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Background

My world has a form of magic that is similar to telekinesis; that is, it makes things move. I also have technology that is essentially the same as today's real world.

Thinking about it, this seems like it could be really useful for powering certain sorts of devices. There is, in fact, a certain type of electrical generator that is powered by magic. It consists of a fixed-magnet rotor surrounded by one or more pairs of coils. The rotor has no axle or bearings; it is spun and held in place solely by magic. As a result, the generator can be fully enclosed with no exposed moving parts; is is very nearly "solid state". Of course, this means it necessarily produces alternating current (DC would require brushes), possibly in several phases, but solid state circuitry can easily rectify this into fairly constant DC.

(Alternatively, the generator has a mobile ring of permanent magnets which spins around a set of fixed coil-wrapped arms. I suspect this might actually work better, especially since the moving portion doesn't need an axle or bearings. The objective is to avoid brushes so as to minimize wear.)

Question

How small can I make such a generator? Can I make it, say, the size of a 2032 watch battery? (At least, small enough to fit in a Smart Watch?) It should be able to generate at least 2 watts. Do any such devices (obviously, unlike my magic-powered generators, they would need an axle that can be externally driven) exist in real life?

Alternatively, is there some other way I can generate the required energy in the available size, using only the ability to make things move?

Extensive Details

This isn't necessarily relevant, but... I've previously asked about this magic system here and here; those might provide some interesting details. In particular, magic is powered by the metabolism of living beings. For our purposes, what this means is that the generator has a constant supply of magic-energy as long as it is on or near a person (or, in some cases, an animal or other living source of magic). The generator isn't a "spin up once and coast" system; it is constantly being supplied with "fresh" kinetic energy via magic (when active; they can be made to have an "off" switch, and for safety and durability reasons, will halt themselves if separated from the person "feeding" the magic or if something happens that the rotor becomes stuck).

Also, and this happens to be extremely convenient, by nature of how generators work (and by how I imagine my magic working), a magneto-type generator will always provide exactly the electricity needed with fairly low waste. Specifically, magic causes the rotor to turn at a fixed speed, which translates into the amount of magic energy used to keep the generator spinning being directly correlated to the amount of electricity being consumed by whatever is connected to it. I can just spend the (very small) amount of extra magic to keep it spinning even if the connected device is off with minimal to negligible consequences. (Conceivably, there would be a delay as the flow of magic adjusts to changing power requirements, but this is what power regulation circuitry is for, and I need that anyway, at least in the form of an AC/DC rectifier.)

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    $\begingroup$ Generators generate exactly as much power as it is consumed. A 2 W generator does not have to generate exactly 2 W: what we mean is that is can generate at most 2 W. An AC generator using permanent magnets is called a magneto. A watch battery is huge -- such a large magneto could have been made in 1900. Today we have very very much smaller electrical generators and electrical motors. See micropower and microelectromechanical systems. $\endgroup$
    – AlexP
    Mar 4, 2020 at 17:42
  • $\begingroup$ "Researchers [...] have built a micro generator 10 mm wide, which [...] produces 1.1 watts." (How tall is it?) Okay, that sounds like 2W in 15-20mm is plausible. I'm vaguely familiar with MEMS, but a) it's less clear what their power generating abilities are (if any?), and b) one of the properties of my magic is that it's actually much harder to make it work on stuff at that scale. The parts that magic acts on will not be microscopic. $\endgroup$
    – Matthew
    Mar 4, 2020 at 17:58
  • $\begingroup$ @Alexander, it doesn't work that way; see edits. I don't really care about the total energy of the system because it isn't meant to work that way; it's meant to provide continuous power given an ongoing input of magic. In fact, because it has no bearings, quickly spinning down while it still can when about to lose its magic input will be an important feature. (That said, I can be flexible on the "no bearings"; the desire to avoid them is a) cost and b) less friction under "normal" operation.) $\endgroup$
    – Matthew
    Mar 4, 2020 at 18:08
  • $\begingroup$ So, a generator like this would leech energy from nearby humans? $\endgroup$
    – Alexander
    Mar 4, 2020 at 18:11
  • $\begingroup$ @Alexander, exactly. There's a reason I gave "smart watches" as an example 😉; they'd be powered by the wearer, and would shut down or require some other power source when taken off. $\endgroup$
    – Matthew
    Mar 4, 2020 at 18:17

1 Answer 1

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One easy generator to make is a faraday disk or homopolar generator. And it should easily fit in your specified 2032 battery dimensions.

enter image description here

In the image the power is developed between the center axis of rotation and the outer edge, as the disk spins. These power sources generate a great deal of current at a low voltage and have been used to power welding machines. The drawback of this design is the rotating contacts.

For your application, since you have magic, you can fix the conductive disk, and rotate the magnets generating the field B shown in the image. Any permanent magnets will work for your application. To generate more power, spin the magnets disk faster. The faster you go, the more power you get. To get 2W out of the generate, you'll need to put in at least 2W of kinetic energy spinning the magnets plus any losses resistive losses in the disk.

The thickness of the disk is determined by airflow for cooling and the current flow delivered to the load. 1 mm of gold can carry over 40 Amperes of current safely, which is much higher than you are going to need.

Your buck-boost or resonant power converters can convert the output voltage to whatever value and waveform you need.

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  • $\begingroup$ "you can fix the conductive disk, and rotate the magnets"... I'm not actually sure this is correct? One source appears to state otherwise. $\endgroup$
    – Matthew
    Mar 6, 2020 at 21:59
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    $\begingroup$ @Matthew, It doesn't matter whether a conductor moves threw a magnetic flux or if the flux moves through the conductor, as long as the geometry of the disk and the magnetic flux are correct, a current will be induced in the conductor. I read your link and it doesn't contradict my position. The later comments by Tinman seem to be discussing a homopolar motor design where the magnets get rotated by a battery applied between the axis of rotation and edge of the disk. And, he correctly observes that it doesn't work, and it shouldn't $\endgroup$
    – EDL
    Mar 6, 2020 at 22:54

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