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enter image description here

I'm new to electronics and I've made a permanent magnet brushed DC generator, but when I spin it (using a drill to get high RPM) I get an output of less than 1 volt.

I've used it as a motor and it runs perfectly fine. I'm wondering if maybe it's the size of the magnets?

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  • \$\begingroup\$ What load do you have on the output of the generator? Have you made sure the brushes are kept in contact with the contacts at the high speed the drill will turn it at? \$\endgroup\$ – Puffafish Dec 10 '19 at 9:41
  • \$\begingroup\$ Ive tried no load (just to test voltage) ,and some LEDS and yes i have. \$\endgroup\$ – Liam Stoermer Dec 10 '19 at 10:37
  • \$\begingroup\$ @LiamStoermer Congratulations on your creation !!! :-). It's great to see such incredible enthusiasm in an age where such things seem to be much less of interest. ||| Noting what Andy said - if you use "rare earth" magnets which are MUCH stronger you will get proportionally more voltage AND while an iron yoke will help it is MUCH less needed. A modern RE magnet will give about 1 Tesla field at half its depth from the pole. So minimising the airgap makes a vey substantial difference. \$\endgroup\$ – Russell McMahon Dec 10 '19 at 11:47
  • \$\begingroup\$ V= 4.44 N.F. Bmax. A | N = turns, Bmax = max flux, f = frequency = coils past magnets/second, A = area at which flux applies. || Screws are a start but a solid iron core will help heaps. Soft iron wire good. A soft iron strip along the screws should help - with magnets adjusted to JUST not touch. | Coil area outside magnet faces is wasted. Extend magnets with steel strip or shorten coil if possible with more turns and same wire. All that should make a vast difference. || Head positioners in old disk drives will supply superb magnets. | Report back. \$\endgroup\$ – Russell McMahon Dec 10 '19 at 11:54
  • \$\begingroup\$ Old question, but I'll post this anyway as it appears to be a significant problem. The way you have your coils wound, it appears you've just done long windings around the screws. The problem with this is that as the coil passes a magnet, for most of the time it passes, both sides of the coil are being exposed to the same pole. You're only generating power based on the difference in field on that same pole, instead of based on the difference in field between a north and a south pole. What you'll want to do is rewind your coils so that when one side of the coil passes a north pole \$\endgroup\$ – K H Jan 12 at 1:00
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It appears in your picture that you have two magnets that are not magnetically connected with a low reluctance material: -

enter image description here

Without the yoke (frame) the magnets in the picture above will not be very effective. The magnetizable material in the yoke ensures that field lines pass more effectively into the rotor.

I'm not ruling out other problems either because it's hard to tell from just a basic picture. Picture source: -

The yoke is the outer cover of the machine supporting and protecting the internal parts. It is made of low reluctance ’ material like silicon steel or cast iron, Since, it has to carry the magnetic flux i.e., to provide the closed path for the flux produced through the poles.

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  • \$\begingroup\$ Would tin work fine for the yoke? And should i add bigger and/or stronger magnets? \$\endgroup\$ – Liam Stoermer Dec 10 '19 at 10:42
  • \$\begingroup\$ Is tin magnetizable?. Fix the yoke first but remember I'm not ruling out other problems/weaknesses in your design. \$\endgroup\$ – Andy aka Dec 10 '19 at 11:21
  • \$\begingroup\$ @LiamStoermer You want a high-permeability metal for the yoke. Iron alloys are generally the best option, though I imagine you could probably use nickel or cobalt if you really wanted to, or a ferrite. Realistically, steel is the best option for a homemade one. \$\endgroup\$ – Hearth Dec 10 '19 at 12:37
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Congratulations on your creation !!! :-).
It's great to see such incredible enthusiasm in an age where such things seem to be much less of interest.

Noting what Andy said - if you use "rare earth" magnets which are MUCH stronger you will get proportionally more voltage AND while an iron yoke will help it is MUCH less needed.

