KC8QVO

All,

I am curious if anyone has ever seen an alternator design for a bike using magnets attached to a wheel/spokes somehow with a coil, or set of coils, on the fork leg?

I have seen several designs of homebrew wind turbines, some based on car brake rotors/bearings/axle shafts, that look like the combination could be adapted to fit a bike wheel. However, it might be hard to get a complete circle of coils so a couple of them may be easier to mount.

My thought on the idea is to get actual DC charging current for a 12v (13.4v or so full charge) battery. The dyno hubs and the old-school generators that have spring pressure holding them against a tire don't put out any respectable voltage/current. They may work for low wattage applications like lights or 5v to run a USB charger, but that won't charge a 12v battery.

If the generator is AC it could be transformed to step up the voltage and rectified to push DC (for charging the waveform doesn't matter so if it isn't exactly "clean" DC it wouldn't hurt - the important thing would be higher voltage than that of the battery so what ever current it is able to push can have enough steam to get in to the battery). However, the low wattage to start with of most "bicycle style" generators won't be enough to make it through the transformation and rectification with much energy left to charge from.

For what it is worth, I run solar panels on some of my trips. One of the ones I use is about 550mA in full sun. That has been enough to float one of my batteries while I have my electronics running at the same time. When I am stopped I have a bigger panel I can run and combined they are about 1.5A. That helps top things off.

Back to the wheel alternator idea - it doesn't seem like it would be too hard to spin magnets around the wheel and have coils on the fork to generate some power. I am not talking too many amps, but certainly much more than your typical hub dynos - and higher voltage off the bat.

It looks like most dynos are in the single digits on watts. If you figure 1 amp at 14v (charge voltage) that is 14 watts - several times more wattage than the most powerful hub dynos. Low power wind turbines (magnitudes smaller than the ones I referenced using car parts) can still be several hundred watts. Taking in to account the lower efficiency from using less coils - say a design uses 9 coils (3x coils per phase, 3-phase) and produces 200 watts, trimming that down to 3 coils at the top of the fork (1 coil per phase) you could potentially get 66 watts. Or, if you don't need that much power and have the ability to switch coils have it to where you can run 1 coil for 22 watts, etc, etc. If you are cruising down a hill and gravity can give you a surplus - switch in all the coils and max out the power generation.

I realize when talking "wattage" and coming from a bicycle wheel the power source for the bike, and the electricity generated, usually comes back to the one peddling the bike so generating higher wattage on flat ground, or worse - up hill, is probably a pretty dumb idea. However, I'm sure there are opportunities for more current.

Going back to the single digit wattage hubs - comparing them with my smaller solar panel - I can get roughly (550mA x 14v) = 7.7 watts of charge power out of that - while riding and in full sun. That is double a lot of dyno hubs - and doesn't add rolling resistance. The hard part is I'm not always riding in full sun and the only opportunity for more power is when stopped when I can combine power from both panels - and it is still dependent on the intensity of sun light (though, the panels do work in lower light - just much less power output).

For discussion's sake on the coil idea on a fork - below is a build thread on a stator for a wind turbine in the 800-1000w range that shows how the coils are wound and phased/connected. The theory could be down-sized, though the coils would have to be modified quite a bit if only 3 are used (they run 3 coils in series in each phase - having only 1 coil in each phase would mean each coil would have to generate the voltage = more windings to compensate).

https://otherpower.com/stator.html

rifraf

Are you yet familiar with the "Forumslader" of which I'm sure I read somewhere of a 12v version.

Some reference in this post of a local forum:
https://bicycles.net.au/forums/viewt...hp?f=9&t=90754


For myself I find my bikes dynohub/E-werk combo fine for my needs.


In the Thorn bicycle forums, there is mention I think pics of a poster Danneaux's extra wheel trailer upside down after the days touring and the dynohub wheel spinning via the breeze, utilising fabric blades.


I liked the idea so much I bought a second dynamo hub/ e-werk for my extra wheel trailer, though I 've yet to use it in anger.


I'm currently looking for a link for you as the youtube clip was interesting (well to me at least).


Struggling to find it and today a little pushed for time but here is the basic idea:

I figure if a days touring isn't offering up enough power, even if you've not got the trailer, the front dynamo hub'd wheel of the bike could be used the same.

He'd added home made "sails" to the spokes to catch the wind during the time he's sleeping.

KC8QVO

I am not familiar with the Forumslader. I looked it up but can't find any electrical specs. They reference downloading a chart that has that data in it, but the only one I found is not in english so I can't read it.

As for the dyno hub on a trailer wheel and making a wind turbine out of it - that is an interesting idea and it makes sense to be able to make some electricity when you are stopped and the wind is blowing. However, I would hope I wasn't stuck in enough wind to make any respectable electricity!

On the other side of the wheel wind generator idea - the clinch point for that vs what I was getting at with charging a 12v battery (13.4 or so volts when charged) is that the conventional bike dynos don't produce enough wattage.

