Engine for small generator 3-30V

[bigwater]

Hello, I am a new user here but have found links to this site in many of my Google quests so I thought this may be the place to start a more focused search for information.

I am doing a project with my son, we have created some spinning discs using magnets as a power source. I am interested in attaching a small electric engine (or coil) to produce electricity that could power a small LED or compact fluorescent light bulb.
I am assuming it would be better to buy a small engine and reverse it than to build the coil because it would probably be better produced and more efficient. But I am not completely understanding how much torque and rpm's are needed to generate electricity from said motors and what tools should be used to measure the performance of the set-up. I was considering buying the Greenlee DM-300 multimeter because of its low voltage accuracy and maybe trying this method of measuring rpm's <http://www.fastcharged.org/electronics/handyrpm> unless there is an easier cost effective way.
Also are their numbers I should look at to find out what rpm and torque is needed to spin the coil? Can anybody recommend a good engine for this?

Thanks much in advance.

 

[NascentOxygen]

A small electric motor that is easiest to get hold of would be one that you salvage from a toy, such as a cheap slot car or battery-operated toy train. You can feel the torque needed to turn them by turning the shaft between two fingers. You should be able to detect a generated voltage if it is spun at even a low speed of a few revs/sec.

BTW, if it runs off electricity it is a "motor", if it burns gasoline it's an "engine".

The cooling fan inside the case of desktop PCs is very easy to spin, and is powered by a low voltage. I've never examined them closely so don't know whether they can be used as a generator. You could salvage one from a dumped PC and try blowing through the fan to see if it will generate DC.

 

[russ_watters]

The way to actually calculate the torque is to equate the electrical output to the mechanical input and add 10% or so. power = torque * rpm.

 

[jim hardy]

English units:

horsepower = 2PI X torque X rpm / 33,000 ; torque in ft-lbs.

should be easy swap to SI units.