Building a Superb Bike Generator

In the old days, if you wanted lights on your bicycle, you headed off to the corner bike shop. There you equipped yourself with a ‘dynamo’ (actually, an alternator) and front and rear lights, both of which used incandescent light bulbs. But these days, generator systems are way out of fashion, replaced by flashing front and rear LEDs powered by standalone AA cells. 

Which is fine if you don’t really want to see where you’re going, and you don’t really want to be seen by other road users…

OK, OK, that’s not quite the case: there are some excellent high intensity LED tail-lights available. And as for seeing where you’re going, if you’re rich, miniature halogen headlights with their own rechargeable battery packs can be purchased.  These latter systems, some of which retail at AUD$300 or more, provide excellent illumination but in addition to the high price, there‘s another downside - the battery pack needs to be frequently re-charged. 

In fact, if you ride for more than an hour a night, the battery may well have insufficient capacity to last for the full length of the journey. 

Even high powered LED headlights and tail-lights are limited in lighting duration if you’re away from a mains or car power source – eg if you’re into long distance, remote touring. 

In short, if you want a lot of light over a long period, you either carry heavy battery packs – or alternatively, generate your own electricity as you ride along.

Generating Power

A traditional bicycle ‘bottle’ alternator uses an 8-pole circular permanent magnet that spins between two coils. The power rating is generally 3W at 6V. In all designs that aren’t electronically controlled, voltage output increases with speed. To prevent the bulb filament burning-out at high speed, the output is ‘governed’ by a relatively high (eg 14 ohms) internal coil resistance. In other words, go really fast and you’re putting lots more energy in, without getting any more out!

A more expensive approach – one that isn’t normally used in bicycle applications - is to run a stepper motor as an alternator. Taking this approach has the advantage that a high output can be gained at low speeds without unduly compromising the output at higher speeds, and that the total power output can be much greater than is achievable with a traditional bike dynamo. 

If the stepper motor alternator is used to recharge a battery pack, the output voltage of the alternator will also vary little over a wide range of speeds.

And, while they may be expensive to buy new, suitably sized stepper motors are available for nothing from a wide range of discarded goods – photocopiers, large printers, big old electric typewriters…

So, how high an output are we talking about from a stepper motor alternator on a bike? Well, it’s not a bike (it’s actually a 63-speed recumbent trike) but on my machine, I’ve measured an absolute maximum output of 54 watts! Say that again? Fifty-four watts! That’s something like 18 times the power of a normal bike generator… 

And even when charging a 12V battery pack, it’s possible to achieve a continuous power output of 10W at normal road speeds – over three times the output of a conventional bike alternator!

So how do you do it?

Selecting the Stepper

The best stepper motor for this application can be found by going through a pile that you’ve salvaged – it’s unlikely that the first you find will be ideal. But stepper motors often look much the same - how do you pick the most suitable?

Firstly, go for a stepper that is decently sized. For example, the one I use is 55mm in both length and diameter. This size of stepper also normally has sealed ball bearings rather than plain bushes – pull it apart to make sure that’s the case.

Most steppers will be six wire designs, with as this diagram shows, two electrically separate centre-tapped windings. 

Use a multimeter to measure the resistances of the coils until you ascertain which wires are which. 

Then place a normal high intensity LED across a winding (eg connections Common 1 and 2 in the diagram) and give the stepper a spin by hand. The stepper you want will light the LED brightly, even with a slow shaft speed. (Nope, you don’t need a rectifier – the LED will still light on the AC voltage.)

Next, short those two wires together. The stepper you want will be now much harder to turn, with a distinct gritty ‘cogging’ action. 

Then measure the DC resistance across the same two wires. The stepper that’s best suited will have the lowest winding resistance – eg less than 5 ohms.

After all of that, do a final check. Connect four 1N4004 diodes to the output windings and wire-in a load - eg a normal bicycle headlight. Use an electric drill to spin the stepper motor (now a generator!) and measure the loaded output voltage and current.

The higher the output, the better, but as a guide, the stepper used in this story developed 8.4V DC at 0.6A when running a 6V 3W filament bulb and being rotated by the electric drill at a nominal 900 rpm. 

Installing the Alternator

To drive the alternator from the tyre you’ll need a knurled aluminium or steel roller that’s about 30 - 60mm in diameter and is a press-fit on the shaft of the stepper. 

Said in one sentence that sounds easy but the reality is often much different to that. I have a metal-turning lathe and so the task of making the roller was straightforward, but if you aren’t so equipped, you might need to approach a local engineering works or tech teaching school. (Also, for an easy solution, see the “Video Head Roller” breakout box below.) 

It is imperative that the roller is both perfectly round and is concentric with the shaft. The diameter of the roller is also important - we’ll come back to this in a moment.

