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Cheap and easy electric generator design

  1. Mar 10, 2015 #1
    I would like to build, from scratch, a small generator to be driven by a small windmill, for experimental purposes.
    One such generator on the internet has 8 NIB disk magnets (about the size of a quarter coin), flat sides facing out, mounted around a plastic rotor, with axes aligned along the radii, and 8 air core coils in Series around a stator, with axes aligned along the radii, surrounding the rotor.

    It is a simple, easy to build generator, with easy to wind air core coils, about 1/2 inch in diameter and 1/2 inch in length, on plastic bobbins.

    It appears the idea for air core was to eliminate the pull or resistance to rotation that iron cores would have. This allows the rotor to be spun easier, and faster, without the magnets getting "stuck" in alignment with the coils. It appears to lower the starting torque required to initiate rotation.

    Is this an acceptably efficient type of generator, or is there a significantly more efficient design, that is still easy to build, with very low start-up torque?
  2. jcsd
  3. Mar 11, 2015 #2


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    Cheap and easy probably negates the ability to make it efficient or practical. What do you plan to do with it? It should be able to provide some level of lighting (if you use LED low voltage lamps ie flashlight type bulbs). Yes.
    Directly charging some type of battery? Maybe, esp if small, possibly as much as a car battery, if it only supports a ham radio for a couple of hours a day.

    Powering a real load ie refrigerator? NO.
  4. Mar 11, 2015 #3


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    I really hope not. Isn't that what engineering is all about? Weren't all the great Victorian engineering projects about just that?

    Perhaps that's why so many big projects fail or go way over budget these days: people really believe the route to a good solution is money and complexity.

    Anyhow, "What do you plan to do with it? " That's the crucial question.

    I'm not sure I'd agree with CalcNerd about exactly what is and isn't possible, but maybe he's heading in the right direction.
    My first reaction was to think, no: without a ferromagnetic core you are reducing your flux in the coil maybe 10x or more.
    But when I thought further about your principal requirement, low starting torque, I thought air core coils is the only simple solution. So then the question is, what are the problems with this and how significant are they?

    Since the induced emf is proportional to the rate of change of flux and the number of turns, then if you reduce your available flux by 10, then you need to increase the number of turns in your coil by a similar factor. So far no great problem, if all you want is to sense the rotation.
    The problems start when you draw current. If you have 10x the turns, then to fit them in the same space, the wire has 1/10th the area (roughly). So the resistance has gone up 10x for the length and 10x for the area, 100x all told. Now for every bit of current you draw the voltage drops 100x what it would have done with a core that allowed 10x the flux.
    You could offset these issues by allowing the coil to be bigger and by accepting a lower output power. It depends what you want.

    On a more positive note.
    The problem of the magnets getting stuck between the poles ("detent") is only a problem at start and at very slow speeds. Once the rotor is moving, magnets are pulled into the poles almost as much as they are restrained from leaving them and the force averages out to very little. Energy loss is small.
    Since you are concerned about starting torque, you have some control over this in choosing the radius of the rotor. For any given strength of magnetic force between the magnets and the poles, the torque will be proportional to the radius. Reduce the radius, lower the starting torque. Of course, as you reduce the radius, you reduce the speed of the magnets and thus the rate of change of flux. Depending on how much you need to reduce the torque, maybe this is a good compromise: don't get rid of detent altogether at a cost of 10x (or more) flux reduction, just reduce it by 2x or 4x and accept this smaller factor reduction in magnet speed.

    Maybe room for experimentation here? Perhaps that is what you intend? At some point, if the wind can't overcome some detent, then it's not going to be capable of giving any significant power in any case. As soon as you start drawing current, even from your air coils, you will get electromagnetic drag.

    Edit - PS. My factor of 10 is just a guess, based on reducing the field's average air path by 90% or so. My feeling is it could easily be more.
    Last edited: Mar 11, 2015
  5. Mar 17, 2015 #4
    Thank you CalcNerd. It is just for experimentation, and tiny current, low power LED loads.
  6. Mar 17, 2015 #5
    Thank you Merlin3189. Yes, the no iron, no attraction, low torque was what I was thinking. Your reminder that the magnetic pull in force is equal on the approach to the iron core, and on the departure, may explain why permanent magnet only motors don't work. But BOTH of those attractions, though it seems they may cancel out as far as any net rotational force, ONCE ROTATING, they don't seem to cancel as far as holding the magnet in alignment with the iron core when stationary. Yet it seems you are probably right that AFTER the START UP force pulls the magnets away, the overall drag on rotation may be lower than I had imagined.

    Your comment about the rate of change of flux is something to think about.
    And your overall thought that the iron cores are probably well worth the drag rings very true.

    Lowering that START UP force could be accomplished by having only two iron cores in the outer stator, 180 degrees apart, and 3 magents on the rotor, 120 degrees apart. That way only one magnet can be locked on one iron core at a time. This seems to require less START UP force, since the other 2 magnets are much farther away from the remaining iron core, and oriented obliquely. So this SEEMS to me to have less LOCKED IN force, than, for example, two iron cores, and two or four magnets, in which cases there would always be two core-magnet pairs LOCKED IN when at rest, requiring twice the START UP force.

    So thank you again for your observations.
  7. Mar 18, 2015 #6


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    Yes, this seems like a good idea. And going further having even more magnets might make it even better? One magnet is aligned, but another may be just approaching a pole, so as soon as you try to move towards that second magnet it helps pull away from the first. Like the more bumps there are, the more they even out?
    I'm not sure what the downside might be. More coils which are not in phase will add to the electrical complexity - more rectifiers. More magnets or more iron cores means more weight.
    On another tack, I have been looking at ironless rotor motors and these do seem to be a practical proposition, so I wonder whether I've missed something about iron cores. But I don't know much about these and I haven't worked it all out yet. Maybe the efficiency requirements are different for these.
  8. Mar 18, 2015 #7
    "More coils which are not in phase will add to the electrical complexity - more rectifiers. "
    @Merlin3189, Yes, the complexity is overwhelming me; the number of magnets and cores, the distance between magnets, the orientation of the magnets and their angles, the optimum rotational speed for highest production, iron core or air core, etc. I think I will just buy a generator, or use a motor, and gear the system so the wind turbine can start turning it in a very light wind, and keep increasing the motor size until the next size up overloads the wind turbine, causing it to stop.

    Since the original idea was that, in my area, there are light winds much more often than there are medium ones, designing the most efficient generator for low speed rotation, with the very least resistive torque, would generate electricity more constantly. It seems that would create more overall battery charging current than 5 or 10 short periods of medium wind gusts. Higher rotational speed giving higher voltage and more current, the gentle, constant light breezes need to rotate the generator, at some rated speed to get the voltage required to charge the battery. So to charge a 12 volt battery, I hear 14.4 volts is best, overall. Therefore, the generator needs to produce 14.4 or more volts in the lightest wind, not loading down the wind turbine so it doesn't spin. Hence the need for a generator producing the most voltage with the least torque. Gearing up or down can only go so far, and is basically irrelevant, since, regardless of gear ratios, the very most efficient generator is what is needed. Again, thanks for helping :-)
  9. Mar 26, 2015 #8
    hm,I think we have pdf plans for free download on the net for building your own generator driven by a windmill. check the net once.

    tsheten dorjee
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