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Salvage It! BY JULIAN EDGAR Building a super bicycle light alternator The traditional bicycle alternator or “dynamo” is not very effective. Here’s how to turn a salvaged stepper motor into a high-power alternator for really effective lighting, even at low speed. I N 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. These days, however, generatorpowered lighting systems are 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, that’s not quite the case – there are some excellent high-intensity LED tail-lights available on the market. 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 $300 or more, provide excellent illumination but there‘s a downside – the battery pack needs to be frequently re-charged. In fact, if you ride for more than an hour at night, the battery may well have insufficient capacity to last the full length of the journey. Even Luxeon LED headlights and tail-lights (see the “Universal High-Energy LED Lighting System” in the April & May 2006 issues) are limited in lighting duration if you’re away from a mains or car power source. In short, if you want a lot of light over a long period, you must either carry a heavy battery pack or, alternatively, generate your own electricity as you ride along. Generating power ➌ ➎ ➋ ➏ ➍ ➊ The main components of the author’s bike alternator system are clearly shown in this photo: (1) knurled aluminium roller made from a video drum (the white centre cap is from the top of a vitamin jar); (2) alternator support frame; (3) stepper motor (used as an alternator); (4) cover over end of video drum bearing (the cover is the cap from a deodorant bottle); (5) bearing support and (6) bike support frame. siliconchip.com.au A traditional bicycle “bottle” alternator uses an 8-pole circular permanent magnet that spins between two coils. Their power rating is generally around 3 watts at 6V. In all designs that aren’t electronically controlled, the output voltage increases with speed. As a result, the output is “governed” by a relatively high (eg, 14W) internal coil resistance to prevent the bulb’s filament burningout at high speed. In other words, go really fast and you’re putting in lots more energy without getting any more out of the alternator. A more expensive approach – one that isn’t normally used in bicycle applications – is to use a stepper motor as an alternator. This approach has two main advantages: (1) a high output can be gained at low speeds without unduly compromising the output at higher speeds and (2) the total power output is much greater than can be October 2006  89 Fig.1: 6-wire stepper motors have internal wiring that looks like this. When quickly sorting through a batch of possible stepper motors, placing a LED directly across a pair of wires (eg, connections 1 and 2) and spinning the stepper by hand will give a quick and easy indication of its potential power output. achieved with a traditional bike dynamo. Another advantage is that if the stepper motor alternator is used to recharge a battery pack, its output voltage will remain relatively constant over a wide range of speeds. Finally, while they may be expensive to buy new, suitably-sized stepper motors are available for nothing from Fig.2: a further test of the alternator’s output can be made by driving it with an electric power drill. As shown here, the alternators output is rectified and connected to a suitable load such as a 6V 3W incandescent bulb. The higher the output, the better but as a guide, the stepper shown on these pages developed 8.4V DC at 0.6A when rotated by the electric drill at a nominal 900 RPM. a wide range of discarded goods, such as photocopiers, large printers and old electric typewriters. So if you can scrounge one from somewhere, you’ll save heaps. So how much output can be obtained from a stepper motor alternator on a bike? Well, on my machine – which is actually a 63-speed recumbent trike – I’ve measured an absolute maximum output of 54 watts! That’s right – 54 watts or about 18 times the output from a normal bike alternator! Even when charging a 12V battery pack, it’s possible to achieve a continuous power output of 10 watts at normal road speeds – over three times the output of a conventional bike alternator! So how do go about getting one working on your bike. Selecting the stepper The brackets that locate the alternator were made from aluminium offcuts purchased for next to nothing from a scrap metal dealer. A large number of holes were drilled in these brackets to give a very light weight while still maintaining sufficient strength and rigidity. 