Is it possible to power a small dc motor with a bicycle dynamo?
 

Is it possible to power a small dc motor (3-6v) with a 6v/3watt bicycle dynamo generator? im not a cyclist or electrician so simple answers would be greatly appreciated

Im an artist currently working on kinetic sculpture, the piece is intended to be a pedal powered fan which requires more effort than the resulting activity should require. my work often has this kind of anticlimactic nature which i find interesting however ive not made anything quite of this nature before, all suggestions and comments are very welcome :)

Cheers
Rob

Sure. Voltage is voltage. You'll want to get a decent motor, not one of those cruddy little $2 Radio Shack hobby motors. If nothing else, so that you can put something substantial on it, or run gears or belts off of it.

As far as not getting as much out as you put in, that's absolutely guaranteed by the laws of thermodynamics.

3W is awful little energy to power a fan. Well, maybe a small fan with like.. mmmm, lemme take a wild stab... 2 inch rotor diameter size.

Also, the dynamo is AC, so you need an AC motor, or you have to put a rectifier bridge and a capacitor between the dynamo and a DC motor.

i'll try the rectifier bridge idea, thanks very much this should hopefully solve my problem, i think :S

So you tried a mechanically-driven fan using a bicycle for power, and it was hard to drive? That indicates it somehow needed rather more power than the 75 watts even a casual cyclist can put on the ground. An electrical solution to drive that same fan/belt combo will need to deliver about that same amount of power, barring inefficiencies. A hub generator only produces 3 watts by spec, somewhat more in reality, but nowhere near 75 watts. I predict you won't get any motion out of that same fan using a hub dyno, even after rectifying.

to Road Fan,

correct :)

When you say "fan" what do you mean? I'm afraid that I just automatically assumed that you would pick a fan that was sized appropriately for the few watts of power that you would have available; to me, that means a tiny little thing perhaps 2 or 3 inches across, turning lazily on a high efficiency motor.

My guess is you will lose too much converting energy from mechanical to electrical and back.

Not too much, just a certain efficiency ratio. Best you're gonna get from an alternator & motor is about 80% each, so 0.80 *0.80 = 0.64% return on your initial power-input. And I think small dynamos and motors are even less than that. I didn't even add in the rectifier stage... Without knowing the specifications of the OP's design, we can't really answer the question.

yes its a tiny little motor, the fan blades are a couple of inches diameter, ive given up with a clever electrical circuit to change the ac to dc, as lack of knowledge of electrical components became an issue. instead i have added another dynamo, and have connected them directly to 6v 3 watt halogen bulbs, then pointed those at a solar panel which has a dc output, i can get up to 0.92v which isnt enough to power the small motor still, my solution is yet more lights and dynamos or a smaller motor, my deadline for this is in a couple of days so only quick solutions will be of any use, cheers : )

Thats a pretty ridiculous setup youre describing there...

why don't you use a mechanical fan and run it off a belt? with proper gearing that is by far the most efficient solution. and guess what, bikes have gears. ;)

Hmm... He said he had a couple of days left for this almost a month ago when the last post from the OP was made.

Old Union Bottle dynamos I thought were DC, Modern stuff is alternators , so an AC motor , or one of the new USB adaptors would be the alternative

I think they designed in the rectifier circuits to have a DC output from AC source..

charge up cell phones with those ..

lol. I love it! About 50 dynamos and lights ought to do it :)

Theres belt driven squirrel cage fans attached to training rollers to cool you while you are working hard

and going nowhere, indoors.

you can push a lot of air with a mechanical linkage from a bike to a fan. I attached a squirrel cage fan from my junk pile to my rollers, and the resulting blast of air was too much, I got too cold riding it that way. A bicycle light dynamo would be the limiting factor. You could get a more powerful generator and power a pretty big fan.

try a 12 volt 6 watt dymano....