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Human-powered Electrical Generation
|General Overview||Converting a Bicycle to Generate Electricity||Converting a Treadmill to Generate Electricity|
Truly, much potential energy is wasted every day on so-called exercise or fitness machines. While some kinds of physical exercise in a gym or at home don't lend themselves well to electrical generation, two activities most certainly do. I am not the first to discover this, as other hobbyists have experimented with the ideas I'm about to speak of.
My interest in the subject of converting physical exercise into electricity came about because my wife engaged in a daily routine of pedaling a stationary bicycle and walking on a motorized treadmill. Now, I'm the sort of person who hates to see energy go to waste, so a number of years ago, following a lengthy power blackout, I explored the possibility of connecting some sort of generator to the exercise bike that could be used to charge a 12-Volt battery. The bicycle in question is a very old one with a chain-driven front wheel. Tension is applied to the large wheel by tightening a much smaller one against it, thus creating friction and making the bicycle harder to pedal. I began the transformation by replacing the original idler wheel, which was of a rather hard material, with one of a similar size that was made of soft rubber. Had an appropriate part not been available, I suppose I might have stretched a section cut from a bicycle inner tube over the hard wheel and superglued it in place. A soft surface was needed because I wanted to use the idler to drive the shaft of a small permanent magnet (PM) motor by friction against it. This motor was secured to the bicycle frame using some steel strapping and a couple of large U-bolts.
Although we generally don't think of motors as being generators of electricity, this particular type of motor lends itself well to the application. A DC (direct current) PM motor contains permanent magnets inside that create a field through which one or more coils of wire are rotated on an armature. This motion creates an electrical current in the coils, which is generally brought to wires outside via a pair of carbon brushes that make contact with copper segments on the commutator at the rear of the armature as it rotates. Most motors that operate from AC (alternating current) do not contain magnets and are thus not particularly useful as generators. At any rate, the setup I describe worked reasonably well until the foam that was helping to push the brushes against the commutator deteriorated and the contact became intermittant with a resulting loss of current, especially at high RPMs. I eventually replaced the motor-generator with a 3-phase AC alternator I modified out of a servo-motor scavenged from a broadcast-type 3/4-inch videocassette recorder. This interesting device contained three rotating magnets and three stationary coils, thus eliminating the need for any potentially troublesome slip rings or brushes in order to bring the electricity to the outside. Since the current induced into each of the coils was alternating in polarity and 120 degrees out of phase with its mates, I had to install a diode in series with each to rectify the output prior to connecting the three of them together. This setup is still in use and will produce several Amps of current at a nominal 14 Volts with a moderate pedal speed.
Unfortunately, not all exercise bicycles lend themselves to conversion and you can forget about those that operate a fan blade. However, anything that converts pedal motion into the spinning of a large-diameter wheel has potential. A large diameter is needed because the shaft on the typical generating device will have to spin at a high rate of speed in order to generate sufficient voltage to charge a battery. Fortunately for us, the difference in wheel size and shaft (or pulley) size gives a favorable ratio that multiplies the speed of the latter many times over. The transfer of energy can be accomplished either by the friction method, as I did, or with some sort of long belt, where this is possible.
Lacking an exercise bike, an ordinary bicycle can be adapted by purchasing a commercially-made stand for it or constructing one out of wood. The same techniques for transfering energy to the shaft of a generator apply, although a tire-less rim suggests a greater possibility for using a belt drive. In addition, a multi-speed bicycle offers the advantage of being able to select from a variety of rotational speeds for the rear wheel that may better match the needs of the generating device than a single-speed would.
The permanent magnet DC motor from an old treadmill might make a good bicycle-powered generator. If you're using some sort of DC motor as a generator, a blocking diode should be installed so that power flowing back from the battery doesn't cause the motor to spin. Also, be sure to use a meter or other means to determine which wire is positive when the bicycle is being pedaled. This will change if the device is switched from one side of the bike to the other, thus reversing the direction of shaft rotation. Some have used an old 12 Volt car alternator with a bicycle. It should be noted that the latter requires the internal "field coils" be energized from the battery being charged in order for electrical generation to take place. These electromagnetic coils take the place of costly permanent magnets. I personally have tried hooking a small, 25-Amp automobile generator to an exercycle that lent itself well to the purpose. However, I found the effort needed to produce several Amps of current to be excessive and would not recommend this route to others. Part of the problem is the relatively high speed that the generator pulley needs to spin at, which makes for a lot of resistance at the pedals from the high ratio. Also, a fair amount of the electricity being generated goes into energizing the field coils, increasing the resistance. That would not be the case with a PM device. I suspect this same problem would be encountered when using an alternator, since the application is similar. If you do attempt to use something from an automobile, I'd advise picking the smallest size available.
Converting an electrically-operated treadmill to function as a generator of electricity is perhaps the easiest option to pursue if you're interested in harnessing human power. This is because most treadmills use a DC permanent magnet (PM) motor in them. This particular type of motor is used because its speed can be controlled by varying the timing of an electrical pulse, a technique that results in a fairly high and constant torque regardless of how fast the armature is rotating. From an experimenter's point of view, treadmill motors of this type make an excellent generator for 12 Volt applications, especially since they tend to be rated at 70 or more Volts DC, meaning that their shafts do not have to rotate particularly fast to start charging a 12 Volt system. In fact, these motors are now fetching a high price on the large Internet auction site now that they have become a popular item for home-made wind turbines. Left mounted in their original treadmill, they are an excellent way of generating several Amps of charging current with very little modification. In fact, at its simplest, one simply disconnects the motor's two leads from whatever terminals they are connected to, splicing them to a longer cord that can be plugged into a charging circuit or hooked up to a battery. Again, you'll need to determine which lead from the motor is the positive. Don't rely on the color, as the black wire was positive in my case. Ideally, a blocking diode should be connected in series with one of the leads to prevent current coming from the battery from operating the motor. The diode should be rated for at least 10 Amps, as it is quite possible for a physically fit person to generate a considerable amount of electricity - in short bursts, anyway. I don't belong in that category, and I have achieved an output of 6 Amps on the treadmill my wife uses. Note that high-capacity diodes generally must be mounted to a metal heatsink in order to dissipate their rated power.
Not every treadmill employs a motor that is easily adaptable to electrical generation, but most do. Look for the words "DC" on the motor's label. In the interest of safety, I would advise that the prongs of the AC power plug be wrapped with tape or otherwise disabled so power can't be applied. To generate electricity, one simply walks on the treadmill in the usual way, the difference being that it is a harder workout as force must be applied by the body to move the belt, rather than just trying to keep up with it. You will find that elevating the front of the machine, either with its incline adjustment or by placing blocks of wood underneath it, will reduce the amount of backward force needed to keep the belt moving. In my own application, 2 Amps of charging current are sent to the 12 Volt battery simply by walking on the treadmill at a normal pace. I should say, however, that electrical generation using a treadmill is not as efficient as with an exercise bicycle, probably due to gravity acting on the upright body.
My wife says she gets a better workout now that she's generating electricity and it's more enjoyable because she's doing something constructive with the energy being expended. Perhaps someday gyms will have bicycles, treadmills and other fitness equipment that converts members' energy into electricity that they can use, in part, to charge their personal electronic devices while they are working out. Wouldn't that be something?
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