This was really hard!! I’m not even sure that I did it right! But it kinda sorta works!
For our midterm in Jeff Fedderson’s Sustainable Energy class we were asked to harvest a DC motor from something to create a generator capable of creating enough electricity to power something that we use on a daily basis. I chose to use a DC motor that I pulled out of an old scanner/printer and tried to attach it to my wife’s workout bike to recharge our Roomba vacuum cleaner. I thought it would be a good candidate because it was a bi-polar stepper and it had some gears attached, giving me the notion that I could pump out enough juice for the tiny vacuum.
It turns out that gears like to eat themselves if there’s too much action. It also turns out that putting a regular sized bike wheel against a tiny rubber wheel with a further 90:1 turn ratio against the motor is a bit of overkill. With the original model that I first built (mounted on wood with a spindle leading to gears), I was able to draw double power from the motor (so 6-24v instead of just 3-12v) but the layout was a little awkward, making it difficult to get the motor to sync up with the bike wheel. The resistance on the motor was so great against the bike wheel that it was difficult to impossible to maintain a reasonable pace riding the bike. It’s like when the landing gear of an airplane goes from 0 to 350mph in half a second. This was very disappointing to find out, especially since there had been so much time taken to tune the variable voltage regulator to make it put out just under 22V at 1.25A consistently. So in fine grad school fashion, I pulled the whole thing apart and started over at midnight the day before it was due – always a good idea….
I yanked the gears, remounted the contact wheel and the motor, made a stand for the whole thing…. only to find that now the motor is only putting out a about 5V consistently – but that’s hand-powered. If I could measure and bike at the same time, I might find something more suitable but given the hour, 5V is the way of the world.
I am pretty happy with the project. I learned much about the potential of extracting energy from trashed parts. I’m also happy that even with all of my trouble in understanding how this electricity business is supposed to work, I was able to get a consistent voltage out of the machine which is something I knew was going to be difficult.
Unfortunate side note; the replacement battery for my Roomba vacuum has still not arrived (after being lost in the mail for a few days) but I’m glad that I had the opportunity to pull a mulligan on the design, lest I explode a large battery in my living in a desperate “let’s just see what happens” mood. :)
The scheme is basic. The bi-polar stepper motor’s two pair of wires each went through full bridge rectifiers, then through a 1microFerrad 50V capacitor, then a variable voltage regulator (which was controlled by a cermet potentiometer with 220ohms resistance), and fed through a 100microFerrad 35V capacitor. This would then go up the line to the Roomba dock which was rated at 24V1A (but tested at 20V1.25A) where the Roomba sits till it’s ready to vacuum my apartment. I suspect I’m missing a zero or two with my big capacitors….
Here are some links to similar projects based on collecting energy from stationary bikes;