As a pet project I'd like to make a small windmill. I'm in the very beginning stages so I'm still researching components to use and have a lot of questions so I came here to start my journey. I was thinking of using a brushless motor (as small as possible) without the control circuitry, and connect the windmill blades to the shaft. Let's say I expect 50-200 CFM of airflow, and I just want to output as much power output as I can. First, are there small brushless motors with a high enough efficiency to produce a considerable amount of voltage say 5, or even 12V, also is there a rule of thumb regarding current output for different sized motors? Also again, when looking for brushless motors without the extra circuitry inside what applications should I look for, brushless motors for RC drones, or planes?

Now let's say I found the perfect motor for my needs and I am doing the calculations for the EMF and the terminal voltage, do I need knowledge of how large my load is? if not how would I calculate the terminal voltage, because I though the formula was V = emf - (load current * armature resistance). and emf = number of poles * rotation speed * magnetic field strength * number of conductors / number of return paths. If there's a certain amount of torque required to move the shaft is that information available on the the device datasheet? will it have a noticeable effect on the output voltage? is the only to know for me to make a test jig and run some experiments?

Thanks for all the help, sorry if this is a noob post.


closed as too broad by mkeith, Dave Tweed Dec 7 '18 at 15:24

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    \$\begingroup\$ There is a lot to this. As the wind speed varies, you have to vary the generator rpm to keep the blades in an efficient generating zone, so to speak. But the back emf also varies with speed. So you can't just rectify the motor output and connect it to a battery or something. Probably the simplest thing would be to use a motor speed controller capable of regen, and then use a wind speed sensor to set motor rpm's. The controller will be trying to slow down the blades, and this will put the motor in regen, and charge the battery that it is running from. \$\endgroup\$ – mkeith Dec 2 '18 at 4:49
  • \$\begingroup\$ Most motors are going to spin too fast for this application. You can either use a reduction gear, or find a slow motor. The propeller blade design is also a critical component of the whole thing. \$\endgroup\$ – mkeith Dec 2 '18 at 4:51
  • \$\begingroup\$ What height are you thinking of mounting your wind turbine - ground level is easy but relatively poor compared to higher... \$\endgroup\$ – Solar Mike Dec 2 '18 at 7:13
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    \$\begingroup\$ Here is probably the best design process. First pick a blade diameter. Then find an actual blade you can purchase. NOW, based on blade specifications (pitch, etc), calculate the torque and speed of the blade in the maximum wind speed you want to operate in. NOW you can select a motor that is rated a bit higher than the torque and speed you are looking for, and find a motor controller. \$\endgroup\$ – mkeith Dec 2 '18 at 7:17
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    \$\begingroup\$ First find the average wind speed in the area you will mount the turbine... that sets you up with the power available... \$\endgroup\$ – Solar Mike Dec 2 '18 at 7:51

When you indicate CFM, that implies a ducted flow, and not open air. If so, a propellor-type rotor would fill the duct cross section. A more directed flow could hit one edge of a squirrel-cage or cup-type rotor.
Sailboats use smaller wind turbines. You could choose a good rotor design and working speed.


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