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I am building a wind generator with a car alternator without a gearbox. I know that the alternator is designed to spin at 5-10k RPM and the blades runs at 60-120 RPM so the alternator generates less than 1 volt. My question is: Can I use a voltage multiplier such as a Cockcroft-Walton generator to get useable voltages at reasonable efficiencies?

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  • \$\begingroup\$ To be clear, you can convert your voltage, up or down, but a CW multiplier is probably not the best way to do it. Motor/generator design/selection will have an impact on performance. Disc generators are popular DIY windmill solutions, and you can also repurpose many other kinds of motor if you want to run directly at low speed. If you are fond of the alternator, adding a gearbox, despite the losses in the gears. For brushless permanent magnet motors anyway, you're be looking for low kV ratings. I suspect motors from those explodey "hoverboards" would be good. \$\endgroup\$ – K H Aug 26 '18 at 1:34
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You probably need to connect your field winding up to get more output but running at this dismal speed is going to be a poor generating solution. No, a cockcroft walton multiplier won't help much here because they need several volts p-p to make any headway due to the internal diode volt drops. Drop the idea of the CWM and stick with understanding how to set the field winding current. I believe a lot of automobiles use the field winding current to control output voltage hence the final alternator output is regulated.

If you are not using the field winding then you are relying on residual magnetism in the rotor for generating your output voltage so it will be low.

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  • \$\begingroup\$ All alternators use the filed winding, the question is: Is the OP using a self excited alternator or a separately excited alternator. \$\endgroup\$ – Jack Creasey Aug 25 '18 at 19:20
  • \$\begingroup\$ So I need to work on the field winding current but i'm worried about the fact tha the field winding could absorb more energy than it produces \$\endgroup\$ – Alessandro Carinelli Aug 25 '18 at 19:38
  • \$\begingroup\$ P.S.My alternator is separately excited \$\endgroup\$ – Alessandro Carinelli Aug 25 '18 at 19:40
  • \$\begingroup\$ Don’t worry about it, try it. If it used more that what is produced then that’s because it isn’t rotating and it would be a poor generator if it did that when spinning. \$\endgroup\$ – Andy aka Aug 25 '18 at 19:42
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Alternators on cars need 2000 to 2500 rpm to begin charging, this is why the pulley diameter is about 1/3 of the crank pulley.

Also the ratio of the pulleys is chosen so the max rpm of the alternator is not exceeded at max engine rpm - the rotor can contact the stator and seize since the clearance is minimal...

You should look at gearing or making your own alternator.

Here is a link to some really good information about making wind turbines, and yes, this guy has been there and done it... http://scoraigwind.co.uk/a-wind-turbine-recipe-book/

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