Rather than take the traditional approach of the roller pushing against the sidewall of the tyre, I chose to run the roller against the (semi-slick) tread of the tyre. (This allows the use of a larger diameter roller while still letting the roller run ‘true’.) 

However, there is a problem with this approach. Most salvaged stepper motors have only a short length of protruding shaft. If you mount a wide roller on this, much of the roller isn’t supported by the shaft and so the roller will have a tendency to wobble. 

I chose to use a narrower roller that is better supported by the shaft but bears against only the centre of the tyre tread. This works very well – even in wet conditions, there’s no detectable slippage. (If the bike is to be used in muddy conditions or has a treaded tyre, a smaller roller should be used that bears against the tyre sidewall.)

The alternator/roller combination needs to be mounted so the assembly can pivot, pushing the roller against the tyre. At its simplest, this requires only a few brackets and a normal door hinge but I chose to make a more elaborate mount.

I used the components from a couple of video drum assemblies salvaged from VHS video cassette recorders. The main shaft support - which contains two widely spaced bearings - was reduced in diameter, as was the spinning head. (All video drum components except the shaft and bearings are made from easily ‘worked’ aluminium.) 

The stepper motor was attached to the cut-down spinning head via a bracket made from aluminium angle. The other part of the drum assembly, comprising the precision sealed ball bearings and support, was attached to another aluminium angle which in turn was bolted to a plate. The plate was attached to the cycle carrier. The aluminium plates and angle were drilled for lightness.

The video drum shaft running in bearings thus forms the pivot on which the stepper motor/roller assembly rotates, allowing it to be pressed against the tyre while rigidly keeping the stepper motor shaft and the cycle wheel axle parallel. 

Because the of the large diameter roller (with the high output at low speeds of the stepper motor alternator, a small roller is not needed), the roller does not need to be heavily pushed against the tyre. A light spring will do the job without great tyre deflection - and so without the frictional and hysteretic tyre losses that would otherwise result. 

I also added an ‘over-centre’ linkage in parallel with the spring that allowed the alternator to be held captive in a lifted position as required. 

Roller Diameters

In addition to the characteristics of the stepper motor and load, the electrical output depends on how fast the alternator turns. Alternator speed is determined by alternator drive roller diameter and how fast you ride. (This latter point is often forgotten, but if you seldom exceed 10 km/h, the gearing of the alternator will need to be quite different than if you frequently ride at 25 km/h.)

An alternator subjected to a load will have a current output that initially rises with speed then plateaus as the speed rises further. 

If the alternator is geared too high, the output current will plateau early. This is bad as you’ll be pedalling hard but getting no more out of the alternator. 

On the other hand, if the alternator is geared too low, the electrical output will always be less than it could otherwise be! 

Making it even more complex is that the slower the alternator turns, the lower are its internal losses – but of course, the lower also is the electrical output of the alternator. Because the optimal alternator gearing depends on the load, the characteristics of the alternator and how fast you ride, the best approach is to try some different diameter rollers.

The first roller that I made was 33mm in diameter. This gave excellent electrical output but the pedalling effort (even with no current draw) was relatively high. (This parasitic load is due to the internal hysteresis losses.) 

Using this roller on a nominally 20-inch tyre, 0.8A at 12.7V was available at 15 km/h when charging a nominally 9.6V ni-cad battery pack.

At over 10W output there was power to spare, so it was decided to try a larger 63mm diameter roller to slow the alternator and so decrease the parasitic losses. With the new roller, the pedalling load was reduced but the electrical power output remains very respectable.

Conclusion

It’s not a five minute job but turning a salvaged stepper motor into a high-powered bike lighting alternator is satisfying and very effective.  

Video Head Roller As we’ve covered previously in AutoSpeed (see More Parts for Nothing!), video heads from VHS video cassette recorders are very good items to salvage. In fact, one can even be used to make the roller that drives the bike alternator.  When you pull the video drum assembly apart, you’ll find a hardened steel shaft that runs on sealed ball bearings. At one end of the shaft is a brass collar that is a push-fit on the shaft.  Bolted to the collar is the part of the drum that spins. This comprises a 61mm diameter, 12mm wide aluminium disc.  The shaft of the video drum is a little smaller in diameter than the shaft of most medium sized stepper motors. So if the brass collar is removed (easily done by using a vice to support the collar, and a drift and a hammer to push the shaft through it) the collar can be carefully drilled-out to become a push-fit on the shaft of the stepper. If the hole in the brass collar ends up a fraction too large to be a genuine push-fit, squeeze the shaft of the stepper in the hardened steel jaws of a vice. This will raise bumps which will then grip the collar very well. For additional security, apply some Loctite.  The drive surface of the aluminium disc can be knurled in a lathe (or have lateral striations cut across it with a file or hacksaw) and then bolted to the brass collar.