How to use electric motor in windmill? - RC Groups
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Aug 10, 2003, 09:14 PM
Registered User

How to use electric motor in windmill?

Hi Guys,

For my daughter's science fair, she needs to make a windmill with the intention of charging a nicad with the energy generated. To avoid needing a rectifier circuit, I tough I could use an electric DC motor to generate DC voltage and current.
I did a test with a speed 400, using my glow starter and was able to measure >2volts (no idea of current generated). Of course,it was at a quite high RPM.

A few questions:

1- Is there any formula to translate motor RPM to voltage generated at terminals? what about current?
2- How do I know which motor size is best (speed 400,480, 500, 600, etc)?
3- For the windmill to work, I will have to probably use a geared motor. Any idea how to figure out the right gearing?

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Aug 10, 2003, 10:44 PM
Registered User
It'd be easier and more convincing if the output was hooked to a voltmeter. Then there'd be no doubt that the windmill was generating elec. And then just suggest this elec. could recharge batt. or have other uses. Doing this would lessen the engineering process of choosing the right motor and gearing. Just my two cents
Aug 11, 2003, 02:12 AM
Registered User
shaneyee's Avatar
The Voltage generate is given by the KV of the motor. The Motocalc database is a good place for this info.. eg the Graupner 7.2V has a KV of 2227 rpm/V. If you turn it at 2227 rpm you'll get 1V at the terminals. Once you draw current, the effort to turn it will increase and if you are using wind, the rpm will decrease and the voltage will drop. The rest you'll have to experiment and get some data.....It might be easier to pick an application like powering a light bulb which is tolerant of voltage variability rather than charging a battery which requires a voltage regulator and other circuitry.

An interesting thing about windmills and real life use is that the windmill has a safeguard in that if the wind gets too strong the cloth panels flap and create turbulence therby restricting the rpm and saving the mill mechanism. Modern propeller windmills have to have variable pitch otherwise they'll burn out their windings in a storm.

Last edited by shaneyee; Aug 11, 2003 at 02:15 AM.
Aug 11, 2003, 04:50 AM
rroback's Avatar
You'll need to do some serious gearing if you want to generate more then a couple of volts. The windwill will only spin at like 70 rpm ( generally) and you obviously need the motor to spin probabily 20 thous. rpm. good luck gearing but i'd like to see photos,

Aug 11, 2003, 07:26 AM
Registered User
Thanks for the help.

I only need to generate a couple of volts, since I need to match it as closely as posible to a solar panel that can generate 2 volts, 350ma (Project is to compare solar vs wind performance at several locations in our country).
The voltmeter is great,but will need a way to log the voltage data it generates in order to be able to make the comparison. That's why I though I could maybe charge one empty cell and use a cycler to "read" the stored energy. I though I could just connect the motor terminals to the cell with a diode in the circuit. Will this work?
Thanks again,
Aug 11, 2003, 09:46 AM
Registered User
shaneyee's Avatar
Kind of like comparing apples to oranges isnt it? Depends more on the efficiency of the machine than the location...think you'll need some simple limiting parameters like for $50 how much electricity could you produce at location A using solar vs wind. Or given a portable system weighing less than 1 lb how much.....

Aug 13, 2003, 01:55 PM
Registered User
E-Challenged's Avatar
I was able to barely light a 1.5 volt penlite bulb with a Speed 280 run by my Jensen 75 steam engine at as high rpm as I could get. A small windmill would have to have some real strong wind to spin a 280 fast enough to get a reading on a voltmeter. Can motors do not make efficient generators, Jensen sells a dedicated generator accessory for it's steam engines.
Aug 13, 2003, 04:12 PM
Registered User
poobs's Avatar
Sounds like a fun project but DC motors do not put out DC current.

You would be better off showing the output of the motor on a cheap volt meter.

Motors do not make good generators and as others mentioned, you would need gearing to speed up the motor for some suitable voltage.
Aug 13, 2003, 05:14 PM
Registered User
Andy W's Avatar
You will still need a diode, or the cell will turn the motor!
May 19, 2005, 08:17 PM
Registered User
Most home size windmills run at about 2 to 3 hundred rpm.
If you're trying to do an actual study of any use on the pro's and cons of wind vs solar you would be better of deploying a series of anaeometers and solar cells connected to data loggers around the country.
This will give you standardised scientific data rather than questionable data from a home built rig.
You will probably find that while wind speed will vary with relief and distance from the coast, sunlight hours will be relatively constant.