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Reelight - like generator for a bike ...

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Evalon

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Hi,

I'm in the process of making a magnet powered generator to mount on my bike. I've made this setup (see picture) to test the idea and am surprised to find that it generates only about 26 mV when rotating at high speed! I'm using a 2 mm thick neodymium magnet and a coil with app. 20 windings.

How can this be optimized to provide ~ 40 volts (!) ? Do I need to orient the coil differently relative to the magnet or is it mainly a question of more powerful magnets and more windings? Will it help adding an iron core to the coils or something similar?

Any insights appreciated :)

Greetings,

Jesper
 

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Hi,

I'm in the process of making a magnet powered generator to mount on my bike. I've made this setup (see picture) to test the idea and am surprised to find that it generates only about 26 mV when rotating at high speed! I'm using a 2 mm thick neodymium magnet and a coil with app. 20 windings.

That's not an alternator, it's just a pulse generator, 25mV sounds about right - it's the sort of thing you'd use to feed a speedo.
 
That's not an alternator, it's just a pulse generator, 25mV sounds about right - it's the sort of thing you'd use to feed a speedo.

Hi Nigel,

Yes, I guess it would be a pulse generator but had hoped for a higher level pulse. Do you know the Reelights, Reelight - Frontpage ? They generate energy for an LED light using this principle, however, I do need a higher wattage output, about 6 watts. Do you reckon there's any chance I can do that using this principle - or maybe there's a better principle that still doesn't require mechanical contact?

Cheers,

Jesper
 
Hi Nigel,

Yes, I guess it would be a pulse generator but had hoped for a higher level pulse. Do you know the Reelights, Reelight - Frontpage ? They generate energy for an LED light using this principle, however, I do need a higher wattage output, about 6 watts. Do you reckon there's any chance I can do that using this principle - or maybe there's a better principle that still doesn't require mechanical contact?

No, 6W of energy output requires greater than 6W of extra mechanical energy provided by you.
 
Hi 3v0

- thanks for answering. No, I don't think 6 watts is unrealistic in terms of the power I can generate (can be up to a 1000 watts peak). And cannot use a hub dynamo as I need higher voltage and also a special type of hub. The problem appears to be maybe the number of windings or the magnets' strength..... ? At this link Wooden Low-RPM Alternator he built an alternator using sort of the same principles as I do but got much more power & higher voltage. Don't exactly know why ...

Best Jesper
 
.... the hub dynamos may also rotate at a lower angular speed, meaning that the induced current (voltage?) may be lower than if the magnets and coil are mounted on the spoke at a distance from the hub ...
 
.... the hub dynamos may also rotate at a lower angular speed, meaning that the induced current (voltage?) may be lower than if the magnets and coil are mounted on the spoke at a distance from the hub ...

You missed the point. It takes physical energy to produce power. While you may be able to build a 6 watt generator, the question is can you peddle it ?
 
Hi again 3v0 ... hmmm... have you ever tried being in a training center? From my experience my long-term output power is about 150 watts which I can output for many hours. 6 watts additional (excluding losses) would not mean much ...

Best Jesper
 
@ Nigel - just tried it with a core, but the magnets are attracted to it ( :) they like eachother :) ) so my guess is that it will pull the spokes on the wheel quite heavily. Trying out more winding and then maybe a transformer can do the rest.

Best Jesper
 
Hi again 3v0 ... hmmm... have you ever tried being in a training center? From my experience my long-term output power is about 150 watts which I can output for many hours. 6 watts additional (excluding losses) would not mean much ...

Best Jesper

How much of your power goes to powering the bike ? How much is wasted by the generator.

If you think you can do better then the units being sold good luck to you. I am not going to debate it.
 
How much of your power goes to powering the bike ? How much is wasted by the generator.

If you think you can do better then the units being sold good luck to you. I am not going to debate it.

Hi 3v0 - I agree ... but thanks for reading & posting.

Best regards,

Jesper
 
My advice would be to buy a commercial unit, and build a step up converter :p.

I still think 6 Watts is a bit optimistic shall we say ;) no matter the route taken. Whatever happens your going to need an extra >6 watts to pedal.

I guess it would be wise to check the poles of the magnet, the most current generated from the wire will be when the field is perpendicular to the wire windings. But i guess you have already done that :D
 
Hey Rich,

Thanks for posting and considering.

For your information I've attached a screen-dump from one of the bicycle power calculators available on the internet. Speed is moderate and the power is about 114 watts. 6 watts+ will add to the load, however, I can switch it off and I do have experience with riding heavy loads. So I consider the challenge of delivering ~6 watts to be manageable from my perspective ... Also, as I mentioned in a previous post it is not an option to use a hub dynamo ....

Whether or not it's possible to generate 6 watts using this kind of "generator" I don't know ... That's what I'm trying to find out :)

.... now I've tried with more windings (3x - altogether about 60 windings now) and output is increased to 170 mVs. However, it's not a very "stiff" voltage as loading with 6,8 ohms reduces output voltage significantly and beyond the voltage drop caused by the internal resistance of the wires.

I've also checked the magnet field orientation, as you suggested Rich, and it is perpendicular to the windings - rotating the field 90 degrees reduces output significantly.

I've also tried to add another set of magnets - double thickness - but it doesn't increase output (probably relevant that the distance to the coils is increased by a few millimeters as well).

Angular velocity is about 1 meter/s which will increase to about 3 meters/s riding the bike at 20 km/h. If that's a linear relationship that would increase voltage by a factor of 3.

Maybe it's a question of larger magnets, placing them closer to the coils, increase the number of coils, and increasing wire diameter? That I can do, yet if one of you have inputs on correct size and placement of the coils relative to magnet size, I'd appreciate this ...

Evening is approaching here in Denmark - the best for your evening,

Jesper
 
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I would say stronger magnets would help (more of them around the rim too) and get them as close as possible. I was always taught in the respect of designing electromagnets, not the other way round but more turns is definately a good idea.

Not sure on iron cores, I think they are used to get the inductance up by constraining the field, but analogue electronics and phsyics is not my profession. They use them in old radios, so I presume it is probably going to help, the old radios must have picked up H (magnetic fields) rather than E fields (electric) or there would be little purpose for the ferrite rod (this is dubious but its my thinking).

Just out of interest how are you measuring the voltage? If it's an average/RMS (depends on the meter) the peak voltage could be quite high. Might be worth looking at it on a scope. Could always make a simple peak detector if you only have a multi meter.

Good evening too :)
 
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Hi Rich (that is your name right :) ?),

Now that you mention it: As far as I know the cores for coils are also for raising the inductance. Don't know if it also increases the voltage of the coils ...

Your question about how I measure the voltage may indeed be spot on. I have a Fluke 85 meter where I use the 100 ms max hold function. It's not an RMS meter, though, so some averaging probably takes place. Glad you mention it - I had it in my thoughts and then it wents away again. However, I don't have an oscilloscope so cannot verify the peak voltages. Would you guess that they are much higher than what I measure on my DMM?

Best - Jesper
 
Hi Jesper, yes Rich is the name :).

I would try a core. It must help in both ways, as DC motor armatures have laminate cores for each pole. I seem to remember my old cars Dynamo (pre alternator) having a some metal in the armature too.

100ms is quite a long time, I guess it averages over .1 seconds and holds the largest value to date. If you have some diodes and capacitors lying about (1n4148 and 100nF at a first estimate the capacitor size may need tweaking but your meter should be high impedance anyway) I would suggest trying the following circuit (is it ok to link?).

This little rectifier will alllow you to measure roughly the peak with your meter. You dont really need the resistor either.

Six watts is a lot of energy to collect though :(
 
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