Weekly online: 7th September 1960. We've scanned the very first edition so you can enjoy it.
Read the very first edition »
Electronics Weekly teams up with RS Grass Roots to highlight the brightest young electronic engineers in the UK today.
Read our special supplement celebrating 60 years of Electronics Weekly and looking ahead to the future of the industry.
Read the Electronics Weekly @ 60 supplement »
Read the first ever Electronics Weekly online: 7th September 1960. We've scanned the very first edition so you can enjoy it.
Read the very first edition »
Tune into this Xilinx interview: Responding to platform-based embedded design
Tune into this podcast to hear from Chetan Khona (Director Industrial, Vision, Healthcare & Sciences at Xilinx) about how Xilinx and the semiconductor industry is responding to customer demands.
By using this website you are consenting to the use of cookies. Electronics Weekly is owned by Metropolis International Group Limited, a member of the Metropolis Group; you can view our privacy and cookies policy here.