I would like to learn some stuff about building generators.

March 12, 2011 10:40 PM

I'm looking to make a few generators Ideally I want to make a contactless generator for a bicycle to be able to add a usb charger port to the bike. Can anyone provide any information on winding the coils and magnet configurations for a generator.

Seems easy enough to setup but there seems to be sparse documentation on it either that or I'm just looking in the wrong places. I guess that this is becoming a usual for me I dont ask here unless I've been beating my head off the wall for a while on a subject.

Anyways The idea is to have a generator that will generally charge usb and add little to no mechanical drag.

posted by Chamunks to technology (14 answers total)

Maybe buy a used hub generator and disassemble it to learn how it is wired. This is the contact-less variety of generator.
posted by Blazecock Pileon at 11:33 PM on March 12, 2011

You're not the first person to try this. Very detailed information at this site.
posted by tim_in_oz at 1:01 AM on March 13, 2011

Or a pre-fab solution here.
posted by tim_in_oz at 1:11 AM on March 13, 2011

I started the same project. The bike came to an untimely end before I got very far, and I had a hub generator built into the replacement bike, so I haven't done the entire process, but I had it figured out, so I think I can help.

First of all, ReeLights are a contactless generator at about the right voltage. So I bought some. They come with a couple of magnets that mount in the spokes (the arc of the magnets is not actual magnet - they are a tough plastic to screw onto the spokes, and a strong rare-earth magnet is mounted in the center). The light's generator circuit itself is what you'd expect - a coil going through a rectifier into a capacitor, then some electronics to power a blinking light off the capacitor once it has charged to an appropriate voltage.

The coil is very fine wire (hair thin) with thousands of turns. I don't advise winding them yourself. I don't have measurements, but you can get visually similar coils in terms of gauge and turns from a $1 quatz analogue clock at a thrift store, or from a low voltage solenoid (where low voltage means 9V or less).

For the capacitor, you want an electrolytic cap in parallel with a supercap bank. The ReeLights use just one memory-style supercap, and my bike fried them - or at least 3 out of my 4 ReeLights had their supercap fail on me, and I believe it was because the cap was rated at 5.5V and so sustained periods of high speed continually overcharged them. So I suggest either a bank of caps so that their voltage rating gives a solid safety need, AND a mechanics to protect them in case there is no load and the coils just keep charging them higher and higher...

The electrolytic cap is fast, so it can catch the pulses from the coil, and the supercaps are slow but with great storage, so they can be charged by the electrolytic and act as a battery that never wears out or runs out of charge/discharge cycles.

More on the coil. The core was iron, and it folded out around the coil as well (eg it makes the shape of the numeral 8 from a 7-segment LED display, with the windings around the middle element). The iron core has to pass about 3mm from the magnet surface. That means the bracket holding it has to be strong else the magnet will pull it into contact and the surfaces will rub. Also, even with the strong bracket that the reelights come with, you can hear a kind of clicking in the frame as the wheel turns due to the rhythmical sudden stress. Move the coil further from the magnets eliminates this, but also generates less power. Closer to the magnets generates more power, but as noted you risk grinding at low speeds as the magnets have time to flex the parts into each other.

ReeLights use two magnets, and don't generate as much power as you probably want (but on the bright side, you cannot feel any increase in drag at all), so I imagine that copying their coil will also produce less power than what you want, however the output can be increased by mounting more than two magnets around the wheel. Four magnets means twice the power.

This system will not produce comparable power to a generator hub, but the drag will be much lower.
Something I wondered about - the mechanical stress and vibration of the magnets and coils passing might have a deleterious effect on the spokes over long term. I'm not good at keeping my spokes in good shape anyway, so when things go wrong, I can't tell if the lights had anything to do with it, or if it was all the other abuses I subject them too :)
posted by -harlequin- at 3:35 AM on March 13, 2011 [2 favorites]

> This system will not produce comparable power to a generator hub, but the drag will be much lower.