There was mention from another thread a while back on the subject of using a "Joule Thief" circuit to step up the voltage. However, no matter how the voltage is boosted from a conventional low-wattage bike dyno the result is the same - even less wattage available at 14v to charge off of because of the losses in the circuitry to get there.

That was where I got the thought of the wind turbine coil/stator idea. You could mount magnets on the wheel somehow (disk tied to the spokes, for example) that, when the wheel is turning, pass the magnets across a series of coils the same way an alternator on a wind turbine works. The magnets (and disk + wheel they are mounted to) are entirely passive. The variable is the coil design. As long as strong magnets are used the power available (voltage and amperage, and read this as possibly available) will be determined by the coil design (number of windings, size of wire). The speed of the bike will only fall within a certain range (say 7-28mph, unless you are flying down hill even faster, then what ever the linear speed the diameter of the stator is based on the tire size and that speed range - speed and tire size determines RPM which will determine the stator speed). So for the low end of the speed range (say, 7mph) - if the voltage that could be generated is 14v then the rest of the range can push charge current.

In the above theory - the base voltage is 14v (DC, so after rectification - voltage purity/regulation is irrelevant) at 7mph. That would be the starting design spec. Depending on how much current is generated through the range it would be conceivable at the higher end of the spectrum coil windings could over-heat if they are allowed to run that hard.

Just thinking out loud here.

unterhausen

obviously you can do this. See as a simple example the reelight. Most people don't want the drag, as you mentioned.

KC8QVO

I happened to come across this light system called Magten. See the following link. It uses a magnet ring on the wheel and a pickup/dyno mounted to the fork leg. However, it is a smaller set up than I envision and is only intended to power enough juice for lights - not to power accessories, let alone charge a 12v class battery.

Magtenlight ? zero battery bike light illuminates your way with magnetism | The Red Ferret Journal

In any event, that at least is a little bit better "visual" of the concept I was trying to explain between a wind turbine stator and modifying the theory to fit a bicycle wheel - just on a smaller scale. I'm thinking having 3x coils/pickups in a ring at the fork leg to still make a 3-phase generator. As to how "big" the coils individually would have to be - I don't know - but using 1 coil per phase as opposed to 3 in the wind turbine examples I presume there would have to be somewhere around 3x the turns count as each of the 3 coils per phase has the coils wired in series (increasing the voltage like 3x AA batteries in series). As to how the windings would actually work out for the voltage being combined in 1 coil vs 3 - I would imagine it will end up being different than an exact tripling of the turns due to resistance and losses somewhere (gaining or loosing will change things).

KC8QVO

I did some experimenting today with an old bike dyno - the kind that has a spring latch where it will press a small wheel against the tire. The ones I have were from the early to mid 90's I think. They have 2 ports on them - one for a headlight and one for a tail light. The headlight port had higher voltage so that is what I am working off of (that and ground).

With the headlight hooked up and spun at 1600rpm (my drill's speed on high) I get about 5v and a dim light. So I presume it would be higher than that on a bike.

I was able to rectify the AC with diodes and a cap to filter. Unloaded DC (open circuit) voltage is 9.2v. Not bad!

I found an old transformer from a 12v wall wart. I used the 12v output of the transformer as the input and the wall plug side as the output. With the transformer on the AC side of the test circuit (before the rectifier diodes) I was able to get 60 some volts DC. So that was plenty to push current in to a battery with (just has to be over the voltage of the battery).

The kicker to it is when I pushed charge current in to a battery that was a bit over 12v (I want to say it was 12.6v) I was only able to get 24mA of charge current. But - that is something.

If you do the math on it - 24mA at 12.6v is (.024 x 12.6) = .3024 watts. That is less than 1/3 of a watt, let alone a couple watts.

So going back to my small solar panel at 550mA - so far this dyno experiment is just that - an experiment not worthy of putting on the bike unless I can find a way to multiply the wattage.

If you think about the charge current - even just my cell phone charging is pulling way more power than that - 500-600mA with a 12v battery power source. If I charge my tablet and anything else (bike computer, lights, handheld radio, etc) the power could easily get to 2 amps. Of course, the batteries can supply that - and any charge current that can be presented will decrease the draw on the battery - but 24mA in to that draw just isn't worth the weight on the bike, let alone the resistance of the generator on the wheel LOL.

Interesting to see, though... If I can get a coil that will push over 14v by itself (no step up transformer) I can get around some of the losses I have now. Then if I can run 3 of them that is even more power.

KC8QVO

Pictures of what I was dealing with - dyno and oscilloscope showing the voltage on the blue trace - unloaded DC 9v (RMS, max/peak shows 9.4 but all in the same ballpark). Yellow trace is the AC side, but I had it disconnected so its flat.

CliffordK

Have you looked at REELIGHT?

https://www.reelight.com/

Spoke mounted magnetic generator for headlights/taillights.




