Homemade generator

I’m wanting to build a generator. Due to high rotational speeds i intend using a diametrically opposed cylinder magnet rotating on a shaft. Im looking to generate a maximum of 12 volts and 120 Amps. It’s only for a prototype so just something that demonstrates the ability of the unit. Rotational speed is expected to be 20,000 rpm upwards. I’ve looked on line but I’m still confused by winding a coil. Are both coils connected. And how should they be connected. Should they be connected from the outside of one coil to the inside of the other? Will the current be ac or dc ? Is there a formula to find power output and what size wire to use for the coil windings. Due to the high rate of flux change I won’t be using a iron core for the windings if that makes any difference

Hi Rodney,

Sounds like an interesting project! There’s an awful lot of energy involved in something that fast and powerful so it’d probably be best to start with something small scale so that you understand the principles before scaling up to something larger.

I’ll always remember the photo in my thermodynamics textbook of the carnage in a room after a centrifuge let go - it looked like a hand grenade had gone off (and the energy released was about the same!).

Here’s a spinny thing letting go:

You don’t want to be anywhere near them when they fail!

The output of a rotational generator will always have some AC component thanks to trigonometry. Coils in generators are usually connected in opposing pairs.

I’d recommend grabbing one of these little water turbines and doing some reverse engineering to understand how they work:

They’re actually a 3 phase AC generator with an onboard rectifier and regulator. Another good go to is to use an alternator from a vehicle. Modern vehicles often have alternators capable of providing more than 10 Amps (and at a lot less than 20,000 RPM, so they’re a lot safer).

What’s your application? The only thing I can think of is a direct drive generator from a gas turbine, either for a large RC aircraft, or just curiosity from a turbo-supercharger gas turbine.

It’s being driven by an experimental turbine. The turbine itself will be stress tested and the generator will be housed securely. The way the turbine works means a it needs a very solid structure housing it.

Can you describe opposing pairs. The turbine itself needs to be doing 20,000 rpm to get useful power so using an alternator is difficult unless gearing is applied which increases the complexity of the build. An alternator is the way to go if the turbine proves itself and warrants extra engineering dollars spent.

Hi Rodney,

Yes, but I’ll reiterate I’d strongly recommend starting slow and small before attempting the engineering required for a high-speed generator.

Adding a generator is going to introduce electromagnetic resonances which may well result in a catastrophic failure. My background is mechanical engineering and unfortunately this is well outside my area of comfort in providing advice - the risks are too high. I can only recommend engaging an engineering consultancy for this one if you do need to go directly to a fully functioning high speed generator.

FYI though, here are a couple of academic papers on the topic:


ICEMS2017_paper_162.pdf (3.6 MB)
Paper04_Gieras_Jonsson.pdf (372.4 KB)

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