Exercise Bike Generator Demonstration

Tuesday, March 31st, ©2009 Marcus Brooks
A Power Generation Demo

A Power Generation Demo (Photo by Anne Castello)

If anybody is reading my posts, you may have noticed gaps between them. I have several ideas for topics, but so far I’ve had a surprising amount of trouble finding time to write entries.

Last week’s distraction was a quick little project that my brother Bill asked me to do. I didn’t catch all the details, but a couple of his colleagues at Frontier Associates were planning a weekend demonstration for a group of 5th-6th graders. The idea was to mount a generator on an exercise  bike and use it to power an ordinary 100-Watt bulb. The kids would pedal both with and without the load to see how much actual work was required to create the electricity. This sounded like a fun little project, so naturally I took the job.

A quick search found a DC Bicycle Pedal Power Generator, from which I freely cribbed ideas. Fortunately my demo was to be simpler, so I discarded the battery and the charge controller. My major components were:

  • An exercise cycle (from Bill’s attic).
  • A 100W, 24V brushed DC motor.
  • A 170W, 12V to 120V inverter.
  • A 100W, 120V light bulb.

My first try for a generator was a nice 24V German Fan from American Science and Surplus. Ain’t surplus great? For less than $18 total I got a really nice blower. Alas, it seems to be a brushless or AC induction motor, ’cause it won’t generate at all! (Open up the back, and instead of brushes you see a very fancy and expensive-looking controller board. Totally useless.)

Punting, I ordered electricscooterparts.com‘s EMOT-100, little brother to my wheelchair’s two 500W motors. Not quite as cheap as the blower, but this one still cost less than $50 with priority mail shipping. The important part: it’s a simple DC motor—the kind with brushes, magnets, and windings. Windings moving in a magnetic field make electricity, so spinning a plain DC motor makes it a generator!

The pedals on Bill’s exercise bike drive a big “semi pneumatic” (solid-rubber) tire. For resistance, a small adjustable pinch roller drags on the tire. I thought it would be a simple matter to convert the pinch roller to a pick-up roller, driving the motor to generate current. But the setup turned out to be too complex to describe here; it involves some steel bits, a hose clamp, a 3″ bolt and other hardware, spare skateboard bearings (blankdecks.com), some chain, and a #25 sprocket (McMaster-Carr). If you really want details, post a comment to say so!

The motor puts out 12-24V DC, but the plan was to drive a 100W household light bulb at 100-120 volts. AC really isn’t necessary, but I figure a DC to AC inverter is the most practical way to get 120V from 12V anyway. I found one on clearance that looks like an XPower Inverter 175 Plus produced under a former brand, and it works well.

This is a modified sine wave (MSW) inverter. That’s code for “square wave.” MSW offends audio systems and the like, but most appliances don’t care—especially not a light bulb. A socket-to-plug adapter plugs the light bulb directly into the inverter’s outlet. Since I don’t have any regulation or storage in the circuit, the inverter beeps to complain about low voltage as soon as you start pedaling, but then it goes quiet and a second later the bulb lights. It’s kind of clunky, but I’m told the kids enjoyed regulating the voltage themselves by watching the attached digital multimeter ($3 on sale from Harbor Freight).

Just in case, I mounted four 12V bulbs in a spare project box to make a 12-24V load. I used two bulbs each of 23W and 13W. One pair was wired in parallel, the other in series. This creates a sort of “bar graph” effect, with one bulb lighting first, then each bulb brightening in turn as you pedal faster. I suspected the kids might pedal fast enough to burn out a parallel-wired bulb, and sure enough, they did. I might go back and put a voltage limiter in the circuit, but apparently the kids preferred the inverter demo anyway.

This setup is not without worries. The 12V bulbs can get pretty hot, the sprockets are exposed, and the 120V bulb should really be in a cage. But I’m told the event went off without a hitch, and that the generator bike was a huge success. They’re even planning to take it to other educational events.

Not a bad distraction, at that.

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14 Responses to “Exercise Bike Generator Demonstration”

  1. Robert Smith says:

    I found your blog and read a few of the posts. Keep up the good work. I am looking forward to checking out more from you in the future.

  2. Hello Marcus,
    Thanks for this cool blog entry. Care to post some drawings on the setup of this power generator? I plan to do the same at my school on a science fair, and this is exactly what I have been looking for. 🙂
    My other plan is to do the same but with a smaller solar panel, so kids can see how powerful these things are.

  3. marcus says:

    Sorry I didn’t reply to this before. I got distracted. As pictured here, there’s nothing very special about the setup. The motor is wired directly to the load. For a serious application, you’d want to add a storage battery, at least. This would be wired in parallel to the load. If there’s any chance of exceeding a safe operating or battery charge voltage, you’d also want some sort of voltage regulation. That’s a problem I carefully avoided, though.

  4. Keith says:

    My first bicycle as an adult had a built-in dynamo generator in its front hub and an included light. It was only 2.4W; I can’ t imagine powering much more than that. In fact, the light wouldn’t even work going uphill, so I had to purchase a 10W halogen light so that I wouldn’t be riding illegal at night.

  5. marcus says:

    True, parasitic generators aren’t so good on a riding bike, especially now that battery-powered lights can be so efficient. It takes a lot of energy to overcome wind and rolling resistance, and, as you say, to climb hills. All that energy is usually wasted on a stationary bike, though, so it makes some sense to try using it. The kids using this rig don’t have much trouble cranking out 100 W or more for short periods. I’ve heard of folks using a similar rig (with a regulator and battery) to power their kids’ TV, video game, or computer; as incentive for exercise. I seem to recall that top cyclists can generate several hundred watts for extended periods. (YMMV!)

  6. kenzo says:

    another great article where stationary bikes are used to generate energy! I’ve seen quite a lot lately but I have been fascinated on the different concepts where they have taken this like what this article dogengine.com/us/used-stationary-bikes.php (go at the bottom of the page and read “Stationary Bikes Used For Generating Energy”) states the summary of the different concepts used in generating energy using bikes.

  7. lizzy says:

    are you hooking the motor up directly to the inverter?

  8. marcus says:

    Yes. The kids regulate the voltage by watching a voltmeter. It doesn’t work all that well, but it probably gets the point across better than a battery and regulator setup would allow. (Pedal faster, more volts, and vice-versa.)

  9. I’m looking for a bike? like this to? make my own generator. Is this bike frame something unique?

  10. marcus says:

    The new exercise bikes that I see around probably wouldn’t work, because there isn’t an exposed roller to mount the pick-up shaft and sprocket on. I think this bike was at least ten or twenty years old. Check out flea markets, thrift stores, Freecycle, garage sales, etc. and you might find something similar. Or maybe you can mount an actual old bicycle on a stand and start with that.

  11. lizzy says:

    I’m getting 12 volts when I pedal and the inverter is turning on when nothing is plugged into it, but the second I plug in a lamp into it, I get a low voltage error and nothing happens, even if I’m still generating 12 volts. any suggestions?

  12. Ignacia Holtsclaw says:

    Before you read this comment on, we’ll have to agree to disagree.

  13. marcus says:

    No. I refuse to disagree. You’ll just have to live with that.

  14. Oh, that’s a creative idea. But can you charge battery?

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