Where to buy a low friction wheel with external hub and a tiny electric generator?

December 7, 2008 4:59 PM

Where can I find a super low-friction wheel with a ring around it and also a little generator? I am making one of those silly home-brew perpetual motion machines with magnets.

The magnets will spin the wheel which will generate electricity to light a light bulb. Is it true that you can take a normal motor (like from a desk fan) and run it backwards to turn it into a generator?

As for the wheel, I need something low friction that will spin easily, and I need a ring that will go on the outside of the wheel, separated by about 1 inch. For example a bike tire inside a hula hoop. But I need it to be a closer fit, and preferably much smaller than a bike tire. Something that will look cooler and more professional.

BTW, I know about how perpetual motion or "free energy" isn't physically possible--this is just a toy to impress friends. (Although it might have a practical application as a non-chemical battery!)

posted by brenton to technology (3 comments total)

Any high-quality bike wheel will have very low friction.

The Lightspin is a low-drag bike-light generator. I haven't used it myself, but from what I've read, you can spin its little drive wheel and that will keep spinning for a little while. With the momentum of a free-spinning bike wheel, it should keep spinning for quite a while.

If you evenly weight the rim of the wheel, it'll have a lot of rotational inertia and keep spinning for a long time indeed. You could use an old bike fork as the frame for your rig.
posted by adamrice at 5:39 PM on December 7, 2008

Is it true that you can take a normal motor (like from a desk fan) and run it backwards to turn it into a generator?

Partially. A desk fan will have an induction motor in it, and those don't work well as generators. The best motors for Weird Science projects are discarded stepper motors.

If you're going to use the momentum of a spinning bicycle wheel as your main energy store, though, you will want to simplify the drive train as much as possible to cut friction. The simplest possible drive train is to have the wheel and a brushless generator share the same bearings, and the simplest way to achieve that is with a bicycle hub alternator.
posted by flabdablet at 9:30 PM on December 7, 2008 [1 favorite]

The magnets will spin the wheel

By the way: no they won't, unless they're interacting with a varying magnetic field and absorbing energy from that interaction, in which case your wheel with magnets is nothing more than a (probably sub-optimal) electric motor.

If you're making a fake perpetual motion machine to impress your friends, you would need to hide a bit of electronics and an electromagnet somewhere in the base to provide the varying magnetic field to interact with your wheel magnets to make the motor spin; and once you have that, you may as well use the power supply that powers the electromagnet to light up your bulbs as well, since you will get a lot more juice straight from the power supply than you will from a generator running off your wheel.

Alternatively, you could put a showy arrangement of permanent magnets around the wheel rim to "interact" with the magnets on the wheel, and then power the whole device by feeding alternating current (at suitably low voltage) into the hub generator and the lights. That way, the only thing you need to hide is the power supply and the wiring connecting it to the generator and lights. You should be able to hack up something with a few AAA cells and a timer IC (possibly tucked inside one of the bicycle forks?) that will get the job done.
posted by flabdablet at 4:25 PM on December 8, 2008