90  Silicon Chip Stepper motors often look much the same, so how do you pick the best one if you’ve got lots to choose from? First, go for a stepper that’s decently sized. For example, the one I use is 55mm in both length and diameter. This size of stepper normally has sealed ball bearings rather than plain bushes but you should pull it apart to make sure. Most steppers will be 6-wire designs with two separate centre-tapped windings – see Fig.1. Use a multimeter to measure the resistances of the coils to determine which wires are which. That done, connect a high-intensity LED across one of the windings winding (eg, connections 1 & 2 in Fig.1) and spin the stepper by hand. The stepper you want will light the LED brightly, even with a slow shaft speed (no, you don’t need a rectifier – the LED will still light on the AC voltage). Next, short those two wires together. The stepper should be now much harder to turn, with a distinct “cogging” action. siliconchip.com.au ➍ ➊ ➌ ➋ Here’s another view of the author’s system: (1) stepper motor; (2) “over-centre” link to allow roller to be locked in lifted position; (3) spring to pull roller against tyre and (4) bike support frame. Now measure the DC resistance between these same two wires. The stepper that’s best suited will have the lowest winding resistance – eg, less than 5W. Now for a final check. First, connect four 1N4004 diodes to the output windings as shown in Fig.2 and connect a load – eg, a normal 6V 3 watt bicycle headlight. That done, use an electric drill to spin the stepper motor (which is now an alternator) and measure the output voltage and current with the load in place. The higher the output, the better but as a guide, the stepper shown in the photos developed 8.4V DC at 0.6A when running a 6V 3 watt filament bulb and being rotated by the electric drill at a nominal 900 RPM. Installing the Alternator In order to drive the alternator from a bicycle tyre, you’ll need to press-fit siliconchip.com.au a knurled aluminium or steel roller that’s about 30-60mm in diameter to the shaft of the stepper. That might sound easy but the reality is often quite different. In my case, I have a metal-turning lathe and so the task of making the roller was straightforward (see the accompanying “Video Head Roller” panel). If you don’t have a lathe, then you might need to approach a local engineering works to make one for you. Note that it’s 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 Fig.3: the current achieved when charging a nominal 4.8V NiMh battery pack with a 6-wire stepper motor with these specifications: 4V, 1.8 ° per step, 1.8A per phase. The alternator uses a 63mm diameter knurled roller contacting the tread of a 20-inch slick tyre. Note the high output at very low road speeds – even when using the large diameter roller, 800mA charging is achieved at just 9km/h. 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. In my case, I chose to use a narrow roller that is better supported by the shaft but bears against only the centre of the tyre tread. This works very well, with no detectable slippage, even in wet conditions. However, if the bike is to be used in muddy conditions or has a treaded tyre, a smaller roller that bears against the tyre sidewall should be used. The alternator/roller combination needs to be mounted so the assembly can pivot, so as to push the roller Stationary Power Station Another application for a converted stepper motor alternator is on an exercise bicycle. In this case, a small diameter roller should be used and by feeding the output into a suitable charger, you can recharge batteries while you exercise. That’s a lot more useful than just dissipating your energy into a friction brake! October 2006  91 Using The Luxeon High Energy Lighting System Fig.4: a very effective bike lighting system can be made by using the alternator to charge the battery in SILICON CHIP’s Universal High Energy LED Lighting System. As shown here, the alternator is directly connected to the battery pack via a 50°C series temperature cut-out, the latter mounted on the battery pack. In addition, a 5A fuse is added in series with the Luxeon output and the battery fuse is upgraded from 5A to 10A. The most best light sources for bike lighting systems are Luxeon LEDs. And in my opinion, the best control system for Luxeons is the Universal High-Energy LED Lighting System described in SILICON CHIP for April & May 2006. In addition to efficiently operating LEDs up to 6 watts, the Universal High Energy LED Lighting System has specific bike light modes that alter flashing rates according to the ambient light levels. However, you can’t just connect the rectified output of the alternator to the charging socket of the Luxeon system to recharge the batteries. Why not? Well, since the no-load output of the alternator can be as high as 80V, this would destroy critical parts in the charging circuit. This occurs because once the input voltage exceeds 18.6V, charging automatically stops, and so the alternator sees a no-load condition and its output voltage skyrockets. 