Note that if there's no drag, there's no power generated. It's gotta come from somewhere. A well set up bottle dynamo (I like Nordlichts) has remarkably low drag. Best of the non-hub dynamos I've used was the Sanyo bottom-bracket type.
posted by scruss at 6:44 AM on March 13, 2011

Hmm these suggestions are great but they still kindof dont give me much information on Coil wrapping... hmm
posted by Chamunks at 8:52 AM on March 13, 2011

I think one of the problems you're running into is DIY Generators seem to break down into two broad catagories:

• Science experiment - 1900's era tech, works but absolutely miserable output in a very controlled situation.
• Real-world functional - High tech using ridiculously hard to home-fab windings and precsision-gapped magnets that will produce some sort of usable energy in real-world conditions.

I went looking for an example of a home-wound generator in Popular Science, and realized that they were scavenging generators as far back as 1945.

If you seriously want to understand how to construct a generator for your specific application that will produce good output reliably, There are textbooks that explain how. But I'm going to make an undereducated guess and say you're probably not going to like what you get.
posted by Orb2069 at 11:32 AM on March 13, 2011

scruss - I've used both types of bottle generator and don't like them, the mechanical drag (ie non productive) if not insignificant.

Chamunks - what about coil wrapping do you want to know? Start with a 4mm drinking straw, put a couple of card discs on it about 20mm apart, that's your spindle. Get your wire (thickness of human hair, insulated) and wind (and glue if you like) until the coil diameter is about 14mm. Done!
IIRC, the core is actually a 3 shape, not an 8, I can check. But make your core out of scrap iron or steel, eg bend a plate into a |_| shape, drill a hole in the center and screw a bolt into it to make the core.
posted by -harlequin- at 5:00 PM on March 13, 2011

A friend of mine keeps spouting off about ferrite cores :S I'm soo in the dark but it really cant be as difficult as everyone makes it seem. I've seen lots of instructibles.com on it... Just that noone really says anything about the coils they just say salvage some..

I've got nothing like this laying around but I have some technical skills just seems as though the coil thing eludes me.
posted by Chamunks at 5:05 PM on March 13, 2011

I guess more indepth questions to ask would be how does wrap direction affect current flow.

How should I wrap once I've reached the end of the spindle? Just start wrapping a second layer ontop in towards the first end of the spindle.
|      «Layer Two.........|
|........Layer one» ........| "." = wire
|???? CORE CORE??????|
|......Layer One» ........|
|      «Layer Two.........|

Maybe a spoonfeeding is in order I seem to be over thinking basic concept here.
posted by Chamunks at 5:19 PM on March 13, 2011

Don't use ferrite for the core. Use iron. Ferrite is berger for transformers, iron is better for magnetism.

The only wrap direction that matters in this case is that you pick one rotational direction and stay with it. Ie mount the spindle in a power drill, turn on the drill and let it wind on the wire however you like, so long as you don't reverse the drill.
posted by -harlequin- at 5:24 PM on March 13, 2011

So essentially a coil is similar to just having a small spool of magwire? Sorry for asking the same question with different wordings I just think that I've been overthinking this for far too long.

You said wrap it over a straw that would essentially leave it with an air core.

How come the core instead of just no core at all?
posted by Chamunks at 5:28 PM on March 13, 2011

Yes, this is just a spool of wire, ie an electromagnet.

The straw creates an air core, yes, but then you slide the iron into it. This is strictly for convenience of assembly. You can wind directly onto metal (and it's better to have the wire closer to the core) but sometimes this creates difficulty with the shape of the core (eg the "3" shaped core would preventing you from using a drill to wind it as the arms of the core would snag the wire as they turned. Likewise, winding onto just a bolt without the arms is good provided you can attach the bolt into the rest of the core later without damaging the coil.
Personally, I estimate the approach that would give me the highest convenience and reliability is by building the core and coil separately then combining them later. But this depends entirely on how you're making it.

The core better shapes and conducts the magnetic field. I think the 3 shape is get more magnet passes - the magnet passes over arm-center-arm as it moves past the coil.

Wikipedia on electromagnets - this explains a lot of the principles, even if we're using it in reverse.
posted by -harlequin- at 9:16 PM on March 13, 2011

I think you would find the hamster power generator interesting reading. Practical info about magnets, winding coils, etc. in a project quite similar to yours.
posted by exphysicist345 at 12:32 PM on March 14, 2011