They also have zero contact, magnetless rim eddy current generators.

KC8QVO

How are you going to convert one of their products to charge a 12v battery?

Going back to my last post - charging would imply a surplus of power than what is consumed. So if I pull 2 amps from my devices combined I would need more than 2 amps to charge. Just 2 amps and I am simply keeping up with demand. Less than 2 amps and I am still running the battery down as the charge current can't keep up with demand.

If you do the math on that - we'll use 2 amps at 13v (neither fully charged, neither near empty battery) - it comes out to 26 watts. The dyno I tested today was supplying about .3 watts (that's 3 tenths of a single watt). If you look at commercial hub dynos - they are still only in the range of up to about 3 watts - at 6 volts AC (stepping that up and rectifying would present less as you loose on both fronts - step up transformation and rectification). That is a far cry from 26 watts.

The concept that REELIGHT is using is interesting. However, trying to increase the power output, I suspect, to what I am looking for is a fruitless effort.

unterhausen

it's interesting that the reelight magnets have moved further out on the wheel from the originals. The original reelights were pretty weak. If you really want to get the kind of power you are talking about in this thread, you need magnets out by the rim. And you need a lot of magnets.

All of your experiments should be done with a load. Dynos are a current source and need a completed circuit to have predictable performance.

CliffordK

I'm not sure placement in or out will make much difference for a single magnet or a couple of magnets. However, for high power, one needs to have multiple pole reversals. So, it might make sense to build a hoop that would mount near the nipples, perhaps even with more than one magnet per spoke. And, potentially a pickup on both sides of the wheel.

There is a lot of discussion about generators on windmill forums.

A few issues. First of all, the voltage specified by a dynamo company is an estimate, and will vary with speed. Some kind of MPPT will help with converting your generator voltage to the actual system voltage. There are converters designed for bikes, but generally at the power levels rated for bike generators.

I do think that if you get up to 20W or so, you'll definitely feel the drag.

KC8QVO

Thanks for the thoughts. And yes, a ring of magnets around the wheel is what I had envisioned.

As to the voltage varying and the dynamo being a "current" source, you are correct. That is the exact same principal a wind turbine runs on.

I suppose I should ground my comment on "wind turbine" principal though. I am referring to small alternative energy systems that use battery banks to store energy, then run inverters to convert the battery power to AC power. In this case the wind turbine is pushing current in to the battery bank. The battery bank will govern the voltage, not the wind turbine.

The above having been said, in either the bike generator case I am thinking through or the wind turbine example, the voltage ability of the alternator has to be simply "higher" than that of the battery. With a protection diode to only allow current flow from the generator to the battery there will be no current flow until the voltage of the generator surpasses that of the battery.

It is like a bicycle tire pump. When you hook up the nozzle to the tire the pressure gauge will read the system pressure (tire and air hose up to the gauge). When you push on the pump you are overcoming the pressure of the system to put more pressure in. When you pull up on the handle to reset the pump the pressure doesn't leak out - it stays put because of a check valve. If you put light pressure on the handle and don't overcome the pressure already there - no air goes in to the tire.

When I came up with the 24mA yesterday - that was actual charge current pushing in to a battery that was under 13 volts (the SLA battery the dynamo is sitting next to on the table). For what it is worth, the voltage of the battery did increase so it was working - just the equivalent of a gnat fart compared to the draw of even just my cell phone charging.

alias5000

As for the forumslader, here is a power vs. ground speed curve for a 700C wheel (all curves labeled "FL_" are forumslader, and the FL_alt_low (green) probably is from an older version ('alt' = 'old'): link.
Depending on what level of design you are speaking, I would rather speak in terms of power, weight and efficiency - rather than volts, amps, mAh. DC voltages nowadays can easily be transformed using DC/DC converters (power electronics, the stuff I earn my living with) at efficiencies beyond 90%. The power limitation of hub dynamos probably is mostly a result of design choices. Maybe it is possible to alter the generator design in those hubs to allow a higher current without overloading (what is the power limit on these? Impedances, wire gauges, ...? What breaks first when I start loading a hub dynamo with more power?). I suspect that any solution based on a hub dynamo and a power electronic converter (AC/DC, plus DC/DC if necessary) will be superior in terms of weight, and efficiency, compared to a wind turbine or spoke/rim magnet generator (that basically is just a hub dynamo realized without enclosure and with a larger radius).
For a wind turbine, I'd be very skeptical about efficiency, as the theoretical maximum efficiency for standard wind turbines (wind to rotational power) is 59.3% (Betz's law). A Rim/Spoke-based generator will have more tolerances than an enclosed hub dynamo (worse catch of magnetic field) and may be more exposed to the troubles of the elements.
So, to end my purely academic talk, I'd stay focused on squeezing out more juice from hub dynamos (unless you really want your 66W!).

On the non-academic note: I hope the forumslader chart helped. Should you be running something like a 20' wheel, you'd be getting that 10W peak already at 22km/h!