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. As shown in the photos, I used the parts from a couple of video drum assemblies (salvaged from VCRs) to make 92  Silicon Chip The best way to integrate the Luxeon system with the alternator is shown in Fig.4. As shown, the alternator’s rectified output is directly connected to the battery pack through a 50°C series temperature cut-out (ie, the input charging circuit is bypassed). The temperature cut-out is mounted on the battery pack and prevents overcharging (the battery pack get hot if over-charged). In addition, a 5A fuse is added in series with the Luxeon LEDs, while the existing 5A battery pack fuse (F2) is upgraded to 10A. These fuse changes prevent a scenario where when the Luxeon output is shorted, the battery fuse blows and the rest of the circuit sees 80V courtesy of the unloaded alternator. In practice, the new charging cable from the alternator can be routed through the existing cable gland (there’s just enough room for the two cables). Note that when using this revised configuration, the coloured LED a suitable assembly. First, one part of a video drum was used for the roller itself (see panel). That done, the main shaft support – which contains two widely spaced bearings – was reduced in diameter, as was the spinning head (note: all video drum components ex- will constantly show battery level – it won’t change to indicate when alternator charging is occurring. If required, “top-up” charging of the battery pack can still be carried out using an external plugpack and in this situation, the charge LED will work as usual. When charging the Luxeon system’s NiMH battery pack, the alternator used by the author gave a measured output as shown in Fig.3. Note how as the road speed (and thus the alternator speed) increases, the rate of current increase begins to flatten out. The trick is to gear the alternator so that there’s still plenty of power available at low speeds but without the current output reaching a plateau early in the normal speed range. Another point to note is that the author’s alternator was internally current limited to 1A. So in this case, when charging a battery pack at about 5V, the peak power obtainable from the alternator was 5 watts. cept the shaft and bearings are made from easily worked aluminium). The stepper motor was attached to a cut-down spinning head via a bracket made from aluminium angle. The other part of the drum assembly, comprising the precision sealed ball siliconchip.com.au bearings and support, was attached to another aluminium bracket which in turn was bolted to a plate. This plate was then attached to the cycle carrier (note: the aluminium plates and angle brackets were drilled for lightness). The video drum shaft and it bearings form the pivot on which the stepper motor/roller assembly rotates. This arrangement allows the roller to be pressed against the tyre while rigidly keeping the stepper motor shaft in parallel with the wheel axle. Because the roller has a relatively large diameter, it doesn’t need to be pushed hard against the tyre. A light spring will do the job, without an appreciable tyre deflection - and without the frictional losses that would otherwise result. (Note: because the stepper has a high output at low speeds, a small roller is not needed). I also added an “over-centre” linkage in parallel with the spring which allows the alternator to be held captive in a lifted position if required. Roller diameters It is not just the characteristics of the stepper motor and the load that determine the electrical output from the stepper – it also depends on how fast the alternator turns. In practice, the alternator speed is determined by tyre diameter, the drive roller diameter and how fast you ride. This latter point is often forgotten, but if you seldom exceed 10km/h, the gearing of the alternator will need to be quite different than if you frequently ride at 25km/h. An alternator subjected to a load will have an output current that initially rises with speed and then levels off as the speed rises further. If the alternator is geared too high, the output current will limit early. This is bad because 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. 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 siliconchip.com.au ➋ ➊ When the over-centre lever (1) is released by turning the knob clockwise, the alternator/roller assembly pivots so that the roller contacts the tyre and is held there by a spring. The pivot is formed from a cut-down video drum assembly (2) that uses high quality ball bearings and a precision shaft. Note that a strong spring is not required as the large diameter knurled roller grips the slick tyre quite well. internal hysteresis losses). Using this roller on a 20-inch tyre gave an output of 12.7V and 0.8A when pedalling at 15km/h. This output was used to charge a 9.6V nicad battery pack. At over 10 watts output, there was power to spare, so I decided to try a larger 63mm diameter roller to slow the alternator and decrease the parasitic losses. This new roller reduced the pedalling effort and the electrical power output remains quite respectable. Conclusion It’s not a five minute job but with a little time and patience, a salvaged stepper motor can be turned into a very effective high-power bike lightSC ing alternator. Using A Video Drum As A Roller As described in Salvage It! for December 2005, the drum assemblies from VCRs are worthwhile salvaging. In fact, one can 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 tapping the shaft with a hammer), it 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 corrugations in the metal which will then grip the collar quite well. You can then apply some Loctite for additional security. 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. October 2006  93