A modern Rare Earth magnet will give about 1 Tesla field at half its depth from the pole.
So minimising the airgap makes a very substantial difference.

V= 4.44 N.F. Bmax.A
N = turns,
Bmax = max flux,
f = frequency = coils past magnets/second/2
A = area at which flux applies.

Screws are a start but a solid iron core will help heaps.
Soft iron wire good.
A soft iron strip along the screws should help - with magnets adjusted to JUST not touch.

Coil area outside magnet faces is wasted.
Extend magnets with iron strip or shorten coil if possible, getting more turns with same wire.

All that should make a vast difference.

Head positioners in old disk drives will supply superb magnets.

Report back.

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  • \$\begingroup\$ The trouble with not having a proper yoke is that the field from the north pole will easily return back through the same rotor coils and the induced voltage will largely tend to cancel. Ditto the south pole. Ideally, you want the magnetic field to push across the rotor from north of magnet 1 to south of magnet 2. Without a yoke that is impossible. A yoke is minimizing the air gap. \$\endgroup\$ – Andy aka Dec 10 '19 at 12:49
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There's a major problem with the way your coils are wound relative to the position of the magnets. I made some images to illustrate. These images show what happens with the voltage induced in your coil as they pass the face of a magnet. Coils are simplified to a single loop of wire for simplicity.

enter image description here

In this image you can see that the face of the north magnet is passing one side of the coil. A voltage is induced on half of the coil, and none on the other half, and you get an induced voltage, but not as much as you would have if both sides of the coil were working together.

The magnet continues to move and we have our second image:

enter image description here

Here you can see that both halves of the coil are about equal distance from the center of the pole face. As a result, the strength of the voltage induced in each half of the coil is equal. In addition to this, you can see that the voltages induced are pushing against eachother!(Both are pushing upwards toward the top of the loop). We can see in the center that the sum of these two vectors working against eachother is zero. The magnet continues to move past the coil and we have our third image:

enter image description here

Similar to the first image, the magnet is now passing by only one half of the coil and the induced voltage again has a sum effect, except now in the opposite direction. Note that as a result of the interactions you can see, a sine wave of sorts is still produced, even though we have only looked at what happens when the coil passes one magnet face.

Here's one last image to illustrate what you actually want to be happening:

enter image description here

This image shows a north and south pole working together on a coil, and you can see that because the north pole pushes up in the diagram and the south pole pushes downward, in the same direction along the loop, their vectors sum together for a much greater induced voltage.

So at this point you should see why your generator was producing close to no voltage, and you can see an awkward problem with the way your coil is wound. Because the coils are so narrow, both halves of the coil fit on one magnet face, so even if you had a complete ring of N and S magnet poles around your generator, you wouldn't get much induced voltage for the reasons I've explained above. (you would get more, volage, but not an even sine wave. You are using a single north and a single south pole opposite to each other, so you need to rewind your coils so that each half of each coil are on opposite sides of the spinning cylinder. As a result your method of using screws driven into a smaller center cylinder to support the coils is unlikely to work. You may wish to salvage a motor core off an old cordless drill(or other power tool) motor and do your windings on it. Brushed DC motors come with commutators fixed to the shaft that you can repurpose. Broken cordless power tools are cheap and have nice cores in them for hobby motor winding. Nice thing about using a factory made core is they are made out of laminate silicon steel to reduce eddy currents, and this will give you superior performance to any core you could build yourself. You can find small cores with a variety of numbers of slots, so you can wind 3 phase or single phase generators depending on the number of slots in your core. You probably want a core with an even number of slots so you can make a single phase generator to get started. I've rebuilt one 8 slot motor core into a generator personally. I wound my own coils onto the core and replaced the original magnets with rare earth magnets. It works really good for easy to build hobby stuff.

Sorry this answer is more than a year late, but I believe I've illustrated a fundamental problem that could come up in other hobby motor or generator builds.

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