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I'm designing and will be building a wind turbine. What characteristics do I look for in choosing a motor to use as a generator?

I know this might not be enough information, I'm not an electrical engineer. What I'm really looking for is the details I need to look for when choosing a suitable motor for the job as generator.

For example: AC or DC, brushless, RPM, high torque or not?

I want to generate the most power possible, right now I don't know what rate the wind turbine is going to spin at.

My design for the prop at the top of the wind turbine:

enter image description here

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    \$\begingroup\$ That's it? That's the whole question? Isn't this a little like saying, "I'm building a vehicle. What do I look for in choosing a chassis?" \$\endgroup\$
    – jonk
    Apr 30 at 6:29
  • \$\begingroup\$ The characteristic you want is that it makes the wind turbine run as a fan in the same way you expect/want it to run when operating as a wind turbine. \$\endgroup\$
    – DKNguyen
    Apr 30 at 6:39
  • \$\begingroup\$ @jonk, I'm not an electrical engineer, so the question is very broad because I don't know exactly what to ask...I'll edit and see what else I can inject. \$\endgroup\$
    – SPlatten
    Apr 30 at 6:55
  • \$\begingroup\$ @SPlatten I'm not intending to be mean in the comment. I just want you to realize that when you engage in a topic for which you are new, you will have a great deal to learn if you will master the topic sufficiently to succeed well. I don't mean you get from "zero" to "perfection," either. There will be lots of mistakes along the way unless you are very, very exceptional and can grasp a lot as you move forward, step by step. I'm no expert in this area, but I recognize when I need to spend time -- lots of it -- learning. And this is one of those. You'll need to grasp the key ideas first. \$\endgroup\$
    – jonk
    Apr 30 at 7:00
  • \$\begingroup\$ @SPlatten The very very very first thing you need to do is to measure your situation. What do you have in terms of area, height, and wind direction patterns and velocity patterns over a year's period? What do you know about the specific location you are at? A great deal (perhaps more than you recognize) depends very heavily on the wind speeds and patterns over a year's cycle. Why don't you realize this aspect and why haven't you already performed an evaluation of your site? How do you expect to make other decisions before this data has arrived and been evaluated? \$\endgroup\$
    – jonk
    Apr 30 at 7:03
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If you are going to generate the 'most power possible', you need a motor that is large enough to absorb all the power the blades can generate, at the rotation rate of the turbine.

If you don't know the expected rate, and you don't know the expected power, then any motor will do, while you get doing some experiments. Make sure the motor is easy to change, as when you are disappointed by your first go, you'll want to change it to something bigger/better/different.

A permanent magnet motor is easiest to use, generating DC directly. A brushless motor will need a 3 phase rectifier, but will be more compact for any given power. Use schottkys rather than silicon diodes to make best use of the low voltage output.

Most motors tend to spin quite quickly to deliver their rated power, any practical sized turbine will spin way slower than those. So if you have a choice, choose a 'low speed' motor. A wheel-hub motor intended for a powered bicycle might be a good match. You might be able to find a geared motor, though the gearbox will absorb some of your wind power.

At the low speeds involved, it might be practical and fun to make your own brushless generator, maybe integrated right into the turbine. There are several videos on youtube showing people making axial generators by gluing high strength magnets to flat steel plates.

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You haven't drawn a scale on your diagram, so we have no way of estimating how big it is, but the field of wind turbines is pretty mature and there are calculators available e.g.: https://rechneronline.de/wind-power/

Your design is weird and will therefore be less efficient than a conventional turbine of the same size.

Common small generators for this kind of thing are bicycle dynamos and car alternators, both of which are designed for generation and often available secondhand or as scrap.

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  • \$\begingroup\$ Weird? Different :), I haven't printed it yet, the size will be another thing that comes out of the R&D process. The benefit of this design is that the Wind Turbine will not have to rotate to find the find direction. \$\endgroup\$
    – SPlatten
    Apr 30 at 9:59
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    \$\begingroup\$ That's a common property of vertical axis (VAWT) turbines. Yours has quite a lot more material in and no through path for air in the center. \$\endgroup\$
    – pjc50
    Apr 30 at 10:12
  • \$\begingroup\$ Note there are VAWTs with a decent TSR unlike the Savonius. They are still an inefficient use of material because the blades have to resist bending, not just tension. See Darrieus for a reasonable compromise, the catenary curve puts the blade mostly in tension. \$\endgroup\$ Apr 30 at 12:20
  • \$\begingroup\$ reuk.co.uk/wordpress/wind/darrieus-wind-turbines ... not selfstarting : I'd start one with a Savonius on a freewheel. \$\endgroup\$ Apr 30 at 12:27
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What characteristics do I look for in choosing a motor to use as a generator?

  • A brushed DC motor will generate voltage even at low RPM. Obviously, it produces DC. A downside of such a motor is that it requires periodic maintenance, related to brush wear. If the DC motor does not have permanent magnets, then it may require an external source of power to start current in the "field" winding. Some DC motors will start generating with residual magnetism from previous operation, but unless you know for sure that is the case, you shouldn't rely upon it.

  • A (three-phase) induction motor will generate (three-phase) AC. Furthermore, this AC can be naturally synchronized with an external power grid. It does not have brush related maintenance issues. A down side is that it has a minimum speed that it must be running at to generate electricity. Below that speed, it acts as a motor, and consumes power. Thus, it needs to be turned on and off depending upon wind conditions.

  • An automotive alternator is designed for rather rugged conditions, and will work well provided the power required is within the alternator's range. It actually generates AC, but generally is used with a rectifier/regulator circuit that gives a fairly stable DC output. By "fairly stable", I mean that the output is both electrically noisy, and may fluctuate depending upon conditions such as heating etc. Much of the electrical noise and voltage fluctuations can be alleviated by connecting the alternator in parallel with a lead acid battery.

  • A brushless DC motor, as is common in electric automobiles, can be used for power generation, but will require some complex circuitry. Such circuitry may be obtainable in the automotive parts market. Designing your own would be non-trivial.

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